Catalytical reactions and environmental chemistry

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Catalytical reactions and environmental chemistrymodifications as seen by synchrotron radiation

NAP-XPSAnthony Boucly

To cite this version:Anthony Boucly. Catalytical reactions and environmental chemistry modifications as seen by syn-chrotron radiation NAP-XPS. Analytical chemistry. Université Pierre et Marie Curie - Paris VI,2017. English. �NNT : 2017PA066237�. �tel-01770411�

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THЀSE DE DOCTORAT DE

L’UNIVERSITÉ PIERRE ET MARIE CURIE

Spécialité

Chimie physique et chimie analytique de Paris Centre

Présenté par

Anthony BOUCLY

Pour obtenir le grade de

DOCTEUR de l’UNIVERSITÉ PIERRE ET MARIE CURIE

Catalytical Reactions and Environmental

Chemistry Modifications as seen by

Synchrotron Radiation NAP-XPS

Soutenue le 17 Octobre 2017 devant le jury composé de :

M. Joachim SCHNADT Professeur Rapporteur

M. Markus AMMANN Professeur Rapporteur

M. Jean DAILLANT Directeur de SOLEIL Examinateur

Mme Virginie MARRY Professeur Examinateur

Mme Sophie LE CAER Directeur de Recherche Examinateur

M. Jean-Jacques GALLET Maître de conférences Examinateur

M. Ahmed NAITABDI Maître de conférences Examinateur

M. François ROCHET Professeur Directeur de thèse

2

3

Acknowledgment

First I would like to thank Alain Dubois director of the LCPMR for welcoming me for

those three years inside his laboratory.

Then to my wonderful PhD supervisor François Rochet for giving me the opportunity

to work with the state of the art NAP-XPS, for his continuous support during those

three years on a complex yet interesting subject, I could have not imagined having a

better supervisor for my P.h.D.

I would also like to express my gratitude to my co-supervisors Jean-Jacques Gallet

and Fabrice Bournel who have always been here for me, but also for their insightful

comments and continuous encouragement during my PhD.

A great thank to Ahmed Naitabdi for allowing me to work with him on the subject of

the nanoparticle and for always being a source of motivation and dynamism for me.

I would like also to thank the members of the jury for accepting to evaluate my work

and their useful comments.

A special mention to our scientific partners at the PHENIX laboratory, for their

continuous help during our collaboration on clays.

I thank my fellow PhD student (Quentin, Gildas, Marie, Sevan, Antoine, Alessandra,

Solène and all the other) for the help they offered throughout the thesis but also for

all the fun we have had during those three years.

Finally for my family and my friend who have always supported me and I know will

always be here for me, I owe you everything.

4

Table of Contents

1

Table of Contents

Nomenclature ..........................................................................................................................................3

Introduction .............................................................................................................................................4

Chapter 1: NAP-XPS through history .................................................................................................7

1) Principle of NAP-XPS ....................................................................................................................7

1.a) The basic principle: XPS .......................................................................................................7

1.b) Evolution towards NAP-XPS ............................................................................................. 14

1.c) Main topics of study ......................................................................................................... 16

2) The NAP-XPS at the TEMPO beamline...................................................................................... 17

Chapter 2: Study of swelling clays .................................................................................................. 20

1) Introduction .............................................................................................................................. 20

2) Hydration of clays studied by NAP-XPS .................................................................................... 27

2.a) Experimental procedure ................................................................................................... 27

2.b) Assessing charging effect ................................................................................................. 28

2.b.1) With Au4f.................................................................................................................. 29

2.b.2) With C1s as a reference peak ................................................................................... 30

2.b.3) Controlling charging effect ....................................................................................... 32

2.c) Hydration of clays ............................................................................................................. 35

2.c.1) Cs+ samples ............................................................................................................... 35

2.c.2) Na+ samples .............................................................................................................. 40

2.c.3) Sr2+samples ............................................................................................................... 47

2.c.4) Binding Energy Shift interpretation .......................................................................... 50

3) Insertion of Pyridine ................................................................................................................. 52

3.a) Experimental procedure ................................................................................................... 53

3.b) Hydration state of the Sr2+-hectorite ................................................................................ 54

3.c) Adsorption/sorption of pyridine at 7% RH ....................................................................... 57

3.d) Mineralization of adsorbed/sorbed pyridine under synchrotron beam irradiation ........ 61

4) Conclusion ................................................................................................................................ 68

Chapter 3: Catalytic reaction on Nanoparticle ............................................................................... 70

1) Introduction .............................................................................................................................. 70

2) Morphology study with STM .................................................................................................... 77

3) Pure oxidation on pure Pt and PtZn NPs .................................................................................. 79

3.a) Metallic Peak .................................................................................................................... 79

Table of Contents

2

3.a.1) On pure Pt Nanoparticle ........................................................................................... 79

3.a.2) On ZnPt nanoparticle ................................................................................................ 83

3.b) C 1s and O 1s Peak ............................................................................................................ 86

3.b.1) On pure Pt Nanoparticles ......................................................................................... 87

3.b.2) On PtZn Nanoparticles .............................................................................................. 89

4) CO Oxidation reaction on Pt, Zn and PtZn NPs with CO:O2 at ¼ ratio ...................................... 91

4.a) C 1s and O 1s spectrum .................................................................................................... 91

4.b) Metallic peak .................................................................................................................... 97

4.b.1) Pure Pt NPs ............................................................................................................... 97

4.b.1) Pure Zn NPs ............................................................................................................. 100

4.b.2) ZnPt NPs .................................................................................................................. 103

4.b.3) Assessing the Nanoparticles activity ...................................................................... 109

5) Conclusion .............................................................................................................................. 113

Conclusion and Perspectives .............................................................................................................. 116

Appendix ............................................................................................................................................. 120

A. Hydration of swelling clay ...................................................................................................... 120

B. Insertion and mineralization of Pyridine ................................................................................ 126

References .......................................................................................................................................... 131

Figure Contents .................................................................................................................................. 158

Table Contents .................................................................................................................................... 163

Nomenclature

3

Nomenclature

1W: One water Layer state, I.E. first hydration sphere

2W: Second water Layer state,I .E. second hydration sphere

BE: Binding Energy

CLS: Core Level Shift

FWHM: Full Width at Half Maximum

IMFP: Inelastic Mean Free Path

IE: Ionization Energy

KE: Kinetic Energy

NAP-XPS: Near Ambient Pressure X-ray Photoelectron Spectroscopy

NPs: Nanoparticles

RH: Relative Humidity

STM: Scanning Tunneling Microscopy

UHV: Ultra High Vacuum

VL: Vacuum Level

XPS: X-ray Photoelectron Spectroscopy

XRD: X-ray Diffraction

Introduction

4

Introduction

Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) that

operates in the tens of mbar range has recently emerged as a powerful tool for

studying in situ chemical reactions under environmental conditions. Indeed, its major

advantage is the synoptical view, within a given core-level window, of the gas and

condensed phase species.1 This is highly valuable when chemical reactions are

occurring, especially in heterogeneous catalysis, as gas phase reactants and

products, on the one hand, and the electronic structure/chemical state of the surface,

on the other hand, can be put into relation. Similarly, environmental sciences which

integrate into their models chemical reactions taking place at vapor/solid or

vapor/liquid interfaces2–5 also benefit greatly from NAP-XPS, especially in the case

of water, as its vapor pressure is ~20 mbar at room temperature.

In addition, one of the main advantages of NAP-XPS is its sensitivity to the

material surface which is due to the small electron mean free path of electrons in a

solid (below 1 nm at kinetic energies of interest). This means that the only

photoelectrons that can escape the sample are those coming from the uppermost

layer of the sample surface. XPS has traditionally been conducted under ultra-high

vacuum (UHV, P≈10−10 mbar) conditions due to the electron analyzers designed to

work under UHV conditions. A recent revolution in the design of electron analyzers

permitted to overcome the question of the photoelectron yield attenuation in the gas

phase and the protection of the detector. The implementation of Near Ambient

Pressure (NAP) conditions (10 mbar) has been an impressive step forward in

addressing real-world issues with XPS, whose contribution to surface chemistry is

widely recognized.

This transition of UHV-XPS to NAP-XPS environment has been led by a few

groups around the world. In particular, a Berkeley group conducted pioneering work

in this field, and operates currently two such setups installed at the Advanced Light

Source synchrotron.6 Their first setup was able to operate at pressures in the range

of 1 mbar, about 7 orders of magnitude higher than the pressure limit of conventional

UHV equipment. This was achieved thanks to ingenious electron optics and efficient

differential pumping stages which allowed the sample to remain at environmental

Introduction

5

pressures, while reducing the sample/analyzer distance to maximize the

photoelectron yield. At about the same time, the Fritz Haber Institute (in collaboration

with Berkeley and SPECS-Berlin) built also an improved version of the Berkeley

prototype, and installed it at BESSY synchrotron (Berlin), where it is operated since

2002.7 Since then, several NAP-XPS instruments are in construction or already

operational all around the world.8

A similar system manufactured by SPECS and described in details in Chapter

1, was installed in February 2013 at the TEMPO beamline of the French synchrotron

facility SOLEIL. This instrument is operated by the LCPMR (Dr Jean-Jacques Gallet

is the principal operating officer), with Prof. François Rochet as scientific director of

the Functional and Environmental Surface Group. During my PhD thesis that started

in November 2014, I have been involved in two different projects where the NAP-

XPS is used as the main investigating tool, and where the gas/solid synoptical

approach has proved of prime importance. The first one is related to environmental

chemistry and concerns the chemistry of hydrated swelling clays. The second one is

dedicated to the catalytic oxidation of CO over metallic nanoparticles (NPs).

The first project detailed in chapter 2 concerns the study of the water

interaction inside the confined space of swelling clays in collaboration with Virginie

Marry and her coworkers of the PHENIX laboratory (UPMC). Clay minerals have

various industrial applications, beginning with their use in ceramic ware, paper-

making, removal of impurities, decolorizing and more recently as molecular sieves, in

catalysis. In relation with environmental issues, wind-blown clay dusts are the site of

key chemical reactions in atmospheric chemistry.9 Note that clays are a major

component of the geological storage of long half-time radioactive waste.10–13

Therefore the interaction the clay and water in the confined spaces of these layered

2D oxides, and the associated mobility of the inserted alkali or alkaline earth ions

inserted therein (of whom cesium and strontium) is of fundamental interest. The

observation of radiolytic reactions in a hydrated smectite induced by the soft X-ray

synchrotron beam is another contribution to this question.

Introduction

6

Finally in chapter 3, I will provide the results of a study conducted in close

collaboration with Dr Ahmed Naitabdi (LCPMR) on the reactivity of metallic and

bimetallic catalyst material in the form of nanoparticles. In particular, the in situ study

of CO oxidation, which is a paradigmatic reaction, in Pt, Zn and PtZn, NPs was

performed using NAP-XPS as the main tool. The present work demonstrates the

effectiveness of NAP-XPS in the study of complex catalytic processes operating on

realistic systems. The investigation of nanocatalysts under more realistic working

conditions of pressures and temperatures represents a real strategy toward a deeper

understanding of their chemical reactivity. Note that CO depollution is major issue, in

automotive industry, due to the toxicity of CO, and in the application of hydrogen fuel

cells, until the main production of hydrogen used in fuel cell is not done via

hydrocarbon cracking.

Chapter 1:1)Principle of NAP-XPS

7

Chapter 1: NAP-XPS through history

1) Principle of NAP-XPS

X ray photoemission spectroscopy (XPS) is an elementary surface analysis

method. The first XPS experience was done by Kai Siegbahn at the Uppsala

University in Sweden who won a physics Nobel Prize in 1981 for his work14.

1.a) The basic principle: XPS

An XPS measurement consists in irradiating the surface of a sample with a

monochromatic X-ray. If the energy of the photon is sufficient, it can tear away

electrons from the sample’s atoms creating the so called photoelectron which are

then emitted from the surface of the sample. Usually the range of excitation energy

for XPS is between 100 and 2000 eV. The Ultra Violet Photoelectron Spectroscopy

(UPS) uses the same principle than XPS but has lower excitation energy and is used

for probing the valence band of the sample while XPS is for the core electron.

The working principle of XPS can be described by the three steps model [Figure 1].

First step: Excitation and creation of a photoelectron

An atom absorbs a photon with an energy=hν sufficient to ionize it by

transferring its energy to an electron. The electron is promoted from a bound

state (Ei) to a free state (Ef) and the excess of energy gives its kinetic energy.

This step is linked to the cross section, the probability for a photon to promote

an electron on a given orbital.

Second step: Transport of the photoelectron to the surface of the sample

The surface sensitivity of the technique appears in this step where the

photoelectron strongly interacts with the matter before escaping to the surface.

This step is driven by the electron Inelastic Mean Free Path (IMFP), which is

the characteristic length that an electron can travel over an infinitesimal

distance without suffering an inelastic scattering.


8

Third step: Escape of the electron to the vacuum

Reaching the surface, the electron still need to overcome a barrier potential

that exist at the surface in order to reach the vacuum level. That barrier is

called the sample work function (w). If the electron arrives with an energy< w

then it will not escape and be reabsorbed into the matter.

Figure 1: Three step model: 1) Photoelectron creation, 2) Transport to surface, 3) Escape to vacuum

During step 2, the interaction of the electron and the matter leads to four

different outcomes for the electron [Figure 2].

First: Electrons which have been elastically scattered escape the surface of

the sample without any kinetic energy loss. Those electrons will create the main XPS

core level peak (also called adiabatic peak).

Second: Electrons which have been inelastically scattered escape the sample

surface losing a part of their kinetic energy. Depending of the interaction it can lead

to a broadening of the core level peak or the creation of satellite peaks.

Third: Electron which have been inelastically scattered many times and have

lost most of their kinetic energy having barely enough to overcome the sample work

function. Those are secondary electrons.

Fourth: Electrons that have lost too much kinetic energy due to inelastic

scattering and cannot overcome the surface potential (ws) and are reabsorbed by the

matter.


9

Figure 2: The different interaction between electrons and matter leading to 4 final states

Emitted electrons are then collected by the analyzer and are counted up by

the detector. The measure of the electron kinetic energy (KE) allows us to determine

the electronic structure of the sample [Figure 3].

Figure 3: Principle of XPS diagram

We can determine the binding energy of the electron coming from the sample by the

following energy balance equation: BE=hν-KE-ws

With:

BE : Binding Energy

ℎ𝜈 : Photon energy

KE : Kinetic Energy in vacuum

ws : Sample work function

Sample surface

Photons II

I III

IV

Electron

BE: Binding Energy

ws: Work Function

KE: Kinectic Enegy

Energy Balance

hv= Photon energy

Electron

Nucleus

=


10

The Kinetic energy measured by the analyzer (EKm) take into account the

work function of the analyzer φAna. By grounding both the analyzer and the sample

allow to align the fermi level [Figure 4].

We can then rewrite the energy balance: BE = ℎ𝜈 − 𝐸𝐾𝑚 − φ𝐴𝑛𝑎 allowing the

determination of the binding energy of the electron.

Figure 4: Sample and analyzer energy level diagram

The binding energy of a core electron measured by XPS is influenced by the

interactions of this electron with the surrounding environment. It is dominated by

Coulomb interaction with other electrons and the attraction of the nuclei. Changes in

the chemical environment have an effect on the valence electron of the element

leading them to a spatial redistribution influencing the value of the binding energy of

the core electron. For example a loss of a valence electron increases the binding

energy while the addition of an electron decreases the binding energy. Those

variations are observed as a shift on the XPS peak of the core electron, called

chemical shift (∆ξ). When the Fermi level of the sample and the analyzer are aligned

and taken as a reference, the chemical shift can be described by the following

equation:

∆ξ=-∆εi-∆ER+∆EF

With


11

Δεi: Initial state contribution

ΔER: Final state shift

ΔEF: Modification of the Fermi level position

The initial state contribution term characterize the potential modification due to

the chemical bond between atoms influencing the electrons. It can be described by a

simple model where we consider a point charge e representing the core hole

surrounded by the valence electron at an approximate distance r from the nucleus;

with the Gauss theorem we have the following relation:

∆εi∝𝐶𝛿𝑞

𝑟

With

C (constant)>0

δq: The partial charge transferred between the valence band and the surrounding

environment

When δq is positive, there is a charge loss due to bonding more electronegative

ligand to the atom and the initial state energy decrease. When δq is negative the

initial state energy increases.

The final state shift is linked with the screening capacity of the core holes by

the environment which is the extra atomic relaxation. In the case of a solid element,

the relaxation can be approximated by the electrostatic polarization energy around

the core hole. The influence of the hole can be described as a point charge in a

dielectric environment which will polarize its surrounding volume akin to a screening

cloud surrounding the positive charge. The energy associated with that situation is

given by the following equation:

ΔER= 1

4πε0(1-εr-1)

e2

2r0

With

εr: the static relative dielectric constant

e: the electronic charge

r0: The effective screening radius


12

The spin orbit splitting (or J-J coupling) is also a final state effect. All orbital levels

(except s) will have a spin orbit splitting giving two peaks, i.e. a doublet. This is due

to the fact that when an electron is removed from the orbital; it will make the orbital

composed of an unpaired number of electrons leading to a coupling between the

unpaired spin and the orbital angular momenta. Those peaks will have a specific

area ratio based on the degeneracy of the spin state. The total angular momentum

(J) is given by the following formula:

J=l+S with the angular momentum quantum number (1 for p, 2 for d and 3 for f

orbital) and S the spin orbit angular momentum of the electron (±1/2)

This will give two values corresponding to the two peaks appearing due to the

splitting (i.e. np1/2 and np3/2 or nd3/2 and nd5/2 with n the principal quantum number)

The relative intensity for the peaks is given by 2J+1 for both peaks. Thus for a 2p

orbital with a doublet composed of 2p1/2 and 2p3/2, the intensity will be ½ (2∗(

1

2)+1

2∗(3

2)+1

)

between 2p1/2 (less intense) and 2p3/2 (more intense). Due to a better screening

effect, the most intense peak of the doublet will always be at a lower Binding Energy.

The modification of the Fermi level depends on the nature of the material. In

the case of a semiconductor, the Fermi level position within the gap depends on the

doping type n or p, the number of dopant and the temperature. A change in

symmetry at the surface will also induce a modification of the Fermi level position by

modifying the band structure and creating new gap states determining the new

position of the Fermi level. Moreover the adsorption of a molecule can also create a

band bending and changes the position of the Fermi level in the gap.

Chemical shifts are easily observable and interpretable in XPS spectra as

core level peaks have generally a well-defined energy position. The ability to

determine different oxidation state and different chemical environment is one of the

major strength of the XPS.

When probing samples with XPS, a useful signal from the surface is obtained

between a depth of about 1 to 100 Å. The thickness of the sample layer that can be


13

analyzed called sampling depth is related to the electron Inelastic Mean Free Path

(IMFP). We remind that the IMFP λ is defined as the characteristic length that an

electron can travel without suffering an inelastic scattering over an infinitesimal

distance dx i.e. dx/λ. The probability for the electron to travel a distance d through

the sample matter without undergoing scattering is given by the following equation:

P(d) = exp (−d

λ)

The evolution of the electron IMFP for various elements as a function of

kinetic energy is available in [Figure 5] and shows its universality and weak

dependence of the element.

Figure 5: IMPF as function of KE for various elements, from Seah, M.P. et Al.15

The short mean free path of the electrons implies the use of Ultra High

Vacuum (UHV) chamber in order to minimize the interaction of the photoelectron

Kinetic Energy (eV)

λm

(Å)


14

with the atmosphere. Indeed after leaving the sample surface, the electron can

experience scattering through the gas phase. The IMFP inside the gas phase

depends on the kinetic energy of the electron and the gas pressure but the count

rate of detected electron decreases exponentially with the increasing pressure

[Figure 6].3 For example, electrons with a kinetic energy of 400eV going through a 1

mbar gas phase have an IMFP around 4mm.

Figure 6: Exponentially decreasing XPS signal vs increasing pressure

1.b) Evolution towards NAP-XPS

The decrease of the signal due to the small inelastic mean free path inside the

gas phase has led to work on new designs that shorten the distance traveled by the

electrons through an high pressure region: the evolution toward NAP-XPS.

Many chemical reaction and treatment at surfaces require the presence of

gases in the range of millibar up to bars. While it is possible to make an XPS

analysis before and after the chemical reaction/surface treatment,16,17 being able to

follow the process in situ can provide more useful insight. It is for answering that

issue that developments toward NAP-XPS have been carried out.

The idea of NAP-XPS has been put forward is in the 70’s by Kai Siegbahn’s

group in Sweden at the Uppsala University18 19 The design at the time introduced the

principle of a differentially pumped analyzer entrance[Figure 7] and used a laboratory

X-ray source allowing measurement up to 1 millibar. In this system Siegbahn used

for the first time a liquid jet inside his chamber allowing the study of liquid water with

XPS. Since then that design has been improved at the Advance Light Source (ALS)

in the Berkeley Laboratory and at BESSY in early 20003,6.


15

In this new system several electrostatic lenses were introduced: In each

differential pumping stage, one electrostatic lens was placed in order collect and

focus the electron towards the next pumping stage up to the hemispherical analyzer

[Figure 7]. This greatly amplifies the quantity of electron collected by the system

giving a better signal. Moreover compared to the setup developed in Uppsala, here

the source was a synchrotron based light source giving a higher flux photon and a

more focalized spot producing more photoelectron than a laboratory source. In order

to use the synchrotron light source, the beamline needs to be kept under UHV

condition, thus a window separate it from the sample environment.

All those improvement have increased the working pressure limit to more than 5

mbar.

Since then a number of synchrotron based NAP-XPS have flourished around

the world, notably at SOLEIL, BESSY, SLS, MAX-lab8,20,21…

In addition to the creation of differentially pumped systems work has been

spend on designing special cell for experiment like electrochemical or microfluidic.

Electrochemical cell were first used in UHV with ionic liquids thanks to their near

zero vapor pressure.22,23 Yet those experiments were limited only to ionic liquid but

Figure 7: Design commonly used since the early 70’s (left) and the new design developed at the Berkeley lab (Right)


16

advancement in NAP-XPS allows now to extend those experiments to non-ionic

liquid requiring a higher vapor pressure for a stable vapor/liquid interface.

Another example is the design of cells with ultrathin membrane. Linked with

the objective to study liquid water with an XPS setup for microfluidic devices or gas

reactant for gas cell, those designs were conceived with the goal of having the fluid

kept inside an ultrathin membrane device under high pressure while the rest of the

chamber is on UHV. The fluids are then probed through the membrane. Those

membrane are usually made of graphene24,25 or silicium.26 One of the main issues

here is to obtain a membrane that is both extremely thin in order to have electron

coming from the solution but also strong enough to not break due to the huge

difference in pressure of several order of magnitude. Moreover due to the necessity

of crossing the membrane for getting substrate signal it usually require high energy

photon of several KeV.

Those are only some examples but each field tends to develop tailored

sample environment in order to maximize the capacity of NAP-XPS in their domain.

1.c) Main topics of study

The most common application for the NAP-XPS is the study of catalytic

reactions. Indeed with a pressure around the millibar, it allows carrying on in situ

catalytic reaction and gives the possibility of determining the oxidation state of the

catalyst under operating condition. This helps to identify the active state of the

catalyst which is critical point. Moreover with NAP-XPS it is possible to gather

information on the surface composition like the different contamination but also to

characterize the different gas phase species.

The other field that benefits the most from the NAP-XPS is the environmental

science. With the pressure range in the milibar, it is possible to condensed liquids

and study the liquid/solid5 and liquid/vapor27–29 interfaces in an equilibrium state. In

particular water interfaces plays an extremely important role in chemistry be it

liquid/ice (Polar ice caps) or liquid/ vapor (ocean and atmospheric interaction)

interfaces. Amine based compounds are other species of important interest due to

Chapter 1:2)The NAP-XPS at the TEMPO beamline

17

their crucial importance as fertilizers but also as main organic soil contamination

species.

Those different fields have led to design new tools for the study like liquid jet

for the study of liquid or the gas cell for the study of catalytic reaction. Each time the

idea is to come closer to the normal condition of operation: Always trying to reach a

higher pressure while being able to do an XPS study.

2) The NAP-XPS at the TEMPO beamline

The NAP-XPS experimental station used for this PhD work is managed by the

LCPMR team attached to the TEMPO beamline of the French national synchrotron

facility (SOLEIL, Saint-Aubin). It is composed of several chambers [Figure 8]:

A loadlock

A transfer/storage chamber

A Catalysis chamber equipped with two gas lines connected to two leak

valves. The pressure inside the catalysis chamber can be as high as 1 bar in

static mode. Moreover two different systems can be used for heating the

sample: A class 4 laser and a button heater. The temperature of the sample is

monitored with a thermocouple.

A preparation chamber equipped with two gas lines connected to the chamber

through leak valves. Typical surface treatment can be done in this chamber:

sputtering with an ion gun, annealing with laser or button heater. The surface

can also be characterized with LEED. Different ports can host evaporators.

An analysis chamber: A gold coated mu metal chamber for XPS experiments

equipped with 7 lines inlets. Five lines are dedicated to gas dosing (four for

pure gas and one for mixed gas).The other two lines are reserved for dosing

with liquid like water or pyridine for example. A five axis manipulator is

available with two different heads. The first one is mainly used for catalysis

reaction as the sample can be heat up to 1000°C in presence of the reactive


18

gas ( pressure up to 20 mbar) allowing to follow in situ the catalytic reaction.

The second head is a Peltier cooler allowing cooling down the sample to -

20°C. This manipulator is mainly used for environmental chemistry study as it

allows lowering the vapor pressure making possible to condensed notably

liquid water and studying the vapor/liquid interface.

Figure 8: The NAP-XPS at the TEMPO beam-light

The analyzer of the experimental station is a Phoibos 150 NAP manufactured

by SPECS [Figure 9].30 To maximize the photoelectron signal intensity, an

electrostatic lens (NAP-lens) is added to the classical system three stage differential

pumping. The analyzer aperture (A nozzle cone ending with a diameter of 0.3 mm)

separating the analysis chamber and the first pumping stage is brought close to the

sample surface at a distance of ~1 mm in order for the electron to have a small

distance to travel through in the mbar range before being in the range of 10-3 mbar

behind the nozzle, in the first pumping stage. The first and second parts of the

differential pumping system are separated by apertures of 4mm diameter putting the

electron in a further decreased pressure. The second and third pumping stages are


19

separated by an iris. The fourth and last pumping stage operating at a pressure as

low as 10-7 mbar contains a true 180° hemispherical energy analyzer with 150 mm

mean radius and a 3D delay line detector system: time and two lateral dimensions.

The acceptance angle of the analyzer, as given by the manufacturer, is ±22° but

considering that we work at a distance of 1 mm from the sample the acceptance

angle is decreased from the nominal value (a value of ±16° is found assuming a

point X-ray spot).

Figure 9: Phoibos NAP 150 Analyzer with its differential pumping system30

The beam entrance compared to other systems where there is usually a

Si3N4,Al window in order to kept the beamline in UHV here we also use a differential

pumping system made up of three stage keeping the beamline in the lower range of

10-9 mbar. The beam spot in the analysis chamber has a diameter of 100 μm and

makes an angle of 54° with the analyzer axis. As the radiation polarization is

horizontal, the angle between the electric field and the axis of the analyzer lens axis

is 36°.

Chapter 2:1)Introduction

20

Chapter 2: Study of swelling clays

1) Introduction

Clays are composed of micrometric size phyllosilicates particle. In terms of

structure, they can be described as stacked phyllosilicate layers separated by an

interlayer space. Those phyllosilicate layers may have some element substituted

inside the structure inducing a charge which is compensated by the presence of

couterions inside the interlayer space. It is the presence of those counterions prone

to be hydrated that drive the adsorption of water inside the interlayer space inducing

a swelling of the clay. This phenomenon is influenced by the nature of the counterion

(charge, size) and the type of substitution inside the structure (charge, location of

charge).

The hydration behavior of clays has already been studied with various

techniques like X-ray diffraction (XRD), adsorption gravimetry31–33, infrared

spectroscopy34 and neutron (spin echo, time of fight and backscattering)35 giving an

overall good understanding of the thermodynamic, crystallography and dynamic

properties of clays but they still leave the question of the electronic structure most

importantly the counterion evolution of its electronic environment during hydration

open. This is where the use of a NAP-XPS station can bring interesting information.

Indeed it has already demonstrated its ability to monitor the binding energy position

of cation, their distribution with a depth profiling showing the possibility to study ion in

solution with the NAP-XPS.36 In the case of clays, the NAP-XPS can show a link

between the evolution in binding energy of the counterion and its hydration but also

the evolution of the electronic environment in structure component showing the utility

of the NAP-XPS to study the electronic structure of ions in confined water.

As natural clays are often non homogenous systems, here we have chosen to

use two synthetic clays from the smectite groups: hydroxy-hectorite and hydroxy-

saponite (shortened as hectorite and saponite after).

This is mainly done in order to get a good control of the charge inside the clay.

Smectite clays have a TOT structure (or 2:1)37. It means that they are constituted of

2 silicate tetrahedral sheets and one hydroxide octahedral sheet. In this system, the


21

tetrahedral sites forming a sheet are arranged in a hexagonal pattern sharing three

of the four oxygen corners and form a planar structure. The unshared oxygen of the

two tetrahedral sites faces each other in the central area of the clay layer forming the

octahedral sheet with additional groups like hydroxyl or Fluor. Those octahedral sites

can be occupied by several elements such as Mg2+, Li+, Al3. It is those kinds of

substitutions that create charges inside the layer leading to the presence of

counterions within the interlayer space.

In the case of our hectorite, all the tetrahedral sites are occupied by Si4+ and

are in a neutral state. But in the octahedral sites usually occupied by Mg2+, some of

them are substituted by Li+ creating a negative charge. Naturally this charge is

compensated by the presence of cations (Cat) in the interlayer space [Figure 10(a)].

The general formula per unit cell for the hectorite (we have only used hydroxyl ones)

clay is37:

Cat0.8/z[Mg5.2Li0.8]Si8O20(OH)4

with z the charge of the cation and the clay charge fixed at 0.8 per unit cell. In some

synthetic hectorite, the OH groups are substituted by F, giving Fluoro-hectorite

instead of hydroxy-hectorite ,a more hydrophobic clay compared to the hydroxyl one,

For the saponite clay, all octahedral sites are occupied by Mg2+, here

substitutions occur in the tetrahedral sheet where Si4+ are substituted by Al3+. Those

negative charges located in the surface are compensated as in hectorite by the

presence of cation in the interlayer space [Figure 10(b)].

The general formula per unit cell for the saponite clay is33,37:

Cat0.8/zMg6[Si7.2Al0.8]O20(OH)4

with z the charge of the cation and the clay charge fixed at 0.8 per unit cell.

Those two kinds of clays have the same charge in order to ease the

interpretation of the result (the value of the charge having an influence on the

hydration of the clay).


22

Figure 10: Structure of a hectorite (a) and a saponite (b)

The density of the substitution, their location and the nature of the counterion

in the interlayer are critical factors33,38,39 that determine the hydration behavior of the

clay, the mobility and the possible exchange of the counterion which are important

properties of the swelling clays. Indeed, for swelling, the charge of clay needs to be

moderate. An excessively low charge will mean too few couterions in the interlayer

space and consequently water will not be enough attracted within the interlayer

space thus preventing swelling. In the contrary with a charge too high, the interaction

between the counterion and the clay layers will be too strong and prevent the

swelling to occur, see Table 112

Name Charge (unit cell) Type of substitution Swelling/Ion Exchange

Pyrophyllite 0 None None

Smectites 0.4-1.2 Tetrahedral/Octahedral Yes

Vermiculites 1.2-1.8 Tetrahedral/Octahedral Yes

Micas 2-4 Tetrahedral None

Table 1: Classification of 2.1 clays swelling ability as function of layer charge

Concerning the clay materials we use, AFM (Atomic Force Microscopy) study

has shown that the typical lateral size of the phyllosilicate lathes (of thickness 0.96

nm) is usually around a micrometer. They have a strong tendency to form stacks of

layer with the adjacent layer being held by Van der Waals forces which are important

with the presence of charged sheets and cation. Those stacked sheets form

aggregates at the mesoscopic level. The typical thickness of a layer from the top

tetrahedral sheet of one layer to the top tetrahedral sheet of the adjacent bottom

layer (one layer + one interlayer space) is called basal spacing and range for 1 nm


23

(dehydrated state) to 1.5 nm (two water layer around the counterion, fully hydrated

state).

The swelling properties of clays have already been investigated through the

use of XRD39–42, water adsorption gravimetry31–33 (often both)32,33 infrared

spectroscopy measurement34 but also with simulation43–45. XRD and gravimetric

adsorption have been the most widely used experimental methods as they give

access directly to the evolution of the basal spacing as a function of the increased

RH giving the moment where water enter inside the interlayer while the gravimetry

adsorption gives the amount water adsorbed by the sample. Those measurements

are limited by the heterogeneous nature of clays when it comes stacking the layer

making it difficult to distinguished between water adsorbed inside the interlayer from

those on external surfaces due to the aggregate nature of the system creating in

addition to the interlayer :mesopore, macropore.46

As mentioned previously, the swelling behavior of the clay depends on the

clay nature (charge density, charge location, nature of the counterion…). Usually at a

certain RH when water is absorbed inside the interlayer, the counterion hydrate and

forms its first hydration sphere (also called one water layer state or 1W) and the

basal spacing goes from 0.9 to 1.2 nm [Figure 11 (a)]. A second entry of water can

happen at a higher RH and the counterion forms its second hydration sphere (also

called second water layer state or 2W) and the basal spacing increased from 1.2 nm

to 1.5 nm [Figure 11 (b)]. In this configuration we have what is called an outer-sphere

surface complex.43,47

Figure 11: Hydration of the couterion in the interlayer space with the formation of the first hydration sphere (a) and second hydration sphere (b)


24

Another possibility is that the counterion sticks on the layer surface instead of

going in the middle of the interlayer space and forms only a partial hydration sphere

[Figure 12] which is called inner sphere surface complex.48 This is mainly due to the

low enthalpy hydration of the cation and a strong interaction with the clay surface.

This effect is more important in the case of saponite than hectorite because the

negative charge is localized within the tetrahedral sheet.

Figure 12: Formation of a partial hydration sphere with couterion staying in interaction with the clay surface

For this study on hydration three different counterions were chosen: Na+, Cs+

and Sr2+with their main characteristic given in

Table 2: Several common counterion main characteristics

Cs+ and Na+ while having the same charge, have a very different radius with

Cs+ being far bigger and having a hydration enthalpy lower. Sr2+ has a radius larger

than Na+ but has a double charge making him more hydrophilic than sodium having

a hydration enthalpy much larger. It should be noted that those hydration enthalpy

are given for ion in bulk water. In clays we are under the condition of confined water

and a behavior different than in bulk can be expected even though it should follow

the same trend. 51 It was indeed demonstrated that for several cations their hydration

energy were smaller in confined space than in bulk water. This can be due to two

Cation Charge Ionic radii49 Hydration enthalpy of gaseous ion50

Na+ 1+ 116 pm -406 kJ/mol

Cs+ 1+ 181 pm -264 kJ/mol

Sr2+ 2+ 132 pm -1446 kJ/mol

Ca2+ 2+ 114 pm -1579 kJ/mol


25

factors. First there needs to be an additional energy given to the cation in order to

separate it from the clay layer. Second due to the restrained space, the hydration

sphere contains less water molecule than in bulk water.

For Na+-hectorite Sorption curve and XRD experiment gives a first entry of

water in the interlayer space with the formation of the first hydration solvation

between RH=0% and RH=20% as shown in [Figure 13].38,52,53 Then there is a

plateau until RH=40% where a second swelling occurs with the formation of the

second hydration state between RH=40% and RH=60%.52,53

Figure 13: Water sorption curve of several hectorite with Na+

-hydroxyhectorite in blue, Na+ Fluorohectorite in purple

and Na+ Fluorohectorite in purple which has received a thermal treatment before the experiment in order to fully

desorb the water trap in the interlayer space. From Dazas, B. et al. 52

In the case of Na+-saponite the behavior is similar to Na+-hectorite[Figure 14]:

the formation of the first hydration sphere is between RH=0 and 15%, the second

hydration sphere is formed between RH=50 and 60%.33,41,53

Figure 14: Na+

-saponite sorption curve with the evolution of the interlayer space as a function of relative pression in

sorption (black filled point, plain line) and desorption (empty point , dotted line) from Michot, L. et al. 33


26

It is expected that the Cs+-hectorite and Cs+-saponite does not fully hydrate.

Indeed the layer spacing for Cs+-smectite remains in the range of 0.12 nm

(equivalent to 1 water layer) even at high RH54–57 and after immersion in an

aqueous54. There is only formation of 2/3 of a monolayer with no more than 2.7 H2O

molecules per smectite unit cell48. In fact the water adsorption isotherm still show an

increase of adsorbed water55–57 but this adsorption is mostly done in the micropore

instead of having additional water inside the interlayer space.55,57 Cs-smectite forms

only a partial hydration sphere similar to the one show in [Figure 12].

The situation for Sr2+-hectorite and Sr2+-saponite hydration is more difficult to

assess as no full information on their hydration behavior is available. Yet information

on Sr2+-montmorillonite which is a dioctahedral structure (both saponite and hectorite

are trioctahedral) are available and the behavior should be similar. For the Ca2+-

Montmorillonite the formation of the first hydration occurs at low pressure, at

RH=5%58 where the basal spacing has value typical of a one water layer complex

(d(001)=12.1 Å) and is on a two water layer state at RH= 40 %.58,39 Yet it should be

noted that the transition from one water layer to two water layer is continuous, the

one water layer is instable and the system tends to favor the formation of a two layer

state.58,39 Moreover even in dry condition (RH=0%) the basal spacing as a value

near a one layer state (d(001)=11.72 Å)39 indicating that the Montmorillonite is not fully

dehydrated. When we compare those values for the Ca2+-hectorite, the hydration

behavior is similar with a one/two layer state at low relative humidity (under 40%)

and a two water layer state at RH=40%.38

For the Sr2+-Montmorillonite, it has one water layer also at low RH (RH=5%)58

and forms its second hydration sphere at a slightly higher relative pressure (RH=35-

40%) compared to the Ca2+-Montmorillonite.58,39 Considering that both Ca2+-

Montmorillonite and Ca2+-hectorite behave similarly we expect that the Sr2+-hectorite

will have the same behavior than the Sr2+-Montmorillonite and shows a two hydration

step with a first water layer formed around RH=5% and a second water layer formed

at RH=30-40% and it will certainly be bihydrated at RH=43%.59

Chapter 2:2)Hydration of clays studied by NAP-XPS

27

For saponite it is more complicated as Ca2+-saponite shows a two layer state

at RH=20%, a lower RH than Ca2+-montmorillonite.38,41,53 making the observation of

a two-step hydration difficult.

Moreover Considering that Ca2+ samples may not be fully dehydrated,60 and

considering the relative similarity in between Ca2+ and Sr2+ cation, we can expect

that it may also be the case for Sr2+-hectorite and Sr2+-saponite to not be fully

dehydrated and showing only a one step hydration to the two layer state.

Now we will describe the experimental procedure used for studying the

hydration of the swelling then the issue of studying clays, a nonconductive material

with XPS which creates charging effect and how to counter it. Then we will discuss

the hydration of the cesium samples then the sodium sample and finally the

strontium samples and explain the binding energy shift observed.

Finally we will show results of adsorption and synchrotron induced radiolysis

of pyridine in a Sr2+-hectorite.

2) Hydration of clays studied by NAP-XPS

2.a) Experimental procedure

For NAP-XPS analysis, clays are deposited in thin layers on a gold coated

silicon surface to limit the charge effect during the photoemission acquisition (see

below). The deposition is done by evaporation of a water solution containing

synthetic hydoxyhectorite38 clay platelets on a gold surface. The shape of the clay

lathes is irregular with a typical lateral dimension of ∼ 1 μm, as determined by atomic

force microscopy. The preparation of the clay dispersion solution used for drop-

casting is available in reference.33 Due to the slow evaporation, the clay platelets

tend to lie flat on the substrate, with a disinclination angle smaller than 20°.

Inside the analysis chamber the sample was cooled down to 275K with a

Peltier effect in order to reduce the condensation pressure to ~7 mbar (down from 23

mbar at 20°C). Water was then introduced through a leak valve until reaching the

desired pressure. As the pressure increased the signal received from the sample


28

became lower and lower due to the decreasing inelastic mean free path of the

electron inside the gas phase. For easing the comparison of the spectrum, the

signals have been amplified by a factor indicated in the figure. The experiment

stopped when liquid water was formed at the sample surface. This formation was

monitored by a camera installed inside the analysis chamber. All peak presented

were acquired at an excitation energy of ℎ =750 eV.

All spectra shown in this chapter were fitted with Gaussian function and used

a Shirley background.

As we are working in with synchrotron radiation we need to determine a

reference peak in order to compare spectra recorded at different energy and

between different samples. Indeed compared to laboratory XPS using Al Kα and/or

Mg Kα where the excitation energy is well known due to the ray used (1486.7 eV for

Al Kα and 1253.6 eV for Mg Kα)61, in synchrotron radiation there is a certain

uncertainty concerning the excitation energy. The determination of the reference

peak was complicated due to the nature of the sample. Indeed, one major issue with

studying clays through NAP-XPS or XPS is the possible presence of differential

charging effect

2.b) Assessing charging effect

Clays being inorganic and nonconductive material the issue here is to refill the

holes made at the surface by the emission of the electron as the electrons coming

from the bottom of sample have to go through the whole clay structure. This difficulty

to refill holes leads to the positive charging of the sample surface due to persistence

of holes. This translates in terms of XPS measurement to peaks appearing at a

higher binding. Indeed with fewer electrons present around the nuclei, the screening

effect is reduced the remaining electron are tighter bounded. Moreover with non-

homogenous sample which is the case with clay another effect can be present: the

differential charging effect. This is due to some part of the sample being in better

contact with the ground resulting in a better hole refilling than other probed part of

the sample. This has the effect of having same elements being in a different

charging state and giving different XPS signal.


29

2.b.1) With Au4f

Usually a grounded metal with strong spectral lines is used as a reference for

the binding energy calibration like gold, silver or cooper62,63. As we have a gold layer

in our substrate, our first idea was to use the Au 4f7/2 peak as a reference by

correcting its measured position to its true binding energy position at BE=84 eV62

with respect to the Fermi level and realign all other peaks studied (Mg, Si,

counterion…) with the same correcting factor. This allows having the binding energy

of peaks being directly related to the Fermi level. Yet gold being grounded will never

be charged compared to clay studied resulting in all peaks constantly shifting due to

the variation of the charging effect.

An example of differential charging with gold as a reference is displayed in

[Figure 15].

Figure 15: Evolution of the Mg 2p peak position referenced to the Fermi level with increasing pressure for a Cs+

-saponite

sample

On this figure we can see the magnesium peak (Mg 2p core level, the

resolution is not sufficient enough to distinguish its two spin, 2p1/2 and 2p3/2 as they

are separated by only 0.38 eV) when aligned with the Fermi level completely

Inte

nsity (

Arb

. U

nits)

60eV 58 56 54 52 50 48

Binding Energy (eV)

hv=750 Mg 2p

UHV

0.24mbar

0.6mbar

1.2mbar

2.5mbar

4.5mbar

6mbar

8mbar

T= 275 K

hv= 750 eV

T= 275 K


30

distorted and having a part shifted to higher binding energy under UHV and at low

pressure. When the pressure reaches 1.2 mbar, Mg 2p appears as a sharper single

peak and is fitted with a single Gaussian due to the small difference between Mg

2p1/3 and Mg 2p3/2: 0.38 eV. As we see here, differential charging effect can have a

strong influence on the XPS spectrum shape. In order to ease the analysis work and

take into account the eventual residual charging effect even at higher pressure, the

Mg 2p peak was chosen as an internal reference, its position will be set at the stable

one reached at 2.5 mbar in this example. This peak was chosen due to its central

position inside the clay structure: it will not see the effect of water entering inside the

interlayer (compared to Si which is at the surface) and being a structure element its

position will take into account an eventual charging effect present in the clay. The

decrease of the charging effect by the increased pressure is linked to the creation of

negative species (ion, electron) inside the gas phase due to the interaction with the

photon beam. Those negative species then reach the surface and compensate the

positive charge. As the gas pressure increases more negative species are created

and the discharging effect is increased. One way to monitor the evolution of the

charging effect while having Mg 2p as the reference peak is to follow its width. A

decrease points to a reduction of the charging effect and in contrary when the peak

is broader it means that the charging effect is stronger. Yet another possibility exists

by using the adventitious carbon with the C 1s core level as a reference peak.

2.b.2) With C1s as a reference peak

One of the most common proxies for following the charging effect is to use the

carbon contamination with the C 1s core level as a reference peak.36,64 Indeed its

main component (corresponding to an aliphatic carbon: C-C) is given at a position of

284.8±0.2 eV 36,64–66 in binding energy with respect to the Fermi level. Being on the

top of the sample it should follow in binding energy the evolution of the charging

effect inside the structure. In [Figure 16] we observe the evolution of the aliphatic

carbon binding energy for several samples with increasing water pressure.


31

Figure 16: Evolution of the C 1s C-C component binding energy with respect to the Fermi level as a function of the pressure for several samples

First in UHV almost all sample show a higher binding energy than expected

for this peak (284.8 eV). Then with increased water pressure, the position decrease

until being near 285.15eV around 2 mbar. Yet when the water pressure keeps

increasing, its position starts rising and stabilizes around 285.3 eV. We observed

that at the higher pressure, the carbon contamination is oxidized. This oxidation is

visible in [Figure 17] where the decomposition of the overall C 1s peak into 3 peaks

ascribe to the C-C component (peak 1), COH/C-O-C (peak 2) and carbonyl (peak

3).67 The decomposition shows a different repartition between the different

component at 1.2 mbar and 8.4 mbar of water for the Cs+-hectorite. The carbonyl

component (peak 3) is more important at the higher pressure with a spectra weight

going from 6% to 21 % while the aliphatic component (peak 1) decreases from 78%

to 59 %.The COH/C-O-C component also increase from 16% to 20%. In this

example, the binding energy of the aliphatic peak at 8.4 mbar ends at the

aforementioned position of 285.3 eV describing the overall oxidized state of the

contamination carbon.


32

Figure 17: C 1s peak of the Cs+

-hectorite decomposition at 1.2 mbar (a) and 8.4 mbar (b)

This uncontrolled oxidation at higher pressure makes the C 1s an unreliable

reference peak in this case for monitoring the hydration of clay while taking into

account the charging effect. This effectively leaves the Mg 2p as the best candidate

for a reference peak.

2.b.3) Controlling charging effect

As seen previously, charging effect have a strong impact on the shape of the

XPS spectrum. While the first two counterions samples (Na+ and Cs+) were analyzed

without trying to drastically diminish the charging effect (exempt increasing the water

pressure), with Sr2+ samples we have decided to insert a neutral gas in order to help

discharge the surface while acquiring our UHV scan. Thus, in order to reduce the

charging effect impact on our spectra we have explored two methods:

Biasing the sample with respect to the ground by +30 V in order to attract a

maximum of secondary electron/negative species in order to discharge the

sample surface.

The introduction of gas pressure as it was shown previously to have a

beneficial effect once an order of 1.5 mbar was reach (augmentation of the

number of negative species).

First the idea of positively biasing the sample is to increase the surface

attraction of the surrounding secondary electron and/or negative species (like HO-


33

with the presence of the water gas phase) in order to discharge the surface directly

from the top instead of having the electron coming from the bottom of the sample

when grounded. This different behavior is schematized in [Figure 18].

Figure 18: Difference in charging effect between a grounded sample (a) and a positively biased one (b)

Figure 19: Evolution of the Mg 2p FWHM in UHV with increasing biased voltage at hv=750 eV

The effect of a positive biasing of the sample is displayed in [Figure 19] where

we observed a decreasing FWHM of the Mg 2p peak from 2.41 eV when grounded

to 2.05 eV at +40 V. Moreover its position in binding energy with respect to the Fermi

level decreased from 50.5 eV to 50.35 eV position usually seen only around 1.5

mbar of water when biasing is not used.

Inte

nsity (

Arb

. U

nit)

54 52 50 48 46

Binding Energy (eV)

0V FWHM=2.407eV 10V FWHM=2.152eV 20V FWHM=2.107eV 30V FWHM=2.080eV 40V FWHM=2.050eV

Mg2p


34

Yet we thought that combining the beneficial effect of biasing the sample and

putting a 1 mbar pressure of argon could further increased the discharge at the

sample surface This gas was chosen as it has the highest cross section of all the

neutral gas (N2, Ne…) at our working excitation energy with its 2p core level having

0.3 Mbarn68 at ℎ =750 eV while Ne has its maximum cross section for its 2s orbital

with a value of 0.02 Mbarn.68 For nitrogen the maximum cross section is for its 1 s

orbital with 0.155 Mbarn.68 Having a high cross section is important for our goal as it

directly determine the quantity of secondary electron produce in the gas phase.

Indeed the cross section describes the probability for an electron at a given orbital to

be ionized by a photon at a given energy. Thus the higher cross section means a

higher yield of electron emitted inside the gas phase. It was shown by Mueller et al.69

that being above or under the O 1s edge has an effect on the attenuation of the

incident photon beam with the beam being more attenuated above the O 1s edge

meaning a stronger creation of electrons from oxygen element in the gas phase.

In [Figure 20] we observe the evolution of the Mg 2p FWHM under a +30

biased condition with increased Argon pressure at ℎ =750 eV, it shows that the

addition of the gas has indeed a good influence on FWHM of the Mg 2p as

decreased from 2.08 to 2.01 eV, a further diminution of almost 0.1 eV compared to

when we used only the biasing condition under UHV.

Figure 20: Evolution of the Mg 2p peak with increased Ar pressure under a +30 V biased at hv=750 eV

Inte

nsity (

Arb

. U

nit)

54 52 50 48 46

Binding Energy (eV)

UHV Pol 30 FWHM=2.080eV 0.5mbar Ar Pol 30 FWHM=2.050eV 1 mbar Ar Pol 30 FWHM=2.005eV

Mg2p


35

Thus our recipe for diminishing charging effect which is applied on our

strontium sample is:

Use a +30 V biased on the sample

Insert 1 mbar of Argon

This is mostly done in order to obtain a good UHV spectrum, for the following

spectrum with increased water pressure no additional partial pressure of neutral gas

was added in order to read the absolute pressure value of water on the gauge. Yet

as seen the effect of biasing alone should be sufficient to obtain good charging effect

reduction at low water pressure.

2.c) Hydration of clays

In this part we will follow the hydration of hectorite and saponite clays starting

by the cesium counterion (the smallest enthalpy hydration) then clays with sodium as

a counterion (highest hydration enthalpy of monovalent cation) and we will finish with

the strontium counterion (Divalent cation) where we try to minimize the charging

effect.

2.c.1) Cs+ samples

We will start by analyzing the Cs 4d spectrum of the Cs+-hectorite sample

shown in [Figure 21]. On the left of the figure is display the Cs 4d core level aligned

with the reference peak Mg 2p display on the right at a fixed position of 50.4

eV±0.2eV.


36

Figure 21: Evolution of the Cs 4d core level peak binding aligned with Mg 2p as a function of the pressure and relative humidity for the hectorite sample at hv=750 eV. Fitting lines are shown in red

The UHV and lower pressure spectrum are not shown due to strong charging

effect. The charging is sufficiently diminished at 1.2 mbar (RH=17%) and the

spectrum are exploitable. The Cs 4d is fitted with one doublet separated by 2.3 eV

and an intensity ratio of 2/3 between the 4d3/5 component (smallest on the left) and

the 4d5/2 component (the most intense one on right). The 5/2 component starts at a

position of 76.1 eV (at a distance of 25.7 eV from Mg 2p). For an easier positioning

of the peak, Mg 2p was fitted with a single Gaussian. From 17% to 67% relative

humidity there is no change in the spectrum except for the signal attenuation due to

the gas phase. It is only at a high relative humidity (83%) that we see an evolution in

the Cs 4d peak, as it shifts by 0.2 eV towards higher binding energy (no liquid water

droplets are yet formed). This indicates that in hectorite, cesium feels the presence

of water only when the condensation pressure is near. This is not surprising as

cesium has difficulty to form its first hydration sphere and most certainly forms an

inner sphere surface complex. When macroscopic droplets of water are formed at

the sample surface (relative humidity of 100 %) [Figure 22], the shift relative to Mg

2p still increases for a total shift of 0.35 eV. However if we keep increasing the water

pressure beyond RH=100% there is no other shift. Surface sensitive condition

(ℎ =400 eV, see appendix A.1) does not show a single shift but the signal of the

Inte

nsity (

Arb

. U

nits)

54 52 50 48

Binding Energy (eV)

82 80 78 76 74 72

m

hv=750 eV

1,2mbar RH=17%

2,4mbar RH=34%

3,6mbar RH=50%

4,8mbar RH=67%

6mbar RH=83%

7,2mbar RH=100%

8,4mbar

Evolution of hectorite Cs4d core level binding energy with increase water pressure

Cs 4d

Mg 2p

T=275 KShift:0.35eV

X2


37

clay is lost quickly (RH=68%) due to the increased pressure and attenuation of the

signal.

Figure 22: Image of the Cs+

-hectorite sample at 7.2 mbar (RH=100%) with water beads up on its surface

The variation of the Si 2p peak with respect to Mg 2p as shown in [Figure 23]

is also worth studying as a function of relative humidity. The Si 2p peak is fitted with

a doublet separated by 0.6 eV and an intensity ratio of 1/2 between the Si 2p1/2

component and the Si 2p3/2 component. In fact, we observe the same relative shift in

binding energy as seen for the Cs 4d spectrum. This shift of 0.35 eV happens at the

same relative humidity (83%) as for Cs 4d. As silicon lies inside the tetrahedral

sheets which are at the surface of the sample, it indicates that both silicon and

cesium share the same evolution of their chemical environment. This may be the

indication of the formation of the inner-sphere surface complex.48

Figure 23: Evolution of the Si 2p core level peak binding energy as a function of the pressure and relative humidity for

the Cs+

-hectorite sample. Fitting lines are shown in red

Inte

nsity (

Arb

. U

nits)

108 106 104 102 100 98

Binding Energy (eV)

hv=750 eV

1,2mbar RH=17%

2,4mbar RH=33%

3,6mbar RH=50%

4,8mbar RH=67%

6mbar RH=83%

7,2mbar RH=100%

8,4mbar

Evolution of Si2p core level binding energy with increase water pressure

T=275 K

X5

X3

X1.5

Si 2p


38

We will now consider the Cs+-saponite sample. In this sample, the Cs 4d

signal overlaps with the Al 2p peak (Al substitutes Si in the tetrahedral sheets [Figure

24]. Yet the Cs 4d3/2 component is still undisturbed allowing us to follow the shift of

the Cs 4d peaks. Like with the previous sample (Cs+-hectorite), the lower pressure

spectra are not available due to the strong differential charging effect and we

observe a shift of 0.3 eV towards higher binding energy. As well as for hectorite the

shift starts to occur only at high relative humidity (83%). This means that the

negative charge location in the lath (octahedral substitution for hectorite vs

tetrahedral substitution for saponite) does not have any remarkable impact on

cesium hydration. Also in surface sensitive condition (Appendix A.2) there does not

seem to be a shift but the signal is overall low.

Figure 24: Evolution of the Cs 4d core level peak binding energy as a function of the pressure and relative humidity for the saponite sample. Fitting lines are shown in red

Moreover we also observe the same shift on the Si 2p peaks [Figure 25] as for

hectorite. This suggests that cesium and silicon share the same environment, also in

saponite. For both clays, hydration occurs while cesium stays clung to the surface of

the clay forming an inner sphere surface complex.

Inte

nsity (

Arb

. U

nits)

54 52 50 48

Binding Energy (eV)

80 78 76 74 72

hv=750 eV

1,2mbar RH=17%

2,5mbar RH=35%

4,5mbar RH=63%

6mbar RH=83%

8mbar RH=100%

Mg 2p

Cs 4d3/2

Cs 4d5/2+Al 2p

Evolution of saponite Cs4d core level binding energy with increase water pressure

T=275 K

Shift:0.3eV

X1.5

X5

X5

X8

X1.5

X3

X4

X5


39

Figure 25: Evolution of the Si 2p core level peak binding energy aligned with Mg 2p as a function of the pressure and relative humidity for the saponite sample. Fitting lines are shown in red

In [Figure 26] we report the evolution of the distance between the counter ion

core level peak (Cs 4d5/2 component) and its reference peak (Mg 2p). We observed

that as shown before, the evolution of the Cs 4d peak follow the same trend for both

clays as it shifts only at high relative humidity with the water condensation. What is

surprising in this figure is the different distance between the conterion and the

reference peak. Indeed the distance is constantly ~0.4 eV higher for hectorite. It is

here that we observe the spectroscopic difference between the saponite and

hectorite structure. This difference is due to the location of the negative charges

(which are due to the substitutions) inside the clay structure. In the case of hectorite,

it is located inside the octahedral sheet, in the middle of the clay structure far from

the counterion while in the case of saponite the negative charge is located inside the

tetrahedral layer, much closer to the counterion and farther away from magnesium.

This means that the counterion in the case of saponite is in an environment more

negatively charged than in the case of hectorite (the negative charge is nearer in the

case of saponite than hectorite) translating in a better screening effect for the

saponite thus lowering the binding energy of the counterion core level peak making it

nearer to the Mg 2p peak.

Inte

nsity (

Arb

. U

nits)

108 106 104 102 100 98

Binding Energy (eV)

hv=750 eV

1,2mbar RH=20%

2,5mbar RH=35%

4,5mbar RH=63%

6mbar RH=83%

8mbar RH=100%

Evolution of saponite Si2p core level binding energy with increase water pressure

T=275 K

X5

X3

X1.5

Si 2p


40

Figure 26: Evolution of the distance between Cs 4d5/2 peak and Mg 2p as a function of the relative humidity for Cs+

-

hectorite (red) and Cs+

-saponite (blue)

2.c.2) Na+ samples

We will now consider the sodium based samples. In [Figure 27] is displayed

the evolution of the Na 2s core level spectra with increased pressure and aligned

with the Mg 2p peak for the hectorite sample. Both Na 2s and Mg 2p are fitted with a

single Gaussian.

Figure 27: Evolution of the Na 2s core level peak binding energy as a function of the pressure and relative humidity for the hectorite sample.

Inte

nsity (

Arb

. U

nits)

54 52 50 48Binding Energy (eV)

68 66 64 62

hv=750eVMg 2p

Na 2s

4,8mbar RH=68%

2,4mbar RH=34%

1,2mbar RH=17%

0,6mbar RH=9%

UHV

0,2mbar RH=3%

Evolution of the Hectorite Na2s core level binding energy with increase water pressure

T=275°K

Shift:-0.5eV

X3

X2

X1.5

X3

X3

X2

X5

X2X3


41

The binding energy shift of this small Na+ counterion is notably different from

that observed in Cs+-hectorite. Indeed, the sodium shifts of 0.5 eV towards lower

binding energy at low relative humidity (between 0 and 17%). This shift occurs at the

pressure range where we expect sodium to form its first hydration sphere.52,53

Moreover it was shown by Winter et al70 that when an ion hydrates it should have its

core level peaks shift to lower binding energy. This shift is also visible on surface

sensitive conditions (Appendix A.3).

Interestingly the sodium signal disappears when a pressure of 4.8 mbar is

reached, corresponding to a relative humidity of 68 %. This disappearance of the

signal happens when we expect sodium to form its second hydration sphere. 52,53

With its second hydration sphere formed sodium is now more mobile than cesium.71

At the time of writing the reason why this happens is not fully understood. We can

propose various conjectures:

Being mobile we suspect that sodium migrate laterally out of the X-ray

spot zone due to the charging effect creating a repulsive electrical field

(the center of the irradiated zone is more positively charged than the

periphery). This can be called lateral charging effect. The potential drop

would be of a few hundreds of meV over the typical of the beam (0.1

mm). The associated electric field should be of the order of 10 V/cm. Is

it sufficient to drag the sodium ion out? We indeed observe with the

widening of the FWHM of Mg 2p (going from 1.65 eV at 2.4 mbar to

1.79 eV at 4.8 mbar) that the charging effect has increased (see Table

3). It should be noted that this increase might be a consequence of the

sodium disappearance as with the loss of a conductive species in the

sample makes it less conductive thus increasing the charging. It is

possible that even at those high gas pressures, the charging effect are

not fully mitigated and are sufficient to force a migration of sodium.


42

Relative Humidity FWHM (eV)

0 (UHV) 2.4

3% 2

9% 1.78

17% 1.77

34% 1.65

68% 1.79

Table 3: Evolution of the Mg 2p FWHM with increased RH for the Na+-hectorite sample

The same effect but in depth. Due to the escape depth (~1 nm) of the

photoelectrons and Auger electrons generated by the spot and their

emission in the gas phase, the surface layer should be more charged

than the inner layers. The resulting electric field would of ~106 V/cm.

This is likely sufficient to drag the ions down to the bulk of the clay

deposit. Na 2s spectra measured at 1050 eV [Figure 28] show that the

species does not disappear at this depth. Moreover, the Mg 2p FWHM

is lower at this higher excitation energy (1.68 eV vs 1.79 eV for ℎ =750

eV) indicating that the charging effect is less important. Based on the

IMFP of MgO and SiO272 we can estimate that the increased probing

depth is around 6Å

Figure 28: Na 2s core level peak binding energy at hv=1050 eV for RH=68% and 100% for the hectorite sample

Inte

nsity (

Arb

. U

nits)

54 52 50 48d


RH=68%

RH=100%

Na 2s

Mg 2phv= 1050 eVT=275 K


43

A radiolytic effect that leads to the formation of H+ (H3O+) and the

replacement of the sodium ions. As discussed at length in the

mineralization of pyridine section, we have averted radiolytic effects

that lead to the mineralization of an organic species, pyridine. Valence

holes and electrons are generated by the photoelectrons and Auger

electrons produced by the X-ray and travelling in the clay. Holes can

react with water to give H+ and the hydroxide radical. H+ would replace

Na+ if the latter is sufficiently mobile. (On the other hand, electrons can

react with water to give OH- and H radicals.) However, it seems that

the 2W hydration state is not the most favorable one to ensure the

breaking of the water molecule, when compared to the 1W one.73

The behavior of the saponite sample is very similar to that of the hectorite for

the Na 2s core level [Figure 29]. We observe a shift of 0.7 eV towards lower binding

at a low relative humidity (0-14%) which is also visible in surface sensitive conditions

(Appendix A.4) and the loss of signal at 4.8 mbar (RH=68%). It confirms the data

obtained with hectorite. Here the shift and disappearance of signal also occurs when

sodium should form its first (shift) and second hydration sphere (lost signal).33,41,53

Figure 29: Evolution of the Na 2s core level peak binding energy as a function of the pressure and relative humidity for the saponite sample

Inte

nsity (

Arb

. U

nits)


68 66 64 62

UHV

0,032mbar

RH=0.5%

0,34mbar RH=5%

0,96mbar RH=14%

2mbar RH=29%

3mbar RH=43%

4,8mbarRH=68%

Na 2s

Evolution of saponite Na2s core level binding energy with increase water pressure

hv=750 eV

T=275°K

Shift:-0.7eV

X3

X3X3X3X3

X10

X50

Mg 2p

X14

X5

X3

X2


44

Moreover when we consider the evolution of the Na/Mg intensity ratio [Figure

30] compared to the Si/Mg we noticed that the first one see an evolution at low

relative humidity until it stabilizes before the signal of the sodium is lost while the

Si/Mg ratio is stable throughout the experiment. The increase of the Na intensity

signal maybe link to the reduction of the charging effect inside the sample as the

pressure increases, making the sodium peak sharper and thus more easily

distinguished from the background signal resulting in an increase of the signal while

the Mg and Si peak being already intense do not show such an improvement. Here

the question is whether the signal attenuation due to the increasing gas pressure

and perhaps also water adsorption at the surface is sufficient to lose the Na signal

because it is not within the detection limit anymore. Here the answer is no, the ratio

Si/Mg does not change so Na/Mg should also not have changed. Moreover if we

consider the Na 2s intensity (by judging with the diminution of the Mg 2p intensity)

that it should have at higher RH, we would still get a signal ten times higher than the

one recorded at ℎ =1050 eV [Figure 28] which is still visible. Thus the

disappearance of the sodium signal cannot be explained by the signal being lost due

to the attenuation.

Figure 30:Evolution of the the Na/Mg ratio as a function of the relative humidity for Na+-hectorite (red) and Na+-saponite (blue)

Another question that arises is if the sodium reappears after pumping. In fact

the scan after pumping shows no sign of sodium being back but the signals of all the

structure components are low due to a strong carbon contamination and the shape

of the spectra is distorted by strong differential charging effect. The excess of carbon

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Ratio N

a/M

g

100806040200Relative Humidity (%)

Signal Lost


45

comes from chamber and are brought on top of the sample by the water; this creates

a strong layer of contamination when water is pump out that we have to go through

before probing the clay. The strong differential charging effect is due to having less

uniformed sample at the end of the experiment compared to starting condition. The

samples are prepared with a slow evaporation of the water droplets containing Clays

in order to allow a good organization of layered stacks. In our experiment when we

pump out, the water is desorbed quickly from the interlayer space leading to an

unorganized collapse of clays stack. Moreover we have found that the following

experiment have traces of clay contamination indicating that some clay layers are

torn away from the sample while pumping out. This is mostly visible in the form of a

silicon peak being present.

This time compared to the Cs+ based samples, the Si 2p does not follow the

counter ion CLS [Figure 31]. In the case of the Na+ based sample it does not shift at

all. This suggests that the sodium is being hydrated in the middle of the interlayer as

it is expected to do and does not stay clung at the clay surface.

Figure 31: Evolution of the Si 2p core level peak binding energy aligned with Mg 2p as a function of the pressure and relative humidity for the Na

+-saponite sample

When we look at the evolution of the distance between the counter ion core

level peak (Na 2s) and its reference peak (Mg 2p) in [Figure 32], we observe the

same different starting position between saponite and hectorite samples with

hectorite having a higher difference between the two peaks. Moreover the shift is

stronger in the case of the saponite. This might be related to the counterion being

Inte

nsity (

Arb

. U

nits)

108 106 104 102 100 98Binding Energy (eV)

UHV

0,032mbarRH=0.5%

0,34mbar RH=5%

0,96mbar RH=14%

2mbar RH=29%

3mbar RH=43%

4,8mbar RH=68%

Evolution of saponite Si2p core level binding energy with increase water pressure

hv=750 eV

T=275 K

X2

X3

X5

8,4mbar RH=100%

X20

Si 2p


46

nearer to the negative charge in the case of saponite (and thus being more

electrostatically attracted) than in the case of hectorite. In the saponite, sodium

would see a more drastic environment change as it leaves the lath surface and goes

hydrated in the middle of the interlayer space.

Figure 32: Evolution of the distance between Na 2s peak and Mg 2p as a function of the relative humidity for Na+-

hectorite (red) and Na+-saponite (blue)

It is interesting that the sodium disappears long before the first droplet of liquid

water is formed at the sample surface [Figure 33].

Figure 33: Image of the sample when Na+

disappears from the XPS spectrum at RH=68 %

To sum up, the behavior of the sodium based element are much different than

those with cesium. We observe a shift to lower binding energy with the hydration of

sodium and the formation of its first hydration sphere in an outer shell complex in the

13.6

13.4

13.2

13.0

12.8

Dis

tance f

rom

Mg2p (

eV

)

50403020100RH(%)

Evolution of Na2s position related to Mg2p

Na Hectorite OH

Na Saponite OH


47

middle of interlayer space and its disappearance certainly linked to a migration due

to charging effect still present inside the sample

2.c.3) Sr2+samples

With those last samples we have greatly improved the mitigation of the

charging effect that affects dramatically the core-level spectral shape in UHV by

applying a +30 V bias and by introducing a chemically neutral gas in the mbar range.

In this part the aim was to study a divalent cation, which is easily solvable yet

much bigger than sodium: strontium. Like with the precedent sample all peaks are

aligned with the Mg 2p peak. The strontium core level peak that was chosen to follow

by NAP-XPS is the Sr 3d peak. The peak is fitted with a Gaussian doublet separated

by 1.8 eV with a 2/3 ratio between the 3/2 and 5/2 component. In [Figure 34] is

displayed the evolution of Sr 3d binding at various RH for the saponite sample with

the UHV spectrum acquired under a pressure of 1 mbar of argon. All spectra have

been acquired under a +30 V biasing. We observe a single shift of 0.5 eV towards

lower binding energy between 2 and 15% of relative humidity. This shift certainly

indicates the formation of two water layer around cation. There are two possibilities

that can explain why there is a single shift. First it is possible that the sample was not

fully dehydrated and retains enough water for a partial first water and then when the

relative humidity is sufficiently high it forms its two water layer state. The second

possibility is that the saponite does not have a two-step hydration and forms

immediately its two hydration sphere. Moreover when we consider the behavior of

Ca2+-saponite (discussed previously) which has its two solvation sphere at RH=20%

and an unstable one water layer state below, it is possible that it is the same case for

the Sr2+-saponite and that it forms directly its two hydration sphere at low RH.


48

Figure 34: Evolution of the Sr 3d core level peak binding energy as a function of the pressure and relative humidity for the saponite sample at hv=750 eV

Nevertheless that shift towards lower binding energy confirms the observation

made on the Na+ samples where a shift to lower binding energy indicates the

hydration of counterion. Yet here the counterion are still present on sample and does

not migrate like sodium while being hydrated showing the strong mobility of Na+

compared to the other counterion studied.

Concerning the Sr2+-hectorite sample unfortunately due to a misalignment of

the setup, the focal point is further away from the sample than in previous in

experiment limiting the max pressure at 3 mbar (RH=48%). The spectra acquired

under biased condition are shown in [Figure 35].

In this figure we observe a first shift of 0.3 eV towards lower binding energy as

soon as a water pressure is introduced. In surface sensitive conditions this shift is

also seen (Appendix A.5). As with the sodium sample this indicate the hydration of

the counterion where it certainly forms its first hydration sphere. A second small shift

of 0.15 eV seems to occurs at RH=48% indicating the formation of the second

hydration sphere as the Sr2+-hectorite should be in a two water layer state after

RH=43%.59 Yet the signal coming from the strontium (as well as the rest of the clay

element) is extremely low with a strong noise in background leading to a large

uncertainty about the width and position of the peaks.

Inte

nsity(A

rb. U

nit)

54 53 52 51 50 49 48 47140 138 136 134 132 130

Binding Energy (eV)

UHV

0.1 mbar RH=2%

0.7 mbar RH=15%

1.7 mbar RH=35%

3 mbar RH=63%

4.8mbar RH=100%

X5

X4

X8

X10

Mg2pSr3d


49

Figure 35: Evolution of the Sr 3d core level peak binding energy as a function of the pressure and relative humidity for the hectorite sample

Concerning the Si 2p peak [Figure 36] the situation is similar than in the case

of sodium samples with no visible shift of the peak. Thus it is probable that the

strontium like the sodium forms an outer sphere surface complex and has no

interaction with the clay surface.

From this experiment we shown that the NAP-XPS is able to follow the

hydration of the counterions inside swelling clays and monitor the evolution of their

electronic environment. The hydration of sodium and strontium sample shows the

formation of the hydration sphere as a shift of the counterion core level to lower

binding energy. Moreover the loss of the sodium signal compared to the other

counterion shows its relatively high mobility. The case of cesium is more complex as

it never fully hydrates and forms an inner sphere complex with the clay surfaces

shown by a shift was towards higher binding energy showing of both cesium and

silicon.

Inte

nsity (

Arb

. U

nit)

52 50 48 Binding Energy (eV)

140 136 132

X80

X20

X6

X0.4

0.7 mbarRH=11%

1.7 mbarRH=27%

3 mbarRH=48%

hv=750 eVT=274 KSr 3d

UHV

hv=750 eVT=275 KSr 3d

Mg 2p

0.1 mbar RH=2%


50

Figure 36: Evolution of the Si 2p core level peak binding energy aligned with Mg 2p as a function of the pressure and relative humidity for the saponite sample at hv=750 eV

2.c.4) Binding Energy Shift interpretation

As explained in chapter 1 the binding energy shift can be due to three factor,

Initial state, final state and changes in Fermi level position. As we do not have

observed changes in Fermi level position, this factor does not contribute to the

binding energy shift, leaving only to consider the initial and final state contribution.

The initial state factor is linked to the electrostatic potential felt by electron.

Our systems can be described as a stack of positive layer between negative charged

layers, and can be considered as a condenser. With the hydration of the cation, the

conductivity and the electrostatic fields of material change. Moreover with the

hydration, the swelling of clays occurs increasing the distance between the negative

layers which has also an impact on the electrostatic field. Simulation to quantify the

initial state felt by the cation between the dry and its hydrated states are being

carried out at the PHENIX Laboratory (UPMC).

Inte

nsity (

Arb

. U

nit)

106 104 102 100

Binding Energy (eV)

UHV

RH=2%

RH=15%

RH=35%

RH=63%

RH=100%

Si2p

Si2pSi 2p


51

The final state here is linked to the modification of the relaxation energy when

the cation hydrates and is surrounded by water molecules which modify the

screening effect. We remind that the classical equation is:

ΔER= 1

4πε0(1-εr

-1)e2

2r0

The main change with the presence of an hydration sphere that is mobile

compared to the dried state were cation are in fixed position is that the

photoemission process is faster than the required time for the molecule in the

hydration sphere to reorganize themselves. For a neutral element, when a hole is

created, a new positive charge appears and the water molecule will reorient to

present their oxygen to the now positively charged element. Thus for the above

equation the optical relative permittivity should be used to take into account this

repositioning.70

Yet Winter et al showed that for cation, the water molecules are pre-oriented

allows to use the static relative permittivity although the photoionization process is

faster than the relaxation of the water dipole.70 This pre-orientation is due to the

cation already having a positive charge orienting the water molecule to present their

oxygen to the cation. Thus when a hole is created the water molecule doesn’t need

to reorient.

In the dry state the permittivity of the system is: εr=εclayε0 with εclay~5 (3-6 for

mica74 and 4 for silicon dioxide75 the tetrahedral component). After hydration the

permittivity become εr=εwaterε0 for one water layer. The static dielectric constant of

one water layer can be estimated to 10 by using the relation between dielectric

constant and basal spacing of 2:1 phyllosilicates.74 With εwater>εclay the dielectric

constant of the cation environment increases with the formation of the hydration

sphere and ΔER is positive thus the binding energy decreases.

This is why in the case of Na+ and Sr2+ samples (hectorite and saponite) the

hydration is visible as a shift to lower binding energy. Yet it does not explain the shift

to higher binding energy of the Cs+ samples. Cesium has the lowest charge density

of the cation and may not fully pre-orient the water molecule due to its diffuse charge.

Yet when a hole is created, its charge density increases reorienting the water

molecule. Thus using static the dielectric constant for cesium may not be relevant. If

Chapter 2:3)Insertion of Pyridine

52

we instead consider that optical permittivity of water is nearer to infinite frequency

(εwater~2)76 we have a slight decrease of the relaxation energy and thus an increase

of the binding energy.

It should be noted that those reflections on the relaxation energy are only a

part of the explication of the chemical shift as the initial state part still has to be

determined.

3) Insertion of Pyridine

The interaction of organic molecules with clay minerals is a far reaching topic

that intersects with catalysis,77–82 and environmental sciences in the broadest

sense.60,83–94 We are focusing here specifically on the insertion of pyridine into

strontium-exchanged synthetic hectorite, under controlled conditions of relative

humidity (RH), using near-ambient pressure x-ray photoelectron spectroscopy (NAP-

XPS). The choice of Sr2+ as a counter-ion is motivated by its high water hydration

energy that drives the swelling of this clay by insertion of water layers with increasing

relative humidity (RH). 95,51 strontium is also of particular interest because its isotope

(90Sr, a β- emitter) is a principal component of many radioactive wastes.96,97 Finally

let us emphasize that the great advantage of synthetic hectorite, with respect to its

natural analogs (like montmorillonite), is to impoverish the chemistry of the side

surfaces of its crystals because of the absence of aluminum and di- and trivalent iron

cations in its structure.

For its part, pyridine (C5H5N, see [Figure 37 (b)]. is a heterocyclic molecule

with a lone-pair bearing nitrogen atom that can make H bond with water molecule. It

is a weak base, widely used to probe the Lewis and Brønsted acidic sites of minerals

in general, 98–101 and clays in particular. 60,90,100,102–104 Much beyond its use as an

acidity probe, pyridine is a simple, prototypical chemical species, as because of its

nitrogen heteroatom, it presents similarities with common herbicides (e.g. pyridine

carboxylic acid herbicides), and with nucleobases. Therefore, the study of its

insertion in clays is relevant to such distant fields as environmental pollution.


53

Figure 37: (a) The schematic structure of Sr2+

hectorite, (b) The pyridine molecule (blue: nitrogen, black: carbon, white: hydrogen)

3.a) Experimental procedure

We have monitored the chemistry of Sr2+-hectorite and its reactivity with in-

situ, real-time synchrotron radiation NAP-XPS. The clay sample was kept at 2°C and

exposed to a partial pressure of H2O (0.5 mbar), to which a partial pressure of

pyridine, from 0.1 to 0.5 mbar, was added. Therefore, with respect to other

techniques like NMR60 or infrared spectroscopy104 applied previously to

pyridine/water/clay systems, the sample environment of NAP-XPS allows the choice

of well-defined conditions of RH (7%) and organic vapor pressure, leading to an

immediate equilibrium in the material probed layers. 2,3,105–107 The analysis relies on

core-level binding energies of species both in the gas phase (water, pyridine and any

other gas present) and in the solid phase (clay, adsorbed pyridine, and any other

compound present).1 In particular the adsorption state of pyridine and its interaction

with water confined in the interlayer could be determined. The great advantage of

NAP-XPS with respect to other spectroscopic techniques is the gas/solid synoptic,

real-time view, of considerable importance in catalytic studies.1 This has enabled us

to determine the pyridine pressure conditions leading to the mineralization of pyridine,

that we interpreted as due to the radiolysis of water (more detail later),73,108–111

resulting from the synchrotron X-ray beam interaction with the oxide material.

Due to the low-conductivity of the material, differential charging under X-ray

irradiation affects the core-level binding energies.112 To eliminate this problem that


54

can lead to core-level shift misinterpretation, the sample is biased positively (+30 V),

to create a flood gun effect discharging the surface sample surface, as described in

more detail in the experimental section 113. Another valuable effect of biasing is the

elimination of the gas phase contribution114, which leaves no ambiguity as to the

attribution of the spectral components. All spectra are aligned with the Mg 2p peaks

fixed at a position of 50 eV in binding energy then with the O 1s peaks fixed at a

position of 532.35 eV when the signal of Mg 2p disappears due to the increased

pressure and adsorption of pyridine.

Photon energies of 450 eV, 750 eV and 1050 eV are used, to vary the probed

depth. The latter is typically three times the inelastic mean free path (IMFP) in the

solid of the first, which depends on the photoelectron kinetic energy (KE).72,115,116 The

clay lathes tend to show a preferential flat orientation on the gold substrate (less than

20° with respect to the substrate plane). Therefore, the relative contributions of the

outer surface and of the interlayer can be changed by varying the X-ray photon

energy, which is enabled by the tunability of synchrotron radiation. All fitting

parameters are given in Appendix B.2

3.b) Hydration state of the Sr2+-hectorite

The Sr 3d and Mg 2p spectrum of the biased sample measured in ultra-high

vacuum (UHV) conditions at ℎ =450, 750 and 1050 eV, are shown in [Figure 38].

The depth profiling shows that while the Mg 2p FWHM is constant at 2 eV, the

FWHM of Sr 3d increases from 1.72 eV (surface sensitive conditions, IMFP of ~

1.2nm) 72,115 at ℎ =450 eV to 1.82 eV(IMFP ~1.9 nm at ℎ =750 eV) 72,115 and finally

to 2.04 eV (more bulk condition, IMFP of ~2.5 nm) 72,115 at ℎ =1050 when fitted with

a single doublet. Details of IMFP are shown in Appendix A.7. Vertical differential

charging is excluded as the FWHM of Mg 2p remains constant. The broadening of

the Sr 3d doublet is due to different chemical environments that cannot be resolved

experimentally. Surface Sr2+ are not exactly in the same chemical environment as

interlayer ones, because they are dehydrated, while the interlayer ones are still in

interaction with some water trapped inside the clay. The relaxation energy117 of

surface Sr2+ is also affected as it lies at the interface between an oxide (with an

infinite frequency dielectric constant for silicon dioxide of about ~4) 118 and the


55

vacuum. This is not surprising as in fact, strontium-exchanged hectorite, as well as

calcium-exchanged hectorite,60 are not fully dehydrated, even after a prolonged stay

in high vacuum at room temperature and one may have expected that the alkaline

earth ions sitting at the surface could be distinguished from those in the interlayer,

due to smaller relaxation energy for the former ones (in a dielectric response scheme,

the upper half space is the vacuum) and because more water could be retained in

the interlayer than at the surface.

Figure 38: Evolution of the FWHM of Mg 2p and Sr 3d with different excitation energy (450, 750 and 1050 eV) under UHV condition.

In [Figure 39] is shown the evolution of the Sr 3d and Mg 2p at ℎ =750 with

increased pressure of water and pyridine. The estimated72,115 IMFP (more on

Appendix A.7) for those orbital are ~1.9 nm for the Sr 3d photoelectrons of kinetic

energy (KE) ~615 eV and ~2.1 nm for Mg 2p (KE=700 eV). We recall that the

thickness of the phyllosilicate lath is 0.9 nm thick and that the basal plane spacing is

between 1.07 nm (zero inserted water, denoted 0W) and 1.29 nm (one inserted

water layer, denoted 1W). The Sr 3d spectrum is still fitted with a single 3d3/2/3d5/2

doublet separated by 1.8 eV, with the 3d5/2 component positioned at a BEFL of 133.95

eV. As mentioned previously, this fitting suggests a single chemical environment for

Inte

nsity (

arb

.un

it)

136eV 132a

54 52 50 48 46Binding energy (eV)

Biased by +30VSr 3dT=275 K UHV

Mg 2p

hv=450

hv=1050

hv=750

FHWM=2.04

FHWM=1.72

FHWM=1.82

FHWM=2.05

FHWM=2.03

FHWM=1.99


56

the strontium ion while there is different environment for the counterion inside the

interlayer and the one present at surface but the resolution is not sufficient to

distinguish between them.

Figure 39: Sr 3d and Mg 2p spectra measured at hν=750 eV with a sample bias of +30 V (the binding energies are corrected)

When the water pressure is raised to 0.5 mbar (RH=7%), the Sr 3d5/2Mg 2p

energy difference diminishes by ~0.3 eV, a shift with the same value was observed

during the study of the Sr2+-hectorite hydration previously in this chapter and we can

admit that a 1W state is also reached in the present case. Only for RH>40% does

Sr2+-hectorite swell to a two-water-layer hydration state (denoted 2W).119 The Sr 3d

chemical shift can also be here interpreted in terms of an initial state effect, i.e. a

change in the electrostatic energy due to swelling felt by the strontium atom, or by a

final-state effect, i.e. a change in the dielectric screening due to the hydration of Sr2+,

or to both effects.112

Inte

nsity

(Arb

. U

nit)

140 136 132Binding Energy (eV)

52 50 48

Sr 3dhv=750 eVT=275 K

Mg 2p

UHV

0.5 mbar water

0.5 mbar Water+0.1 Pyridine

x4

x0.5

Biased by +30V

FWHM: 1.99 eV

FWHM=1.66 eV

FWHM=1.62 eV


57

3.c) Adsorption/sorption of pyridine at 7% RH

When 0.1 mbar of pyridine is added to the water base pressure of 0.5 mbar

[Figure 39], the Sr 3d5/2 Mg 2p energy difference also diminish by 0.2 eV.

Consequently, the Sr2+ ion feels the presence of the pyridine molecule that sticks to

the clay external surface and penetrates in the interlayer. For the higher partial

pressures of pyridine (0.3 mbar and above), the Sr 3d signal becomes very weak,

almost non-measurable. This originates from two conjugated effects, the attenuation

of the photoemission signal (the IMFP of the photoelectrons in the gas phase is

inversely proportional to the pressure3), and the adsorption/sorption of pyridine. The

latter phenomenon is clearly demonstrated by the (unbiased) O 1s spectrum of

[Figure 40] which remains measurable up to a pyridine pressure of 0.5 mbar.

Figure 40: O 1s spectra of the unbiased, grounded sample measured at hν=750 eV. The grey component corresponds to the clay lattice oxygen and to confined water, while the blue one corresponds to gas phase water

The grey component corresponds to the clay lattice oxygen and to confined

water, while the blue one corresponds to gas phase water. In all cases, the sample is

kept at +2°C. The water pressure is 0.5 mbar (RH=7%). Pyridine gas is added

(partial pressures are indicated, from 0.1 to 0.5 mbar) while the water pressure is

kept constant. The UHV conditions correspond to a base pressure in the low 10-8

Inte

nsity

(Arb

. U

nit)

570 560 550 540 530 520

Binding Energy (eV)

X0.2

X0.5

X4

X6

X2

NOx(g)

H2O gas phase

0.5 mbar Water

0.5 mbar water+ 0.1 pyridine



After Pumping

UHV

hv=750 eVT=275 K

O 1s

Grounded

OBulkOH2O

=8.34

OBulkOH2O

=5.4

OBulkOH2O

=1.66

OBulkOH2O

=3.39


58

mbar range. After pumping down the gas mixture, a pressure of ~10-7 mbar is

recovered. All spectra are aligned with respect to the Mg 2p.

As the spectra are recorded under a constant water pressure, the solid

(phyllosilicate) intensity can be normalized by dividing it by the gas (water vapor) O

1s intensity. The solid/gas O 1s ratio, reported in this figure decreases regularly as

the pyridine pressure increases. This means that the phyllosilicate O 1s contribution

is damped by the adsorption of pyridine at the external surface of the layers and

within the layers. The swelling of the clay could be appreciable, as the basal plane

distance changes (e.g. in Ca2+-hectorite60) from 1.29 nm (1W) to 2.13 nm when

pyridine gets into the interlayer. This lattice expansion is comparable to the relatively

small IMFP of the O 1s photoelectrons, ~0.9 nm.72,115 at a kinetic energy of ~220 eV.

The N 1s spectra in measured at ℎ =750 eV [Figure 41(a,b)] and at ℎ = 450

eV in [Figure 41(c)] give clues both about the chemical state of adsorbed/sorbed

pyridine and its reactivity with the clay. In all cases, the sample is kept at +2°C. The

water pressure is 0.5 mbar (RH=7%). Pyridine gas is added (partial pressures are

indicated, from 0.1 to 0.5 mbar) while the water pressure is kept constant. After

pumping down the gas mixture, a pressure of 10-7 mbar is recovered. All spectra are

aligned with respect to the Mg 2p maximum at 50.00 eV and the O 1s maximum at

532.32 eV The IMFP in the clay is estimated to be ~1.3 nm for N 1s photoelectrons

of kinetic energy ~350 eV at ℎ =750 eV and 0.8 nm at ℎ =450 eV.

The signals relative to both the gas-phase and solid-phase species are

present in the unbiased spectrum of [Figure 41(a,c)]. In the biased spectra of [Figure

41(b)], the gas-phase contribution is practically eliminated, appearing as a small

background that increases to lower binding energies. By comparing panels (a) and

(b), one sees immediately that under a partial pyridine pressure of 0.1 mbar, the two

components at BEFL of 400.4 eV (golden component) and 405.35 eV (blue

component) pertain to the gas phase. This point will be addressed in detail later on.


59

Figure 41: N 1s spectra in measured at hν=750 eV under grounded condition (a), at hv=750 eV in biased condition (b) and at hv=450 eV under grounded conditions(c)

Thus, let us first focus on the two solid-phase components of [Figure 41]. The

low binding energy component at 399.5 eV (dark purple) is attributed to

adsorbed/sorbed pyridine molecules with free N lone-pairs, i.e. that are not engaged

in dative bonding120,121 nor in hydrogen bonding.122–126 The high binding energy

component at 401.5 eV (light purple) is attributed to pyridine molecules engaged in

hydrogen bonds with adsorbed/sorbed water molecules (forming the hydration shell

of the Sr2+ ions). Pyridine, a Lewis base, can act as an H acceptor. The effect on the

N 1s binding energy is essentially electrostatic in nature,125 inducing a shift to higher

binding with respect to the free lone-pair case. In the specific case of isonicotinic

acid, a pyridine carboxylic acid, a sizeable binding energy shift of 1.7 eV between

non hydrogen bonded nitrogen atoms and nitrogen atoms in acceptor OHN

bonds.124 However, the presence of pyridinium ions is excluded as the 2 eV binding

energy difference observed between the two N 1s components is significantly

smaller than that between neutral pyridine and the pyridinium ion (about 2.65 eV127).

Therefore, water molecules in Sr2+-hectorite are less acidic than in Mg2+

smectites90,128 where the protonation of pyridine is observed (and within the

(b) (a) (c)

Inte

nsity

(Arb

. U

nit)

408 404 400 396

Binding Energy (eV)

UHV

0.5 mbar water+ 0.1 Pyridine



After PumpingX0.5

Pyridine Gas phase

AdsorbedPyridine

Pyridine in Hydrogen bond

N2 gas phase

N 1shv=750 eVT=275 KGrounded

Inte

nsity

(Arb

. U

nit)

408 404 400 396

Binding Energy (eV)

X0.5

0.5 mbar water+0.1 Pyridine



After Pumping

Pyridine in hydrogen bond

Adsorbed Pyridine

N 1shv=750 eVT=275 KBiased by +30V

Inte

nsity

(Arb

. U

nit)

408 404 400 396

Binding Energy (eV)




After Pumping

X0.2

X0.2

N2 gas phase

Pyridine gas phase

Adsorbedpyridinepyridine in hydrogen bond

hv=450 eVT= 275 KGrounded

N1s


60

detection limit of NMR in Ca2+--hectorite60), because of the bigger size of the

strontium cation.

At ℎ =750 eV [Figure 41 (a,b)], the “H-bond” to “non H-bond” intensity ratio is

0.15 and does not change much during the experiment when the pyridine pressure is

raised to 0.3 mbar and then to 0.5 mbar. This ratio is notably different when the

spectra are recorded at ℎ =450 eV, see [Figure 41 (c)]. At this excitation energy the

IMFP in the clay is minimal, in the range of 0.8 nm,72,115,129 nearly a factor of two

smaller than at ℎ =750 eV, and practically equal to the phyllosilicate lath thickness.

Consequently, more surface sensitive conditions are reached. Note that the too

small kinetic energy prevents the application of the +30 V positive bias, and the

elimination of the gaseous pyridine component. Nevertheless, as long as the latter

one is not too intense (i.e. for pyridine partial pressure below 0.3 mbar), the “H-bond”

to “non H-bond” intensity ratio can be precisely obtained by curve fitting. At ℎ =450

eV and under a pyridine partial pressure of 0.1 mbar, this ratio is 0.47, compared to

0.16 at ℎ =750 eV. Consequently, in surface sensitive conditions, the spectral

weight of the H-bonded pyridine increases in a sizeable manner. This suggests that

the “non H-bonded” pyridine is preferentially found in the interlayer space, or in pores

deeper in the deposit.

Figure 42: The pyridine hydrated cation (1W) complex, according to Refs.14 and 16

2,1

3 n

m


61

H-bonded complexes in the interlayer causing the clay expansion to 2.13 nm

were detected by NMR in Ca2+-hectorite.15 A model was previously proposed,90

based on infrared spectroscopy [Figure 42]: four pyridine molecules are placed

around the first hydration shell of the cation, making acceptor H bond with water, the

molecular plane is orthogonal to the phyllosilicate plane, and the molecular C2 axis

makes an angle 45° with the latter. Advanced simulations of pyridine bonded on the

surface of the smectite lathes, on the edges, or within the interlayer, have been

carried out for the dry materials,130 but, unfortunately they are lacking when a

hydration shell around the cation is considered. In Ca2+-hectorite, NMR also detected

strong signals due to pyridine making no H bonds, mobile intercalated pyridine and

pyridine physisorbed on the outer surface, or pores. Considering that the average

free area around one strontium ion is 1.21 nm² (corresponding to an average

distance between Sr2+ ions of 1.1 nm). In the model given in Refs. 90 and 60, the

basal spacing increases by 1.2 nm (swelling due to water and pyridine), the free

space around the strontium ion is 1.45 nm3. Now the dimensions of the complex

between the hydrated Sr2+ and pyridine can be estimated to be 1.2 nm (height) ×0.9

nm (width) ×0.26 nm (depth, i.e. the size of the Sr2+ solvation sphere131,132). The

complex occupies a volume of 0.28 nm3. Consequently, of the 1.45 nm3 available

space, 0.28 nm3 are occupied by the complex, leaving 1.17 nm3 free for non H

bonded pyridine molecules. Pyridine is a planar molecule with a diameter of 0.5 nm,

whose minimal occupation volume can be estimated to be 0.063 nm3 taking a face-

to-face - stacking of 0.32 nm. This means that a maximum of 19 non-H bonded

pyridine molecules per strontium ion can be inserted in the available space.

Therefore the maximum non H bonded to H bonded ratio is 19/4, that is 4.75 ~5.

This value compares with the ratio measured in bulk sensitive conditions (~6.7), but

is notably higher than that measured in surface sensitive conditions (~2.1). For

physisorbed molecules, the adsorption energy of pyridine on the outer surface of the

lath is likely smaller than in the interstice between the lathes.

3.d) Mineralization of adsorbed/sorbed pyridine under synchrotron beam

irradiation


62

We now consider the gas-phase components at BEFL of 400.4 eV (golden

component) and 405.35 eV (blue component) in the unbiased spectrum of [Figure 41

(a,c)]. The lower energy one (at 400.4 eV) is ascribed to pyridine in gas phase as it

increases with pyridine partial pressure. These two components arise from core-

ionized molecules present in gas volume in contact with the solid. The vacuum level

of the molecules is pinned to the vacuum level of the solid, and thus BEFL is simply

the difference between the gas phase ionization energy referenced to the vacuum

level (IEVL) and the solid work function, neglecting the contact potential difference

gradient between sample and analyzer. As the N 1s IEVL of pyridine 404.9 eV,133 the

other gas phase component seen at higher binding when the pyridine partial

pressure is 0.1 mbar, corresponds to an IEVL of 409.95 eV. This is precisely that of

dinitrogen (N2).133 Gaseous NO is also excluded. NO being a radical, should exhibit

a N 1s doublet at IEVL 410.1 and 411.5 eV(i.e. at BEFL 405.5eV and 406.9eV) and an

O 1s doublet at IEVL= 543.2 and 543.8 eV133 (i.e. at a BEFL of 538.6 and 539.2 eV) ,

observed neither in the N 1s window [Figure 41(a,c)] nor in the O 1s one [Figure 40].

When the partial pressure of pyridine reaches 0.3 mbar, the production of gas

phase N2 stops. We can find an explanation for this effect considering the C 1s

spectra of [Figure 43]. Indeed, once the gaseous N2 signal disappears from the N 1s

energy window, a C 1s component at a BEFL of 292.15 eV starts to grow both in the

biased and grounded condition spectrum, showing it must be attributed to a solid

phase species. This BEFL is significantly higher than that of CO32- in SrCO3 (289.0-

289.6 eV) or of HCO3- (290 eV) in NaHCO3.

134 However it corresponds to the C 1s

BEFL of CO2 physisorbed on oxide surfaces (291.8 eV)135. The signal still increases

when the pyridine partial pressure reaches 0.5 mbar. However, when the gas

mixture is pumped down, the component disappears from the C 1s window, showing

that the species is weakly bonded, being only produced when the pyridine partial

pressure reaches 0.3 mbar. Note that the UV-induced complete degradation136 of

pyridine in aqueous solutions over TiO2 yielded ammonium and nitrate ions

(unobserved here), apart from CO2 (we do observe). Gaseous N2 maybe the result of

a further decomposition of NH4+ and NO3

-. If that is the case then the reaction of

decomposition should be extremely fast as no NH4+ and NO3- are observed, their

lifetime in the system must be low.


63

Figure 43: C 1s spectra measured at hv=750 eV while the sample is biased to +30 V

The identification of N2 in the gas phase (under a pyridine partial pressure of

0.1 mbar) and of adsorbed CO2 (in the 0.3-0.5 mbar range) indicates that pyridine is

partly mineralized. A temperature of +2°C is not the in range where thermally

activated reactions are expected to occur. Therefore, the synchrotron X-ray beam

certainly creates highly reactive oxidizing species that attack the adsorbed/sorbed

molecule. Such species are not produced in the gas phase, as the N2 yield is zero

when the pyridine partial pressure reaches 0.3 mbar. In fact, the observed

phenomenon occurs in the solid phase.

The physics of radiation damage induced by X-rays in insulating solids was

given by Cazaux.137 In these materials, photoemission and Auger electron lose

energy during their transport in the insulating material by creating electron-hole (eh)

valence pairs. The number of eh pair produced by one (absorbed) photon of energy

h and having ionized a given core-level is approximately h/E(eh), where E(eh) the

Inte

nsity (A

rb.

Unit)

294 292 290

Binding Energy (eV)



Inte

nsity

(Arb

. U

nit)

295 290 285 280 275

Binding Energy (eV)

X2

X0.5

CO2(ads)

0.5mbar water




After Pumping

UHV

C 1shv=750 eVT=275 KBiased by +30V


64

energy for creating a eh pair (E(eh) is two to three times the bandgap). The pair

generation factor per unit volume and per unit time 𝑔(𝑒ℎ)may also be expressed as a

function of Φ0the incident photon flux, and 𝜇 the linear absorption coefficient of the

specimen for the X-ray photons of interest as:

𝑔(𝑒ℎ) =ℎ Φ0𝜇

𝐸(𝑒ℎ) (in cm-3 s-1)

The mass absorption coefficient μ* in dry Sr2+-hectorite of composition

Sr0.4Mg5.2Li0.8Si8.0O20(OH)4 can be calculated from the mass absorption coefficients

of its atomic constituents (see Table 4.) Then the linear mass absorption knowing the

mass density of Sr2+- hectorite(~2.5 g/cm3) as μ=μ*×ρ

Element Atomic mass μ* at hν=750ev138 μ* at hν=450ev138

Si 28.09 3514 cm²/g 12950 cm²/g

Mg 24.31 1936 cm²/g 7655 cm²/g

Li 6.94 542 cm²/g 2544 cm²/g

Sr 87.62 6643 cm²/g 18288 cm²/g

O 15.999 9673 cm²/g 1778 cm²/g

H 1.008 17,6 cm²/g 95 cm²/g

Clay (unit cell) 779.74 6393 cm²/g 6689 cm²/g

μ - 15981 cm-1 16723 cm-1

Table 4: Mass adsorption coefficient and linear adsorption value for clay

At ℎ=750 eV (450 ev), Φ0= 4×1016 photon/s/cm2 (2×1016 photon/s/cm2). With

𝐸(𝑒ℎ) =17 eV (same as silica) and μ =15981 cm-1 (16723 cm-1), we find

g(eh) =2.8×1022 cm-3 s-1 (8.85×1021 cm-3 s-1). A typical N 1s spectrum is taken within

a time ti of 25 second, so 7×1023 (2.2×1023)eh pairs per unit volume are generated

during the acquisition.

In the specific case of clays, the presence of strong electric fields in the

lamellar structure may efficiently separate holes and electrons, and reduce their

recombination probability. However, as emphasized by Cazaux, not all experimental

data can be explained by transport and eh pair generation only. The Auger decay,

that leads to a change of the normal charge of a given atom (think of an O atom

excited above the K edge of 530 eV, that ends in a 2h valence state) may have

considerable implication in X-ray beam damage, as the normal charge cannot be


65

restored quickly (contrary to metals) because of the lack of a significant density of

conduction electrons. Bond rupture (covalent systems) or atom desorption (ionic

system) can be envisaged. For instance, the O KLL Auger decay in

adsorbed/inserted water molecules can lead to a H2O2+ species, that dissociates via

Coulomb explosion, or react with neutral water (an acid-base re-equilibration

reaction) to give H3O+ plus atomic O.139 Indeed water ice exposed to the synchrotron

white beam sees the appearance of O, HO2 and O2 species in the NEXAFS O 1s

spectrum, besides OH and H2O2.140

Considering now the specific case of oxide/water interfaces, there is an

abundant literature on water radiolysis under pulsed high energy electron

beams,73,109–111 (or under irradiation by other high energy particles, see 141 and

references therein). The wide interest for this topic stems from the fact that water

radiolysis leads to H2 production, with obvious safety issues. As discussed by

Cazaux, electrons of kinetic energy EKE and photons of energy ℎ =EKE ) are

equivalent for the number of electron hole pairs generated during the electron

transport. However a major difference is the dominant interaction : for incident X-rays

it is the excitation of core-electrons (followed by the Auger decay), while for incident

electrons it is the formation of valence eh pairs, without Auger decay (due to low

core-ionization cross-sections).137 Another difference between synchrotron X-ray

irradiation and electron pulse irradiation is the received dose. The radiation dose

received at the sample surface can be estimated with the following formula

D=Φ0h μ* ti , i.e. 7.68×107 Gy for ℎ=750 eV and 2.41×107 Gy for ℎ=450 eV.

Considering that we are working in the so-called top-up mode with 28 ps wide pulses

each separated by 2.84 ns (352 MHz),142 each pulse emits a radiation dose of

8.81×10-3 (2.76×10-3) Gy at ℎ =750 eV (450 eV). Let us now compare with the

pulsed electron irradiation experiments of Refs. 111 and 73. 10 MeV electron pulses of

10 ns duration at a repetition rate of 2 Hz are used. The dose per pulse is 30 Gy.

Typical doses received by the samples are 1.5×103 Gy, four orders of magnitude

smaller than during our typical XPS scan of 25 s.

In any case, the lessons drawn from electron irradiation studies are worth

being exposed. In swelling clays (montmorillonite, saponite), water confinement has


66

a strong influence on the H2 radiolytic yield induced by electron irradiation, being

maximum for one water layer (1W) in the interlayer space, and diminishing when the

number of inserted water layers increase.73 The following explanation is offered in

Ref. 73. Valence electron and holes generated by irradiation migrate to the interlayer

space. Holes react with confined water to give H+ and HO,73 all the more easily

because in the 1W hydration state, the OH bond of water molecules, strongly

polarized by the counterion, is weakened. Electrons are hydrated (eaq), then

recombine to produce HO- species and H2 (hence the potential dangerousness of

irradiated water). They can also react with H+ to give H (that recombines to give H2).

The literature concerning pyridine radiolysis in bulk water indicates that 𝑒𝑎𝑞− is

efficiently scavenged by pyridine to give a pyridine radical anion,143,144 and that OH

radicals react by addition to the pyridine ring.143,145 In the case of our hydrated clay

exposed to soft X-rays, the observation of gaseous N2 and then of adsorbed/sorbed

CO2 suggests far more advanced reactions leading to mineralization. The reaction

pathways to N2 and CO2 are not documented in the current radiolysis literature on

pyridine,143,144,146 and thus it may be specific of the soft X-ray irradiation of pyridine in

hydrated clays, both because of the different irradiation doses and of the physical

nature of the system.

Let us first consider the steady-state production of N2 at a pyridine partial

pressure of 0.1 mbar. Pyridine dissociation to N2 needs the breaking of the aromatic

ring. As no NOx products are seen, we consider primarily the H radical. Both the

abstraction of a pyridine H atom by the H radical, and its addition to the ring can be

envisaged, but the latter is favored over the former, at least in the gas phase.147 In a

hydrogenolyse/denitrogenation scheme, the pyridine ring is first fully hydrogenated

by the H, forming a piperidine molecule. Hydrogenolysis leads to n-pentyl amine

(ring opening) and then C5 hydrocarbons (that could be hidden in the gas and solid

phase main peak of pyridine, see above) plus ammonia. N2 is observed instead in

the N 1s spectrum, which would imply that ammonia is radiolytically decomposed in

the clay to give N2 and H2.148,149 The source of H is the reaction of hydration water

with electrons according to :

e + (Sr2+)…OH2 HO¯ + H (1)


67

The electrostatic field (the phyllosilicate layer is negatively charged and the

cations are in the interlayer) should facilitate the injection of electrons in the

interlayer, and one could consider that the production of Hdominates that of HO

that is due to the reaction of a hole with water according to :

h + (Sr2+)…OH2 HO + H+ (2)

HO is a strongly oxidizing species. However, the more difficult injection of

holes in the interlayer and the eh recombination may explain why the inserted

species are not oxidized (HO can indeed add to pyridine) and why no NOx are seen

in the gas phase.

Now the very interesting point is that the steady-state N2 production stops

when the pyridine partial pressure is raised to 0.3 mbar. We interpret this pressure

effect as due to pyridine scavenging the electrons falling from the clay conduction

band into its LUMO to form a radical anion. The elimination of the electrons diminish

the recombination probability with the holes, thus more holes are available and

therefore the yield of HO radicals via reaction (2) increases. The latter ones can

now oxydize the sorbed CxHy carbon remaining from the denitrogenation reaction

discussed here above. Consequently, adsorbed CO2 appears in the C 1s spectrum,

its intensity increasing with pyridine pressure.

Finally, in the context of the questions raised by Cazaux,137 i.e. the respective

importance of photoelectron/ Auger electron transport (i.e. eh pair formation) and of

multiply ionized Auger final state in beam damage, we have observed that the

mineralization yield of pyridine depends on the irradiation conditions, photon flux and

photon energy. The intensity ratio 𝑔𝑎𝑠𝑒𝑜𝑢𝑠 𝑁2 𝑝𝑒𝑎𝑘

𝑔𝑎𝑠𝑒𝑜𝑢𝑠 𝑝𝑦𝑟𝑖𝑑𝑖𝑛𝑒 𝑝𝑒𝑎𝑘 is 1.57 at ℎ =750 eV and only

0.21 at ℎ =450 eV [Figure 41]. The ratio increases by a factor of 7.5, at ℎ =750 eV,

i.e. above the O K edge of 530 eV. When we calculate the overall ionization cross-

sections (see Appendix B.1) for the clay compound of formula

Sr0.4Mg5.2Li0.8Si8.0O20(OH)4 we found that is it practically the same at 450 eV (8.22

Mbarn) and 750 eV (8.29 (Mbarn). Between 450 eV and 750 eV, the number of

generated eh pairs being proportional to ℎ 137 should increase by a factor 1.7. As

the photon flux at ℎ =750 eV is twice that at ℎ =450 eV, the yield of eh pairs should

Chapter 2:4)Conclusion

68

increase by a factor of 3.4 from ℎ =450 eV to ℎ =750 eV, a value that compares

with the observed increase in the N2 yield by a factor 7.5. However, it is not excluded

that water dissociation from the doubly ionized Auger state should be taken into

consideration. Against the effectiveness of the Auger decay, one can consider that

the NAP conditions, where the material is practically discharged by negative species

coming from the gas phase, are not conducive to the formation of long timescale 2h

Auger decay species.

4) Conclusion

We have shown in this chapter the sensibility of the NAP-XPS to the changes

and evolution of the chemical environment of the counterion inside the clay interlayer.

Indeed we have shown drastic different behavior linked to the nature of the

counterion. With cesium based samples, a reaction to the presence of water is only

seen at high relative humidity when liquid water is about to form at the sample

surface. The reaction is under the form of a shift to higher binding energy for both Si

and Cs element linking to an outer-sphere surface complex. In contrary the sodium

based element shows a shift to the lower binding due to an augmentation of the

relaxation energy with the hydration of the counterion. Here there is no shift in the Si

element and the sodium is expected to form an inner-sphere surface complex during

hydration. When sodium is supposed to have formed its second hydration sphere, it

disappear from the spotting region probably due to an electrostatic repulsion caused

by residual charging effect within the sample and showing the mobility of the sodium

in confined space even before the first droplet of water is formed. With the strontium

samples we have a hydration behavior similar to sodium with shifts to lower binding

due the increased relaxation energy but compared to sodium, strontium is stays in

the spotting region even when macroscopic liquid water is formed at the sample

surface. The spectroscopic signature between the two structures used (saponite and

hectorite) is observable with a shorter distance between the counterion core level

and the referenced peak (Mg 2p) for saponite and a higher distance for hectorite.


69

Moreover, NAP-XPS in the mbar range was used for the first time to study in

real time the adsorption/sorption pyridine in a smectite, Sr2+ hectorite, kept at a

constant relative humidity of 7% (1W hydration state). Within the range of used

photon energies, both the external surface of the lamellar material and the interlayer

where the cation sits were probed. XPS could distinguish pyridine molecules making

hydrogen bonds with water molecule forming the first coordination shell of the cation,

and molecules making no hydrogen bonds, i.e. adsorbed on the siloxane planes or

accumulating in mesopores. No pyridinium species were detected, an indication of

the weak acidity of water molecules forming the hydration shell of Sr2+.

Remarkably when we inserted pyridine in a prehydrated with one water layer

Sr2+-hectorite, we observed a steady-state catalytic reaction triggered by the

synchrotron soft X-ray probe. The synoptic view of the gas and solid phase, and the

biasing procedure we used to distinguish unambiguously the two contribution, lead to

the conclusion that pyridine is mineralized to (at least) two species, N2 that escapes

to gas phase, and CO2 that accumulates in the solid. N2 is detected until the pyridine

partial pressure overcomes a threshold, and we attribute this effect to the

scavenging of conduction electrons by the pyridine molecule, and hence the

quenching of H radical production leading to the hydrodenitrogenation of the

molecule.

As the soft X-ray irradiation of hydrated clays leads to formation of highly

active species that are able to decompose a molecule of prototypical of more

complexes substances (nucleobases, herbicides), the contribution of the present

work to soil sciences or pollution remediation of waste water is obvious. It may also

be relevant to the conservation of biomolecules in the smectite-rich150 soils of a

planet like Mars that receives more radiation than Earth,151 and whose atmosphere

and variable humidity152,153 could be simulated conveniently in NAP-XPS setups.

More generally, our work underlines the effects of synchrotron beam irradiation with

water confined in a wide band-gap material, and raises the question of the detailed

physics of water radiolysis in this systems.75 and whose atmosphere and variable

humidity could be simulated conveniently in NAP-XPS setups.


70

Chapter 3: Catalytic reaction on Nanoparticle

1) Introduction

The increase of the standard of living throughout the world has led to the

inevitable increase in global energy consumption. This trend is further amplified by

the growing of world population. Overall the energy consumption is expected to

increase by an order of 40% over the next 20 years [Figure 44].154

Figure 44: Evolution of energy consumption in quadrillion Btu with time

While it is possible to meet those requirements with fossil energy sources, the

issue of air pollution and climate change will amplify. In particular, this will lead to a

further increase of CO2 emissions which are already at their highest level with 36 Gt

CO2 emitted per year.155 We will reach twice the amount of CO2 in the atmosphere

than the pre anthropogenic (260 ppm) values without action to limit those

emissions.156 Indeed up to now fossil sources (coal, petroleum, and natural gas)


71

have been the overwhelming dominant sources of energy, making them the heaviest

producer of CO2 gases [Figure 45].154

Figure 45: Evolution of CO2 emission in metric tons with time for different country

Strategies are now emerging in order to provide environmentally-friendly

sources of energy and limit the emission of CO2. The objective of those limitations is

to keep the increase of the global warming under 2°C at the end of the century. In

order to achieve this goal, new highly, efficient and reliable technologies to supply

the increasing energy consumption are needed.157–159

One of those technologies is the proton exchange membrane fuel cell

(PEMFC or PEM). Indeed this technology has a relatively low operating temperature

(80-100°C), a high energy density, a low emission of pollutants and particles as it

emits only waters.160,161

The main constituents of a fuel cell can be described as two electrodes

separated by an electrolyte (the proton exchange membrane) [Figure 46]. Those

electrodes have to be designed to facilitate three transport processes:

The transport of proton through the membrane

The transport and collection of electron from the collector

The transport of the reactant gases to and from the electrode (O2 for the

cathode and H2 for the anode)


72

Figure 46: Working principle of a fuel cell

The reaction that produces energy can be summarized as follow:

The hydrogen stream that arrives at the anode is separated into

electron and proton. This is the hydrogen oxidation reaction (HOR).

The newly created protons go to the anode through the porous

membrane while the electrons travel to the cathode through an external

circuit which creates the energy output of the fuel cell. The half-cell

reaction equation is:

H2→2H+ +2e-

At the cathode, the oxygen molecules catalytically react with the

incoming protons and electrons from the anode and forms water. This

is the Oxygen Reduction Reaction (ORR). The half-cell reaction

equation of the cathode is

1

2O2+2H+ +2e-→H2O

The overall reaction of the PEMFC being:

H2+1

2O2 →H2O

Here we see that the main point of the fuel cell is to use abundant material like

O2 and H2 in order to produce energy while rejecting only water and unreacted gases.


73

One of the crucial points of that fuel cell is the nature of the catalyst being in

general platinum. In fact, the quest for environmentally friendly energy generation

solutions like fuel cell which emphasizes the role of Pt-based materials as major

catalysts has to be considered. 162–166 Indeed Platinum is one of the most prominent

transition metal catalysts used in heterogeneous catalysis thanks to its relevance in

a wide range of industrial applications.167–169 While Platinum shows excellent

catalytic performance, it also suffers from two issues: the CO poisoning creating a

rapid deactivation and from material dissolution.170,171 Indeed, when the CO strongly

adsorbed on the surface of the platinum, it blocks the active sites and prevents the

electro-oxidation reaction of H2 at the anode and the oxygen reduction reaction

(ORR) at the cathode of the fuel cell to take place. The contamination with CO

comes from the way H2 is produced; mainly from fossil fuels where CO is a

byproduct of the production and may not be fully eliminated.172 This susceptibility of

the platinum to the CO poisoning lies primarily on the slow kinetics of the ORR at the

cathode.162 Thus, in order to have an optimal operation of the cathode process there

needs to be a high number of available Pt active sites for the reaction. Strategies to

mitigate the CO poisoning of Pt active sites and thus improve the stability of catalysts

include the design of improved CO-tolerant catalysts with enhanced resilience

against decomposition.171,173

A way to make the fuel cell more cost effective was to use nanoparticles of

platinum thanks to their higher surface contact area. They exhibit higher catalytic

activity while using the same amount of metal than a more standard bulk electrode.

Yet issues mentioned before still apply to those nanoparticle(CO poisoning and

material dissolution). The conventional road to reach better efficiency by minimizing

those issues is to design Platinum based binary metal nanocatalysts.171,173–177 The

formation of a binary metal compound leads to a fundamental alteration in the

properties of the Pt-M (M = metal) catalysts.158,176–181 Especially changes in

electronic structure and Pt lattice (Pt-Pt bond distance) are key factors for increasing

the activity of Pt-based catalysts.181 Moreover, Pt stabilization against material loss

and rapid deactivation can be achieved by having synergetic effects due to the

presence of a second component in the catalyst.179 One example is Pt oxygen-

reduction fuel-cell electrocatalysts which when modified with Au clusters exhibit an


74

enhanced stability against dissolution. This stabilizing was attributed to high Pt

oxidation potential in the presence of Au.

Similarly, it was shown that in the Pt-based catalyst, the formation of a thin

transition metal-oxide layer on Pt promote the activation of O2, and keep unstable

the adsorption of CO on the active sites.182,183 These interfaces with confined

unsaturated sites present stronger resilience against CO poisoning and an increased

catalytic performances. Moreover, designing oxide-on-metal nanoparticles obtained

from the formation of a metal-oxide layer (shell) on Pt (core) represents an

interesting way to drastically reduce the Platinum loads in the catalysts while

maintaining their optimum efficiency as due to the Platinum being expensive, large

scale development of fuel cells remains hindered.184 Thus, Zinc which is an

abundant metal more affordable than platinum represents a good choice for the

creation of bi-metallic nanoparticles less expensive than pure platinum but with the

same catalytic activity. The incorporation of Zn on Pt has to be performed so that an

ultrathin ZnO layer is formed on metallic Pt. This is obtained when Pt-Zn NPs were

exposed to an oxidizing atmosphere. The formation of a ZnO layer on the shell

protects the metallic Pt. Pt/Zn has been shown to be an interesting catalytic system

as for example Pt-Zn catalysts demonstrated activities for methanol oxidation

comparable to pure Pt, which is relevant for fuel cells application but with excellent

poisoning tolerance.185 In addition, highly dispersed bimetallic Pt-Zn on carbon-

supported nanoparticle electrocatalysts have shown high performance activity for the

borohydride-hydrogen peroxide fuel cell, illustrating the advantages of Zinc

insertion.158 It should be noted that the first fuel cell that converts chemical energy to

electricity was invented by William Robert Grove. He was inspired by Christian

Schönbein’ research work and his fuel cell built in 1839 was made from Pt-Zn

material.186

It was shown that ultra-thin metal oxide films grown on Pt-group metals

supports exhibit large catalytic activity most notably for CO oxidation reaction.187

These inverse model catalysts, have contributed significantly to catch part of the

complexity of real-world catalysts systems and to better understand their reaction

mechanisms. Indeed, the metal/oxide interface, where important phenomena such

as charge transfer take place, are considered to be the prevailing catalytic sites. For

example, the increased reactivity on ultra-thin ZnO films supported on platinum


75

substrate was attributed to reactions occurring at the oxide/metal boundaries.188,189

The fact that superior CO oxidation reaction is seen on partially covered-Pt with ZnO

islands compared to the fully-covered surface one, at near-atmospheric pressures,

underlines that the interfacial oxide/metal is the most likely active sites.188,189 The

aforementioned studies emphasize the importance of ZnO/Pt systems for catalysis

applications and provide also suitable systems to elucidate the fundamental

processes that control the reactivity meta/oxide interfaces.

Thus, investigating these nanocatalysts under their working conditions of

pressures and temperatures is of prime importance in order to obtain a realistic

description of their catalytic activity associated properties.3,190 Due to their inherent

flexibility, nanocatalysts undergo significant structural and chemical changes under

reactive gases.191 This implies that their properties before and after the reaction may

differ noticeably. Moreover, dynamic and reversible processes occurring during the

reaction, such as element segregation but also surface restructuring and oxidation

states; make unpredictable the identification of the active structure/phase within

those catalysts.

Recently, a large set of in situ techniques has been used to overcome this

issue.192–197Therefore, the ability to monitor in situ the oxidation states of

nanocatalysts constituent, has provided valuable information on the understanding of

chemical and structural information. Besides, it has contributed to bridge the

pressure gap between the model catalysts investigated in UHV and those operating

under realistic conditions (high pressure exposures). In particular, Synchrotron

radiation Near Ambient Pressure Photoelectron Spectroscopy (NAP-XPS)198–200 is

especially suitable to monitor in situ the presence of the gas phase at high pressures

the evolution of oxidation states, active phase, elemental composition and catalytic

surface reactions.

These recent developments of in-situ techniques have brought about

remarkable achievements that helped better understand fundamental phenomena

that control surface catalyzed reactions.3,201,202 They also revived studies on close-

packed Pt single crystals (flat or stepped high Miller index) even though that not long

ago those systems were considered inadequate to reach a close representation of

real-world catalysts operating mechanisms and their related issues when


76

investigated in UHV conditions.181,203–207 Thus, the oxidation of Pt(111) at near

ambient O2 pressures (in the millibar range), differs from that under low oxygen

pressure (UHV). Indeed it has been shown to proceed via a PtO-like surface-oxide

phase emphasizing the effect of large chemisorbed O2 induced by the high O2

pressure coverage.205 Exposure of Pt(110) to elevated pressure of pure O2 induced

the formation of reconstructed nano-islands of PtO2-like surface oxide along with

chemisorbed oxygen. Both oxygen species were relieved by the CO gas phase.203

Moreover, the study at near ambient conditions of high Miller index Pt surfaces can

serve to link model-surface catalysts (single crystal) to supported nanoparticles that

are close to realistic catalysts.202,206,207

Thus, in order to move closer towards real-world catalysts, the study of the

chemical and structural evolution of surface catalysts under chemical reaction

condition on Pt-based nanoparticles, represents an appropriate approach in this

regard.190,208

The following described experiences are twofold. First we have studied the

oxidation of nanoparticles under pure oxygen exposure. Second we have carried out

the CO oxidation reaction as a well suited prototypal reaction behavior before

moving to the ones occurring in a real fuel cell reaction. Core level photoemission

measurements were performed using the synchrotron NAP-XPS in isobar condition

as a function of step-wise annealing temperature. The size and morphology of

nanoparticles were studied through Scanning Tunneling Microscopy (STM)

experiments.

The nanoparticle were fabricated by electron beam evaporation from a metal

source in UHV onto a rutile TiO2(110) single crystal substrate(length 10 mm × width

5 mm × thickness 0.5 mm) substrate. The TiO2(110) surface is prepared by cycles

(more than 20) of Ar+ sputtering (for 15 min) and annealing at 800 K (for 15 min).

The deposition was carried out while the surface was kept at room temperature. Pt is

deposited from a high purity Pt rod (diameter = 2 mm, purity = 99.5%), and Zn from

high purity Zn wires (diameter = 2 mm, purity = 99.5%) contained in a Mo crucible.

For the bimetallic nanoparticle, Pt is deposited first and Zn second with the objective

of having a Zn/Pt ratio close to one.

Chapter 3:2)Morphology study with STM

77

2) Morphology study with STM

The STM setup, comprises two connected chambers, one containing the

microscope (a variable temperature XA microscope from Omicron NanoTechnology)

and one the preparation tools, both with a base pressure better than 3×10-11 mbar.

STM imaging is carried out at room temperature. To prepare the tip, we follow the

standard procedure, i.e. the DC electrochemical etching of a W wire (of diameter

0.25 mm) in a 2M NaOH electrolyte. Then the tip is cleaned by flash heating, via

direct current annealing in the UHV system prior to its use. In the STM experiments,

the deposits are examined after deposition.

In [Figure 47(a)] we show that the pure Platinum nanoparticles (NPs) are

distributed homogenously on the support with a surface density of 7.8 x 1012 NPs

/cm2. The free TiO2 area fraction left uncovered after Pt deposition is around 65 %.

The apparent average height of those nanoparticles is 0.50 ± 0.15 nm (note that the

(001) and (111) interplanar distances in cfc Pt are 0.192 nm and 0.226 nm,

respectively). The diameter of the NPs [Figure 47(d)] is 2.3 ± 0.7 nm. While we could

not identify in the images platinum atoms dispersed on the TiO2 surface, their

presence should be considered. Assuming a cap shape for the NPs, a rough

estimate of the surface density of Pt atoms is ~4 x 1014 /cm2.

The Zn NPs as shown in [Figure 47(b)] are also distributed homogeneously on

the support. The density of Zn NPs is 3.0 x 1012 NPs/cm2. The TiO2 free area is

~84% of the surface. The average apparent height of the Zn NPs is 0.74 ± 0.14 nm

(the (0001) interplanar distance in hcp Zn is 0.247 nm). The diameter of the NPs is

2.1 ± 0.9 nm [Figure 47(e)]. Consequently the Zn NPs shape and density is similar to

that of the Pt NPs. Assuming a cap shape for the Zn NPs an estimate of the Zn atom

surface density is ~3 x 1014 /cm2, a value very close to the estimated amount of

platinum in the pure Pt NPs.

To form the ZnPt bimetallic NPs with an average stoichiometry close to 1:1,

we first deposit Pt, and then Zn in exactly the same conditions as for the pure

monometallic NPs. Pt is deposited first because it exhibits a stronger interaction with

TiO2(110) than Zn and can serve as seeds for the nucleation of Zn. The resulting

overall density of the bimetallic NPs [Figure 47(c)] is 10.8 x 1012 NPs /cm2.

Chapter 3:2)Morphology study with STM

78

Compared with the pure Pt NPs, we observed an increase in NPs density by 38%.

This increase is relatively moderate, as it is much less than an expected double

value if pure Zn NPs stay apart from pure Pt NPs. Therefore the STM images show

that Zn atoms are attracted by the pre-existing Pt NPs and forms bimetallic NPs.

Naturally some pure Zn NPs may also appear on the surface and some pure Pt NPs

might still exist. Compared with pure platinum NPs the apparent average height of

ZnPt NPs has increased to 0.7±0.17 nm. Although heights do not only depend on the

physical shape but also on the electronic density, it may be a further indication that

Zn is added to the Pt mounds. The average diameter [Figure 47(f)] of the NPs

measured after Zn deposition is 2.6 ± 1.1 nm. The distribution width is broader than

that of the pure Pt NPs but similar to that of Zn NPs.

Figure 47: 3D empty states STM of pure pt(a), pure Zn(b), PtZn (c) and their histogram distribution of the nanoparticle diameter (d,e,f) respectively

d)

e)

f)

Chapter 3:3)Pure oxidation on pure Pt and PtZn NPs

79

With those STM experiment we are confident that we can produce bimetallic

NPs and observe their evolution in situ with the NAP XPS.

3) Pure oxidation on pure Pt and PtZn NPs

One of the main issues with studying catalyst under synchrotron radiation is

the beam damage. Beam damage can alterate the nature of the catalysts such as

molecular dissociation of oxidation. The beam damage is also more important when

experiments are carried out beyond the ionization value of the oxygen 1S orbital.

Thus in order to minimize those damages we have changed the spotting region each

time we changed the excitation energy of the beam photon. Moreover for excitation

energy higher than the ionization potential of the O 1s orbital, we have detuned the

undulator in order to reduce the photon flux.

3.a) Metallic Peak

The oxidation of Pt and ZnPt samples under 1 mbar of pure oxygen was

monitored via in situ NAP-XPS investigations. The evolution of Pt 4f spectra of pure

Pt and bimetallic PtZn NPs, which gives the information on the oxidation state of

platinum, (the catalyst) was followed in UHV and under an oxygen pressure of 1

mbar as a function of the temperature from RT up to 475 K. In order to monitor the

evolution of oxidation states and elemental composition on the surface of NPs and

inside the bulk, two photon energies, 400 eV and 165 eV, were used. Thus, at a low

photon energy of 165 eV, more sensitive to the topmost surface layer of the NPs, the

IMFP of photoelectrons is about 0.42 nm for the Pt 4f core level and 0.45 nm for Zn

3p,72 which compared with the size of our NPs (between 2 and 2.5 nm) doesn’t

allows us to completely probe of NPs giving mostly surface information. In contrary,

photons with a high energy of 400 eV, the IMFP will be about 0.64 and 0.78nm for Pt

4f and Zn 3p core level peak respectively.72 As 75% of the signal comes within 3λ, at

this higher excitation energy we will be able to probe completely our NPs.

3.a.1) On pure Pt Nanoparticle


80

The investigation of the oxidation of pure Pt NPs on reduced TiO2(110)

through the Pt 4f core-level under an oxygen pressure at 1 mbar of O2 and at

different temperature are shown in [Figure 48]. Prior to the exposure, we have first

measured in UHV at room temperature the Pt 4f core-level region of the Pt NPs as

deposited.

Figure 48: Core level Pt 4f of pure Pt NPs on TiO2(110)-(1x1) under 1 mbar of O2 gas at different temperature taken at hv=400 eV (a) and 165 eV(b)


81

The XPS measurements in UHV conditions of the Pt 4f region shows one

single peak at a binding energy (BE) of 71.3 eV [Figure 48] and its spin orbit

component at a binding energy of 74.6 eV. Those peaks were fitted with a Doniach-

Sunjic function for the metallic component and the separation between the two spin

orbits was always fixed with a distance of 3.3eV and the intensity ratio between the

two spin orbits is 3/4. This value of the Pt 4f7/2 peak, indicates that the Pt NPs as

prepared are in a metallic state (Pt0) although the BEs commonly reported for the

bulk metallic Pt are found at 71.0 eV - 71.2 eV.203,206,209,210

The slightly higher BE (+ 0.1 eV - + 0.3 eV) measured in UHV for these small

Pt NPs may come from both an initial and a final state effect.211 First, when metallic

nanoclusters are deposited on an n-type semiconducting clean surface, such as in

our reduced TiO2(110), a shift in the BE may occur due to an upward band bending

effect.212,213 Indeed on clean TiO2, the surface is filled by electrons coming from the

bulk inducing a negative charge at the surface which creates an upward band

bending. This original band bending is further aggravated by the deposition of Pt as it

increases the negative charge at the surface.212,213

Figure 49: (a) Band-bending diagram of TiO2(110) surface and (b) core level spectra of Ti 2p at three states different states: clean (reduced), after Pt deposition and after subsequent exposure to 1 mbar of O2


82

Moreover this band bending can be further aggravated with the introduction of

oxygen. Indeed, as shown in the above STM section, there are still TiO2(110)

surface free of nanoparticles after Pt deposition which contains oxygen vacancies.

Since the surface was not fully covered, oxygen vacancies were still present at the

TiO2 surface, so that the interactions with O2 may occur leading to their suppression.

Thus, the absence of O2 vacancies together with the increase of the O2 molecular

adsorption will led to an increase of the band bending.214 Those successive

evolutions of the band bending are schematized in [Figure 49(a)]. A better way to

follow that band bending evolution is to follow the Ti 2p core level peak [Figure 49(b)].

Indeed we observed in those spectra the diminution of the Ti3+ state and a shift of

the Ti4+ (main peak) of 0.2 eV to lower binding energy when Pt is deposited The

filling of Ti3+ is completed with the introduction of O2 leading to the suppression of the

oxygen vacancies and another shift of 0.1 eV of the Ti4+ peak to lower binding

energy is observed.

In contrast to the Ti 2p peak, the Pt 4f7/2 metallic peak has a core level shift of

0.7 eV towards higher binding energy (opposite direction of Ti 2p) with the

introduction of the O2 gas [Figure 48]. This shift is not linked to the TiO2 band

bending change due to the O2 introduction, this shift is related to the chemical

modification of the nanoparticle. This is illustrated by the apparition of an additional

component at 72 eV with a core level shift of + 0.7 eV with respect to the BE of Pt

4f7/2 metallic peak. This new component was fitted with a Voight function; the

distance between the two spin orbits was fixed at 3.3 eV and used an intensity ratio

of ¾.

This additional peak is attributed to Pt surface oxide which formed due to the

presence of chemisorbed O2. The presence of this additional component at a higher

binding energy is consistent with results reported in the literature on the Pt surface-

oxide small clusters on stepped single crystals206 and on Pt atoms bound to

chemisorbed O2.203 This spontaneous formation of Pt surface oxide on the NPs at

room temperature (RT) after exposure to 1 mbar of O2 can be attributed to their

small size (average apparent height 5 Å on STM measurement). Coordinately

unsaturated Pt atoms are highly reactive and inherently present on the surface of

these small NPs. They can be readily oxidized by O2 at high dosing pressure even at

RT. The coexistence of both a Pt surface oxide component and a metallic Pt one


83

suggests the possible formation of a thin oxide shell around the metallic core of the

Pt NPs. However, there is no other peak at higher binding energies for our Pt NPs.

This implies that the presence of bulk Pt oxides can be excluded since the

corresponding Pt 4f7/2 BEs were usually reported at CLS shift of + 1.3 eV with

respect to the metallic component and typically above 72.8 eV in binding

energy.203,209,215,216 The large cohesive energy in 5 d transition metals (5.84 eV for

Pt)217 hinders the formation of bulk oxide but it does not impact the formation of

surface oxide. With an increasing temperature up to 475 K under O2 and under a

pressure of 1 mbar, no noticeable change was seen in the BE of the Pt surface oxide

component located at 72.0 eV.

As the temperature increases during the experiment we observed a drastic

attenuation of the Pt 4f spectra at 475 K [Figure 48(a)]. It is certain that a desorption

of platinum at this rather low annealing temperature can be ruled out due to the high

melting temperature of platinum and the fact that thin oxide films formed on their

metal surface analogues are thermodynamically very stable.218 Indeed, Pt oxide

clusters on the surface has been shown to be stable up to 1250 K.216 Therefore, the

strong decrease in the Pt signal, which was seen to amplify rapidly above 400 K

under O2, has to be attributed to the encapsulation of Pt NPs by TiOx (0≲x≲

2compounds. Although, that phenomenon occurs at a much higher temperatures

(above 670 K) in UHV conditions, it takes place at much lower temperatures under

high O2 atmosphere due to the reaction of the O2 with the reduced TiO2(110)

surface.214 The encapsulation effect is very consistent with the fact that at 475 K, the

Pt signal was still visible with the measurements performed at photon energy ℎ =

400 eV [Figure 48(a)] while it was almost negligible for those acquired at ℎ = 165 eV

[Figure 48(b)]. Indeed, photon energy of 400 eV allows a deeper probing than at 165

eV. Platinum is buried in the TiO2 and only an excitation energy of 400 eV can reach

the buried element. The photon energy at 165 eV corresponds to a photoelectron

kinetic energy of just ~ 93.7 eV and it only allows the access to the topmost surface

which is depleted by Pt with an inelastic mean free path of 4.15 Å.72

3.a.2) On ZnPt nanoparticle

The results on ZnPt NPs exhibit important differences compared to those

obtained on pure Pt NPs. Indeed, the core level spectrum measured in UHV


84

conditions on the Pt 4f region [Figure 50] was convoluted with two components

corresponding to BEs at 71.6 eV and 72.2 eV, indicating a major effect associated

with Zn contribution. The first component at 71.6 eV (blue shaded) is attributed to the

metallic Pt 4f7/2 contribution. The second component located at 72.2 eV (green

shaded) which was not seen in pure Pt results is associated with Pt 4f7/2 in the Pt-Zn

alloy phase.219


85

Figure 50: Core level Pt 4f of PtZn NPs on TiO2(110)-(1x1) under 1 mbar of O2 gas at different temperature taken at hv=400 eV (a) and 165 eV(b)

The coexistence of these two contributions indicates the formation of a Pt-Zn

interface in the PtZn NPs. The extra CLS of + 0.3 eV observed in the Pt 4f7/2 for the

metallic component in PtZn NPs with respect to the Pt 4f7/2 measured for pure Pt

NPs, indicates the presence of an additional band bending effect as a result of Zinc

deposition. We have seen from the STM image that there is a significant increase in

the density of NPs after the deposition of Zn on the Pt NPs /TiO2(110) surface. Thus,

more metallic NPs were in contact with the reduced TiO2(110) surface leading to an

extra bend bending compared with the case of pure Pt NPs. This effect is consistent

with the absence of a core level shift after the exposure to 1 mbar of O2 [Figure 50].

Indeed, the presence of Zn resulted in less exposed reduced TiO2(110)

surface that can interact with O2. However, the band bending is completely

suppressed with the thermal treatment since at 475 K a CLS of – 0.4 eV was

measured as the metallic Pt 4f7/2 was stable at 71.2 eV. The presence of a third

component at 72.6 eV, with a CLS of + 1.2 eV with respect to the metallic one, after

exposure to O2 is attributed to Pt surface oxide. The large CLS of + 1.2 eV measured

for this component in PtZn with respect to the value (+ 0.7 eV) seen in the case of

pure Pt NPs, is associated with a different bounding environment for Pt atoms. This

is due to the presence of the neighboring ZnO interface. The Pt-Zn alloy and the Pt

surface oxide components were present in the Pt 4f spectra up to 440 K.

While there is no detectable Pt 4f signal on pure Pt NPs samples at 440 K

when measured with 165 eV photon energy, it is still present in PtZn NPs sample in

the same conditions [Figure 50(b)]. Therefore, the presence of Zn contributes to the

stabilization of Pt and help at reducing the encapsulation effect by TiOx species.


86

Figure 51: Evolution of the platinum metallicity of both Pt and PtZn NPs through the experiment

In [Figure 51] we show the evolution of Platinum metallic component, as

measured from the integral intensity of the photoemission core level spectra shown

in [Figure 48] and [Figure 50], with respect to the Pt surface oxide contribution. While

the ratio (Ptmet /(Ptmet + PtOx)) evolution shows almost the same values for both

400 eV and 165 eV in the case of pure Pt NPs and gave the same proportion of

oxide and metallic part at both energy, it exhibits substantial differences in the case

of PtZn NPs. Indeed in the case of ZnPt nanoparticle, the bulk sensitive condition

(ℎ = 400 eV) shows a higher degree of metallicity in the nanoparticle than in surface

sensitive condition (ℎ =165 eV). This can be explained by the reduced platinum

surfaces exposed to oxygen due to the presence of a Zn layer on top of it. In addition

the Zinc stronger affinity to O2 makes it more susceptible to oxidize before platinum

and thus acts as a sacrificial element.

Those XPS measurement in the metallic peak have shown the beneficial

effect of adding zinc to platinum: it limits the platinum oxidation and reduces the

platinum susceptibility to encapsulation.

3.b) C 1s and O 1s Peak

We will now discuss the C 1s and O 1s spectra for both samples. In pure

oxidation conditions, it is the O 1s that will provide most valuable information on the

1.0

0.8

0.6

0.4

0.2

0.0

Pt m

et/ (

Pt m

et+

PtO

x)

450400350300

Temperature (K)

293

1mBar O2 UHV

Pt NPs: 400 eV Pt NPs: 165 eV PtZn NPs: 400 eV PtZn NPs: 165 eV


87

oxidation states of the surfaces. The C 1s will provide mainly the information on the

residual carbon in the UHV chamber.

3.b.1) On pure Pt Nanoparticles

First we will discuss the C 1s spectra [Figure 52] recorded for the pure Pt NPs

sample. In this spectrum, we observe in the UHV condition the presence of an initial

carbon contamination. It is composed of two peaks, a first one located at a binding of

284.8 eV (attributed to C-C liaison) and a second one at a higher binding energy of

286.6 eV.220 This second component is ascribed to C-O-C liaison.

The introduction of the O2 gas leads to an Initial shift of the carbon

contamination and its oxidization. In addition, new peaks situated at 289 eV and at

290 eV have appeared (noted Carb*). This binding energy region corresponds to the

area where carbonate and carboxyl species were commonly reported.220 Moreover

we observe a core level shift of 0.3 eV at higher energy for both the C-C and C-O-C

component. This shift may be linked to the band bending as they follow the trend

observed with the Pt 4f peaks.

Figure 52: C 1s core level spectra under 1 mbar of O2 at different temperature for pure Pt NPs

XP

S Inte

nsity (

arb

. units)

292 290 288 286 284 282

Binding Energy (eV)

UHV293 K

293 K

370 K

475 K

C 1s

h = 400 eV

O2: 1mbarCarb*

C(gr)


88

With the heating treatment we observe the gradual disappearance of the

carbon species due to desorption. A binding energy downward shift of the peaks

which follows the same trend as Pt 4f is also observed.

At high temperature (475 K) all carbon and platinum signals have disappeared.

The Pt 4f vanishing is due to the encapsulation by TiOx species. Indeed at this

temperature in surface sensitive condition (ℎ =165) there no more signal from the

Platinum. The platinum seems to be the adsorption site of the carbon species.

We will now consider the O1s spectra [Figure 53]

Figure 53: O 1s core level spectra under 1 mbar of O2 at different temperature for pure Pt NPs

Here, in UHV condition we just observe a peak at 530.25 eV corresponding to

the the oxygen coming from the substrate: TiO2.221,222 When O2 is introduced, we

see the apparition of the appearance of the two characteristic peaks of the gas

phase (O2(g)) at a binding energy comprised between 540 and 538 eV, a carbonate

component at 532.3 eV and a shift of the O 1s in the TiO2 associated to the evolution

of the band bending as discussed earlier. The O2(g) signal is made of two

XP

S Inte

nsity (

arb

. units)

542 540 538 536 534 532 530 528

Binding Energy (eV)

O 1s

h = 635 eV

293 K

293 K

370 K

475 K

UHV

O2: 1mbarO2(g)


89

components. The main one at lower binding energy is a quartet state (4). The

smaller one shifted by ~1 eV to higher binding energy is a doublet state (2).223

The O2(g) signal does not exhibit clear vibrational progressions, due to vibrational

spacing of only ~0.18 eV (4) and ~0.13 eV (2). The leading component of the 4

peak is the adiabatic peak.223

With the increased temperature we see a gradual diminution of the

carbonates up to their almost total desorption at 475K. We also see a shift of the

oxygen gas phase that is linked to the variation of the sample’s surface work function.

Indeed with the desorption of adsorbed species, its value has certainly changed

leading to a shift of the gas phase.

3.b.2) On PtZn Nanoparticles

We will now discuss the evolution seen in the ZnPt NPs sample starting with

the C1s spectra [Figure 54].

Figure 54: C 1s core level spectra under 1 mbar of O2 gas at different temperature for ZnPt NPs

XP

S Inte

nsity (

arb

. units)

292 290 288 286 284 282

Binding Energy (eV)

C 1s

h = 400 eV

293 K

293 K

370 K

400 K

440 K

475 K

O2: 1mbar

UHVCarb*

C(gr)


90

First in UHV condition we observe the presence of an initial carbon

contamination with an aliphatic and a C-O-C component (grey shaded). Those are

also observed in the case of pure Pt Nps. Yet here there is also a few adsorbed

carbonate species (orange component) present. In this case considering that we

have zinc which stabilizes carbonate; this component is more susceptible to be some

carbonate rather than carboxyl even though the latter one can be an intermediate

leading to carbonate with a short lifetime.

When the O2 gas is introduced, there is a strong increase of the carbonate

signal and a decrease of the carbon contamination. With increasing temperature we

observe a desorption of all species but compared with the pure Pt case here we still

see the presence of the carbonate species at the highest temperature (475 K). This

is due to the presence of zinc in the bimetallic NPs. The carbonate species

associated to zinc will further discussed in the next part in presence of CO gas.

Figure 55: O 1s core level spectra under 1 mbar of O2 gas at different temperature for ZnPt NPs

XP

S I

nte

nsity (

arb

. u

nits)

542 540 538 536 534 532 530 528

Binding Energy (eV)

293 K

293 K

370 K

400 K

440 K

475 K

UHV

O2: 1mbar

O 1s

h = 635 eV

O2(g)

O(TiO2)

Carb*

OH

Chapter 3:4)CO Oxidation reaction on Pt, Zn and PtZn NPs with CO:O2 at ¼ ratio

91

For the O 1s spectra [Figure 55] in UHV condition there is the presence of the

oxygen from the substrate TiO2 but we also observe a shoulder at higher energy

(cyan component). This shoulder is linked to OH species bonded with titanium from

dissociation of residual water molecules.

When O2 is introduced, we see the appearance of the gas phase (O2(g)) with a

binding energy comprised between 540 and 538 eV, a carbonate component at 532

eV and we observe a shift of the O 1s in then TiO2 component linked to the band

bending. With increasing temperature we see the diminution of the carbonate

component due to partial desorption but it is still present even at 475 K. This

confirms the observation mentioned in the C 1s spectra.

As with the pure Pt NPs there is a shift of the gas phase due to the variation of

the sample’s surface work function resulting from the desorption of species from the

surface.

4) CO Oxidation reaction on Pt, Zn and PtZn NPs with CO:O2 at ¼ ratio

The CO oxidation reaction (2CO + O2 2CO2) on pure Pt, pure Zn and ZnPt

NPs samples was monitored in situ through the evolution of both C 1s and O 1s

core-level spectra. The C 1s and O 1s core level spectra give information on CO, O2

and produced CO2 in the gas phase. In addition, they provid information on adsorbed

species such as CO and carbonates. The total pressure of the CO:O2 mixture (partial

pressure ratio of 1:4) was set to 1 mbar. The system was then heated with a button

heater from room temperature (293 K) to 450 K in a step-wise manner. The oxidation

state of the NPs were monitored through the Pt 4f core level peak for NPs having

platinum and through the Zn 3p core level for NPs containing Zinc.

4.a) C 1s and O 1s spectrum

Let us first consider the C 1s [Figure 56] core-level spectra. Those spectra are

recorded at a photon energy ℎ = 400 eV for the three different NPs (pure Pt

nanoparticles (a), pure Zn nanoparticles (b) and bimetallic ZnPt nanoparticles (c)) as

a function of the annealing temperature. The UHV spectrum was recorded at room

temperature (293 K) on the samples as prepared before introducing gases. All the

other spectra were recorded in isobaric conditions under a pressure of 1 mbar with a


92

CO:O2 ratio of 1:4 (oxygen rich mixture). The component noted CO(g) is associated

with the CO in gas phase. COad corresponds to adsorbed CO species. CO2(g) is the

CO2 produced by the reaction in gas phase. Carb* corresponds to diverse

carbonates species adsorbed on the sample surface. In the C 1s spectrum [Figure

56], C(gr) is ascribed to the graphitic carbon coming from the initial carbon

contamination at 284.6 eV.224 This contamination is observed for all three samples

and will be strongly reduced with the increased temperature and oxidative conditions.

Ultimately it will be eliminated at the higher temperature for the Pt and Znpt sample

while a small amount will still be observable for Zn NPs. Three other components are

seen at higher binding energy (285.9 eV, 286.7 eV and 287.3 eV). The component at

286.7 can be attributed to a COC contamination.220 The cyan structure at the highest

binding energy (left) is the K 2p3/2 component of a potassium impurity commonly

seen in TiO2(110).225,226 The K 2p3/2 should be visible at a binding energy of 296 eV

not visible due to a short energy window. The possibility of it being a carbon based

component is to be excluded due to the fact that this peak stays stable thought all

the experiment for fall three samples while all the other carbon based component

change. Thus it can be used as a yardstick to compare signal intensities. The

binding energies (measured from the substrate Fermi level) of the gaseous species

CO and O2, (noted CO(g) and O2(g), respectively), and of the reaction product CO2

(noted CO2(g)) are found at significantly higher binding energy values than the

adsorbed surface species like adsorbed CO (noted CO(ad)) or carbonates (noted

Carb*). Indeed, in the C 1s spectra the peaks above 290 eV are related to the gas

phase, for the O 1s spectra it’s the peak above 534 eV which are related to the gas

phase.3,227–229


93

Figure 56: C 1s core level spectra under 1 mbar of CO-O2 gas mixture (1:4 ratio) at different temperature for pure Pt NPs (a), Pure Zn NPs (b) and ZnPt NPs (c)

This ability to distinguish the gas phase and solid phases makes the NAP-

XPS particularly suited for the monitoring of this reaction product. For example, it

allows an immediate identification of the activation temperature of CO oxidation.1,3

The CO(g) core-level spectrum is visible at a binding energy of about 291 eV.229 The

produced CO2(g), appears in the C 1s spectra [Figure 56] at a higher energy than

CO(g) in the energy range 292.1 eV to 292.7 eV.227–229 The overall energy resolution

(~200 meV) available during the experiment is sufficient to make the vibrational fine

structure of the CO(g) C 1s peak observable[Figure 57]. Indeed the peak was fitted

with three components equally separated by ~300 meV and with a full half width

maximum of 0.3 eV in accordance with the literature.230


94

Figure 57: CO gas phase (CO(g)) in C 1s spectrum measured at hv=400 eV fitted with a vibrational progression

In the O 1s spectra [Figure 58] recorded at ℎ =635 eV under the same

condition as the C 1s spectra, the CO2 gas phase appears as an unstructured peak

in the 535 eV - 536 eV range at lower binding energy than both O2(g) and CO(g). The

O2(g) peak is found in the 537 eV to 539 eV binding energy range, and the CO(g) peak

is visible as a small shoulder to the right of the O2(g) peak. It should be noted that the

gas phase binding energies shift with temperature, phase binding energies depend

on the work function of the sample surface.2,36,231


95

Figure 58: O 1s core level spectra under 1 mbar of CO-O2 gas mixture (1:4 ratio) at different temperature for pure Pt NPs (a), Pure Zn NPs (b) and ZnPt NPs (c)

The onset of CO oxidation reaction is characterized by the appearance of the

CO2(g) peak in the C 1s and O 1s spectrum windows. For the pure Pt NPs and ZnPt

NPs samples the product CO2(g) is detected at ~380 K, with the reactants (CO(g) and

O2(g)) still visible. The pure Zn NPs sample also demonstrates the ability to activate

the CO oxidation reaction. Yet on this sample the onset of CO2(g) production is seen

at higher temperature of 396 K. In all samples, the activation of CO oxidation

coincides with a significant decrease in the intensities of CO(g) peak. At the next

heating step, the CO(g) peak is no longer detected (better seen in the C 1s window, in

[Figure 56]). The temperature at which the CO(g) signal disappears is indicative of the

renowned mass transport phenomena in the vicinity of the reactive surface. This

illustrates also the high catalytic activity of these samples, i.e; high concentration of


96

the produced CO2 gas phase. The temperature of the mass transport limit was

detected at 396 K for both the pure Pt and ZnPt NPs samples, and at 450 K for the

pure Zn NPs sample. Owing to this high catalytic activity CO molecule arriving “in the

vicinity of the surface” are quickly transformed into CO2, depleting the gas layers

close to the active surface from CO molecules. This phenomenon of mass transport

limitation was also observed by Blomberg and coworkers229 in their NAP-XPS study

of CO oxidation on a single crystal of Pd(100).

The CO oxidation activity was monitored via the monitoring of the CO2(g)

signal evolution as a function of the temperature. Thus, since the O2(g) peak is

always present throughout the experiment in the O 1s spectrum [Figure 58] and a

signal of CO2(g) is also visible when the CO oxidation starts, we can plot the ratio

ICO2(g)

IO2(g)

against the heating temperatures [Figure 59], with ICO2(g)and IO2(g)

respectively

the intensities of the CO2(g) and O2(g) peak in the O 1s spectra window. It shows, that

the CO2(g) yield of the pure Pt NPs and ZnPt samples are very similar (reaction onset

at 380 K, maximum CO2 yield at 396 K). In contrast, the pure Zn NPs sample is less

efficient than the surface with Pt-containing catalysts at all temperatures, up to 450 K.

Figure 59: Evolution of CO2/O2 gas phase ratio as a function of temperature

Now that we have described the gas phase spectra and the temperature

conditions that lead to CO oxidation, we can turn our attention to the correlative

changes in the “solid-state” core-levels.


97

4.b) Metallic peak

First we shall consider the pure Pt and pure Zn NPs cases. Then we shall

discuss the behavior of the bimetallic NPs. Indeed this more complex bimetallic

system will be discussed against the information gathered on the “pure” NPs

systems.

4.b.1) Pure Pt NPs

The Pt 4f spectra [Figure 60(a)] were measured at ℎ =400 eV.

Figure 60: Pt 4f core level spectrum (a) and the C 1s core level spectrum (b) recorded at hv=400 eV for the pure Pt NPs

In the UHV conditions at room temperature (prior to exposure to the gas

mixture) the peak is metallic (Ptmet) similar to the one seen in the pure oxidation part.

However when the CO:O2 mixture is introduced at room temperature (293 K), a new

component appears at a higher binding energy (+0.9 eV, orange-shaded) with


98

respect to the metallic peak (Ptmet, violet shaded). We attribute this new component

to Pt-CO bonds. This corresponds to the adsorption of CO on the Pt NPs, as

indicated by the strong CO(ad) signal in the C 1s spectrum [Figure 60(b)].

Indeed, as shown in Ref. 232, In their Pt 4f spectra of the Pt(111) surface the

CO(ad) induces a sizeable shift (to higher binding energy of the Pt 4f) with respect to

the bulk component (~+1 eV for the atop bonding, ~+0.3 eV for the bridge). The

calculated chemisorption energies of CO on atop sites are in the 1.51-1.54 eV range

for the Pt(111) surface.233 This is a considerably greater energy than on perfect (110)

rutile TiO2 which is in the range of 0.2-0.36 eV range234. Moreover, a possible

trapping effect of Ti3+ present on the TiO2 surface defects on CO should be negligible

due their quenching after Pt deposition [Figure 61].235

Figure 61: Ti 2p core level spectra of pure Pt NPs sample recorded at hv=560 eV

Naturally, here our Pt NPs present various facets, as suggested by the 3D

STM images [Figure 47(a)]. CO C 1s XPS data are available on Pt(111) single

crystals.236,237 In those data, the CO molecule occupies sequentially the atop and the

bridge sites. The C 1s binding energies of CO in atop and bridge sites are 286.7 and


99

286.0 eV, respectively.238 Assuming that (111) Pt facets dominate, then the

introduction of the gas mixture leads to the preferential occupation of atop positions

by CO, with minority occupation of the bridge sites. Yet other sites must be

envisaged due to the nanometer size of our Pt NPs, CO could adsorb on steps

where under-coordinated Pt atoms are present. With this consideration the

component at 287.6 eV was attributed to CO bonded to the under-coordinated Pt

atoms. For their part, the O 1s binding energies of CO of bridge, atop and under-

coordinated Pt sites are reported at 531.1, 532.7 and 533.1 eV, respectively.202 Yet

they are overshadowed by the intense TiO2 component and surface carbonates as

these components fall in within the same binding energy range, see [Figure 58(a)].

At 380 K, the CO(ad) signal in the Pt 4f spectrum tends to decrease, but the C

1s binding energies (and relative intensities) of the two main CO(ad) components

(attributed to atop sites and under-coordinated Pt sites) do not change. The spectral

weight of Ptmet also increases in correlation with the decrease of the CO(ad) signal.

However when the CO oxidation reaches its maximum [Figure 59] at 396 K, the

CO(ad) and CO(g) signal vanishes completely [Figure 60(b)]. As the reaction between

CO and O2 takes place on the surface, the CO has a very short residence time on

the surface as it is immediately oxidized into CO2(g). The CO(ad) signal of adsorbed

CO on platinum disappears from the O 1s window [Figure 58 (a)] as atomic

oxygen.232

Both C 1s [Figure 60(b)] and O 1s [[Figure 58](a)] spectra also show the

surprising presence of carbonate species on the pure Pt NP sample when the gas

mixture is introduced at room temperature. The main carbonate component is found

at a binding energy of ~289 eV.227,228 The carbonate signal diminishes when

temperature increases, and almost completely vanishes at 380 K while a sizeable

CO(ad) signal remains. The two phenomena are certainly uncorrelated. Indeed we

see here that with platinum there is a strong CO adsorption but weak carbonate

while in the case of Zn NPs (more in next part) we have a strong carbonate

adsorption but without a CO adsorption. The formation of carbonates could follow

this reaction: PtIVO2+COPtCO3. However, the PtIV (PtO2) component which is

expected to have a binding energy shift of +3.7 eV from the main metallic Pt 4f, was

not detected in our samples. Thus, another hypothesis must be envisaged. Since Pt


100

deposition is performed at room temperature, dispersed Pt atoms could be present

at the surface. Indeed, in the similar case of Pt NP deposited on Al2O3, Newton et

al.239 favor the hypothesis of a reaction of CO over atomically dispersed oxidic Pt

centers adsorbed on the alumina substrate. Here, the interaction between Pt atoms

and TiO2 is so strong that their surface mobility is limited.

4.b.1) Pure Zn NPs

In sharp contrast with the pure Pt NPs sample, the deposition of zinc at room

temperature in UHV conditions does not quench the Ti3+ state (see the arrow in

[Figure 62]). This means that Zn has no particular affinity for the surface defects

(oxygen vacancies) associated to Ti3+, contrary to Pt.

Figure 62: Ti 2p core level spectra of pure Zn NPs recorded at hv=560 eV

In UHV conditions, the C 1s spectrum [Figure 63] shows components due to

surface contamination at 284 eV-285 eV. It should be noted that we do not see the

same higher energy component as seen in the case of pure Pt NPs. Indeed those

were adsorbed residual CO on platinum site which are absent on this sample. The

small peak at ~289 eV is due to carbonates resulting from the adsorption of CO from


101

the residual and its oxidation on the surface. Its counterpart in the O 1s spectrum

[Figure 58(b)] (about +1.8 eV from the main O 1s in TiO2 peak) is also very weak

under UHV.

Figure 63: C 1s core level spectra of pure Zn NPs recorded at hv=400 eV

At room temperature, the presence of O2 in the CO:O2 gas mixture leads to

the filling of the oxygen vacancies, thus the Ti3+ component disappears in the Ti 2p

spectrum [Figure 62]. While CO is not adsorbed on metallic zinc,240 O2 reacts with

metallic Zn. It has been reported that Zn reacted readily with oxygenalready under

10-7 mbar at room temperature.241 Here, we do find evidence of oxidation in our Zn

3p spectra measured at photon energies of 165 and 400 eV through the widening of

the FHWM. In [Figure 64], we plot the corresponding changes in the binding energy

(Zn 3p3/2).

Therefore, following the introduction of the gas mixture at room temperature,

the Zn 3p binding energy increases by 0.5-0.6 eV. This is the characteristic binding


102

energy shift from metallic Zn to ZnO.242 Moreover the increase of the FWHM of 0.4

eV is another indication of the zinc oxidation. In metallic Zn (ZnO oxide) the inelastic

mean free path λ is 0.447 nm (0.483 nm) at ℎ =165 eV and 0.775 nm (0.868 nm) at

ℎ =400 eV, to be compared with the average height 0.74 nm 0.14 nm of the

metallic Zn NPs. This indicates that at 400 eV the zinc oxidation states of zinc on

whole NPs were seen while at 165 eV it was mostly the surface contribution that was

monitored. Importantly, we find no signal related to CO(ad) in [Figure 63]. This is not

much of a surprise as DFT calculations show that CO interacts weakly with the ZnO

surfaces.243

The broad C 1s peak (between 288.5 and 289.0 eV) ascribed to carbonates

grows substantially at room temperature. It is made of two components separated by

~1 eV. While the component at 288.5 eV in binding energy is the most intense at low

temperatures, the one at higher binding (289 eV) becomes preponderant at 450 K. It

suggests that there exist two different bonding configurations for CO2 on oxidized

zinc, with different chemisorption energies. C. Wöll has given a review244 of the

chemistry and physics of zinc oxide surfaces which gives the binding energies of CO

and CO2 adsorbed on O-ZnO and ZnO(1010) leading to “CO3” surface compounds.

Moreover, in other literature work, there are indications that CO can be oxidized on

the ZnO surfaces. Indeed four decades ago, Esser and coworkers245 have observed

the thermal desorption of CO and CO2 after CO exposure to stoichiometric ZnO

powders at 300 K. They also noted that the coadsorption of O2 on ZnO increased the

production of CO2. More recently, infra-red spectroscopy246 demonstrated that there

are carbonates irreversibly adsorbed on the surface of finely divided oxidized ZnO

powders at 300 K. The formation of those carbonates was also enhanced by the

addition of O2.

In [Figure 59] we show that the CO2(g) yield increases from 396 K to 450 K. At

this high temperature, the CO(g) signal [Figure 63] disappears (we have reached the

mass transport limitation regime). In this pure Zn NPs sample we reach the mass

transport regime at a higher temperature (450 K) compared with that of pure Pt NPs

(396 K) as discussed previously. Moreover contrary to the pure Pt NPs sample, the

carbonate signal is still present for pure Zn NPs, which appear as a single peak at

~290 eV in the C 1s spectrum and as a shoulder at +1.8 eV to the left of the main


103

peak in the O 1s spectrum [Figure 58(b)]. The fact that the carbonate signal in the C

1s spectrum [Figure 63] does not vanish when CO oxidation conditions are met

suggests that CO2 is strongly bonded to the oxidized zinc compound. This

hypothesis is supported by other works where, a high resolution electron energy loss

spectroscopy (HREELS) study,247 supported by DFT calculations, did show that a

tridentate CO3δ− species is formed on the Zn (1010) face (the most stable and non-

polar surface face) of ZnO). Moreover, recent DFT calculations248 indicate that the

chemisorption energy for CO2 on the same face (ZnO(1010)) is significant ( 0.83 eV),

but is small on the polar faces (in the 0.06-0.09 eV range).

The in-situ NAP-XPS study shows that CO is oxidized into CO2 even at room

temperature on the oxidized zinc surface when exposed to the CO:O2 gas mixture

and is then readily bonded as a carbonate. The evolution into CO2(g) is only possible

when the temperature is raised and the energy given to the system is sufficient to

desorb bonded CO2. This mechanism is believed to proceed when a steady-state

regime is reached: CO2 molecules were continuously formed on the ZnO and desorb

afterward. Our oxidized Zn NPs behave in a way very similar to prismatic ZnO(1010)

surface (the major orientation found in ZnO powders). Indeed, Esser et al.245 found

that the stationary regime of CO oxidation starts at 400 K and is maximum at ~530 K

in a CO:O2 (1:1) mixture, under a pressure of 1.610-5 mbar. The type of reaction

mechanism is possibly a the Mars–van Krevelen one.249 In this mechanism, a

surface oxygen atom oxidizes the CO molecule, leaving an oxygen vacancy at the

surface, which is then replenished by reaction with an O2 molecule from the gas

phase. However, a detailed mechanistic description is still missing. The only

available parameter is the calculated oxygen vacancy formation energy, often used

as a descriptor of the catalytic activity, which is 3.3 eV for the ZnO(1010) face.250

Similarly, the CO2 strongly bonded carbonate on ZnO is not discussed in the

literature, while it is observed on similar systems, for instance in the case of CO

oxidation by Au/ZnO catalysts.251

4.b.2) ZnPt NPs

The ZnPt has substantial difference in behavior compared with the pure Zn

NPs, in particular the evolution of the binding energy of the Zn 3p in both samples as


104

shown in [Figure 64]. Indeed in this case, the binding energy does not shift

noticeably once the CO:O2 gas mixture is introduced at room temperature for PtZn

samples, while it increases clearly for pure Zn NPs.

Figure 64: Binding energy shift of Zn 3p core level peak for pure Zn NPS and ZnPt NPs

This indicates that metallic Zn is protected from any contact with the oxygen

molecules in PtZn NPs. In fact, due to its oxophilicity, if it was in contact with oxygen,

it would have immediately oxidized and would have its binding energy shift to higher

binding energy. Moreover given the absence of CO bonding to this metal, we

suspect that most of the zinc atoms must have penetrated inside the pre-existing Pt

NPs, it is the only explanation for the lack of bonding with either CO or O2 molecule.

This happened despite the fact that we have deposited Zn after Pt and that the

surface energy of Zn(0001) (0.993 J.m-2) is less than that of Pt(111) (2.489 J.m-

2).180,252 Therefore, the Zn diffusion into the core of the ZnPt NPs is a chemically

driven phenomenon that drags the platinum atoms to the surface of the bimetallic

nanoparticles. We attribute this phenomenon to the formation of Pt-CO bonds.

Indeed, in the C 1s spectrum [Figure 65(b)], the CO(ad) signal “titrates” the platinum

atoms in contact with the gas mixture and we see two CO(ad) peaks (shaded in green


105

color) at 286.3 eV and 287.2 eV at room temperature. Using the K 2p3/2 peak (in

cyan, a TiO2 contamination) as a reference, both their intensities are smaller than in

the case of the pure Pt NPs sample under the same conditions. When we look at the

pure Zn NPs under the same conditions, we remarked that the CO(ad) signal is

absent. This is a further confirmation that CO(ad) is bonded to platinum and its

evolution is identical on both pure Pt and ZnPt NPs: CO(ad) is present as soon as the

gas mixture is introduced. It was still present at the onset of the oxidation reaction

(380 K), but disappeared at 396 K together with at the vanishing of CO(g) signal at

the oxidation reaction regime.

Figure 65: Pt 4f core level spectrum (a) and the C 1s core level spectrum (b) recorded at hv=400 eV for the ZnPt NPs

The Zn 3p binding energy in ZnPt NPs was constant until both CO(g) and

CO(ad) signal disappear, then it increased suddenly, by 0.5-0.6 eV. This is the same

shift as for pure Zn NPs when the gas was introduced. This indicates that when the


106

CO, previously bonded to Pt has left the NPs surface, Zn get oxidized into ZnO. This

change corresponds to the onset of the CO oxidation reaction regime. The Pt 4f

spectra [Figure 65 (a)] provide interesting information on the chemical bonding. They

are fitted with three components: The main metallic (Ptmet) component which is

violet-shaded, a smaller green-shaded at +0.5 eV from the main peak attributed to

Zinc and an orange shaded one corresponding to CO-Pt contribution at + 1.0 eV

from the main peak.

The metallic Pt (Ptmet) spectral weight [Figure 66] also gives interesting

information on the chemical environment of the NPs. This spectral weight is obtained

by dividing the intensity of the platinum metallic peak (Ptmet) by all the component of

the Pt 4f peak (Ptmet, PtOx, Pt-CO). It gives information on the metallicity of the peak.

Indeed it shows that the Ptmet spectral weight decreases due to the adsorption of CO

in a temperature window comprised between room temperature and 380 K. At 396 K,

when the CO(ad) signal disappears and Zn is oxidized into ZnO, Ptmet starts to

increase again until it reaches 100% (Platinum is fully metallic) at 450 K. This can be

explained by Zn segregating at the surface of the NP and forming an oxidized layer

compound. This strongly suggests that in oxidative conditions, a ZnO shell (with

adsorbed carbonates at the surface) is formed over a metallic Pt core. Therefore, for

the highest temperatures of the experience, zinc plays the role we expected it to

have in ZnPt NPs: it forms a sacrificial oxide layer that protects platinum form being

oxidized.

When comparing with the spectral weight of Ptmet from the pure Pt NPs we

see that the behavior is different. Indeed, we observe that it decreases from 100% to

80% with the introduction of the gas mixture. This value is less important (less

metallic) than with the ZnPt NPs as there are more accessible Pt sites in the case of

the pure NPs to CO molecule. As the temperature increases, the CO(ad) signal

decreases and Ptmet increases. When the CO(ad) signal disappears from the C 1s

window as atomic oxygen, then Ptmet decreases again due to oxidation and reaches

its lower value, nearly 70%,a value far lower than in the case of the bimetallic NPs

indicating the beneficial effect of adding Zinc to Platinum.


107

Figure 66: Metallic Pt (Ptmet) spectral weight versus temperature obtained from the Pt 4f XPS spectra for pure Pt NPs and ZnPt NPs

Additional information on the ZnPt NPs structure (Pt/(Pt+Zn)) can be gained

with the photon energy dependence of the platinum atomic weight displayed in

[Figure 67]. The Pt atomic weight is obtained from the intensity of the Pt 4f and Zn 3p

spectral peak. Those intensities are then corrected from variations linked to the

photon energy with the atomic cross sections and the asymmetry factor.253 Those

atomic cross section and asymmetry factor describe the probability of a photon with

a given energy to ionize the atom by ejecting an electron from the considered

electronic level. The formula for correcting those intensities is the following:

Icorrected=I

σ(1+0963*β)

With

I: The measured peak intensity in XPS

Icorrected : The corrected intensity

σ: The cross section factor

β: The asymmetry factor


108

Figure 67: Pt 4f and Zn 3p binding energy window of ZnPt NPs samples measured at hv = 400 eV (a) and Pt atomic weight (Pt/(Pt+Zn))ratio (b)

The Pt/(Pt+Zn) atomic weight after a drop after the gas introduction, remains

stable then systematically decreases when the temperature increases. In particular,

the decrease above 380 K indicates the Pt surface depletion This is fully coherent

with the idea of an outer ZnO shell with carbonates adsorbed at its surface and a

metallic Pt core and are coherent with the analysis of the Pt 4f and Zn 3p core-level

binding energies [Figure 64, Figure 65, Figure 66]. This segregation phenomenon of

Zn as an oxide can easily be explained by thermochemical effects, as mentioned

previously. Indeed Pt enrichment was observed by Sode et al.254 on an

electrochemically prepared PtZn alloy after extended exposure to CO while a Zn

enrichment was observed with the same alloy in under exposure to O2. Therefore,

the phenomena observed in our samples, is a chemically-driven phase separation


109

leading to the segregation, that has be distinguished from a segregation process that

is only due to surface energy considerations.

4.b.3) Assessing the Nanoparticles activity

The surface behavior of the three studied NPs is summarized in a scheme

[Figure 68]. The behavior in the case of pure Pt NPs is in a way similar to the

crystalline surfaces of bulk platinum (or bulk palladium)229 exposed to a CO:O2

mixture. First the Pt NPs surfaces are poisoned by CO then, when the temperature

increases and reaches the critical value of 380 K where the CO oxidation reaction

starts, CO is consumed by the reaction and transformed in CO2. The, the oxygen is

able to reach the surface of NPs which leads to platinum surface oxidation. When

the mass transport limit is attained, all previous CO adsorbed sites are replaced by

atomic oxygen fully oxidizing the surface platinum. The formation of PtO leads to

TiOx encapsulation.

Figure 68: Comparison of chemical changes in pure Pt, pure Zn and ZnPt NPs


110

Compared with pure Pt NPs, the pure Zn NPs sample exhibits a very different

behavior. Indeed in this case, CO does not bond to metallic Zn: there is no adsorbed

CO. In fact, Zn is immediately oxidized into ZnO when the CO:O2 gas mixture is

introduced at room temperature. The Zinc oxide provides O species to oxidize the

CO molecules (in a Mars van Krevelen scheme), and the newly created CO2

molecule remains bounded with ZnO as a carbonate. At a critical temperature, a

steady-state regime is reached. In this regime, carbonates are continuously both

destroyed when CO2 desorb and formed again on the Zinc oxide surface following

the Mars van Krevelen scheme. This observation of an appreciable catalytic activity

for the pure Zn NPs at 450 K raises the important question of which crystallographic

faces are exposed to the gas mixture when the catalytic activity of ZnO is observed.

The observation of carbonates is only possible if non-polar faces like ZnO(1010) and

(1120) are in contact with the gas mixture. Indeed the calculated chemisorption

energies of CO2 are large (0.83 and 0.79 eV respectively).248 In contrary, on the ZnO

polar faces, the calculated chemisorption energy for CO2 is very low (<0.1 eV).248

Moreover, the absence of CO oxidation activity on a continuous film of ZnO

exhibiting only a polar surface to the gas mixture was indeed noticed by Martynova

et al..188 This phenomenon illustrates the important aspect of ZnO in the form of

nanoparticles.

The case of the ZnPt bimetallic NPs, a more complex behavior was noticed.

While Pt is deposited first and Zinc after, the bimetallic NPs are Pt-terminated at

room temperature. Thus CO can poison the NP by bonding to platinum. At a critical

temperature (380 K) when the reaction starts and CO2 is produced, Zn moves to the

external surface where it forms an oxide, while Pt goes to the core where it remains

metallic. Then in oxidative conditions where CO2 production is maximum and the

adsorbed CO disappeared, we have the NPs composed of a ZnO outer layer and a

platinum core. As in the case of pure Zn NPs due to the ZnO shell, carbonates are

observed and they likely play a role in CO oxidation. Yet the role of platinum that

remains metallic (in contrast with the pure Pt NP sample that is oxygen-covered)

remains to be fully elucidated. We observed that the onset of CO oxidation occurs at

380 K, a temperature similar to pure Pt NP and lower than that of pure Zn NP case

(396 K). Therefore, the presence of platinum is clearly beneficial if one considers the

onset of the CO oxidation reaction. Indeed the metallic platinum core can provide


111

electrons to the ZnO shell in a “Cabrera-Mott picture” facilitating the dissociation of

O2 molecules. Moreover, we may also consider the fact that Pt doping decreases the

oxygen vacancy formation energy at prismatic ZnO surfaces.250 This means that

more atomic oxygen could be provided to oxidize CO in a Mars van Krevelen

scheme.

Ideally, the Pt 4f and Zn 3p spectral intensities when normalized with respect

to the photon flux received at the sample surface and at a given photon energy,

should give crucial information on the interaction with the rutile TiO2 substrate like

wetting/dewetting, sintering and encapsulation process by the substrate. But this

process is suitable when done in UHV experiment. However, here in NAP-XPS

conditions, a sample current proportional to the photon flux cannot be measured,

due to the charged species (ions and electrons) produced in the gas phase by the

beam. Thus, we have chosen to normalize the Pt 4f and Zn 3p core-level intensities

by dividing them by that of Ti 3p, a substrate peak. For this comparison, two photon

energies are used: ℎ =165 eV (surface sensitive conditions) and ℎ =400 eV (more

bulk sensitive conditions). Those normalized intensity evolution are showed in

[Figure 69]

We start first with the Pt 4f intensity for the pure Pt NPs. In this case, the

intensity decreases when CO oxidation starts. Here, platinum becomes oxidized as a

result of CO desorption and the presence of oxidizing environment (oxygen rich). As

discussed above, the formation of Pt surface oxide enhances the encapsulation by

TiOx. At 450 K, the Pt 4f intensity is very small at ℎ =165 eV, almost non visible,

although it remains measurable at ℎ = 400 eV. The encapsulation of Pt NPs by

TiOx is a very well documented phenomenon (for more information see reference

235,255,256). It is generally attributed to the surface energy of the metallic platinum

clusters (2.489 J.m-2 for Pt(111)) being higher than that of non-stoichiometric titania

in order to minimize the surface energy.

For the pure Zn NPs case, the normalized Zn 3p intensity remains rather

constant with the increasing temperature until 396 K where its intensity strat to

increase. This increased intensity can be attributed to the destruction of the

carbonate species adsorbed at the Zinc surface. This carbonates destruction

corresponds to the apparition of CO2 in the gas-phase. This diminution of the surface


112

adsorbed species (the top layer of the systems) leaves more Zn surface visible. In

any case, it is clear that the pure Zn NPs are not encapsulated by TiOx. The

experimental surface energy of ZnO, averaged over the existing faces, is relatively

high (2.55 ± 0.23 J.m-2), see Ref.257 and references therein. However calculations

show that while the (0001)-Zn terminated surface has a high surface energy (~2.4

J.m-2), the (0001)-O terminated surface has a rather low surface energy (~0.96 J.m-

2).107 as well as the (1010) and (1120) surfaces (0.839 J.m-2 and 0.872 J. m-2,

respectively).258 Those (1010) and (1120) surfaces are the ones that are more likely

to be available here as it is on those surface that the observed absorbed carbonate

are present and stable. Thus we have an overall ZnO surface with a low surface

energy making it less prone to the encapsulation by TiO2 than platinum.

In the ZnPt NPs case, we observe that the Pt 4f intensity has the same

behavior than in the pure Pt NPs case: it is quite stable until the start of the CO

oxidation reaction at 380K where it decreases in the same way as for the pure Pt

NPs. However here, the Zn 3p signal behaves in stark contrast with the pure Zn NP

sample case. Indeed, here it decreases also from 380 K, at the same temperature

where Platinum signal also diminishes. The fact that zinc shares the fate of platinum

is surprising. Indeed we have shown previously that as soon as the CO oxidation

reaction starts, Zn elements are driven at surface creating a core platinum with a

ZnO shell NPs. Moreover we have seen that the oxidized pure Zn NPs are not

encapsulated by the support and could expect that oxidized zinc would protect Pt

from encapsulation. Taking into account only surface energy considerations, one

could argue that the ZnO surface of ZnPt NPs has a much higher surface energy

than that of oxidized pure Zn NPs (for instance surface doping by Pt atoms could

increases the surface energy and favor the encapsulation by TiOx). Nevertheless,

the ZnPt NPs seem to be less prone to encapsulation than the pure Pt ones. Indeed,

at the highest working temperature, 450 K, the normalized Pt signal (ℎ =400 eV) is

greater for bimetallic NPs, and non-measurable for pure Pt NPs.


113

Figure 69: Evolution of the ratio of integrated intensities of the XPS peaks of Pt 4f, Zn 3p (Ipeak) with respect to that of the Ti 3p

5) Conclusion

We have shown with STM measurements that the growth of small Pt NPs with

large densities was achieved due to their strong bonding with the reduced TiO2(110)

substrate. The large density of Pt cluster is an important effect since these NPs were

used as seed nucleation sites for the Zinc deposition. This deposition strategy

promotes the formation of bimetallic ZnPt NPs while the formation of pure Pt and

pure Zn nanoclusters was limited.

NAP-XPS measurements show the formation of stable Pt surface oxide at

room temperature for both pure Pt and ZnPt NPs after the exposure to a partial

pressure of 1 mbar of O2. In addition, the Pt 4f spectra show the presence of a Pt-Zn

alloy phase which forms readily at room temperature in ZnPt NPs. While a drastic

TiOx encapsulation took place for pure Pt NPs after annealing at 440 K under 1 mbar

of O2, a better resilience towards this effect was noticed for ZnPt NPs. The C 1s and

O 1s spectrum show the formation and stabilization of carbonate in the case of the

ZnPt NPs.


114

In the study of the CO oxidation reaction we have examined whether this

inverse oxide/metal catalyst films system can be transposed to the case of

nanometer-size bimetallic ZnPt nanoparticles (with a Zn/Pt ratio close to 1). To gain

a comprehensive understanding of the phenomena occurring at the atomic scale, a

study of the CO oxidation catalytic activity of pure Pt and pure Zn nanoparticles in

addition to the ZnPt NPs system deposited on TiO2 was conducted.

All three type of samples (Pure Pt, Pure Zn and ZnPt NPs) were submitted to

the same experimental conditions of gas composition (CO:O2 gas mixture with a 1:4

pressure ratio), pressure (1 mbar) and temperature (293 K – 450 K range), while the

surface reactivity was monitored using NAP-XPS. The NAP-XPS enables the

collection of photoelectrons from both the gas and solid phases. With this we could

relate the gas phase composition (reactants and compound), with the chemical state

of the surface such as adsorbed molecular species (CO, carbonates), but also the

oxidation state of the metals, and their depth distribution, using the tunability of the

synchrotron excitation source (with surface or bulk sensitive condition).

Working in isobaric conditions, we have determined the temperatures at which

the CO oxidation starts in all three samples. We observed that CO oxidation reaction

is so effective that the CO gas signal disappears, at temperatures of 396 K for the

pure Pt and the ZnPt nanoparticles, and at 450 K for the pure Zn nanoparticles. We

have reached the mass transport limitation regime. NAP-XPS shows that there is a

CO poisoning of the pure Pt NPs at room temperature until CO oxidation starts. At

this moment it is replaced by atomic oxygen as the temperature is raised.

The behavior of the pure Zn NPs is in stark contrast with pure Pt NPs. They

are oxidized when the gas mixture is introduced at room temperature and

carbonates are readily formed on their surface. Then when the critical temperature is

reached, a gaseous CO2 production and a steady-regime are achieved with the

constant carbonate formation and CO2 desorption. This unexpected efficiency of Zn

nanoparticles emphasizes the role of non-polar ZnO orientations in CO oxidation

reaction as they favor the formation of carbonate. Moreover Zn NPs are not

encapsulated by TiO2.

In the case of ZnPt NPs, the Pt-terminated nanoparticles are poisoned by

adsorbed CO below the CO oxidation onset temperature. When oxidative conditions


115

are reached, Zn is segregated to the surface forming an oxide layer (covered with

carbonates), while the core of Pt remains metallic. Pure Pt and bimetallic ZnPt

nanoparticles show similar behaviors in the oxidative regime as both are prone to be

encapsulated by the support. Yet the addition of zinc makes the phenomenon more

difficult. Pure Pt and bimetallic ZnPt also show a very similar activity towards the CO

oxidation reaction, as deduced from the gas phase photoemission intensity of the

product and reactants. The strong synergetic effect brought by ZnO/Pt boundaries

that Martynova et al.188 have reported at 450 K is not clearly observed here. This

may be simply a consequence of the mass transport limitation regime that we have

reached thus preventing the control of the CO partial pressure that occurs at a lower

temperature in the present case. This can be also due to the interaction of the

bimetallic NP with the rutile substrate.

Here we can see the tremendous complexity of the chemistry of supported

bimetallic nanoparticles, which is highlighted by NAP-XPS in great detail. It makes

the transposition of the observations made for much simpler single crystal systems

to the NPs not particularly straightforward. Yet the present work raises new

interesting questions about the reactivity of zinc oxide and zinc oxide / platinum

interfaces that NAP-XPS is able to tackle in a very relevant way using the

“reductionist” approach of single crystal surface studies.

Conclusion and Perspectives

116


During my PhD, I had the opportunity to conduct my research studies using

NAP-XPS as the main investigating experimental technique. This instrument

represents a powerful tool for the chemical surface analysis as it offers a synoptical

view of the whole reacting system, gaseous reactants and products, on the one hand,

the reactive solid surface, on the other hand. This specificity was used in two

different kinds of experiments, one dedicated to environmental chemistry and the

other one to heterogeneous catalysis (supported metallic nanoparticles). In this

regard, although the overlap between the gas phase and the solid phase

components is generally limited, the application of a positive voltage to the sample

was decisive to remove any ambiguity regarding the assignment of signals. By the

same token, the positive biasing (eventually associated to the presence of a neutral,

unreactive gas with a pressure in the mbar range) was proved to be highly beneficial

to strongly mitigate charging effects in poorly conducting materials like clays. This

simple “trick”, which we thought of, compensates the impossibility of using flood guns

in NAP conditions.

My first research study, the hydration of swelling clays, illustrates the ability of

the NAP XPS to follow the evolution of the chemical environment around a

counterion in two types of smectite clays (hectorite and saponite), as a function of

the relative humidity. In particular, I have shown that different behaviors are

observed depending on the counterion nature. Indeed, with sodium and strontium

counterions the hydration process occurs and is revealed by a clear chemical shift of

their core level binding energies (Na 2s, Sr 3d) to lower values. These shifts point to

the formation of their hydration spheres, and to a correlated increase of the

relaxation energy. Moreover, I have observed the migration of the sodium counterion

outside the analyzed region. This outdiffusion was tentatively attributed to a water-

driven enhanced ionic mobility, associated to a drift under an electric field resulting

from a small residual (a few hundreds of meV) charging effect. In stark contrast, in

the case of the cesium counterion, a slight chemical shift of the binding energy Cs 4d


117

towards higher energies was measured at the onset of water condensation,

indicating a decrease of the relaxation energy.

Thanks to the aforementioned gas/solid synoptical view, we have made the

first NAP-XPS observation of the soft X-ray radiolytical mineralization of pyridine

inserted in hydrated Sr2+-hectorite. The major observation was that the N2 yield stops

when a given pyridine partial pressure threshold is overcome. We have tentatively

proposed a model in which the hydrodenitrogenation of pyridine stops when X-ray

irradiation induced valence electrons (that trigger the production of H radicals from

water) are scavenged by pyridine itself.

In the second research topic dedicated to the catalytic properties of metallic

nanoparticles, we have first shown the beneficial effect of adding zinc to platinum

nanoparticles (NPs) as they limit the oxidation of the platinum. This process was

based on a chemically-driven effect for which Zn gets oxidized preferentially in the

presence of Pt in order to form a stable ZnO (shell) Pt (core) NP. Moreover, the

presence of Zn prevents the problematic encapsulation of the catalysts by the

support. To investigate possible synergetic effects reported for ZnO epitaxied on

Pt(111) single crystals,188 CO oxidation reaction was investigated in in situ conditions

on pure metallic Zn and Pt NPs and in their bimetallic counterparts ZnPt. As NAP-

XPS allows the collection of photoelectrons from both the gas and solid phase, I

have been able to relate the gas phase composition (reactants and compound), with

the chemical state of the surface, i.e. adsorbed molecular species (CO, carbonates),

the oxidation state of the metals, and their depth distribution, using the tunability of

the synchrotron excitation source. Working in isobaric conditions, I have determined

the temperatures at which the CO oxidation starts. In all cases CO oxidation is so

effective that the CO gas signal disappears from the gas phase (mass transport

limitation regime), at temperatures of 396 K for the pure Pt and the ZnPt

nanoparticles, and at 450 K for the pure Zn NPs. Thus we have also demonstrated

that pure zinc NPs are also catalytically active for the CO-oxidation reaction and are

able to form CO2 through a steady state reaction, even if it occurs at a somewhat

higher onset temperature. The tremendous complexity of the chemistry of supported

bimetallic nanoparticles, highlighted by NAP-XPS, makes that the transposition of

the observations made for much simpler single crystal systems is anything but

straightforward. On the other hand, we believe that the present work raises new


118

interesting questions about the reactivity of zinc oxide and zinc oxide / platinum

interfaces that NAP-XPS is able to tackle in a very relevant way using the

“reductionist” approach of single crystal surface studies.

The present PhD work has generated a lot of interrogations and opened new

perspectives. Among the new issues, the question of the electronic structure of the

solid/aqueous solution interface is of prime importance. It was motivated by a

previous NAP-XPS study of alkali halide solutions where the question of the

gold/solution interface was raised. With NAP-XPS, it would be interesting to study

the energy alignment of electrochemical solution.36 As the Fermi level in the solution

and the redox potential are one and the same thing,259,260 the level should align at

thermodynamic equilibrium. Therefore it is crucial to study solutions with well-defined

redox couples and their energy level alignments. A comparison between NAP-XPS

investigations and liquid jets experiments is technically and fundamentally of prime

importance. Indeed, liquid jets present several advantages over the NAP-XPS: (i)

easy control of solute concentration, where the liquid surface is less prone to

contamination, (ii) and a better control of beam damage as the material is constantly

regenerated. On the other hand, one can raise the question of thermodynamic

equilibrium between the flowing liquid and some electrode (Fermi level equilibration),

on the one hand, and of thermodynamic equilibrium at the surface of liquid jets when

the base pressure in the chamber is around 10-5 mbar. All these points make the

comparison between the two techniques extremely interesting, leaving aside the

prejudices about the superiority of one technique over the other. At the time of

writing, this XPS analysis of metal/solution interfaces has not yet been carried out,

because of delays in the allocation of beam-time. In any case, experiments, that I am

going to take part in, are scheduled in the 2nd semester of 2017 at Pléiades beamline,

SOLEIL.

Second, the study of the hydration of clays and of the chemical reactions that

could be sustained in them opens new opportunities in space science. Considering

the pressure range of NAP-XPS (1-10 mbar), this technique could be used to

simulate the atmosphere of planets like Mars or the study of the chemical reactivity

of silicate particles in warmed interstellar clouds261,262 can be easily managed. In this

regard, preliminary experiments were realized at SOLEIL by Rudolf’s group

(Groningen). The study of photocatalytic process in hydrated clays should be also


119

pursued. In fact, the relevance with radioactive waste storage is obvious, and the

search for efficient organic molecules inhibitors of H2 production should be an

interesting and potentially fundable research theme. Coming back to the application

of synchrotron radiation XPS in water-containing systems studied in (near) ambient

pressure conditions, water radiolysis by the beam raises serious issues more

especially when cells25,263 containing aqueous solutions are used (the famous H2

bubbling issue264,265). Clearly more beam-time and more instrumentation research

should be devoted to find ways to mitigate beam damage.

Third, the study of CO oxidation over ZnOPt NPs was a transposition of a

previous surface science study using the classic reductionist methodology of single

crystals and epitaxied layers, as developed by Freund’s group in the Fritz Haber

Institute of Berlin. Strangely, no XPS, and above all, no NAP-XPS study was made

on this catalytic system where the Zn/Pt synergy was seemingly observed. Shifting

away from the desire of bridging the so-called “material gap”, as we did before by

using NPs that are closer to real catalysts, the group will study the reactivity of ZnO

overlayers on Pt(111) with NAP-XPS in the 2nd semester of 2017 (the beam-time

was finally allocated). I am also strongly motivated to participate to this measurement

in SOLEIL, the last one of my PhD thesis experience.

Appendix

120

Appendix

A. Hydration of swelling clay A.1 Cs+-hectorite

Figure A1: Evolution of the Cs 4d core level peak binding aligned with Mg 2p as a function of the pressure and relative humidity for the hectorite sample at hv=400 eV

P (mbar) hν (eV) Cs 4d5/2 BE (eV)

Cs 4d5/2 FWHM (eV)

Mg 2p BE (eV)

Mg 2p FWHM (eV)

Si 2p3/2 BE (eV)

Si 2p FWHM (eV)

1.2 750 76,31 1,75 50,64 1,88 103,34 1,82

2.4 750 76,31 1,55 50,64 1,72 103,37 1,66

3.6 750 76,25 1,56 50,64 1,69 103,38 1,67

4.8 750 76,25 1,55 50,64 1,75 103,36 1,71

6 750 76,45 1,55 50,64 1,91 103,47 1,75

7.2 750 76,71 1,56 50,64 1,90 103,63 1,80

8,4 750 76,72 1,60 50,64 1,92 103,59 1,84

1,2 400 76,30 1,87 50,63 1,88

2,4 400 76,32 1,436 50,61 1,61

3,6 400 76,21 1,42 50,58 1,59 Table A1: Cs 4d, Mg 2p and Si 2p fitting parameters at hv=750 and 400eV for Cs

+-hectorite at various pressure

All Spectra are fitted with Gaussian peaks and Shirley background. Cs 4d

doublet is separated by 2.3 eV and Si 2p by 0.6 eV

Inte

nsity (

Arb

. U

nit)

82 80 78 76 74 72 54 53 52 51 50 49 48 47

Binding Energy (eV)

Mg 2p

Cs 4d

hv=400 eV

T=275 K

1,2mbar RH=17%

2,4mbar RH=34%

3,6mbar RH=50%

4,8mbar RH=67%

Appendix

121

A.2 Cs+-saponite

Figure A2: Evolution of the Cs 4d core level peak binding aligned with Mg 2p as a function of the pressure and relative humidity for the saponite sample at hv=400 eV

P (mbar) hν (eV) Cs 4d5/2 BE (eV)

Cs 4d5/2 FWHM (eV)

Al 2p3/2 BE (eV)

Mg 2p BE (eV)

Mg 2p FWHM (eV)

Si 2p3/2 BE (eV)

Si 2p FWHM (eV)

2.5 750 75,75 1,58 74.42 50,49 1,74 102,91 1,76

4.5 750 75,70 1,57 74.37 50,47 1,75 102,89 1,73

6 750 75,81 1,55 74.61 50,43 1,88 102,86 1,90

8 750 76,00 1,62 74.8 50,43 1,93 103,03 1,91

2.5 400 75,80 1,53 74.48 50,71 1,62 - -

4.5 400 75,54 1,98 74.42 50,60 1,69 - -

6 400 75,96 1,49 74.48 50,61 1,66 - - Table A2: Cs 4d, Mg 2p and Si 2p fitting parameters at hv=750 and 400eV for Cs

+-saponite at various pressure

All Spectra are fitted with Gaussian peaks and Shirley background. Cs 4d

doublet is separated by 2.3 eV and Si 2p by 0.6 eV. The Al 2p peak is fitted with a

doublet separated by 0.4 eV

Inte

nsity (

Arb

. U

nit)

80 78 76 74 72 7053 52 51 50 49 48

Binding Energy (eV)

Cs 4d5/2

and Al 2p

Mg 2p

2,5mbar RH=35%

4,5mbar RH=63%

6mbar RH=83%Cs 4d3/2

hv=400 eV

T=275 K

Appendix

122

A.3 Na+-hectorite

Figure A3: Evolution of the Na 2s core level peak binding aligned with Mg 2p as a function of the pressure and relative humidity for the hecorite sample at hv=400 eV

P (mbar)

hν (eV)

Na 2s BE (eV)

Na 2s FWHM (eV)

Mg 2p BE (eV)

Mg 2p FWHM (eV)

UHV 750 64,37 2,47 50,63 2,45

0,2 750 64,17 2,65 50,58 1,55

0,6 750 63,88 2,16 50,64 1,78

1,2 750 63,89 1,90 50,63 1,77

2,4 750 63,87 1,86 50,63 1,65

4.8 750 - - 50,64 1,79

6.7 750 - - 50,63 1,62

0.2 400 64,28 2,43 50.66 1.83

0,6 400 63,80 2,07 50.68 1.82

1.2 400 63,82 1,77 50,61 1,63

2.4 400 63,76 1,71 50,64 1,55 Table A3: Na 2s and Mg 2p fitting parameters at hv=750 and 400eV for Na

+-hectorite at various pressure

Spectra fitted with Gaussian peaks and Shirley background, for the first two

pressure at ℎ =400 (0.2 and 0.6 mbar) a linear background was first substracted

from the spectrum. Si element was not monitored during this experiment.

Inte

nsity (

Arb

. U

nit)

66 64 62 60 52 50 48Binding Energy (eV)

Mg 2p

Na 2s

hv=400eV

T=275 K

0,2mbar RH=3%

1,2mbar RH=17%

2,4mbar RH=34%

0,6mbar RH=9%

Appendix

123

A.4 Na+-saponite

Figure A4: Evolution of the Na 2s core level peak binding aligned with Mg 2p as a function of the pressure and relative humidity for the saponite sample at hv=400 eV

P (mbar) hν (eV) Na 2s

BE (eV) Na 2s

FWHM (eV) Mg 2p BE (eV)

Mg 2p FWHM (eV)

Si 2p3/2 BE (eV)

Si 2p FWHM (eV)

UHV 750 64,12 2,29 50,64 2,52 103,10 2,40

0,032 750 63,81 2,02 50,65 2,36 103,09 2,29

0,37 750 63,49 2,17 50,65 1,97 103,08 1,98

0,96 750 63,41 1,76 50,64 1,77 103,09 1,84

2 750 63,43 1,55 50,64 1,69 103,09 1,67

3 750 63,42 1,84 50,65 1,70 103,08 1,63

4.8 750 - - 50,65 1,83 103,05 1,87

8.4 750 - - 50,64 1,85 103,06 1,80

UHV 400 63,67 2,98 50,71 2,46 103,12 2,44

0,032 400 64,14 2,49 50,74 2,42 103,15 2,40

0,37 400 62,88 2,14 50,70 1,94 103,18 1,98

0,96 400 63,29 1,78 50,62 1,66 103,09 1,73

2 400 63,61 1,56 50,67 1,66 103,12 1,73

3 400 63,79 1,94 50,67 1,54 103,06 1,69 Table A4: Na 2s, Mg 2p and Si 2p fitting parameters at hv=750 and 400eV for Na

+-saponite at various pressure

Spectra fitted with Gaussian peaks and Shirley background for the first three

pressure at ℎ =400 (0.032, 0.37 and 0.96 mbar) a linear background was first

substracted from the spectrum. Si 2p doublet is separated by 0.6 eV

Inte

nsity (

Arb

. U

nit)

68 66 64 62 60 54 52 50 48 46

Binding Energy (eV)

Mg 2pNa2s

UHV

0,032mbar RH=0.5%

0,34mbar RH=5%

2mbar RH=29%

3mbar RH=43%

0,96mbar RH=14%

hv=400 eV

T=275 K

Appendix

124

A.5 Sr2+-hectorite

Figure A5: Evolution of the Sr 3d core level peak binding aligned with Mg 2p as a function of the pressure and relative humidity for the hectorite sample at hv=450 eV

P (mbar) hν (eV) Sr 3d5/2 BE (eV)

Sr 3d5/2 FWHM (eV)

Mg 2p BE (eV)

Mg 2p FWHM (eV)

Si 2p3/2 BE (eV)

Si 2p FWHM (eV)

1.8 (Ar) 750 134,01 1,94 50,10 1,96 102,51 1,88

0,1 750 133,75 1,68 50.10 1,75 102,51 1,85

0,7 750 133,76 1,72 50,10 1,86 102,59 1,93

1,7 750 133,71 1,78 50.10 1,75 102,54 1,79

3 750 133,72 1,61 50.09 1,78 102,51 1,76

1.8 (Ar) 450 133,92 1,77 50,10 1,79 102,58 1,762

0,1 450 133,74 1,71 50,10 2,11 102,68 2,125

0,7 450 133,74 1,48 50,11 1,98 102,64 2,119

1,7 450 133,72 1,37 50,11 1,73 102,64 1,767

3 450 133,71 1,97 50,12 1,41 102,65 1,621 Table A5: Sr 3d, Mg 2p and Si 2p fitting parameters at hv=750 and 450eV for Sr

2+-hectorite at various pressure

All Spectra are fitted with Gaussian peaks and Shirley background. Sr 3d


Inte

nsity (

Arb

. U

nit)

140 138 136 134 132 130 52 50 48

Binding Energy (eV)

X0.2

X2

X10

X40

X150

1.8 mbar Ar

0.1 mbar

0.7 mbar

1.7 mbar

3 mbar

Mg 2phv=450 eVT=275 KSr 3d

Appendix

125

A.6 Sr2+-saponite

P (mbar) hν (eV) Sr 3d5/2 BE (eV)

Sr 3d5/2 FWHM (eV)

Mg 2p BE (eV)

Mg 2p FWHM (eV)

Si 2p3/2 BE (eV)

Si 2p FWHM (eV)

1 (Ar) 750 134,41 1.99 50,59 1.95 103,34 1.89

0.1 750 134,48 1.72 50,59 1.71 103,35 1.84

0.7 750 133,89 1.68 50,59 1.75 103,32 1.92

1.7 750 133,90 1.73 50,59 1.7 103,32 1.79

3 750 133,88 1.67 50,59 1.72 103,32 1.77

4.8 750 134,04 1.72 50,59 1.74 103,32 1.8 Table A6: Sr 3d, Mg 2p and Si 2p fitting parameters at hv=750 for Sr

2+-hectorite at various pressure

All Spectra are fitted with Gaussian peaks and Shirley background. Sr 3d


A.7 Electron IMFP

Element hv=750 eV hv=450 eV hv=400 eV

Mg 2p 2,1 nm (KE=700 eV) 1,35 nm (KE=400 eV) 1,2 nm (KE=350 eV)

Si 2p 1,95 nm(KE=650 eV) 1,2 nm (KE=350 eV) 1,1 nm(KE=300 eV)

Cs 4d 2 nm(KE=670 eV) - 1,15 nm(KE=320 eV)

Na 2s 2,05 nm(KE=685 eV) - 1,05(KE=285 eV)

Sr 3d 1,85 nm (KE=615 eV) 1,15nm (KE=315 eV) 1 nm (KE=265 eV)

N 1s 1,2 nm (KE=350 eV) 0,8 nm (KE=50 eV) -

C 1s 1,5 nm (KE=460 eV) - -

O 1s 0,9 nm (KE=220 eV) - - Table A 7: Electron IMFP estimation for various elements at the excitation energy used on the hydration and pyridine study.

Those values were estimated from Tanuma et al.72 and Akkerman et al.115 for

silicon dioxide and magnesium oxide using a 8/14 ratio for silicon dioxide and 6/14

for magnesium considering the stoichiometry of clay structure (8 Si elements for 6

Mg).

Cs 4d and Na 2s IMFP are not given at ℎ =450 as they were not study at this

excitation energy. Indeed ℎ =450 was used as a replacement for Sr2+-hectorite

experiment in order to have a very surface sensitive condition for the N 1s spectra

(impossible with ℎ =400 as KE~0).

Appendix

126

B. Insertion and mineralization of Pyridine

B.1 Ionization cross sections at h=450 and h=750 eV

In order to discuss the eh pair generation by the soft X-ray beam, we first calculate

the total cross sections corresponding to the Sr0.4Mg5.2Li0.8Si8.0O20(OH)4 formula at

ℎ =450 eV and ℎ =750 eV, see Table S1. Note that both are very close. At ℎ =750

eV, the O 1s core-level is ionized with a high cross-section, but the metal atom

contribution is less than at ℎ =450 eV. We give also the total cross-sections for the

pyridine molecule. As at ℎ =750 eV the energy distances from the C and N K edges

are greater than at 450 eV, the total cross-section is smaller.

Orbital Cross-sections in Mbarn at

hv=450 eV (cross section for a

unit cell)

Cross-sections in Mbarn at

hv=750 eV (cross section for a

unit cell)

O 2p

O 2s

O 1s

0.015 (0.36)

0.036 (0.864)

-

0.0028 (0.067)

0.015 (0.36)

0.24 (5.76)

Si 3s

Si 3p

Si 2p

Si 2s

0.013 (0.10)

0.006 (0.048)

0.4 (3.2)

0.14 (1.1)

0.0047 (0.038)

0.0015 (0.012)

0.093 (0.744)

0.055 (0.44)

Mg 3s

Mg 2p

Mg 2s

0.005 (0.026)

0.18 (0.94)

0.1 (0.52)

0.0018 (0.01)

0.041 (0.21)

0.036 (0.19)

Li 2s

Li 1s

0.0004 (0.0003)

0.03 (0.024)

8 10-5

(6 10-5

)

0.006 (0.005)

Appendix

127

Sr 5s

Sr 4p

Sr 4s

Sr 3d

Sr 3p

Sr 3s

0.0027 (0.0011)

0.09 (0.036)

0.03 (0.012)

1.9 (0.76)

0.59 (0.24)

0.15 (0.06)

0.0011 (0.0005)

0.039 (0.016)

0.013 (0.005)

0.53 (0.21)

0.31 (0.12)

0.077 (0.031)

Total cross-section

corresponding to

Sr0.4Mg5.2Li0.8Si8.0O20(OH)4 formula

8.29 8.22

N 2p

N 2s

N 1s

0.0055 (0.0055)

0.0245 (0.0245)

0.545 (0.545)

0.001 (0.0055)

0.007 (0.007)

0.0155 (0.0155)

C 2p

C 2s

C 1s

0.0015 (0.0075)

0.015 (0.075)

0.35 (1.75)

0.0003 (0.0015)

0.004 (0.02)

0.093 (0.465)

Pyridine total cross section

corresponding to C5H5N

formula

2.4 0.50

Table B1: Photoionization cross-sections, from Ref. 68

of the clay constituent atoms (for numbers between brackets, the

stoichiometry Sr0.4Mg5.2Li0.8Si8.0O20(OH)4 is accounted for). Photoionization cross-sections of the pyridine constituent atoms (for numbers between brackets the stoichiometry of the molecule is accounted for)

Appendix

128

B.2Fitting parameters

Table B2: Sr 3d and Mg 2p fitting parameters under grounded and biased condition at hv=750 eV

hv=750 Grounded hv=750 biased (+30V)

O Bulk BE (FWHM) H2O Gas Phase BE (FWHM) O Bulk BE (FWHM)

UHV 532.13 (2.78) -

531.58 (2.5)

0.5 mbar H2O 532.37 (2.71) 535.28 (1.2)

532.31 (2.7)

0,5 mbar H2O + 0,1 pyridine 532.38 (2.49) 535.36 (0.99)

532.32 (2.7)

0,5 mbar H2O + 0,3 pyridine 532.35 (2.49) 535.54 (0.93)

532.28 (2.5)

0,5 mbar H2O + 0,5 pyridine 532.31 (2.22) 535.38 (0.89)

532.30 (2.48)

After Pumping 532.34 (2.89) -

532.32 (2.94)

Table B3: O 1s at hv=750 eV fitting parameters under grounded condition

BE and (FWHM) evolution of Sr 3d 5/2 BE and (FWHM evolution of Mg 2p

Grounded Biased (+30V) Grounded Biased (+30V

UHV 133.84 (1.65)

135.11 (1.65)

133.95 (1.82) 50.02 (2.21) 50.01 (1.98)

1 mbar H2O 133.86 (1.97) 133.71 (1.6) 50.03 (1.81) 50.03 (1.66

0,5 mbar H2O

+ 0,1 pyridine

133.65 (1.7) 133.54 (1.7) 50.01 (1.61) 50.01 (1.62)

Appendix

129

Table B4: N 1s at hv=750 eV fitting parameters in grounded and biased condition

Table B5: N 1s at hv=450 eV fitting parameters in grounded condition

hv=750 Grounded hv=750 biased (+30V)

Pyr (ads) Pyr gas phase Pyr H-bond N2 gas phase Pyr (ads) Pyr-H bond

0,5 mbar H2O

+ 0,1 pyridine

399.46 (1.85) 400.41 (0.92) 401.53 (1.85) 405.27 (0.8) 399.36 (1.94) 401.55 (1.94)

0,5 mbar H2O

+ 0,3 pyridine

399.46 (1.56) 400.46 (0.94) 401.57 (1.56) - 399.42 (1.82) 401.51 (1.81)

0,5 mbar H2O

+ 0,5 pyridine

399.43 (1.49) 400.39 (1.85) 401.61 (1.49) - 399.43 (1.85) 401.53 (1.85)

After Pumping 399.46 (2.31) - 401.56 (2.31) - 399.47 (2.28) 401.57 (2.28)

hv=450 Grounded

Pyr (ads) Pyr gas phase Pyr H-bond N2 gas phase

0,5 mbar H2O

+ 0,1 pyridine

399.5 (1.68) 400.35 (0.95) 401.3 (1.68 405.47 (0.83)

0,5 mbar H2O

+ 0,3 pyridine

399.55 (0.88) 400.52 (0.89) 401.66 (0.88) -

0,5 mbar H2O

+ 0,5 pyridine

399.56 (0.98) 400.56 (0.81) 401.65 (0.98) -

After Pumping 399.54 (2.27) - 401.54 (2.2) -

Appendix

130

C-C BE (FWHM) eV COC BE (FWHM) eV C=O BE (FWHM) eV CO2 BE (FWHM) eV

UHV 284.84 (1.93) 286.48 (1.93) 288.41 (1.93) -

0.5 mbar H2O 284.84 (1.67) 286.36 (1.67) 288.03 (1.67) -

0,5 mbar H2O

+ 0,1 pyridine

284.86 (1.64) 286.37 (1.64) 287.98 (1.64) -

0,5 mbar H2O

+ 0,3 pyridine

284.90 (1.63) 286.18 (1.63) 287.75 (1.63) 292.25 (1.63)

0,5 mbar H2O

+ 0,5 pyridine

284.94 (1.9) 286.51 (1.9) 287.74 (1.9) 292.23 (1.87)

After Pumping 285.00 (1.81) 286.64 (1.81) 288.15 (1.81) -

Table B6: C 1s at hv=750 eV fitting parameters in biased condition

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Figure Contents

158

Figure Contents

Figure 1: Three step model: 1) Photoelectron creation, 2) Transport to surface, 3) Escape to

vacuum ....................................................................................................................................... 8

Figure 2: The different interaction between electrons and matter leading to 4 final states .... 9

Figure 3: Principle of XPS diagram ............................................................................................. 9

Figure 4: Sample and analyzer energy level diagram .............................................................. 10

Figure 5: IMPF as function of KE for various elements, from Seah, M.P. et Al.15 .................... 13

Figure 6: Exponentially decreasing XPS signal vs increasing pressure .................................... 14

Figure 7: Design commonly used since the early 70’s (left) and the new design developed at

the Berkeley lab (Right) ............................................................................................................ 15

Figure 8: The NAP-XPS at the TEMPO beam-light ................................................................... 18

Figure 9: Phoibos NAP 150 Analyzer with its differential pumping system30 .......................... 19

Figure 10: Structure of a hectorite (a) and a saponite (b) ....................................................... 22

Figure 11: Hydration of the couterion in the interlayer space with the formation of the first

hydration sphere (a) and second hydration sphere (b) ........................................................... 23

Figure 12: Formation of a partial hydration sphere with couterion staying in interaction with

the clay surface ........................................................................................................................ 24

Figure 13: Water sorption curve of several hectorite with Na+-hydroxyhectorite in blue, Na+

Fluorohectorite in purple and Na+ Fluorohectorite in purple which has received a thermal

treatment before the experiment in order to fully desorb the water trap in the interlayer

space. From Dazas, B. et al. 52 .................................................................................................. 25

Figure 14: Na+-saponite sorption curve with the evolution of the interlayer space as a

function of relative pression in sorption (black filled point, plain line) and desorption (empty

point , dotted line) from Michot, L. et al. 33 ............................................................................. 25

Figure 15: Evolution of the Mg 2p peak position referenced to the Fermi level with

increasing pressure for a Cs+-saponite sample ........................................................................ 29

Figure 16: Evolution of the C 1s C-C component binding energy with respect to the Fermi as

a function of the pressure for several samples ....................................................................... 31

Figure 17: C 1s peak of the Cs+-hectorite decomposition at 1.2 mbar (a) and 8.4 mbar (b) .. 32

Figure 18: Difference in charging effect between a grounded sample (a) and a positively

biased one (b) .......................................................................................................................... 33

Figure Contents

159

Figure 19: Evolution of the Mg 2p FWHM in UHV with increasing biased voltage at hv=750

eV ............................................................................................................................................. 33

Figure 20: Evolution of the Mg 2p peak with increased Ar pressure under a +30 V biased at

hv=750 eV ................................................................................................................................ 34

Figure 21: Evolution of the Cs 4d core level peak binding aligned with Mg 2p as a function of

the pressure and relative humidity for the hectorite sample at hv=750 eV. Fitting lines are

shown in red ............................................................................................................................ 36

Figure 22: Image of the Cs+-hectorite sample at 7.2 mbar (RH=100%) with water beads up on

its surface ................................................................................................................................. 37

Figure 23: Evolution of the Si 2p core level peak binding energy as a function of the pressure

and relative humidity for the Cs+-hectorite sample. Fitting lines are shown in red ............... 37

Figure 24: Evolution of the Cs 4d core level peak binding energy as a function of the pressure

and relative humidity for the saponite sample. Fitting lines are shown in red ...................... 38

Figure 25: Evolution of the Si 2p core level peak binding energy aligned with Mg 2p as a

function of the pressure and relative humidity for the saponite sample. Fitting lines are

shown in red ............................................................................................................................ 39

Figure 26: Evolution of the distance between Cs 4d5/2 peak and Mg 2p as a function of the

relative humidity for Cs+-hectorite (red) and Cs+-saponite (blue) .......................................... 40

Figure 27: Evolution of the Na 2s core level peak binding energy as a function of the

pressure and relative humidity for the hectorite sample. ...................................................... 40

Figure 28: Na 2s core level peak binding energy at hv=1050 eV for RH=68% and 100% for the

hectorite sample ...................................................................................................................... 42

Figure 29: Evolution of the Na 2s core level peak binding energy as a function of the

pressure and relative humidity for the saponite sample ........................................................ 43

Figure 30:Evolution of the the Na/Mg ratio as a function of the relative humidity for Na+-

hectorite (red) and Na+-saponite (blue) .................................................................................. 44


function of the pressure and relative humidity for the Na+-saponite sample ........................ 45

Figure 32: Evolution of the distance between Na 2s peak and Mg 2p as a function of the

relative humidity for Na+-hectorite (red) and Na+-saponite (blue) ......................................... 46

Figure 33: Image of the sample when Na+ disappears from the XPS spectrum at RH=68 % .. 46

Figure 34: Evolution of the Sr 3d core level peak binding energy as a function of the pressure

and relative humidity for the saponite sample at hv=750 eV ................................................. 48

Figure 35: Evolution of the Sr 3d core level peak binding energy as a function of the pressure

and relative humidity for the hectorite sample ...................................................................... 49

Figure Contents

160


function of the pressure and relative humidity for the saponite sample ............................... 50

Figure 37: (a) The schematic structure of Sr2+ hectorite, (b) The pyridine molecule (blue:

nitrogen, black: carbon, white: hydrogen) .............................................................................. 53

Figure 38: Evolution of the FWHM of Mg 2p and Sr 3d with different excitation energy (450,

750 and 1050 eV) under UHV condition. ................................................................................. 55

Figure 39: Sr 3d and Mg 2p spectra measured at hν=750 eV with a sample bias of +30 V (the

binding energies are corrected) ............................................................................................... 56

Figure 40: O 1s spectra of the unbiased, grounded sample measured at hν=750 eV. The grey

component corresponds to the clay lattice oxygen and to confined water, while the blue one

corresponds to gas phase water .............................................................................................. 57

Figure 41: N 1s spectra in measured at hν=750 eV under grounded condition (a), at hv=750

eV in biased condition (b) and at hv=450 eV under grounded conditions(c) .......................... 59

Figure 42: The pyridine hydrated cation (1W) complex, according to Refs.14 and 16 ........... 60

Figure 43: C 1s spectra measured at hv=750 eV while the sample is biased to +30 V ............ 63

Figure 44: Evolution of energy consumption in quadrillion Btu with time ............................. 70

Figure 45: Evolution of CO2 emission in metric tons with time for different country............. 71

Figure 46: Working principle of a fuel cell ............................................................................... 72

Figure 47: 3D empty states STM of pure pt(a), pure Zn(b), PtZn (c) and their histogram

distribution of the nanoparticle diameter (d,e,f) respectively ................................................ 78

Figure 48: Core level Pt 4f of pure Pt NPs on TiO2(110)-(1x1) under 1 mbar of O2 gas at

different temperature taken at hv=400 eV (a) and 165 eV(b) ................................................ 80

Figure 49: (a) Band-bending diagram of TiO2(110) surface and (b) core level spectra of Ti 2p

at three states different states: clean (reduced), after Pt deposition and after subsequent

exposure to 1 mbar of O2 ......................................................................................................... 81

Figure 50: Core level Pt 4f of PtZn NPs on TiO2(110)-(1x1) under 1 mbar of O2 gas at different

temperature taken at hv=400 eV (a) and 165 eV(b) ................................................................ 85

Figure 51: Evolution of the platinum metallicity of both Pt and PtZn NPs through the

experiment ............................................................................................................................... 86

Figure 52: C 1s core level spectra under 1 mbar of O2 at different temperature for pure Pt

NPs ........................................................................................................................................... 87

Figure 53: O 1s core level spectra under 1 mbar of O2 at different temperature for pure Pt

NPs ........................................................................................................................................... 88

Figure 54: C 1s core level spectra under 1 mbar of O2 gas at different temperature for ZnPt

NPs ........................................................................................................................................... 89

Figure Contents

161

Figure 55: O 1s core level spectra under 1 mbar of O2 gas at different temperature for ZnPt

NPs ........................................................................................................................................... 90

Figure 56: C 1s core level spectra under 1 mbar of CO-O2 gas mixture (1:4 ratio) at different

temperature for pure Pt NPs (a), Pure Zn NPs (b) and ZnPt NPs (c) ........................................ 93

Figure 57: CO gas phase (CO(g)) in C 1s spectrum measured at hv=400 eV fitted with a

vibrational progression ............................................................................................................ 94

Figure 58: O 1s core level spectra under 1 mbar of CO-O2 gas mixture (1:4 ratio) at different

temperature for pure Pt NPs (a), Pure Zn NPs (b) and ZnPt NPs (c) ........................................ 95

Figure 59: Evolution of CO2/O2 gas phase ratio as a function of temperature ....................... 96

Figure 60: Pt 4f core level spectrum (a) and the C 1s core level spectrum (b) recorded at

hv=400 eV for the pure Pt NPs................................................................................................. 97

Figure 61: Ti 2p core level spectra of pure Pt NPs sample recorded at hv=560 eV ................ 98

Figure 62: Ti 2p core level spectra of pure Zn NPs recorded at hv=560 eV .......................... 100

Figure 63: C 1s core level spectra of pure Zn NPs recorded at hv=400 eV ............................ 101

Figure 64: Binding energy shift of Zn 3p core level peak for pure Zn NPS and ZnPt NPs ...... 104

Figure 65: Pt 4f core level spectrum (a) and the C 1s core level spectrum (b) recorded at

hv=400 eV for the ZnPt NPs ................................................................................................... 105

Figure 66: Metallic Pt (Ptmet) spectral weight versus temperature obtained from the Pt 4f XPS

spectra for pure Pt NPs and ZnPt NPs.................................................................................... 107

Figure 67: Pt 4f and Zn 3p binding energy window of ZnPt NPs samples measured at hv = 400

eV (a) and Pt atomic weight (Pt/(Pt+Zn))ratio (b) ................................................................. 108

Figure 68: Comparison of chemical changes in pure Pt, pure Zn and ZnPt NPs .................... 109

Figure 69: Evolution of the ratio of integrated intensities of the XPS peaks of Pt 4f, Zn 3p

(Ipeak) with respect to that of the Ti 3p ................................................................................ 113

Appendix

Figure A1: Evolution of the Cs 4d core level peak binding aligned with Mg 2p as a function of

the pressure and relative humidity for the hectorite sample at hv=400 eV ......................... 120

Figure A2: Evolution of the Cs 4d core level peak binding aligned with Mg 2p as a function of

the pressure and relative humidity for the saponite sample at hv=400 eV .......................... 121

Figure A3: Evolution of the Na 2s core level peak binding aligned with Mg 2p as a function of

the pressure and relative humidity for the hecorite sample at hv=400 eV .......................... 122

Figure Contents

162

Figure A4: Evolution of the Na 2s core level peak binding aligned with Mg 2p as a function of

the pressure and relative humidity for the saponite sample at hv=400 eV .......................... 123

Figure A5: Evolution of the Sr 3d core level peak binding aligned with Mg 2p as a function of

the pressure and relative humidity for the hectorite sample at hv=450 eV ......................... 124

Table Contents

163

Table Contents

Table 1: Classification of 2.1 clays swelling ability as function of layer charge ....................... 22

Table 2: Several common counterion main characteristics .................................................... 24

Table 3: Evolution of the Mg 2p FWHM with increased RH for the Na+-hectorite sample ..... 42

Table 4: Mass adsorption coefficient and linear adsorption value for clay ............................ 64

Appendix

Table A1: Cs 4d, Mg 2p and Si 2p fitting parameters at hv=750 and 400eV for Cs+-hectorite at

various pressure ..................................................................................................................... 120

Table A2: Cs 4d, Mg 2p and Si 2p fitting parameters at hv=750 and 400eV for Cs+-saponite at


Table A3: Na 2s and Mg 2p fitting parameters at hv=750 and 400eV for Na+-hectorite at


Table A4: Na 2s, Mg 2p and Si 2p fitting parameters at hv=750 and 400eV for Na+-saponite at


Table A5: Sr 3d, Mg 2p and Si 2p fitting parameters at hv=750 and 450eV for Sr2+-hectorite

at various pressure................................................................................................................. 124

Table A6: Sr 3d, Mg 2p and Si 2p fitting parameters at hv=750 for Sr2+-hectorite at various

pressure ................................................................................................................................. 125

Table A 7: Electron IMFP estimation for various elements at the excitation energy used on

the hydration and pyridine study. ......................................................................................... 125

Table B1: Photoionization cross-sections, from Ref. 68 of the clay constituent atoms (for

numbers between brackets, the stoichiometry Sr0.4Mg5.2Li0.8Si8.0O20(OH)4 is accounted for).

Photoionization cross-sections of the pyridine constituent atoms (for numbers between

brackets the stoichiometry of the molecule is accounted for) ............................................. 127

Table B2: Sr 3d and Mg 2p fitting parameters under grounded and biased condition at

hv=750 eV .............................................................................................................................. 128

Table B3: O 1s at hv=750 eV fitting parameters under grounded condition ........................ 128

Table B4: N 1s at hv=750 eV fitting parameters in grounded and biased condition............. 129

Table B5: N 1s at hv=450 eV fitting parameters in grounded condition ............................... 129

Table B6: C 1s at hv=750 eV fitting parameters in biased condition ..................................... 130

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