Surface Science 549 (2004) 134–142

www.elsevier.com/locate/susc

Dissociative adsorption of NO on TiO2(1 1 0) argonion bombarded surfaces

Jose Abad *, Oliver B€oohme, Elisa Rom�aan

Institute of Materials Science of Madrid, ICMM-CSIC, Cantoblanco, 28049 Madrid, Spain

Received 6 August 2003; accepted for publication 17 November 2003

Abstract

The interaction of NO with TiO2(1 1 0) Arþ-ion-bombarded surfaces has been studied by X-ray photoelectron

spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy. Surfaces with different degrees of

defects have been characterized by monitoring the evolution of the electronic structure of the surface, with the aim of

studying the influence of the surface defects on the interaction with NO. The interaction was studied for exposures up to

500 L. However, the main effects occur already in the first 10 L. The exposure of the surfaces to NO resulted in the

removal of defect sites without adsorption of N.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Titanium oxide; Nitrogen oxides; X-ray photoelectron spectroscopy; Visible and ultraviolet photoelectron spectroscopy;

Auger electron spectroscopy; Oxidation

1. Introduction

The interaction of NO with TiO2 has recently

received much attention, due to the interesting

applications of this system in catalysis [1], photo-

catalysis [2–4], and NO sensors [5,6]. Boccuzzi et al.

[7] have studied the adsorption of NO on TiO2

powders using Fourier transform infrared (FTIR)

spectroscopy concluding that NO molecules aredissociated on reduced TiO2 surfaces. On the other

hand, Sorescu et al. [8], using the Thermal Pro-

grammed Desorption (TPD) technique and theo-

retical calculations, have studied the adsorption of

* Corresponding author. Tel.: +34-91-334-9000; fax: +34-91-

372-0623.

E-mail address: jabad@icmm.csic.es (J. Abad).

0039-6028/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2003.11.031

NO at 118 K on oxidized TiO2(1 1 0) surfaces,concluding that molecular NO adsorbs weakly and

desorbs at 127 K.

Lu et al. [9] have studied the interaction at 100–

250 K between NO and reduced TiO2(1 1 0) single

crystal surfaces, using TPD technique. Surface

defect sites were created by thermal annealing.

They concluded that desorption takes place at 120

K for molecular NO, at 169 K for molecular N2Obonded on (Ti3þ) defects, and at 250 K for

molecular N2O bonded on steps. Li et al. [10] have

theoretically studied the adsorption of NO on a

TiO2(1 1 0)-(1 · 1) surface defect and conclude thatNO is adsorbed between two fourfold coordinated

Ti3þ sites. However, to our knowledge, no exper-

imental study has been made on the interaction of

NO with TiO2(1 1 0) single crystals at room tem-perature.

ed.

J. Abad et al. / Surface Science 549 (2004) 134–142 135

It is known that defects play a fundamental role

in the interaction of molecules with oxide surfaces,

since defects act as active sites for the adsorption

and dissociation of molecules on the surface. In

TiO2(1 1 0) surfaces, these defects, mainly Ti3þ

species (oxygen vacancies), appear in ultravioletphotoelectron spectroscopy (UPS) as band gap

states at 0.7 eV below the conduction band edge

[1,11,12], in ELS as an energy loss peak in the

region of 1–2 eV related with Ti3d–Ti3d transi-

tions [1,11], and in X-ray photoelectron spectro-

scopy (XPS) as a shoulder in the lower binding

energy side of the Ti 2p line [1,12,13].

In this paper, we study the interaction of NOwith TiO2(1 1 0) stoichiometric and Ar

þ-ion-bom-

barded surfaces, at room temperature (RT). This

sputtering preferably removes O and produces

topographically complex surfaces (disordered de-

fects), with Ti3þ and Ti2þ species appearing in

several layers inside the material, depending on the

sputtering conditions. In this work we present for

the first time a study of the adsorption of NO onTiO2(1 1 0) surfaces with different subsurface de-

fect densities at room temperature, combining

techniques with different information depths (XPS,

AES, and UPS), with the aim of studying the

importance of surface and subsurface defects in

the surface reactivity.

Table 1

Sputtering conditions for surfaces I and II

Energy

[keV]

Current density

[lA/cm2]

Time

[min]

Surface I 0.5 0.5 40

Surface II 0.5 0.5 85

0.8 0.8 30

Initial: Stoichiometric TiO2(1 1 0)-(1· 1) single crystal surface.

2. Experimental

Experiments were carried out in an ultra-high

vacuum (UHV) vessel with a base pressure of

2 · 10�10 mbar. The system was equipped with the

necessary instrumentation to perform Auger elec-

tron spectroscopy (AES), ELS, XPS, UPS, and

LEED. The UHV system was also fitted with anion gun for sample cleaning and preparation, and

a quadruple mass spectrometer to check the purity

of NO and Ar.

TiO2(1 1 0) rutile single crystals (10 · 10 · 1 mm3

and both sides polished), provided by PI-KEM

Ltd., UK, were used in this study. The sample was

attached to a Ta holder with resistive heating. The

surfaces were cleaned by repeated cycles of 3 keVArþ ion bombardment (the angle of incidence of

the ion gun relative to sample normal was 70�) at

room temperature for 30 min followed by 1–2 h of

heating to 800 K, until no impurities were detected

by AES and XPS. The sample temperature was

measured using an optical pyrometer. After this

treatment the sample exhibited a sharp (1 · 1)LEED pattern.Three types of surfaces were investigated in

these experiments: the stoichiometric (1 1 0)-(1 · 1)surface, and two sputtered surfaces. In order to

prepare samples with different surface and sub-

surface defects, the crystal was bombarded with

Arþ ions at the conditions indicated in Table 1.

Surface II was prepared starting from the steady

state corresponding to the conditions of surface I(increasing the sputtering time until 85 min) since a

steady state is always reached where the removal

of O and Ti from the surface is determined by the

composition of the bulk [14]. All the surfaces were

exposed to NO at 1–7 · 10�8 mbar for the lowerdoses and 3 · 10�7 mbar for the highest dose, insequential doses up to 500 Langmuir (L) (1

L¼ 1.33 · 10�6 mbar s).AE, UP, XP, and EL spectra were taken using a

double-pass cylindrical mirror analyzer (CMA).

Auger spectra were performed with an electron

beam energy of 3 keV and a current of 33 nA. EL

spectra were carried out with incident beam ener-

gies of 100 and 250 eV, varying the current in the

range 1–5 nA. Nonmonochromatized MgKa

(1253.6 eV) X-rays were used in XPS. Narrow-scan spectra were taken at an analyzer pass energy

of 50 eV, providing a resolution of 1 eV. Before

the XPS data were analyzed, the contribution of

the MgKa satellite lines were subtracted and the

spectra were subjected to a Shirley background

subtraction formalism. The spectra were normal-

ized to the integral intensity of the Ti 2p peak of

the stoichiometric sample, since the density of Ti in

Table 2

Binding energies, areas and FWHM for Gaussian fits of XP Ti 2p spectra

Sample Peak Binding energy

(eV)

FWHM (eV) % of Ti 2p3=2

TiO2(1 1 0)-(1· 1) Ti 2p3=2(a) Ti4þ 458.5 1.8 100

Satellite 461.0 2.6

Ti 2p1=2 (Ti4þ) 464.2 2.4

TiO2(1 1 0) surface I Ti 2p3=2 Clean After 500 L NO

(a) Ti4þ 458.5 2.2 63 78

(b) Ti3þ 456.8 2.2 27 14

(c) Ti2þ 455.4 2.2 10 8

Satellite 461.0 3.3

Ti 2p1=2 (Ti4þ) 464.2 3.0

TiO2(1 1 0) surface II Ti 2p3=2(d) Ti4þ 458.5 2.2 54 69

(e) Ti3þ 456.8 2.2 35 24

(f) Ti2þ 455.4 2.2 11 7

Satellite 461.0 3.3

Ti 2p1=2 (Ti4þ) 464.2 3.0

136 J. Abad et al. / Surface Science 549 (2004) 134–142

the sputtered layers is comparable to that of thefull-oxidized surfaces [14]. Fitting of the Ti 2p

peaks was performed using Gaussian doublets

with a spin-orbit splitting of 5.7 eV and intensity

ratio of approximately 0.5, in agreement with the

literature [15–17]. The full widths at half maximum

(FWHM) were fixed and broadening in sputtered

samples taken into account [18], see Table 2. The

fitting with Gaussian functions is justified, in ourcase, because of the limited resolution (1 eV) of the

energy analyzer [19]. UP spectra were excited with

He I (21.2 eV) and He II (40.8 eV) radiation. Data

were recorded with a resolution of 0.3 eV for He I

valence band spectra, 0.1 eV for the work function

(/) measurements, and 0.2 eV for the He II valenceband spectra. The position of the Fermi level was

determined from the spectrum of a Ta foil at-tached to the sample.

3. Results and discussion

3.1. Characterization of the clean surfaces

In Fig. 1, XP Ti 2p spectra fitted with Gaussianpeaks are shown for the clean surfaces: (c) stoi-

chiometric surface, (b) surface I and (a) surface II

(open circles are experimental data and solidcurves are Gaussian fits). Ti species present in

sputtered surfaces are Ti4þ, Ti3þ and Ti2þ. The

difference in binding energy between Ti3þ and Ti4þ

is 1.7 eV [13,20] and that between Ti2þ and Ti4þ is

3.1 eV [13,20]. For the best fit of the experimental

data it was necessary to introduce a satellite

(dotted curves in Fig. 1) located at 2.5 eV from the

Ti4þ 2p3=2 peak, see Table 2; corresponding satel-lites of Ti3þ and Ti2þ species are not shown for

reasons of clarity [21].

In Table 2, binding energies, normalized inten-

sities, and FWHM are shown for the different

species in Ti 2p lines fitted with Gaussian peaks. As

we can also observe in Fig. 1, the amount of Ti4þ

species decreases whereas the Ti3þ and Ti2þ species

increase in amounts depending on the sputteringconditions. From these data, we can calculate the

extent of reduction d which is related to the O/Tiratio of the surface by d ¼ 2� 0:5 · (O/Ti). Theparameter d is calculated by summing the fractionsof cations in each oxidation state, weighted by the

difference in valence state relative to Ti(IV)¼Ti4þ[22,23]: d ¼ 0:01 · [Ti3þ] + 0.02 · [Ti2þ] ¼ 4 )0.04 · [Ti4þ] ) 0.03 · [Ti3þ] ) 0.02 · [Ti2þ] ¼ 4 ) 2 ·(O/Ti), where [Tinþ] are the percentages in Table 2.

These parameters are useful to characterize the

Fig. 1. Ti 2p XP spectra fitted with Gaussian peaks: (a) clean

surface II, (b) clean surface I and (c) stoichiometric clean sur-

face. Open circles are the original data, solid peaks are the

Gaussian peaks and dotted curves are satellite peaks. See Table

2 for details.

Fig. 2. UP He I spectra for clean surfaces. Inset shows defect

band gap states.

J. Abad et al. / Surface Science 549 (2004) 134–142 137

average oxidation state of the surface. The values

of d for surfaces I and II were 0.5 and 0.6, respec-tively. The corresponding O/Ti ratios for surfaces I

and II were 1.75 and 1.70, respectively. These val-

ues are comparable to those in the literature for

similar sputtering conditions [13,14,24]. Thus, thedifference Dd of 0.1 between the two defective

samples studied corresponds to an average de-

crease (DO/Ti) of 0.05 oxygen atoms per Ti withinthe near-surface region probed by XPS, i.e., about

3% of all the O. However, the change in oxygen

concentration at the surface between surfaces I and

II is evidently greater than 3%. We estimate the

fraction of the Ti 2p signal from the topmostmonolayer (ML) in about 14% [25] of the total.

When the average oxygen decrease is normalized

by this value, i.e., ascribing all the O loss to the

surface ML, we obtain 21% of oxygen decrease

between surfaces I and II. However, if we ascribe

all the O loss to the first two surface ML, we obtain

a difference of 11%. This last value is in agreementwith UPS data, see below.

In Fig. 2, valence-band UP He-I spectra are

shown for the clean surfaces: stoichiometric (1 · 1)surface, surface I, and surface II. In the inset of

Fig. 2, the region of the band-gap defect states is

shown in detail. The TiO2(1 1 0) valence band is

derived mainly from O 2p orbitals. However, there

is hybridization between O 2p and Ti 3d orbitals,and the valence band spectrum has been qualita-

tively decomposed into bonding (r) and nonbond-ing (p) components [26,27]. The main difference inthe valence band for the stoichiometric (1 · 1)surface is a pronounced valley around 7 eV while

in sputtered surfaces this valley does not exist

[11,28,29]. The stoichiometric surface has higher

emission intensity from O 2p nonbonding orbitals(lower binding energy) and lower emission inten-

sity from bonding orbitals (higher binding energy).

The emission intensity ratio between bonding and

nonbonding orbitals is very sensitive to Ti 3d

population [26,30,31]. Thereby, the emission de-

crease of the nonbonding orbitals in Fig. 2, indi-

cates that oxygen vacancies have been produced

Fig. 3. Ti 2p XP spectra fitted with Gaussian peaks: (a) clean

surface II, (b) surface II after 2 L of NO and (c) surface II after

500 L of NO. Notation is the same as in Fig. 1.

138 J. Abad et al. / Surface Science 549 (2004) 134–142

on the surface (surface reduction), and the Ti 3d

population has increased. This point is confirmed

by the emission increase of the band gap states for

sputtered surfaces, inset of Fig. 2. Band gap states

are of Ti 3d character [1].

From the normalization of curves reported inFig. 2, it can be observed that the defect-induced

band-gap emission defects for surface I is around

13% less than for surface II, while the O 2p emis-

sion is increased in the same amount. We can also

observe that the top of the valence band moves

away from the Fermi level by 0.2 eV for surface I

and 0.4 eV for surface II with respect to the stoi-

chiometric surface. This is consistent with previousresults reported by Henrich et al. [11].

It is known that sputtering of the stoichiometric

(1 1 0) surface decreases its work function, due to

an increase in the surface free electron concentra-

tion [28]. In the present study, the work function

change (D/) with respect to the stoichiometric

surface measured for surfaces I and II were )0.4and )0.7 eV, respectively.To summarize, the stoichiometric surface is

characterized by the presence of Ti4þ and the ab-

sence of Ti3þ and Ti2þ species (XPS results) and

the absence of band gap states (UPS results).

Surfaces I and II are characterized by the presence

of Ti3þ and Ti2þ species and band gap states.

Analyzing the XPS and UPS results, we observe

that the amount of defects probed by XPS (7.3 MLor 23.7 �AA [25]) for both surfaces is very similar (the

difference is 3%). However, if we consider a model

in which the defects are located in the first two ML

the difference in the amount of defects between the

two surfaces is about 11%. The information depth

of the UPS technique is about 1.4 ML (4.5 �AA)[25,32,33] and the difference in the amount of de-

fects between both surfaces is 13%, in goodagreement with XPS results. Therefore, UPS and

XPS data show that the difference in the amount

of defects between both surfaces is located mainly

in the first two ML.

3.2. NO on TiO2(1 1 0) surfaces

The stoichiometric single crystal surface showedno interaction with NO at room temperature, even

at the highest exposure tested, 500 L. This result is

consistent with a previous study of NO on

TiO2(1 1 0) surfaces [8,9]. They reported that NO

bonds weakly in a molecular adsorption state and

desorbs at 120 K.

Regarding the bombarded surfaces I and II

(with different amount of defects), we have ob-served that both surfaces behaved in a similar way

upon exposure of NO: Attenuation of Ti3þ and

Ti2þ species and no N features were detected even

after 500 L NO exposure. In Fig. 3, Ti 2p XP

spectra fitted with Gaussian peaks are shown for

surface II at the following conditions: (a) clean, (b)

dosed with 2 L of NO and (c) dosed with 500 L of

NO (open circles are experimental data, solidcurves correspond to Tinþ species, and dotted

curves are satellite peaks of Ti4þ). Ti 2p XP spectra

J. Abad et al. / Surface Science 549 (2004) 134–142 139

for surface I exposed to NO are not shown because

their behavior is similar to that of surface II. As we

can clearly see in Fig. 3, Ti4þ species increase

(while Ti3þ and Ti2þ species decrease) with expo-

sure to NO. The relative Ti concentrations in each

oxidation state following NO adsorption are pre-sented in Table 2. It should be noted that even

after 500 L exposure, suboxides remain about 60%

of their initial clean-surface value, for both sur-

faces.

In order to check the presence of N at the sur-

face, AES measurements were performed because

AES is expected to be more sensitive to adsorbed

N than XPS, partly due to its has lower detectionlimit [34]. Fig. 4 shows Auger Ti LMM and Ti

LMV transitions for clean surface II and after 500

L of NO. The Auger electron emission from N (N

KVV at 379 eV [35]) occurs at an energy that

overlaps the Ti LMM transition, this fact com-

plicates the quantitative aspects of AES studies of

N on TiO2. However, we can qualitatively estimate

the presence of N on the surface by comparing theline shape of Auger spectra before and after

adsorption of NO. Fig. 4 shows that there are no

changes in the line shape of Ti LMM transition

after NO exposure in the N region. The main

deference is in the high-energy side of the Ti LMV

Auger transition, where a decrease in the intensity

is observed. This transition shows a pronounced

sensitivity to detect small amounts of point defects

Fig. 4. Ti LMM and Ti LMV Auger transitions for the clean

surface II and after 500 L of NO.

[13]. Thus, the behavior observed could be attrib-

uted to a depopulation of the Ti 3d electrons after

NO exposure.

In Fig. 5, the valence-band He I and He II UP

spectra for surface II exposed up to 500 L of NO is

shown. Surface I showed a similar behavior (notshown). The largest change in the spectra occurred

at 2 L of NO. The spectra show a significant de-

crease in the signal of the defect band-gap states,

that has not completely disappeared after the

higher dose studied (500 L). This is revealed in

more detail in the inset (a) of Fig. 5. For the first 2

L, there is also an upward band bending of 0.2 eV

(this band bending is lower for surface I) and anincreased emission in the region of 5 eV binding

energy (O 2p nonbonding orbitals), relative to the

clean surface.

Molecular NO in the gas phase is characterized

by 1p and 5r (�15.6 eV) and the 2p (9.3 eV) va-lence orbitals with respect to the vacuum level [36].

After adsorption on the TiO2 surface, these states

are expected to appear at 10.4 and 4.1 eV withrespect to the TiO2 Fermi level (subtraction of

work function 5.2 eV). However, no significant

change in the valence band emission in the vicinity

Fig. 5. UP He I spectra for the surface II exposed to NO. Inset

(a) shows defect band gap states and inset (b) shows UP He II

spectra for the surface II exposed to 500 L of NO.

Fig. 7. Work function change (D/) variation with NO exposure

for the surface II.

140 J. Abad et al. / Surface Science 549 (2004) 134–142

of 10 eV was observed, see Fig. 5. All the results

indicate dissociative adsorption of NO at defects

sites, followed by the incorporation of O into the

surface, oxidation of Ti3þ and Ti2þ cations to Ti4þ

and N going in the gas phase, although from the

present data we can not determine the mechanismfor N desorption.

A summary of the NO adsorption experiments

for surfaces I and II is shown in Figs. 6 and 7. Fig.

6 (upper part) shows the evolution of the O/Ti

ratio (computed from the extent of reduction (d)defined in Section 3.1) as a function of NO expo-

sure for both sputtered surfaces. In the first 10 L

the O/Ti ratio shows a rapid increase, followed bya slower increase which does not lead to the ex-

pected value of 2 even at the highest exposure of

Fig. 6. Upper part, evolution of the O/Ti ratio from XPS as a

function of NO exposure for both sputtered surfaces. Lower

part, evolution of the normalized intensity of UPS He I defect

band gap state with NO exposure for both sputtered surfaces.

Lines are drawn to guide the eye.

the experiment, 500 L Fig. 6 (upper part). The

maximum values obtained at this stage were 1.8and 1.85 for surfaces I and II, respectively. The O/

Ti ratio increased with NO dosage, indicating that

oxygen adsorbs on these surfaces at 300 K. It can

be noticed that the difference between both sur-

faces remains constant. These findings suggest that

the adsorbate was not able to eliminate all the Tidþ

sites in the surface and subsurface probed by XPS

(7.3 ML or 23.7 �AA) and confirms the presence ofsubsurface defects, see below.

Fig. 6 (lower part) shows the area of the Ti 3d

defect states normalized by the area of the Ti 3d

state of surface II as a function of NO exposure,

for both sputtered surfaces. The almost complete

overlap of two curves after 2 L of NO should be

noted. In addition, at 10 L the intensity of the Ti

3d state is reduced down to 30% of the referencevalue (it is initial value for surface II), and at 500 L

it corresponds to 20% of that value for both sur-

faces. These results show that NO was able to

eliminate Ti3þ defects in the UPS penetration

depth (1.4 ML or 4.5 �AA). These results are quali-tatively similar to those reported in the literature

[37] for the adsorption of electronegative mole-

cules such as O2, SO2 and N2O, which can removesurface defects. Defect intensities were greatly re-

duced after exposing the argon defective surface to

the above-mentioned molecules up to about 10 L

exposure. Above this value the reaction became

weaker. For the Ti 3d emission intensity to fall to

J. Abad et al. / Surface Science 549 (2004) 134–142 141

5% of its clean surface value, this requires for

example 107 L of O2 but only 10 L of SO2 [38,39].

However, Rodr�ııguez et al. [40] found that after adose of 10 L of NO2 at 300 K the Ti 2p spectra

resemble a full stoichiometric surface. This differ-

ence between O2, SO2, N2O, NO2 and NO reac-tivity could be explained with the difference in

electron affinity EAO2 ¼ 0:45 eV, EASO2 ¼ 1:11 eV,EAN2O ¼ 0:22 eV, EANO2 ¼ 2:273 eV whereas

electron affinity for NO is 0.026 eV [41]. Since

there is an extra electron associated with each Ti3þ

defect site, it seems reasonable that the interaction

could occur through a charge transfer from the

defect to an electronegative molecule. This elec-tron transfer effect is also observed in the work

function increase. The work function of surface II

relative to the stoichiometric surface (D/) changesas a function of NO exposure, as reported in Fig. 7

(the curve is not shown for surface I because D/change is inside the error bars). D/ shows a

maximum increase of 0.4 eV with NO exposures

indicating that electrons were transferred from thesurface defect sites to the adsorbed species. The

evolution of D/ undergoes saturation at 10 L, but

/ does not recover the value of the stoichiometric

surface even after 500 L NO exposure.

4. Conclusions

The interaction of NO with stoichiometric

(1 · 1) and two kinds of defective TiO2(1 1 0) sur-

face have been studied by XPS, UPS, AES, and

LEED at room temperature. It has been shownthat defects are crucial in the interaction of NO

with TiO2. No reaction was observed on the defect

free TiO2(1 1 0)-(1 · 1) surface exposed up to 500L. On the defective surfaces, the present results

show that NO heals the Ti3þ defect. No N signa-

ture was seen on the surface after defect healing by

NO indicating that this occurs through dissocia-

tion. Above 10 L the reaction becomes quite weak,primarily because there is no longer a significant

number of defect-created Ti3þ cations on top of

the surface with which NO can react. In addi-

tion, NO does not interact with subsurface defects

in the penetration information depth of XPS

(23.7 �AA).

Acknowledgements

The authors acknowledge the support from

Consejo Superior de Investigaciones Cient�ııficas(CSIC) and ESPE Dental AG.

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