Proton polarons in HxWO3 by synchrotron photoemission and

DFT modelling

Emanuel Billeter,1, 2, ∗ Andrea Sterzi,1, ∗ Olga Sambalova,1, 3 Ren

Wick-Joliat,2 Cesare Grazioli,4 Marcello Coreno,5 Yongqiang

Cheng,6 Anibal J. Ramirez-Cuesta,6 and Andreas Borgschulte1, 2, †

1Laboratory for Advanced Analytical Technologies,

Empa - Swiss Federal Laboratories for Material Science and Technology,

berlandstrasse 129, 8600 Dbendorf, Switzerland

2Department of Chemistry, University of Zurich,

Winterthurerstrasse 190, 8057 Zrich, Switzerland

3Department of Chemistry, University of Zrich,

Winterthurerstrasse 190, 8057 Zrich, Switzerland

4IOM-CNR, Laboratorio TASC, Basovizza SS-14, km 163.5, 34149 Trieste, Italy

5ISM-CNR, Instituto di Struttura della Materia, LD2 Unit, 34149 Trieste, Italy

6Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

(Dated: June 24, 2020)

1

arX

iv:2

006.

1311

3v1

[co

nd-m

at.m

trl-

sci]

23

Jun

2020

Abstract

The measurement of hydrogen induced changes on the electronic structure of transition metal

oxides by X-ray photoelectron spectroscopy is a challenging endeavor, since the origin of the pho-

toelectron cannot be unambiguously assigned to hydrogen. The H-induced electronic structure

changes in tungsten trioxide have been known for more than 100 years, but are still being con-

troversially debated. The controversy stems from the difficulty in disentangling effects due to

hydrogenation from the effects of oxygen deficiencies. Using a membrane approach to X-ray photo-

electron spectroscopy, in combination with tuneable synchrotron radiation we measure simultane-

ously core levels and valence band up to a hydrogen pressure of 1000 mbar. Upon hydrogenation,

the intensities of the W5+ core level and a state close to the Fermi level increase following the

pressure-composition isotherm curve of bulk HxWO3. Combining experimental data and density-

functional theory the description of the hydrogen induced coloration by a proton polaron model is

corroborated. Although hydrogen is the origin of the electronic structure changes near the Fermi

edge, the valence band edge is now dominated by tungsten orbitals instead of oxygen as is the case

for the pristine oxide having wider implication for its use as (photo-electrochemical) catalyst.

∗These two authors contributed equally†Electronic address: andreas.borgschulte@empa.ch

2

I. INTRODUCTION

Hydrogen is the ubiquitous element in the environment. The element plays a key role in

biology, chemistry and physics: It is involved in numerous chemical reactions, from photo-

synthesis to the combustion of its products and plays an essential role in corrosion processes.

The fast diffusion of hydrogen in most materials including non-organic matter such as oxides,

makes hydrogen an omnipresent impurity [1]. Due to its ambivalent chemical character, hy-

drogen in matter may be present as proton, neutral atom, or as anion. As a consequence of

its atomic number, hydrogen has only one electron. Particularly this property is a challenge

for many analytical tools based on the interaction with electrons: core-level spectroscopies

such as X-ray photoelectron spectroscopy (XPS) cannot be used as a quantitative method for

hydrogen, because a photo-emitted electron cannot be assigned to a hydrogen atom without

any doubt, H+ (e.g. OH) has no electrons, H− has two, and in covalent bonds the H electron

has a high probability density between the binding atoms, i.e., the location of hydrogen elec-

trons depends on the electronic structure of the material. The electronic structure, though,

is the key to understand the physical and chemical properties of matter, and photoemission

spectroscopy is the standard experimental method to unravel it. Obviously, materials, in

which the electronic structure is decisively influenced by hydrogen are particularly difficult.

The archetypical example of this materials class is hydrogen intercalated tungsten trioxide.

Already in the 19th century, Berzelius noticed a color change when hydrogen gas was passed

over tungsten trioxide [2]. Similar reversible optical and electrical changes are observed upon

electrochemical insertion of alkali metals into WO3 [3]. The discovery of the electrochromic

properties of thin WO3 films led to the development of a number of applications such as

smart windows, displays and variable mirrors [4, 5]. As the above discussed constraints for

hydrogen do not apply here, the electronic structure of alkali-metal intercalated tungsten

bronzes, e.g. NaxWO3, are well characterized [6, 7]. Together with electronic structure

modeling [8, 9], there is consensus about the underlying physical phenomena of the chem-

ical modification induced by alkali-metal intercalation [10]. In short, the electron of the

alkali-metal forms a new state in the band gap (conducting phase), but the phase remains

insulating due to Coulomb interactions. In NaxWO3, a metal-insulator transition occurs at

x = 0.24; similar behavior is found for the other alkali-metal intercalated tungsten bronzes

[9, 11]. Although intuitively similar, the changes taking place during hydrogen intercalation

3

are different and controversially debated. Simplified, there are two models: hydrogen is in-

tercalated into WO3 and remains there, most likely as hydroxide; the corresponding electron

affects the valence band similar to the ones originating from alkali-metals [8]. The second

model proposes that hydrogen can form water with oxygen from WO3, and leave the now

substoichiometric crystal [12]. The corresponding oxygen vacancies are filled with electrons.

The loss of oxygen from the lattice induces distortions, leading to the localization of the

electrons, that are described by an electron polaron model. Summarizing, the origin of the

controversy stems from the inability to detect and characterize hydrogen in the oxide.

In contrast to the electronic structure, the crystal structures of hydrogen and alkaline

metal intercalated WO3 are well studied. Most alkaline metal bronzes have a perovskite

structure, where the alkaline metal occupies the central position [3]. The structure deter-

mination of hydrogen intercalated WO3 was determined by as shown by X-ray diffraction

(XRD) and neutron diffraction on deuterium analogs [13, 14]. It revealed a distorted cubic

structure, where the hydrogen occupies a position 1.1 A from the oxygen along the diagonal

through the central position. This was supported by DFT calculations on cubic WO3 and

HWO3 finding the minimum energy position of hydrogen in the WO3 lattice at a distance

of 1.03 A from the oxygen atoms [8]. Electronic structures have been calculated for hy-

drogen and alkali metal intercalation explaining the observed changes at high intercalant

concentration [8, 15].

Experimentally, the intercalation process can be performed in two different ways. Elec-

trochemical hydrogen insertion is relatively facile, but the electrochemical surface changes

due to the aqueous environment hinder photoemission measurements. Gaseous hydrogen in-

tercalation into WO3 does not change the surface but is feasible only at UHV-incompatible

pressures and in the presence of dissociatively active sites [16], limiting photoemission ex-

periments to the study of post-mortem samples.

In this study, we present in-situ photoemission data obtained by the membrane pho-

toemission approach [17]: we employ a sample holder that enables varying the hydrogen

pressure up to one bar while keeping the tungsten oxide thin film under UHV conditions

necessary for photoelectron spectroscopy experiments [17] (see Fig. 1). This allows to mea-

sure the pressure-composition isotherm of WO3 by photoelectron spectroscopy. Synchrotron

light enables probing of oxygen and tungsten core levels and the valence band states as a

function of hydrogen content. The operando approach facilitates interpretation, as effects

4

FIG. 1: Sketch of the membrane XPS sample holder and the experimental setup. The enlarged

area shows the hydrogen permeation and the hydrogen chemical potential though the membrane.

from unavoidable substoichiometries of the sample and other experimental uncertainties

can be separated from the effects expected from hydrogen intercalation. Furthermore we

compare the photoemission results with DFT calculations, supporting the interpretation of

experimental results in light of a polaron model.

II. EXPERIMENTAL

A. Sample Preparation

WO3 was deposited on palladium by electrodeposition from a H2WO4 solution. The

H2WO4 solution was prepared in a round bottom flask by suspending 0.92 g tungsten

powder (99.9 %, Sigma-Aldrich) in 7.5 ml water. The suspension was heated to 60 C

and 3.5 ml H2O2 (30 %, Merck) were added. Tungsten was oxidized to tungstic acid

accompanied with vigorous gas formation (H2) and after 2 minutes a colorless, clear

solution was obtained. A platinum wire was added to decompose the excess of H2O2

until no more oxygen bubbles evolved, which took 8 hours at 60 C. A final tungstic acid

5

concentration of 50 mM was obtained by adding 50 ml of isopropyl alcohol and filling up

with water to a total volume of 100 ml. Electrodeposition was performed in a two electrode

setup. A 2x2 cm foil of Pd with 47 µm thickness (purity 99.95 %, Goodfellow) with its

backside covered with Kapton tape served as working electrode and a Pt wire as counter

electrode. Since the WO3 deposition on Pd competes with H2 formation, relatively high

current densities of approximately -2 mA/cm2 were applied in chronopotentiometry mode

for 10 minutes, which resulted in an amorphous blue film of approximately 30 nm thickness,

as determined by XRD and EDX respectively. The sample was then dried in air on a hot

plate at 200 C for 10 minutes.

B. Membrane Photoemission

The high pressure XPS study reported here is based on the Pd membrane approach that

we previously presented [18]. The photo-electron spectroscopy measurements were carried

out at the GasPhase beamline of the Elettra Synchrotron light source in Trieste (Italy). The

beamline is equipped with a dedicated differential pumping system [19]. The combination of

a high resolution monochromator with the high transmission of the electron analyzer allowed

to collect spectra with a minimal energy resolution of 50 meV [20].

Briefly, the idea behind the embedded membrane approach is the following. The sample

holder sketched in Fig. 1 consists of a hydrogen permeable Pd membrane which is exposed at

one side to high hydrogen pressure (up to 1 bar) and at the other side to UHV (10−8 mbar).

Hydrogen is fluxed at the high pressure side of the membrane and, following permeation,

it reaches the surface exposed to vacuum in the form of atomic hydrogen. The tungsten

oxide film, which is deposited on the analyzed side of the membrane, is thereby hydrogenated.

Photoemission measurements require this system to be in a pressure-temperature equilibrium

state. In order to realize that, both the gas pressure and the membrane temperature were

adjusted. The membrane was heated up to 160C by means of a heating filament and the

hydrogen flux was controlled through a valve and a pressure gauge. Pressure-dependent

spectra were obtained as follows. At a fixed temperature, hydrogen was constantly fluxed

inside the sample holder. Depending on the temperature, an equilibrium condition is reached

when the amount of injected gas compensates the desorption process which occurs at the

6

UHV side of the membrane [18]. When this steady state had been reached, the pressures

at the two sides of the membrane are stable and the measurement can be performed. The

hydrogen amount inside the experimental chamber and hence the desorption process were

monitored by means of a quadrupole residual gas analyser. To prepare the sample surface

argon ion-sputtering and annealing were performed in-situ.

All experiments described herein were performed with a photon energy of 104 eV, al-

lowing for the simultaneous detection of tungsten 4f core levels and the valence band. The

photoemission spectra were recorded continuously in single sweep mode, with each spectrum

taking approximately 120 s. The applied hydrogen pressure was measured by an external

pressure gauge. As soon as the partial hydrogen pressure in the chamber was constant,

the external hydrogen pressure was increased. This lead to approximately ten spectra be-

ing recorded for each external hydrogen pressure. Data analysis was performed using the

CasaXPS software, employing the GL(30) line shape, Shirley background subtraction and

a 2.1 eV spin orbit splitting for both W5+ and W6+ doublets [21]. The binding energy

calibration was per-formed by shifting the W6+ 4f 7/2 peak to 35.8 eV.

C. Electronic structure calculation

Calculation of electronic structure by Density Functional Theory (DFT) and hybrid-

DFT was performed using the Vienna Ab initio Simulation Package (VASP) [22–25]. The

calculation used Projector Augmented Wave (PAW) method [26, 27] to describe the effects of

core elec-trons, and Perdew-Burke-Ernzerhof (PBE) [28] implementation of the Generalized

Gradient Approximation (GGA) for the exchange-correlation functional. Energy cutoff was

600 eV for the plane-wave basis of the valence electrons. The electronic structure was

calculated on a 15x15x15 Γ-centered mesh for WO3 (unit cell), and a 4x4x4 Γ-centered mesh

for HxWO3 (2x2x2 supercell). The total energy tolerance for electronic energy minimization

was 10−6 eV, and for structure optimization it was 10−5 eV. The maxi-mum interatomic force

after relaxation was below 0.01 eV/. After structural optimization, hybrid-DFT calculation

of band structure was performed using the HSE06 hybrid functional [29, 30] with a mixing

parameter of 25% and a screening parameter of 0.2 A−1.

Hydrogen free WO3 crystallizes in the monoclinic structure with space group P21/n with

a = 7.304 A, b = 7.536 A, c = 7.691 A and β = 90.85 [31]. Other polymorphs exist

7

depending on temperature [32]. However, intercalation of hydrogen leads to the formation

of cubic structures of HxWO3 for x = 0.5.[14] We thus simplified the calculations using the

cubic ReO3 structure where the metal is surrounded by six oxygen atoms in a octahedron

(see Fig. 3) for all compositions including hydrogen free WO3. With this constraint, the

calculated electronic structure is that of an artificial structure with slight deviations from

reality. A simple quality parameter is the optical gap [15, 33]. The calculated direct band

gap of simple cubic WO3 with around 2.3 eV match the experimental values of 2.6 to 3.3 eV

for the direct band gap.[12, 33, 34] This difference is due to the general underestimation of

the optical gap by GGA and WDA methods, [35] with WO3 being particularly notorious [36].

Furthermore, calculations of the gap of the simple cubic phase yield smaller gaps than the

ones calculated for the monoclinic phase of bulk tungsten trioxide.[8, 15, 37] Values vary from

0.69 eV (RPBE) and 2.25 eV (PBE8) for the simple cubic system to 1.3 eV (RPBE) and 3.68

eV (PBE8) for the monoclinic system with functional indicated in brackets [36]. However,

the simplicity of a simple cubic system allows the modelling of the partially hydrogenated

bronzes HxWO3 with x = 0, 0.25, 0.75, and x = 1.0. For x = 0.25, two hydrogens with

anti-parallel spin configuration are placed in a simple cubic 8-fold unit cell of WO3 (see

Fig. 3). Three different hydrogen constellations were calculated. The difference between

these constellations turned out to be negligible (not shown). The hydrogen and oxygen

positions were released in the subunit to find the H-O position with minimum total energy.

We found an optimum for dO−H = 1.03A, in good agreement with literature.[8] In addition,

the oxygen tetrahedrons were distorted (Fig. 3). Similar calculations were performed for

H0.75WO3 using 6 hydrogens per eight-fold unit cell. For better comparison, the calculated

total density of electron states are broadened by 0.5 eV to match the experimental resolution

of the photoemission spectroscopy (see Fig. 4).

III. RESULTS

Hydrogen intercalation was probed at room temperature and slightly elevated tempera-

tures (160 C). Figure 2 shows a typical photoemission spectrum of the W4f core levels. By

peak fitting, we extract the oxidation states of tungsten, here W5+ and W6+ [38]. There is

no evidence of W4+ in any of the W4f core level spectra, as has been proposed elsewhere

[12]. At room temperature, core level as well as valence band spectra did not show signif-

8

FIG. 2: (a) A typical W4f core level spectrum is shown in green where two pairs of spin-orbit

doublets are fitted in gray and purple respectively. The black dashed line represents the sum of the

fitted peaks. (b) The fraction of W5+ is shown as a function of time (black dots) and compared to

the applied hydrogen pressure (red dots). It was obtained from the peak intensities as extracted

from normalized and background subtracted W4f spectra (inset). The linear fits (dashed lines)

show the changes in W5+ fraction during increasing hydrogen pressure and in the static pressure

regime, respectively.

icant changes upon hydrogen pressure exposure due to slow kinetics. Berzins et al. report

faster reversible hydrogen exchange at elevated temperatures [16]. We therefore focus on the

changes taking place at 160 C. The fraction of W5+ is shown as a function of time (spectrum

number), during which the hydrogen pressure increased from 0 to 1000 mbars (Fig. 2). The

concentration of hydrogen in WO3 is proportional to the square root of the applied hydro-

gen pressure [16]. This allows us to estimate the hydrogen content during photoemission

experiments. The change rate of the W5+/W6+ ratio slows drastically when the hydrogen

pressure is kept constant for a longer period of time. It should be noted that the initial

W5+ to W6+ ratio is not zero, meaning that the sample had already been partially reduced.

The presence of W5+ before hydrogenation is due to the fact that amorphous WO3 thin

films such as ours tend to be substoichiometric (i.e WO3−δ). This oxygen deficiency leads

to the formation of W5+ pairs [39]. Even in crystalline bulk WO3, oxygen vacancies cannot

be avoided [40]. The corresponding experimental difficulty is overcome by our experimental

setup, as we probe the changes of the electronic structure as a function of hydrogen content,

and modifications due to specific preparation details are canceled out.

The measurements of valence band spectra were performed in parallel to the core-levels,

with a binding energy between 34-40 eV (Fig. 2). However, the actual hydrogen content

9

cannot be easily quantified. The H1s state is both core-level as well as valence state, and

thus cannot be directly used for quantification. This problem requires a more in-depth

analysis of the valence band spectra with the help of DFT calculations.

DFT calculations were performed on simple cubic WO3, H0.25WO3 and H0.75WO3 to

illustrate the effects of hydrogenation on the electronic structure. WO3 exhibits typical

features of a semiconductor with a direct band gap of about 2.3 eV and an indirect one of

1.3 eV as derived from band structure calculations (see also Fig. 3). As discussed in section

II C, the gap is underestimated (experimental gap of monoclinic WO3 is 3.2 eV [12, 34]) due

to the assumption of a cubic lattice, which simplifies the calculation of hydrogen intercalated

WO3. However, the main result of the valence band edge dominated by oxygen orbitals is

in perfect agreement with literature (Fig. 3). The intercalation of hydrogen into WO3 was

modelled by placing the hydrogens in the center of the WO3 subunit and optimizing the

position of the hydrogens simple cubic structure by energy minimization. Minimum was

found with hydrogen occupying a position near one oxygen site with a distance of 1.04

A identical to the calculations of Hjelm et al. [8], and very similar to the bond length of an

OH ion (Fig. 4). Simultaneously the lattice is slightly distorted. The presence of hydrogen

accompanied by the lattice distortion leads to two main changes in the electronic (see Fig.

3): the valence band broadens due to the bonding of oxygen with hydrogen (bonding states

around 9 eV) and, as one extra electron per hydrogen is brought into the system, EF is

shifted into the conduction band mainly consisting of consist of mainly tungsten states.

This means that the valence band edge is now dominated by tungsten orbitals. Electronic

conduction is now equal to moving one electron originating from one tungsten atom to

the neighbor one with various consequences (see Fig. 3 and discussion later on). However,

the aim of this paper is to confirm these effects experimentally. As we do not start at

zero hydrogen content in WO3, we calculated the electronic structures of H0.25WO3 and

H0.75WO3 to mimick increasing hydrogen content without effects of symmetry changes. The

effects of going from H0.25WO3 to H0.75WO3 are less drastic (see Fig. 4). Most striking is the

increasing hydrogen - oxygen bonding states and corresponding broadening of the oxygen

states, which leads to a decrease of DOS near 3 eV binding energy. Second effect most

relevant to this paper is the increase of tungsten states near EF with increasing hydrogen

content. These two changes match the experimental spectra exceptionally well (Fig. 4). In

10

FIG. 3: Top: Illustration of the cubic crystal structure of WO3 and of one polymorph of H0.25WO3

as used for modelling. As an example, tungsten atoms (grey) exclusively surrounded by oxygen

atoms (blue atoms) are highlighted with green corona, the one surrounded by five oxygens and one

OH group (hydrogen in red) is highlighted with a purple corona. Moving an electron from this

W5+ atom to a W6+ as indicated by the purple arrow includes crystallographic rearrangements.

Bottom: Modelled partial and total DOS of cubic WO3 (black lines) and HxWO3 (red line) with

hydrogen concentrations as small as 0.25. EG = 1.3 eV is the indirect band gap in WO3 derived

from the onsets of valence and conduction bands. Full band structure calculations give the direct

band gap of 2.3 eV (not shown).

particular, the calculations confirm the existence of hydrogen induced states near EF , which

are associated to tungsten states.

The normalized peak area at 0.4 eV binding energy scales with pressure (Fig. 5). It

is important to note that it does not evolve any further with time at constant pressure,

which excludes its origin induced by radiation damage and confirms that the measured state

is in thermodynamic equilibrium with hydrogen gas. With this, the hydrogen content of

the thin films can be derived by comparison with the bulk pressure-composition isotherm

11

FIG. 4: Top panel: Calculated partial and total DOS of H0.25WO3 (black) and H0.75WO3 (red).

Bottom panel: Measured photoemission spectra of minimum and maximum hydrogen content in

black and red. The hydrogen contents as labelled are derived from the peak intensities of the peak

evolving with hydrogen pressure as shown in the inset (see Fig. 5 and text for more information).

of Pt:WO3 reported in literature [16]. From the overlay of both data (bulk H content and

photoemission), we extract that the intial hydrogen content in our thin films is H0.1WO3

increasing to H0.25WO3 at 1000 mbar.

IV. DISCUSSION

The similarity of the optical absorption in crystalline HxWO3 to that in amorphous

HxWO3 indicates that the crystallinity plays only a minor role for electronic structure

12

FIG. 5: The area of the peak at 0.4 eV binding energy (see Fig. 4b) as a function of time (black)

correlates with the applied hydrogen pressure (red) and is compared to hydrogen content (blue

triangles) extracted from bulk pressure composition isotherms reported in literature (see insert,

from [16]).

changes upon hydrogenation [40]. The optical absorption band coincides with a peak near

to the Fermi level as detected by photoelectron spectroscopy [41]. Hollinger et al. state

that coloration in amorphous tungsten oxide films is connected to an increase of localized

electronic states (W5+), but could not disentangle the origin of the effect to be from oxygen

defects caused by UV-exposure or from hydrogen intercalation.

Our measurements confirm most of the literature data on photoemission in HxWO3, in

particular the emergence of the peak near to the Fermi level [21, 38, 41, 42]. However,

due to the experimental procedure we can exclude that the peak is due to UV irradiation.

Although crucial for photoemission, prolonged exposure to the photons used to excite the

photoelectrons (104 eV) did not result in significant changes, compared to the changes

observed upon hydrogenation (Figs. 2 and 5).

However, attributing this peak to hydrogen is too short-viewed: as already outlined in

the results section (Fig. 3b), DFT assignes this peak to originate from tungsten electronic

orbitals. This result can also be experimentally rationalized: there are 3 · 6 O2p states (we

neglect here the small contribution of W5d), and at most 0.3 hydrogens. In addition, we

13

have to consider the photoionization cross section of electrons localized on hydrogen and

oxygen of 0.02 Mb and 1 Mb, respectively at around 104 eV [43]. The expected intensity

ratio is 0.016 · 0.02 = 3.2 ·10−4. It is thus unlikely that the detected photoelectrons stem

from hydrogen in good agreement with the results from DFT.

However, conductivity and optical experiments evidence that hydrogen intercalated WO3

is not an electronic conductor at small hydrogen content as suggested by the calculated DOS

(Fig. 3b): there are definitively states at EF . This discrepancy was already found in the past,

and explained by correlation models.[44] We depict the electronic structure using the simple

model of in-gap states depending on hydrogenation assuming that the overall electronic

structure is predominantly given by WO3. Hydrogen intercalation gives:

W6+O2–3

HW5+(OH)–O2–

2 (1)

The movement tungsten-state like electron is correlated with the hopping of the proton,

which requires crystallographic rearrangements as the oxygen orientations around the tung-

sten atoms are slightly different (see Fig. 3). Thus, despite having electrons near the Fermi

level (Fig. 4) HxWO3 does not exhibit metallic character (no Drude absorption [12]). Cran-

dall and Faughnan [44] find a metal-insulator transition around hydrogen concentrations of

approximately 0.3, which is close to that predicted by percolation theory. At the percolation

threshold, the hopping between the tungsten will form a continuous chain throughout the

crystal.

Further evidence for the localization of these electrons was provided by optical absorption

spectroscopy where amorphous HxWO3 exhibits an absorption maximum in the near infrared

range [4, 45]. The optically induced jumping of the electron from one W site (Wa) to the

other (Wb) is a special charge transfer excitation, which is well described by the so-called

polaron model [12]:

W5+a (OH)+W6+

b O~ω

W6+a O+W5+

b (OH) (2)

In contrast to Saenger et al., who attribute the corresponding interaction of this electron

with the surrounding distorted oxygen deficient lattice, we propose the interaction to be

between the proton and the electron on W5+. This so-called proton polaron has been pro-

posed for describing the proton transport in the similar system hydrated BaCe0.8Y0.2O3−δ

[46]. The general underlying principle, the required crystalline rearrangements linked to

14

charge transfer (”move of distortions”, see Fig. 3), remains the same. The metal oxygen

rearrangement proceeds through vibrations of the WO6 octahedra and O-H vibrations. The

mobility of protons may thus be in both cases a result of phonon-assisted jumps.

The main outcome of this study is the change of the electronic structure near the Fermi

edge from from oxygen dominated orbitals to tungsten dominated orbitals (see Fig. 3), which

has drastic consequences on (electro-) catalysis of oxygen related reactions in addition to

the more physical properties electron conductivity and optical properties as discussed above.

WO3 is one of the oxides considered to be a suitable photo electro-chemical water splitting

catalysts [47]. In particular, it shows a high activity for water oxidation to oxygen in the

presence of a suitable electron acceptor [47, 48]. Hydrogen treatment of WO3 can increase

photocurrent significantly [48, 49]. The authors interpreted the increase to the formation of

oxygen vacancies, and thus defect states. With our findings, we cannot exclude the existence

of oxygen defect states in parallel to proton polarons. However, the more pronounced effect

of hydrogen treatment than vacuum annealing [48, 49] suggests that generally hydrogen

related states are the main origin of the effect.

Similarly, the hydrogen induced electronic structure near the Fermi edge explains the

rather small effect on superconductivity in tungsten oxides, if compared to metal doping

[50, 51]. If hydrogen vibrations with H-PDOS at EF contributed to the superconductivity,

a strong positive effect may be expected. However, the superconductivity in bulk WO3 is

currently understood to originate from a weak-coupling state sustained by soft vibrational

modes of the WO6 octahedra [51], and the formation of OH instead creation of oxygen

vacancies upon hydrogenation is in line of this argumentation.

V. CONCLUSIONS

We measured the palladium assisted hydrogenation of tungsten trioxide by operando

membrane XPS at hydrogen pressures up to 1 bar. The combination of the membrane XPS

with synchrotron radiation allows to measure both core levels and valence band simultane-

ously up to high hydrogen pressures minimizing the effect of initial preparation parameters,

beam damage and exposure to the reducing UHV environment that would occur with con-

secutive XPS/UPS measurements. Analysis of the tungsten 4f core levels shows an increase

15

of W5+ with increasing hydrogen pressure. At the same time a state appears close to the

Fermi level, that is connected to the hydrogen content in the compound. Using previous

reference measurements it is possible to create a pressure-composition isotherm from the

spectroscopy data. The photoemission measurements together with DFT calculations cor-

roborate that the coloration of the films by hydrogen can be explained by a proton polaron

model.

Acknowledgments

This work was partly supported by the UZH-UFSP program LightChEC. Financial sup-

port from the Swiss National Science Foundation (grant number 172662) is greatly acknowl-

edged. We thank Dr. C. Puglia (Uppsala University, Sweden) and the Carl Tygger Foun-

dation for the availability of VG Scienta’s SES-200 photoelectron analyzer at the GasPhase

beamline.

[1] J. Volkl and H. Wipf, Hyperfine Interact. 8, 631 (1981).

[2] F. Wohler, Ann. Phys. 78, 345 (1824), ISSN 00033804, URL http://doi.wiley.com/10.

1002/andp.18240781202.

[3] E. O. Brimm, J. C. Brantley, J. H. Lorenz, and M. H. Jellinek, J. Am. Chem. Soc. 73, 5427

(1951), ISSN 0002-7863, URL https://pubs.acs.org/doi/abs/10.1021/ja01155a121.

[4] S. K. Deb, Philos. Mag. 27, 801 (1973), ISSN 0031-8086, URL http://www.tandfonline.

com/doi/abs/10.1080/14786437308227562.

[5] C. G. Granqvist, Sol. Energy Mat. Sol. C. 60, 201 (2000), ISSN 0927-0248, URL https:

//www.sciencedirect.com/science/article/pii/S0927024899000884.

[6] H. Hochst, R. Bringans, H. Shanks, and P. Steiner, Solid State Commun. 37, 41

(1981), ISSN 0038-1098, URL https://www.sciencedirect.com/science/article/pii/

003810988190884X.

[7] S. Raj, T. Sato, S. Souma, T. Takahashi, D. D. Sarma, and P. Mahadevan, Mod. Phys. Lett.

B 23, 2819 (2009), ISSN 02179849.

[8] A. Hjelm, C. G. Granqvist, and J. M. Wills, Phys. Rev. B 54, 2436 (1996), ISSN 0163-1829,

16

URL https://link.aps.org/doi/10.1103/PhysRevB.54.2436.

[9] A. L. Larsson, B. E. Sernelius, and G. A. Niklasson, Solid State Ionics 165, 35 (2003), ISSN

01672738.

[10] C. G. Granqvist, in Handbook of inorganic electrochromic materials (Elsevier, 1995), chap. 2,

p. 633, ISBN 9780444899309.

[11] H. Hochst, R. D. Bringans, and H. R. Shanks, Phys. Rev. B 26, 1702 (1982), ISSN 0163-1829,

URL https://link.aps.org/doi/10.1103/PhysRevB.26.1702.

[12] M. F. Saenger, T. Hoing, B. W. Robertson, R. B. Billa, T. Hofmann, E. Schubert, and

M. Schubert, Phys. Rev. B 78, 245205 (2008), ISSN 1098-0121, URL https://link.aps.

org/doi/10.1103/PhysRevB.78.245205.

[13] P. G. Dickens and R. J. Hurditch, Nature 215, 1266 (1967), ISSN 0028-0836, URL http:

//www.nature.com/articles/2151266a0.

[14] P. Wiseman and P. Dickens, J. Solid State Chem. 6, 374 (1973), ISSN 0022-4596, URL https:

//www.sciencedirect.com/science/article/pii/0022459673902259.

[15] G. A. de Wijs and R. A. de Groot, Phys. Rev. B 60, 16463 (1999), ISSN 0163-1829, URL

https://link.aps.org/doi/10.1103/PhysRevB.60.16463.

[16] A. R. Berzins and P. A. Sermon, Nature 303, 506 (1983), ISSN 0028-0836, URL http:

//www.nature.com/articles/303506a0.

[17] R. Delmelle, B. Probst, R. Alberto, A. Zuttel, D. Bleiner, and A. Borgschulte, Rev. Sci. Instr.

86, 053104 (2015), ISSN 10897623.

[18] O. Sambalova and A. Borgschulte, J. Alloy. Compd. 742, 518 (2018), ISSN 09258388, URL

https://doi.org/10.1016/j.jallcom.2018.01.160.

[19] R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, R. Krempaska, J. Slezak, K. Prince,

R. Richter, M. Vondracek, R. Camilloni, et al., J. Electron Spectrosc. 101-103, 959

(1999), ISSN 0368-2048, URL https://www.sciencedirect.com/science/article/pii/

S0368204898003818.

[20] C. Grazioli, Ph.D. thesis, Universita degli Studi di Trieste (2017).

[21] O. Bouvard, A. Krammer, and A. Schuler, Surf. Interface Anal. 48, 660 (2016), ISSN 01422421,

URL http://doi.wiley.com/10.1002/sia.5927.

[22] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993).

[23] G. Kresse and J. Hafner, Phys. Rev. B 49, 14251 (1994).

17

[24] G. Kresse and J. Furthmller, Comput. Mat. Sci. 6, 15 (1996).

[25] G. Kresse and J. Furthmller, Phys. Rev. B 54, 11169 (1996).

[26] P. E. Blochl, Phys. Rev. B 50, 17953 (1994).

[27] G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (199).

[28] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996).

[29] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003).

[30] A. V. Krukau, O. A. Vydrov, A. F. Izmaylov, and G. E. Scuseria, J. Chem. Phys. 125, 224106

(2006).

[31] E. Lassner and W.-D. Schubert, Tungsten Properties, Chemistry, Technology of the Element,

Alloys, and Chemical Compounds (Springer US, New York, 1999), 1st ed., URL http://link.

springer.com/10.1007/978-1-4615-4907-9_10.

[32] H. A. Wriedt, Bulletin of Alloy Phase Diagrams 10, 368 (1989), ISSN 01970216.

[33] P. P. Gonzlez-Borrero, F. Sato, A. N. Medina, M. L. Baesso, A. C. Bento, G. Baldissera,

C. Persson, G. A. Niklasson, C. G. Granqvist, and A. Ferreira da Silva, Applied Physics

Letters 96, 061909 (2010).

[34] M. Green and Z. Hussain, J. Appl. Phys. 69, 7788 (1991).

[35] K. Xiong, J. Robertson, and S. J. Clark, J. Appl. Phys. 102, 083710 (2007).

[36] F. Wang, C. Di Valentin, and G. Pacchioni, J. Phys. Chem. C 115, 8345 (2011).

[37] F. Cora, A. Patel, N. M. Harrison, R. Dovesi, and C. R. A. Catlow, J. Am. Chem. Soc. 118,

12174 (1996).

[38] B. A. De Angelis and M. Schiavello, J. Solid State Chem. 21, 67 (1977), ISSN 1095726X.

[39] G. A. de Wijs and R. A. de Groot, Electrochim. Acta 46, 1989 (2001), ISSN 0013-4686, URL

https://www.sciencedirect.com/science/article/pii/S0013468601003772.

[40] O. F. Schirmer and E. Salje, J. Phys. C Solid State 13, L1067 (1980), ISSN

0022-3719, URL http://stacks.iop.org/0022-3719/13/i=36/a=005?key=crossref.

8bdd77b697d629d1b05b177c9e545f82.

[41] G. Hollinger, T. Minh Duc, and A. Deneuville, Phys. Rev. Lett. 37, 1564 (1976), ISSN 0031-

9007, URL https://link.aps.org/doi/10.1103/PhysRevLett.37.1564.

[42] F. Bussolotti, L. Lozzi, M. Passacantando, S. La Rosa, S. Santucci, and L. Ottaviano, Surf.

Sci. 538, 113 (2003), ISSN 0039-6028, URL https://www.sciencedirect.com/science/

article/pii/S0039602803006964.

18

[43] J. J. Yeh and I. Lindau, Atom. Data. Nucl. Data 32, 1 (1985).

[44] R. S. Crandall and B. W. Faughnan, Phys. Rev. Lett. 39, 232 (1977), URL https://link.

aps.org/doi/10.1103/PhysRevLett.39.232.

[45] V. Wittwer, O. F. Schirmer, and P. Schlotter, Solid State Commun. 25, 977 (1978), ISSN

00381098.

[46] A. Braun and Q. Chen, Nat. Commun. 8, 15830 (2017), ISSN 2041-1723, URL http://www.

nature.com/articles/ncomms15830.

[47] S. S. Kalanur, L. T. Duy, and H. Seo, Top. Catal. 61, 10431076 (2018)).

[48] J. Yan, T. Wang, G. Wu, W. Dai, N. Guan, L. Li, and J. Gong, Adv. Mat. 27, 1580 (2015).

[49] T. Singh, R. Mller, J. Singh, and S. Mathur, Applied Surface Science 347, 448 (2015).

[50] S. Reich, G. Leitus, R. Popovitz-Biro, A. Goldbourt, and S. Vega, J. Supercond. Nov. Magn.

22, 343346 (2009).

[51] C. Pellegrini, H. Glawe, and A. Sanna, Phys. Rev. Materials 3, 064804 (2019), URL https:

//link.aps.org/doi/10.1103/PhysRevMaterials.3.064804.

19