# Hydrogen in tungsten trioxide by membrane photoemission and DFT modelling

Emanuel Billeter  Laboratory for Advanced Analytical Technologies, Empa -
Swiss Federal Laboratories for Materials Science and Technology,
Überlandstrasse 129, 8600 Dübendorf, Switzerland  Department of Chemistry,
University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
Andrea Sterzi  Laboratory for Advanced Analytical Technologies, Empa - Swiss
Federal Laboratories for Materials Science and Technology, Überlandstrasse
129, 8600 Dübendorf, Switzerland  Olga Sambalova  Laboratory for Advanced
Analytical Technologies, Empa - Swiss Federal Laboratories for Materials
Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Department of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057
Zürich, Switzerland  René Wick-Joliat  Department of Chemistry, University of
Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland  Cesare Grazioli
IOM-CNR, Laboratorio TASC, Basovizza SS-14, km 163.5, 34149 Trieste, Italy
Marcello Coreno  ISM-CNR, Istituto di Struttura della Materia, LD2 Unit, 34149
Trieste, Italy  Yongqiang Cheng  Neutron Scattering Division, Oak Ridge
National Laboratory, Oak Ridge, TN 37831, USA  Anibal J. Ramirez-Cuesta
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN
37831, USA  Andreas Borgschulte<EMAIL_ADDRESS>Laboratory for
Advanced Analytical Technologies, Empa - Swiss Federal Laboratories for
Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf,
Switzerland  Department of Chemistry, University of Zürich,
Winterthurerstrasse 190, 8057 Zürich, Switzerland

###### 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 no photoelectron can 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 controversially 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 photoelectron spectroscopy, in combination with tunable
synchrotron radiation we measure simultaneously 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 a (photo-
electrochemical) catalyst.

###### pacs:

Valid PACS appear here

##  I Introduction

Hydrogen is an ubiquitous element in the environment. The element plays a key
role in biology, chemistry and physics: It is involved in numerous chemical
reactions, from photosynthesis 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 polyvalent chemical character, hydrogen
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 formally 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 electrons 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 to analyze. The archetypal
example of this materials class is hydrogen intercalated tungsten trioxide,
HxWO3. 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]. With the help of electronic structure modeling [8, 9],
there is consensus about the underlying physical phenomena of the chemical
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 initially 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 [11, 9]. Although intuitively similar, the changes taking place during
hydrogen intercalation are different and controversially debated. Simplified,
there are two models: hydrogen is intercalated into WO3 and remains there,
most likely as hydroxide; the corresponding electron affects the valence band
in a similar way as the ones originating from alkali-metals [8, 12]. The
second model proposes that hydrogen can form water with oxygen from WO3, and
leave the now substoichiometric crystal [13]. 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 [14]. 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 of hydrogen intercalated WO3 was
determined by X-ray diffraction (XRD) and neutron diffraction on deuterium
analogs [15, 16]. It revealed a distorted cubic structure, where the hydrogen
occupies a position 1.1 Å 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 Å from the oxygen atoms [8]. Electronic structures have been
calculated for hydrogen and alkali metal intercalation explaining the observed
changes at high intercalant concentration [8, 17].

Experimentally, the intercalation process can be performed in two different
ways. Electrochemical hydrogen insertion is relatively facile, but the
electrochemical surface changes due to the aqueous environment hinder
photoemission measurements. Gaseous hydrogen intercalation into WO3 does not
change the surface but is feasible only at UHV-incompatible pressures and in
the presence of dissociatively active sites [18], limiting photoemission
experiments to the study of post-mortem samples.

In this study, we present in-situ photoemission data obtained by the membrane
photoemission approach [19]: 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 [19]
(see Fig. 1). This allows to measure 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 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.

Figure 1:  Sketch of the membrane XPS sample holder and the experimental
setup. The enlarged area shows the hydrogen permeation and the hydrogen
chemical potential through the membrane. At steady state, the chemical
potential at the feed side of the membrane is nearly equal to that at the
vacuum side allowing the measurement of hydrogen concentrations in the
material corresponding up to atmospheric pressures by surface science
techniques [19, 20].

##  II  Experimental

###  II.1 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 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 $\mu$m 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.

###  II.2  Membrane sample holder

The high pressure XPS study reported here is based on the Pd membrane approach
that we previously presented [20]. The photoelectron 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 [21]. 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 [22].

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. Since the hydrogen permeation in the palladium membrane is much
faster than in tungsten oxide, the oxide layer blocks hydrogen permeation.
This creates a constant hydrogen concentration, and therefore a constant
chemical potential, in the palladium membrane [19]. It is thereby possible to
control the chemical potential of hydrogen at the UHV side of the membrane by
the applied hydrogen pressure (see Fig. 1). The validity of the assumption of
the equality of chemical potentials is experimentally evidenced by the
permeation kinetics: the analysis gives a surface limited permeation (see
appendix, Fig. 9).

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
160$~{}^{\circ}$C 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. Hydrogen was constantly fluxed inside the sample
holder, an equilibrium condition is reached when the amount of absorbed gas
compensates the desorption process which occurs at the UHV side of the
membrane [20]. When this steady state has 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 (see
appendix, Fig. 9).

###  II.3  Photoelectron spectroscopy

In-situ synchrotron measurements were recorded using 700 eV photon energy for
the survey spectra and 104 eV for the simultaneous detection of tungsten 4f
core levels and the valence band. During hydrogenation experiments, 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 being recorded for each
external hydrogen pressure. These spectra were averaged for the subsequent
data evaluation to improve signal to noise ratios. 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 [23]. The W5p3/2 core level is not fitted in the spectra recorded at
104 eV, due to its small photoionization cross section at this photon energy
[24]. The binding energy calibration was performed on the gold coated sample
holder, setting the Au 4f7/2 peak to 84.0 eV. Due to small photon energy
instabilities and work function changes during hydrogenation, all spectra were
subsequently aligned by shifting the W6+ 4f7/2 peak to 35.8 eV. These shifts
are on the order of 0.2-0.3 eV.

Ex-situ measurements were performed using a PHI Quantes spectrometer (ULVAC-
PHI) equipped with a monochromatic Al K$\alpha$ (1486.6 eV) X-ray source.
Charge neutralization was accomplished by a dual beam charge neutralization
system, employing low energy electron and argon ion beams. Detailed
acquisition parameters are given in table 1.

###  II.4 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)
[25, 26, 27, 28]. The calculation used Projector Augmented Wave (PAW) method
[29, 30] to describe the effects of core electrons, and Perdew-Burke-Ernzerhof
(PBE) [31] 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 $\Gamma$-centered mesh for WO3 (unit cell), and a 4x4x4
$\Gamma$-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 maximum interatomic force after relaxation was below 0.01
eV/Å. After structural optimization, hybrid-DFT calculation of band structure
was performed using the HSE06 hybrid functional [32, 33] with a mixing
parameter of 25% and a screening parameter of 0.2 Å-1.

Hydrogen free WO3 crystallizes in the monoclinic structure with space group
P21/n with a = 7.304 Å, b = 7.536 Å, c = 7.691 Å and $\beta$ = 90.85∘ [34].
Other polymorphs exist depending on temperature [35]. However, intercalation
of hydrogen leads to the formation of cubic structures of HxWO3 for $x=0.5$
[16]. 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 [17,
36]. 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 [37,
13, 36]. This difference is due to the general underestimation of the optical
gap by GGA and WDA methods [38], with WO3 being particularly notorious [39].
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 [40, 8, 17]. 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 [39]. However, the simplicity of
a 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
antiparallel spin configuration are placed in a simple cubic eightfold unit
cell of WO3 (see Figs. 3, 10). Nine different hydrogen constellations were
calculated, with the two hydrogen atoms placed in the unit cells along the
100, 110 and 111 direction respectively. The difference between these
constellations turned out to be negligible (Appendix, Fig. 10). The hydrogen
and oxygen positions were released in the sub-unit to find the H-O position
with minimum total energy. We found an optimum for $d_{O-H}=1.03$ Å, 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 photoemission spectroscopy (see Fig. 5).

##  III Results

For characterization, XPS measurements employing Al-K$\alpha$ radiation were
performed on the tungsten oxide coated palladium membrane pre and post
hydrogenation (see Appendix Fig. 7). The results are in perfect agreement with
literature; i.e., we find mainly W6+ and some W5+ as extracted by peak fitting
of the W4f states [41], which is typical for amorphous WO3 thin films [42].
There is no significant difference between the data taken ex-situ before and
after hydrogen exposure. The survey spectra show less carbon contamination
after hydrogenation and the presence of Pd3d peaks, that are attributed to
inadvertent removal of the thin WO3 layer during mounting or dismounting from
the membrane sample holder.

The same procedure was applied in-situ directly before and after hydrogen
exposure (Appendix, Fig. 8). After hydrogen exposure the ratio
W${}^{5+}/W^{6+}$ increases. The intensity of the higher binding energy
shoulder of the O1s peak, indicative of OH and/or carbonate species [42],
increases in parallel (Appendix, Fig. 8). These changes were followed in-situ
at various hydrogen pressures applied to the membrane. We observed continuous
variations of core levels (Fig. 2) as well as the valence band (Fig. 5b)
indicative of hydrogen intercalation into WO3. However, in addition to the
effects assigned to hydrogen exposure, we found a higher W5+/W6+ ratio derived
from in-situ measurements at 700 eV than the one derived ex-situ at 1486.6 eV
photon energy, both measured on the sample before hydrogen exposure. This
effect is attributed to the prolonged UHV exposure necessary to align the
experimental setup. During alignment and optimization of the acquisition
parameters, the membrane was exposed to UV radiation of 104 eV photon energy.
It is well known that UV radiation induces the reduction of WO3, leading to
larger fractions of W5+ [43]. This effect is a major detriment of
photoelectron spectroscopy applied to such systems in general.

The in-situ hydrogenation opens the possibility to disentangle the effects
from radiation damage and hydrogen exposure by following the evolution of the
spectra with and without hydrogen. In Fig. 2, the WO3 thin film was exposed to
increasing hydrogen back pressure up to 1 bar and then kept under these
conditions for a long period. The increase of the W5+ fraction shows two
different slopes. With increasing the hydrogen back pressure, the amount of
W5+ grows fast. When the hydrogen back pressure is high but constant, the
growth of W5+ slows down markedly. The amount of W5+ purely from hydrogenation
may then be the difference between the two dashed fitting curves, neglecting a
mutual enhancement of the two effects. The hydrogen content in HxWO3 has been
shown to be proportional to the square root of the applied hydrogen pressure
[18, 44]. With this relation, the amount of W5+ is proportional to
$\sqrt{p_{H_{2}}}$ and thus to the amount of hydrogen $x$ in HxWO3-δ. An
uncertainty is the number of oxygen vacancies $\delta$. The measured
differences of the O to W elemental ratio between pre- and post hydrogenation
is less than 3% (Appendix, (Fig. 8)). This lies within the accuracy of the
measurements. Within this limitation, there is no oxygen loss during
hydrogenation.

Figure 2: The fraction as derived from peak fitting shown in the appendix
(Fig. 7 ) of W5+ in the sample is shown in black, with the linear
interpolation of the slope shown by the dashed gray line during the increasing
hydrogen pressure and the constant hydrogen pressure phases, shown in red. The
gray scale spectra show the changes in the W4f spectra during the increasing
hydrogen pressure phase.

The measurement of valence band spectra was performed in parallel to the W4f
core levels, showing subtle changes with hydrogenation as well (6). However,
in contrast to the binding energy shifts of core levels, which can be
interpreted rather easily [45, 46], the H1s state is both core level as well
as valence state, and thus not easily characterized. This challenge is tackled
by an in-depth analysis of the valence band spectra with the help of DFT
calculations.

DFT calculations were performed on simple cubic WO3, H0.25WO3, H0.75WO3, and
H1WO3 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 Fig. 4). As discussed in section II.4, the gap is
underestimated (experimental gap of monoclinic WO3 is 3.2 eV [37, 13]) due to
the assumption of a cubic lattice, which simplifies the calculation of
hydrogen intercalated WO3. Nevertheless, the overall electronic structure with
the valence band edge dominated by oxygen orbitals is in perfect agreement
with literature. This is also in agreement with experimental observations: the
optical absorption in crystalline HxWO3 is very similar to that in amorphous
HxWO3 indicating that the crystallinity plays only a minor role for electronic
structure changes upon hydrogenation [47].

Figure 3:  Illustration of the cubic crystal structure of WO3 (left) and of
one polymorph of H0.25WO3 (right) as used for modelling. Note the distortion
of the oxygen atoms induced by hydrogen amounts as little as $x=0.25$. This
distortion is local: as an example, two tungsten atoms (grey) exclusively
surrounded by oxygen atoms (blue atoms) are highlighted with green corona, one
surrounded by five oxygens and one OH group (hydrogen in red) is highlighted
with a purple corona. In a local picture, the latter atoms are W5+, the ones
surrounded solely by oxygen W6+. Moving an electron and hydrogen (arrow)
requires structural re-arrangements  Figure 4:  Modelled partial and total DOS
of cubic WO3 (black lines) and HxWO3 (red lines) with hydrogen concentrations
as small as $x=0.25$. $E_{G}=1.3$ 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). Note the minimal
contribution of hydrogen electrons (purple dashed line) to the total density
of states in H0.25WO3.

The intercalation of hydrogen into WO3 was modelled by placing hydrogen in the
center of the WO3 subunit and optimizing its position in the simple cubic
structure by energy minimization. The minimum was found with hydrogen
occupying a position near one oxygen site with a distance of 1.04 Å identical
to the calculations of Hjelm et al. [8], and very similar to the bond length
of an OH ion. Simultaneously, the lattice is slightly distorted (Fig. 3). The
presence of hydrogen accompanied by the lattice distortion leads to two main
changes in the electronic structure (see Fig. 4): the valence band broadens
due to the bonding of oxygen with hydrogen (bonding states around -11 eV) and
the conduction band minimum dominated by tungsten states is being occupied by
electrons donated by the hydrogen and pushed below the Fermi level. It is
expected that these changes lead to large increases in the electronic
conductivity, which will be discussed in section IV. However, the aim of this
paper is to confirm these effects by photoelectron spectroscopy, as theses
states should be observable.

Figure 5:  a) Calculated partial and total DOS of H0.25WO3 (black) and
H0.75WO3 (red). b) 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. 6 and text for more information).

Figure 5 shows the calculated DOS for H0.25WO3 and H0.75WO3 that have been
broadened to account for the natural linewidth of the photoemission process
and valence band spectra at low and high applied hydrogen back pressure. The
effects of going from H0.25WO3 to H0.75WO3 (see Fig. 5) are less drastic
compared to the difference between WO3 and HxWO3 (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 $E_{F}$ with increasing hydrogen content. These two
changes match the experimental spectra exceptionally well (Fig. 5). In
particular, the calculations confirm the existence of hydrogen induced states
near $E_{F}$, which are associated with tungsten states.

The normalized peak area of these hydrogen induced tungsten states at 0.4 eV
binding energy scales with the square root of the hydrogen back pressure (Fig.
6). 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. Since the number of hydrogen induced tungsten states is directly related
to the applied hydrogen back pressure by employing our membrane approach, we
plot it together with the bulk pressure-composition isotherm of Pt:WO3
reported in literature [18] using the common pressure axis. Using this
calibration we extract that the initial hydrogen content in our thin films is
H0.04WO3 increasing to H0.14WO3 at 1000 mbar.

Figure 6:  The intensity of hydrogen induced, conduction band like tungsten
states (black) are shown as a function of time and with hydrogen back pressure
(red). Full symbols indicate increasing hydrogen pressure, empty ones mean
constant hydrogen back pressure. The bulk pressure-composition isotherm data
on Pt:WO3 (blue) at 423 K are plotted on the same pressure axis [18]. Squares
are absorption whereas triangles are desorption data.

##  IV Discussion

Already in the past, Hollinger et al. proposed that coloration in amorphous
tungsten oxide films upon hydrogenation 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 [48]. The optical absorption band corresponds to a peak near to
the Fermi level detected by photoelectron spectroscopy.

With this, our measurements confirm most of the literature data on electronic
structure changes in HxWO3 or substoichiometric WO3-δ, in particular the
evolution of the peak near to the Fermi level [48, 41, 43, 23, 14]. However,
with our experimental procedure we can disentangle the contributions from
oxygen deficiency and hydrogen incorporation. The membrane photoemission
measurements show that hydrogen interaction with WO3 leads to band gap states
without the loss of oxygen (Appendix, Fig. 8).

Attributing these states near to the Fermi level to hydrogen is too short-
viewed. Even without sophisticated band structure calculations we can estimate
the potential effect: the valence band of HxWO3 consists of $3\cdot 6$ O2p
states (we neglect here the small contribution of W5d), and at most 0.2
hydrogens. In addition, we 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 [24]. The expected intensity ratio between
hydrogen states and total valence states is $0.2/(3\cdot 6)\cdot 0.02/1=2.2$
$\cdot 10^{-4}$. It is thus unlikely that the photoelectrons near $E_{F}$ stem
from hydrogen. Instead, the DFT modelling shows that hydrogenation of WO3
leads to the formation of OH with hydrogen states appearing at -11 eV and
tungsten like conduction band states near the Fermi level (Fig. 5).

Conductivity and optical experiments evidence that hydrogen intercalated WO3
is not an electronic conductor at small hydrogen content in contrast to what
is suggested by the calculated DOS (Fig. 4), which shows states at $E_{F}$.
This discrepancy was already found in the past, and explained by correlation
models [49]. In local picture, hydrogen intercalation gives:

W^6+ O_3^2- -¿[ H ] W^5+ (OH)^- O_2^2-, (1)

with the electron from the hydrogen localized by the OH bond. With this, the
nearby tungsten atom is different from the ones surrounded solely by oxygen
atoms (Fig. 3). This local picture is still compatible with the band
structure, however, dynamic properties will depend on the correlation of the
corresponding electrons. Such calculations are beyond the scope of this paper.
We restrict ourselves to the following simplified picture explaining the
observation that despite having electrons near the Fermi level (Fig. 5) HxWO3
does not exhibit metallic character (no Drude absorption [13]): The movement
of electrons associated with the tungsten atom nearby the OH group (W5+) is
correlated with the hopping of the proton, which requires crystallographic
rearrangement as the oxygen orientations around the tungsten atoms are
slightly different (see Fig. 3).

Crandall and Faughnan [49] 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, 50]. 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 [13]:

W_a^5+ (OH) + W_b^6+ O -¿[$\hbar\omega$] W_a^6+ O + W_b^5+ (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+ also
leading to a distorted lattice. This so-called proton polaron has been
proposed for describing the proton transport in the similar system, hydrated
BaCe0.8Y0.2O3-δ [51]. The general underlying principle, the required
crystalline rearrangements linked to charge transfer (”movement of
distortions”, see Fig. 3, and energy dependence on hydrogen constellation,
Appendix, Fig. 10), 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
changes of the electronic structure by a proton-oxygen bond also explains the
difference to changes induced by oxygen vacancies. Here, recent DFT
calculations provide arguments that oxygen vacancies in WO3 introduce ’free’
carriers to the conduction band, because the corresponding electrons are
delocalized over a large area [52]. In our case, the OH bond pins the similar
electron brought into the system by hydrogen intercalation. From an electron
spectroscopy point of view, this difference is indistinguishable, but the
localized picture is in good agreement with other physical properties of
HxWO3-δ (see discussion above).

Finally, tackling the difficulties of probing the electronic structure in
HxWO3-δ by static electron spectroscopy by following dynamic changes in-situ
may be extended to ultrafast spectroscopy. With this, the degree of
localization is directly accessible.

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. 4). This has drastic consequences on (electro-) catalysis
of oxygen related reactions in addition to the physical properties like
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 [53]. In particular, it shows a high activity for water oxidation in
the presence of a suitable electron acceptor [54, 53]. Hydrogen treatment of
WO3 can increase photocurrent significantly [54, 55], which was interpreted as
an increase in the number 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 [54, 55] 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 [56, 57]. If hydrogen vibrations with H-PDOS at
$E_{F}$ 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 [57], and the formation of OH instead of the creation of
oxygen vacancies upon hydrogenation is in line with 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 simultaneously 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 consecutive XPS/UPS
measurements. Analysis of the tungsten 4f core levels shows an increase 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.
Although induced by hydrogen, the corresponding electrons are allocated to
tungsten d-states. Using previous reference measurements it is possible to
create a pressure-composition isotherm from the spectroscopy data. The
photoemission measurements together with DFT calculations corroborate that the
coloration of the films by hydrogen can be explained by a proton polaron
model.

###### Acknowledgements.

This work was partly supported by the UZH-UFSP program LightChEC. Financial
support from the Swiss National Science Foundation (grant number 172662 and
Requip grant 182987) is greatly acknowledged. We thank Dr. C. Puglia (Uppsala
University, Sweden) and the Carl Tygger Foundation for the availability of VG
Scienta’s SES-200 photoelectron analyzer at the GasPhase beamline.

##  VI Appendix

###  VI.1 Experimental details of the photoemission experiments

The table 1 shows acquisition parameters for XPS and synchrotron photoelectron
spectroscopy used.

| Energy Range | Pass Energy | Energy Stepsize | total acquisition time / point
---|---|---|---|---
ex-situ XPS Survey | 1100-0 eV | 112 eV | 100 meV | 0.4 s
ex-situ XPS narrow scan | variable | 55 eV | 50 meV | 1.2 s
in-situ XPS Survey | 550-0 eV | 20 eV | 200 meV | 0.6 s
in-situ XPS narrow scan | variable | 20 eV | 50 meV | 0.1 s
Table 1: Acquisition parameters for XPS and synchrotron photoelectron
spectroscopy.

Figures 7 and 8 compare the survey and W4f core level spectra as measured ex-
situ (XPS) and in-situ (synchrotron photoemission) before and after
hydrogenation. In addition, the deconvolution of the spectra to derive the W6+
and W5+ fractions are shown for one archetypal example. From the intensity
ratio of the O1s to W4f, one can estimate that the oxygen loss is below 3%.

Figure 7:  a) The pre- and post-hydrogenation XPS survey spectra are shown in
black and red respectively. b) The W5p3/2 (brown) and W4f (grey/violet) core
levels of the electrodeposited WO3 as prepared, as described in section II.1,
measured by Al K$\alpha$ XPS. The spectra are fitted with two pairs of
doublets giving the W6+ and W5+ fractions, respectively. The peak fitting
indicates the sample being almost completely oxidized, with a small fraction
of W5+ states present ($\simeq 7.7\%$).

Figure 8: W4f (a) and O1s (b) core level spectra recorded using 700 eV photon
energy. The spectra were recorded in-situ before and after exposure to
hydrogen (black and red respectively). The W5+ fraction increases from 37.6%
to 51.2% and the oxygen defect fraction from 38.8% to 52.0% upon
hydrogenation. The intensity ratio of W to O remains nearly constant with
$I(O_{latt})/I(W)=1.06\rightarrow 1.03$.

The rate-limiting step of the permeation of hydrogen through the membrane can
be derived from the pressure dependence on both sides of the membrane:

$p_{UHV}\propto p_{appl}^{r}$ (1)

For ideal cases, surface limited permeation gives an exponent $r=1$, bulk
diffusion gives $r=\frac{1}{2}$.[19, 20] A fit to the double log plot of the
measured hydrogen pressure in the vacuum chamber as a function of the applied
feed pressure yields an exponent of 0.79 (Fig. 9), indicative for surface
limited kinetics.

Figure 9:  Double-log plot of the measured hydrogen pressure in the vacuum
chamber as a function of the applied feed pressure. A fit yields a slope of
0.79.

###  VI.2 Hydrogen constellations used for DFT calculations

Substoichiometric concentrations ($x<1$) in HxWO3 are difficult to model. For
$x=0.25$, the unit cell of H0.25WO3 is extended to a 2x2x2 supercell of the
simple unit cell of H1WO3 based on one H in the WO3 cell (Fig. 10). Not all of
the now possible H-position are occupied to reflect the substoichiometry
giving additional freedom where to place these hydrogen atoms relative to each
other. We modelled nine archetypal constellations, where the hydrogen atoms
are placed in the subcells along the 100, 110 and 111 directions and with
different spin constellations. The corresponding energy shifts are of the
order of 0.3 eV. This difference is neglected within the interpretation of the
photo-electron spectra, however, the non-zero energy difference will
contribute to the energy barrier for hydrogen motion in HxWO3.

Figure 10: Top: as an example, two different hydrogen constellations in
H0.25WO3, denoted as +1+1 and +1-1, before relaxation. Bottom: DFT of nine
different constellations. The right graph is an enlargement around the band
gap.

## References

  * Völkl and Wipf [1981] J. Völkl and H. Wipf, Diffusion of Hydrogen in Metals, Hyperfine Interact. 8, 631 (1981).
  * Wöhler [1824] F. Wöhler, Ueber das Wolfram, Ann. Phys. 78, 345 (1824).
  * Brimm  _et al._ [1951] E. O. Brimm, J. C. Brantley, J. H. Lorenz, and M. H. Jellinek, Sodium and Potassium Tungsten Bronzes, J. Am. Chem. Soc. 73, 5427 (1951).
  * Deb [1973] S. K. Deb, Optical and photoelectric properties and colour centres in thin films of tungsten oxide, Philos. Mag. 27, 801 (1973).
  * Granqvist [2000] C. G. Granqvist, Electrochromic tungsten oxide films: Review of progress 1993–1998, Sol. Energy Mat. Sol. C. 60, 201 (2000).
  * Höchst  _et al._ [1981] H. Höchst, R. Bringans, H. Shanks, and P. Steiner, Failure of the rigid band model in NaxWO3: An XPS study, Solid State Commun. 37, 41 (1981).
  * Raj  _et al._ [2009] S. Raj, T. Sato, S. Souma, T. Takahashi, D. D. Sarma, and P. Mahadevan, Metal-insulator transition of NaxWO3 studied by angle-resolved photoemission spectroscopy, Mod. Phys. Lett. B 23, 2819 (2009).
  * Hjelm  _et al._ [1996] A. Hjelm, C. G. Granqvist, and J. M. Wills, Electronic structure and optical properties of WO3, LiWO3, NaWO3, and HWO3, Phys. Rev. B 54, 2436 (1996).
  * Larsson  _et al._ [2003] A. L. Larsson, B. E. Sernelius, and G. A. Niklasson, Optical absorption of Li-intercalated polycrystalline tungsten oxide films: Comparison to large polaron theory, Solid State Ionics 165, 35 (2003).
  * Granqvist [1995] C. G. Granqvist, Bulk crystalline tungsten oxide, in  _Handbook of inorganic electrochromic materials_ (Elsevier, 1995) Chap. 2, p. 633.
  * Höchst  _et al._ [1982] H. Höchst, R. D. Bringans, and H. R. Shanks, Electronic structure of NaxWO3 : A photoemission study covering the entire concentration range, Phys. Rev. B 26, 1702 (1982).
  * Leng  _et al._ [2017] X. Leng, J. Pereiro, J. Strle, G. Dubuis, A. T. Bollinger, A. Gozar, J. Wu, N. Litombe, C. Panagopoulos, D. Pavuna, and I. Božović, Insulator to metal transition in WO3 induced by electrolyte gating, npj Quantum Materials 2, 35 (2017).
  * Saenger  _et al._ [2008] M. F. Saenger, T. Höing, B. W. Robertson, R. B. Billa, T. Hofmann, E. Schubert, and M. Schubert, Polaron and phonon properties in proton intercalated amorphous tungsten oxide thin films, Phys. Rev. B 78, 245205 (2008).
  * Johansson  _et al._ [2016] M. B. Johansson, P. T. Kristiansen, L. Duda, G. A. Niklasson, and L. Österlund, Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy and optical spectrophotometry, Journal of Physics: Condensed Matter 28, 475802 (2016).
  * Dickens and Hurditch [1967] P. G. Dickens and R. J. Hurditch, X-ray and Neutron Diffraction Studies of a Tetragonal Hydrogen Bronze HxWO3, Nature 215, 1266 (1967).
  * Wiseman and Dickens [1973] P. Wiseman and P. Dickens, The crystal structure of cubic hydrogen tungsten bronze, J. Solid State Chem. 6, 374 (1973).
  * de Wijs and de Groot [1999] G. A. de Wijs and R. A. de Groot, Structure and electronic properties of amorphous WO3, Phys. Rev. B 60, 16463 (1999).
  * Berzins and Sermon [1983] A. R. Berzins and P. A. Sermon, Reversible isothermal sorption of hydrogen by tungsten trioxide in presence of platinum, Nature 303, 506 (1983).
  * Delmelle  _et al._ [2015] R. Delmelle, B. Probst, R. Alberto, A. Züttel, D. Bleiner, and A. Borgschulte, Closing the pressure gap in x-ray photoelectron spectroscopy by membrane hydrogenation, Rev. Sci. Instr. 86, 053104 (2015).
  * Sambalova and Borgschulte [2018] O. Sambalova and A. Borgschulte, Membrane concept for environmental surface science, J. Alloy. Compd. 742, 518 (2018).
  * Blyth  _et al._ [1999] R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, R. Krempaska, J. Slezak, K. Prince, R. Richter, M. Vondracek, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani, C. Furlani, M. de Simone, S. Stranges, and M.-Y. Adam, The high resolution Gas Phase Photoemission beamline, Elettra, J. Electron Spectrosc. 101-103, 959 (1999).
  * Grazioli [2017] C. Grazioli,  _Photoionization experiments in the study of energy transfer in nanostructured materials and their precursors_ , Ph.D. thesis, Università degli Studi di Trieste (2017).
  * Bouvard  _et al._ [2016] O. Bouvard, A. Krammer, and A. Schüler, In situ core-level and valence-band photoelectron spectroscopy of reactively sputtered tungsten oxide films, Surf. Interface Anal. 48, 660 (2016).
  * Yeh and Lindau [1985] J. J. Yeh and I. Lindau, Atomic subshell photoionization cross sections and asymmetry parameter: 1 < z < 103, Atom. Data. Nucl. Data 32, 1 (1985).
  * Kresse and Hafner [1993] G. Kresse and J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47, 558 (1993).
  * Kresse and Hafner [1994] G. Kresse and J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium, Phys. Rev. B 49, 14251 (1994).
  * Kresse and Furthmüller [1996a] G. Kresse and J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mat. Sci. 6, 15 (1996a).
  * Kresse and Furthmüller [1996b] G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54, 11169 (1996b).
  * Blochl [1994] P. E. Blochl, Projector augmented-wave method, Phys. Rev. B 50, 17953 (1994).
  * Kresse and Joubert [199] G. Kresse and D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B 59, 1758 (199).
  * Perdew  _et al._ [1996] J. P. Perdew, K. Burke, and M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77, 3865 (1996).
  * Heyd  _et al._ [2003] J. Heyd, G. E. Scuseria, and M. Ernzerhof, Hybrid functionals based on a screened Coulomb potential, J. Chem. Phys. 118, 8207 (2003).
  * Krukau  _et al._ [2006] A. V. Krukau, O. A. Vydrov, A. F. Izmaylov, and G. E. Scuseria, Influence of the exchange screening parameter on the performance of screened hybrid functionals, J. Chem. Phys. 125, 224106 (2006).
  * Lassner and Schubert [1999] E. Lassner and W.-D. Schubert,  _Tungsten Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds_, 1st ed. (Springer US, New York, 1999).
  * Wriedt [1989] H. A. Wriedt, The O-W (oxygen-tungsten) system, Bulletin of Alloy Phase Diagrams 10, 368 (1989).
  * González-Borrero  _et al._ [2010] P. P. González-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, Optical band-gap determination of nanostructured WO3 film, Appl. Phys. Lett. 96, 061909 (2010).
  * Green and Hussain [1991] M. Green and Z. Hussain, Optical properties of dilute hydrogen tungsten bronze thin films, J. Appl. Phys. 69, 7788 (1991).
  * Xiong  _et al._ [2007] K. Xiong, J. Robertson, and S. J. Clark, Behavior of hydrogen in wide band gap oxides, J. Appl. Phys. 102, 083710 (2007).
  * Wang  _et al._ [2011] F. Wang, C. Di Valentin, and G. Pacchioni, Electronic and Structural Properties of WO3: A Systematic Hybrid DFT Study, J. Phys. Chem. C 115, 8345 (2011).
  * Cora  _et al._ [1996] F. Cora, A. Patel, N. M. Harrison, R. Dovesi, and C. R. A. Catlow, An ab Initio Hartree-Fock Study of the Cubic and Tetragonal Phases of Bulk Tungsten Trioxide, J. Am. Chem. Soc. 118, 12174 (1996).
  * De Angelis and Schiavello [1977] B. A. De Angelis and M. Schiavello, X-ray photoelectron spectroscopy study of nonstoichiometric tungsten oxides, J. Solid State Chem. 21, 67 (1977).
  * Barreca  _et al._ [2001] D. Barreca, G. Carta, A. Gasparotto, G. Rossetto, E. Tondello, and P. Zanella, A Study of Nanophase Tungsten Oxides Thin Films by XPS, Surface Science Spectra 8, 258 (2001).
  * Bussolotti  _et al._ [2003] F. Bussolotti, L. Lozzi, M. Passacantando, S. La Rosa, S. Santucci, and L. Ottaviano, Surface electronic properties of polycrystalline WO3 thin films: a study by core level and valence band photoemission, Surf. Sci. 538, 113 (2003).
  * Fripiat and Lin [1992] J. J. Fripiat and X. Lin, Hydrogen intercalation within transition metal oxides: entropy, enthalpy, and charge transfer, The Journal of Physical Chemistry 96, 1437 (1992).
  * Kerber  _et al._ [1996] S. J. Kerber, J. J. Bruckner, K. Wozniak, S. Seal, S. Hardcastle, and T. L. Barr, The nature of hydrogen in x‐ray photoelectron spectroscopy: General patterns from hydroxides to hydrogen bonding, J. Vac. Sci. Technol. A 14, 1314 (1996).
  * Bourque  _et al._ [2016] J. L. Bourque, M. C. Biesinger, and K. M. Baines, Chemical state determination of molecular gallium compounds using XPS, Dalton Trans. 45, 7678 (2016).
  * Schirmer and Salje [1980] O. F. Schirmer and E. Salje, Conduction bipolarons in low-temperature crystalline WO3-x, J. Phys. C Solid State 13, L1067 (1980).
  * Hollinger  _et al._ [1976] G. Hollinger, T. Minh Duc, and A. Deneuville, Charge Transfer in Amorphous Colored WO3 Films Observed by X-Ray Photoelectron Spectroscopy, Phys. Rev. Lett. 37, 1564 (1976).
  * Crandall and Faughnan [1977] R. S. Crandall and B. W. Faughnan, Electronic transport in amorphous ${\mathrm{H}}_{x}\mathrm{W}{\mathrm{O}}_{3}$, Phys. Rev. Lett. 39, 232 (1977).
  * Wittwer  _et al._ [1978] V. Wittwer, O. F. Schirmer, and P. Schlotter, Disorder dependence and optical detection of the Anderson transition in amorphous HxWO3 bronzes, Solid State Commun. 25, 977 (1978).
  * Braun and Chen [2017] A. Braun and Q. Chen, Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors, Nat. Commun. 8, 15830 (2017).
  * Wang  _et al._ [2016] W. Wang, A. Janotti, and C. G. Van De Walle, Role of oxygen vacancies in crystalline WO3, J. Mat. Chem. C 4, 6641 (2016).
  * Kalanur  _et al._ [2018] S. S. Kalanur, L. T. Duy, and H. Seo, Recent progress in photoelectrochemical water splitting activity WO3 photoanodes, Top. Catal. 61, 1043–1076 (2018)).
  * Yan  _et al._ [2015] J. Yan, T. Wang, G. Wu, W. Dai, N. Guan, L. Li, and J. Gong, Tungsten oxide single crystal nanosheets for enhanced multichannel solar light harvesting, Adv. Mat. 27, 1580 (2015).
  * Singh  _et al._ [2015] T. Singh, R. Müller, J. Singh, and S. Mathur, Tailoring surface states in WO3 photoanodes for efficient photoelectrochemical water splitting, Appl. Surf. Sci. 347, 448 (2015).
  * Reich  _et al._ [2009] S. Reich, G. Leitus, R. Popovitz-Biro, A. Goldbourt, and S. Vega, A Possible 2D HxWO3 Superconductor with a Tc of 120K, J. Supercond. Nov. Magn. 22, 343–346 (2009).
  * Pellegrini  _et al._ [2019] C. Pellegrini, H. Glawe, and A. Sanna, Density functional theory of superconductivity in doped tungsten oxides, Phys. Rev. Materials 3, 064804 (2019).