Hydrogen in tungsten trioxide by membrane photoemission and density functional theory modeling

The measurement of hydrogen-induced changes in 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 the 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 implications for its use as a (photoelectrochemical) catalyst.