Proton polarons in HxWO3 by synchrotron photoemission and DFT modelling

TL;DR

This study combines synchrotron photoemission and DFT modeling to clarify hydrogen's role in electronic structure changes in HxWO3, revealing proton polarons as key to coloration and catalytic properties.

Contribution

It introduces a membrane-based X-ray photoelectron spectroscopy method combined with DFT to unambiguously analyze hydrogen effects in tungsten trioxide.

Findings

Hydrogenation increases W$^{5+}$ core level intensity.

Valence band near Fermi level is influenced by hydrogen-induced proton polarons.

Valence band edge shifts from oxygen to tungsten orbitals after hydrogenation.

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 photoelectron 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 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 tuneable synchrotron radiation we measure simultaneously core levels and valence band up to a hydrogen pressure of 1000 mbar. Upon hydrogenation, the intensities of the W$^{5+}$ core level and a state close to the Fermi level increase following the pressure-composition isotherm curve of bulk H$_x$WO$_3$. 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.