Electronic surface level positions of WO3 thin films for photoelectrochemical hydrogen production

Polycrystalline WO3 thin films for photoelectrochemical hydrogen production were investigated using photoelectron spectroscopy and inverse photoemission. First, we report on a careful study to minimize X-ray and electron beam-induced degradation. Second, we combined ultraviolet photoelectron spectroscopy and inverse photoemission to determine the surface positions of the valence and conduction band edges, respectively, and the work function (i.e., the position of the vacuum level). This allows us to paint a completely experiment-based picture of the WO3 surface level positions, which are of central relevance for the photoelectrochemical activity of such surfaces. We find the WO3 surface to be wide gap [3.28 (+/- 0.14) eV] and n-type, with the conduction band minimum 0.39 (+/- 0.10) eV above the Fermi level and 0.31 (+/- 0.11) eV above the H+/H-2 reduction potential. The valence band maximum is 2.89 (+/- 0.10) eV below the Fermi level and 1.74 (+/- 0.11) eV below the H2O/O-2 oxidation potential.