Electronic structure of hexagonal tungsten trioxide: XPS, XES, and XAS studies

Abstract

X-Ray photoelectron (XPS), emission (XES) and absorption (XAS) spectroscopy methods were used to study the electronic structure of hexagonal tungsten trioxide, h-WO 3. Its precursor, hexagonal hydrogen tungsten bronze, H x WO 3, and the monoclinic form of tungsten trioxide, m-WO 3, were also studied. For the mentioned compounds, both the XPS valence-band and core-level spectra, as well as the O Kα emission bands and the W L III absorption edges were derived. It was established that, binding energies of both the W 4f and O 1s core-level electrons do not change when going from the tungsten trioxides to H x WO 3. A high-energy shift of the inflection point of the XAS W L III spectrum of the H x WO 3 bronze, with respect to its position on the spectrum of pure metallic tungsten, was found to be close to those of the spectra of the two WO 3 forms studied. Half-widths of both the XPS valence-band spectra and the O Kα bands increase somewhat in the sequence m-WO 3→h-WO 3→H x WO 3. The formation of a near-Fermi sub-band, which is absent for both the modifications of WO 3, was observed on the XPS valence-band spectrum of hexagonal H x WO 3. The energy positions of the centers of gravity of the O Kα band remain constant for all the compounds studied.