X-ray Photoelectron Spectroscopic Study of Reduced Alkali Tungsten Oxides with Localized and Delocalized Electrons

In a series of Cs-doped tungsten oxides that underwent different degrees of reduction, new components and behaviors were found in X-ray photoelectron spectroscopy (XPS) signals of O-1s, W-4f, and Cs-3d and analyzed in terms of oxygen vacancies (VOs) and electron localization with the aid of first-principles calculations. Orthorhombic Cs4W11O35 was reduced at high temperature to transform it to hexagonal tungsten bronze with increasing W5+ trapped electrons, as detected in W-4f. The binding energy of W5+-4f showed a distinct redshift toward low saturation values that was implied to be due to W5+ alignment on the hexagonal prismatic planes. The VO enthalpy of formation and the Bader charge calculated for each atom site supported this view, by identifying the preferred sites for VO and W5+ on (020) in Cs4W11O35. The W5+ component was newly admitted in O-1s at 531.25-531.94 eV and 532.35-533.04 eV, while the carbonation contributions were eliminated using C-1s deconvolution. In Cs-3d, a VO-related extra component was found on the high-energy side, which grew with increasing reduction time. These observations and calculations supported the proposition that electrons emitted from Cs should be delocalized, and those from VOs should be both localized and delocalized in Cs-doped tungsten oxides.