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 (V_Os) 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 V_O enthalpy of formation and the Bader charge calculated for each atom site supported this view, by identifying the preferred sites for V_O 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 V_O-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 V_Os should be both localized and delocalized in Cs-doped tungsten oxides.