HxWO3

HxWO3, or hydrogen tungsten oxide, is a hydrogen-intercalated oxide of tungsten. It belongs to the family of tungsten bronzes, formed by inserting protons into the lattice of WO3 to give a nonstoichiometric compound with hydrogen content x (0 ≤ x ≤ 1). The insertion is accompanied by reduction of some tungsten centers from W6+ to W5+, which enhances electronic conductivity and alters optical properties.

Structure and composition: In the WO3 framework, hydrogen ions occupy interstitial sites in channels or voids created by the WO6 octahedra. The hydrogen content can vary with synthesis, and the material exhibits mixed valence W(V)/W(VI). The hydrogen is mobile at moderate temperatures, enabling ionic transport and dynamic changes in properties.

Preparation: HxWO3 is typically prepared by electrochemical insertion (proton intercalation) of WO3 films or powders from acidic electrolytes, applying a negative potential to drive H+ and electrons into the lattice. Chemical routes include hydrogenation of WO3 in hydrogen-containing atmospheres or reduction in hydrogen with suitable catalysts. Intercalation can be partial or near-complete up to high x values, depending on conditions.

Properties: The material shows electrochromism, with color change upon reduction as protons and electrons are inserted. It exhibits increased electrical conductivity relative to pristine WO3 due to electron injection and polaron hopping. The exact optical and electronic properties depend on hydrogen content and arrangement within the lattice.

Applications: HxWO3 is studied for electrochromic devices such as smart windows and displays, where color change can be controlled by voltage. It is also of interest for reversible proton intercalation and potential hydrogen storage, as well as fundamental studies of tungsten oxide bronzes and their conductive behavior.