WO3x

WO3x denotes a non-stoichiometric oxide of tungsten in which some oxygen atoms are missing from the ideal WO3 lattice. The parameter x represents the number of oxygen vacancies per WO3 unit and typically ranges from near zero up to about 0.3–0.5 for materials produced under strong reduction. The oxygen vacancies donate electrons to the lattice, generating W5+ centers and n-type conductivity.

Tungsten trioxide exists in several polymorphs (monoclinic, tetragonal, cubic) depending on temperature and synthesis. Oxygen vacancies locally distort the lattice and introduce defect states within the band gap. These vacancies can be mobile, interact with dopants, and influence color, optical absorption, and electrical transport. Defect chemistry plays a central role in the material’s electronic structure and physical properties.

Synthesis and control of WO3−x are achieved by reducing WO3 in hydrogen, through vacuum annealing, chemical reducing agents, or electrochemical reduction. The extent of reduction is controlled by temperature, duration, gas composition, and ambient pressure; post-treatment can affect vacancy distribution and valence states. Precursors and processing conditions determine the final vacancy concentration and uniformity.

Properties of WO3−x include increased electrical conductivity with higher vacancy content and a bluish coloration associated with tungsten bronzes. The material exhibits tunable optical properties, including absorption in the visible range, and enhanced electrochemical activity. Stability under cycling and environmental sensitivity can be concerns at high vacancy levels, requiring careful control for device use.

Applications span electrochromic devices and smart windows, where vacancy modulation affects transmittance; gas sensing; and electrochemical energy storage and catalysis, where defect states facilitate charge transfer and surface reactions.