Tungsten Oxide-based Materials: Synthesis, Properties, and Applications

Abstract

Non-stoichiometric compounds of tungsten oxides that are abundant on the earth received extensive attention in electrochromic devices, electrochemistry, photothermal conversion, gas sensors, and photocatalysis because of their unique physicochemical structures and highly tunable structures. Because of its high colouring efficiency, quick optical reaction time, and reversible colour changes, tungsten oxide (WO3 ), along with various oxidation states of oxygen-deficient tungsten oxide (WO3-x), has been identified as one of the mainly effective possibilities for electrochromic materials among transition metal oxides. For example, one-dimensional nanostructures of WO3-x such as nanotubes, nanowires, and nanorods, have gotten huge attention due to their unique electrical, optoelectrical, and optical properties. Additionally, distinct oxygen-deficient tungsten oxides (WO3-x) - WO2.72, WO2.83, and WO2.9, for example - have significant light absorption characteristics up to the near-infrared (NIR) region and a stable crystal phase, which sets them apart from surface-reduced WO3 . Out of these, reduced tungsten oxide (WO2.72) is one of the most studied owing to promising properties such as unusual defect structures and abundant oxygen vacancies. These oxygen vacancies create new discrete energy bands below the conduction band thus narrowing its bandgap The other series of tungsten-based materials is tungsten bronze (MxWO3 , where M= Na, Cs, K, Rb, etc) which has mixed-valence states. These are the materials that are explored less but have excellent properties as compared to WO3 . So, in this review, the applications, synthesis methods, and properties of MxWO3 and WO2.72 are studied in detail.