WO3: a review of synthesis techniques, nanocomposite materials and their morphological effects for gas sensing application

Metal oxides constitute a large and crucial class of chemical compounds. They form one of the most fascinating classes of functional materials. Owing to their unique physical and chemical properties, metal oxides are undoubtedly ideal candidates for a wide variety of applications in the field of sensing, energy conversion and storage, ceramics, catalysis, etc. Tungsten oxide (WO3) is one such n-type semiconductor with intriguing properties making it highly useful for various technological applications, especially gas sensing of various volatile organic compounds, environmental and toxic gases. Gas sensing has gained a lot of interest among researchers over the years and has proved to be one of the most promising application areas for semiconductor metal oxides, as they react to oxidizing and reducing gases leading to considerable change in their electrical conductivity. This review mainly focuses on the synthesis techniques of WO3 and the use of WO3 for gas sensing applications. Moreover, the morphology of WO3 obtained at a nanoscale also has a significant effect on its sensing efficiency, as the gas sensing mechanism is mainly based on the reaction occurring on the sensor surface. This review also provides a detailed overview of the effect of various morphologies on gas sensor parameters as well as a glimpse into the vast variety of WO3 nanocomposite materials and their applications for gas sensing.