Tungsten oxide-based visible light-driven photocatalysts: crystal and electronic structures and strategies for photocatalytic efficiency enhancement

Photocatalysis (PC) technology has received global attention due to its high potential of addressing both environmental and energy issues using only solar light as energy input. However, large-scale commercialization of PC technology is still far from expectation, which is primarily limited by low efficiency. The development of PC technology depends crucially on photocatalyst materials. In the past half century, TiO2 has been mostly investigated and developed as a benchmark photocatalyst. However, TiO2 responds intrinsically only to UV light, which has limited its efficient utilization of solar energy and restrained its applications to where UV light is not available, e.g., indoor air purification. The development of novel intrinsically visible light-driven photocatalysts has been a new trend of PC technology. Amongst the various visible-light responsive candidates, tungsten oxides (WOX, X <= 3) have attracted much attention due to their diversely tunable stoichiometries and structures, suitable band gaps, chemical stability and Earth-abundance. However, bare WOX exhibits comparatively low efficiency because of the fast recombination rate of photogenerated electrons and holes. Various strategies have been developed to enhance the photocatalytic efficiency of WOX, including the controls in the crystal phase, stoichiometry/oxygen-vacancy, active facet and morphology, elemental doping, loading of noble metal nanoparticles, hybridization with carbon materials and coupling with other semiconductors to construct heterojunctions. This review summarizes firstly the fundamentals of WOX (i.e., crystal and electronic structures and optical properties) and then highlights the strategies for the enhancement of the photocatalytic efficiency of WOX-based photocatalysts. The related synthesis methods are also briefly discussed. It is anticipated that this paper could offer a comprehensive understanding of WOX-based photocatalysts and serve as a guideline for future designs of highly active visible light-driven WOX-based photocatalysts.