Synthesis and applications of WO(3) nanosheets: the importance of phase, stoichiometry, and aspect ratio

Tungsten trioxide (WO(3)) is an abundant, versatile oxide that is widely explored for catalysis, sensing, electrochromic devices, and numerous other applications. The exploitation of WO(3) in nanosheet form provides potential advantages in many of these fields because the 2D structures have high surface area and preferentially exposed facets. Relative to bulk WO(3), nanosheets expose more active sites for surface-sensitive sensing/catalytic reactions, and improve reaction kinetics in cases where ionic diffusion is a limiting factor (e.g. electrochromic or charge storage). Synthesis of high aspect ratio WO(3) nanosheets, however, is more challenging than other 2D materials because bulk WO(3) is not an intrinsically layered material, making the widely-studied sonication-based exfoliation methods used for other 2D materials not well-suited to WO(3). WO(3) is also highly complex in terms of how the synthesis method affects the properties of the final material. Depending on the route used and subsequent post-synthesis treatments, a wide variety of different morphologies, phases, exposed facets, and defect structures are created, all of which must be carefully considered for the chosen application. In this review, the recent developments in WO(3) nanosheet synthesis and their impact on performance in various applications are summarized and critically analyzed.