Hydrothermal Synthesis of Monoclinic VO Microparticles without Use of Hazardous Reagents: A Key Role for the W-Dopant.

Monoclinic vanadium dioxide (VO (M)) is a promising material for various applications ranging from sensing to signature management and smart windows. Most applications rely on its reversible structural phase transition to rutile VO (VO (R)), which is accompanied by a metal-to-insulator transition. Bottom-up hydrothermal synthesis has proven to yield high quality monoclinic VO but requires toxic and highly reactive reducing agents that cannot be used outside of a research lab. Here, we present a new hydrothermal synthesis method using nontoxic and safe-to-use oxalic acid as a reducing agent for VO to produce VO (M). In early stages of the process, polymorphs VO (A) and VO (B) were formed, which subsequently recrystallized to VO (M). Without the presence of W, this recrystallization did not occur. After a reaction time of 96 h at 230 °C in the presence of (NH)HWO in Teflon-lined rotated autoclaves, we realized highly crystalline, phase pure W-doped VO (M) microparticles of uniform size and asterisk shape (Δ = 28.30 J·g, arm length = 6.7 ± 0.4 μm, arm width = 0.46 ± 0.06 μm). We extensively investigated the role of W in the kinetics of formation of VO (M) and the thermodynamics of its structural phase transition.