Hydrothermal Synthesis of Monoclinic VO2 Microparticles without Use of Hazardous Reagents: A Key Role for the W-Dopant.

Monoclinic vanadium dioxide (VO2 (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 VO2 (VO2 (R)), which is accompanied by a metal-to-insulator transition. Bottom-up hydrothermal synthesis has proven to yield high quality monoclinic VO2 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 V2O5 to produce VO2 (M). In early stages of the process, polymorphs VO2 (A) and VO2 (B) were formed, which subsequently recrystallized to VO2 (M). Without the presence of W6+, this recrystallization did not occur. After a reaction time of 96 h at 230 °C in the presence of (NH4)6H2W12O40 in Teflon-lined rotated autoclaves, we realized highly crystalline, phase pure W-doped VO2 (M) microparticles of uniform size and asterisk shape (ΔH = 28.30 J·g-1, arm length = 6.7 ± 0.4 μm, arm width = 0.46 ± 0.06 μm). We extensively investigated the role of W6+ in the kinetics of formation of VO2 (M) and the thermodynamics of its structural phase transition.