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

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.