Hydrothermal Synthesis of Monoclinic VO 2 Microparticles without Use of Hazardous Reagents: A Key Role for the W-Dopant.

Monoclinic vanadium dioxide (VO 2 (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 2 (VO 2 (R)), which is accompanied by a metal-to-insulator transition. Bottom-up hydrothermal synthesis has proven to yield high quality monoclinic VO 2 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 V 2 O 5 to produce VO 2 (M). In early stages of the process, polymorphs VO 2 (A) and VO 2 (B) were formed, which subsequently recrystallized to VO 2 (M). Without the presence of W 6+ , this recrystallization did not occur. After a reaction time of 96 h at 230 °C in the presence of (NH 4 ) 6 H 2 W 12 O 40 in Teflon-lined rotated autoclaves, we realized highly crystalline, phase pure W-doped VO 2 (M) microparticles of uniform size and asterisk shape (Δ H = 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 W 6+ in the kinetics of formation of VO 2 (M) and the thermodynamics of its structural phase transition.