Effects of precursor chemistry and tungsten doping on structure, phase-transition thermodynamics, and thermochromic performance of VO2(M) nanoparticles synthesized by supercritical hydrothermal flow synthesis

Monoclinic vanadium dioxide (VO 2 (M)) nanoparticles were synthesized by continuous hydrothermal flow synthesis (CHFS) without reducing agents or post-annealing, and the effects of the precursor chemistry and tungsten incorporation on the structural evolution, phase-transition thermodynamics, and thermochromic performance were investigated. Precursor suspensions prepared using NH 4 HCO 3 , NH 3 ·H 2 O, and NaOH exhibited distinct thermal decomposition behaviors. Corresponding differences were also observed in the crystallization and particle formation under CHFS conditions. Among the three systems, the NH 3 -derived precursor produced relatively small and homogeneous nanoparticles with a high crystallinity. The resulting particles also exhibited the highest transition enthalpy and thermochromic modulation. W-doped VO 2 nanoparticles were subsequently prepared using the NH 3 -derived precursor. W incorporation systematically lowered the phase-transition temperature at rates of 13.3 and 11.9 °C at.% -1 during the heating and cooling phases, respectively. At 1.5 at.% W, the heating transition temperature decreased from 68.3 to 38.1 °C, while a high transition enthalpy was retained (42.0 to 39.9 J g −1 ). The near-infrared modulation at 1200 nm was likewise preserved from 47 % to 45 %. At higher W concentrations, both the transition enthalpy and thermochromic modulation rapidly decreased. These results demonstrated that retaining a high transition enthalpy and near-infrared modulation is as important as lowering the transition temperature when designing high-performance thermochromic VO 2 nanoparticles.