Compositional inhomogeneity and its impact on the phase transition kinetics of hydrothermally synthesized W-doped VO2

Tungsten-doped vanadium dioxide is a promising thermochromic material for energy-efficient smart windows. Its performance depends on the thermodynamics and kinetics of the first-order structural phase transition (SPT) between semiconducting monoclinic VO 2 and metallic rutile VO 2 . In this study, W-doped VO 2 microparticles (17–34 μm) were synthesized via a hydrazine-free hydrothermal route using oxalic acid as the reducing agent. The influence of tungsten incorporation on phase formation and on the thermodynamics and kinetics of the SPT was investigated. Structural and morphological analyses show that W 6+ promotes the formation of phase-pure monoclinic and rutile VO 2 . Differential scanning calorimetry revealed switching enthalpies between 15 and 36 J g –1 and a decrease in phase transition temperature with increasing W content. Broad transition profiles and deviations from ideal switching behavior were attributed to compositional and structural inhomogeneities arising from non-uniform tungsten distribution and particle size variation. Isoconversional kinetic analysis demonstrated that the SPT is governed by nucleation kinetics, resulting in a temperature-dependent activation barrier that increases strongly as the temperature approaches the critical switching temperature. Activation energies ranging from 10 3 to 10 4 kJ mol –1 were obtained, indicating that substantial overheating or undercooling is required to achieve rapid thermochromic switching.