An In-Depth Study of the Thermodynamics and Kinetics of the Structural Phase Transition of Hydrothermally Synthesized W/VO2 Microparticles

Abstract

Thermochromic vanadium dioxide (VO2) is widely recognized for its reversible structural phase transition (SPT) from a semiconducting monoclinic VO2(M) phase to a metallic rutile VO2(R) phase near 68 °C, accompanied by a sharp modulation in infrared (IR) transmittance.1,2 This unique feature has positioned VO2 as a strong candidate for smart window coatings in energy-efficient buildings.3 However, the high intrinsic transition temperature and optical limitations of bulk VO2 have motivated intensive research into lowering the transition temperature and improving phase transition behaviour through metal doping and particle engineering.4,5 In this study, we present a comprehensive investigation of both the thermodynamic and kinetic properties of the SPT in hydrothermally synthesized, tungsten-doped W:VO2 microparticles. A benign, oxalic acid-based hydrothermal method was employed to produce phase-pure VO2(M) particles with W concentrations ranging from 0.5 to 2.5 at.%. Differential scanning calorimetry (DSC) was used to extract key transition parameters, including transition temperature (Tspt), enthalpy (ΔHspt), and hysteresis width, while isoconversional kinetic analysis provided activation energy estimates for both heating and cooling transitions. Our analysis shows that W-doping successfully lowers the transition temperature (Tspt) across the full doping range. A reduction of approximately 22 °C per atomic percent of W was expected, and while a decrease was observed for all samples, inhomogeneous W incorporation into the VO2 lattice resulted in higher transition temperatures than theoretically predicted for the respective doping levels. High transition enthalpies (ΔHspt) were achieved across all compositions, indicative of a highly crystalline material. However, inhomogeneous W distribution and broad particle size distributions contributed to relatively wide hysteresis widths. Calorimetric and kinetic data indicate measurable differences in activation energy barriers, suggesting that dopant level and microstructure influence switching behaviour. Results are compared to the kinetic characteristics of VO2 materials prepared via classic bead milling, as reported in the literature by Calvi et al., to contextualize these findings. This work addresses a critical knowledge gap in VO2 synthesis and transformation kinetics, supporting future design strategies for VO2-based smart coatings that are optically viable, thermally responsive, and kinetically stable under repeated environmental exposure.