Thermal kinetic analysis of metal–insulator transition mechanism in W-doped VO2

Tungsten (W) doping can decrease the phase transition temperature of VO2, but underlining reasons are not clear. In this study, differential scanning calorimetry was employed to investigate the kinetics of the solid–solid transition in hydrothermal-synthesized W (X = 0.50, 1.09, 2.58 and 3.81 %)-doped VO2 nanoparticles. We firstly revealed the W-doping mechanism by combining the classical nucleation kinetics model with isoconversional kinetic analysis and applied them on the solid–solid transition taking place in doping. The experimentally observed large lag in the cooling stage and asymmetry effects of the decreasing temperature on insulator–metal transition and metal–insulator transition can be rationally explained. In the heating stage, W doping decreases free energy barrier (ΔG*) for homogenous nucleation and reduces geometrical factor (f(Θ)) and both factors promote the transition and thus lower the phase transition temperature quickly. However, in cooling stage, the free energy barrier (ΔG het * ) for heterogeneous nucleation was much larger than that of heating stage due to lacking of proper nucleation sites. The effect of decreasing geometrical factor was accompanied with the effect of increasing free energy barrier for homogenous nucleation by doping W. Such a competition mechanism slows down the trend of reducing temperature. It is important to unravel interaction mechanisms of doped W on different VO2 phases, which is helpful to further tailor kinetic properties of VO2 phase transition.