Boron doped M-phase VO2 nanoparticles with low metal-insulator phase transition temperature for smart windows

Vanadium dioxide (VO2) is considered to be a promising candidate for energy-efficient smart windows because of its special reversible Metal-Insulator Transition (MIT) near the ambient temperature. However, its use is constrained by its high transition temperature (TC) relative to the room temperature. In this paper, VO2 doped by boron, could achieve an outstanding metal-insulator phase transition property with a low TC (28.1 °C) close to the room temperature. This enhancement strongly contributes to the studies of the VO2-based smart windows. A limit doping level of around 9.0 at% is observed for the boron-doped VO2. Moreover, the particle size is getting smaller and more uniform and the particle distribution becomes more equal and compact with the continued increase in the doping content. Such uniform grain size and grain boundary conditions suppress the extension of the hysteresis loop (ΔT decreases from 25 °C to 7 °C). In addition, the TC first declines with the increase in the boron content and it starts to increase after reaching its minima of 28.1 °C at 6.0 at% doping level. This feature is the consequence of the competition between the inhibition on the phase transition caused by the V5+ and the promotion on the phase transition caused by the heterogeneous defect-nucleation sites. VO2 doped with 6.0 at% boron exhibits a favorable thermochromic performance with ΔTsol of 12.5% and Tlum up to 54.3%, which is promising for the smart windows.