Significant changes in phase-transition hysteresis for Ti-doped VO2 films prepared by polymer-assisted deposition

This study is one of a series of researches on polymer-assisted deposition (PAD) of thermochromic VO2 film. This paper describes the synthesis of Ti-doped VO2 films (V1−xTixO2, x=0–0.167) and a systematic investigation of the effects of Ti doping on morphology, crystalline phase and optical properties (visible transmittance, transmittance changes in near infrared (NIR) light across the metal–insulator transition (MIT), absorption edge, MIT temperatures and sharpness) of V1−xTixO2 films. The films showed excellent visible transmittance and large changes (∼50% at 2000 nm) in the NIR-light transmittance before and after MIT for samples with a wide range of Ti contents (0–10%). The width of the hysteresis loop was severely reduced, which is in agreement with a qualitative analysis using the nucleation theory, while the MIT sharpness was not obviously influenced. An increase in the MIT temperature was observed for all samples with Ti doping, in particular 18.5 °C for 2% Ti doping, which is quite large compared with those in the literature. However, the MIT seemed saturated at around 80 °C. The morphology evolution of Ti-doped VO2 films was investigated for the first time. Interestingly, Ti doping could remarkably reduce the particle size of VO2 films due to Ti enhanced heterogeneous nucleation process of VO2 particles, indicating a new method for the growth of nanostructured VO2 films.