Synthesis of Tungsten-Doped Vanadium Dioxide Using a Modified Polyol Method Involving 1-Dodecanol

The doping of tungsten into VO₂ (M) via a polyol process that is based on oligomerization of ammonium metavanadate and ethylene glycol (EG) to synthesize a vanadyl ethylene glycolate (VEG) followed by postcalcination was carried out by simply adding 1-dodecanol and the tungsten source tungstenoxytetrachloride (WOCl₄). Tungsten-doped VEGs (W-VEGs) and their calcinated compounds (W_xVO₂) were prepared with varying mixing ratios of EG to 1-dodecanol and WOCl₄ concentrations. Characterizations of W-VEGs by powder X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and infrared and transmittance spectroscopy showed that tungsten elements were successfully doped into W_xVO₂, thereby decreasing the metal-insulator transition temperature from 68 down to 51 °C. Our results suggested that WOCl₄ variously combined with 1-dodecanol might interrupt the linear growth of W-VEGs, but that such an interruption might be alleviated at the optimal 1:1 mixing ratio of EG to 1-dodecanol, resulting in the successful W doping. The difference in the solar modulations of a W_0.0207VO₂ dispersion measured at 20 and 70 °C was increased to 21.8% while that of a pure VO₂ dispersion was 2.5%. It was suggested that WOCl₄ coupled with both EG and 1-dodecanol at an optimal mixing ratio could improve the formation of W-VEG and W_xVO₂ and that the bulky dodecyl chains might act as defects to decrease crystallinity.