In a typical experiment, 0.3 g V2O5 (Wako, Japan) powder was dissolved in 10 ml solution of hydrogen peroxide (5wt%)(H2O2, Wako, Japan) in a 25 ml Teflon vessel. The exothermic reaction leads to the partial decomposition of hydrogen peroxide that in turn leads to the release of oxygen with rigorous bubbling and the formation of V(V) peroxo complexes. A clear orange solution was formed after about 10min (with the final pH about 1.5) and it progressively turned into a red-brown gelatinous flocculate forms which appears mainly amorphous by X-ray diffraction after ageing for 24 h. An appropriate N2H4 _ H2O (Wako,Japan) was added drop-wisely to this gel under continuously stirring. It can be observed that with the addition of hydrazine the gel turn black quickly, which means that some V2O5 was reduced. After that the Teflon vessel was sealed in a stainless steel autoclave and then followed by hydrothermal treatment. It has been found that high temperature prefers the formation of rutile VO2 phase. So most hydrothermal treatments were carried out at 260 C for 24h in this study. After hydrothermal treatment, the suspension was centrifuged and rinsed several times with distilled water and anhydrous alcohol. The final product was dried at 60 C for 10 h in desiccator.
To tune down the phase transition temperature of VO2, doping technology was used. In our experiments, high valence elements— tungsten from the source of ðNH4Þ10W12O41 _ 5H2O (Wako,Japan) was used for cation substitution. For this purpose, the doped samples were fabricated by adding appropriate amount of dopants just before sealing the Teflon vessel and then followed by hydrothermal treatment.