Fig. 7 showed the optical properties for films coated with as-prepared VO2 nanopowder. The tested films were prepared through coating nano-sized VO2 powders on the high transparency Teflon tape.The average amount of VO2 nanopowder coated on one square metre tape was about (0.15710%) g, which was calculated by weighting the change of the quantity of Teflon tape before and after coating. Fig. 7(a) was a typical SEM image for one of these films and the transmittance spectra corresponding to 20 C and 80 C for samples with different doping amount of tungsten were illustrated at Fig. 7(b). Table 2 listed the optical properties for films coated by VO2 powders with different doping amount of tungsten. The comprehensive properties of our powder and film were very nice. It reached a sufficiently high luminous transmittance (45–60%) and keeping the thermochromic effect in the infrared area very well. VO2 powders with such high quality had not been reported until now. Generally, its optical properties are comparable with high quality single layer VO2 film deposited by PVD. Slightly doping with tungsten did not decrease the optical properties while highly doped samples showed lower thermochromic effect in the infrared area. According to the model of Tang et al. [49], the incorporation of W atoms leads to the loss of V4þ–V4þ pairs, which are essential for the crystal structure of the semiconducting VO2 phase [3,50]. Elements doping destabilize the semiconducting phase and consequently lower the semicon-ductor–metal phase transition temperature. At the same time, doping with W atoms increase the concentration of free electrons and the switching property will deteriorate with high doping content. Given the relatively lower starting point of tungsten doping for decreasing the tc of VO2, our prepared powders can approach the practicable temperature by slightly doping of tungsten without deteriorating the thermochromic properties, which made this powder very suitable for practical application.