Electrochromic effect of nanostructured WO3 bulk

WO3 films are well known for the change of their color through electro-, photo- and thermo-chromism. We report here that the nanostructured WO3 bulk has an obvious electrochromic effect, which does not exist in ordinary WO3 bulk. The sample turns to a deep blue color upon changing from anode to cathode when external voltage is applied. There is no difference of the microstructure and phase structure between the colored and uncolored samples. X-ray photoelectron spectroscopy analysis shows that low valence state of tungsten ion (W5+) existed in the colored samples. Based on the experimental results, it is concluded that the coloration is an electron-injection effect. The high valence state of tungsten ion (W6+) in nanostructured WO3 bulk has enough activity to combine with electron and turns to the low valence state counterpart because of the surface effects of nanostructured materials. As a consequence, the sample changes its color.