Blue tungsten oxide (BTO) is reduced by ammonium bromide at 390-430℃. The so-called blue tungsten oxide does not have a unified molecular formula, which can be written in WO2.9.
From X-ray photoelectron spectrum (XPS) can analyze the surface of the valence of blue tungsten oxide. Reducing of oxygen causes W5+ and W6+ valences of blue tungsten oxide. With the temperature rising and the oxygen content decreasing, W6+ tungsten oxide decreases and blue tungsten oxide change from deep blue to violet blue. W4+ tungsten oxide appears in the photoelectron spectrum at 32.6 eV of the peak.
XPS is a technology using information of photoelectron intensity and electrons angular distribution to study the electronic structure of atomic, molecular and condensed phase, especially solid surface.