Pronounced photochromism of titanium oxide hydrates (hydrous TiO2)

Titanium oxide hydrates are a class of materials that attracted strong scientific interest in the first half of the 19th century, but nowadays find use virtually only as precursors for the preparation of crystalline TiO2. We re-visited these inorganic species and show here that they display highly useful properties. We demonstrate that dispersions of titanium oxide hydrates, prepared by low-temperature hydrolysis of titanium tetrachloride, TiCl4, or titanium tetraisopropoxide (TTIP) exhibit pronounced photochromism upon irradiation with UV light-especially when in the presence of glycerol. X-band electron-spin resonance spectroscopy reveals that the photochromism originates in the formation of Ti-III species. The glycerol also considerably stabilises the dispersions, preventing aggregation of the titanium oxide hydrates and, thus, permitting preparation of transparent systems of high content of such inorganic polymers. When fully converting the Ti species to crystalline TiO2, the photochromic response is significantly reduced-if not entirely lost-indicating that titanium oxide hydrates were the true origin for the photochromism reported in the past for "crystalline TiO2". Clearly, the strong photochromic response can be taken advantage of, for instance, for the detection of amorphous fractions in crystalline TiO2 or the fabrication of UV light detectors.