Sol–gel synthesis of WO3 thin films

Abstract

Versatile molecular precursors for the sol–gel synthesis of tungsten oxide thin films can be obtained via the reaction of tungsten oxychloride WOCl₄ with alcohols. Oligomeric species [WOCl_{4 –x}(OR)_x]_n are formed. Their molecular structure is analysed by infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy (¹H, ¹³C, ¹⁸³W), X-ray absorption spectroscopy (EXAFS) and small-angle X-ray scattering (SAXS). Hydrolysis of these precursors leads to the formation of tungsten oxide colloidal solutions. They can be easily deposited by dipcoating. Thin films ca. 3000 Å thick are obtained. Their morphology depends on the nature of the alcohol. Homogeneous films are obtained with bulky alkyl groups such as PrⁱOH. These amorphous tungsten oxide layers exhibit electrochromic properties and could be used for display devices or smart windows. They can also be easily transformed, at room temperature, into crystalline hydrates WO₃·nH₂O (n= 1 or 2) when left in a humid atmosphere.