<title>Microstructure, optical, and electrochromic properties of sol-gel nanoporous tungsten oxide films</title>

Abstract

Porous tungsten oxide films have been prepared by a nonhydrolitic sol-gel method using poly(ethylene glycol) (PEG) as a structure directing agent. The method entails the hydrolysis of an ethanolic solution of tungsten ethoxide (formed by the reaction of WCl₆ with ethanol) followed by condensation and polymerization at the PEG-tungsten oxide oligometers interface. A highly porous WO₃ framework was obtained after PEG was burned off by calcination at a relativley low temperature. AFM images of the films treated thermally show an ordered material rather than microscopic particulates. Both fibrilar nanostructures and striped phase can be obtained via this approach, depending on the concentration of PEG in the coating solution. XRD data from the fibrils indicate that they are crystalline with very small crystals, whereas the striped phases obtained with 20% PEG correspond to two crystalline phases, one, the stoichiometric WO₃ and the other one an oxygen deficient phase, containing larger crystals (~28 nm). The results show that PEG promotes the formation of oxygen deficient phases and delays crystallization. Compared to WO₃ with no PEG, the optical and electrochromic properties of the macroporous tungsten oxide films appear to be significantly improved. The formation of organized nanostructures is tentatively accounted for by the strong hydrogen bonding interactions between PEG and the tungsten oxide oligomers.