Electrochromic Nb2O5and Nb2O5/silicone composite thin films prepared by sol–gel processing

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Abstract
Alcoholysis of NbCl5 in EtOH gives a viscous solution of NbClx(OEt)5 –x which may be used to spin-coat optically transparent electrodes (indium tin oxide, ITO). The electrode is converted to Nb2O5 by immersion in 1 mol dm3 H2SO4. Electron microscopic examination of the resulting 5–10 µm thick films revealed extensive shrinkage and cracking on gel drying. These films proved to be electrochromic (λmaxca. 800 nm) but unstable to voltammetric cycling in MeCN–(0.5 mol dm–3)LiClO4. Greater stability (> 30 cycles) was achieved with a composite Nb2On/silicone electrode, prepared from a solution containing Nb:Si:H2O in the molar ratio 5 : 13 : 82. This increase in durability was achieved with no sacrifice in electrochromic efficiency. The electrochromic colouration efficiency (6 cm2 C–1) compares favourably with that for sputtered Nb2O5 films. The colouration–bleaching cycle of the composite is complete in < 40 s, and the electrochromical kinetics of colouration and bleaching follow a diffusional model with DLi= 6 × 10–8 cm2 s–1 for Nb2O5 and 3.2 × 10–8 cm2 s–1 for the composite. The mechanism of the electrochromic response and its decay are discussed.