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

Abstract

Alcoholysis of NbCl₅ in EtOH gives a viscous solution of NbCl_x(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 (λ_maxca. 800 nm) but unstable to voltammetric cycling in MeCN–(0.5 mol dm^–3)LiClO₄. Greater stability (> 30 cycles) was achieved with a composite Nb₂O_n/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 D_Li= 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.