ELECTROCHROMIC NB2O5 AND NB2O5 SILICONE COMPOSITE THIN-FILMS PREPARED BY SOL-GEL PROCESSING

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 dm-3 H2SO4. Electron microscopic examination of the resulting 5-10-mu-m thick films revealed extensive shrinkage and cracking on gel drying. These films proved to be electrochromic (lambda-max ca. 800 nm) but unstable to voltammetric cycling in MeCN-(0.5 mol dm-3)LiClO4. Greater stability (> 30 cycles) was achieved with a composite Nb2O5/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 x 10(-8) cm2 s-1 for Nb2O5 and 3.2 x 10(-8) cm2 s-1 for the composite. The mechanism of the electrochromic response and its decay are discussed.