Preparation and electrochromic properties of V–Nb mixed-oxide films by evaporation

Abstract

Pentoxide vanadium materials, as a counter electrode of WO3 in electrochromic devices, have been widely investigated using various methods. For example, vanadium oxide films have been prepared by vacuum evaporation [1, 2], sol–gel processes [3], sputtering [4, 5], and chemical vapor deposition [6]. V2O5 films prepared by these methods have been found to exhibit less reversibility than tungsten oxide films (beyond 1200 cycles), as reported by Macek et al. [7]. Another drawback of V2O5 films is their weaker optical properties [8, 9]. V2O5 films darken upon lithium extraction (oxidized state) in the blue and near-U.V. ranges (300– 400 nm) and have weak colouration on lithium insertion (reduced state) in the red and near infrared ranges. Due to the effects of such shortcomings, a compromise has to be made between the transmittance range and the maximum transmittance range in designing electrochromical devices with V2O5 counter electrodes. To solve these problems, researchers have attempted to minimize the near-infrared colouration of reduced V2O5 films by adding dopants such as Ce [10] and Fe [11]. In this study, the electrochromic behaviors of V2O5 films modified by Nb2O5 were investigated. Niobium was chosen because sputtered Nb2O5 films have been observed to exhibit only weak colouration in the visible range and no colouration in the near-infrared range upon lithium insertion [12]. In addition, Nb2O5 films have shown high reversibility under voltammetric cycling [13]. In our work, to decrease the deviation in stoichiometry between V–Nb mixed-oxide films and starting materials, V2O5 and Nb2O5 powders were sintered at 900 C for 2 h before being evaporated. The compositions of these films were investigated by ICP. Their recharge ability and the cycle reversibility of the Li/e insertion/extraction process were studied through cyclic voltammetry. Optical properties of lithium ion intercalated/deintercalated films were investigated by in situ U.V.–VIS. spectroelectrochemical measurement. The film structures were also studied and characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), and spectrofluorometry. 2. Experimental details