Optical and electrochromic properties of sol–gel WO 3 films on conducting glass

Abstract

The precursor containing peroxotungstic acid (PTA) was prepared by the reaction of tungstic acid with H 2O 2 (30%) solution in water–ethanol mixture. This was utilized for the deposition of WO 3· nH 2O film on conducting (F-doped SnO 2 coated) flat glass substrate by the dipping technique. The films were cured in the temperature range of 200–300 °C in air. The optimum thickness of the film (300–500 nm) was obtained by successive operations (3–5 numbers). Films of around 500 nm thickness exhibited 60–70% transmission in the visible region. Electrochromic properties (colouration↔bleaching) of the films were studied by cyclic voltammetry (CV) using a classical three-electrode potentiostatic cell system. The cell system consists of a WO 3-coated sample as working electrode, a platinum rod as counter electrode, Ag/AgCl as reference electrode and 0.5 M LiClO 4 in propylene carbonate as an electrolyte. Several voltammograms were recorded within the voltage range of +1.5 to −1.8 V with a scan rate of 50 mV/s. A continuous curve was observed for the films at a certain voltage sweep where a random insertion of Li + occurred reversibly in a definite crystallographic site [WO 3 (colourless)+ nLi ++ ne −↔ Li n WO 3 (blue)]. A dark blue colouration (% T=20–30% in the visible region) was observed under a constant voltage of −1.8 V whereas bleaching occurred at +1.0 V (% T=60–70%), which was studied simultaneously along with the voltage sweep of CV. The colouration time ( T col) and the bleaching time ( T bl) were almost equal as revealed by the simultaneous study of the second signal (photomultiplier output coupled with the electrochemistry system) during colouration↔bleaching. Coatings of about 500 nm thickness exhibited more that 500 cycles (colouration↔bleaching). The reversibility of the cycle remained good but the intensity of the colouration decreased with the number of cycles. This was possibly due to the structural deformation of WO 3 films for its long time exposure to the electrolytic solution. The cathodic current ( I c) and anodic current ( I a) increased with increasing thickness.