Electrochromism in Self-Assembled Porous Thin Films of Hexagonal Tungsten Trioxide Microspheres Prepared by Aqueous Chemical Growth

摘要

WO3 thin films were grown by self-assembly, at 80-90 C, on un-seeded conductive SnO2:F-glassthrough the method of Aqueous Chemical Growth (ACG). The thin films produced contained urchin- like microspheres of WO3, 1-2 m thick. Scanning Electron Microscopy (SEM) and TransmissionElectron Microscopy showed these microspheres to consist of a thick central core on which grew hair- like protrusions 200-400 nm long and 50-100 nm wide. X-ray Diffraction analysis, post annealing at500 C, revealed WO3 to be in the hexagonal phase while X-ray Photoelectron Spectroscopy confirmed6+ the presence of W in the W state. Focused Ion Beam-Field Emission SEM revealed a porous thinfilm grown for 18-24 h, averagely 3 m thick, that offers a wide surface area and shorter diffusion+ path-ways for H insertion in a 0.1M H2SO4 electrolyte. The fairly fast optical switching times (15- 25s) observed at a scan rate of 50 mV/s for such highly crystalline thin films can be attributed to the+ -11 2 -11 2 porous nature of the films. A diffusion coefficient for H of 1.66 x 10 cm /s and 1.33 x 10 cm /s at+ 50 mV/s was observed during H insertion/de-insertion. We propose that by reducing film growthtime, film thickness can be reduced rendering the method a viable one for producing WO3 thin filmsamenable to electrochromic devices.