Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films

A significant influence of microstructure on the electrochromic and electrochemical performance characteristics of tungsten oxide (WO3) films potentiostatically electrodeposited from a peroxopolytungstic acid (PPTA) sol has been evaluated as a function of annealing temperature. Powerful probes like X-ray diffractometry (XRD), transmission electron microscopy (TEM), UV-vis spectrophotometry, multiple step chronoamperometry and cyclic voltammetry have been employed for the thin film characterization. The as-deposited and the film annealed at 60 degrees C are composed of nanosized grains with a dominant amorphous phase, as well as open structure which ensues from a nanoporous matrix. This ensures a greater number of electroactive sites and a higher reaction area thereby manifesting in electrochromic responses superior to that of the films annealed at higher temperatures. The films annealed at temperatures >= 250 degrees C are characterized by a prominent triclinic crystalline structure and a hexagonal phase co-exists at temperatures >= 400 degrees C. The deleterious effect on the electrochromic properties of the film with annealing is ascribed to the loss of porosity, densification and the increasing crystallinity and grain size. Amongst all films under investigation, the film annealed at 60 degrees C exhibits a high transmission modulation (AT similar to 68%) and coloration efficiency (eta similar to 77.6 cm(2) C-1) at lambda = 632.8 nm, charge storage capacity (Q(ins) similar to 21 MC cm(-2)), diffusion coefficient (6.08 x 10(-10) cm(2) s(-1)), fast color-bleach kinetics (t(c) similar to 275 s and t(b) similar to 12.5 s) and good electrochemical activity, as well as reversibility, for the lithium insertion-extraction process upon cycling. The remarkable potential, which the film annealed at 60 'C has, for practical "smart window" applications has been demonstrated. (c) 2005 Elsevier B.V. All rights reserved.