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 8C 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 8C are characterized by a prominent triclinic crystalline structure and a hexagonal phase co-exists at temperatures 400 8C. 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 8C exhibits a high transmission modulation (DT 68%) and coloration efficiency (h 77.6 cm2 C 1) at l = 632.8 nm, charge storage capacity (Qins 21 mC cm 2), diffusion coefficient (6.08 10 10 cm2 s 1), fast color-bleach kinetics (tc 275 s and tb 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 8C has, for practical ‘‘smart window’’ applications has been demonstrated