High colouring efficiency, optical density and inserted charge in sol-gel derived electrochromic titania nanostructures

A pure TiO2 thin film (100-120 nm) was made from a green aqueous sol-gel precursor on FTO glass and calcined at 430 1C. It was a mix of amorphous, anatase, rutile and brookite TiO2 phases, and exhibited very good electrochromic properties over visible and NIR wavelengths with an applied bias of +0.1 V to -1.5 V. It was highly transparent showing excellent coloration with applied voltage, with transmittance modulation (?T) = 69.7% at 550 nm, 86% at 700 nm and an overall ?T between 400-1650 nm of 60%, giving a very large change in optical density (?OD) of 1.4 at 550 nm and 2.4 at 700 nm. Cyclic voltammograms had typical peaks for TiO2 at -1.3 V for colouration and -0.9 V for bleaching, with a high separation of 0.37 V between peaks, and a charge density after charging for 25 min of Qc = 50 mC cm-2. After only 60 s and 120 s at -1.5 V, inserted charge values of 17.6 and 22 mC cm-2 were observed, leading to a high colouration efficiency (CE) of 55.9 cm2 C-1 at 550 nm. These ?OD, ?T, Qc and CE values are superior to any previously reported for crystalline sol-gel TiO2 films. They also possessed rapid switching times for bleaching and colouring of ?b90% = 10 s and ?c90% = 55 s, comparable to the best previously reported sol-gel anatase-based TiO2 films. This makes this nanomaterial an excellent candidate for smart windows and other electrochromic devices and applications.