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

A pure TiO 2 thin film (100–120 nm) was made from a green aqueous sol–gel precursor on FTO glass and calcined at 430 °C. It was a mix of amorphous, anatase, rutile and brookite TiO 2 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 (Δ T ) = 69.7% at 550 nm, 86% at 700 nm and an overall Δ T between 400–1650 nm of 60%, giving a very large change in optical density (ΔOD) of 1.4 at 550 nm and 2.4 at 700 nm. Cyclic voltammograms had typical peaks for TiO 2 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 Q c = 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 cm 2 C −1 at 550 nm. These ΔOD, Δ T , Q c and CE values are superior to any previously reported for crystalline sol–gel TiO 2 films. They also possessed rapid switching times for bleaching and colouring of τ b90% = 10 s and τ c90% = 55 s, comparable to the best previously reported sol–gel anatase-based TiO 2 films. This makes this nanomaterial an excellent candidate for smart windows and other electrochromic devices and applications.