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 °C. 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 (ΔT) = 69.7% at 550 nm, 86% at 700 nm and an overall ΔT 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 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 ΔOD, ΔT, 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 τb90% = 10 s and τc90% = 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.