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

Abstract

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 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 cm² C⁻¹ at 550 nm. These ΔOD, ΔT, Q_c 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 τ_b90% = 10 s and τ_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.