Formation of AgI/TiO2 nanocomposite leads to excellent thermochromic reversibility and photostability

Abstract

AgI/TiO₂ nanocomposite was prepared and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and diffusive reflectance UV-vis (DRUV-vis) absorption spectra at different temperatures. The AgI/TiO₂ nanocomposite exhibits thermochromic switching phenomena at the phase β–α transition temperature of AgI with the evolution of its color from light yellow at ambient temperature to dark khaki at 150 °C. It was found for the first time that the formation of the AgI/TiO₂ nanocomposite not only results in a considerable reduction of thermochromic transition temperature of AgI as well as excellent thermochromic reversibility, but also leads to excellent photostability under illumination with an indoor fluorescent lamp. Based on the characterization results by photoluminescence, transient photocurrent decay and positron annihilation spectra, it is concluded that large surface defects, such as I⁻ vacancy clusters (e.g.Ag⁺–I⁻_V–Ag⁺–I⁻_V–Ag⁺) on the surface of AgI as deep electron traps, are responsible for the photodecomposition of pure AgI. The excellent photostability of the AgI/TiO₂ nanocomposite is due to the following reason: the surface I⁻ vacancy clusters as deep electron traps in pure AgI are replaced by Ag⁺–O²⁻–Ti⁴⁺ bonds on the interface of AgI/TiO₂, thus Ag_ncluster formation through the surface migration of photogenerated Ag atoms is completely inhibited, resulting in the inhibition of photodecomposition of AgI.