Realizing temperature-gated photochromic NaYTiO4:Bi3+ for a time–temperature indicator

Abstract

Time–temperature indicator (TTI) technologies enable real-time quality monitoring of perishable products during transportation–storage. Photochromic material-guided TTI offers significant benefits in terms of nondestructive and convenient visualization. However, photochromic materials with low-temperature-range dependency are rare, limiting the development of cryogenic temperature-responsive TTI methods. This work proposes a novel temperature-gated bicolor photochromic material. Using NaYTiO₄ as the matrix, which has a native blue color center related to the intrinsic deep trap, Bi³⁺ ions are incorporated to create extremely shallow trap levels (100–230 K) along with an associated longwavelength absorption color center. By combining these two color centers with extended full-spectrum absorption, NaYTiO₄:Bi³⁺ converts to dark gray upon ultraviolet irradiation below 233 K. With increasing temperature, the trapped electrons in shallow traps are released first, and the color state becomes yellowish until it completely fades back to white above 573 K. The liberation of thermally activated charge carriers is positively correlated with the storage duration and the ambient temperature. For the first time, we realize a TTI method based on photochromic materials at extremely low temperatures, which allows for direct visualization of quality management for cryogenic products without further information extraction and conversion. This work demonstrates the significant ability of photochromic materials as advanced information-recording materials in the next generation of smart TTIs.