A 20-Year Review of Inorganic Photochromic Materials: Design Consideration, Synthesis Methods, Classifications, Optical Properties, Mechanism Models, and Emerging Applications

Inorganic photochromic materials (i.e., in-PCMs) are developed as a research hot point in the lighting field due to their excellent optical characteristics, impressive  body color modulation, etc. The aim of this work is to make a review on the developments of multi-metals in-PCMs (i.e., min-PCMs) in the past 20 years. The review is structured based on the reversible coloration, crystal hosts, doped ions, PC properties, and mechanism of min-PCMs and their applications. The synthesis methods of min-PCMs are reviewed first and then the min-PCMs are classified according to the types of the doped ions (i.e., single rare-earth (i.e., RE) ion, multi RE ions, non-RE ions, and/or the coupled arrangement of these ions). Meanwhile, the min-PCMs co-doped with RE and non-RE ions, together with RE- and/or non-RE-doped and co-doped dual crystal matrix, are also disclosed. In the 4th section, we underline the PC mechanisms and show the perspectives on how to produce the PC and obtain the related min-PCMs. After that, diverse emerging applications of min-PCMs are summarized, which include optical storage, anti-counterfeiting, bio-imaging, light detection, temperature sensing, photocatalysis, and smart windows. In the end, we conclude this review and forecast the promising opportunities and potential challenges of min-PCMs in the future.