Light-to-color conversion on MoO3, WO3, and Bi2WO6: from mechanism to materials and applications

Because of their good chemical stability and excellent optical properties, MoO3, WO3, and Bi2WO6 are important in photochromism. Their light-to-color conversion is highly dependent on the electronic band structure and charge transfer, and they obey the mechanism of electron accumulation in semiconductors when excited within the bandgap. Pure semiconductors face limitations in practical applications due to insufficient light absorption, charge carrier recombination, and low charge capacity. Diverse forms of photochromic hybrids (nanopowders, films, hydrogels, and multilayer structures) with rapid change, repeatability, and reversibility are possible via nanocustomization, surface/interface engineering, heterojunction fabrication, and complexing organic ligands. Manipulating the function of photochromic systems through light stimulation is becoming an attractive paradigm, divided into two branches: light-color complementarity and photoconductivity. This review examines the widely accepted photoresponsive principles and the still controversial energy transfer models. We emphasize the correlation between material properties and performance enhancement to inspire the rational structure design. The bottlenecks in current development are identified by analyzing application-specific innovation concepts, fabrication processes, and performance metrics. In addition, we present several perspectives to encourage meaningful multidisciplinary collaboration.