“Bi–O” vacancy-pairs induced photochromic behavior in Bi2WO6 ultrathin nanosheets

Developing cheap and efficient materials for enhancing the photochromic performance upon visible or solar light irradiation is a hot-topic in the field of solar energy conversion. However, few inorganic photochromic materials can provide satisfying applications due to their low coloring and bleaching efficiency as well as slight color change. Here, we synthesized the bismuth tungstate (Bi2WO6) ultrathin nanosheets with the thickness of 1.29 nm via co-precipitation-hydrothermal method using cetyltrimethylammonium bromide (CTAB) surfactant. The prepared ultrathin nanosheets show the similar crystalline structure but a narrowed band gap of 2.73 eV, as compared with the 2.86 eV of pristine Bi2WO6. Additionally, the numerous [WO6–xBr2x] species and abundant “Bi–O″ vacancy pairs on the surface can serve as active sites that suppress the recombination of photogenerated electron-hole pairs and capture the photogenerated electrons to form the color trapping centers. As a result, Bi2WO6 ultrathin nanosheets exhibit excellent photochromic behavior upon solar light irradiation (coloring time of 5 s) and under dark atmospheric treatment (bleaching time of 35 min) compared with WO3-based photochromic materials (coloring time of 20–60 min and bleaching time of 3 h days).