Defect engineering of BiOX (X = Cl, Br, I) based photocatalysts for energy and environmental applications: Current progress and future perspectives

The emerging layered BiOX (X = Cl, Br, I) materials have been widely used for energy and environmental applications. The BiOX crystal structure has been arranged by stacking of alternate [Bi2O2] slabs, double slabs of halogen, Bi–O, and Bi–X bond linked by sturdy covalent bonds and fragile interactions of van der Waals force between X and X. The layered ultrathin structures of BiOX are relatively obtained by van der Waals interactions of BiOX. In general, BiOX has high photocatalytic activity attributed to its indirect bandgap (optical transition) and layered crystalline structures.The existence of defects such as oxygen vacancy, halide vacancy and bismuth-rich in BiOX tunes the facets of its layered materials and boosts the physicochemical properties, thereby enhancing the photocatalytic activity in energy conversion and environmental remediation applications. Furthermore, various synthesis routes, confirmation of defect formation with different analytical characterisations and significance of defects in BiOX have been comprehensively reviewed. Apart from materials synthesis, the role of defect engineering on the performance enhancement in photocatalytic dye degradation, water splitting, CO2 reduction, organic transformation, ammonia synthesis, degradation of antibiotics is also discussed in detail. The current research trends, challenges and future research opportunities of defective BiOX are prospected to advance the growing field in energy and environmental science.