Advances in BiOX-based ternary photocatalysts for water technology and energy storage applications: Research trends, challenges, solutions, and ways forward

BiOX (bismuth oxyhalides) are a ternary structure (V–VI–VII) of semiconductor materials with tunable band gaps, a low recombination rate, wide light absorption range, electro-magnetical and optical properties due to their unique crystallinity with tetragonal matlockite configuration. This article critically reviews the applicability of BiOX-based photocatalysts for water treatment and/or energy storage applications. To enhance their photocatalytic activities under visible light, a particular focus is given to the formation of heterojunctions, or plasmonic nanoparticles. Their technological strengths and limitations are evaluated and compared. Synthesis techniques such as precipitation, solvothermal, hydrolysis, and doping strategy of self-assembling BiOX including heterojunctions with other semiconductors in enhancing photocatalytic performance are presented. Research direction, challenges, and perspectives of BiOX-based photocatalysts for practical applications are elaborated. It is evident from a literature survey of 227 published articles (1972–2022) that the physico-chemical properties of BiOX-based photocatalysts such as energy band structures and anisotropic layered structure are responsible for UV light-driven photocatalytic performance. The hybridized valence band of O 2p and Bi 6s² orbitals in the Bi(III)-based compounds upshifts their valence band (vb) that narrows energy bandgap and expands the absorption of visible light range. Among the BiOX, BiOI and BiOCl are the most outstanding photocatalysts under UV Vis irradiation due to their narrow bandgaps (Eᵒ = 2.0 and 3.4 eV, respectively), large surface area, and strong light absorption. It is important to note that technical applicability, target pollutants, and cost-effectiveness represent the key factors in selecting the most appropriate BiOX-based photocatalysts for water treatment and/or energy storage applications.