Photocatalytic applications of alkali-transition metal oxides from environmental remediation to solar-fuel generation: A review

Photocatalysis has emerged as a promising strategy to address global challenges in environmental remediation and energy sustainability. Research has mainly focused on tuning the electronic structure of typical semiconductors, whereas the role of surface phenomena, particularly adsorption, remains underexplored. Alkali-transition metal oxides (A x M y O z , where A = Li, Na, or K; and M is a first-row transition metal) show enhanced surface basicity due to alkali-cation incorporation within the crystal structure, modifying the electronic density and strength of Lewis and Brønsted acid/basic sites. These features influence adsorption behavior toward key reactant species such as CO 2 , H 2 O, and organic pollutants. In this review, alkali-transition metal oxides are systematically organized by transition metal component (Ti, V, Mn, Fe, and Co), scrutinizing their photocatalytic performance across dye degradation, pharmaceutical, and hazardous pollutant removal, hydrogen photoproduction, and CO 2 valorization. Alkali-titanates are the most extensively studied family, whereas alkali-manganates, ferrites, vanadates, and cobaltates remain comparatively underexplored despite promising theoretical and experimental results. Across these systems, surface basicity consistently correlates with improved adsorption and photocatalytic efficiency. Nevertheless, these properties are rarely explicitly intertwined in the literature, with performance improvements only attributed to charge-carrier dynamics. Building on these findings, four research directions are proposed: correlating pH, surface charge, and sacrificial agent speciation in H 2 evolution; replacing conventional sacrificial agents with biomass- or glycerol-derived alternatives; developing photocatalytic methane reforming; and recycling spent lithium-battery cathode materials as photocatalysts. These future directions aim to establish a unified framework linking composition, surface chemistry, and photocatalytic performance within a circular economy approach.