Solar energy is one of Earth’s most abundant and cleanest resources. It is vital for future energy systems. Being one such abundant energy resource, its efficient usage relies on the development of advanced materials for solar energy conversion. Among the various materials, metal sulfide-based photocatalysts have gained significant attention for their promising potential in solar energy to chemical energy conversion. These unique physical, electronic, and chemical properties make them ideal candidates for various photocatalytic applications involving water splitting, CO₂ reduction, pollutant degradation, and nitrogen fixation. In the last decade, great efforts have been focused on the design and optimization of metal sulfide photocatalysts with the aim of achieving highly efficient, stable, and selective solar energy conversion processes. This review article updates the recent progress made on metal sulfide photocatalysts and discusses their possibilities in securing a sustainable energy future.
Metal Sulfide Photocatalysts
Metal sulfides are semiconductors that contain metal cations and sulfur anions. Because they can absorb light and catalyze redox reactions, metal sulfides have become the focus of many studies concerning their potential applications in various photocatalytic processes. The excellent performance of metal sulfides is due to their tunable band gaps, strong light absorption, high electron mobility, and abundant active sites. These materials show good efficiency in solar spectrum harvesting, including UV and visible light, hence very efficient solar energy conversion.
One of the major challenges for solar energy conversion lies in the efficient separation of the photogenerated electron-hole pairs. The problem is solved in metal sulfide photocatalysts by strategies of bandgap engineering, surface modification, and heterojunction formation. In this process, the electronic structure of the material can be optimized toward superior charge separation and transfer behavior, hence further improving the photocatalytic performance. Besides, most metal sulfides have high chemical stability, which provides a guarantee for long-term catalytic activity in harsh conditions.
Solar Water Splitting with Metal Sulfides
One of the fascinating applications of metal sulfide photocatalysts is water splitting into hydrogen (H₂) and oxygen (O₂) using solar energy. Hydrogen can be regarded as a clean and renewable energy carrier, and its production by solar water splitting may ignite an energy revolution. Metal sulfides like MoS2, ZnIn2S4, and NiS2 are some of the catalysts that have gained wide studies as active catalysts toward the HER (Hydrogen Evolution Reaction) and OER (Oxygen Evolution Reaction) of water-splitting processes.
For example, ZnIn₂S₄, a layered metal sulfide, has shown very excellent photocatalytic activity toward hydrogen production via water splitting. By internal electric field engineering that boosts the polarity of ZnIn₂S₄, its charge carrier separation has been eminently improved while the electron-hole recombination is reduced, hence recording astonishing hydrogen generation rates and positioning ZnIn₂S₄ among the leading hopefuls for hydrogen production on an industrial scale.
Other than ZnIn₂S₄, MoS₂ has also been considered one of the most promising materials for water splitting. MoS₂-based catalysts have been characterized by large surface areas and excellent electrocatalytic properties, hence being highly active toward HER (Hydrogen Evolution Reaction). The coupling of MoS₂ is also allowed with other materials, such as NiP₃, to form heterostructures with enhanced catalytic performance. This synergistic effect in MoS₂/NiP₃ improves hydrogen production efficiency, hence presenting a viable pathway toward solar-driven water splitting.