Perovskite-based photocatalytic materials have emerged as promising candidates for hydrogen production due to their tunable optoelectronic properties, high charge carrier mobility, and cost-effective synthesis methods. These materials are broadly classified into oxide perovskites and halide perovskites, each offering distinct advantages and challenges in photocatalytic water splitting.

Oxide perovskites, such as strontium titanate (SrTiO3) and lanthanum cobaltite (LaCoO3), exhibit excellent chemical stability and favorable band edge positions for water reduction and oxidation. SrTiO3, with a bandgap of approximately 3.2 eV, is particularly notable for its strong reduction potential, making it suitable for hydrogen evolution. However, its wide bandgap limits visible light absorption. LaCoO3, with a narrower bandgap around 2.1 eV, extends light absorption into the visible spectrum but often suffers from rapid charge recombination.

Halide perovskites, including cesium lead bromide (CsPbBr3), possess superior light-harvesting capabilities due to their direct bandgaps and high extinction coefficients. CsPbBr3, with a bandgap of around 2.3 eV, demonstrates efficient charge separation and transport properties. However, halide perovskites face significant stability issues in aqueous environments, where they degrade due to ion leaching and structural collapse.

Bandgap engineering is a critical strategy to optimize the performance of perovskite photocatalysts. For oxide perovskites, doping with transition metals or rare-earth elements can introduce intermediate energy levels, reducing the effective bandgap. For example, nitrogen doping in SrTiO3 creates mid-gap states that enhance visible light absorption. Similarly, substituting cobalt in LaCoO3 with iron or nickel alters the electronic structure, improving charge separation.

In halide perovskites, compositional tuning via partial substitution of halides or cations enables precise control over the bandgap. Mixed halide perovskites, such as CsPb(Br/I)3, exhibit tunable bandgaps ranging from 1.7 to 2.3 eV, allowing optimization for specific solar spectra. However, phase segregation under illumination remains a challenge, necessitating further stabilization efforts.

Defects play a dual role in perovskite photocatalysts. Oxygen vacancies in oxide perovskites, such as SrTiO3, can act as electron traps, reducing recombination and enhancing photocatalytic activity. However, excessive defects may introduce recombination centers, degrading performance. In halide perovskites, controlled defect engineering via post-synthetic treatments can passivate surface traps, improving charge carrier lifetimes.

Stability remains a major hurdle, particularly for halide perovskites in aqueous environments. Protective coatings, such as thin layers of carbon or metal oxides, have been explored to shield the perovskite from water-induced degradation. Alternatively, hybrid systems combining halide perovskites with stable oxides or organic semiconductors can mitigate instability while maintaining high activity. For example, coupling CsPbBr3 with TiO2 forms a heterojunction that enhances charge separation and protects the perovskite core.

Tandem configurations represent a significant advancement in solar-to-hydrogen conversion efficiency. By stacking multiple photocatalysts with complementary bandgaps, a broader range of the solar spectrum can be harnessed. A common approach involves pairing a wide-bandgap oxide perovskite (e.g., SrTiO3) with a narrow-bandgap halide perovskite (e.g., CsPbBr3). The oxide perovskite absorbs UV light to generate oxygen, while the halide perovskite utilizes visible light for hydrogen evolution. Recent studies report solar-to-hydrogen efficiencies exceeding 5% in such tandem systems, a notable improvement over single-component photocatalysts.

Recent research has also explored the integration of cocatalysts to further enhance performance. Noble metals like platinum or non-precious alternatives such as nickel or cobalt phosphides are commonly deposited on perovskite surfaces to lower the overpotential for hydrogen evolution. In oxide perovskites, these cocatalysts facilitate proton reduction, while in halide perovskites, they also help mitigate degradation by providing reactive sites away from the perovskite lattice.

Despite these advances, scalability and long-term durability remain critical challenges. Large-scale synthesis of defect-controlled perovskites with consistent performance is still under development. Additionally, the economic viability of using rare or toxic elements in perovskites must be addressed through material substitution or recycling strategies.

In summary, perovskite-based photocatalytic materials offer a versatile platform for hydrogen production, with oxide and halide variants providing complementary advantages. Bandgap tuning, defect engineering, and protective strategies are key to optimizing their performance, while tandem configurations and cocatalyst integration push the boundaries of solar-to-hydrogen efficiency. Ongoing research aims to overcome stability and scalability barriers, paving the way for practical implementation in renewable energy systems.