Recent advances in enhancing MoO3-based photocatalysts for hydrogen production via water splitting

Molybdenum trioxide (MoO₃) is an n-type semiconductor belonging to the family of metal oxides and has been explored for use in energy applications and environmental remediation due to its abundance, low cost, stability, and non-toxicity, with the potential to mitigate the use of expensive noble metals in catalysis. However, inadequacies due to positive conduction band (CB) potential and broad bandgap limit its ability for wide visible absorption photocatalytic application and thus prevent it from performing as a single photocatalyst but rather as a support. The synthesis approach is one of the methods that can help in the industrial scaling of the photocatalysts. This review highlights some of the adopted approaches, including facile exfoliation, hydrothermal, sol-gel, spray pyrolysis, and vapor deposition methods that yield MoO₃ with the desired nanostructure. Important photocatalytic properties, such as electronic, optical, and structural properties, and how the properties contribute to photocatalytic activities (like light absorption, electron densities, and their arrangements) are highlighted. The effect of each of the crystal phases of MoO₃, including orthorhombic (α-MoO₃), monoclinic (β-MoO₃), and hexagonal (h-MoO₃)_, in catalytic applications for hydrogen (H₂) production is discussed. Improvement strategies for strengthened performance in photocatalytic systems by morphology control, introducing defect or oxygen vacancy, modifying crystal structure or phase, and different heterojunction formations are studied. Future perspectives are provided to pave the path for the design and development of the strong composite photocatalyst for energy (H₂) generation via water splitting. Therefore, this review focuses on heterojunctions and non-stoichiometric formation of MoO₃ as a cocatalyst in order to provide insight into developing a low-cost and efficient photocatalyst for better H₂ evolution performance via photocatalytic water splitting for more equitable energy production using renewable sources.