Exploring Metal- and Porphyrin-Modified TiO2-Based Photocatalysts for Efficient and Sustainable Hydrogen Production

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Abstract
Photocatalytic H2 production is one of the most promising approaches for sustainable energy. The literature presents a plethora of carefully designed systems aimed at harnessing solar energy and converting it into chemical energy. However, the main drawback of the reported photocatalysts is their stability. Thus, the development of a cost-effective and stable photocatalyst, suitable for real-world applications remains a challenge. An ideal photocatalyst for H2 production must possess appropriate band-edge energy positions, an effective sacrificial agent, and a suitable cocatalyst. Among the various photocatalysts studied, TiO2 stands out due to its stability, abundance, and non-toxicity. However, its efficiency in the visible spectrum is limited by its wide bandgap. Metal doping is an effective strategy to enhance electron–hole separation and improve light absorption efficiency, thereby boosting H2 synthesis. Common metal cocatalysts used as TiO2 dopants include platinum (Pt), gold (Au), copper (Cu), nickel (Ni), cobalt (Co), ruthenium (Ru), iron (Fe), and silver (Ag), as well as bimetallic combinations such as Ni-Fe, Ni-Cu, Nb-Ta, and Ni-Pt. In all cases, doped TiO2 exhibits higher H2 production performance compared to undoped TiO2, as metals provide additional reaction sites and enhance charge separation. The use of bimetallic dopants further optimizes the hydrogen evolution reaction. Additionally, porphyrins, with their strong visible light absorption and efficient electron transfer properties, have demonstrated potential in TiO2 photocatalysis. Their incorporation expands the photocatalyst’s light absorption range into the visible spectrum, enhancing H2 production efficiency. This review paper explores the principles and advancements in metal- and porphyrin-doped TiO2 photocatalysts, highlighting their potential for sustainable hydrogen production.

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