Apostolos Salepis

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.