Photocatalytic Hydrogen Evolution

Solar energy conversion is one of the sustainable technologies that tackles the global warming and energy crisis. Photocatalytic hydrogen (H₂) production is a clean technology to produce eco-friendly fuel with the help of semiconductor nanoparticles and abundant sunlight irradiation. Titanium oxide (TiO₂), graphitic-carbon nitride (g-C₃N₄) and cadmium sulfide (CdS) are the most widely explored photocatalysts in recent decades for water splitting.

As the guest editors, we have comprehensively investigated the role of sacrificial agents on the H₂ production efficiency of TiO₂, g-C₃N₄ and CdS photocatalysts [1]. The activity of the catalysts was evaluated without any noble metal co-catalysts. The effects of the most widely reported sacrificial agents were evaluated in this work. The activity of the catalysts was influenced by the number of hydroxyl groups, alpha hydrogen and carbon chain length of the sacrificial agent. We found that glucose and glycerol are the most suitable sacrificial agents to produce H₂ with minimum toxicity to the solution. The findings of this study would be highly favorable for the selection of a suitable sacrificial agent for photocatalytic H₂ production.

Hong et al. demonstrated the photoelectrochemical (PEC) efficiency of MoSe₂/Si nanostructures for H₂ production and carbon dioxide (CO₂) reduction [2]. PEC deposition coupled with the rapid thermal annealing method was applied to fabricate the electrodes on the Si substrate. PEC H₂ evolution and CO₂ conversion efficiencies of the MoSe₂/Si electrode were higher in visible light irradiation as compared to dark conditions.

Kim et al. synthesised monodispersed spherical TiO₂ particles with a disordered rutile surface for photocatalytic H₂ production [3]. The photocatalyst was synthesised through sol-gel and a chemical reduction technique using Li/ethylenediamine (Li/EDA) solution. The samples were calcined at various temperatures to tune the anatase to the rutile phase ratio. The disordered rutile surface and mixed crystalline phase of TiO₂ significantly increased the H₂ production under solar light irradiation.

Idrees et al. reported the photocatalytic activity of Nb₂O₅/g-C₃N₄ heterostructures for molecular H₂ production under simulated solar light irradiation [4]. A hydrothermal technique was utilised to develop the three dimensional Nb₂O₅/g-C₃N₄ heterostructure with a high surface area. H₂ production efficiency of Nb₂O₅/g-C₃N₄ (10 wt. %) was higher than that of pure Nb₂O₅ and g-C₃N₄. The photogenerated electron hole pairs were successfully separated through a direct Z-scheme mechanism at the heterojunction.

Kim and Woodward described the band gap modulation of tantalum (V) perovskite by anion control [5]. Perovskites such as BaTaO₂N, SrTaO₂N, CaTaO₂N, KTaO₃, NaTaO₃ and TaO₂F were studied in this work. Pt-loaded CaTaO₂N was utilised as a visible-light-driven photocatalyst for H₂ production using CH₃OH as the sacrificial agent.

Son et al. reported the impact of sulfur defects on the H₂ production efficiency of a CuS@CuGaS₂ heterojunction under visible light irradiation [6]. The activity of the CuS@CuGaS₂ heterojunction was higher as compared to pure CuS. This was ascribed to the introduction of structural defects to promote the photo-generated electron hole separation.

The recent accomplishments in the synthesis and application of various photocatalysts for H₂ production are briefly reviewed by Zhang et al. [7]

Tremendous efforts should be taken in the future to commercialise this photocatalytic technology at the industry level. The studies should also be performed with cheap materials, industrial wastewater and seawater for H₂ production.

Finally, we would like to convey our sincere thanks to all the authors for their significant contributions in this special issue.