There are several common water splitting techniques for solar hydrogen generation. These include indirect PEC splitting of H₂O molecules using electricity generated by photovoltaic cells, and direct splitting of H₂O molecules using photocatalyst materials, such as TiO₂, CdS, etc., or light absorbing semiconductor materials and structures as photoelectrode. The articles presented in this Feature Issue focus primarily on the studies of direct PEC water splitting techniques using new materials or structures to enhance energy conversion efficiency, and to improve stability.

For wide technological adoption, solar hydrogen production via PEC techniques needs to tackle challenges related to the achievement of high solar-to-hydrogen (STH) energy conversion efficiency, high stability, low cost, and scalability of the hydrogen generation system. In PEC water splitting, the bandgap of semiconductor materials, band-edge tuning, oxygen/hydrogen evolution potentials, solar energy harvesting efficiency and stability must be satisfied simultaneously to improve the overall STH efficiency. The invited review paper by Ronglei Fan et al. gave a general overview of recent advances of silicon photoelectrodes in PEC water splitting, and particularly focused on various methods for enhancing conversion efficiency and stability of silicon photoelectrodes.

The surface passivation of semiconductor nanostructure, for mitigating Shockley-Read-Hall recombination and for protecting the semiconductor surface from corrosion, is important for reducing carrier loss through surface recombination and for improving material stability. Recent progress on the integration of various co-catalyst and passivation layer on silicon based photocathodes and photoanodes were discussed by Ronglei Fan et al. and Haseong Kim et al.. The application of Al₂O₃ overlayer on GaN photoelectrodes to protect the GaN-photoelectrodes was found to significantly increase the stability of the PEC water splitting system. Enhancement of STH efficiency and high stability PEC water splitting were observed from MoS₂-Si heterojunction photocathode, as reported by Abeer Alarwai et al.. GaP photoanode coated with TiO₂ followed by a layer of CoO_x co-catalyst was found to remain stable for over 24 hours, as reported by Mahdi AlQahtani et al.

GaN-based nanowires photoelectrodes grown on various substrate materials were found to be reliable and effective for PEC water splitting. Huafan Zhang et al. studied the different combination of axial heterojunction and the associate net electron/hole carriers concentrations in InGaN/GaN photoelectrode on hydrogen production. The study found that it is essential to optimize the doping levels of the Pt-coated p-InGaN/p-GaN axial nanowires to achieve both high efficiency and high stability.

In the case of PEC water splitting using metal-oxide catalyst and photoelectrode methods, Rowena Yew et al. found that oxygen-vacancies in the macroporous TiO₂ inverse opal (TiO₂-IO) material enhances STH efficiency. In this study, the lifetime of photogenerated carriers was found to increase in the presence of oxygen vacancies. This improves the conductivity of TiO₂, and hence improving the PEC water splitting performance.

Low cost solutions to the PEC water splitting techniques were studied by Nam-Woon Kim et al., and Mostafa Afifi Hassan et al. Kim et al. reported the fabrication of high-density CuBi₂O₄ thin film photocathodes using a low cost solution-based process. Hassan et al. reported the synthesis of ZnO/ZnS core-shall nanowire photoanode on silicon substrate using MOCVD method. Compared to the ZnO nanowire structures, a significant performance improvement was observed from the ZnO/ZnS core-shall nanowire structures.

Novel hydrogen generation method using Mg/MgO plasmonic method was explored by Yael Gutierres et al. Preliminary studies on the basic optical properties of Mg, MgH₂ and MgO were performed.

In summary, this issue of Optics Express features 9 articles reporting various aspects of solar fuel generation using PEC water splitting methods with special emphasis on enhancing STH efficiency, increasing stability and utilization of lower materials cost. We hope that this feature issue provides not only a technical forum for presenting some latest research in solar hydrogen generation via PEC water splitting but also contribute to catalyzing future submission of article in this field to Optics Express.

We would like to thank all the authors who have contributed to this issue. We would also like to thank former Editor In-Chief Andrew Weiner, Associate Editor Christian Seassal, Editorial Director Dan McDonald, and Peer Review Manager Carmelita Washington for their valuable supports of this project.

This feature issue aims to highlight the recent progress and trends in general fields related to solar-hydrogen and solar fuel generation via technique such as PEC, photochemical, artificial photosynthesis, etc. Specific areas of interest include but not limited to: