The alarming depletion rate of reserved fossil fuel associated with rapid increase in environmental pollution has caused an urgent need to develop efficient clean and renewable energy resources. In this regard, many different approaches have been followed up. The sun is a free, clean, sustainable and easy access energy source, and the solar produced hydrogen, which can be used in fuel cells to generate electricity or changed directly in combustion engines, makes no pollution except water. Hence, to obtain hydrogen as a clean energy carrier, the scenario of a renewable hydrogen economy has attracted much attention from researchers recently. Because of low operation temperature and strong synergies with contemporary researches in the field of photovoltaic and nanomaterials, photoelectrochemical (PEC) water splitting is an emerging technology for the future world hydrogen generation. The concept of this method is based on a semiconductor photoelectrode device which excites with sunlight irradiation, oxidizes/reduces H₂O molecules by generated h+/e− pair and finally converted them to chemical energy (H₂ gas). For efficient PEC reaction, the selected semiconductor photoanodes should exhibit chemical stability, suitable band edge positions for absorbing sunlight and also participating in water oxidation/reduction, high charge carrier mobility and also variety of low cost synthesis methods.

Photoanodes based on n-type metal oxides such as TiO₂,WO₃, ZnO, and Fe₂O₃ have been extensively investigated due to their chemical and physical stability in aqueous solution under evolving oxygen, ease of fabrication, and reasonably high incident light to current generation when operated in a PEC cell. Different approaches have been followed to prohibit charge recombination and improve visible response of these photoanodes. For example, size-controlled noble metal nanoparticles can enhance visible light absorption of the semiconductor photoanodes as a result of their surface plasmonic effects. Moreover, junctions formed between metal nanoparticles and semiconductor nanomaterials can facilitate interfacial charge transfer processes and, hence, cause retardation of electron−hole recombination. Many interesting reports and reviews are available in this context which present recent strategies and trends to improve solar hydrogen production systems in PEC, EC or PC approaches.

1- Seo et al. Visible‐Light‐Responsive Photoanodes for Highly Active, Stable Water Oxidation, 2017.

2- Kumaravel et al. Photocatalytic hydrogen production using metal doped TiO₂: A review of recent advances, 2019.

3- Kegel et al. Zinc oxide for solar water splitting: A brief review of the material’s challenges and associated opportunities, 2018.

4- Sharma et al. Key Strategies to Advance the Photoelectrochemical Water Splitting Performance of α‐Fe₂O₃ Photoanode, 2018. 

5- Viory et al. Low-dimensional catalysts for hydrogen evolution and CO₂ reduction, 2018.

Considering the depletion of hydrocarbon reservoirs and adverse effects of their combustion, providing a renewable and environmentally friendly resource of energy is necessary for human societies. Photoelectrochemical (PEC) and electrochemical (EC) water splitting into O₂ and H₂ at the surface of a proper material (photoactive semiconductor or electrocatalysts) is a promising solution to produce solar fuel as a green and sustainable source of energy. In both these approaches, each half-reaction, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), must be optimized to achieve the highest efficiency.
The challenging step is the OER since its Gibbs free energy is 1.23 eV per electron while kinetic barriers (known as overpotential) may increase this amount up to ~2 eV. In order to reduce the overpotential, a proper electrocatalyst should be applied to facilitate surface hole transfer and raise reaction kinetics. In this context, materials employed as cocatalysts in PEC systems must be chosen among earth abundant and chemically stable compounds. According to extensive researches in this area, iridium and ruthenium-based co-catalysts are found to exhibit the lowest overpotential but not applicable due to their high cost and rarity. As proposed by well-known volcano plot, the most attracting candidates for oxygen evolution reaction are first row transition metals such as Co and Ni in the form of oxide/hydroxide nanostructures which are abundant with variety of low cost preparation methods. Recently, efficient electrocatalysts for HER are also proposed based on 2D MoS₂ nanosheets. In the following review papers, basic information about parameters as well as recent trends can be found.

1- Roger et al. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting, 2017.

2- Kanan et al. Cobalt–phosphate oxygen-evolving compound, 2009. 

3- Chen et al. Recent Progress on Nickel‐Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction, 2018.

4- Lyu et al. Noble‐Metal‐Free Electrocatalysts for Oxygen Evolution, 2018. 

5- Guo et al. Phosphate‐Based Electrocatalysts for Water Splitting: Recent Progress, 2018.

6- Zhao et al. Heterostructures for Electrochemical Hydrogen Evolution Reaction: A Review, 2018.

7- Wang et al. Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting, 2018. 

8- Li et al. Transition‐Metal‐Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review, 2018. 

One‐dimensional nanostructures including nanowires (NWs), nanotubes, and nanofibers have shown promising PEC solar water splitting performance owing to their enhanced light absorption, higher surface area, reduced carrier recombination, and improved charge collection efficiency compared to their bulk counterparts. However, one‐dimensional structures still show insufficient specific surface area for water‐splitting reaction compared to mesoporous films. The hierarchically branched nanorod structure is a model architecture for efficient PEC application because it simultaneously offers a large seminconductor/electrolyte contact, excellent light‐trapping characteristics, and a highly conductive pathway for charge‐carrier collection. For example, branched TiO₂ and ZnO nanorod arrays have been constructed exhibiting superior PEC water‐splitting performance. Here mire details and examples are available.

1- Cho et al. Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production, 2011.

2- Zhang et al. Fabrication and spectroscopic investigation of branched silver nanowires and nanomeshworks, 2012.

3- Zhang et al. Branched Tungsten Oxide Nanorod Arrays Synthesized by Controlled Phase Transformation for Solar Water Oxidation, 2016.

4- Lin et al. Nanophotonic perovskite solar cell architecture with a three-dimensional TiO2 nanodendrite scaffold for light trapping and electron collection, 2016.