The depletion of fossil fuels is a worldwide problem that has led to the discovery of alternative energy sources. Solar energy is the focus of numerous studies due to its huge potential power and environmentally friendly nature. Furthermore, one such area of study is the production of hydrogen energy by engaging photocatalysts using the photoelectrochemical (PEC) method. 3-D ZnO superstructures are extensively explored, showing high solar light-harvesting efficiency, more reaction sites, great electron transportation, and low electron-hole recombination. However, further development requires the consideration of several aspects, including the morphological effects of 3D-ZnO on water-splitting performance. This study reviewed various 3D-ZnO superstructures fabricated through different synthesis methods and crystal growth modifiers, as well as their advantages and limitations. Additionally, a recent modification by carbon-based material for enhanced water-splitting efficiency has been discussed. Finally, the review provides some challenging issues and future perspectives on the improvement of vectorial charge carrier migration and separation between ZnO as well as carbon-based material, using rare earth metals, which appears to be exciting for water-splitting.

The mononuclear ruthenium 1-Cl and dinuclear ruthenium 2-Cl complexes undergo a photo-induced ligand exchange in water, affording the corresponding 1-H₂O and 2-H₂O complexes. The use of indium tin oxide nanoparticles (nanoITOs) to explore the photo-electrochemistry of the in situ-generated 1-H₂O and 2-H₂O in solution revealed greater photocurrents produced by these two complexes when compared with an experiment using a buffer only. Interestingly, the high photocurrent shown by the dinuclear complex 2-H₂O was accompanied by the deposition of its higher oxidation state (H₂O)Ru^II–Ru^III(OH), as evidenced with cyclic voltammetry, SEM and XPS. The IPCE and spectro-electrochemistry studies supported by TD-DFT calculations revealed the visible light harvesting ability of 1-H₂O and 2-H₂O in solution and the subsequent electron injection into the conduction band of the nanoITOs, enhanced in 2-H₂O via a plausible chelating effect.