The emission of greenhouse gases from fossil fuels creates devastating effects on Earth’s atmosphere. Therefore, a clean energy source is required to fulfill the energy demand. Hydrogen is considered an energy vector, and the production of green hydrogen is a promising approach. Photoelectrochemical (PEC) water splitting is the best approach to produced green hydrogen, but the efficiency is low. To produce hydrogen by PEC splitting water, semiconductor photocatalysts have received an enormous amount of academic research in recent years. A new class of co-catalysts based on transition metals has emerged as a powerful tool for reducing charge transfer barriers and enhancing photoelectrochemical (PEC) efficiency. In this study, copper oxide (CuO) and tin oxide ($Sn{O}_2$) multilayer thin films were prepared by thermal evaporation to create an energy gradient between $Sn{O}_2$ and CuO semiconductors for better charge transfer. To improve the crystallinity and reduce the defects, the prepared films were annealed in a tube furnace at 400 °C, 500 °C, and 600 °C in an argon inert gas environment. XRD results showed that $Sn{O}_2$/CuO-600 °C exhibited strong peaks, indicating the transformation from amorphous to polycrystalline. SEM images showed the transformation of smooth dense film to a granular structure by annealing, which is better for charge transfer from electrode to electrolyte. Optical properties showed that the bandgap was decreased by annealing, which might be diffusion of Cu and Sn atoms at the interface. PEC results showed that the $Sn{O}_2$/CuO-600 °C heterostructure exhibits the solar light-to-hydrogen (STH%) conversion efficiency of 0.25%.

Optically Transparent multilayer NiO/Ag and NiO/Al heat mirrors were prepared,which allow the visible radiation to pass through themand reflect the Infrared radiation. NiO thin filmswere deposited by the sol-gel spin coating technique, and Ag and Al thin films were prepared by thermal evaporation. XRD analysis showed the formation of a cubic structure of NiO. The chemical analysis reveals that the Ni2O3phase is also present along with the NiO phase. The transmittance spectra of NiO/Ag and NiO/Al coatingsshowed good transmittance in the visible region,while being highly reflective in the infrared region. XRDanalysis of the film showed the formation of a cubic structure of NiO. The chemical analysis showed that some peaks belongto another phase of the NiO. The transmittance spectra of multilayersof NiO/Ag and NiO/Al films are transparent in the visible region and reflect well in the infrared region.

The transition to clean and renewable energy sources is of prime importance in addressing environmental challenges related to the consumption of fossil fuels. Hydrogen, being a clean fuel with a high energy density, has huge potential, especially when it can be obtained through solar-driven PEC water splitting. Herein, $Mo{S}_2$ photocatalysts were synthesized by sulfurizing $Mo{O}_3$ thin films, using a CVD technique. The deposited $Mo{O}_3$ films by thermal evaporation at 450 °C were further sulfurized at 500 °C, 550 °C, and 600 °C. XRD results confirmed the successful conversion of $Mo{O}_3$ into $Mo{S}_2$. The optical properties showed a bandgap reduction from 2.50 eV to 1.30 eV, which leads to better absorption of light in the visible region.The photoelectrochemical experiment shows that the S-$Mo{O}_3$-600 °C thin film has the best performance, and the solar-to-hydrogen conversion efficiency reaches 0.11385% at an applied bias of 1.0 V versus Ag/AgCl, which is about 189 times higher than that of the pristine $Mo{O}_3$ thin film.

In recent years, significant research has been done on semiconductor heterostructures to produce hydrogen by water splitting. The absorption of visible light and photoelectrochemical properties of CuO thin film areenhanced by the selenization. The selenization of CuO thin film is done by chemical vapor deposition (CVD) at various temperatures. The structural properties of the prepared samples were characterized by XRD,and the morphological properties of the prepared film werecharacterizedby scanning electron microscopy. Optical properties reveal that the bandgap was decreased by increasing the selenization temperature. The solar light to hydrogen conversion efficiency of the CuSe-500oC, CuSe-600oC,and CuSe-650oC films were estimated by using three-electrode cells. It was noticed that CuSe-650oC showed much better STH% compared to pristine CuO thin film.