Hydrogen (H 2 ) generation through photoelectrochemical (PEC) water splitting is a promising approach to reducing energy crises and associated environmental problems. Among currently available semiconductors, 2D nanostructured metal oxides have attracted wide interest in PEC applications because of their low-cost synthesis routes, stability in aqueous media, unique configuration and features, and favorable band edge positions. However, metal oxides still suffer from the severe recombination of photogenerated charge carriers and the low consumption of visible light. One solution is to introduce co-catalysts to enhance photocatalytic H 2 generation by improving charge separation and transfer, extending light absorption, and lowering the activation energy. This review summarizes the fundamentals and recent developments of co-catalysts on metal oxides which can be classified into the following categories: metal cocatalysts, metal sulfide co-catalysts, metal hydroxide co-catalysts, metal phosphide co-catalysts, carbon-based co-catalysts, and dual co-catalysts. Charge transfer mechanisms of co-catalysts on metal oxides in photocatalytic H 2 generation and present future trends for co-catalysts and strategies in enhancing PEC water splitting are also presented.

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Three-dimensional (3D) structures made of graphene sheets have been developed recently, and have resulted in the development of a new class of graphene materials known as 3D graphene materials. High-quality free-standing 3D graphene foam has been synthesized by chemical vapor deposition (CVD) on nickel foam followed by a chemical etching process to remove the nickel foam as a template. Field-emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), and Raman spectroscopy measurements were performed to investigate the morphologies, crystal phase, and the structure of nickel foam (NF), graphene/nickel foam (Gr/NF), and 3D graphene (3D Gr). In this study, the influence of etching solution and etching time on Gr/NF to produce free-standing 3D Gr was investigated. XRD spectroscopy showed that the mixed solutions of 1M FeCl₃:1M HCl at 80 °C for 3 h can significantly remove the NF and no peaks of NF are observed, thus indicating a high crystal quality of 3D Gr was obtained. In addition, XRD spectroscopy revealed that by increasing the etching time beyond 3 h, the intensity of diffraction peaks decreases, thus degrading graphene quality. This research emphasizes the significance of proper selections of etching solution and etching time in removing the NF to maintain the characteristic, quality, and surface morphology of 3D Gr after the etching process.

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