The cost‐effective, robust, and efficient electrocatalysts for photoelectrochemical (PEC) water‐splitting has been extensively studied over the past decade to address a solution for the energy crisis. The interesting physicochemical properties of CuO have introduced this promising photocathodic material among the few photocatalysts with a narrow bandgap. This photocatalyst has a high activity for the PEC hydrogen evolution reaction (HER) under simulated sunlight irradiation. Here, the recent advancements of CuO‐based photoelectrodes, including undoped CuO, doped CuO, and CuO composites, in the PEC water‐splitting field, are comprehensively studied. Moreover, the synthesis methods, characterization, and fundamental factors of each classification are discussed in detail. Apart from the exclusive characteristics of CuO‐based photoelectrodes, the PEC properties of CuO/2D materials, as groups of the growing nanocomposites in photocurrent‐generating devices, are discussed in separate sections. Regarding the particular attention paid to the CuO heterostructure photocathodes, the PEC water splitting application is reviewed and the properties of each group such as electronic structures, defects, bandgap, and hierarchical structures are critically assessed.

Vertically aligned multi-wall carbon nanotube (CNT) arrays were fabricated in tip-growth mode on Ni/Si substrates using plasma enhanced chemical vapor deposition. In a purification process including hydrogenation and acid washing of the Ni/CNTs, the oxygen-containing functional groups were substantially reduced and a wide hollow core at the tip of the CNTs was formed by removing the Ni seeds. Sol–gel silver nanoparticles were deposited on the surface of the unpurified Ni/CNTs, while they could also be embedded within the hollow core of the Ni-removed CNTs. The persistency of the silver ions in the Ni-removed Ag–CNTs in comparison to the release of the silver ions from the Ag–Ni/CNTs in a dilute HNO₃ solution confirmed the effective embedding of the Ag nanoparticles within the hollow core of the Ni-removed CNTs. The Ni-removed Ag–CNTs showed a strong antibacterial activity against Escherichia coli bacteria in the dark (inactivation of 99.8 ± 0.1% of the bacteria as compared to the Ni-removed CNTs and the Ag–Ni/CNTs with 62 ± 11 and 47 ± 18% inactivation in 120 min), while the bacteria even exhibited some proliferation on the surface of the unpurified Ni/CNT arrays. In the context of cell membrane rupture of the bacteria through a direct contact mechanism, it was proposed that the Ni seeds acted as obstacles for an effective contact between the bacteria and the tips of the CNT arrays.