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.

The current state of thin film heterojunction solar cells based on cuprous oxide (Cu₂O), cupric oxide (CuO) and copper (III) oxide (Cu₄O₃) is reviewed. These p-type semiconducting oxides prepared by Cu oxidation, sputtering or electrochemical deposition are non-toxic, sustainable photovoltaic materials with application potential for solar electricity. However, defects at the copper oxide heterojunction and film quality are still major constraining factors for achieving high power conversion efficiency, η. Amongst the Cu₂O heterojunction devices, a maximum η of 6.1% has been obtained by using pulsed laser deposition (PLD) of Al_xGa_1−xO onto thermal Cu₂O doped with Na. The performance of CuO/n-Si heterojunction solar cells formed by magnetron sputtering of CuO is presently limited by both native oxide and Cu rich copper oxide layers at the heterointerface. These interfacial layers can be reduced by using a two-step sputtering process. A high η of 2.88% for CuO heterojunction solar cells has been achieved by incorporation of mixed phase CuO/Cu₂O nanopowder. CuO/Cu₂O heterojunction solar cells fabricated by electrodeposition and electrochemical doping has a maximum efficiency of 0.64% after surface defect passivation and annealing. Finally, early stage study of Cu₄O₃/GaN deposited on sapphire substrate has shown a photovoltaic effect and an η of ~10⁻²%.

A high recombination rate and low charge collection are the main limiting factors of copper oxides (cupric and cuprous oxide) for the photocatalytic degradation of organic pollutants. In this paper, a high performance copper oxide photocatalyst was developed by integrating cupric oxide (CuO) and cuprous oxide (Cu₂O) thin films, which showed superior performance for the photocatalytic degradation of methylene blue (MB) compared to the control CuO and Cu₂O photocatalyst. Our results show that a heterojunction photocatalyst of CuO–Cu₂O thin films could significantly increase the charge collection, reduce the recombination rate, and improve the photocatalytic activity.

The poor photocorrosion stability and low photovoltage of cupric oxide (CuO) are the main limiting factors of CuO-based photocathodes for solar-driven photoelectrochemical (PEC) water splitting and hydrogen evolution. In this paper, we demonstrate a highly efficient CuO-based photocathode fabricated on a glass substrate coated with fluorine-doped tin oxide (FTO). Incorporating aluminum (Al) into the thin CuO film (CuO:Al) and inserting a thin CuO interfacial layer between the CuO:Al film and the FTO-coated glass substrate is shown to improve the photocorrosion stability of the CuO and increase the photocurrent without increasing the dark current. We also demonstrate that depositing a layer of ZnO to form a buried p-(CuO/CuO:Al)/n-ZnO:Al heterojunction and controlling the carrier concentration and conductivity of the ZnO through the incorporation of Al can significantly improve the photovoltage and PEC activity of the photocathode, leading to a record-high photovoltage of ∼0.53 V_RHE. By capping the photocathode with a crystal-engineered TiO₂ protective layer, we are able to significantly stabilize the photocathode against photocorrosion and further improve the PEC activity of the final p-(CuO/CuO:Al)/n-ZnO:Al/TiO₂/Au–Pd photocathode, resulting in record-high photocurrent density of ∼5.4 mA cm⁻² and photocorrosion stability of ∼87% after 5 hours.

The high recombination rate of photogenerated electron–hole pairs, poor photocorrosion stability, and the discrepancy between the optical absorption length and charge collection efficiency of cupric oxide (CuO) are the main limiting factors of visible-light-driven CuO photocatalysts for hydrogen evolution and photocatalytic degradation of organic pollutants. In this paper, we demonstrate a novel thin CuO film photocatalyst on a fluorine doped tin oxide (FTO) coated glass substrate with low back contact resistivity, high charge collection efficiency, high optical absorption, and high photocorrosion stability for hydrogen production and photocatalytic degradation of organic pollutants. This photocatalyst was fabricated by incorporating palladium (Pd) nanostructures into CuO to form a CuO:Pd light trapping thin film. The CuO:Pd light trapping thin film was then sandwiched between a nitrogen doped cupric oxide [CuO(N)] hole transporting layer and a CuO capping layer [CuO(N)–CuO:Pd–CuO]. The performance of the CuO(N)–CuO:Pd–CuO photocatalyst is further improved by incorporating a ZnO buffer layer and TiO₂ protective layer, and decorating with a AuPd co-catalyst. Moreover, we demonstrate a significant improvement of photocorrosion stability and photocatalytic degradation efficiency of the CuO(N)–CuO:Pd–CuO–ZnO–TiO₂ photocatalyst through plasma assisted in situ nano-crystal engineering of the ZnO buffer layer and TiO₂ protective layer. The fabricated novel photocatalyst could retain 95% of the initial photocurrent density after 6 hours of standard illumination with solar light and could give a record high photocurrent density of ∼8 mA cm⁻² for the CuO photocatalyst.