The global energy demand is rising daily, while the supply comes from the same non-renewable energy sources [1]. Carbon emissions and environmental damage are liabilities of the energy derived from fossil fuels. However, the diminishing fossil fuel sources do not provide a steady fuel supply to meet the growing population and demand. Alternative energy sources should be considered to mitigate the impact of carbon emissions and meet the energy demand. H₂ is considered a fuel for future generations with a high energy density and a lower molecular weight. At the moment, the majority of H₂ is being produced using non-renewable sources of energy processes, such as steam methane reforming, coal gasification, and coal pyrolysis. In comparison, renewable sources of H₂ production include biomass gasification & pyrolysis, thermochemical, solar-driven plasmolysis, and photo-electrolysis [2]. Presently, renewable energy sources are being scrutinized for developing reliable energy sources with no carbon emissions and a sustainable environment ^[3][4][3,4]. Solar energy-based technologies are attracting a vast number of applications in the form of renewable sources of energy. Solar-driven water splitting, or photoelectrochemical (PEC) water splitting, is one of the most attractive techniques used to decompose water into hydrogen (H₂) and oxygen (O₂) ^[5][6][5,6]. The energy falling on the earth’s surface from solar radiation is a driving force for H₂ production. PEC cells use photoactive materials coated over a transparent conductive oxide surface, mainly known as a photoanode (where oxidation of water occurs on the surface resulting in the oxygen evolution reaction OER) and photocathode (where water reduction occurs on the surface, resulting in hydrogen evolution reaction (HER)) ^[7][8][7,8]. The photoactive material coated films over the photoanode are generally n-type semiconductor materials and p-type semiconductor materials on the photocathode.

PEC water splitting is also known as “artificial photosynthesis.” The concept of this phenomenon is nature-inspired plant leaves that convert solar light into carbohydrates. The sunlight absorbed by plant leaves is utilized by photosystem I (PS I) and photosystem II (PS II) to generate and transfer electrons through a series of transfer processes across several redox systems, to reduce CO₂ to hydrocarbons [9]. Similarly, semiconductors, absorbing the minimum threshold wavelength of light equivalent to their bandgaps, generate photoexcited electron/hole (e⁻/h⁺) pairs, and the photoexcited electrons are diffused to the conduction band. Holes remain in the valence band of semiconductors, and electrons travel to the surface of the photocathode. The HER and OER occur on the surface of the photoelectrodes, and the active surface sites govern the kinetics of the reaction. The active surface sites contribute toward the adsorption of H⁺ and OH− molecules on the surface of the photoelectrodes ^[10][11][12][10,11,12].

The seminal report on PEC water splitting, in 1972, used a thin layer of TiO₂ (E_g ~ 3.2 eV) as a photoanode for a PEC half-cell. Since then, photoanodes have been studied extensively with several material modifications and fabrication techniques. The OER takes place on the photoanode while the H₂ evolution occurs on the counter electrode [13]. The n-type semiconductors, such asTiO₂ (3.2–3.4 eV) [14], BiVO₄ (2.2–2.4 eV) [15], WO₃ (2.6–3 eV) [16], g-C₃N₄ (2.5–2.8 eV) [17], CdS (2.2–2.4 eV) [18], SrTiO₃ (3.2–3.4 eV), and Fe₂O₃ (2–2.2 eV) have been widely applied for the photoanode application in the PEC cell. Among all of these materials, TiO₂ comes out as the best material in terms of stability, while BiVO4 is the best material in abundance and light-absorbing capacity in the visible region [19].

Moreover, photocathodes have received very little attention when compared to photoanodes. Meanwhile, the direct evolution is more crucial than indirect evolution of H_2, which occurs in the photoanodic half-cells. Developing a p-type semiconductor material with a suitable bandgap, band alignment, and stability toward the oxidizing environment is paramount. Along with these physical and optical characteristics, the semiconductor must be tested for applications using non-precious and earth-abundant metals that are neither poisonous nor harmful with a sustainable approach^{[20][21][22][23][24]}[20,21,22,23,24].

Copper-based oxide/chalcogenide semiconductor materials are promising materials owing to their tunable band gaps, band alignment with respect to water reduction potential (Figure 1), absorption coefficients, and various synthesis procedures. In this rentry, the researchers hview, we have discussed recent advancements in the synthesis and fabrication of earth-abundant copper-based oxides/chalcogenides as a functional photocathode material for solar-driven water splitting, in light of their prospective use in PEC solar-to-hydrogen systems. In addition, several modifications that took place over a period and critically assess the ongoing debates in this area, have been discussed. Finally, this evaluation provides a chance to compare Cu-based semiconductor materials to give increasing emphasis to visible light-driven solar water electrolysis.

Photocathodes are responsible for the reduction in a PEC cell serving the purpose of the direct evolution of hydrogen in a 2e⁻ transfer process over its surface. The minority charge carriers drive the reaction on the surface of the photoelectrode. To date, a number of materials have been investigated as efficient photocathodes for hydrogen generation such as p-Si (E_g = 1.1 eV) with a theoretical maximum photocurrent density −44 mA/cm^{2 [25]}[26], InP (E_g = 1.3 eV) with a theoretical maximum photocurrent of −35 mA/cm^{2 [26]}[27], p-GaP (E_g = 2.2 eV) ^[27][28] Sb₂Se₃ (E_g = 1.2 eV) ^[28][29], BiSbS₃, a p-type material with narrow bandgap (E_g = 2.2 eV)^[29][30]. Cu-based photocathode materials have a suitable conduction band maximum for the reduction of H⁺ ions and bandgap. The non-toxicity, earth abundance, low-cost, high absorption coefficient, and ease of synthesis make them a promising and sustainable component for photoabsorber materials. The sluggish reaction kinetics of the HER and the recombinations require the engineering of the interfaces with the different layers which give several advantages.

Copper-based metal oxides, such as Cu₂O and CuO are among the studied metal oxides, owing to their non-toxicity, availability, nature, and bandgaps. Due to their unique optoelectronic properties, they have been extensively used in energy conversion, storage, and sensing devices. However, the chemical instability of these materials restricts their PEC performance ^[30][31]. Cu₂O has a narrow bandgap (E_g = 2.0 eV) with a theoretical photocurrent density of −14 mA/cm² and a 18% photoconversion efficiency (PCE) owing to its light absorption in the AM 1.5 spectrum ^[31][32]. Cu₂O, as a photocatalyst, showed a stable H₂/O₂ evolution for 1900 h in visible light (λ > 460 nm) ^[32][33]. Although Cu₂O presents an immense opportunity for the hydrogen evolution, the photodegradation and recombination rate limits its photoactive material usage. Several strategies, including surface passivation, interfacial engineering, and cocatalyst decoration, have been investigated to overcome photodegradation and suppress the recombination to fabricate efficient photocathodes ^[33][34].

A Cu₂O/CuO heterojunction photocathode modified with a Cu₂S-Pt composite exhibited an enhanced hydrogen evolution. The optimum Cu₂S and Pt used as cocatalysts deposited using SILAR and sputtering sequentially, facilitated the charge transport and suppressed recombination. Cu₂O/CuO/Cu₂S-9/Pt delivered J = −5.7 mA/cm² at 0 V_RHE (2.5 times of Cu₂O/CuO), while a higher onset potential (E_op) for Cu₂O/CuO/Cu₂S−9/Pt was ~0.64 V_RHE observed, compared to Cu₂O/CuO (E_op = 0.54 V_RHE). The excessive Cu₂S deposition resulted in a parasitic light absorption and the creation of the recombination centers, resulting in J_ph fading ^[34][38]. The back contact material is essential in suppressing the recombination and charge transfer. Au is the best contact material for Cu₂O due to its large work function aligned with the valence band of Cu₂O. Recently, a CuO/NiO-based composite was investigated in place of Au using sputtering and aerial oxidation for the back contact with Cu₂O. A CuO/NiO thin film significantly improved and enhanced the transparency and hole collection (electron blocking) at the back contact of the Cu₂O photocathodes ^[35][39]. CuO is a p-type semiconductor with a narrow bandgap (E_g = 1.3–1.7 eV) suitable for light absorption and H₂ evolution. However, the photo corrosion of CuO in the presence of light is a significant drawback for the application. The majority of J_ph for the CuO photocathode is due to the photo corrosion of CuO to Cu with a low faradaic efficiency of the HER (0.01%) ^[36][43]. The deposition of CdS (buffer layer) and TiO₂ (as a protective layer), and Pt as a cocatalyst sequentially resulted in an enhanced PEC performance and faradaic efficiency (~100%) ^[36][43]. Various methods have been developed to synthesize CuO having different morphologies and bandgaps. The synthesis of CuO layers using sputtering and using variable power and thickness, exhibited a different photoactivity for hydrogen evolution. The variable power of sputtering (i.e., 30, 100, 200, and 300 W) in the synthesis resulted in a different morphology of the fabricated films, while the thickness of the layer of CuO played a role in light absorption. PXRD patterns showed the poor crystallinity of CuO due to low kinetic energy for diffusion and formation of Cu-O bond, while at higher the sputtering power, good crystalline films were obtained ^[37][44]. The deposition of a protective and stable metal oxide layer has proven to improve the faradaic efficiency of the H₂ evolution. The CuO/CuFe₂O₄ heterostructure was fabricated using the ion impregnation method. Although the faradaic efficiency increased from 45% for the bare CuO nanowires to 100% for CuO/CuFe₂O₄ with an improved stability, Jph decreased to one-third of the initial J_ph ^[38][45]. Generating Defects in the crystal structure (e.g., copper vacancies (V_Cu) in CuO) is an effective way to improve the charge separation and transfer. A study showed that V_Cu in CuO can be tuned by changing the O₂ partial pressure in the annealing process. The creation of vacancies in CuBi₂O₄ and CuFe₂O₄ resulted in an enhanced carrier concentration ^[39][46]. The incorporation of a foreign metal ion in the crystal structure can mitigate the issue of a photoinduced degradation. Several mixed metal oxides have been synthesized, such as CuBi₂O₄ ^[40][47], CuFe₂O₄ ^[41][48], CuAl₂O₄ ^[42][49], CuNbO₃ ^[43][50], and CuCrO₂ ^[44][51]_, and tested for the PEC photocathode application. CuBi₂O₄ has a bandgap of 1.5–1.8 eV and is a p-type mixed-metal oxide semiconductor that can be produced at low cost and with little environmental impact. By contributing their conduction band (CB) from the secondary metal rather than Cu 3d, ternary oxides have an advantage over the binary copper oxides in that they improve the photostability by preventing the conversion of Cu²⁺ to Cu⁰. CuBi₂O₄ is regarded as a good photocathode material because of its advantageous narrow bandgap, band positions, low cost, visible light absorption, non-toxicity, and high flat band potential (E_fb) values (>1 V vs. RHE), which are advantageous for unaided solar water splitting with a small bias given by the tandem photoanode. The theoretical photocurrent density reported for CuBi₂O₄ under AM 1.5 G (19.5–24.5 mA/cm²) is very far from achieved ^[45][52]. The practical photocurrent densities of the CuBi₂O₄ photocathodes are lower due to their poor charge transport, lower absorption coefficient, and higher charge recombination rates. Doping Ag⁺ in CuBi₂O₄ replacing Bi³⁺ increased the hole concentration, suppressing the anodic photo corrosion ^[46][53]. The sandwiched metal NPs between the heterojunction is a new approach for different configurations of the photocathode, which have shown potential in boosting the PEC performance. The PEC performance of the N,Cu-Codoped Carbon Nanosheets/Au/CuBi₂O₄ photocathode was examined. Au served as a plasmonic sensitizer and electron relay to transfer the charge from CuBi₂O₄ to the N,Cu-Codoped Carbon nanosheets ^[47][54].  Ferrites are considered one of the best candidates, owing to their merits, including earth abundance, non-toxicity, and stability in aqueous solutions. They have narrow band gaps and suitable band positions to drive the redox reaction over their surface ^[48][55]. Copper ferrite (CuFe₂O₄) is a mixed metal oxide and p-type semiconductor material for the PEC photocathode with a narrow bandgap (E_g = 1.5–1.9 eV). Theoretically, it can yield a high J_ph (~27 mA/cm²) and a STH efficiency (~ 33%). Although CuFe₂O₄ is an excellent p-type material, the crystallization temperature is high (~800–1000 °C), concerning the glass transition temperature of FTO (~564 °C). Fabrication of the heterojunction with CuFe₂O₄ has been extensively examined for the enhanced charge separation. CuFe₂O₄/Amorphous MnO₂ (AMO) was examined for the H₂ evolution in the neutral electrolyte, and 502.8 μmol H₂ was evolved in 90 min for a 1:4 ratio of CuFe₂O₄/AMO, which is higher than CuFe₂O₄/TiO₂ (130 μmol/h) and CuFe₂O₄/g-C₃N₄ (76 μmol/h) ^[49][57]. Photonic crystals (PC), consisting of CuFeO₂ decorated microspheres served as self-light harvesting architectures, allowing a high transmittance (~76%) and an amplified light absorption. The synthesis proceeded over the silica microspheres and the polymer-assisted synthesis. The novel design exhibited −0.2 mA/cm² at 0.6 V_RHE ^[50][58].