Cu-Based Materials as Photocatalysts for Solar Light Artiﬁcial Photosynthesis: Aspects of Engineering Performance, Stability, Selectivity

: Cu-oxide nanophases (CuO, Cu 2 O, Cu 0 ) constitute highly potent nanoplatforms for the development of efﬁcient Artiﬁcial Photosynthesis catalysts. The highly reducing conduction band edge of the d -electrons in Cu 2 O dictates its efﬁciency towards CO 2 reduction under sunlight excitation. In the present review, we discuss aspects interlinking the stability under photocorrosion of the (CuO/Cu 2 O/Cu 0 ) nanophase equilibria, and performance in H 2 -production/CO 2 -reduction. Converging literature evidence shows that, because of photocorrosion, single-phase Cu-oxides would not be favorable to be used as a standalone cathodic catalyst/electrode; however, their heterojunctions and the coupling with proper partner materials is an encouraging approach. Distinction between the role of various factors is required to protect the material from photocorrosion, e.g


Introduction
Over the last decades, the negative impact of greenhouse gas emissions on the environment is strongly correlated with air pollution, climate change, and global warming. Carbon dioxide (CO 2 ) is considered one of the main greenhouse gases, thus strategies need to be developed in order to decrease CO 2 levels in the atmosphere. The "Artificial Photosynthesis" approach [1] aims to exploit photocatalytic technology, that is the use of solar photons to produce hydrogen (H 2 ) and-ideally-couple it to CO 2 reduction towards carbon-based fuels. Economically and environmentally, this is a sustainable, circular economy approach since CO 2 reduction can result in useful products such as formic acid (HCOOH), formaldehyde (HCHO), methanol (CH 3 OH), methane (CH 4 ), and carbon monoxide (CO), to name a few [2].
Both photocatalytic H 2 evolution from H 2 O and photocatalytic CO 2 reduction reactions (CO 2 RR) share some common steps, with their difference being on the specific surface reactions of the photogenerated electrons. These steps include, see Figure 1: photonadsorption by the semiconductor, electron-hole separation and their migration to the semiconductor surface [3], and subsequent incorporation of the electron to a H + , leading towards H 2 or to one CO 2 radical, leading towards the HCOOH/HCOH/CH 3 OH/CH 4 chain. H 2 production can be identified as a less-complex process, compared to the CO 2 reduction, the latter involving CO 2 adsorption and activation, product formation, and desorption. Thus, CO 2 reduction is a multi-proton and multi-electron transfer process and selectivity of the products remains a major challenge (see Figure 1). The difficulty in activating the inert CO2 molecule lies in its closed-shell electronic configuration, linear geometry, and D∞h symmetry [2]. Thus, it requires a highly negative reduction potential of −1.9 V (vs. NHE at pH 7) to activate CO2 and form the CO2 − radical, which the vast majority of semiconductors cannot provide [2]. Fujishima and Honda in 1972, [4], paved the way for light-induced water splitting by TiO2 and, since then, numerous studies on photocatalysts were presented, and in particular TiO2 [5][6][7]. Moreover, research interest in photocatalytic and photo-electrochemical applications has increased, prompting successful engineering of various semiconducting photocatalysts [5, [8][9][10]. So far, TiO2 remains the most studied photocatalyst, due to its chemical stability, low cost, and availability [11]. However, the large energy gap of TiO2 (3.2 eV) [3] limits its photoactivity exclusively in the ultraviolet radiation range, typically λ < 360 nm, which accounts for 2-5% of the sunlight [12]. Moreover, the conduction band (CB) edge of TiO2 is ECB = −160 mV vs. NHE (pH = 0) or ECB = −50 mV (pH = 7) [3], which disfavors its photoexcited electrons to efficiently achieve reduction of CO2 towards the key intermediate CO2 − . In this context, p-type Cu2O is considered as highly promising and attractive since it can absorb visible-light photons, Eg in the range 2.0-2.2 eV [13][14][15][16], and mostly due to the highly reducing energy positioning of its conduction-band edge, which is approximately ECB = −1000 mV vs NHE (pH = 0) (see Figure  2) [10]. The difficulty in activating the inert CO 2 molecule lies in its closed-shell electronic configuration, linear geometry, and D ∞h symmetry [2]. Thus, it requires a highly negative reduction potential of −1.9 V (vs. NHE at pH 7) to activate CO 2 and form the CO 2 − radical, which the vast majority of semiconductors cannot provide [2]. Fujishima and Honda in 1972, [4], paved the way for light-induced water splitting by TiO 2 and, since then, numerous studies on photocatalysts were presented, and in particular TiO 2 [5][6][7]. Moreover, research interest in photocatalytic and photo-electrochemical applications has increased, prompting successful engineering of various semiconducting photocatalysts [5, [8][9][10]. So far, TiO 2 remains the most studied photocatalyst, due to its chemical stability, low cost, and availability [11]. However, the large energy gap of TiO 2 (3.2 eV) [3] limits its photoactivity exclusively in the ultraviolet radiation range, typically λ < 360 nm, which accounts for 2-5% of the sunlight [12]. Moreover, the conduction band (CB) edge of TiO 2 is E CB = −160 mV vs. NHE (pH = 0) or E CB = −50 mV (pH = 7) [3], which disfavors its photoexcited electrons to efficiently achieve reduction of CO 2 towards the key intermediate CO 2 − . In this context, p-type Cu 2 O is considered as highly promising and attractive since it can absorb visible-light photons, E g in the range 2.0-2.2 eV [13][14][15][16], and mostly due to the highly reducing energy positioning of its conduction-band edge, which is approximately E CB = −1000 mV vs NHE (pH = 0) (see Figure 2) [10]. desorption. Thus, CO2 reduction is a multi-proton and multi-electron transfer process and selectivity of the products remains a major challenge (see Figure 1). The difficulty in activating the inert CO2 molecule lies in its closed-shell electronic configuration, linear geometry, and D∞h symmetry [2]. Thus, it requires a highly negative reduction potential of −1.9 V (vs. NHE at pH 7) to activate CO2 and form the CO2 − radical, which the vast majority of semiconductors cannot provide [2].
Fujishima and Honda in 1972, [4], paved the way for light-induced water splitting by TiO2 and, since then, numerous studies on photocatalysts were presented, and in particular TiO2 [5][6][7]. Moreover, research interest in photocatalytic and photo-electrochemical applications has increased, prompting successful engineering of various semiconducting photocatalysts [5, [8][9][10]. So far, TiO2 remains the most studied photocatalyst, due to its chemical stability, low cost, and availability [11]. However, the large energy gap of TiO2 (3.2 eV) [3] limits its photoactivity exclusively in the ultraviolet radiation range, typically λ < 360 nm, which accounts for 2-5% of the sunlight [12]. Moreover, the conduction band (CB) edge of TiO2 is ECB = −160 mV vs. NHE (pH = 0) or ECB = −50 mV (pH = 7) [3], which disfavors its photoexcited electrons to efficiently achieve reduction of CO2 towards the key intermediate CO2 − . In this context, p-type Cu2O is considered as highly promising and attractive since it can absorb visible-light photons, Eg in the range 2.0-2.2 eV [13][14][15][16], and mostly due to the highly reducing energy positioning of its conduction-band edge, which is approximately ECB = −1000 mV vs NHE (pH = 0) (see Figure  2) [10]. The photocorrosion pathways can involve either "self−oxidation" or "self−reduction" of Cu 2 O. This is related to the positions of the redox couples Cu 1+ /Cu 2+ and Cu 1+ /Cu 0 relative to the E VBand and E CBand edges. CuO can be an effective photocatalyst and a photocathode if the self-reduction problem of the Cu 2+ sites to Cu 1+ is solved via a heterojunction with a n-type semiconductor such as TiO2 [19,34]. This TiO2@CuO core-shell scheme can act as a protective layer, see Figure 3 (Case-Study A). The efficiency of this approach can be influenced by the preparation method. Masudy-Panah et al. [24], via control of the sputtering power, achieved to optimize the TiO2-layer over CuO, and this in turn had a beneficial effect on the photocorrosion stability of CuO-based photocathodes [24]. Under five hours of photocorrosion stability tests, increasing the sputtering power was found to significantly suppresses the formation of Ti 3+ , which increased the Hydrogen Evolution Reaction (HER) efficiency of the photocathode [24]. The Nyquist-curves recorded by Electrochemical Impedance Spectroscopy (EIS) of thin TiO2-film indicated easier transfer of photoinduced electrons to the electrolyte solution [24]. Xing et al. [26] reported that a protective layer of TiO2 formed by sol-gel method, can be used to protect the CuO photocathode [26]. In this case, they had achieved improved photocurrent density by TiO2@CuO vs. bare CuO photocathode. In [26], the long-term stability was impaired due to a less-homogeneous protective TiO2 layer on the CuO photocathode [26]. Specifically, in the TiO2@CuO core-shell, it was suggested [26] that photoexcited electrons of CuO and TiO2 easily migrate to the CB of TiO2 and the holes from VB of TiO2 transfer to the VB of  CuO can be an effective photocatalyst and a photocathode if the self-reduction problem of the Cu 2+ sites to Cu 1+ is solved via a heterojunction with a n-type semiconductor such as TiO 2 [19,34]. This TiO 2 @CuO core-shell scheme can act as a protective layer, see Figure 3 (Case-Study A). The efficiency of this approach can be influenced by the preparation method. Masudy-Panah et al. [24], via control of the sputtering power, achieved to optimize the TiO 2 -layer over CuO, and this in turn had a beneficial effect on the photocorrosion stability of CuO-based photocathodes [24]. Under five hours of photocorrosion stability tests, increasing the sputtering power was found to significantly suppresses the formation of Ti 3+ , which increased the Hydrogen Evolution Reaction (HER) efficiency of the photocathode [24]. The Nyquist-curves recorded by Electrochemical Impedance Spectroscopy (EIS) of thin TiO 2-film indicated easier transfer of photoinduced electrons to the electrolyte solution [24]. Xing et al. [26] reported that a protective layer of TiO 2 formed by sol-gel method, can be used to protect the CuO photocathode [26]. In this case, they had achieved improved photocurrent density by TiO 2 @CuO vs. bare CuO photocathode. In [26], the long-term stability was impaired due to a less-homogeneous protective TiO 2 layer on the CuO photocathode [26]. Specifically, in the TiO 2 @CuO core-shell, it was suggested [26] that photoexcited electrons of CuO and TiO 2 easily migrate to the Solar 2023, 3 91 CB of TiO 2 and the holes from VB of TiO 2 transfer to the VB of CuO, while the internal electric field further promotes the separation and transfer of photogenerated carriers in the two components of the Z-heterojunction direction, resulting to a more stable and efficient CuO photocathode [26]. Similarly, the results of [35,36] can be understood as being due to a Z-scheme configuration [37] that is considered to improve electron-hole separation and transfer them to the CB of TiO 2 and VB of Cu 2 O, respectively.

Case−Study
In [23], precise control of Oxygen-rich CuO and Cu-rich CuO regions on the same material was investigated via sputtering on Fluorine Tin Oxide (FTO)-coated glass. It was observed that the stability of CuO electrodes was considerably influenced by the precise local balance of Cu or O elements, with O-rich CuO electrodes to be most stable against photocorrosion [23]. O-rich materials show higher stability against photocorrosion, which might be a result from slower self-reduction of CuO to Cu 2 O. EIS data showed that, in O-rich CuO electrodes, charge transfer resistance was decreased [23], thus improving interfacial charge transport and photocatalytic performance.
Jeong et al. [38] had fabricated BiVO 4 /CuO heterojunction electrodes by spin-coating capping layers of BiVO 4 on CuO photoelectrodes, in order to prevent self-reduction of CuO to Cu 2 O. Using X-Ray Photoelectron Spectroscopy (XPS) data, they have observed that CuO photoelectrodes with poor photostability occurred when self-reduction of CuO to Cu 2 O was occurring, while after repeated BiVO 4 deposition cycles, the photocorrosion was suppressed, i.e., a > 76% photostability was reported [38].
In conclusion, heterojunction of Cu-oxide semiconductors with appropriate n-type semiconductors can be a promising strategy to address the issue of Cu 2+ /Cu 1+ selfreduction. Properly accounting for the relative CB and VB positioning, and the ensuing band-bending, is of primary importance in this approach. In another approach, to prevent Cu 2+ to Cu 1+ self-reduction [23], an oxygen-rich CuO film was coated with a noble-metal nanolayer, e.g., Pt or Pd-Au, using sputteringtechnology. Improved stability was confirmed by EIS, showing enhanced CuO stability to correlate with decrease of the resistance [23], i.e., photoinduced electrons had higher mobility [23]. Noticeably, in that work, electron-hole generation was improved via the interaction of plasmonic Au with the semiconductor [23]. In a similar approach, adding Pt as an electron accepting catalyst on a CuO was reported in [26], using an electro-deposition method [26] (see Figure 3 Case-Study B). By adding a metal with higher work function than CuO [Φ = 5.3 eV] e.g. such as Pt [Φ = 5.65 eV], the photogenerated electrons can migrate from CuO to the Pt layer, thus be trapped there, until thermodynamic equilibrium is established, forming an inner electric-field between CuO and Pt [39]. The benefit of this approach, is that, by decreasing electron accumulation on CuO, it minimizes the self-reductive corrosion [26].
In an alternative approach, Zhang et al. [40] had anchored single Cu-atoms (Cu-SA) on TiO 2 structure by a bottom-up approach, where a metal organic framework (MOF) MIL-125 was used as a substrate to ensure atomic dispersion of atomic cocatalyst and enabled the highest loading amount of approximately 1.5 wt.% Cu. Anchoring Cu on the TiO 2 matrix significantly decreased the charge carrier recombination. Loading single Cu-atoms on TiO 2 versus pristine TiO 2 indicated that the Cu-SA loading might effectively facilitate the transfer of photogenerated electrons from TiO 2 to the Cu active sites [40]. This can be attributed to the fact that the redox-potential of Cu 2+ /Cu 1+ (0.16 V vs. NHE) is more positive than CB of TiO 2 (−0.1 V vs. NHE). Electrochemical Impedance Spectroscopy Nyquist curves of Cu-SA/TiO 2 verify that the Cu-SA species can act as electron acceptors, thus facilitating the interfacial charge separation [40]. The decreased electron-hole recombination was also evident by the photoluminescence data [40], i.e., in Cu-SA TiO 2 the recombination of electrons and holes was slower. As a result, long-term stability was reported, i.e., 380 days with minor loss on the catalytic performance [40]. The aforementioned examples, indicate that fast transfer of electrons out from the Cu-oxide matrix, in a non-reversible way, is the key-condition to improve Cu 2+ to Cu 1+ self-reduction. Thus, the redox positioning of the electron-acceptors vs. the CB of the Cu-atoms in Cu-oxides, is one of the key-parameters to be considered.

Case−Study C: The Use of Hole Scavengers
As exemplified in Figure 2, in Cu 2 O, self-oxidation of Cu 1+ to Cu 2+ by photogenerated holes, can be a primary photocorrosion source [31]. The highly negative potential CB-edge of Cu 2 O, typically −1000 mV vs. NHE (pH = 0) [10] boosts the electron transfer to interfacial H + , towards H 2 production. Concomitantly, h + are gradually accumulated and this may promote photocorrosion, oxidation of Cu 1+ to Cu 2+ , since the low level of valence band (VB) prevents h + to participate in water oxidation. Thus, an efficient hole scavenger would be required to achieve a rapid withdrawal of h + in order to suppress this Cu 2 O self-oxidation (see Figure 3 Case-Study C inset). The presence of a h + scavenger with a suitable oxidizing potential (see Figure 4) facilitates the h + transfer, thus improving the photostability of Cu 2 O. In this context, careful selection of a sacrificial hole-scavenger may improve stability by preventing self-photo-oxidation of Cu 2 O. This in turn, is expected to enhance the overall photocatalytic H 2 evolution by Cu 2 O, permitting an educing potential to be built-up. In this context, Toe et al. [31] demonstrated that among some hole scavengers, e.g., Na 2 SO 3, methanol, and ethanol, Na 2 SO 3 was the most effective in photocatalytic H 2 evolution by Cu 2 O catalysts [31]. Solar 2023, 3, FOR PEER REVIEW 6 The aforementioned examples, indicate that fast transfer of electrons out from the Cu-oxide matrix, in a non-reversible way, is the key-condition to improve Cu 2+ to Cu 1+ self-reduction. Thus, the redox positioning of the electron-acceptors vs. the CB of the Cu-atoms in Cu-oxides, is one of the key-parameters to be considered.

Case−Study C: The Use of Hole Scavengers
As exemplified in Figure 2, in Cu2O, self-oxidation of Cu 1+ to Cu 2+ by photogenerated holes, can be a primary photocorrosion source [31]. The highly negative potential CB-edge of Cu2O, typically −1000 mV vs. NHE (pH = 0) [10] boosts the electron transfer to interfacial H + , towards H2 production. Concomitantly, h + are gradually accumulated and this may promote photocorrosion, oxidation of Cu 1+ to Cu 2+ , since the low level of valence band (VB) prevents h + to participate in water oxidation. Thus, an efficient hole scavenger would be required to achieve a rapid withdrawal of h + in order to suppress this Cu2O self-oxidation (see Figure 3 Case-Study C inset). The presence of a h + scavenger with a suitable oxidizing potential (see Figure 4) facilitates the h + transfer, thus improving the photostability of Cu2O. In this context, careful selection of a sacrificial hole-scavenger may improve stability by preventing self-photo-oxidation of Cu2O. This in turn, is expected to enhance the overall photocatalytic H2 evolution by Cu2O, permitting an educing potential to be built-up. In this context, Toe et al. [31] demonstrated that among some hole scavengers, e.g., Na2SO3, methanol, and ethanol, Na2SO3 was the most effective in photocatalytic H2 evolution by Cu2O catalysts [31]. Importantly, this study showed that via this approach [31], photocatalytic H2-production from [H2O+scavenger] could be accomplished without any secondary components compared, for example, to the common case of use of alcohols (methanol, isopropanol) as hole scavengers. In [31], Na2SO3 enabled h + scavenging to oxidize SO3 2-into SO4 2-, leaving free ein the conduction band for HER. Importantly, Na2SO3 allowed an effect 12-fold better than ethanol [31], allowing a 15 h production of H2 by the Cu2O/Na2SO3, without any signs of photocorrosion [31]. This approach is interesting since it highlights the possibility to engineer Cu2O without addition of metal or cocatalysts. Importantly, this study showed that via this approach [31], photocatalytic H 2 -production from [H 2 O+scavenger] could be accomplished without any secondary components compared, for example, to the common case of use of alcohols (methanol, isopropanol) as hole scavengers. In [31], Na 2 SO 3 enabled h + scavenging to oxidize SO 3 2− into SO 4 2− , leaving free e − in the conduction band for HER. Importantly, Na 2 SO 3 allowed an effect 12-fold better than ethanol [31], allowing a 15 h production of H 2 by the Cu 2 O/Na 2 SO 3 , without any signs of photocorrosion [31]. This approach is interesting since it highlights the possibility to engineer Cu 2 O without addition of metal or cocatalysts.

Case−Study D: The Effect of Carbonaceous Materials
Reduced graphene oxide (rGO) can act as a protecting agent. An et al. [32] have studied the effect of rGO interfaced with different exposed facets of Cu 2 O with (100) to be better for CO 2 R than (100) [32]. The proposed mechanism is described in Figure 3  Case-Study D). They measured Cu-leaching using ICP (inductively coupled plasma optical emission spectrometry) and they demonstrated a Cu-atom leaching of~3% after 3 h of light-irradiated Cu 2 O/rGO [32]. EIS data showed a smaller resistance indicating better charge-transfer in Cu 2 O/rGO vs. Cu 2 O, as can be seen in Figure 5a. In a Mott-Schottky analysis they had a negative slope [32], which indicates p-type semiconductor properties in Cu 2 O/rGO and an increased donor-density vs. Cu 2 O.
A different carbonaceous substrate, such as g-C 3 N 4 /Vulcan Carbon, was prepared by Hussain et al. [42], protecting the (111) facets of Cu 2 O, resulting in improved stability of Cu 2 O nanoparticles. A different approach was given by Sun et al. [43], who developed a heterostructure of Cu 2 O quantum dots (QDs) supported on a 3D g-C 3 N 4 foam. In this work, the authors suggested that the (111)Cu 2 O facet, when interfaced with g-C 3 N 4 , allowed creation of additional DOS at the Cu 2 O/g-C 3 N 4 interface [43]. In such heterojunction, the photoexcited electrons are suggested to be transferred from g-C 3 N 4 to Cu 2 O QDs, thus avoiding Cu 2 O photocorrosion and enhance photocatalytic activity. Enhanced stability was also evidenced by the reusability of this Cu 2 O/g-C 3 N 4 heterojunction up to five times [43].

Case−Study D: The Effect of Carbonaceous Materials
Reduced graphene oxide (rGO) can act as a protecting agent. An et al. [32] have studied the effect of rGO interfaced with different exposed facets of Cu2O with (100) to be better for CO2R than (100) [32]. The proposed mechanism is described in Figure 3 Case-Study D). They measured Cu-leaching using ICP (inductively coupled plasma optical emission spectrometry) and they demonstrated a Cu-atom leaching of ~3% after 3 h of light-irradiated Cu2O/rGO [32]. EIS data showed a smaller resistance indicating better charge-transfer in Cu2O/rGO vs. Cu2O, as can be seen in Figure 5a. In a Mott-Schottky analysis they had a negative slope [32], which indicates p-type semiconductor properties in Cu2O/rGO and an increased donor-density vs. Cu2O.
A different carbonaceous substrate, such as g-C3N4/Vulcan Carbon, was prepared by Hussain et al. [42], protecting the (111) facets of Cu2O, resulting in improved stability of Cu2O nanoparticles. A different approach was given by Sun et al. [43], who developed a heterostructure of Cu2O quantum dots (QDs) supported on a 3D g-C3N4 foam. In this work, the authors suggested that the (111)Cu2O facet, when interfaced with g-C3N4, allowed creation of additional DOS at the Cu2O/g-C3N4 interface [43]. In such heterojunction, the photoexcited electrons are suggested to be transferred from g-C3N4 to Cu2O QDs, thus avoiding Cu2O photocorrosion and enhance photocatalytic activity. Enhanced stability was also evidenced by the reusability of this Cu2O/g-C3N4 heterojunction up to five times [43].  The protective potential of interfacial carbon was also evidenced by Wang et al., who synthesized Cu 2 O/graphited carbon interfaces, using a polyol-method, to produce Cu 2 O nano-flowers on graphene [45]. Increased porosity of the catalyst achieved by 90 • C treatment improved the internal charge transfer, as evidenced by EIS Nyquist data and as can be seen in Figure 5c. [45]. Overall, carbon-nanomatrices can act as protecting agents for Cu-oxides. The enhanced electron-storage capacity of graphitized carbon, together with their high chemical resilience are key-beneficial parameters that renders this approach as highly promising towards industrial exploitability.

Case−Study E: Core-Shell and Alloys
Besides the conventional approach of core-shell structures, in the case of Cuoxides, one can create core-shell structures by starting from a fully reduced Cu state and result to a reduced metallic-core, with a stable oxidized layer. Yang et al. [18] proposed a twostep method, firstly by electrodeposition of Cu 2 O on FTO film and a subsequent thermal oxidation (time was the factor that controlled the thickness of the outer layer) in which the outer film was transformed to CuO. The so-formed Cu 2 O/CuO bilayer was tested for its photostability, and Electrochemical Impedance Spectroscopy data showed very small resistance under illumination, while the Mott-Schottky measurements demonstrated large carrier density and a high flat-band potential, evidencing good conductivity and a high degree of band bending [18]. This approach places the Cu 2 O/CuO composite as promising photocathode candidate for HER, according to its good stability in alkaline media, high photocurrent, and the ease of method.
A self-supported Cu/Cu 2 O-CuO/rGO nanowire array [44] showed good stability for more than 15 h in alkaline medium, as can be seen in Figure 5b, in accordance with low resistance evidenced by EIS-loops, which indicates fast charge transfer. This provides evidence that the presence of metallic Cu 0 as a hetero-component-not a standalone onecan offer high-speed paths for electron transportation. In addition, the highly reductive rGO shell also provides a conductive network that may promote charge transfer, which are beneficial for the Hydrogen Evolution Reaction process.
A highly efficient and low cost Cu 2 O-SnO 2 core-shell electrocatalyst has been reported in [46] via control of the shell thickness. An optimized 5 nm thick SnO 2 shell gave the faradaic efficiency, i.e., >90% of CO 2 to CO reduction [46]. These Cu 2 O-SnO 2 electrocatalyst achieved a good stability over 18 h of test at −0.6 V vs. RHE in 0.5 M KHCO 3 electrolyte.
A three-dimensional nanoporous Cu-Ru alloy as a Pt-free catalyst for HER, was prepared by Wu et al. [28] using a dealloying process, as shown schematically in Figure 6a,c. They demonstrated that the so-obtained nanoporous Cu 53 Ru 47 exhibits long-term operation stability for more than 24 h in neutral pH (1.0 M PbS) and, after the first 3 h, fully stable at the alkaline pH (1 M KOH), as can be seen in Figure 6b.
Shen et al. started with a fundamental basis, which relies on electronic modification of a graphitic shell [33]. As reported from other groups too, with a graphitic layer to act as a protective shell for the core, this method can improve significantly the durability of the catalyst. Therefore, this group [33] synthesized a nickel-copper alloy (core) encapsuled in graphitic layer (shell) (NiCu@C) via chemical vapor deposition method, using CH 4 as a carbon source. This allowed them to obtain a tunable C-thicknesses. In this way, they had obtained three materials: NiCu@C-1 had mostly single-layered (78.4%) C-coating, NiCu@C-2 a well-defined, 2-3 nm thickness, core-shell structure, corresponding to 5-10 graphitic layers, and NiCu@C-3 with an 8-15 nm shell. Raman Spectroscopy, see Figure 5d, shows the three characteristic peaks, D, G, and 2D, that increased upon increase the graphitic shell (C) [32]. Shift in Raman peaks could be related to the modification of electronic structures of graphitic layers arisen from the interaction of carbon shell NiCu core. The stability of the catalyst was evaluated by chronoamperometric measurements, which verify that NiCu@C catalysts show slower decay in both acidic and alkaline pH than in pristine NiCu. From EIS, they demonstrated [32] also that the charge transfer resistance follows the same rule, as the thickness is increasing, the resistance increasing too, which can be explained by slower reaction kinetics and electron transport at the thicker graphitic shells (see Figure 3 Case-Study E-inset). Shen et al. started with a fundamental basis, which relies on electronic modification of a graphitic shell [33]. As reported from other groups too, with a graphitic layer to act as a protective shell for the core, this method can improve significantly the durability of the catalyst. Therefore, this group [33] synthesized a nickel-copper alloy (core) encapsuled in graphitic layer (shell) (NiCu@C) via chemical vapor deposition method, using CH4 as a carbon source. This allowed them to obtain a tunable C-thicknesses. In this way, they had obtained three materials: NiCu@C-1 had mostly single-layered (78.4%) C-coating, NiCu@C-2 a well-defined, 2-3 nm thickness, core-shell structure, corresponding to 5-10 graphitic layers, and NiCu@C-3 with an 8-15 nm shell. Raman Spectroscopy, see Figure  5d, shows the three characteristic peaks, D, G, and 2D, that increased upon increase the graphitic shell (C) [32]. Shift in Raman peaks could be related to the modification of electronic structures of graphitic layers arisen from the interaction of carbon shell NiCu core. The stability of the catalyst was evaluated by chronoamperometric measurements, which verify that NiCu@C catalysts show slower decay in both acidic and alkaline pH than in pristine NiCu. From EIS, they demonstrated [32] also that the charge transfer resistance follows the same rule, as the thickness is increasing, the resistance increasing too, which can be explained by slower reaction kinetics and electron transport at the thicker graphitic shells (see Figure 3 Case-Study E-inset).

Case−Study F: Lattice-Size-Shape-Facets
Lattice-strain engineering can be effective on altering the DOS near the Fermi level, thus regulating adsorption energy of catalysts [27]. This was exemplified by Kang et al. [27], who used plasma spraying (PS) to prepare the coating layer in self-supported Cu electrode with intensive strain. The electrode's stability was significant for more than 30 h at 100 mA cm −2 (see Figure 3 Case-Study F). EIS data show that their PS-Cu had the lowest resistance of all tested samples, and the stability was best at acid medium and the worst at alkaline medium.
In addition, lattice-strain engineering may improve interaction between catalyst and intermediates [47]. For instance, tensile strain [48] could increase the interatomic distance

Case−Study F: Lattice-Size-Shape-Facets
Lattice-strain engineering can be effective on altering the DOS near the Fermi level, thus regulating adsorption energy of catalysts [27]. This was exemplified by Kang et al. [27], who used plasma spraying (PS) to prepare the coating layer in self-supported Cu electrode with intensive strain. The electrode's stability was significant for more than 30 h at 100 mA cm −2 (see Figure 3 Case-Study F). EIS data show that their PS-Cu had the lowest resistance of all tested samples, and the stability was best at acid medium and the worst at alkaline medium.
In addition, lattice-strain engineering may improve interaction between catalyst and intermediates [47]. For instance, tensile strain [48] could increase the interatomic distance inside the lattice catalyst, thus decreasing the overlap of d-orbitals, and/or upshift the d-band center and intermediates [49]. In this context, so far, several methodologies have been exploited to generate lattice-strain, such as, introducing interfaces with lattice mismatch [47], creating defects [48], and preparing ultra-thin nanosheets [50].
The different crystallite-facets of metal oxide nanoparticles may affect the activity and stability of photocatalysts for HER and CO 2 RR, thus controlling preferential-facet growth can be a fruitful strategy. In a commonly applied methodology, the differential affinity of certain surfactants for certain Cu 2 O-facets may allow control of surface oxidation extent, oxophilicity, hydrophilicity, and their activity to HER [51]. In a refined work, Gao et al. [52] studied Cu 2 O nanoparticles (NPs) with (100) facets, octahedral Cu 2 O NPs with exposed (111) and t-Cu 2 O with both (111) and (100) facets. Studies on these Cu 2 O NPs showed that the types of interfaced crystal facets exhibited different stabilities and different catalytic activities [53]. Shape of Cu 2 O particles has been shown to be correlated with stability during the water splitting process [54]. More specifically, when comparing Cu 2 O cubes with (100) facets, Cu 2 O octahedra with (111) facets and Cu 2 O rhombic dodecahedra with (110) facets, it was concluded that-practically-standalone Cu 2 O, irrespective of the faced exposure, could not be very stable, no matter its facet engineering [53].
From the above-mentioned examples, it may be anticipated that, after finding which facet is best for the catalyst's activity, we should protect that facet, by an appropriate method, e.g., p-n heterojunctions or coating or by mixing it with carbon materials as graphene.

Hydrogen Evolution by Cu-Oxide Based Materials
Both Cu 2 O and CuO are p-type semiconductors [13][14][15][16] with band gaps 2.0-2.5 eV and 1.3-1.7 eV, respectively, see Figure 2, thus, absorbing a significant proportion of the solarspectrum. However, their stability issues, when combine with fast e − -h + recombination phenomena, make them rather unfavorable photocatalysts for H 2 evolution from H 2 O [54]. Nonetheless, recent studies demonstrate that we can minimize these disadvantages by coupling copper oxides with other, more stable and suitable semiconductors. Doing so, the electrical and optical properties of Cu 2 O and CuO are significantly changed by the material they are coupled with [24,28,33].
Hydrogen Evolution Reaction (HER) [55] is considered an extraordinary reaction in electrochemistry for two reasons: firstly, for its simplicity, because it is always a 2e − process, no matter the pH [55], and secondly because it is a direct process to produce high-purity H 2 , considered to be the key fuel for the future. Mechanistically, the basis of HER is described by the reactions in Table 1 [56]. HER in electrochemistry is a 2e − process [55], which is divided mainly in two steps: first is the Volmer step (1.i in Table 1) that includes the adsorption of one H-atom on the catalyst surface, by transferring one proton H + from the acid electrolyte which combines with an electron e − transferred from the catalyst surface to form an adsorbed hydrogen atom (H* = H + + e − ); second is the Heyrovsky step (Equation (1.ii)) in which this H* combines with one electron e − and one H + to form a H 2 molecule, see also Figure 7. In addition, there is possibility to form one H 2 molecule through the Tafel step (1.iii) i.e., the combination of two H* [55].   Table 2 (photocatalytic H2 production), Table 3 (electrocatalytic H2 production) and Table 4 (photoelectrocatalytic H2 production). Hereafter, we highlight some engineering aspects related to the control/improvement of hydrogen evolution by such catalysts.
In acidic electrolytes, it has been generally accepted that the differences in reaction rates and activity are closely related to the differences of free-energy of hydrogen-adsorption on different materials [26]. In alkaline media, due to the lack of H + , the reaction must begin from dissociation of H2O molecules, see Volmer-Heyrovsky steps (Equations 2.i and 2.ii). In addition, in alkaline media, as in the case of acidic media, there is the possibility to form a H2 molecule through the Tafel step (2.iii), i.e., the combination of two H*. This additional step of proton-generation from H2O in alkaline media intro-  Table 2 (photocatalytic H 2 production), Table 3 (electrocatalytic H 2 production) and Table 4 (photoelectrocatalytic H 2 production). Hereafter, we highlight some engineering aspects related to the control/improvement of hydrogen evolution by such catalysts.
In acidic electrolytes, it has been generally accepted that the differences in reaction rates and activity are closely related to the differences of free-energy of hydrogen-adsorption on Solar 2023, 3 97 different materials [26]. In alkaline media, due to the lack of H + , the reaction must begin from dissociation of H 2 O molecules, see Volmer-Heyrovsky steps (Equations (2.i) and (2.ii)). In addition, in alkaline media, as in the case of acidic media, there is the possibility to form a H 2 molecule through the Tafel step (2.iii), i.e., the combination of two H*. This additional step of proton-generation from H 2 O in alkaline media introduces an additional energy barrier and affects the whole reaction kinetics and that is the main reason why acid medium is preferred in this reaction. In alkaline environments, there are two approaches to start with: [i] by the single descriptor of hydrogen binding energy [57], or [ii] the bifunctional nature of HER, a good catalyst needs both a beneficial OH ad energetics (as it is slower when it has to do with metal; thus, is the rate-limiting step) and beneficial H ad energetics [51].
A summary of pertinent Cu-based semiconducting materials is listed in Table 2 (photocatalytic H 2 production), Table 3 (electrocatalytic H 2 production) and Table 4 (photoelectrocatalytic H 2 production). Hereafter, we highlight some engineering aspects related to the control/improvement of hydrogen evolution by such catalysts. The shape of Cu 2 O nanocrystals can affect the photocatalytic H 2 production performance [54]. Cu 2 O nanopowders with different morphologies, i.e., cubes with (100) facets, octahedra with (111) facets, or rhombic dodecahedra with (110) facets, which were evaluated for long-term stability on hydrogen production as well as for overall water-splitting. All materials presented photocorrosion phenomena under irradiation in the form of phase transformation of Cu 2 O to CuO, verified both by XPS and X-Ray Diffraction (XRD) measurements. Noteworthy, the stability trend was completely different under dark conditions. Moreover, the absence of sacrificial agent during water splitting led to an absence of O 2 production, as normally H 2 and O 2 should have been produced in a 2:1 ratio. Thus, in [54], the photogenerated holes of Cu 2 O that were supposed to oxidize H 2 O and produce O 2 were consumed in the oxidation of Cu 2 O to CuO. Furthermore, the best performing material, rhombic dodecahedra Cu 2 O, was tested for long-term stability where at 48 h the produced H 2 was 8-fold less and at 72 h no H 2 was detected. To prevent the Cu 2 O oxidation, the nanocrystals were coated with a TiIrO x layer [54]. This hybrid TiIrO x /Cu 2 O material had a better photocatalytic performance with a H 2 :O 2 ratio of 2:1 , indicating that TiIrO x Solar 2023, 3 98 facilitated the hole transfer, thus preventing photocorrosion of Cu 2 O, at the same time enabling O 2 -production from water oxidation.

The case of Cu-TiO 2
Another strategy is the coupling of CuO or Cu 2 O with other semiconductors and the establishment heterojunctions or Z-scheme mechanism [37]. Coupling copper oxides with other semiconductors, such as TiO 2 or ZnO with lower energy band positions can lead to the formation of Z-Scheme or Type-II electron transfer mechanism between the two materials. Therefore, Cu 2 O/TiO 2 hybrid systems have been widely investigated in the fields of photocatalysis for both H 2 evolution [60] and CO 2 reduction [67] and degradation of organics [68]. Lv et al. [58] coupled Cu 2 O nanoparticles onto the surface of TiO 2 and showed that, due to the Z-scheme mechanism, carriers with strong oxidation and reduction ability were confined to TiO 2 , inhibiting the photocorrosion of the Cu 2 O. XPS and EPR data, see Figure 8a-c, indicated presence of surface lattice defects, oxygen vacancies (O V ), which can act both as electron traps and simultaneously enhance the adsorption capability of H + ions in the hydrogen evolution reaction [58]. Moreover, they notice an upward shift and a downward shift on the XPS spe Ti2p and Cu2p, respectively [58], indicating a directional carrier transport from T Cu2O (Figure 8a,b). Regarding the photocatalytic activity, their Cu2O/TiO2 compos hibited high H2 production rates, 11 mmol g −1 h −1 in water and 5.1 mmol g −1 h −1 i water [57]. Compared with the pristine Cu2O, yielding 0.5 mmol g −1 h −1 and 0.3 mm h −1 in water and seawater, respectively, the Cu2O/TiO2 composite showed a cle provement. This significant improvement of photocatalytic activity and stability where under illumination metallic Cu NPs enhance H 2 production due to SPR. Reprinted from [59] with permission from Elsevier. Moreover, they notice an upward shift and a downward shift on the XPS spectra of Ti 2p and Cu 2p , respectively [58], indicating a directional carrier transport from TiO 2 to Cu 2 O (Figure 8a,b). Regarding the photocatalytic activity, their Cu 2 O/TiO 2 composite exhibited high H 2 production rates, 11 mmol g −1 h −1 in water and 5.1 mmol g −1 h −1 in seawater [57]. Compared with the pristine Cu 2 O, yielding 0.5 mmol g −1 h −1 and 0.3 mmol g −1 h −1 in water and seawater, respectively, the Cu 2 O/TiO 2 composite showed a clear improvement. This significant improvement of photocatalytic activity and stability is attributed on the consumption of the highly oxidizing holes of Cu 2 O from the O V 's of TiO 2 and the Z-scheme mechanism [58] (Figure 8d). Generally, stabilizing CuO or Cu 2 O nanoparticles on TiO 2 is a common approach, Tian et al. have prepared a TiO 2 photocatalyst loaded with Cu NPs through an ion exchange (IE) process, where they report the co-existence of Cu 2+ , Cu 1+ , and Cu 0 , based on XPS-data. The final composite material gave a purple hue which persisted during the photocatalytic experiments, indicating that copper was at the metallic state [60]. In another study, Jung et al. also used TiO 2 as a substrate to deposit small, finely-dispersed CuO NPs (0.5-2.5 nm) [59] (Figure 8e). During H 2 generation, they made two observations, [i] there was a color change from dark-grey to deep-violet, attributed to change of the Cu oxidation state, and [ii] there was an initial delay of the hydrogen production. XPS and AES profile at different illumination times show the slow transformation of Cu oxidation states from Cu 2+ to Cu 1+ . The authors [59] suggest that the photogenerated electrons are consumed to reduce CuO to Cu 2 O instead of reducing H + and produce H 2 (Figure 8f,g). When the reduction of copper has been accomplished, these photogenerated electrons participate in H 2 production. In the case of Cu 0 , as in the cases of noble metal cocatalysts, there is a continuum of states within the TiO 2 energy gap where Cu 0 attracts the photogenerated electron from TiO 2 , thus facilitating electron transfer to protons (see Figure 8h).

Coupling of Cu with Non-TiO 2 Semiconductors
Besides TiO 2 , other semiconductors, especially n-type, have been also used for the stabilization of Cu NPs. In this context, Hu et al. coupled p-type Cu 2 O with n-type WO 3 , forming a p-n heterojunction [29]. Unlike pristine Cu 2 O, which did not produce H 2 during water splitting, Cu 2 O/WO 3 heterojunction facilitated H 2 production, due to efficient charge separation. Lou et al. designed a Cu 0 @Cu 2 O/ZnO heterostructure in order to stabilize the plasmonic Cu core, by decorating it with Cu 2 O shell, and finally stabilizing this core-shell structure onto ZnO nanorods [62] (See Figure 9a). This hybrid nanostructure exhibited better stability even after four catalytic cycles (Figure 9b), and superior photocatalytic performance, i.e., 1472µmol g 1 h −1 compared with the pristine Cu@Cu 2 O material which yields 5 µmol g −1 h -1 (See Figure 9c). This improvement on the photocatalytic activity was attributed on the Localized Surface Plasmon Resonance (LSPR) caused by photoexcitation of metallic Cu 0 particles [69]. Arguably, the LSPR was suggested to be linked with hot-electron transfer, as evidenced by Photoluminescence (PL) data. Specifically, the PL emission spectra indicated that Cu@Cu 2 O/ZnO nanocomposites could delay the recombination rate of e − -h + pairs. In the proposed model, see Figure 9d, surface plasmons are photoexcited upon irradiation and the hot electrons occupy the surface plasmon states above the Fermi energy. A fraction of these hot electrons are injected onto the CB of Cu 2 O, by overcoming the Schottky barrier formed at the Cu 2 O-Cu 0 interface, and finally they leap on the CB of ZnO. This mechanistic path for the electrons is believed to intercept photocorrosion phenomena [62].

Carbon-Based Materials and Core-Shell Structures
Use of carbonaceous materials is another interesting approach towards the stabilization and the improvement of the photocatalytic performance of copper nanocatalysts. Lin et al. has decorated 50 nm Cu 2 O with small~3 nm nanodiamonds (NDs), and the optimal loading was found to be 3 wt.% enabling a H 2 production rate of 1597 µmol g −1 h −1 with a solar-tohydrogen efficiency of 0.85%. This nanocomposite exhibited high photocatalytic stability attributed to the narrower band gap, compared with pristine Cu 2 O, which improves the ability to harvest solar light [65] (See Figure 9e). As for the proposed photocatalytic mechanism, the photoexcited electrons from the NDs surface were injected into the Cu 2 O and at the same time the photogenerated holes of Cu 2 O migrated to the NDs (See Figure 9f). The excess of electrons on Cu 2 O performed the reduction of H 2 O to H 2 and the excess of holes on the NDs oxidized the ethanol. Another material of great interest is graphitic carbon nitride (g-C 3 N 4 ). Liu et al. synthesized a Cu 2 O@g-C 3 N 4 core-shell structure onto the Cu 2 O octahedra with exposed (111) facets. The efficiency of the composite material Solar 2023, 3 100 under visible light irradiation was attributed to the synergistic effect at the interface of Cu 2 O and g-C 3 N 4 , i.e., transfer of the photogenerated electrons of Cu 2 O to the g-C 3 N 4 shell. In contrast, UV irradiation on the Cu 2 O crystals had a negative effect, and this was attributed to a mechanism where O V in Cu 2 O act as electron traps, leading to the reduction of Cu 2 O to Cu 0 exhibiting lower photocatalytic activity [66].
photoexcitation of metallic Cu 0 particles [69]. Arguably, the LSPR was suggested linked with hot-electron transfer, as evidenced by Photoluminescence (PL) data. Sp cally, the PL emission spectra indicated that Cu@Cu2O/ZnO nanocomposites could d the recombination rate of e − -h + pairs. In the proposed model, see Figure 9d, su plasmons are photoexcited upon irradiation and the hot electrons occupy the su plasmon states above the Fermi energy. A fraction of these hot electrons are injected the CB of Cu2O, by overcoming the Schottky barrier formed at the Cu2O-Cu 0 inter and finally they leap on the CB of ZnO. This mechanistic path for the electrons is beli to intercept photocorrosion phenomena [62].

Carbon-Based Materials and Core-Shell Structures
Use of carbonaceous materials is another interesting approach towards the st zation and the improvement of the photocatalytic performance of copper nanocata Lin et al. has decorated 50 nm Cu2O with small ~3 nm nanodiamonds (NDs), and th timal loading was found to be 3 wt.% enabling a H2 production rate of 1597 μmol g with a solar-to-hydrogen efficiency of 0.85%. This nanocomposite exhibited high p catalytic stability attributed to the narrower band gap, compared with pristine C which improves the ability to harvest solar light [65] (See Figure 9e). As for the prop photocatalytic mechanism, the photoexcited electrons from the NDs surface wer

The Case of Cu-Single Atoms
Lastly, single-atom catalysts (SACs) are gaining more and more interest owing to their ability in maximizing reaction active sites [70]. These isolated metal atoms anchored in the surface of photocatalysts may offer more reaction active sites, but their stability and tendency to aggregate during photocatalysis is still an issue to address. Very recently, Lee et al. [64] designed atomically-dispersed Cu on TiO 2 by incorporating site-specific single atoms of Cu. Interestingly, it was shown that photogenerated electrons can be transferred from the CB of TiO 2 to the d-orbitals of the isolated Cu 2+ atoms. These trapped electrons induced a polarization field resulting in lattice distortion of TiO 2 , that was suggested to be linked with the enhanced photocatalytic H 2 activity. This phenomenon was reversible when the material was exposed to O 2 without irradiation. They also report a change of color during photocatalysis, in accordance with other studies on TiO 2 -based nanocatalysts [58,59]. In another very recent study, Zhang et al. managed to deposit higher amounts of CuSA (>1 wt%) on TiO 2 , at this point we should note that normally the loading percentages of SACs is near 0.1-0.3 wt% [40]. Their optimized material exhibited the impressive H 2 evolution rate of 101.7 mmol g −1 h −1 , which in their case was higher than PtSA-TiO 2 (95.3 mmol g −1 h −1 ). This impressive photocatalytic performance is attributed to the larger amount of Cu SACs as well as to the high dispersion onto the TiO 2 surface. Regarding the photocatalytic mechanism, efficient electron transfer is attributed to the reversible Solar 2023, 3 101 redox process between Cu 2+ and Cu 1+ atoms. These findings are supported by Electron Paramagnetic Resonance (EPR) data, showing a strong Cu 2+ EPR signal before irradiation, which during irradiation was converted to Cu + , which was re-oxidized to Cu 2+ after exposure to air O 2 [40]. This in situ self-healing process enables CuSA-TiO 2 to achieve these remarkable H 2 production rates. Furthermore, long-term H 2 evolution experiments, i.e., 380 days later, showed no loss of the photocatalytic performance [40].

Electrocatalytic Hydrogen Production
Electrochemistry permits quantitative comparisons of activity stability, while photoelectrochemistry permits more advanced parameters to be monitored, i.e., photocurrent density, photo-response, photostability and photocorrosion. In brief, the tools used can be [56]: [i] exploring the intrinsic activity of catalysts by measuring the capacitance of double layer Cdl to detect the electrochemical active surface area (ESCA) [71], [ii] overpotential needs to be the lowest possible, at 10 mA/cm2 [72], [iii] the Tafel slope shows the rate of adsorption-desorption kinetics, where the electrochemical desorption step is rate-determining according to the Volmer-Heyrovsky mechanism [73].
[iv] through EIS measurements the charge transfer resistance can easily be found [56].
[v] Mott-Schottky plots can provide information about flat band potential and the populations of donors and acceptors [74].
The double-layer capacitance which can be determined by cyclic voltammetry measurements, the electrochemically active surface area, current density [56], and the faradaic efficiency of the sample [75]. Linear Sweep Voltammetry (LSV) provides a measure of overpotentials necessary to achieve j = 10 mA/cm 2 , and after 2 h to check the stability, or even a long-term stability test which can last up to 24 h [76].
In Table 3, we summarize pertinent values for these electrochemical parameters for selected Cu-oxide catalysts. Table 3. Summary of Electrocatalytic Parameters, related to Hydrogen Evolution Reaction, for Cu-based catalysts.

Catalyst
Electrolyte or pH C dl (mF/cm 2 ) η(mV) @ −10 mA/cm 2 Tafel Slope (mV dec −1 ) Ref. In Table 3, pertinent literature works are presented. In all cases, we mention works where they did not use noble metals as co-catalysts, but use only Cu-based electrocatalysts. Some of them had merged the electrolytes with graphene as a core-shell structure in order to protect them from corrosion [77], while others [44,78] used carbon nanotubes with Cu or Cu/Cu 2 O/CuO structures merged in reduced graphene oxide. An important contribution is a strain-activated copper catalyst [27], operating under a wide pH range. In [27] the current density achieved was1.987 A cm −2 at −1.2 V vs. RHE, which is 2.85 times vs. a reference Pt foil.
Moreover, a three-dimensional nanoporous Cu-Ru alloy as an outstanding Pt-free catalyst for HER has been prepared by a dealloying process by Wu et al. [28]. This catalyst had a good activity with a very low overpotential and very low Tafel slopes in alkaline and neutral environments. By alloying those two metals, Cu and Ru, the incorporation of Ru atoms in the Cu matrix enhanced the strength of the Cu-H interaction, while it weakened the Ru-H interaction, but improved the overall activity [27]. The Electrochemical impedance spectroscopy measurements of np-Cu 53 Ru 47 further confirmed that the incorporation of Ru atoms into the Cu matrix brings about small internal resistance and fast charge-transfer behavior. The same group, [27], demonstrated with Density Functional Theory (DFT) that the incorporation of Ru atoms into the Cu matrix could effectively optimize the delectron domination of Cu and Ru atoms and decrease the energy barrier and improving the HER activity.
Another approach [79] presented a complex heterostructure of NiCo layered double hydroxide wrapped around Cu nanowires grown on top of commercially available Cu mesh (Cu-m). This Cu-m/Cu-W/NiCo-LDH material outperformed the benchmark 40 wt% Pt/C at a wide pH range [79]. As shown in Figure 10a,b, the required overpotential was smaller for alkaline medium than 40%Pt/C, confirmed by the Tafel plots at Figure 10c. While in acidic environment, as shown in Figure 10d,e, the Cu-m/Cu-W/NiCo-LDH had increased overpotential versus 40%Pt/C at −10 mA cm −2 , while the Tafel slope in Figure 10f shows a decreased value for Cu-m/Cu-W/NiCo-LDH. Figure 10h shows the required potential across the pH range to reach −10 mA cm −2 for Cu-m/Cu-W/NiCo-LDH versus 40 wt% Pt/C. In Figure 10g, high-resolution transmission electron microscopy (HRTEM) shows the (111) exposed facets of Cu-m and Cu-Ws, and the latter's growth direction is along (111). mesh (Cu-m). This Cu-m/Cu-W/NiCo-LDH material outperformed the benchmark 40 wt% Pt/C at a wide pH range [79]. As shown in Figure 10a,b, the required overpotential was smaller for alkaline medium than 40%Pt/C, confirmed by the Tafel plots at Figure  10c. While in acidic environment, as shown in Figure 10d,e, the Cu-m/Cu-W/NiCo-LDH had increased overpotential versus 40%Pt/C at −10 mA cm −2 , while the Tafel slope in Figure 10f shows a decreased value for Cu-m/Cu-W/NiCo-LDH. Figure 10h shows the required potential across the pH range to reach −10 mA cm −2 for Cu-m/Cu-W/NiCo-LDH versus 40 wt% Pt/C. In Figure 10g, high-resolution transmission electron microscopy (HRTEM) shows the (111) exposed facets of Cu-m and Cu-Ws, and the latter's growth direction is along (111).

Photoelectrochemical Hydrogen Evolution Reaction
Photoelectrochemical cells (PEC) are widely considered potent for solar water splitting devices [80] because they combine solar energy collection with water electrolysis and because they spatially separate two half-cell reactions, HER and OER. For a PEC cell, the process depends on three steps [55]: (i) light absorption by a semiconductor material, (ii) electron-hole pair generation, and, as a result, (iii) photoinduced electrons (or holes) are driven, by space-charge field, to the semiconductor/solution interface where they reduce or oxidize water.
Usually, a PEC cell has a n-type semiconductors, such as TiO 2 , ZnO, BiVO 4 , WO 3 , Fe 2 O 3 [4,[81][82][83] whose valence band edges are more positive than the potential of the H 2 O/O 2 redox couple, that makes them ideal for OER photoanodes. On the other hand, p-type semiconductors such as p-CdS, p-WSe, p-InP and p-type Copper Oxides [25,84,85], whose conduction band edge is more negative than the H 2 O/H 2 potential, are ideal for HER photocathodes (see Figure 11). In Table 4, we selected some interesting works from several groups, who have presented durable and stable photocathodes for a PEC cell. In addition, very important for these catalysts is to have good light response and photocurrent to benchmark their photocatalytic activity.
ting devices [80] because they combine solar energy collection with water electrolysis a because they spatially separate two half-cell reactions, HER and OER. For a PEC cell, process depends on three steps [55]: (i) light absorption by a semiconductor material, electron-hole pair generation, and, as a result, (iii) photoinduced electrons (or holes) driven, by space-charge field, to the semiconductor/solution interface where they redu or oxidize water.
Usually, a PEC cell has a n-type semiconductors, such as TiO2, ZnO, BiVO4, W Fe2O3 [4,[81][82][83] whose valence band edges are more positive than the potential of H2O/O2 redox couple, that makes them ideal for OER photoanodes. On the other ha p-type semiconductors such as p-CdS, p-WSe, p-InP and p-type Copper Oxides [25,84,8 whose conduction band edge is more negative than the H2O/H2 potential, are ideal HER photocathodes (see Figure 11). In Table 4, we selected some interesting works fr several groups, who have presented durable and stable photocathodes for a PEC cell addition, very important for these catalysts is to have good light response and pho current to benchmark their photocatalytic activity. Figure 11. Band positions of various metal oxide and non−oxide semiconductors and 2D materi Reproduced from Ref. [86] with permission from the Royal Society of Chemistry.
In order to explore the photocorrosion of CuO, Xing et al. [26] had tested hete junctions CuO: CuO/TiO2, CuO/Pt, and CuO/TiO2/Pt as photocathodes. Under increas protection of the photocathodes, allowed increase of the photocurrent density at −0.55 as shown in Figure 10j. The composite of CuO/TiO2/Pt had the highest photocurr density. In addition, according to the Figure 10k, CuO/TiO2/Pt seems to have a very go photo-response, at −0.55 V vs. Ag/AgCl applied under 30 s light on/off cycles for m than 300 s. This electrode was stable against photocorrosion in 1M NaOH electrolyte −0.55 V vs. Ag/AgCl under simulated sunlight illumination for 2 h, Figure 10l. The E Nyquist plot, shown in Figure 10m, reveals that the CuO/TiO2/Pt has decreased sistance in charge transfer phenomena, which indicates easier transfer of photoinduc electrons to the electrolyte solution.  In order to explore the photocorrosion of CuO, Xing et al. [26] had tested heterojunctions CuO: CuO/TiO 2 , CuO/Pt, and CuO/TiO 2 /Pt as photocathodes. Under increased protection of the photocathodes, allowed increase of the photocurrent density at −0.55 V, as shown in Figure 10j. The composite of CuO/TiO 2 /Pt had the highest photocurrent density. In addition, according to the Figure 10k, CuO/TiO 2 /Pt seems to have a very good photo-response, at −0.55 V vs. Ag/AgCl applied under 30 s light on/off cycles for more than 300 s. This electrode was stable against photocorrosion in 1 M NaOH electrolyte at −0.55 V vs. Ag/AgCl under simulated sunlight illumination for 2 h, Figure 10l. The EIS Nyquist plot, shown in Figure 10m, reveals that the CuO/TiO 2 /Pt has decreased resistance in charge transfer phenomena, which indicates easier transfer of photoinduced electrons to the electrolyte solution.
As general conclusion, photocorrosion must be scrutinized as another important phenomenon in PEC for solar hydrogen production. This aspect is considered using molecular components such as linkers, non-oxide material modifiers, or catalysts. The most common method, as mentioned in Table 4, is by using TiO 2 as a matrix where CuO, Cu 2 O can be protected by photocorrosion through the Z-scheme mechanism. In the case of Solar 2023, 3 105 single-Cu atoms, there is a different approach, where Cu(II) transforms reversibly to Cu(I) during photoelectrocatalysis as described before.

CO 2 Reduction by Cu-Based Materials
From an economic and environmental point of view, (electro)photocatalytic CO 2 reduction is a forward-looking realm in catalytic technology. Inspired by natural-photosynthesis, scientists are trying to realize "Artificial-Photosynthesis", convert CO 2 into useful and high value chemical fuels, using the highly abundant solar energy (see Figure 1). Inoue together with Fujishima and Honda pioneered the idea of the feasibility of catalytic conversion of CO 2 to C1-fuels using semiconductors [4,89]. Since then, many efforts were made to improve the overall performance and tune the selectivity towards a specific product [9]. In this context, Cu-oxides and metal-Cu particles emerge among the most promising photocatalysts thanks to good electron transfer properties and loosely bound d-electrons, thus having great potential for facilitating CO 2 photoreduction [20]. Herein, we focus on Cubased semiconductors coupled with other oxide semiconductors, carbon-based materials or plain copper catalysts with facet modifications and discuss stability issues and product selectivity (see Table 5).  As in the case of photocatalytic H 2 evolution, see Section 3.1.1, facet engineering of Cu oxides shows high promise towards increased performance in photocatalytic CO 2 reduction [10]. Recently, Wu et al. reported the photocatalytic reduction of CO 2 on facet specific active sites of Cu 2 O [90]. They demonstrated that the (110) facet of a single Cu 2 O particle was the photoactive site towards CO 2 reduction to CH 3 OH, while the (100) facet remained inert [90]. This observation can be attributed to electron-density difference on Cu active sites on the (110) facet and a shift from Cu(I) to Cu(II) due to CO 2 and H 2 O adsorption [90]. In this way, during CO 2 reduction, the Cu 2 O catalyst manages to oxidize H 2 O accompanied with a lattice expansion due to the CO 2 adsorption. This process showed a selectivity towards methanol yielding 1.2 mol CH 3 OH g −1 h −1 and reaching an internal quantum yield of~72% [90].

Coupling with Semiconductors
The Case of Cu-TiO 2 Cu 2 O coupling with TiO 2 has been explored as an efficient strategy to enhance photocatalytic performance. More specifically, Aguirre et al. synthesized a p-n Cu 2 O/TiO 2 heterojunction, where the Z-scheme mechanism of electron transfer [37] enhances the stability of Cu 2 O [67] (see Figure 12a,b). The energetics of this example are educative. TiO 2 alone exhibits very low CO 2 -to-fuel-conversion efficiency under UV excitation, while Cu 2 O possesses a more favorable E CB = −1.4 eV vs. NHE (pH = 7) [103]; however, it suffers from photostability issues [13]. In the p-n Cu 2 O/TiO 2 scheme, Cu(0) was not observed during photocatalytic experiments. In contrast, compared with pure Cu 2 O, an increase of Cu(II)/Cu(I) ratio [67] was observed (see Figure 12c). Liu et al. had prepared a Cu/TiO 2 catalyst tailoring Cu valence and oxygen vacancies of the composite [93]. By thermal treatment under reducing atmosphere (H 2 and He) they exhibit the formation of defect sites, i.e., O V 's and Ti 3+ centers which affected the CO production. Moreover, during the reduction process through calcination, there was a change on the Cu oxidation states from Cu 2+ to Cu 1+ or Cu 0 . These Cu 1+ species can effectively trap electrons due to the more positive reduction potential of Cu + /Cu 0 (+0.52eV) couple vs. that of Cu 2+ /Cu 0 (+0.34 eV) [104]. The Cu + /Cu 0 couple can play a dual role where Cu 1+ species trap electrons, and Cu 0 species can effectively trap holes [93]. Xiong et al. had used TiO 2 crystals as a matrix to deposit Pt and Cu 2 O NPs where Cu 2 O promoted CH 4 production but suppressed H 2 evolution [92]. In that case, Pt favored the activation of H 2 O while Cu 2 O performed the CO 2 reduction. After the photocatalytic reaction, most of the Cu 2 O phase was reduced to Cu 0 , indicating that Pt promotes e --transfer to Cu 2 O during photocatalysis. Afterwards, the so-formed metallic Cu 0 enhanced the selective CH 4 production, a well-known property of metallic Cu [92]. Incorporating copper oxide in carbon-based materials such as rGO, g-C3N4, or carbon coating, is a widely used strategy in photocatalytic CO2 reduction. Graphene-based materials are gaining attention, due to their characteristics, high specific area and adsorption capacity, as well as excellent electron mobility and chemical stability [105]. Gusain et al. prepared rGO-CuO/Cu2O nanohybrids and used them for photocatalytic conversion of CO2 to CH3OH [100] (see Figure 12d). rGO interfaced with semiconducting materials can serve as an electron bridge and enhance the electron transfer process, a step crucial for the photocatalytic reactions [106]. In that work [106], it was suggested that the photogenerated electrons of CuO can be efficiently transferred to rGO, inhibiting in this way the electron hole recombination (see Figure 12e). Moreover, the high surface area and the defects of rGO can enhance CO2 adsorption [100]. Noticeably, CuO (Cu 2+ ) nanorods grafted on rGO exhibited better photocatalytic activity than a rGO/Cu2O (Cu 1+ ) nanocomposite. Stability experiments indicated that after six catalytic runs, there were no significant changes in the morphological and chemical characteristics of the nanocomposite [100] (see Figure 12f). In another work, Chang et al. fabricated Cu2O/gCN nanocomposites with different Cu2O morphologies [101]. The main gaseous product of CO2 reduction was CO with c-Cu2O/gCN being the best performing photocatalyst. The high activity of c-Cu2O/gCN was suggested to be linked with the improved CO2 adsorption as well as the formation of CuO [101].

Conclusions
Herein, we discuss cases of pertinent Cu-based materials, that under appropriate coupling and engineering (facet and strain) aimed to resolve stability problems (corrosion and photocorrosion). There are many factors to be taken into consideration, such as

The Case of Cu Coupling with Non-TiO 2 Semiconductors
As we have already stated, p-n junctions of Cu 2 O with appropriate n-type metal-oxides can exhibit better charge separation and enhanced photocatalytic activity for CO 2 reduction. ZnO as an n-type semiconductor possesses a large energy gap of 3.2-3.3 eV with high electron mobility and low dielectric constant, e.g., compared to TiO 2 . Bae et al. [95] used ZnO-Cu 2 O nanohybrids to reduce CO 2 to CH 4 in an CO 2 -saturated aqueous medium without use of hole scavenger [95]. Once again, the Z-scheme was the proposed mechanism and when compared to TiO 2 (P25)-Cu 2 O, the ZnO-Cu 2 O exhibited superior reaction activity and selectivity. Another work [96] reported the synthesis of a CuO-NaTaO 3 hybrid with the ability to reduce CO 2 to CH 3 OH with a maximum yield of 1302 µmol CH 3 OH g −1 h −1 . As expected, smaller, and more uniform distribution of CuO NPs exhibited better catalytic performance. Noteworthy, in that work [96], CuO was suggested to be the CO 2 reduction site, while NaTaO 3 was the hole-scavenging site, and oxidized isopropanol to acetone. XPS analysis indicated the presence of Cu 2+ species only, i.e., by CuO; however, a post catalytic analysis was missing.

Carbon-Based Materials and Core-Shell Cu-Oxide Structures
Incorporating copper oxide in carbon-based materials such as rGO, g-C 3 N 4 , or carbon coating, is a widely used strategy in photocatalytic CO 2 reduction. Graphene-based materials are gaining attention, due to their characteristics, high specific area and adsorption Solar 2023, 3 108 capacity, as well as excellent electron mobility and chemical stability [105]. Gusain et al. prepared rGO-CuO/Cu 2 O nanohybrids and used them for photocatalytic conversion of CO 2 to CH 3 OH [100] (see Figure 12d). rGO interfaced with semiconducting materials can serve as an electron bridge and enhance the electron transfer process, a step crucial for the photocatalytic reactions [106]. In that work [106], it was suggested that the photogenerated electrons of CuO can be efficiently transferred to rGO, inhibiting in this way the electron hole recombination (see Figure 12e). Moreover, the high surface area and the defects of rGO can enhance CO 2 adsorption [100]. Noticeably, CuO (Cu 2+ ) nanorods grafted on rGO exhibited better photocatalytic activity than a rGO/Cu 2 O (Cu 1+ ) nanocomposite. Stability experiments indicated that after six catalytic runs, there were no significant changes in the morphological and chemical characteristics of the nanocomposite [100] (see Figure 12f). In another work, Chang et al. fabricated Cu 2 O/gCN nanocomposites with different Cu 2 O morphologies [101]. The main gaseous product of CO 2 reduction was CO with c-Cu 2 O/gCN being the best performing photocatalyst. The high activity of c-Cu 2 O/gCN was suggested to be linked with the improved CO 2 adsorption as well as the formation of CuO [101].

Conclusions
Herein, we discuss cases of pertinent Cu-based materials, that under appropriate coupling and engineering (facet and strain) aimed to resolve stability problems (corrosion and photocorrosion). There are many factors to be taken into consideration, such as the catalytic environment, the wavelength and intensity of irradiation source, and primarily the Cu oxidation state, which is proven to be a crucial factor determining the catalytic performance. Fundamental steps towards the stabilization and durability of Cu-based materials include the selection of a proper semiconductor coupling, suitable sacrificial agents, surface protection through core-shell structures and stabilization on defect sites of the substrate matrix. Z-scheme mechanisms could lead to the stabilization of Cu 2 O through enhanced charge-separation of the photogenerated carriers. In addition, the proper selection of Cu oxidation states can tune the selectivity towards specific CO 2 reduction products. It is important to mention that the vast majority of Cu-based nanocatalysts discussed herein were noble-metal free. To conclude, the reviewed works indicate that the Cu-oxide nanophases constitute highly potent nanoplatforms for development of efficient artificial photosynthesis catalysts. There is converging evidence that-probably-pure Cuphases would be difficult to be used as standalone cathodic catalysts or electrodes; however, their heterojunctions with proper partner materials are an encouraging approach. Distinction between the role of numerous factors is required, protection from photocorrosion vs. CO 2 -reduction pathways, band-gap engineering, nano-facet engineering.