Cu-Based Z-Schemes Family Photocatalysts for Solar H₂ Production

Abstract

Solar photocatalytic H₂ production has drawn an increasing amount of attention from the scientific community, industry, and society due to its use of green solar energy and a photocatalyst (semiconductor material) to produce green H₂. Cu-based semiconductors are interesting as photocatalysts for H₂ production because Cu is earth-abundant, cheap, and the synthesis of its copper-containing semiconductors is straightforward. Moreover, Cu-based semiconductors absorb visible light and present an adequate redox potential to perform water splitting reaction. Nevertheless, pristine Cu-based semiconductors exhibit low photoactivity due to the rapid recombination of photo-induced electron-hole (e⁻-h⁺) pairs and are subject to photo corrosion. To remedy these pitfalls, the Cu semiconductor-based Z-scheme family (Z-schemes and S-schemes) presents great interest due to the charge carrier mechanism involved. Due to the interest of Z-scheme photocatalysts in this issue, the basic concepts of the Z-scheme focusing on Cu-based semiconductors are addressed to obtain novel systems with high H₂ photo-catalytic activity. Focusing on H₂ production using Cu-based Z-schemes photocatalyst, the most representative examples are included in the main text. To conclude, an outlook on the future challenges of this topic is addressed.

1. Introduction

Some of the great challenges nowadays in our society are energy production and environmental protection since they have a large negative impact on our society, affecting, among others, air quality, climate, and economy [1,2]. Such problems have been caused by rapid industrialization, uncontrolled environmental pollution, and the current energy scenario based on fossil fuels. This fact needs the development and increase in the use of renewable energies to ensure independence from ‘‘fossil fuels’’ to prevent further environmental degradation [3]. With this scenario, hydrogen (H₂) is considered an ideal solution for changing the current fossil fuel-based technology to a green technology due to the fact that H₂ possesses a high energy density without generating CO₂ emissions [4,5]. Moreover, H is the most abundant element on earth. However, hydrogen in nature is found in combination with other elements (oxygen in water and carbon in natural gas) [6]. Currently, H₂ is mainly produced by non-renewable sources such as steam methane reforming, coal gasification, and coal pyrolysis [7]. To implement the H₂ economy in our society, the development of new technologies is essential to allow the production of H₂ without using fossil fuels [4]. Consequently, the generation of H₂ by water splitting has attracted the attention of the scientific community and society because this reaction does not produce CO₂ [8,9]. However, the water-splitting reaction (Equation (1)) is a non-spontaneous endergonic reaction (ΔG = +237 KJmol⁻¹), which means that energy is needed for the reaction to occur [10]:

$${\mathrm{H}}_2\mathrm{O}\to \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2+{\mathrm{H}}_2(∆\mathrm{G}=237{\mathrm{K}\mathrm{J}\mathrm{m}\mathrm{o}\mathrm{l}}^{-1})$$

(1)

As we can notice from Equation (1), another key point in the water splitting reaction is the generation of oxygen as a sub-product of the reaction, making necessary the purification of the H₂ [11,12].

One alternative to producing H₂ from water splitting using renewable energy is photocatalysis, which requires the use of sunlight and a semiconductor [9,13]. The photo-catalytic mechanism, see Figure 1a, might be described briefly as follows: Initially, a photon with energy equal to or greater than the energy band gap (Eg) of the semiconductor is absorbed by the semiconductor. This phenomenon promotes a (photo-excited) electron from the valence band (VB) to the conduction band (CB), generating an electron–hole pair (e⁻-h⁺). The produced e⁻ and h⁺ can migrate to the surface of the semiconductor and initiate the redox reaction(s) with H₂O or H⁺ to produce O₂ and H_2; nevertheless, the e⁻-h⁺ pair can recombine and hinder the photo-catalytic activity [14,15]. Photo-catalysis technology has been receiving an increased amount of attention for its use in H₂ production after the pioneering work reported by Fujishima and Honda in 1972 based on photoelectrochemical water splitting on a TiO₂ electrode [16]. In this work, the authors showed that it is possible to perform water splitting reaction using TiO₂ as a semiconductor and UV light as an energy source. However, TiO₂ is not active under solar light due to its high band gap (3.1 eV). Additionally, the pristine TiO₂ presents a high e⁻-h⁺ recombination ratio, which is one of the main drawbacks of the use of TiO₂ as a photocatalyst [17,18].

With this in mind, the scientific community has been focused on developing photocatalysts that can use solar light to efficiently and more environmentally friendly produce H₂ instead of using semiconductors with a high band gap [13,19]. Moreover, semiconductors based on earth-abundant and inexpensive metals (Cu, Ni, Fe, etc.) have been developed due to their interesting potential as viable alternatives to rare and expensive noble metal photocatalysts [20,21]. One family of transition-metal semiconductors that is gaining the attention of the scientific community for application in photo-catalysis is the one based on copper “Cu-based semiconductors” such as Cu_xO and Cu_xS, among others [22,23]. Cu-based semiconductors are attractive for their use as photocatalysts because copper is abundant on earth, presents a low cost, and its synthesis is straightforward [24]. Regarding the application of Cu-based semiconductor materials in photo-catalytic H₂ production, they are promising materials due to their tunable band gaps, band alignment with respect to the water reduction potential (Figure 1b), and easy synthetic process, with respect to other semiconductors based on transition metals such as Ni, Fe, or Co [22,25,26,27,28,29]. In this sense, Cu-based oxide/chalcogenide semiconductors present a higher reduction potential than H⁺/H₂ redox potential, indicating that these semiconductors may be active for water splitting [30]. Moreover, Cu-based semiconductors are active under visible light because they have a low band gap between 1.5 eV and 2.2 eV [31]. However, pristine Cu-based semiconductor materials exhibit low photoactivity in H₂ production due to the rapid recombination of photo-induced e⁻-h+ pairs [32]. Also, another important drawback of this family of semiconductors is that the oxidation potential of some materials, such as Cu₂O, CuO, and Cu₃P, is not suitable to perform the overall water splitting reaction [21]. However, the most important drawback of Cu-based semiconductor materials for their possible commercialization is the low stability of these materials under reaction conditions due to photo-corrosion [33,34]. This is caused by the ability of copper to be both reduced (Cu^+I + e⁻ -> Cu⁰) and oxidized (Cu^+I + h⁺ -> Cu^+II) by photogenerated e⁻ and h⁺, respectively [22]. To improve the photo-catalytic efficiency of Cu-based semiconductor materials in H₂ production, several strategies have been developed by the scientific community [34], i.e., doping elements [35], deposition of metals [36], and construction of heterojunctions [37,38]. In this sense, the use of metal and co-catalysts to boost the catalytic activity of the semiconductor has been widely studied by the scientific community [39,40].

One alternative to obtain effective separation of photogenerated e⁻-h⁺ pairs, avoid the photocorrosoin of Cu-based semiconductors, and improve their catalytic activity is mimicking the natural photosynthetic systems (Z-Scheme charge transfer), where H₂O is cleaved into O₂ and the H₂ equivalent species by a double excitation mechanism [41]. In this sense, the two half-reactions are spatially separated in photosystem I (PSI) and photosystem II (PS II), starting a ‘Z-scheme’ charge transfer mechanism (see Figure 2a) [42]. Inspired by natural photosynthesis, in the last years, the scientific community is focused on developing materials like semiconductors to construct the Z-scheme heterojunctions family (Z-Scheme and S-scheme), see Figure 2b, consisting of an H₂ evolution photocatalyst and an O₂ evolution photocatalyst for efficient water splitting [43,44]. Indeed, the Z-scheme family systems present accessibility to photoexcited carriers with greater redox potential, separation of oxidative and reductive hubs, mediator-free charge transfer between the semiconductors, and enhanced light absorption ability [45].

Z-schemes family heterojunctions (Z-schemes and S-schemes) using several semiconductors such as TiO₂, BiVO₄, etc., are applied in relevant photo-catalytic reactions, i.e., environmental applications, energetic applications, transformation of organic compounds into high added-value products, and photo-catalytic biorefineries [47,48]. This review will describe in depth the Z-Schemes family based on Cu due to the relevance of these abundant and cheap metal-based photocatalysts. The mini-review will cover the most relevant works on Cu-based Z-scheme photocatalysts for their use in energy aspects, focusing on a relevant application such as H₂ production. This novel technology based on a low transition metal (Cu) has received the attention of the scientific community, increasing the number of publications about this topic during the last few years.

2. Z-Schemes Family Based on Cu

Semiconductor-based photocatalysis technology is still limited for commercialization by its low quantum yield, low utilization rate of visible light, and high recombination rate of photogenerated e⁻-h⁺ pairs [49,50], as mentioned in the Introduction section. In order to improve the photo-catalytic efficiency of semiconductors, several aspects should be taken into account, such as charge separation and absorption in visible light [13,46]. Cu-based semiconductors are promising photocatalysts, considering their low band gap between 1.5 eV and 2.2 eV and consequent absorption of visible light [31], but they present a high e⁻-h⁺ pairs recombination rate and photo corrosion [32,34] (see Introduction section).

One alternative to solve these drawbacks is to develop heterojunction systems [51,52], which are defined as the ‘junction′ of different semiconductors in intimate contact [44]. The heterojunction photocatalysts can be classified into different categories [53]. Considering the band alignment between the CBs and VBs of the semiconductors composing the heterojunction, this can be classified into three types [54] (see

Table 1. Summary of different types of heterojunctions.

Photo-Catalytic System	Advantages	Drawbacks	Representation of Mechanism
Type I	-	Fast recombination
Type II	Improve charge separation efficiency	Low oxidation and reduction potential
Type III	-	Not synergistic effect between semiconductors
Liquid-phase Z-scheme	Higher redox ability than traditional heterojunction	The reaction is performed in liquid phase. Difficult application
All-solid-state Z-scheme	Strong redox ability in solid state	High cost of noble metals
Direct Z-scheme	Strong redox ability without the use of mediators	The mechanism is controversial
S-scheme	Controllable built-in electric field intensity and stable interfacial carrier transport process, Strong redox ability, and clear mechanism.	Mainly n-type semiconductors

):

(I) Straddling-gap junctions (Type I): In this junction, the semiconductor I (SI) has a higher CB and a lower VB than the semiconductor II (SII). So, the e⁻ and h⁺ move to the CB and VB from SI to SII. Then, the charge carriers are not properly separated due to their aggregation on the same semiconductor, favoring the recombination.

(II) Staggered-gap junctions (Type II): In this junction, the semiconductor I (SI) has a higher CB and VB than the semiconductor II (SII). Then, e⁻ and h⁺ will move to their corresponding low-energy bands. As a result, the photo-catalytic efficiency of the systems will be improved due to the charge carriers’ separation; nevertheless, the redox capacities will be reduced since the e⁻ and h⁺ are in the bands with low energy potentials.

(III) Broken-gap junctions (Type III): The band gaps do not overlap at all. e⁻ and h⁺ stay in their original semiconductors without charge migration, so the two semiconductors work separately without any synergistic effect.

As was mentioned in the classification, Type II junctions present a great interest in photo-catalytic H₂ production due to the charge carriers’ separation, one of the main problems of using Cu-based semiconductors [55]. However, these materials might be useless for this application due to the diminished redox capacity of these materials [56,57]. To solve this challenge, the scientific community has been focusing on exploring novel junction types, namely the Z-scheme system. These heterojunctions aim to mimic the photosynthetic process [41,56]. Both Z-scheme and Type II heterojunctions present the same staggered band alignment, but the charge transfer is different. In Z-schemes, e⁻ of SI are recombined with h⁺ of SII, allowing a stronger redox ability of the Z-scheme heterojunctions compared to the Type II heterojunctions. So, the development of emerging Z-scheme heterojunctions based on Cu is very meaningful and important for achieving high-efficiency photo-catalytic reactions [45,58]. Proposed for the first time by Bard in 1979 [59], this system needs a shuttle redox mediator (electron acceptor/donor pair) to form a liquid-phase Z-scheme (traditional Z-scheme), but its main limitation is the redox mediator reversibility because they can only be applied in the liquid phase [44,60]. In 2006, Tada et al. proposed all-solid-state Z-scheme photocatalysts (second-generation Z-scheme) [61]. These systems are composed of two different semiconductors with a solid-phase electron mediator as noble metal nanoparticles or carbon material (graphene and carbon nanotubes) [62]. The noble metals used as a mediator present a limitation for the efficiency and applicability of the process because they are rare and expensive [56]. In addition, mediators used in the all-solid-state Z-scheme can produce the Schottky junction, restricting the continuous flow of electrons by the Schottky circuit [63]. In 2013, Yu et al. [64] constructed a direct Z-scheme photocatalyst (third-generation Z-Scheme) combining g-C₃N₄ and TiO₂ to solve the inconvenience of using an electron mediator since the interfacial contact between the semiconductors facilitates the direct electron transfer [43]. The main limitation of direct Z-schemes is that the overall concentration of photocarriers in direct Z-scheme photo-catalytic materials is reduced by the charge carriers’ recombination at the interface, resulting in a lower quantum yield of the photocatalyst [44].

To address the above problems of type-II heterojunction, traditional Z-scheme, and all-solid-state Z-scheme systems, with the difficulty of demonstrating the direct Z-scheme mechanism charge transfer, a new step-scheme (S-scheme) heterojunction concept was proposed based on direct Z-scheme heterojunction [44,65]. Then, the development of S-scheme heterojunctions is a historical necessity to describe adequately the charge transfer in direct Z-scheme systems [66]. In the S-scheme [44], a reductant photocatalyst (RP) and oxidant photocatalyst (OP) come into contact; hence, e⁻ in RP with a higher Fermi level will drift to the OP with a lower Fermi level at the interface. In other words, the RP side loses e⁻ and is positively charged. Contrarily, the OP side accepts e⁻ and becomes negatively charged. Additionally, upward or downward band bending occurs at the interface of RP and OP, respectively. Concurrently, a built-in electric field whose direction points from RP to OP is formed. Under light irradiation, said electric field drives the photogenerated e^-, transferring them from CB of OP to VB of RP. Moreover, the Coulombic attraction between e⁻ in OP and h⁺ in RP and band bending also favors this charge transfer. On the contrary, Coulombic repulsion, band bending, and a built-in electric field also inhibit the electron transfer from CB of RP to CB of OP and the hole transfer from VB of OP to VB of RP. Ultimately, the powerful photogenerated e⁻ and h⁺ are reserved in the CB of RP and VB of OP, respectively. Therefore, strong redox ability is earned in the S-scheme photocatalyst. The S-scheme heterojunction system collectively achieves charge separation and exhibits strong photo-redox ability, which results in improved photo-catalytic performance [44].

With this in mind, in this review, we will focus on the scientific works based on all-solid-state Z-scheme, direct Z-scheme, and S-scheme due to the advantages described in this section and their promising future applications. Moreover, this review will be focused on the use of Cu-based semiconductors as a part of Z-scheme photocatalysts. This fact is due to the interesting properties of Cu-based semiconductors in terms of visible light adsorption and suitable redox potentials previously described in the Introduction section to obtain photocatalysts with high activity in photo-catalytic H₂ production.

3. H₂ Production by Z-Schemes Based on Cu

As mentioned in the previous sections, Cu-based direct Z-schemes have been widely studied by scientists for their application in photo-catalytic H₂ production due to the great properties of these systems, such as adequate charge transfer and high redox ability [24]. In this section, the most representative Cu-based Z-schemes (all-solid-state Z-scheme, direct-Z-scheme, and S-scheme) and their use in H₂ production will be presented. Additionally, in the main text, the most relevant aspects of the developed direct Z-schemes based on copper are also described.

3.1. Cu Oxides-Based Z-Schemes (Family)

Cu-based metal oxides, such as Cu₂O and CuO, are among the most studied metal oxides owing to their non-toxicity, availability in nature, and suitable band gaps. Due to their unique optoelectronic properties, they have been extensively used in energy conversion, storage, and sensing devices [30,67].

Cu₂O is a p-semiconductor with a band gap in the visible range (2.2 eV). This characteristic makes it very interesting for photo-catalytic applications, but like many narrow band gap semiconductors, it suffers from poor photostability [33]. For this reason, many heterostructures have been successfully studied in the literature, and some of the Z-schemes family heterojunctions are resumed in Table 2.

Among others, TiO₂ has been extensively applied as a component in heterojunctions for photo-catalytic applications in order to overcome its biggest drawback, i.e., its wide band gap (>3 eV), which brings to inactivity in the visible range [68]. Wei et al. demonstrated that even different facets of TiO₂ are relevant in the stabilization of Cu₂O. Indeed, a Cu₂O/TiO₂ Z-scheme heterostructure with exposed 101-faceted TiO₂ facets and oxygen vacancies exhibited extremely higher activity for solar water splitting and unpredicted photostability compared to a 001-faceted TiO₂ Type II Cu₂O/TiO₂ junction, as shown in Figure 3a. A DFT-based density of states (DOS) was used to explain the impact of defects on the electronic structures, namely the appearance of gap states coming from Ti-located dangling bonds [69]. The improved photocatalytic activity obtained using Cu₂O/TiO₂ was confirmed by Lv et al. In this case, not only could the mesoporous TiO₂ increase the photostability of Cu₂O in a Z-scheme, but also provide more active sites due to its high surface area [70]. Furthermore, in the work of Fu et al., the presence of Ag as an electron mediator enhanced the visible light absorption and the photo-catalytic activity by forming a dual Z-scheme TiO₂/Ag/Cu₂O [71]. Interestingly, the Cu₂O/TiO₂ showed promising activity even in the form of film stripes. Zhu et al. distributed Cu₂O and TiO₂ in parallel on fluorine-doped tin oxide (FTO) to obtain a film photocatalyst as opposed to the more conventional particulate materials. The result was an active photocatalyst for overall water splitting, and it gave insights into the design of Z-scheme photocatalysts [72].

As TiO₂, ZnO can be listed as one of the most used photocatalysts, but it presents the same disadvantage, i.e., a wide band gap of 3.37 eV [73]. Heterostructures composed of Cu₂O and ZnO will be endowed with the positive sides of both components, such as absorption in the visible range and low photo corrosion, respectively. Indeed, the S-scheme Cu₂O/ZnO showed less recombination than the pure components, which was reflected in a higher catalytic activity of the composite [74]. Moreover, in the Z-scheme ZnO/Cu₂O-CuO, the CuO could improve the photocatalytic performance of the composite made only by ZnO and Cu₂O (see Figure 3b) [75].

Carbon-based materials are ideal components in heterojunctions, considering that carbon is one of the most abundant elements on Earth. Thus, g-C₃N₄ has been widely used as a photocatalyst due to its absorption in the visible range and potential activity in H₂ evolution and pollutant degradation [46]. Unfortunately, it suffers from a high recombination rate and synthetic difficulties, which result in too low activity. To solve these issues, different heterojunctions have been studied, and Cu₂O has also been used as part of them to overcome the drawbacks of both materials. Xu et al. synthesized a high surface area Cu₂O/g-C₃N₄ Z-scheme, where the g-C₃N₄ benefitted the presence of Cu²⁺ ions for an easier synthesis, to obtain a highly active photocatalyst for H₂ evolution (see Figure 3c) [76]. While in the previous work, Cu₂O nanoparticles resulted in a Cu₂O/g-C₃N₄ Z-Scheme, Dai et al. could prepare an S-scheme using Cu₂O octahedrons. The obtained Cu₂O/g-C₃N₄ S-scheme demonstrated high activity in H₂ production but also in the reduction of Cr⁶⁺ and oxidation of tetracycline. Charge transfer was illustrated with charge density profiles obtained computationally [77]. Similar Cu₂O octahedrons were used in the all-solid Z-scheme Cu(OH)₂/Cu₂O/g-C₃N₄, where the presence of Cu as an electron mediator and of Cu(OH)₂ as a semiconductor could increase the activity towards H₂ compared to the pure compounds composing the heterojunction [78]. The performance of heterojunctions in photo-catalytic H₂ production can also be improved by adding new sites in the form of doping or decoration. In these cases, it is worth mentioning the works of Gu et al. and Mu et al. The former could prove an increased light stability of Cu₂O by doping it with sulfur in the Z-scheme Cu₂O/g-C₃N₄ [79]. Additionally, the plasmonic effect obtained by decorating with Au nanoparticles could increase the photo-catalytic H₂ evolution and the interfacial charge separation in the S-scheme Au/g-C₃N₄/Cu₂O [80].

Different studies have been made on Cu₂O composites, and the Z-schemes RGO-Cu₂O/Fe₂O₃ and RGO-Cu₂O/Bi₂WO₆, where RGO acts as a mediator, represent good examples of the potential of Cu₂O in photo-catalysis [81,82]. Both heterojunctions were proven to have improved activity and could benefit from the advantages of each component, even without the use of sacrificial agents.

Among copper oxides, CuO definitely has a pivotal role in catalysis and photo-catalysis. It is a p-semiconductor whose band gap ranges from 1.2 eV to 2.0 eV, making it active in visible light [83]. As mentioned above, narrow band gap materials frequently possess a common drawback, i.e., a high recombination rate. On the other hand, these kinds of materials are absorbed in the visible range, arousing the interest of their application in visible light-mediated processes. Hence, CuO has also been used in several Z-scheme family heterojunctions. Even though CuO nanostructures do not always have the correct potential energy for hydrogen evolution, CuO-based heterojunctions are demonstrated to profit from the presence of this material [84]. Chu et al. described a successful Z-scheme formed by CuO and polyimide (PI) for H₂ production. In this case, the presence of CuO decreased the well-known inadequate space charge separation of polymers [85]. In the S-schemes g-C₃N₄/TiO₂/CuO and CuO/CdS/CoWO₄, the improvement in the catalytic activity was demonstrated to be related to the close interface contact in the double heterojunction. The second could avoid all the disadvantages related to the single components, i.e., recombination rate (g-C₃N₄, CdS, CuO), photo corrosion (CdS), or wide band gap (TiO₂) [86,87]. Ahmad et al. proved the importance of the surface plasmon resonance (SPR) effect caused by Au in the S-scheme ZnO/CuO/Au. Indeed, the SPR was considered one of the reasons for the increased photocatalytic activity in H₂ production [88]. The combination of both Cu oxides and ZnO brought to the extremely active Z-scheme ZnO/CuO/Cu/Cu₂O/TiO₂, where the use of Cu could improve the light absorption of TiO₂ [89]. Finally, Kannan et al. showed how the presence of the multilayered 2D structure, Ti₃AlC_2, improved the activity of the CuO/NiO composite. The extremely active S-scheme Ti₃AlC₂/CuO/NiO was prepared, and it was demonstrated to possess a very long charge carrier lifetime (3 ns), as shown in Figure 3d [90].

Figure 3. (a) Schematic diagrams of interfacial charge transfer in defective Cu₂O/TiO₂ Type II (left) and Cu₂O/TiO₂ Z-scheme (right) heterojunctions. Reproduced from [69]. Copyright 2019, ACS Publications. (b) Schematic illustration of the Z-scheme-assisted ZnO/Cu₂O-CuO heterojunction. Reproduced from [75]. Copyright 2020, Elsevier. (c) Synthetic process (top) and H₂ production (bottom) of Cu₂O/g-C₃N₄. Reproduced from [76]. Copyright 2022, ACS Publications. (d) Time-resolved photoluminescence spectrum (top) and plausible photocatalytic mechanism of Ti₃AlC₂/CuO/NiO (bottom). Reproduced from [90]. Copyright 2023, Elsevier.

Figure 3. (a) Schematic diagrams of interfacial charge transfer in defective Cu₂O/TiO₂ Type II (left) and Cu₂O/TiO₂ Z-scheme (right) heterojunctions. Reproduced from [69]. Copyright 2019, ACS Publications. (b) Schematic illustration of the Z-scheme-assisted ZnO/Cu₂O-CuO heterojunction. Reproduced from [75]. Copyright 2020, Elsevier. (c) Synthetic process (top) and H₂ production (bottom) of Cu₂O/g-C₃N₄. Reproduced from [76]. Copyright 2022, ACS Publications. (d) Time-resolved photoluminescence spectrum (top) and plausible photocatalytic mechanism of Ti₃AlC₂/CuO/NiO (bottom). Reproduced from [90]. Copyright 2023, Elsevier.

Up to now, only binary compounds were considered, but the synergistic effect in ternary and quaternary transition metals-based photocatalysts is recently becoming of central relevance due to their more tunable band gaps and decreased photocorrosion compared to the monometallic counterparts. CuFeO₂ or delafossite is a promising mixed oxide with a narrow band gap (1.30 eV), and it was demonstrated to be active in H₂ production in the Z-scheme with Bi₂₀TiO₃₂ [91]. On the other hand, CuBi₂O₄ is a suitable photocatalyst for visible light-mediated processes due to its band gap of 1.5–1.8 eV, but it shows fast recombination. Hence, Mao et al. demonstrated that the construction of the S-scheme CuBi₂O₄/Na-TiO₂ could decrease the recombination in CuBi₂O₄ and increase the visible light absorption of TiO₂ [92].

Finally, polyoxometalates can be mentioned in the family of mixed oxides, considering that they are formed by a metal anion composed of Mo or W and O and a metal cation, in our case, copper. Indeed, these compounds are extremely interesting for energy applications due to their semiconducting properties and the possibility of easily tuning their structure [93]. CuWO₄ has been demonstrated to be applicable in catalysis, but also in photocatalysis to form Z-schemes with TiO₂ and allow high H₂ production in visible light-mediated processes [94,95]. Similarly, copper molybdates have shown great applicability in photocatalysis, and Li et al. demonstrated the decreased recombination of γ-graphyne (γ-GY) in the S-scheme CuMoO₄/γ-GY. In this work, the electronic structure of the heterojunction has been calculated through ab initio calculations; DOS was used to confirm the direction of the charge transfer and give some precision on the location of the e⁻/h⁺ migration [96].

Table 2. Cu oxides-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity and AQE	Reference
Cu2O/TiO2	Photodeposition	Xe lamp (300 W)	32.6 mmol g−1 h−1 AQE: 53.5% (350 nm) SA	[69]
Cu2O/TiO2	Adsorption-reduction	Xe lamp	11 mmol g−1 h−1 AQE: 15.1% (365 nm) SA	[70]
TiO2/Ag/Cu2O	Impregnation-calcination/ Photodeposition	Xe lamp (300 W)	874.7 μmol g−1 h−1 AQE: 2.3% (365 nm) SA	[71]
TiO2/FTO/Cu2O	Impregnation-calcination/ Electrodeposition	Xe lamp (300 W)	200 μmol m−2 AQE: NA SA	[72]
Cu2O/ZnO	Impregnation	Xe lamp (150 W)	208.9 μmol g−1 h−1 AQE: 8.8% (500 nm) SA	[74]
ZnO/Cu2O-CuO	Thermal oxidation	Xe lamp (150 W)	1.1 mmol g−1 h−1 AQE: 3.0% SA	[75]
Cu2O/g-C3N4	Recrystallization–calcination	Xe lamp (300 W, λ ≥ 420 nm)	5.8 mmol g−1 h−1 AQE: 13.4% SA	[76]
Cu2O/g-C3N4	Impregnation	Xe lamp (500 W, λ ≥ 400 nm)	480.6 μmol g−1 h−1 AQE: NA SA	[77]
Cu(OH)2/Cu2O/g-C3N4	Sonoprecipitation/ Impregnation	Metal halide lamp (150 W, UV cut-off)	622.0 μmol g−1 h−1 AQE: NA SA	[78]
S-Cu2O/g-C3N4	Sonication	Xe lamp (300 W, λ ≥ 420 nm)	620.7 μmol g−1 h−1 AQE: NA SA	[79]
Au/g-C3N4/Cu2O	Impregnation	Xe lamp (500 W, λ ≥ 400 nm)	552.6 μmol g−1 h−1 AQE: NA SA	[80]
RGO-Cu2O/Fe2O3	Hydrothermal	Xe lamp (300 W, λ ≥ 420 nm)	4.86 μmol g−1 h−1 AQE: NA	[81]
RGO-Cu2O/Bi2WO6	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	1.80 μmol g−1 h−1 AQE: NA	[82]
CuO/PI	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	104.6 μmol g−1 h−1 AQE: 5.8% (450 nm) SA	[83]
g-C3N4/TiO2/CuO	Impregnation	Xe lamp (300 W)	97.5 μmol g−1 h−1 AQE: NA SA	[84]
CuO/CdS/CoWO4	Microwave	Xe lamp (300 W, λ ≥ 420 nm)	457.9 μmol g−1 h−1 AQE: NA SA	[87]
ZnO/CuO/Au	Sol–gel	Xe lamp (300 W, λ ≥ 400 nm)	4.7 mmol g−1 h−1 AQE: NA SA	[88]
Ti3AlC2/CuO/NiO	Sonication–Calcination	Solar simulator	20.7 mmol g−1 h−1 AQE: 14.2% (365 nm) SA	[90]
IrO2/Bi20TiO32/CuFeO2/rGO	Impregnation	Xe lamp (500 W)	1.1 mmol g−1 h−1 AQE: 4.8%	[91]
CuBi2O4/Na-TiO2	Hydrothermal	Xe lamp (300 W)	2.7 mmol g−1 h−1 AQE: NA SA	[92]
CuWO4/TiO2	Impregnation	Solar simulator	106.7 mmol g−1 h−1 AQE: NA SA	[94]
CuWO4/TiO2	Hydrothermal	Hg lamp (500 W)	9.85 mmol g−1 h−1 AQE: NA SA	[95]
CuMO4/γ-GY	Hot solvent	NA	4 mmol g−1 h−1 AQE: NA SA	[96]

Table 2. Cu oxides-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity and AQE	Reference
Cu2O/TiO2	Photodeposition	Xe lamp (300 W)	32.6 mmol g−1 h−1 AQE: 53.5% (350 nm) SA	[69]
Cu2O/TiO2	Adsorption-reduction	Xe lamp	11 mmol g−1 h−1 AQE: 15.1% (365 nm) SA	[70]
TiO2/Ag/Cu2O	Impregnation-calcination/ Photodeposition	Xe lamp (300 W)	874.7 μmol g−1 h−1 AQE: 2.3% (365 nm) SA	[71]
TiO2/FTO/Cu2O	Impregnation-calcination/ Electrodeposition	Xe lamp (300 W)	200 μmol m−2 AQE: NA SA	[72]
Cu2O/ZnO	Impregnation	Xe lamp (150 W)	208.9 μmol g−1 h−1 AQE: 8.8% (500 nm) SA	[74]
ZnO/Cu2O-CuO	Thermal oxidation	Xe lamp (150 W)	1.1 mmol g−1 h−1 AQE: 3.0% SA	[75]
Cu2O/g-C3N4	Recrystallization–calcination	Xe lamp (300 W, λ ≥ 420 nm)	5.8 mmol g−1 h−1 AQE: 13.4% SA	[76]
Cu2O/g-C3N4	Impregnation	Xe lamp (500 W, λ ≥ 400 nm)	480.6 μmol g−1 h−1 AQE: NA SA	[77]
Cu(OH)2/Cu2O/g-C3N4	Sonoprecipitation/ Impregnation	Metal halide lamp (150 W, UV cut-off)	622.0 μmol g−1 h−1 AQE: NA SA	[78]
S-Cu2O/g-C3N4	Sonication	Xe lamp (300 W, λ ≥ 420 nm)	620.7 μmol g−1 h−1 AQE: NA SA	[79]
Au/g-C3N4/Cu2O	Impregnation	Xe lamp (500 W, λ ≥ 400 nm)	552.6 μmol g−1 h−1 AQE: NA SA	[80]
RGO-Cu2O/Fe2O3	Hydrothermal	Xe lamp (300 W, λ ≥ 420 nm)	4.86 μmol g−1 h−1 AQE: NA	[81]
RGO-Cu2O/Bi2WO6	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	1.80 μmol g−1 h−1 AQE: NA	[82]
CuO/PI	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	104.6 μmol g−1 h−1 AQE: 5.8% (450 nm) SA	[83]
g-C3N4/TiO2/CuO	Impregnation	Xe lamp (300 W)	97.5 μmol g−1 h−1 AQE: NA SA	[84]
CuO/CdS/CoWO4	Microwave	Xe lamp (300 W, λ ≥ 420 nm)	457.9 μmol g−1 h−1 AQE: NA SA	[87]
ZnO/CuO/Au	Sol–gel	Xe lamp (300 W, λ ≥ 400 nm)	4.7 mmol g−1 h−1 AQE: NA SA	[88]
Ti3AlC2/CuO/NiO	Sonication–Calcination	Solar simulator	20.7 mmol g−1 h−1 AQE: 14.2% (365 nm) SA	[90]
IrO2/Bi20TiO32/CuFeO2/rGO	Impregnation	Xe lamp (500 W)	1.1 mmol g−1 h−1 AQE: 4.8%	[91]
CuBi2O4/Na-TiO2	Hydrothermal	Xe lamp (300 W)	2.7 mmol g−1 h−1 AQE: NA SA	[92]
CuWO4/TiO2	Impregnation	Solar simulator	106.7 mmol g−1 h−1 AQE: NA SA	[94]
CuWO4/TiO2	Hydrothermal	Hg lamp (500 W)	9.85 mmol g−1 h−1 AQE: NA SA	[95]
CuMO4/γ-GY	Hot solvent	NA	4 mmol g−1 h−1 AQE: NA SA	[96]

3.2. Cu Sulfides-Based Z-Schemes (Family)

Cu-based sulfides are promising semiconducting materials for several applications, including photovoltaics, photo-electrocatalysis, energy storage, energy conversion, sensing, CO₂ reduction, and organic degradation [97,98]. Some Z-scheme family heterojunctions composed of Cu sulfides are resumed in Table 3.

Among all the copper sulfides, Cu₂S has been studied as a component of Z-scheme family heterojunctions for its positive properties, such as a narrow band gap (2.2 eV) [25]. Ranjith et al. discovered that Cu₂S could extend the visible light absorbance range, promote effective charge carrier separation in the Z-scheme ZnO/ZnS/Cu₂S, and improve H₂ production [99]. ZnCdS compounds have also been used as components in Cu₂S-based heterojunctions due to their tunable band gap and favorable photo-catalytic activity. Indeed, the S-scheme Cu₂S/ZnCdS and the Z-scheme Cu₂S/Zn_0.67C_0.33dS (see Figure 4a) reached quite excellent H₂ production, confirming these materials as extremely promising for energy applications [100,101].

Additionally, CuS has attracted a lot of interest and has been extensively used in photo-catalysis for its narrow band gap (from 1.88 eV to 2.16 eV), which is easily modifiable by changing the morphology of the material [102]. For this reason, it has been used in several heterojunctions to make the most of its properties and decrease its recombination rate. Kumar et al. demonstrated that in the Z-scheme CuS/Ag₂O/g-C₃N_4, CuS could provide not only an effective heterojunction interface but also more contacting sites to facilitate the formation of h⁺ [103]. In the case of the Z-scheme CuS/CdS, both components presented a decreased recombination rate and intimately contacted interfaces (see Figure 4b) to improve the photo-catalytic H₂ production [104]. CuS was also successfully introduced by Liu et al. in an S-scheme with NiCo₂O₄ to improve the charge carriers’ separation and transfer [105].

Cu₇S₄ is also finding its place in Z-schemes construction due to its narrow band gap [106]. Yuan et al. found two main advantages in the use of Cu₇S₄, i.e., the relevance of the Cu vacancies for the formation of the Z-scheme Cu₇S₄/MnS and the SPR effect to improve light harvesting [107]. Additionally, Cu₇S₄ could decrease the photo corrosion of CdS in the Z-scheme Cu₇S₄/CdS, thus improving the photocatalytic activity by enhancing the charge carrier's separation (see Figure 4c) [108].

In the following part, we analyze some of the ternary and quaternary sulfides used in Z-scheme family photocatalysts, which present improved characteristics compared to the monometallic compounds, as commented above. Among them, CuInS₂ represents the most studied and applied due to its light absorption in the visible range and suitable conduction band for H₂ evolution [109]. CuInS₂ has been used to improve the photocatalytic activity of g-C₃N₄ and take advantage of its more abundant active sites to form Z-schemes [110]. The use of noble metal nanoparticles has been demonstrated to improve the photocatalytic activity of the composite CuInS₂/g-C₃N₄ in the Z-scheme Au/CuInS₂/NCN-CN_x, as shown in Figure 4d. DFT calculations have been performed and were shallowly used as support for experimental results [111]. Zhong et al. reported the synthesis of the Z-scheme Pt-CuInS₂/CdS with high photocatalytic activity, which was attributed to the photoinduced electron transfer from the defect-mediated trap states in CuInS₂ [112]. Yang et al. used CuInS₂ to successfully improve the absorption range of Ti₃C₂/TiO₂ to obtain an S-scheme with improved photocatalytic activity [113]. Finally, combining CuInS₂ with another bimetallic sulfide (Cd_0.5Zn_0.5S), an efficient S-scheme could be prepared with a large interfacial contact area, enhanced light absorption, and directional charge transfer and separation [114].

As in the case of ternary compounds, Cu-based quaternary sulfides have been extensively applied in Z-scheme photocatalysts. (CuGa)_1−xZn_2xS₂ is one of the best examples, especially for overall water splitting in combination with semiconductors possessing the right potential to produce O₂ [115,116]. Additionally, Cu₂NiSnS₄ and Cu₂ZnSnS₄ have demonstrated the versatility of quaternary compounds in the preparation of Z-schemes. Indeed, they were shown to be active in H₂ production due to their higher stability and broader absorption spectra [117,118].

Figure 4. (a) Photocatalytic H₂ production of the Cu₂S/Zn_0.67C_0.33dS photocatalysts. Reproduced from [101]. Copyright 2019, RSC Publications. (b) Schematic illustration of CdS/CuS core-shell nanorods with different molar ratios of Cu element. Reproduced from [104]. Copyright 2023, Elsevier. (c) Photocatalytic H₂ production of CdS/Cu₇S₄ heterojunctions with different Cd content under visible-light irradiation. Reproduced from [108]. Copyright 2022, Wiley. (d) Comparison of H₂ production rates of g-C₃N₄, NCN-CNx, CuInS₂/NCN-CNx, and AuNPs/CuInS₂/NCNCNx. Reproduced from [111]. Copyright 2021, ACS Publications.

Figure 4. (a) Photocatalytic H₂ production of the Cu₂S/Zn_0.67C_0.33dS photocatalysts. Reproduced from [101]. Copyright 2019, RSC Publications. (b) Schematic illustration of CdS/CuS core-shell nanorods with different molar ratios of Cu element. Reproduced from [104]. Copyright 2023, Elsevier. (c) Photocatalytic H₂ production of CdS/Cu₇S₄ heterojunctions with different Cd content under visible-light irradiation. Reproduced from [108]. Copyright 2022, Wiley. (d) Comparison of H₂ production rates of g-C₃N₄, NCN-CNx, CuInS₂/NCN-CNx, and AuNPs/CuInS₂/NCNCNx. Reproduced from [111]. Copyright 2021, ACS Publications.

Table 3. Cu sulfides-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity and AQE	Reference
ZnO/ZnS/Cu2S	Sputtering–Sulfidation	Xe lamp (150 W, λ ≤ 400 nm)	436 μmol g−1 h−1 AQE: 0.86% (420 nm) SA	[99]
Cu2S/ZnCdS	Hydrothermal	5 W LED	5.9 mmol g−1 h−1 AQE: 2.13% (400 nm) SA	[100]
Cu2S/Zn0.67C0.33dS	Hydrothermal	Xe lamp (300 W, λ ≥ 420 nm)	15.3 mmol g−1 h−1 AQE: 18.15% (420 nm) SA	[101]
CuS/Ag2O/g-C3N4	Hydrothermal–Precipitation	Solar simulator	1.8 mmol g−1 h−1 AQE: NA SA	[103]
CuS/CdS	Cation exchange	Xe lamp (300 W, λ ≥ 420 nm)	13.4 mmol g−1 h−1 AQE: NA SA	[104]
NiCo2O4/CuS	Electrostatic self-assembly	Solar simulator	6.0 mmol g−1 h−1 AQE: NA SA	[105]
Cu7S4/MnS	Cation exchange	Xe lamp (300 W)	718 μmol g−1 h−1 AQE: 18.8% (420 nm) SA	[107]
Cu7S4/CdS	Cation exchange	Xe lamp (300 W, λ ≥ 420 nm)	21.6 mmol g−1 h−1 AQE: 14.4% SA	[108]
g-C3N4/CuInS2	Hydrothermal	Xe lamp (300 W, λ ≥ 420 nm)	1.3 mmol g−1 h−1 AQE: 5.6% (400 nm) SA	[110]
Au/CuInS2/g-C3N4	Impregnation/Photodeposition	Xe lamp (300 W)	10.7 mmol g−1 h−1 AQE: NA SA	[111]
Pt-CuInS2/CdS	Impregnation	Xe lamp (300 W, λ ≥ 420 nm)	20.5 μmol g−1 h−1 AQE: 0.3% (380 nm) SA	[112]
Ti3C2/TiO2/CuInS2	Hydrothermal	Xe lamp (300 W)	356.3 μmol g−1 h−1 AQE: 1.9% (350 nm) SA	[113]
Cd0.5Zn0.5S/CuInS2	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	7.7 mmol g−1 h−1 AQE: 1.25% (420 nm) SA	[114]
(CuGa)0.5ZnS2/RGO-(CoOx/BiVO4)	Impregnation	Xe lamp (300 W, λ ≥ 420 nm)	128 μmol g−1 h−1 AQE: 0.8% (440 nm) SA	[115]
(CuGa)0.5ZnS2/(CoOx/BiVO4)	Impregnation	Xe lamp (300 W, λ ≥ 420 nm)	44.7 μmol g−1 h−1 AQE: NA SA	[116]
Cu2NiSnS4/TiO2	Hydrothermal	Solar simulator	7.1 mmol g−1 h−1 AQE: NA SA	[117]
Cu2ZnSnS4/Cu2O	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	897 μmol g−1 h−1 AQE: NA SA	[118]

Table 3. Cu sulfides-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity and AQE	Reference
ZnO/ZnS/Cu2S	Sputtering–Sulfidation	Xe lamp (150 W, λ ≤ 400 nm)	436 μmol g−1 h−1 AQE: 0.86% (420 nm) SA	[99]
Cu2S/ZnCdS	Hydrothermal	5 W LED	5.9 mmol g−1 h−1 AQE: 2.13% (400 nm) SA	[100]
Cu2S/Zn0.67C0.33dS	Hydrothermal	Xe lamp (300 W, λ ≥ 420 nm)	15.3 mmol g−1 h−1 AQE: 18.15% (420 nm) SA	[101]
CuS/Ag2O/g-C3N4	Hydrothermal–Precipitation	Solar simulator	1.8 mmol g−1 h−1 AQE: NA SA	[103]
CuS/CdS	Cation exchange	Xe lamp (300 W, λ ≥ 420 nm)	13.4 mmol g−1 h−1 AQE: NA SA	[104]
NiCo2O4/CuS	Electrostatic self-assembly	Solar simulator	6.0 mmol g−1 h−1 AQE: NA SA	[105]
Cu7S4/MnS	Cation exchange	Xe lamp (300 W)	718 μmol g−1 h−1 AQE: 18.8% (420 nm) SA	[107]
Cu7S4/CdS	Cation exchange	Xe lamp (300 W, λ ≥ 420 nm)	21.6 mmol g−1 h−1 AQE: 14.4% SA	[108]
g-C3N4/CuInS2	Hydrothermal	Xe lamp (300 W, λ ≥ 420 nm)	1.3 mmol g−1 h−1 AQE: 5.6% (400 nm) SA	[110]
Au/CuInS2/g-C3N4	Impregnation/Photodeposition	Xe lamp (300 W)	10.7 mmol g−1 h−1 AQE: NA SA	[111]
Pt-CuInS2/CdS	Impregnation	Xe lamp (300 W, λ ≥ 420 nm)	20.5 μmol g−1 h−1 AQE: 0.3% (380 nm) SA	[112]
Ti3C2/TiO2/CuInS2	Hydrothermal	Xe lamp (300 W)	356.3 μmol g−1 h−1 AQE: 1.9% (350 nm) SA	[113]
Cd0.5Zn0.5S/CuInS2	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	7.7 mmol g−1 h−1 AQE: 1.25% (420 nm) SA	[114]
(CuGa)0.5ZnS2/RGO-(CoOx/BiVO4)	Impregnation	Xe lamp (300 W, λ ≥ 420 nm)	128 μmol g−1 h−1 AQE: 0.8% (440 nm) SA	[115]
(CuGa)0.5ZnS2/(CoOx/BiVO4)	Impregnation	Xe lamp (300 W, λ ≥ 420 nm)	44.7 μmol g−1 h−1 AQE: NA SA	[116]
Cu2NiSnS4/TiO2	Hydrothermal	Solar simulator	7.1 mmol g−1 h−1 AQE: NA SA	[117]
Cu2ZnSnS4/Cu2O	Solvothermal	Xe lamp (300 W, λ ≥ 420 nm)	897 μmol g−1 h−1 AQE: NA SA	[118]

3.3. Cu Phosphide-Based Z-Schemes (Family)

In the last five years, copper phosphide (Cu₃P) has attracted attention as a semiconducting (p-type) and plasmonic material for its application in optical, electronic, and photocatalysis applications [119,120]. In this sense, Cu₃P is a p-type semiconductor material with a band gap of about 1.5 eV, indicating absorption in visible light [121]. Moreover, this semiconductor presents a suitable band alignment to perform the H₂ production from water splitting [119,121,122]. However, Cu₃P displays drawbacks for its application as a photocatalyst in overall water splitting reaction such as photocorrosion, rapid recombination of photo-induced e⁻-h⁺ pairs, and high oxidation potential. The fabrication of Z-Scheme heterojunctions represents a solution to solve these drawbacks [123,124]. A. Rauf et al. studied for the first time the pristine Cu₃P catalyst for solar watersplitting applications, indicating its potential. Moreover, they also observed that the construction by the ball milling method of a novel direct Z-scheme based on Cu₃P (Bi₂WO₆–Cu₃P) without a co-catalyst and mediator brings a higher photocatalytic activity in overall water splitting than the pure semiconductors (Bi₂WO₆ and Cu₃P). This work emphasizes the importance of materials design based on appropriate energy level positions to photocatalytically produce H₂ [125]. S. Hua et al. combined Cu₃P with the widely studied g-C₃N₄ nanosheets photocatalyst via chemical deposition and phosphorization, obtaining a novel Z-scheme based on the Cu₃P semiconductor (see Figure 5a). The reaction mechanism of the composite shows that Cu₃P enhances the charge separation process. Indeed, the authors revealed that the photogenerated e⁻ in g-C₃N₄ recombine with the h⁺ in Cu₃P, the photogenerated e⁻ of Cu₃P will be left for H₂ production, improving the photocatalytic activity of the Z-scheme semiconductor (Figure 5a) [126]. Z. Yang et al. observed a boost in photocatalytic H₂ production using a 10 wt% Cu₃P/ZnIn₂S₄ Z-scheme synthesized by the facile solution-phase hybridization method as a photocatalyst, obtaining an apparent quantum yield of 22.3% at 420 nm (Figure 5b). The authors indicated that this fact might be related to the formation of Z-scheme with a built-in electric field within Cu₃P/ZnIn₂S₄ and Z-scheme charge carriers transfer pathway, which results in an efficient separation and migration of charge carriers. DFT-based DOS and PDOS are used to gain information about the electronic structure of the heterojunction. Charge-density profiles were used to indicate the charge transfer [127]. Q. Wang et al. demonstrated that a Z-scheme photocatalyst of Cu₃P with TiO₂ is feasible using a hydrothermal methodology. The authors show that the catalyst was active in visible light; see Figure 5c [128]. G. Ge et al. fabricated Cu₃P/Zn_0.5Cd_0.5S via an in situ phosphating method. The authors also observed that the Cu₃P/Zn_0.5Cd_0.5S Z-scheme heterojunction increased the H₂ production, which is 3.1 folds higher than Zn_0.5Cd_0.5S due to a strong contact between Cu₃P and Zn_0.5Cd_0.5S introduced by in situ phosphidation method [129].

Another alternative to ensure the efficient transmission of photogenerated carriers and further optimize the structure of the Z-scheme family systems is the design of dual-Z-scheme systems. [130]. In this sense, K. Wang et al. designed for the first time a metal-organic frameworks (MOF)-derived dual-S-scheme based on Cu₃P by simple hydrothermal and phosphating methods (Figure 5d). The authors indicated that the optimized CoP/Cu₃P/Ni₂P (CCNP) photocatalyst had a photocatalytic hydrogen evolution activity of 786.58 μmol g⁻¹ h⁻¹, which is roughly 2.64 and 3.33 times higher than that of CoP (CP) and CoP/Ni₂P (CNP), respectively. This work opens the door for the use of dual S-schemes based on the Cu₃P photocatalyst in H₂ production [131] (Table 4).

Figure 5. (a) Synthetic method, photocatalytic H₂ production, and photocatalytic reaction mechanism of the Cu₃P/g-C₃N₄ heterojunction synthesized by phosphorization. Reproduced from [126]. Copyright 2019, Elsevier. (b) Photocatalytic production of Cu₃P/ZnIn₂S₄ photocatalyst and TEM images of the composite showing the intimate contact between the semiconductors. Reproduced from [127]. Copyright 2020, Elsevier. (c) UV-visible spectra of Cu₃P/TiO₂ active photocatalysts. Reproduced from [128]. Copyright 2022, Elsevier. (d) Photocatalytic reaction mechanism of CoP/Cu₃P/Ni₂P dual S-scheme. Reproduced from [131]. Copyright 2022, ACS Publications.

Figure 5. (a) Synthetic method, photocatalytic H₂ production, and photocatalytic reaction mechanism of the Cu₃P/g-C₃N₄ heterojunction synthesized by phosphorization. Reproduced from [126]. Copyright 2019, Elsevier. (b) Photocatalytic production of Cu₃P/ZnIn₂S₄ photocatalyst and TEM images of the composite showing the intimate contact between the semiconductors. Reproduced from [127]. Copyright 2020, Elsevier. (c) UV-visible spectra of Cu₃P/TiO₂ active photocatalysts. Reproduced from [128]. Copyright 2022, Elsevier. (d) Photocatalytic reaction mechanism of CoP/Cu₃P/Ni₂P dual S-scheme. Reproduced from [131]. Copyright 2022, ACS Publications.

Table 4. Cu phosphide-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity (µmol g−1 h−1) and AQE	Reference
Bi2WO6-Cu3P	Mechanical ball milling	Xe lamp (AM 1.5G)	40.6 AQE: NA SA	[125]
Cu3P/g-C3N4	Phosphorization	Xe lamp (300 W, λ ≥ 420 nm)	808 AQE: NA SA	[126]
Cu3P/ZnIn2S4	Solution-phase hybridization method	Xe lamp (300 W, λ ≥ 420 nm)	2561.1 AQE: NA AQE: 22.3% (420 nm) SA	[127]
Cu3P/TiO2	Hydrothermal method	Xe lamp (300 W, λ ≥ 420 nm)	607.5 AQE: NA SA	[128]
Cu3P/Zn0.5Cd0.5S	In situ phosphidation method	Xe lamp (300 W, λ ≥ 420 nm)	2700 AQE: NA SA	[129]
CoP/Cu3P/Ni2P	Hydrothermal and phosphating	5 W LED white light (λ ≥ 420 nm)	786.58 AQE: 3.69% (420 nm) SA	[131]

Table 4. Cu phosphide-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity (µmol g−1 h−1) and AQE	Reference
Bi2WO6-Cu3P	Mechanical ball milling	Xe lamp (AM 1.5G)	40.6 AQE: NA SA	[125]
Cu3P/g-C3N4	Phosphorization	Xe lamp (300 W, λ ≥ 420 nm)	808 AQE: NA SA	[126]
Cu3P/ZnIn2S4	Solution-phase hybridization method	Xe lamp (300 W, λ ≥ 420 nm)	2561.1 AQE: NA AQE: 22.3% (420 nm) SA	[127]
Cu3P/TiO2	Hydrothermal method	Xe lamp (300 W, λ ≥ 420 nm)	607.5 AQE: NA SA	[128]
Cu3P/Zn0.5Cd0.5S	In situ phosphidation method	Xe lamp (300 W, λ ≥ 420 nm)	2700 AQE: NA SA	[129]
CoP/Cu3P/Ni2P	Hydrothermal and phosphating	5 W LED white light (λ ≥ 420 nm)	786.58 AQE: 3.69% (420 nm) SA	[131]

3.4. Other Cu-Based Z-Schemes (Family)

Along with the most common Cu-based compounds described up to now, different materials have been used to construct Z-schemes family heterojunctions, i.e., CuI, Cu-based metal-organic frameworks (MOFs), and multimetallic compounds. In this section, we will briefly describe some of them and their properties. Their photocatalytic performances are resumed in Table 5.

CuI has been drawing attention for its high hole mobility and applicability in optoelectronic applications, but it has a wide band gap (3.1 eV) [132], different from all the compounds that we are observing in this review. This band gap makes mandatory the use of narrow band gap semiconductors if it is intended to use this compound in visible light-mediated processes. Up to now, this interesting material has been used in combination with graphdiyne (GDY) to decrease its recombination rate and simplify the synthetic approach to obtain GDY. It was possible to obtain different S-schemes in combination with several semiconductors, i.e., Co₉S₈ (see Figure 6a) [133], CdS [134], MIL-53 [135], NiTiO₃ [136], Co₃(PO₄)₂ [137], CoFe layered hydroxide (LDH) [138], NiV LDH [139], and MoO₂ [140] active in H₂ production.

MOFs are not semiconductors, but their structural tunability makes them interesting for the efficient use of solar light in several photocatalytic applications [141]. Cu-based MOFs have been used in S-schemes to improve the active sites of different semiconductors. Cao et al. used Cu-MOFs to construct an active heterojunction with Mn_0.05Cd_0.95S by improving the surface chemistry of the materials and its optical properties, as shown in Figure 6b [142]. Similarly, a Cu-MOF was used to enhance the photocatalytic activity of CdS, improving not only the surface area but also the charge carriers’ separation [143].

Finally, the synergistic effect of the use of Cu in the presence of other metals has been exploited not only in oxides and sulfides but also in mixed-transition metal oxysulfides. Sun et al. used La₅Ti₂Cu_0.9Ag_0.1S₅O₇ to improve the photocatalytic activity of BiVO₄ in a Z-scheme for the overall water splitting and decrease the undesired redox reactions common in oxysulfides [144].

Figure 6. (a) Photocatalytic H₂ generation mechanism of the hybrid catalyst GDY/CuI/CdS- R. Reproduced from [144]. Copyright 2022, RSC Publications (b) Photocatalytic mechanism of the Cu-MOFs/Mn_0.05Cd_0.95S. Reproduced from [142]. Copyright 2021, Elsevier.

Figure 6. (a) Photocatalytic H₂ generation mechanism of the hybrid catalyst GDY/CuI/CdS- R. Reproduced from [144]. Copyright 2022, RSC Publications (b) Photocatalytic mechanism of the Cu-MOFs/Mn_0.05Cd_0.95S. Reproduced from [142]. Copyright 2021, Elsevier.

Table 5. Other Cu-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity and AQE	Reference
Co9S8/GDY/CuI	Hydrothermal	5 W LED	1.4 mmol g−1 h−1 AQE: NA SA	[133]
CuI/GDY/CdS-R	Hydrothermal	5 W LED	16.2 mmol g−1 h−1 AQE: 6.11% (450 nm) SA	[134]
GDY/CuI/MIL-53	Ultrasonication	5 W LED	596.8 μmol g−1 h−1 AQE: NA SA	[135]
GDY/CuI/NiTiO3	Impregnation	5 W LED	509.03 μmol g−1 h−1 AQE: NA SA	[136]
Co3(PO4)2/CuI/GDY	Hot solvent	5 W LED	319.4 μmol g−1 h−1 AQE: NA SA	[137]
CoFe LDH/CuI/GDY	Solvothermal	5 W LED	1.2 mmol g−1 h−1 AQE: NA SA	[138]
GDY/CuI/NiV LDH	Hot solvent	5 W LED	2.9 mmol g−1 h−1 AQE: 0.15% (420 nm) SA	[139]
GDY/CuI/MoO2	Hot solvent	5 W LED	820 μmol g−1 h−1 AQE: 4.2% (520 nm) SA	[140]
Cu-MOF/Mn0.05Cd0.95S	Hydrothermal	5 W LED	13.7 mmol g−1 h−1 AQE: 1.83% (450 nm) SA	[142]
Cu-MOF/CdS	Impregnation	5 W LED	4 mmol g−1 h−1 AQE: NA SA	[143]
La5Ti2Cu0.9Ag0.1S5O7/BiVO4	Particle transfer	Xe lamp (300 W, λ ≥ 420 nm)	2.75 μmol cm−2 h−1 AQE: 4.9% (420 nm) SA	[144]

Table 5. Other Cu-based Z-schemes family heterojunctions for H₂ production (AQE: apparent quantum efficiency; SA: sacrificial agent; NA: not available).

Photocatalyst	Fabrication Methodology	Irradiation Source	H2 Production Activity and AQE	Reference
Co9S8/GDY/CuI	Hydrothermal	5 W LED	1.4 mmol g−1 h−1 AQE: NA SA	[133]
CuI/GDY/CdS-R	Hydrothermal	5 W LED	16.2 mmol g−1 h−1 AQE: 6.11% (450 nm) SA	[134]
GDY/CuI/MIL-53	Ultrasonication	5 W LED	596.8 μmol g−1 h−1 AQE: NA SA	[135]
GDY/CuI/NiTiO3	Impregnation	5 W LED	509.03 μmol g−1 h−1 AQE: NA SA	[136]
Co3(PO4)2/CuI/GDY	Hot solvent	5 W LED	319.4 μmol g−1 h−1 AQE: NA SA	[137]
CoFe LDH/CuI/GDY	Solvothermal	5 W LED	1.2 mmol g−1 h−1 AQE: NA SA	[138]
GDY/CuI/NiV LDH	Hot solvent	5 W LED	2.9 mmol g−1 h−1 AQE: 0.15% (420 nm) SA	[139]
GDY/CuI/MoO2	Hot solvent	5 W LED	820 μmol g−1 h−1 AQE: 4.2% (520 nm) SA	[140]
Cu-MOF/Mn0.05Cd0.95S	Hydrothermal	5 W LED	13.7 mmol g−1 h−1 AQE: 1.83% (450 nm) SA	[142]
Cu-MOF/CdS	Impregnation	5 W LED	4 mmol g−1 h−1 AQE: NA SA	[143]
La5Ti2Cu0.9Ag0.1S5O7/BiVO4	Particle transfer	Xe lamp (300 W, λ ≥ 420 nm)	2.75 μmol cm−2 h−1 AQE: 4.9% (420 nm) SA	[144]

4. Conclusions and Outlooks

In conclusion, Cu-based Z-schemes family photocatalysts are an innovative technology to boost the H₂ production activity and to overcome the main drawbacks of the single photocatalysts, i.e., high e⁻-h⁺ recombination rate, photocorrosion, and inadequate redox potentials. Moreover, the use of a transition metal such as Cu favors the possibility of their future commercial application due to the low cost and the photocatalytic activity properties of Cu. In this review, a comprehensive overview of the recent research on Cu-based Z-schemes family photocatalysts for their use in solar photocatalytic H₂ production is described. The scientific community has developed several synthetic methodologies and combinations with different semiconductors to boost the catalytic properties of the synthesized materials. In this sense, researchers combined different compounds of Cu (copper oxide, copper sulfide, etc.) with widely studied semiconductors (TiO₂, g-C₃N₄, MOFs, etc.), and the results described in this review show that the Z-schemes obtained present higher activity than the pristine Cu-based semiconductors. This fact indicates the important role of this system in removing the photocorrosion and improving the charge transfer of the Cu-based semiconductors for their future application in the industry. However, nowadays, the photocatalytic efficiencies are still low, and many catalytic mechanisms remain unclear, being the main drawbacks for their commercialization. With this in mind, the challenges for new researchers in the application of Cu-based Z-schemes family photocatalysts might be in the synthetic methodology, focusing on the interaction between the Cu-based semiconductor and the other semiconductor in the heterojunction. The use of ab initio calculations is not only to describe but also to yield conclusions; neither is it justified to use a partial study to support claims. There is an important need for a thorough study of systems to gain deep insights into the systems studied experimentally. As mentioned in this review, the intimate contact between both components is one of the keys to improving the photocatalytic activity of the heterostructures and obtaining Z-schemes systems. Additionally, the monitoring of the reaction has a central role, but it is sometimes complex due to the formation of by-products or photocorrosion of the catalyst. Hence, it is also necessary to perform in-depth studies of the catalytic reactions’ mechanisms and, to do so, in all the stages of these catalytic processes (adsorption, reaction, and desorption). To this end, ab initio calculations can help in giving directions for the determination of the atomic reactivity of catalysts. This opens the doors to developing novel Cu-based Z-scheme photocatalysts, characterization techniques, and studies of new mechanisms to obtain catalysts capable of being soon used in practical applications using solar light. Another drawback that should be addressed by the scientific community to the commercialization of this technology is the design and the manufacture of stable photocatalysts under reaction conditions since most of the works reported do not study in deep the stability of the catalyst used.