S-Scheme Heterojunction Photocatalysts for CO 2 Reduction

: Photocatalytic technology, which is regarded as a green route to transform solar energy into chemical fuels, plays an important role in the fields of energy and environmental protection. An emerging S-scheme heterojunction with the tightly coupled interface, whose photocatalytic efficiency exceeds those of conventional type II and Z-scheme photocatalysts, has received much attention due to its rapid charge carrier separation and strong redox capacity. This review provides a systematic description of S-scheme heterojunction in the photocatalysis, including its development, reaction mechanisms, preparation, and characterization methods. In addition, S-scheme photocatalysts for CO 2 reduction are described in detail by categorizing them as 0D/1D, 0D/2D, 0D/3D, 2D/2D, and 2D/3D. Finally, some defects of S-scheme heterojunctions are pointed out, and the future development of S-scheme heterojunctions is proposed.


Introduction
The rapid development of industrialization has been met with the increasingly serious energy shortage, and environmental pollution [1].The greenhouse gas (CO 2 ) produced by the massive burning of fuels has caused serious environmental problems.Photosynthesis can utilize solar energy to convert CO 2 into energy-rich organic products.Inspired by plant photosynthesis, semiconductor-based photocatalysts have received widespread attention.Photocatalytic reduction of CO 2 can be divided into the following five steps (Figure 1a) [2,3]: (1) The semiconductor photocatalyst is excited by incident light, and electrons (e − ) transition from the valence band (VB) to the conduction band (CB) and leave the same number of holes (h + ) on the VB.(2) Separation of photogenerated electron-hole pairs occurs.It also faces the problem of photogenerated electron-hole recombination, which leads to the loss of free carriers, and the energy of recombination is released in the form of heat, which lowers the sunlight utilization efficiency.(3) CO 2 adsorption occurs; this is a prerequisite for the transfer of electrons from the photocatalyst to CO 2 .(4) A surface redox reaction occurs, and the photogenerated electrons migrate to the surface to reduce CO 2 .(5) Product desorption occurs.At the end of the reaction, the products need to be released from the catalyst surface in time, otherwise they will cover the active sites and lead to catalyst "poisoning".The CO 2 reduction reaction involves the participation of multiple electrons and protons, and is a complex multi-step process with slow kinetics [4].Different products will be produced via different reduction processes, including CO [5], CH 3 OH [6], HCOOH [7], CH 4 [8], and C 2+ [9], etc.The selectivity of the products mainly depends on the composition and structure of the catalyst and the conditions of the catalytic reaction.Photocatalytic technology is regarded as a green and environmentally friendly route, not only using solar energy to produce valuable fuels, such as H 2 , CO, and CH 4 , etc., but also achieving the goal of carbon neutrality [10][11][12][13][14].Most of the semiconductor-based photocatalysts have the disadvantages of low light absorption and a fast electron-hole pair recombination rate [15][16][17].Their performance is generally enhanced by some strategies, such as surface structure modulation, defect engineering, metal deposition, construction of heterojunctions, and photosensitization treatments [13,[18][19][20].Due to the limitations of the traditional type II heterojunctions and Z-scheme heterojunctions, Yu and co-workers proposed the concept of S-scheme heterojunctions in 2019 (Table 1) [21].The S-scheme heterojunction photocatalyst consists of an oxidation photocatalyst (OP) with a lower Fermi level and a larger work function, coupled with a reduction photocatalyst (RP) with a higher Fermi level and a smaller work function (Figure 1b) [22,23].The photogenerated electron transfer route in the S-scheme heterojunction resembles "step" in the macroscopic viewpoint and the letter "N" in the microscopic viewpoint.When OP and RP are in contact, electrons from RPs with a higher Fermi energy level transfer to OPs with a lower Fermi energy level at the interface (Figure 1c).As a result, RP loses electrons and becomes positively charged, while OP gains electrons and becomes negatively charged, resulting in an internal electric field (IEF) at the interface (direction of the electric field: RP→OP).The CB and VB of RP bend upward due to the formation of electron depletion regions.In contrast, the CB and VB of OP bend downward due to the formation of electron accumulation regions.Under illumination, the electric field drives the transfer of photogenerated electrons from the CB of OP to the VB of RP (Figure 1d).In addition, the Coulomb gravitational force between electrons in OP and holes in RP, and energy band bending, also facilitate this charge transfer, while the electron transfer from the CB of RP to the CB of OP (hole transfer from the VB of OP to the VB of RP) can be preserved to achieve the spatial separation of carriers.Ultimately, the useful photogenerated electrons and holes are retained in the CB of RP and the VB of OP, respectively, and the relatively weak photogenerated carriers are eliminated by recombination.Thus, the S-scheme heterojunction has strong redox and charge separation capabilities [24][25][26][27][28][29].
Catalysts 2024, 14, x FOR PEER REVIEW 2 of Most of the semiconductor-based photocatalysts have the disadvantages of low lig absorption and a fast electron-hole pair recombination rate [15][16][17].Their performance generally enhanced by some strategies, such as surface structure modulation, defe engineering, metal deposition, construction of heterojunctions, and photosensitizati treatments [13,[18][19][20].Due to the limitations of the traditional type II heterojunctions a Z-scheme heterojunctions, Yu and co-workers proposed the concept of S-schem heterojunctions in 2019 (Table 1) [21].The S-scheme heterojunction photocatalyst consi of an oxidation photocatalyst (OP) with a lower Fermi level and a larger work functio coupled with a reduction photocatalyst (RP) with a higher Fermi level and a smaller wo function (Figure 1b) [22,23].The photogenerated electron transfer route in the S-schem heterojunction resembles "step" in the macroscopic viewpoint and the letter "N" in t microscopic viewpoint.When OP and RP are in contact, electrons from RPs with a high Fermi energy level transfer to OPs with a lower Fermi energy level at the interface (Figu 1c).As a result, RP loses electrons and becomes positively charged, while OP gai electrons and becomes negatively charged, resulting in an internal electric field (IEF) the interface (direction of the electric field: RP→OP).The CB and VB of RP bend upwa due to the formation of electron depletion regions.In contrast, the CB and VB of OP be downward due to the formation of electron accumulation regions.Under illuminatio the electric field drives the transfer of photogenerated electrons from the CB of OP to t VB of RP (Figure 1d).In addition, the Coulomb gravitational force between electrons OP and holes in RP, and energy band bending, also facilitate this charge transfer, wh the electron transfer from the CB of RP to the CB of OP (hole transfer from the VB of O to the VB of RP) can be preserved to achieve the spatial separation of carriers.Ultimate the useful photogenerated electrons and holes are retained in the CB of RP and the VB OP, respectively, and the relatively weak photogenerated carriers are eliminated recombination.Thus, the S-scheme heterojunction has strong redox and charge separati capabilities [24][25][26][27][28][29].The excellent properties of S-scheme heterojunction make it widely used in photocatalytic CO 2 reduction, hydrogen hydrolysis, hydrogen peroxide synthesis, nitrogen fixation, degradation of organic pollutants, and organic conversion and sterilization [30][31][32][33][34].The components usually used for constructing S-scheme heterojunctions include g-C 3 N 4 , bismuth halide, metal oxides (TiO 2 , ZnO), metal-sulfur compounds (CdS, CdSe, In 2 S 3 , CdIn 2 S 4 , ZnIn 2 S 4 ), and metal organic frameworks (MOFs), which have been reported in detail in several reviews [35][36][37][38][39][40][41][42][43][44][45].The synergistic effect of materials with different dimensions can solve the problems of the few reactive active sites, the easy aggregation of materials, and the slow electron transfer rate.Hence, the resulting S-scheme heterojunction has unique photochemical properties such as quantum effect, specific surface area, and chemical stability [46][47][48][49].Most of the current reviews on S-scheme heterojunctions are based on semiconductor material, and a few reviews are on S-scheme heterojunctions prepared from nanomaterials of different dimensions.This paper presents a review on multidimensionally assembled S-scheme heterojunctions to conduct a more in-depth study on S-scheme heterojunctions.The development, preparation, and characterization methods of S-scheme heterojunctions are briefly introduced, followed by the S-scheme photocatalysts for CO 2 reduction according to the classification of 0D/1D, 0D/2D, 0D/3D, 2D/2D, and 2D/3D (Figure 2).Finally, an outlook for the development of S-scheme heterojunctions for CO 2 reduction is given.

Preparation of S-Scheme Photocatalysts
The methods commonly used to prepare S-scheme photocatalysts with multidimensional structures include self-assembly [50,51], in situ growth [52][53][54][55], and impregnation [56,57].The three methods are universal in the preparation of S-scheme heterojunctions (0D/1D, 0D/2D, 0D/3D, 2D/2D, and 2D/3D).Self-assembly, in situ growth, and impregnation methods have their own advantages and limitations (Table 2), which can be selected according to the properties of the materials and their own needs during synthesis.In the self-assembly method, the two desired materials are prepared first, followed by amalgamation to form heterojunction through surface interactions (chemical bond, hydrogen bond, electrostatic interactions, etc.) [58].The electrostatic self-assembly commonly used for the preparation of S-scheme heterojunctions typically involves pass-dispersing the two as-prepared photocatalysts in a medium such as water, or an organic solvent, and adjusting the pH of the solution until the surface charges of the two materials are opposite.Driven by the Coulomb force, the two materials form a heterostructure.As shown in Figure 3a, ZnIn 2 S 4 nanosheets and B-C 3 N 4 nanosheets were synthesized first, and then ZnIn 2 S 4 /B-C 3 N 4 composites were obtained by surface charge interactions using the electrostatic self-assembly method [59].Impregnation is a simple method to synthesize heterojunction, in which one component is impregnated on the surface of the other component by mixing the dispersion of the two components under ultrasound assistance.Using ultrasonication not only improves the mass transfer effect, but also enhances the dispersion of nanoparticles on the surface of the substance [60][61][62].Zhang et al. [57] modified ZnCo 2 S 4 nanoparticles on the surface of Bi 2 WO 6 nanosheets using the ultrasonic impregnation method, and constructed the S-scheme heterojunction ZnCo 2 S 4 /Bi 2 WO 6 for photocatalytic hydrogen production (Figure 3b).In situ growth is a method to prepare heterojunction by nucleation and growth on the substrate surface, which is a chemical reaction zone [52,63].The metal ions adsorbed on the surface of the pre-prepared photocatalyst act as nucleation centers and gradually grow into well-established crystals, resulting in a heterojunction with two components in close contact [34,64].Heterojunction via in situ growth can usually be prepared by means of hydrothermal or chemical deposition [65,66].The S-scheme heterojunction photocatalysts were obtained by growing a second photocatalyst on the surface of the pre-prepared photocatalysts by reacting under hydrothermal, solvent-thermal, or oil-bath conditions (Figure 3c).

Characterization Methods of S-Scheme Heterojunctions
Confirmation of the electron transfer pathway is an important part in the study of S-scheme heterojunctions; so far, the commonly used experimental characterizations are electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and in situ irradiated X-ray photoelectron spectroscopy (ISIXPS), Kelvin probe force microscopy (KPFM), and femtosecond transient absorption spectroscopy (FT-AS) [35,[67][68][69][70]. Theoretical calculation includes density functional theory (DFT) calculation, which can predict the type of carrier migration and provide directions for mechanism studies [71].The combination of experimental characterization and theoretical calculations can well reveal the microscopic mechanisms, as well as the dynamics of the separation and transport processes of photogenerated carriers.
Catalysts 2024, 14, x FOR PEER REVIEW 5 of 22 a second photocatalyst on the surface of the pre-prepared photocatalysts by reacting under hydrothermal, solvent-thermal, or oil-bath conditions (Figure 3c).

Characterization Methods of S-Scheme Heterojunctions
Confirmation of the electron transfer pathway is an important part in the study of Sscheme heterojunctions; so far, the commonly used experimental characterizations are electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and in situ irradiated X-ray photoelectron spectroscopy (ISIXPS), Kelvin probe force microscopy (KPFM), and femtosecond transient absorption spectroscopy (FT-AS) [35,[67][68][69][70]. Theoretical calculation includes density functional theory (DFT) calculation, which can predict the type of carrier migration and provide directions for mechanism studies [71].The interfacial electron transfer from RP to OP decreases and increases the electron density of elements in RP and OP, respectively, which varies the binding energies [72].The change in the atomic binding energy can be found in the XPS spectra of the Bi 3 NbO 7 (BNO)/g-C 3 N 4 (UCN) S-scheme heterojunction, for example [73].The formation of the heterostructure results in a negative shift of the characteristic peaks of Bi 1s and Nb 3d of BNO, and an obvious shift to higher energy of the typical peaks of C 1s and N 1s of UCN, indicating that the electrons flow from UCN to BNO when the two components are in contact.The binding energies of the Bi 1s and Nb 3d of BNO are increased, and those of the C 1s and N 1s of UCN are decreased after the irradiation of the surface, which indicates that BNO is the electron donor under the light irradiation (Figure 4a).The XPS results clearly revealed the charge transfer process from UCN (RP) to BNO (OP).The S-scheme charge transfer process can also be reflected in the KPFM results, in which the changes in the surface potentials of RP and OP reveal the electron transfer process [74,75].The potential difference between pyrene-alt-triphenylamine (PT, point A) and CdS (point B) is about 100 mV, suggesting the formation of an internal electric field (A→B).Under light irradiation, the surface potential of point A decreased, while that of point B increased (Figure 4b,c), and the change in surface potential fully revealed the transfer of photogenerated electrons from OP to RP [76].The above two techniques provide direct evidence for the S-scheme charge transfer mechanism, while EPR is an indirect method to verify the S-scheme charge transfer mechanism [77,78] The parameters, such as work functions and differential charge density, can b obtained from DFT calculation (Figure 5a,b).The interfacial electron transfer between R and OP can be visually reflected by charge density differences [80].From the differenti The parameters, such as work functions and differential charge density, can be obtained from DFT calculation (Figure 5a,b).The interfacial electron transfer between RP and OP can be visually reflected by charge density differences [80].From the differential charge density plot of SnO 2 /Cs 3 Bi 2 Br 9 , it can be noticed that electrons are consumed at Cs 3 Bi 2 Br 9 , while they are accumulated at SnO 2 (Figure 5c) [81].The transfer of interfacial electrons from RP to OP after hybridization is consistent with the S-scheme mechanism.Multiple characterizations combined with theoretical calculations can verify the S-scheme charge transfer mechanism.Although the above characterization techniques can characterize the steady-state S-scheme electron transfer, the photocatalytic transient reactions still require other technical tools.Therefore, femtosecond transient absorption spectroscopy was used to monitor ultrafast electron dynamics from the femtosecond to picosecond time scale to analyze the energy level structure and electronic relaxation process of semiconductors [82,83].Figure 5d,e shows the femtosecond transient absorption spectra and FT-AS decay curves of TiO 2 and TiO 2 /dopamine (TiO 2 /PDA).The increased fraction of τ2 in TP0.5 compared to TiO 2 suggests a new electron transfer pathway in TP0.5, which can be attributed to an interfacial electron transfer in the TiO 2 /PDA S-scheme heterojunction.The electron transfer from the CB of TiO 2 to the VB of PDA occurs within ~5 ps, and the time scale of the interfacial electron transfer is much shorter than that of the charge recombination, allowing for an efficient separation of electrons and holes in the TiO 2 /PDA S-scheme heterojunction [84].This characterization method elucidates the mechanism for the enhanced performance of S-scheme photocatalysts from the perspective of interfacial electron transfer kinetics.

S-scheme Photocatalysts for CO2 Reduction
Materials used for S-scheme heterojunctions are categorized as zero-dimensional, one-dimensional, two-dimensional, and three-dimensional.Heterojunctions formed by dimension-specific nanostructures can create high-speed transport channels for carriers inside or on the surface of the material [85,86].Zero-dimensional nanomaterials such as quantum dots can provide unliganded surface sites for reactions, but are prone to aggregation.The short radial distance of one-dimensional geometries (ribbons, tubes,

S-Scheme Photocatalysts for CO 2 Reduction
Materials used for S-scheme heterojunctions are categorized as zero-dimensional, one-dimensional, two-dimensional, and three-dimensional.Heterojunctions formed by dimension-specific nanostructures can create high-speed transport channels for carriers inside or on the surface of the material [85,86].Zero-dimensional nanomaterials such as quantum dots can provide unliganded surface sites for reactions, but are prone to aggregation.The short radial distance of one-dimensional geometries (ribbons, tubes, fibers, rods, and wires) reduces electron-hole recombination, while the high aspect ratio and larger specific surface area significantly improve the light absorption properties [87].Most of the 2D nanomaterials have good electrical conductivity, which facilitates the transfer and separation of photogenerated electrons and holes [88][89][90].As good carriers, 2D and 3D nanomaterials with the large surface area can provide a large number of binding sites for other photocatalysts to form heterojunction.Stabilized nanomaterials as components in the S-scheme heterojunction can bring high stability to the photocatalyst.If one of the components is poorly stabilized, a material with higher stability can be selected to form an interaction with it, so that the stability of the photocatalyst can be improved.S-scheme heterojunctions with multidimensional structures can improve the photocatalytic performance by utilizing the synergistic or superposition effect of materials with different dimensions [91][92][93][94].In this article, we will discuss the S-scheme heterojunction photocatalysts for CO 2 reduction consisting of five different dimensional materials, i.e., 0D/1D, 0D/2D, 0D/3D, 2D/2D, and 2D/3D.

Heterojunction of 0D/1D
Zero-dimensional structures are particles with nanoscale dimensions in three dimensions, such as points without a specific shape.Powdered catalytic particles with nanoscale dimensions can increase the total surface area and the number of available reaction sites.The loose structure of 1D materials allows for better dispersion of nanoparticles and inhibits nanoparticle aggregation.The combination of 0D and 1D nanomaterials allows for more efficient separation of internal and surface carriers [88,95].For example, Wang et al. [96] prepared Ta 2 O 5 /Ag 2 S S-scheme photocatalysts by self-assembly of 0D Ag 2 S nanoparticles into the fibrous Ta 2 O 5 (Figure 6a).The broad light absorption of Ag 2 S compensates for the limited light-harvesting ability of Ta 2 O 5 ; meanwhile, the attachment of Ag 2 S nanoparticles to the surface of Ta 2 O 5 nanofibers solves the weak interfacial interactions.This hybrid fiber showed the enhanced selectivity for CH 4 with a yield of 132.3 µmol g −1 h −1 and a selectivity of 93.1%, which provided a new pathway for the efficient reduction of CO 2 in the diluted CO 2 atmosphere.Similarly, Hu et al. [81] constructed SnO 2 /Cs 3 Bi 2 Br 9 (SC) S-scheme heterojunctions by electrostatically self-assembling SnO 2 nanofibers and Cs 3 Bi 2 Br 9 quantum dots.The loose network structure, the rough surface, and the shortened charge-transport pathway of the SnO 2 nanofibers enable them to be ideal carriers for Cs 3 Bi 2 Br 9 .The selectivity of this catalyst for CH 4 was more than 70%, which was attributed to the much lower Gibbs free energy for the production of *CHO than *CO in the reduction of CO 2 by Cs 3 Bi 2 Br 9 .The selectivity of CH 4 could be further improved.Zhang et al. [97] constructed a CsPbBr 3 /AgBr S-scheme heterojunction, and systematically investigated the S-scheme charge transfer mechanism and dynamics in the heterojunction by using XPS, electron spin resonance, and time-resolved photoluminescence techniques (Figure 6b,c).The electron consumption rate reaches 141.4 µmol g −1 h −1 due to the effective spatial separation of photogenerated charge carriers and the retaining of strongly reductive electrons.

Heterojunction of 0D/2D
Most S-scheme photocatalysts are built based on 2D S-scheme heterostructures, where one or both components consist of 2D materials [64,98].Quantum dots are unstable and prone to self-aggregation, which leads to the recombination of photogenerated charges.By loading quantum dots onto 2D materials, the interaction between the two components can make the quantum dots more dispersed and stable, while the 2D materials reduce the migration distance of charge carriers from the interior to the surface, which greatly reduces the electron-hole recombination during the transfer process [93,99].The 0D/2D heterojunction has a higher structural order than 0D/1D, which is more favorable for improving chemical stability.Zhang et al. [100] prepared S-scheme photocatalysts with CsPbBr3 quantum dots/BiOBr nanosheets (CPB/BiOBr) using a simple self-assembly process (Figure 7a).The large surface area of the BiOBr nanosheets ensured efficient deposition of CsPbBr3 quantum dots on their surface, which led to a close BiOBr and CsPbBr3 contact that was favorable for electron transfer.Although a strategy is provided for modulating the photocatalytic properties of lead halide perovskite-based material in this study, the photocatalytic efficiency is not much improved.Pei et al. [101] elaborated a ZnGa2O4/g-C3N4 heterojunction photocatalyst (Figure 7b), achieving an efficient CO2-to-syngas conversion with a syngas yield as high as 103.3 µmol g −1 h −1 .In addition, the CO/H2 ratio of the syngas can be adjusted in the range of 1:4 to 2:1.The twodimensional structure of g-C3N4 provides a large surface area, which promotes CO2 adsorption.DFT calculations reveal the transfer of electrons from g-C3N4 to ZnGa2O4 through the interface, which is consistent with the XPS results, identifying the formation of S-scheme heterojunction.Tan et al. [102] prepared a series of 0D/2D CsPbBr3/BiVO4 heterojunctions via self-assembly (Figure 7c).The average CO yield of the optimized

Heterojunction of 0D/2D
Most S-scheme photocatalysts are built based on 2D S-scheme heterostructures, where one or both components consist of 2D materials [64,98].Quantum dots are unstable and prone to self-aggregation, which leads to the recombination of photogenerated charges.By loading quantum dots onto 2D materials, the interaction between the two components can make the quantum dots more dispersed and stable, while the 2D materials reduce the migration distance of charge carriers from the interior to the surface, which greatly reduces the electron-hole recombination during the transfer process [93,99].The 0D/2D heterojunction has a higher structural order than 0D/1D, which is more favorable for improving chemical stability.Zhang et al. [100] prepared S-scheme photocatalysts with CsPbBr 3 quantum dots/BiOBr nanosheets (CPB/BiOBr) using a simple self-assembly process (Figure 7a).The large surface area of the BiOBr nanosheets ensured efficient deposition of CsPbBr 3 quantum dots on their surface, which led to a close BiOBr and CsPbBr 3 contact that was favorable for electron transfer.Although a strategy is provided for modulating the photocatalytic properties of lead halide perovskite-based material in this study, the photocatalytic efficiency is not much improved.Pei et al. [101] elaborated a ZnGa 2 O 4 /g-C 3 N 4 heterojunction photocatalyst (Figure 7b), achieving an efficient CO 2 -to-syngas conversion with a syngas yield as high as 103.3 µmol g −1 h −1 .In addition, the CO/H 2 ratio of the syngas can be adjusted in the range of 1:4 to 2:1.The two-dimensional structure of g-C 3 N 4 provides a large surface area, which promotes CO 2 adsorption.DFT calculations reveal the transfer of electrons from g-C 3 N 4 to ZnGa 2 O 4 through the interface, which is consistent with the XPS results, identifying the formation of S-scheme heterojunction.Tan et al. [102] prepared a series of 0D/2D CsPbBr 3 /BiVO 4 heterojunctions via self-assembly (Figure 7c).The average CO yield of the optimized samples could reach 41.02 µmol g −1 h −1 .

Heterojunction of 0D/3D
Heterojunctions combining 0D with 3D nanomaterials can improve not only the dispersion of 0D materials, but also the ability to capture CO2, as well as the presence of more active sites of 0D material for the reaction, thus improving the photocatalytic ability [104].Zhang et al. [105] prepared an S-scheme heterojunction (Cs3Bi2Br9@VO-In2O3) via embedding Cs3Bi2Br9 in mesoporous In2O3 (Figure 8a).After oxygen vacancies (VO) were introduced into the mesoporous In2O3, IEF strength and the charge transfer rate were enhanced (Figure 8b); meanwhile, the activation energy of CO2 reduction was reduced.The S-scheme photogenerated carrier separation mechanism was verified using DFT calculations, in situ irradiation XPS, and KPFM.The weak photogenerated carriers were recombined, and the highly reducing electrons were retained in the CB of Vo-In2O3.The introduction of oxygen vacancies and construction of the S-scheme synergistically improved the photocatalytic activity in the reduction of CO2, with a CO yield of 130.96 µmol g −1 h −1 .Similarly, Dong et al. [106] prepared an S-scheme heterojunction photocatalyst with a CO2 photoreduction yield of 145.28 µmol g −1 h −1 by embedding CsPbBr3 perovskite quantum dots in mesoporous TiO2 beads (MTB) (Figure 8c).From the tauc diagrams of CsPbBr3@MTB composites, the bandgap energies of the heterojunction are different from their intrinsic bandgaps, indicating that the two are in close contact with each other, and that they have strong interactions.The close interfacial contacts and strong interactions brought about by the heterostructure facilitated an effective electron transfer between the interfaces.Wang et al. [104] prepared 0D/3D CsPbBr3/ZnO S-scheme heterojunction hollow spheres using a simple self-assembly method (Figure 8d).The

Heterojunction of 0D/3D
Heterojunctions combining 0D with 3D nanomaterials can improve not only the dispersion of 0D materials, but also the ability to capture CO 2 , as well as the presence of more active sites of 0D material for the reaction, thus improving the photocatalytic ability [104].Zhang et al. [105] prepared an S-scheme heterojunction (Cs 3 Bi 2 Br 9 @V O -In 2 O 3 ) via embedding Cs 3 Bi 2 Br 9 in mesoporous In 2 O 3 (Figure 8a).After oxygen vacancies (V O ) were introduced into the mesoporous In 2 O 3 , IEF strength and the charge transfer rate were enhanced (Figure 8b); meanwhile, the activation energy of CO 2 reduction was reduced.The S-scheme photogenerated carrier separation mechanism was verified using DFT calculations, in situ irradiation XPS, and KPFM.The weak photogenerated carriers were recombined, and the highly reducing electrons were retained in the CB of Vo-In 2 O 3 .The introduction of oxygen vacancies and construction of the S-scheme synergistically improved the photocatalytic activity in the reduction of CO 2 , with a CO yield of 130.96 µmol g −1 h −1 .Similarly, Dong et al. [106] prepared an S-scheme heterojunction photocatalyst with a CO 2 photoreduction yield of 145.28 µmol g −1 h −1 by embedding CsPbBr 3 perovskite quantum dots in mesoporous TiO 2 beads (MTB) (Figure 8c).From the tauc diagrams of CsPbBr 3 @MTB composites, the bandgap energies of the heterojunction are different from their intrinsic bandgaps, indicating that the two are in close contact with each other, and that they have strong interactions.The close interfacial contacts and strong interactions brought about by the heterostructure facilitated an effective electron transfer between the interfaces.Wang et al. [104] prepared 0D/3D CsPbBr 3 /ZnO S-scheme heterojunction hollow spheres using a simple self-assembly method (Figure 8d).The presence of the 3D structure improves the stability of the CsPbBr 3 , which effectively enhances the capture capacity of CO 2 .The hollow sphere structure can not only provide enough active sites, but also improves the light-harvesting efficiency.Su et al. [107] prepared S-scheme heterojunctions based on TiO 2 hollow spheres (TH), in which WO 3 nanoparticles (WP) acted as OPs to form close interfacial contacts with TH.Without adding any sacrificial agent, the yield of CO over the WP/TH was 4.73 µmol g −1 h −1 .
Catalysts 2024, 14, x FOR PEER REVIEW 13 of 22 presence of the 3D structure improves the stability of the CsPbBr3, which effectively enhances the capture capacity of CO2.The hollow sphere structure can not only provide enough active sites, but also improves the light-harvesting efficiency.Su et al. [107] prepared S-scheme heterojunctions based on TiO2 hollow spheres (TH), in which WO3 nanoparticles (WP) acted as OPs to form close interfacial contacts with TH.Without adding any sacrificial agent, the yield of CO over the WP/TH was 4.73 µmol g −1 h −1 .

Heterojunction of 2D/2D
The unique atomic-scale thickness of 2D materials shortens the carrier migration distance and reduces the photogenerated electron-hole recombination, and the large surface area provides plenty of binding sites for the reaction.A larger interfacial contact area can be formed when different 2D materials are combined; thus, more efficient photocatalysts are constructed [26,108].Zhou et al. [109] fabricated H2WO4/Cs2AgBiBr6 (HWO/CABB) S-scheme heterojunction catalysts through the "top-down" acid cleavage of layered Bi2WO6 (Bi2O2-WO4) into 2D H2WO4 (HWO) nanosheets approach, followed by in situ epitaxial growth of 2D Cs2AgBiBr6 (CABB) (Figure 9a).This catalyst has abundant Br vacancies (VBr) on the surface, which lowers the formation energy barrier of CHO* and improves CO2 adsorption and activation.The unique butterfly-shaped carbon nitride/zinc-doped bismuth vanadium oxide (CN-ZnBVO) catalysts were prepared by Li et al. [110], with a maximum CH3OH generation rate of 609.1 µmol g −1 h −1 , and a selectivity of up to 90.5% in the photocatalytic reduction of CO2 to CH3OH under UV-visible light (Figure 9b).With a more open outer surface, which could provide abundant accessible active sites, the CN-ZnBVO photocatalyst enhanced the CO2 adsorption capacity, and had an excellent photocatalytic efficiency.
As organic porous materials, the covalent organic framework (COF) and metal organic framework (MOF) are excellent photocatalytic materials with abundant catalytic sites, which can effectively prevent particle aggregation, and their structures are easy to be modified; furthermore, higher catalytic activity and selectivity can be achieved by modulating their structural compositions [111,112].Niu et al. [113] embedded octahedral

Heterojunction of 2D/2D
The unique atomic-scale thickness of 2D materials shortens the carrier migration distance and reduces the photogenerated electron-hole recombination, and the large surface area provides plenty of binding sites for the reaction.A larger interfacial contact area can be formed when different 2D materials are combined; thus, more efficient photocatalysts are constructed [26,108].Zhou et al. [109] fabricated H 2 WO 4 /Cs 2 AgBiBr 6 (HWO/CABB) S-scheme heterojunction catalysts through the "top-down" acid cleavage of layered Bi 2 WO 6 (Bi 2 O 2 -WO 4 ) into 2D H 2 WO 4 (HWO) nanosheets approach, followed by in situ epitaxial growth of 2D Cs 2 AgBiBr 6 (CABB) (Figure 9a).This catalyst has abundant Br vacancies (V Br ) on the surface, which lowers the formation energy barrier of CHO* and improves CO 2 adsorption and activation.The unique butterfly-shaped carbon nitride/zinc-doped bismuth vanadium oxide (CN-ZnBVO) catalysts were prepared by Li et al. [110], with a maximum CH 3 OH generation rate of 609.1 µmol g −1 h −1 , and a selectivity of up to 90.5% in the photocatalytic reduction of CO 2 to CH 3 OH under UV-visible light (Figure 9b).
With a more open outer surface, which could provide abundant accessible active sites, the CN-ZnBVO photocatalyst enhanced the CO 2 adsorption capacity, and had an excellent photocatalytic efficiency.
As organic porous materials, the covalent organic framework (COF) and metal organic framework (MOF) are excellent photocatalytic materials with abundant catalytic sites, which can effectively prevent particle aggregation, and their structures are easy to be modified; furthermore, higher catalytic activity and selectivity can be achieved by modulating their structural compositions [111,112].Niu et al. [113] embedded octahedral NH 2 −UiO-66 (Zr) (NUZ) on the surface of an olefin (C=C)-linked covalent organic framework (TTCOF) to conduct visible-light-driven CO 2 photoreduction (Figure 9c).The CO yield in the gas-solid system was 6.56 µmol g −1 h −1 when only H 2 O (g) was used as a weak reducing agent under visible light.EPR results indicated that the visible-light-induced electron transfer path in the TTCOF/NUZ followed the S-scheme mechanism.The holes are retained in the VB of NUZ, and the electrons are transferred to the CB of the TTCOF.The useless carriers were rapidly recombined during the S-scheme transfer pathway, while carriers with an excellent redox capacity were retained.Zhao et al. [114] prepared S-scheme heterojunctions of zinc porphyrin-based metal organic frameworks/BiVO 4 nanosheets (Zn-MOF/BVON) (Figure 9d).The two-dimensional zinc porphyrin-based Zn-MOF has the advantages of both the two-dimensional MOF and porphyrin, with a wide visible light absorption range.The Zn-MOF BVON (20 wt%) heterojunction is more favorable for CO 2 adsorption, while the well-dispersed zinc-aluminum nodes Zn 2 (COO) 4 in the Zn-MOF facilitate the activation of CO 2 .Compared to the previously reported BiVO 4 nanosheets (~15 nm), the optimal heterojunction showed a 22-fold increase in photoactivity, even over the conventional g-C 3 N 4 /BiVO 4 heterojunction by ~2-fold.
Catalysts 2024, 14, x FOR PEER REVIEW 14 of 22 NH2−UiO-66 (Zr) (NUZ) on the surface of an olefin (C=C)-linked covalent organic framework (TTCOF) to conduct visible-light-driven CO2 photoreduction (Figure 9c).The CO yield in the gas-solid system was 6.56 µmol g −1 h −1 when only H2O (g) was used as a weak reducing agent under visible light.EPR results indicated that the visible-lightinduced electron transfer path in the TTCOF/NUZ followed the S-scheme mechanism.The holes are retained in the VB of NUZ, and the electrons are transferred to the CB of the TTCOF.The useless carriers were rapidly recombined during the S-scheme transfer pathway, while carriers with an excellent redox capacity were retained.Zhao et al. [114] prepared S-scheme heterojunctions of zinc porphyrin-based metal organic frameworks/BiVO4 nanosheets (Zn-MOF/BVON) (Figure 9d).The two-dimensional zinc porphyrin-based Zn-MOF has the advantages of both the two-dimensional MOF and porphyrin, with a wide visible light absorption range.The Zn-MOF BVON (20 wt%) heterojunction is more favorable for CO2 adsorption, while the well-dispersed zincaluminum nodes Zn2(COO)4 in the Zn-MOF facilitate the activation of CO2.Compared to the previously reported BiVO4 nanosheets (~15 nm), the optimal heterojunction showed a 22-fold increase in photoactivity, even over the conventional g-C3N4/BiVO4 heterojunction by ~2-fold.

Heterojunction of 2D/3D
Three-dimensional structures have higher light absorption efficiency due to multiple reflections and scattering, as well as higher carrier separation efficiency [115].Therefore, the development of S-scheme heterojunctions by combining 2D and 3D materials is regarded as an important research topic.Qaraah et al. [58] prepared hierarchical S-scheme heterojunctions (OCNNb) using different ratios of O-doped g-C3N4 (OCN) and N-doped Nb2O5 (NNBO) (Figure 10a).Firstly, 2D OCN nanosheets were synthesized by one-step polymerization, then 3D NNBO nanoflowers were prepared using the hydrothermal

Heterojunction of 2D/3D
Three-dimensional structures have higher light absorption efficiency due to multiple reflections and scattering, as well as higher carrier separation efficiency [115].Therefore, the development of S-scheme heterojunctions by combining 2D and 3D materials is regarded as an important research topic.Qaraah et al. [58] prepared hierarchical S-scheme heterojunctions (OCNNb) using different ratios of O-doped g-C 3 N 4 (OCN) and N-doped Nb 2 O 5 (NNBO) (Figure 10a).Firstly, 2D OCN nanosheets were synthesized by one-step polymerization, then 3D NNBO nanoflowers were prepared using the hydrothermal method, and finally, NNBO was adhered to the surface of OCN nanosheets by electrostatic selfassembly.The OCN was tightly covered on the surface of NNBO, which showed a kind of smooth, gauze-like shape.This hierarchical S-scheme heterostructure improved the catalytic efficiency, and the CO and CH 4 yields of OCNNb were 253.34 µmol g −1 h −1 and 68.11 µmol g −1 h −1 , respectively.Han et al. also designed a hierarchical S-scheme semiconductor photocatalyst [116].CeO 2 was coated on SiO 2 spheres, and the NiCo-MOF nanosheets were grown in situ, followed by a selenization reaction and etching, to finally obtain the CeO 2 @Ni 1-x Co x Se 2 hollow sphere photocatalyst with oxygen-rich vacancies (Figure 10b).The tight interface between the two nanoshell layers allows for rapid charge transfer, and the formation of S-scheme heterojunction retains photogenerated electrons with high reduction capacity.Hierarchically structured photocatalysts have the advantages of high specific surface area, higher surface accessibility, and better light reflection and scattering.The hierarchical S-scheme heterojunction photocatalysts can have superior catalytic performance.Zhao et al. [117] constructed 3D/2D S-scheme heterojunctions by embedding the Fe-MOF in bipyramidal morphology into a 2D wrinkled porous structure of carbon nitride (Figure 10c).The porous nanosheet structure of C 3 N 4 allows for a larger specific surface area of the C 3 N 4 /Fe-MOF, which serves as an active site to improve CO 2 adsorption capacity.The response of the Fe-MOF in the visible region improves the light-harvesting ability, and there are more non-radiative transfers of long photogenerated charges due to the S-scheme charge transfer mechanism.The in situ FTIR results demonstrated that the C 3 N 4 /Fe-MOF could produce more HCOO − intermediates during the catalytic process, which promoted CO production, and the C 3 N 4 /Fe-MOF showed the highest CO yield (19.17 µmol g −1 ), which was almost 10 times higher than that of C 3 N 4 .
method, and finally, NNBO was adhered to the surface of OCN nanosheets by electrostatic self-assembly.The OCN was tightly covered on the surface of NNBO, which showed a kind of smooth, gauze-like shape.This hierarchical S-scheme heterostructure improved the catalytic efficiency, and the CO and CH4 yields of OCNNb were 253.34 µmol g −1 h −1 and 68.11 µmol g −1 h −1 , respectively.Han et al. also designed a hierarchical S-scheme semiconductor photocatalyst [116].CeO2 was coated on SiO2 spheres, and the NiCo-MOF nanosheets were grown in situ, followed by a selenization reaction and etching, to finally obtain the CeO2@Ni1-xCoxSe2 hollow sphere photocatalyst with oxygen-rich vacancies (Figure 10b).The tight interface between the two nanoshell layers allows for rapid charge transfer, and the formation of S-scheme heterojunction retains photogenerated electrons with high reduction capacity.Hierarchically structured photocatalysts have the advantages of high specific surface area, higher surface accessibility, and better light reflection and scattering.The hierarchical S-scheme heterojunction photocatalysts can have superior catalytic performance.Zhao et al. [117] constructed 3D/2D S-scheme heterojunctions by embedding the Fe-MOF in bipyramidal morphology into a 2D wrinkled porous structure of carbon nitride (Figure 10c).The porous nanosheet structure of C3N4 allows for a larger specific surface area of the C3N4/Fe-MOF, which serves as an active site to improve CO2 adsorption capacity.The response of the Fe-MOF in the visible region improves the light-harvesting ability, and there are more non-radiative transfers of long photogenerated charges due to the S-scheme charge transfer mechanism.The in situ FTIR results demonstrated that the C3N4/Fe-MOF could produce more HCOO − intermediates during the catalytic process, which promoted CO production, and the C3N4/Fe-MOF showed the highest CO yield (19.17 µmol g −1 ), which was almost 10 times higher than that of C3N4.Subsequently, S-scheme heterojunctions for CO2 reduction reported in recent years were summarized in Table 3.  Subsequently, S-scheme heterojunctions for CO 2 reduction reported in recent years were summarized in Table 3.
The choice of S-scheme heterojunction is mainly focused on n-type semiconductors, which limits the development of S-scheme heterojunctions.There is an urgent need to explore a variety of materials to expand the range of S-scheme heterojunction materials.The MOF has the advantages of a controllable structure, large specific surface area, high porosity, and a ligand-metal charge transfer pathway.It is necessary to explore the structure-activity relationship between the structure and performance of MOFs for enhancing the photocatalytic performance.Practicality and cost feasibility need to be considered in the preparation of MOFs, and metal ions and organic ligands with low cost and good stability are selected, which provides a possibility for the commercialization of efficient photocatalysts with S-schemes.The MOF-on-MOF, MOF-COF, and series-connected MOFsbased double S-scheme heterojunctions have been reported as being less up-to-date, which can be explored more deeply to provide new routes for highly effective photocatalysts in CO 2 reduction.

Figure 1 .
Figure 1.(a) Schematic diagram of photocatalytic CO 2 reduction process; schematic illustrations of an S-scheme heterojunction: (b) before contact; (c) after contact; and (d) photogenerated carrier transfer under light irradiation.

Figure 4 .
Figure 4. (a) XPS spectra for BNO, UCN, and BNO/UCN-3 in the dark or under light irradiation [73]; Copyright © 2022 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences.Published by Elsevier B.V. All rights reserved.(b) The line-scanning surface potential from point A to B and the schematic illustration of photoirradiation KPFM [75]; (c) Corresponding surface potential distribution of PT/CdS [76]; Copyright © 2021 Wiley-VCH GmbH.(d) EPR spectra of DMPO−•OH in aqueous dispersions, and DMPO−•O 2 − in methanol dispersions of WO 3 , ZnIn 2 S 4 , and WZ25 [79]; Copyright © 2022 Royal Society of Chemistry.

Figure 10 .
Figure 10.(a) The diagram depicts the 3D S-scheme OCNNb heterostructures production process [58]; Copyright © 2022 Elsevier B.V. All rights reserved.(b) Schematic illustration of the synthesis process of CeO 2 @Ni 1−x Co x Se 2 hollow spheres [116]; Copyright © 2023, American Chemical Society.(c) Schematic of synthesis of C 3 N 4 /Fe-MOF [117]; Copyright © 2022 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences.Published by Elsevier B.V. All rights reserved.

Table 1 .
The advantages and disadvantages of different types of heterojunctions.

Table 2 .
The preparation methods of S-scheme photocatalysts.

Table 2 .
The preparation methods of S-scheme photocatalysts.