2D/2D S-scheme heterojunction photocatalysts: fundamentals, engineering modification strategies, and applications

Abstract

Two-dimensional/two-dimensional (2D/2D) S-scheme heterojunctions provide an effective route to combining efficient charge separation with strong redox capability in photocatalysis. Their planar interfaces shorten carrier transport distances, strengthen interfacial coupling, and facilitate the formation of internal electric fields. Meanwhile, the S-scheme pathway promotes the recombination of low-energy carriers while preserving highly reductive electrons and strongly oxidative holes. This review summarizes recent advances in 2D/2D S-scheme photocatalysts, emphasizing band alignment, interfacial charge transfer, structural advantages, and material selection. Major construction routes, including mixing-assisted assembly, surface chemical regulation, and in-situ growth, are compared in terms of interface formation and coupling strength. The discussion then examines how microscopy, spectroscopy, surface potential mapping, transient spectroscopy, and theoretical calculations resolve interfacial structures and verify charge transfer pathways. Recent advances in interfacial bonding, multidimensional coupling, doping, defect engineering, cocatalyst integration, single atom engineering, strain modulation, and facet control are further assessed for their effects on carrier dynamics, redox activity, and surface reaction kinetics. Finally, representative applications in H₂ evolution, CO₂ reduction, pollutant degradation, and H₂O₂ production are surveyed, and the review concludes by identifying key challenges and outlining future priorities for rational interface design.

Graphical abstract

Keywords

2D/2D S-scheme heterojunction / photocatalysis / environmental purification / interfacial engineering / energy conversion

1. Introduction

With the rapid development of society and the increasing demand for sustainable energy, energy shortages and environmental pollution have become two pressing global challenges.^[1,2] In this context, semiconductor photocatalysis has attracted considerable attention as a promising strategy for solar-energy conversion and environmental remediation.^[2-5] This technology is not only capable of converting solar energy into utilizable fuels, ^[6-8] but also achieves efficient pollutant degradation and waste reduction.^[9,10] Owing to its dual potential in energy conversion and environmental purification, photocatalysis has emerged as an important frontier in contemporary research.^[11,12]

The overall efficiency of semiconductor photocatalysis is governed by several key processes, including light harvesting, charge-carrier separation and transport, surface reaction kinetics, and the thermodynamic driving force for redox reactions. ^[1,13,14] However, single-component photocatalysts often fail to optimize these factors simultaneously. In particular, extending the light-absorption range generally requires a narrow band gap, whereas maintaining strong redox power demands a more negative conduction-band (CB) potential and a more positive valence-band (VB) potential, which are typically associated with a wider band gap.^[15,16] In addition, the rapid recombination of photogenerated electrons and holes further limits charge utilization and overall photocatalytic efficiency.^[17,18] Therefore, developing rational photocatalyst architectures that can simultaneously promote charge separation and preserve sufficient redox driving force remains a central challenge in this field.^[19-21]

Constructing semiconductor heterojunctions has been widely regarded as an effective strategy to overcome the intrinsic limitations of single-component photocatalysts. ^[12,22,23] Nevertheless, conventional heterojunction systems still suffer from several inherent drawbacks. Type-II heterojunctions can promote spatial charge separation, but this charge-transfer pathway usually occurs at the expense of redox capability.^[24,25] Traditional Z-scheme heterojunctions can retain high-energy charge carriers for surface redox reactions; however, their practical implementation is frequently restricted by complicated structural design, possible interfacial transport barriers, and difficulties in unambiguously verifying the charge-transfer pathway.^[26,27] These issues have stimulated the development of more advanced heterojunction models capable of integrating efficient charge separation with strong redox capacity.

In this context, S-scheme heterojunctions have emerged as an advanced charge-transfer model for high-efficiency photocatalysis.^[28-30] By coupling a reduction photocatalyst with an oxidation photocatalyst, S-scheme systems provide a promising route to reconcile kinetically favorable charge separation with thermodynamically favorable redox reactions.^[31,32] Since the performance of S-scheme systems is highly dependent on interfacial charge transfer, the structural configuration and interfacial quality of the coupled components are crucial for realizing their full photocatalytic potential. Accordingly, 2D/2D architectures have attracted increasing attention as ideal platforms for constructing S-scheme heterojunctions, because their extended face-to-face contact provides a favorable structural basis for strong interfacial coupling and efficient charge transfer. ^[33-35]

As illustrated in Fig. 1 a, the bibliometric analysis identifies interfacial engineering, charge-transfer regulation, and photocatalytic applications as the major research hotspots in the development of 2D/2D S-scheme heterojunction photocatalysts. Fig. 1 b shows a steady increase in the number of related publications over the past decade, indicating the growing research interest in this emerging field. Meanwhile, Fig. 1 c presents the chronological evolution of representative 2D/2D S-scheme systems, highlighting the continuous diversification of material systems and photocatalytic applications. Collectively, these trends demonstrate that 2D/2D S-scheme heterojunctions have become an important and rapidly evolving research direction in semiconductor photocatalysis.

Despite these advantages, several critical challenges remain in the practical development of 2D/2D S-scheme heterojunction photocatalysts. ^[36,37] First, the controllable fabrication of stable, intimate, and low-defect 2D/2D interfacial structures remains a major bottleneck, thereby limiting interfacial charge transport and structural/catalytic stability. Second, definitive verification of the S-scheme charge-transfer mechanism remains challenging, because enhanced photocatalytic activity alone cannot provide direct evidence for the proposed charge-migration pathway. Third, the intrinsic relationships among material selection, interfacial configuration, built-in electric field, charge-transfer behavior, and photocatalytic performance have not yet been fully elucidated. Furthermore, generally accepted evaluation criteria and systematic design principles are still lacking, which hinders the further development of this emerging field.

In this review, recent advances in 2D/2D S-scheme heterojunction photocatalysts are systematically summarized, with particular emphasis on their design principles, interfacial construction, mechanism verification, performance regulatory characteristics, charge-transfer mechanisms, design criteria, and representative material systems. Subsequently, synthetic strategies for constructing high-quality 2D/2D interfaces are summarized, followed by a discussion of key characterization techniques for identifying interfacial structures and verifying S-scheme charge-transfer pathways. Recent performance-enhancement strategies are then reviewed, with emphasis on their roles in regulating charge transport, redox ability, and surface reaction kinetics. Representative photocatalytic applications, including H₂ evolution, CO₂ reduction, pollutant degradation, and H₂O₂ production, are further highlighted. Finally, current challenges and future perspectives are discussed to provide guidance for the rational design and further development of high-performance 2D/2D S-scheme heterojunction photocatalysts.

2. Fundamentals of S-scheme heterojunctions

The rational design of 2D/2D S-scheme heterojunction photocatalysts requires a systematic understanding of photocatalytic fundamentals, interfacial charge-transfer mechanisms, structural characteristics, and material-selection principles. The photocatalytic performance of these systems is jointly governed by the intrinsic properties of the coupled semiconductors, band alignment, Fermi-level difference, interfacial contact, built-in electric field, and surface redox kinetics. Accordingly, this section summarizes the evolution from conventional heterojunctions to S-scheme systems, the S-scheme charge-transfer mechanism, the structural advantages of 2D/2D architectures, and the key criteria for OP/RP selection.

2.1 Mechanism of heterogeneous photocatalysis

As shown in Fig.2, semiconductor photocatalysis generally proceeds through three fundamental steps ^[47,48]: light absorption, charge-carrier separation and migration, and surface redox reactions. First, when the incident photon energy is equal to or greater than the band gap of a semiconductor, electrons are excited from the valence band (VB) to the conduction band (CB), leaving holes in the VB. Second, the photogenerated electrons and holes are separated and migrate from the bulk to the surface or reactive sites; during this process, bulk or surface recombination may occur, leading to energy loss and reduced quantum efficiency. Third, the electrons and holes that reach the surface participate in reduction and oxidation reactions, respectively. Specifically, electrons can reduce electron acceptors adsorbed on or near the catalyst surface, such as protons, CO₂, O₂, or other reducible species, whereas holes can oxidize water, hydroxide ions, organic molecules, or pollutants.^[13] Therefore, efficient photocatalysis requires sufficient light-harvesting ability, effective charge separation, rapid carrier transport, accessible surface active sites, favorable surface reaction kinetics, and appropriate band-edge potentials for the target redox reactions.^[18]

However, single-component photocatalysts usually struggle to satisfy these requirements simultaneously. A narrow band gap can enhance light absorption but often compromises redox ability, whereas strong reduction and oxidation abilities generally require a sufficiently negative CB potential and a sufficiently positive VB potential, which are usually associated with a wider band gap.^[15,49]Moreover, rapid recombination of photogenerated electrons and holes further limits charge utilization.^[17] These intrinsic limitations have motivated the construction of semiconductor heterojunctions to improve charge separation and photocatalytic performance.

2.2 Evolution and charge-transfer mechanism of S-scheme heterojunctions

Composite photocatalysts were initially interpreted mainly within the framework of the Type-II heterojunction model (Fig.3 a).^[25] In a typical Type-II system, upon photoexcitation of both semiconductor components, photogenerated electrons migrate from the semiconductor with a more negative conduction-band potential to its counterpart with a less negative conduction-band potential, whereas holes transfer in the opposite direction, from the semiconductor with a more positive valence-band potential to that with a less positive valence-band potential.^[18] Such a staggered band configuration promotes the spatial separation of electrons and holes and thereby suppresses charge recombination from a kinetic perspective. However, from a thermodynamic viewpoint, this charge-transfer pathway drives electrons and holes to band positions with diminished reduction and oxidation potentials, respectively.^[37] As a result, Type-II heterojunctions intrinsically suffer from a trade-off between efficient charge separation and strong redox capability.^[50]

To address this dilemma, liquid-phase (Fig.3 c) and all-solid-state Z-scheme (Fig.3 b) heterojunctions were developed to retain strongly reductive electrons and highly oxidative holes for surface redox reactions. ^[51,52] In principle, the Z-scheme configuration enables the recombination of relatively low-energy electrons and holes while preserving the most energetic charge carriers. Nevertheless, traditional Z-scheme systems remain limited by several factors, including their dependence on redox mediators, possible light-shielding effects, high interfacial charge-transport barriers, complicated structural design, and ambiguous interpretation of the charge-transfer pathway. ^[2,53] These limitations stimulated the development of a more rational heterojunction model capable of simultaneously achieving favorable charge-separation kinetics and retained redox thermodynamics.

In this context, Yu et al. proposed the S-scheme heterojunction concept in 2019.^[28] An S-scheme heterojunction is generally constructed by coupling a reduction photocatalyst (RP) with an oxidation photocatalyst (OP).^[18] Typically, the RP possesses a higher Fermi level and a more negative conduction-band potential, whereas the OP exhibits a lower Fermi level and a more positive valence-band potential. This energetic asymmetry provides the fundamental prerequisite for the formation of an S-scheme charge-transfer pathway. Unlike the Type-II model, the S-scheme mechanism is not a simple downhill transfer of photogenerated carriers toward lower-energy band positions. Instead, it selectively removes thermodynamically less favorable charge carriers while retaining highly reductive electrons and strongly oxidative holes for photocatalytic reactions. ^[54,55]

As schematically shown in Fig. 4, when the RP and OP come into intimate contact, electrons spontaneously transfer from the RP to the OP because of the higher Fermi level of the RP.^[56] This interfacial electron redistribution continues until Fermi-level equilibration is reached, resulting in electron depletion and positive charge accumulation on the RP side, as well as electron enrichment and negative charge accumulation on the OP side.^[57] Consequently, the energy bands of the RP bend upward, whereas those of the OP bend downward.^[58] Meanwhile, an internal electric field directed from the RP to the OP is established across the heterointerface. Therefore, Fermi-level equilibration, interfacial charge redistribution, band bending, and internal electric-field formation collectively constitute the thermodynamic basis of the S-scheme charge-transfer mechanism.^[59]

Under light irradiation, both the RP and OP generate electron-hole pairs. Driven by the internal electric field, interfacial electrostatic interaction, and band bending, electrons in the conduction band of the OP preferentially migrate toward the interface and recombine with holes in the valence band of the RP.^[36] In contrast, the transfer of highly reductive electrons from the conduction band of the RP to the OP is suppressed, while the migration of strongly oxidative holes from the valence band of the OP to the RP is also inhibited.^[56] As a result, highly reductive electrons are retained in the conduction band of the RP, whereas strongly oxidative holes remain in the valence band of the OP.^[60] This selective interfacial recombination pathway eliminates charge carriers with relatively weak redox ability and spatially separates the energetic electrons and holes required for surface reduction and oxidation reactions.^[61,62]

Thus, the heterointerface is central to the S-scheme mechanism, governing charge redistribution, internal electric-field formation, directional carrier migration, and selective recombination of low-energy carriers. The requirement for intimate, clean, and strongly coupled interfaces makes 2D/2D architectures particularly suitable for constructing high-performance S-scheme photocatalysts.

2.3 Structural advantages of 2D/2D architectures

Since its proposal, the S-scheme heterojunction has attracted increasing attention and has been widely applied in various photocatalytic systems owing to its ability to integrate efficient charge separation with strong redox capability.^[10,63] In particular, S-scheme heterojunctions based on two-dimensional (2D) materials are highly attractive because their large specific surface areas, ultrathin layered structures, and abundant exposed active sites favor interfacial charge transfer, photogenerated-carrier utilization, and surface redox reactions.^[2,64,65] The large surface area of 2D materials promotes intimate contact between the oxidation photocatalyst and reduction photocatalyst, thereby facilitating interfacial charge redistribution, internal electric-field formation, and directional carrier migration.^[66,67] Moreover, their ultrathin structure shortens the transport distance of photogenerated carriers to the surface or heterointerface, suppressing bulk recombination and improving charge-utilization efficiency.^[45,68] These advantages make 2D materials suitable building blocks for constructing efficient S-scheme photocatalysts.^[69]

S-scheme heterojunctions containing 2D components can generally be categorized into 0D/2D^[70,71], 1D/ 2D^[72,73], and 2D/2D^[74,75] configurations, as shown in Fig. 5 Compared with point-contact 0D/2D and line-contact 1D/2D heterojunctions, 2D/2D architectures enable extended face-to-face coupling between two nanosheet components, thereby creating enlarged and continuous interfacial contact areas.^[36,76] Such an interface facilitates Fermi-level equilibration, interfacial charge redistribution, and internal electric-field formation, which are essential for S-scheme charge transfer. Furthermore, the face-to-face interface in 2D/2D heterojunctions provides abundant charge-transfer channels, reduces interfacial migration resistance, and shortens the carrier diffusion distance from the bulk phase to the interface.^[2,77] These features promote directional charge migration, suppress nonselective recombination losses, and favor selective recombination of low-energy carriers at the interface. Consequently, 2D/2D architectures enable more efficient charge separation, improved light utilization, and enhanced synergistic effects within heterojunction systems.^[59,78]

2.4 Design principles of 2D/2D S-scheme heterojunctions

Based on the above mechanistic and structural considerations, the rational design of 2D/2D S-scheme heterojunctions should focus on three key aspects: appropriate band alignment, high-quality interfacial coupling, and functional complementarity between the two components. In general, an S-scheme heterojunction is composed of a reduction photocatalyst (RP) and an oxidation photocatalyst (OP).^[18] The conduction-band minimum and Fermi level of the RP should be higher than those of the OP, which represents the primary energetic criterion for establishing an S-scheme charge-transfer pathway.^[59] Once this requirement is met, the conductivity types of the two semiconductors are not strictly constrained, ^[56] allowing the construction of n-n, p-p, p-n, and n-p S-scheme heterojunctions.^[79,80] This flexibility considerably broadens the material scope for designing 2D/2D S-scheme photocatalysts.

Functional complementarity between the RP and OP should also be considered during material selection.^[81] The RP is expected to retain highly reductive electrons and mainly drive reduction reactions, such as H₂ evolution, CO₂ reduction, and N₂ fixation. In contrast, the OP should preserve strongly oxidative holes for oxidation reactions, including water oxidation, organic oxidation, and pollutant degradation.^[82] Therefore, the selection of OP/RP pairs should not rely solely on band-edge positions, but should also consider light-harvesting ability, carrier mobility, surface catalytic activity, chemical stability, and compatibility with the target reaction.

2.5 Representative OP/RP materials and band-structure mapping

The practical implementation of these design criteria ultimately depends on the rational selection of semiconductor pairs with matched electronic structures and complementary redox functions. Among the relevant parameters, band-edge positions provide the most direct indication of whether a given OP/RP combination can establish the required energetic alignment, whereas surface chemistry, catalytic activity, and reaction compatibility determine its practical feasibility under specific photocatalytic conditions. Therefore, a systematic comparison of representative OP and RP photocatalysts in terms of their band structures is essential for guiding the rational construction of 2D/2D S-scheme heterojunctions.

Based on these principles, a broad range of semiconductors have been employed to construct S-scheme heterojunctions, ranging from conventional metal oxides, such as TiO₂ and WO₃, ^[85,86] to emerging semiconductor platforms, including C₃N₄, ^[87] metal-organic frameworks (MOFs), ^[65,88] covalent organic frameworks (COFs), ^[89] and perovskites. From a functional perspective, materials with relatively positive valence-band potentials, such as WO₃, BiVO₄, TiO₂, BiOCl, α-Fe₂O₃, are commonly considered as OP candidates because they are favorable for generating strongly oxidative holes.^[90] By contrast, materials with relatively negative conduction-band potentials, such as CdS, ZnIn₂S₄, g-C₃N₄, C₃N₅, and selected MOFs or COFs, are generally suitable as RP candidates because they can retain highly reductive electrons (Fig. 6). ^[83,84,91] Nevertheless, the role of a given semiconductor as an OP or RP is not absolute, but depends on its band position relative to its counterpart, Fermi-level difference, interfacial configuration, and reaction environment.

3. Interfacial construction strategies for 2D/2D S-scheme heterojunctions

Constructing high-quality 2D/2D interfaces is a prerequisite for achieving efficient S-scheme photocatalytic performance.^[36] The interfacial coupling strength between two 2D materials directly affects charge-transfer efficiency, interfacial stability, and overall catalytic activity. According to the dominant interfacial formation mode, the stage at which the interface is generated, and the resulting interfacial coupling strength, the commonly used construction strategies for 2D/2D S-scheme heterojunctions can be classified into three categories: mixing-assisted assembly, surface chemical regulation and bonding, and in-situ growth.

3.1 Mixing-assisted assembly

Mixing-assisted assembly represents one of the most fundamental and widely employed approaches for constructing 2D/2D heterostructures. In this strategy, two preformed 2D components are integrated through post-synthetic processes such as stirring, sonication, grinding, mechanical blending, or dispersion processing. ^[92,93] The formation of 2D/2D heterointerfaces primarily depends on the processing conditions and the intrinsic physicochemical properties of the constituent materials.

During the mixing process, external physical forces facilitate the dispersion, exfoliation, and spatial proximity of the two 2D components, thereby increasing the probability of interfacial contact. The subsequent evolution of such contact into an effective heterointerface is largely governed by inherent intercomponent interactions, including electrostatic attraction, van der Waals forces, surface-energy matching, hydrophilic/hydrophobic interactions, hydrogen bonding, and π-π stacking interactions.^[94,95] In this context, “self-assembly” should not be considered an independent synthesis route separate from mixing-assisted assembly, but rather an interaction-driven interfacial organization process occurring during post-synthetic mixing. Thus, mixing-assisted assembly can be understood as a broad operational strategy, whereas self-assembly describes the interfacial driving force that enhances the directionality, intimacy, and stability of contact in certain systems.

WO₃/g-C₃N₄^[28] is a representative example of electrostatic interaction-assisted mixing assembly. By tuning the pH of the suspension, Fu et al. induced opposite zeta potentials (Fig. 7 b) on WO₃ and g-C₃N₄, thereby enabling electrostatic attraction to serve as the primary driving force for large-area face-to-face stacking between the two ultrathin nanosheets. This strategy led to the construction of a composite heterostructure with intimate interfacial contact. The TpPa-1-COF/g-C₃N₄^[96] system further extends this strategy to the field of all-organic semiconductors, where interfacial assembly mainly relies on π-π stacking interactions between two-dimensional organic frameworks. TEM (Fig. 7 g) morphological observations and FT-IR (Fig.7 e) characterization confirmed that the two components were effectively integrated while preserving the structural integrity of their respective frameworks, thus providing a structural basis for the subsequent establishment of an S-scheme charge-transfer pathway.

3.2 Interfacial construction via surface chemical regulation

In contrast to mixing strategies, which largely rely on the inherent surface properties of the constituent materials, interfacial engineering based on surface chemical regulation provides a more powerful platform for rational interface design. By means of surface functionalization, incorporation of specific functional groups, elemental doping, and modulation of the local electronic structure, the surface reactivity and interfacial interaction behavior of materials can be deliberately tailored. ^[97,98,99] Consequently, heterointerface formation shifts from passive compatibility between components to directed assembly governed by predesigned interfacial interactions. This evolution enables interfacial regulation to move beyond the adjustment of external assembly conditions toward the precise control of intrinsic interfacial driving forces, thereby substantially enhancing the directionality, stability, and electronic coupling of heterointerfaces.^[100]

In recent studies, Chen et al. ^[97] constructed a two-dimensional/two-dimensional (2D/2D) porous graphitic carbon nitride (Pg-C₃N₄)/CdS-diethylenetriamine (DETA) organic-inorganic composite photocatalytic system. The introduction of DETA groups onto CdS nanosheets simultaneously modulated their crystal growth behavior and electronic band structure, while imparting a positively charged surface. These changes promoted intimate interfacial contact with Pg-C₃N₄ nanosheets and facilitated the formation of an S-scheme heterojunction. The resulting built-in electric field drives the recombination of electrons in the conduction band of CdS with holes in the valence band of Pg-C₃N₄, thereby preserving strongly reducing electrons and oxidizing holes on opposite sides of the interface. This charge-transfer pathway effectively suppresses electron-hole recombination and substantially enhances photocatalytic H₂ evolution. Similarly, the NH₂-TiO₂/ReS₂^[99] system exemplifies this strategy. Functionalization of the TiO₂ surface with APTMS reverses its surface charge from negative to positive, thereby enabling efficient assembly with negatively charged ReS₂ through electrostatic attraction. More importantly, APTMS serves not only as a surface charge regulator but also as a molecular bridge between the two phases, strengthening the interfacial interaction from simple electrostatic adsorption to more stable bridge-mediated coupling. Differential charge density analysis (Fig. 8 f, g) further reveals pronounced charge redistribution at the Si and N sites of the NH₂-TiO₂/ReS₂ interface, corroborating the formation of Si-O and Re-N bridging interactions and the markedly enhanced interfacial coupling. This case demonstrates that surface chemical regulation can actively tailor surface properties and interfacial affinity, allowing components with inherently weak assembly tendencies to form stable and strongly coupled heterojunctions.

Furthermore, in the protonated g-C₃N₄/ZnIn₂S₄^[101] system, surface chemical regulation enables the synergistic optimization of both interfacial structure and electronic behavior through a protonation strategy. Under acidic conditions, protonated CN-H⁺ species are generated on the surface of g-C₃N₄, which modify the local charge distribution (Fig. 8 b, c) and provide coordination sites for the oriented growth of ZnIn₂S₄. As a result, a 2D/2D heterostructure with intimate interfacial contact is established. Meanwhile, the built-in electric field at the interface promotes charge separation and migration, while the optimized interfacial electronic structure lowers the reaction energy barrier, collectively leading to markedly improved photocatalytic CO₂ reduction performance. This system underscores that surface chemical regulation can simultaneously tailor interfacial binding configurations, growth behavior, and electronic structure, thereby enabling the synergistic design of heterojunction structure and function.

3.3 In-situ growth-dominated interfacial construction

Unlike mixing or surface chemical modification, which primarily improve interfacial contact after the individual components have already formed, in-situ growth-driven interfacial engineering moves the regulation of heterointerfaces to the nucleation and growth stages of the constituent materials.^[57] In this strategy, one two-dimensional material typically serves as a substrate or structure-directing scaffold, while the other component is generated directly on its surface through heterogeneous nucleation, followed by subsequent growth or lateral spreading.^[59] As a result, the heterointerface is constructed synchronously with material formation.^[100] Compared with post-synthetic hybridization, this strategy is more conducive to the formation of large-area, continuous, and intimate interfacial contact, thereby minimizing random stacking, local lattice or structural mismatch, and poor interfacial integration.^[102] Therefore, the significance of in-situ growth lies not merely in establishing physical contact, but more importantly in enhancing interfacial continuity, strengthening interfacial coupling, and synergistically optimizing the electronic structure through regulation of the nucleation and growth processes.

CdS/g-C₃N₄^[103] system is a representative example of a 2D/2D S-scheme heterointerface constructed via in-situ growth. In this system, g-C₃N₄ acts as a two-dimensional substrate and provides a reactive surface for CdS formation under hydrothermal conditions. Studies have shown that Cd²⁺ ions can accumulate on the surface of g-C₃N₄ and subsequently react with the generated S²⁻ species, leading to the in-situ growth of CdS nanosheets and the formation of an intimate heterointerface. Compared with routes in which the two components are synthesized separately and then physically combined, the interface in this system is established concurrently with material generation, generally resulting in higher interfacial continuity. This example clearly demonstrates the essential characteristic of the in-situ growth strategy: the heterointerface originates from the in-situ generation of the second phase rather than from post-synthetic physical assembly.

On this basis, the g-C₃N₄—ZnBVO (CN-ZnBVO)^[105] system further illustrates the importance of nucleation control in the in-situ growth process. In this system, CN nanosheets act as the two-dimensional substrate, whereas ZnBVO is formed under solvothermal conditions, with its nucleation and growth mediated by the CTAB surfactant. The study suggests that CTAB facilitates the formation of charged intermediates, such as BiCl⁴⁻-CTA⁺, which provide favorable nucleation sites and thereby promote the heterogeneous nucleation of ZnBVO on the CN surface, followed by the growth of ZnBVO nanostructures. As a result, ZnBVO nanosheets are firmly anchored onto the CN surface, yielding a 2D/2D heterostructure with intimate interfacial contact, as shown in the TEM image in (Fig.9 f). In contrast to the CdS/g-C₃N₄ system, this example more clearly reflects the regulation of nucleation through manipulation of the reaction environment, particularly the surfactant and precursor species, thereby improving interfacial uniformity and interfacial integration.

To further illustrate the potential for the synergistic construction of multiple interfaces, the MX-CdS/WO₃^[104] system represents a more sophisticated example. In this system, two-dimensional WO₃ and Ti₃C₂ MXene nanosheets serve as a composite substrate, on which CdS nanostructures are subsequently grown in-situ via a hydrothermal process, resulting in a hierarchical 2D/2D/2D multi-interfacial architecture, as shown in the TEM image in Fig. 9 (b, c). Within this configuration, an S-scheme heterojunction is established between CdS and WO₃, generating an internal electric field that facilitates directional charge separation, while an ohmic contact is formed between CdS and MXene, providing a low-resistance pathway for electron transport. The simultaneous formation of multiple interfaces within a single growth process enables the integration of distinct interfacial functions, thereby markedly enhancing charge separation and transport efficiency. This system demonstrates that the value of in-situ growth extends beyond the construction of a single high-quality interface; more importantly, it enables the synergistic integration of multiple components, multiple interfaces, and multiple functions during material formation.

4. Mechanistic characterization of 2D/2D S-scheme heterojunctions

A detailed understanding of directional interfacial charge transport is essential for validating the photocatalytic mechanism of 2D/2D S-scheme heterojunctions. To establish a robust mechanistic framework, this section summarizes multidimensional characterization strategies, including structural analysis of interfacial architectures, band-alignment evaluation, and dynamic tracking of charge-transfer processes. These approaches provide critical experimental evidence for identifying S-scheme charge-transfer pathways and assessing interfacial coupling strength in 2D/2D systems.

4.1 Structural characterization of interfacial architectures

The formation of an atomically intimate and continuous heterointerface is a prerequisite for efficient interfacial charge recombination in S-scheme systems. High-quality face-to-face contact enlarges the effective interfacial area, improves active-site accessibility, and shortens both exciton diffusion distances and charge-carrier migration pathways, thereby reducing interfacial contact resistance.^[57,106,107] Therefore, direct visualization of the geometric contact, interlayer stacking configuration, and spatial distribution of each component is essential for clarifying microscopic reaction mechanisms and verifying the establishment of S-scheme charge-transfer channels.

4.1.1 Lattice structure

Transmission electron microscopy (TEM) and related techniques are powerful tools for resolving interfacial morphology and atomic arrangements in heterojunctions. They enable direct visualization of lattice-fringe continuity, crystallographic orientation relationships, and interfacial structural coherence at the atomic scale.^[108] However, because electron-scattering contrast and beam sensitivity vary substantially among different material systems, characterization strategies should be adapted to sample crystallinity, scattering behavior, and structural stability under electron irradiation.^[109]

For highly crystalline systems with long-range order, TEM-based analysis should focus on identifying lattice matching, possible epitaxial relationships, and moiré fringes arising from lattice mismatch. Such information helps exclude interfacial amorphous layers, high-density dislocations, and other structural defects that may serve as charge-recombination centers.^[74,110,111] By contrast, weakly scattering systems often exhibit extremely low mass-thickness contrast in conventional bright-field TEM images, making interface identification difficult.^[109] In these cases, high-angle annular dark-field scanning TEM (HAADF-STEM) can enhance interfacial contrast through Z-contrast imaging. For beam-sensitive organic or hybrid interfaces, cryogenic electron microscopy (Cryo-EM) offers an advanced alternative by preserving native molecular configurations under near-intrinsic solvated or dispersed states.^[112] This strategy effectively reduces beam-induced damage and morphological distortion, enabling high-resolution imaging of fragile organic-organic interfaces down to the sub-angstrom scale.

4.1.2 Three-dimensional surface topography and interlayer structure

TEM images are two-dimensional projections of three-dimensional structures and therefore provide limited information along the out-of-plane direction. In this context, atomic force microscopy (AFM), with its sub-angstrom vertical resolution, is particularly useful for characterizing the surface topography and interlayer stacking features of 2D/2D heterojunctions.^[113] Height-profile analysis enables quantitative determination of the absolute thickness of ultrathin nanosheets. By comparing the thickness increment in heterojunction regions with those of the individual components, ^[114] AFM can verify whether a compact and ordered face-to-face stacking configuration has formed between different nanosheets.^[115] This evidence helps distinguish genuine 2D/2D interfacial integration from simple physical mixing or nonspecific aggregation.

For example, TEM (Fig. 10 d-f) characterization has demonstrated a face-to-face vertically stacked architecture in a BiVO₄/CsPbBr₃ heterojunction.^[40] The quasi-square BiVO₄ nanosheets, featuring well-defined two-dimensional surfaces, act as growth templates for CsPbBr₃. Benefiting from its high surface energy, CsPbBr₃ preferentially nucleates on the BiVO₄ surface to reduce the overall interfacial energy, resulting in intimate interfacial coupling and improved size uniformity of the CsPbBr₃ nanosheets. HRTEM (Fig. 10 g) images further show a sharp heterointerface with clear lattice fringes corresponding to the BiVO₄ (200) / (002) and CsPbBr₃ (001) planes, indicative of an ordered crystalline interface. EDS (Fig. 10 h) elemental mapping further verifies the coexistence and spatial distribution of Bi, V, O, Cs, Pb, and Br in BC2. AFM height profiling (Fig.10 i) and surface-potential mapping (Fig. 10 j) further verify the layered structure and interfacial electronic redistribution, respectively, providing complementary evidence for the successful formation of the 2D/2D S-scheme heterojunction.

4.2 Chemical nature of interfacial interactions

The intimate contact revealed by microscopic imaging provides the physical basis for S-scheme charge-transfer channels, while chemical interactions at the interface create effective pathways for charge migration across the junction.^[116] Whether arising from noncovalent interactions or robust chemical bonding, identifying interfacial interactions beyond simple physical mixing is essential for reducing interfacial contact resistance and promoting efficient S-scheme charge transfer. ^[70,117]

4.2.1 Identifying bonding motifs via vibrational spectroscopy

Fourier transform infrared spectroscopy (FT-IR) is widely used to probe local chemical environments and monitor the evolution of surface functional groups during heterojunction assembly. ^[118,119] In 2D/2D heterojunctions, interfacial coupling can perturb the vibrational modes of specific chemical bonds, and the resulting spectral changes provide insight into the nature and strength of interfacial interactions. For noncovalently assembled systems, such as those involving π-π stacking or hydrogen bonding, the preservation of framework-related bands together with subtle red- or blue-shifts of key functional groups is generally regarded as evidence of effective interfacial coupling. ^[120,121]

For example, Zhang et al. ^[67] used FT-IR (Fig.11 b) spectroscopy to investigate the chemical structure and assembly mode of a BiOI/porphyrin-based coordination polymer composite. The BiOI@PCP samples retained the characteristic BiOI features, including the Bi-O vibration at 490 cm^-1 and the surface hydroxyl band. After Zn²⁺coordination, a new band appeared at approximately 1000 cm^-1, which was assigned to skeletal vibrations of the pyrrolic ring in Zn-porphyrin. These results confirm successful Zn²⁺ coordination and indicate the formation of chemically linked interfacial structures.

4.2.2 Static redistribution of electron density

Interfacial interactions are often accompanied by charge redistribution across the interface. X-ray photoelectron spectroscopy (XPS), by detecting binding-energy shifts of core-level electrons, provides direct evidence for changes in local electron density.^[57,116] Typically, shifts toward higher binding energy suggest reduced local electron density, weakened electron screening, and increased effective nuclear charge. Therefore, XPS binding-energy shifts can be used to infer the direction and degree of interfacial charge transfer, offering strong evidence for electronic coupling between components and helping to distinguish genuine heterojunction formation from simple physical mixing. ^[122,123]

In the BiOI@HZnPCP heterojunction studied by Zhang et al^[67]. XPS analysis showed opposite binding-energy shifts for the two components. The Bi 4f (Fig.11 c) peak of BiOI shifted toward lower binding energies, while the Zn 2p (Fig. 11 d), N 1s (Fig.11 e), peaks of HZnPCP shifted toward higher binding energies. These shifts indicate directional electron transfer from HZnPCP to BiOI and the establishment of an interfacial built-in electric field. Compared with the Zn-free BiOI@HPCP sample, BiOI@HZnPCP exhibited larger binding-energy shifts, suggesting that the Zn center enhances interfacial charge redistribution. Thus, although Zn does not alter the direction of charge transfer, it can function as an electron-transport center that promotes electron delocalization and strengthens interfacial electronic coupling.

4.3 Band alignment and interfacial electric field

In S-scheme photocatalytic systems, the interfacial built-in electric field (IEF) provides the thermodynamic driving force for the directional migration of photogenerated charge carriers and enables selective recombination of low-energy carriers at the interface. Therefore, quantitatively determining the band structures, Fermi-level positions, and IEF direction is essential for validating the S-scheme charge-transfer mechanism.^[113]

4.3.1 Visualization and direction determination of the IEF

Kelvin probe force microscopy (KPFM), an advanced AFM-based technique, is widely used to visualize the IEF and determine its direction at heterojunction interfaces.^[124] By measuring the contact potential difference between a conductive tip and the sample surface, KPFM maps the spatial distribution of surface potential and work-function variations. According to the principle of Fermi-level equilibration, when two components come into contact, electrons spontaneously diffuse from the component with a lower work function, corresponding to a higher Fermi level, to that with a higher work function, corresponding to a lower Fermi level, until thermodynamic equilibrium is reached. This charge redistribution generates a directional interfacial potential gradient, providing the thermodynamic basis for directional charge transfer in S-scheme systems. ^[115,125]

KPFM enables both qualitative visualization of the interfacial built-in electric field (IEF) and quantitative assessment of its magnitude and light-induced response. For field-strength quantification, recent studies on all-organic HOF/COF heterojunctions ^[43] offer a representative example. KPFM (Fig.12 a-c) measurements showed that the surface potential difference across the HOF/COF heterojunction reached 40.0 mV, markedly higher than those of pristine Py-HOF (14.8 mV) and Py-COF (9.0 mV). This enhanced potential difference indicates a strengthened IEF, which provides a thermodynamic driving force for efficient charge separation and transport. Furthermore, light-assisted KPFM, which maps surface photovoltage, enables the monitoring of light-induced charge redistribution. Taking BiOI@HZnPCP as an example ^[67] (Fig.12 d, e), the surface potential under dark conditions was 44.73 mV higher than that of the substrate. Upon light irradiation, the surface potential decreased markedly to 33.07 mV, with no obvious morphological change. This negative potential shift indicates directional transfer of photogenerated electrons from the inner BiOI core to the outer ZnPCP surface under the driving effect of the IEF. Overall, KPFM provides strong evidence for the establishment of the thermodynamic driving force in S-scheme heterojunctions by correlating static surface-potential differences with light-induced potential variations.

4.3.2 Theoretical simulations as auxiliary validation

As an important complement to experimental characterization, density functional theory (DFT) calculations provide atomic-level insights into interfacial charge transfer and the formation of built-in electric fields in 2D/2D S-scheme heterojunctions.^[36] Through structural optimization of heterojunction models and calculations of differential charge density and planar-averaged electrostatic potential, charge redistribution upon interfacial contact can be systematically evaluated.^[126] Differential charge density maps visualize charge accumulation and depletion at the interface, reflecting pronounced electronic coupling between the two components.^[127] Meanwhile, planar-averaged electrostatic potential profiles reveal interfacial potential reconstruction and a pronounced potential gradient, providing theoretical evidence for the formation of a built-in electric field.^[128] When combined with quantitative charge-transfer analysis, these simulations further demonstrate that the built-in electric field can drive directional migration of photogenerated charge carriers, promote the recombination of low-energy electrons and holes, and retain high-energy carriers with strong redox potentials, consistent with the characteristic charge-transfer pathway of S-scheme heterojunctions.^[36]

For example, Teng et al. ^[23] used DFT calculations to clarify the interfacial electronic structure of a Bi₃TiNbO₉@C₄N 2D/2D S-scheme heterojunction. As shown in Fig.13(a), the in-situ encapsulation strategy enables intimate coupling between Bi₃TiNbO₉ and C₄N, providing a structural basis for interfacial charge transfer. The PDOS results in Fig. 13 (c-e) indicate that heterojunction formation modifies the electronic-state distribution near the Fermi level, suggesting strong electronic coupling between the two components. Work-function calculations further show that C₄N possesses a higher Fermi level than Bi₃TiNbO₉, which drives spontaneous electron migration from C₄N to Bi₃TiNbO₉ upon contact and induces an interfacial built-in electric field. This charge redistribution is further supported by the charge-density difference and electron-density isosurface analyses in Fig.13 (h-k), where obvious interfacial electron rearrangement and a transferred charge of 0.332 e are observed. Under irradiation, the built-in electric field and band bending promote S-scheme charge transfer, retaining electrons with strong reduction ability for O₂ reduction and holes with strong oxidation ability for water oxidation. Consequently, the Bi₃TiNbO₉@C₄N heterojunction provides a favorable electronic configuration for artificial photosynthetic H₂O₂ production.

4.4 Kinetic evidence for S-scheme charge transfer

Although the above structural analyses and thermodynamic characterizations can establish the interfacial basis and internal electric field driving force for S-scheme charge transfer, they are not sufficient to fully determine the actual carrier migration pathway under photoexcitation. Direct kinetic evidence is therefore essential for clarifying how photogenerated charges are generated, transferred, recombined, and spatially retained during photocatalysis. In-situ spectroscopic techniques and ultrafast time-resolved measurements provide powerful tools for monitoring carrier migration pathways and lifetimes under working conditions, thereby offering critical evidence for verifying the S-scheme charge-transfer mechanism.

4.4.1 Monitoring electron flow under in-situ illumination

Among the currently available characterization techniques, in-situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) is widely regarded as one of the most direct and persuasive approaches for probing light-induced interfacial charge migration. This technique relies on the high sensitivity of core-level binding energies to the local electron density. In general, electron accumulation enhances electronic shielding and shifts the corresponding core-level peaks toward lower binding energies, whereas electron depletion weakens the shielding effect and leads to positive binding-energy shifts. ^[123,129]. Therefore, by comparing the XPS spectra of individual components and their heterojunctions under dark and illuminated conditions, ISI-XPS can track the light-induced evolution of electron-rich and electron-deficient regions during photocatalysis.

A convincing ISI-XPS analysis should distinguish between interfacial charge redistribution in the dark and photoinduced carrier migration under illumination. In the dark, binding-energy shifts relative to the pristine components mainly reflect Fermi-level equilibration, interfacial electron redistribution, and the formation direction of the built-in electric field. Upon illumination, the reversible shifts of characteristic core levels provide direct information on the migration direction of photogenerated carriers.^[129] For an S-scheme heterojunction, the built-in electric field drives the selective recombination of low-energy electrons and holes at the interface, while preserving strongly reductive electrons and strongly oxidative holes on different components.^[125] This light-induced charge-transfer pattern is fundamentally different from that expected for a conventional type-II heterojunction, and therefore provides strong evidence for an S-scheme pathway.^[123]

Recent studies on COF-based organic/inorganic S-scheme heterojunctions have further demonstrated the diagnostic capability of ISI-XPS. Representative systems, including CYANO-COF/ZnIn₂S₄ ^[130] and COF/ Mn_0.2Cd_0.8S^[131], exhibit reversible light-induced binding-energy shifts, confirming that ISI-XPS can sensitively probe interfacial electron redistribution under photocatalytic conditions. When combined with complementary techniques such as electron paramagnetic resonance (EPR), Kelvin probe force microscopy (KPFM), and femtosecond transient absorption spectroscopy (fs-TAS), ISI-XPS provides a robust criterion for distinguishing S-scheme charge transfer from conventional type-II mechanisms.

This approach is particularly informative for 2D/2D S-scheme heterojunctions, where intimate face-to-face interfaces can strengthen orbital coupling and accelerate interfacial charge migration. For example, in a 2D/2D BN-C₃N₄/O-C₃N₅ S-scheme heterojunction (Fig. 14 e), interfacial B-O bonds function as direct charge-transfer channels.^[132] In-situ irradiated B 1s XPS (Fig. 14 f) showed that the B-O-related peak shifted toward higher binding energy under illumination and returned to its initial position after the light was switched off, whereas the O 1s peak exhibited the opposite shift behavior. These reversible spectral changes indicate light-induced electron migration through interfacial B-O bonds, supporting an S-scheme pathway in which charge carriers with weaker redox ability recombine at the interface, while those with stronger redox ability are preserved for overall water splitting. A similar conclusion was obtained for the 2D/2D COF-316/ TpBpy-COF ^[44] nanosheet heterojunction (Fig.14 a). Under illumination, the pyridinic N signal of TpBpy-COF shifted negatively, whereas the -CN signal of COF-316 shifted positively, indicating electron migration from COF-316 to TpBpy-COF (Fig.14 b). These opposite light-induced XPS shifts provide direct evidence for the formation of a 2D/2D S-scheme charge-transfer channel.

4.4.2 Indirect verification of redox capability

Electron paramagnetic resonance (EPR) spectroscopy cannot independently determine the charge-transfer route, but it provides important supporting evidence by identifying the reactive species generated during photocatalysis. In S-scheme heterojunctions, the built-in electric field and selective recombination of low-energy carriers allow the preservation of electrons and holes with strong redox potentials. Thus, the simultaneous enhancement of DMPO- • O₂⁻ and DMPO-•OH signals usually suggest that the heterojunction retains sufficient reduction and oxidation ability, which is more compatible with an S-scheme pathway than with a conventional type-II mechanism. ^[57,133]

For example, Luan et al. ^[44] used DMPO-assisted EPR measurements (Fig. 14 c, d) to examine ROS generation over COF-316/TpBpy-COF under visible-light irradiation. The composite showed much stronger DMPO-•O₂⁻ and DMPO-•OH signals than either single COF component, indicating enhanced carrier utilization and preserved redox capacity. These results suggest that photogenerated electrons can reduce O₂ to •O₂⁻, while holes can oxidize H₂O/OH⁻ to •OH. When combined with in-situ XPS evidence, the EPR results support the assignment of an S-scheme-dominated charge-transfer mechanism in the COF-316/TpBpy-COF heterojunction.

4.4.3 Ultrafast carrier dynamics

Femtosecond transient absorption spectroscopy (fs-TAS) is a powerful technique for resolving ultrafast carrier dynamics in photocatalytic heterojunctions on femtosecond-to-nanosecond timescales. By monitoring transient signals such as ground-state bleaching (GSB), excited-state absorption (ESA), and their decay kinetics, fs-TAS can provide kinetic evidence for interfacial charge transfer and carrier recombination pathways.^[134,135] In S-scheme heterojunctions, the built-in electric field and band bending promote selective interfacial recombination between low-energy electrons and holes, while preserving high-energy carriers with strong redox potentials. Therefore, variations in transient spectral features and lifetime components, rather than simple acceleration or retardation of decay alone, should be analyzed together with complementary evidence to identify an S-scheme charge-transfer pathway. ^[136,137]

For example, Liu et al. ^[138] used fs-TAS to investigate ultrafast charge dynamics in a Ni-MOF/CdS heterojunction. As shown in Fig. 15 (b-e), the fs-TA maps and transient spectra of NC80 exhibit characteristic CdS-related ground-state bleaching, excited-state absorption, and stimulated-emission features, with the GSB signal mainly assigned to photogenerated electrons. Compared with pristine CdS, NC80 shows prolonged carrier lifetimes in the fitted decay kinetics monitored at 500 nm (Fig.15 e), indicating the retention of long-lived electrons on CdS after heterojunction formation. Meanwhile, the weakened Ni-MOF-related transient features and the proposed carrier-dynamics scheme (Fig.15 g) suggest rapid interfacial recombination between electrons in Ni-MOF and holes in CdS. Together with XPS and TRPL analyses, these results support an S-scheme charge-transfer pathway, in which low-energy carriers are selectively recombined at the interface while high-energy electrons are preserved on CdS for subsequent redox reactions.

In summary, although both S-scheme and type-II heterojunctions can promote spatial charge separation, their charge-transfer consequences are fundamentally different. In a type-II heterojunction, photogenerated electrons and holes are transferred to relatively low-energy band positions, resulting in weakened thermodynamic driving forces for surface redox reactions. In contrast, an S-scheme heterojunction selectively recombines low-energy carriers at the interface while retaining highly reductive electrons and strongly oxidative holes. Therefore, a convincing distinction between S-scheme and type-II mechanisms requires a mutually consistent evidence chain. Structural and spectroscopic characterizations should first confirm the formation of an intimate heterointerface. Band-structure and work-function analyses should then establish the RP/OP configuration, Fermi-level equilibration, band bending, and the built-in electric field. Interfacial charge redistribution can be further supported by dark-state XPS, KPFM, and related electronic-structure analyses. More importantly, in-situ irradiated XPS and time-resolved spectroscopies are needed to track photoinduced charge migration and carrier lifetime evolution, thereby determining whether low-energy carriers recombine through an S-scheme pathway or follow a conventional type-II transfer route. EPR detection of reactive species provides complementary evidence for retained redox ability, particularly when both •O₂⁻ and •OH are generated under conditions where a type-II pathway would be thermodynamically unfavorable. Thus, an S-scheme assignment is reliable only when interfacial structure, band alignment, built-in electric field, light-induced charge migration, and preserved redox capability are mutually supported.

4.5 Reaction-induced dynamic evolution of photocatalytic heterointerfaces

Most discussions of S-scheme heterojunctions rely on static band structures, work-function differences, and interfacial configurations. However, under photocatalytic conditions, the active heterointerface may undergo dynamic electronic evolution induced by light irradiation, charge accumulation, reactant adsorption, intermediate formation, defect activation, and local reaction environments. Such evolution can alter band bending, built-in electric fields, interfacial recombination channels, and surface reaction kinetics.^[139] Therefore, S-scheme heterointerfaces should be regarded as reaction-responsive charge-transfer regions rather than fixed contact boundaries.

A representative example is the BN-C₃N₄/O-C₃N₅ S-scheme heterojunction^[132] reported by Cui et al., in which interfacial B-O bonds act as direct electron-transfer bridges between two carbon nitride components. In-situ irradiated XPS revealed reversible binding-energy shifts of B-O-related signals under illumination, indicating dynamic modulation of the electronic state at the bonded interface. Together with fs-TA and DFT analyses, these results suggest that B-O bonding channels can reduce the interfacial charge-transfer barrier, shorten the migration distance, and facilitate selective S-scheme recombination of low-energy carriers, while preserving carriers with strong redox potentials for overall water splitting. Notably, post-reaction XRD and FTIR spectra showed negligible changes, suggesting that the chemically bonded interface can sustain dynamic charge transfer without obvious structural degradation.

Dynamic interfacial charge redistribution has also been observed in tandem heterojunction systems. In GDY-Cu/WO₃, the WO₃/GDY S-scheme junction and the GDY-Cu ohmic contact establish coupled charge-transfer channels.^[140] Under illumination, in-situ XPS showed a negative shift of the C 1s binding energy and a positive shift of the W_4f binding energy, suggesting directional electron migration from WO₃ to GDY through the S-scheme pathway. Meanwhile, the GDY-Cu ohmic interface provides an additional electron-extraction route toward reduction sites. This system indicates that photocatalytic heterointerfaces can undergo light-induced electronic reconfiguration, in which multiple interfacial electric fields cooperate to regulate charge flow, carrier extraction, and surface redox reactions.

In summary, the catalytic performance of S-scheme heterojunctions is governed by both the initially constructed interface and its dynamic evolution under reaction conditions. Future studies on 2D/2D S-scheme photocatalysts should integrate in-situ /operando XPS, in-situ FTIR spectroscopy, EPR, KPFM, time-resolved spectroscopy, isotope-labeling experiments, and theoretical simulations to distinguish reversible electronic polarization, bond-assisted charge migration, defect activation, adsorbate-induced interfacial changes, and irreversible structural degradation. Such systematic analysis is essential for establishing a reliable structure-evolution-charge-transfer-activity relationship in 2D/2D S-scheme photocatalysts.

5. Engineering modification strategies for 2D/2D S-scheme heterojunctions

With a clearer understanding of the construction principles, charge-transfer mechanisms, and characterization methods of 2D/2D S-scheme heterojunctions, increasing attention has shifted from heterojunction fabrication to interfacial performance optimization. Although face-to-face 2D/2D contact can enlarge the interfacial contact area, shorten carrier migration pathways, and strengthen the built-in electric field, the practical photocatalytic performance of these systems remains constrained by interfacial resistance, discontinuous charge transport, sluggish mass transfer, insufficient active sites, and limited structural stability. Therefore, precise modulation of interfacial structures, charge dynamics, and surface-active sites is essential for overcoming these bottlenecks.

Current optimization strategies mainly involve multidimensional heterointerface engineering, interfacial chemical bonding, element doping, defect engineering, cocatalyst modification, and single-atom regulation. These strategies contribute to enhanced charge transport, stronger interfacial coupling, optimized band and electronic structures, improved reactant activation, and more efficient surface charge utilization. In essence, their shared objective is to translate the intrinsic charge-separation advantage of S-scheme heterojunctions into accelerated surface reaction kinetics and improved photocatalytic performance.

5.1 Construction of multidimensional heterointerface.

Conventional 2D/2D S-scheme heterojunctions primarily rely on a single face-to-face contact interface to achieve charge separation and selective recombination. ^[102,141] However, under complex reaction conditions or high carrier fluxes, a single interface is often required to simultaneously mediate light absorption, charge migration, interfacial recombination, and surface reactions, which may intensify local carrier recombination and limit charge-transport efficiency. Therefore, introducing a third component to construct multidimensional heterointerfaces has emerged as an effective strategy to overcome the charge-transport limitations of single-interface systems. ^[38,142]

The key to multidimensional heterointerface design lies in functional partitioning and multichannel charge management. By incorporating conductive layers, catalytic components, light-harvesting units, or interfacial modulation layers, cascade band structures and additional charge-migration pathways can be established beyond the original RP/OP S-scheme interface. This design facilitates the directional recombination of low-energy charge carriers while preserving highly reductive electrons on the RP and highly oxidative holes on the OP.^[104] Compared with single 2D/2D interfaces, multidimensional heterostructures can simultaneously promote charge separation, alleviate interfacial transport pressure, and improve structural stability and adaptability under complex reaction environments.

For example, multidimensional heterointerface construction provides an effective route to overcoming the charge-transport bottleneck in single 2D/2D S-scheme interfaces. A representative case is the 2D/2D/0D TiO₂/C₃N₄/Ti₃C₂ MXene system, ^[38] in which 2D TiO₂ and ultrathin C₃N₄ form a tightly coupled S-scheme heterointerface (Fig. 16 d), enabling the selective recombination of low-energy carriers while retaining strong redox capability. Meanwhile, Ti₃C₂ MXene quantum dots anchored on the C₃N₄ surface act as electron acceptors and transport channels, accelerating electron extraction and migration from the C₃N₄ conduction band and thereby promoting multielectron CO₂ reduction (Fig. 16 f). This example demonstrates that the incorporation of conductive catalytic components can effectively decouple charge separation from electron transport, thereby improving interfacial charge utilization and photocatalytic reaction efficiency.

Similarly, the cascaded 2D-coupled Ti₃C₂/Zn_0.7 Cd_0.3S/Fe₂O₃ heterojunction (Fig. 16 a) further highlights the merits of multi-interface synergy.^[142] In this architecture, the Zn_0.7Cd_0.3S/Fe₂O₃ S-scheme interface establishes a built-in electric field and preserves charge carriers with strong redox potentials, whereas metallic Ti₃C₂ MXene forms an ohmic contact with Zn_0.7Cd_0.3S, offering a low-resistance channel for rapid electron transport and additional active sites for hydrogen evolution (Fig. 16 b). The optimized ternary composite therefore delivers substantially improved H₂-evolution activity (Fig. 16 c), demonstrating that the coupling of S-scheme and ohmic interfaces can synergistically enhance charge separation, interfacial transport, and surface reaction efficiency.

These findings suggest that multidimensional 2D/2D-based S-scheme heterostructures should be developed through function-oriented interface engineering rather than simple component stacking. In such systems, S-scheme interfaces maintain strong redox capability and promote selective charge separation, conductive components such as MXene accelerate electron extraction and transport, and catalytic or active-site interfaces facilitate surface reactions. Accordingly, future efforts should focus on coordinated band alignment, work-function matching, intimate interfacial coupling, and directional charge migration to construct well-defined multi-interface S-scheme photocatalysts.

5.2 Interfacial bonding engineering.

Interfacial quality is crucial for determining the charge-transfer efficiency and structural stability of 2D/2D S-scheme heterojunctions.^[143] Although face-to-face contact enlarges the interfacial area, interfaces assembled merely through van der Waals interactions, electrostatic adsorption, or physical mixing often exhibit weak coupling, high charge-transfer resistance, and insufficient structural stability. In S-scheme heterojunctions, the interface serves as both the region for built-in electric field formation and charge redistribution and the pathway for selective recombination of low-energy carriers while retaining charge carriers with strong redox potentials.^[144,145] Thus, constructing chemically coupled interfaces through interfacial bonding, molecular bridging, or ligand-assisted assembly is essential for improving 2D/2D S-scheme systems. Interfacial bonding motifs can establish direct atomic-scale connections across heterointerfaces, thereby enhancing interfacial coupling and promoting localized charge redistribution. These effects reduce interfacial charge-transfer resistance, regulate the work-function difference and built-in electric field, and ultimately promote S-scheme charge-transfer dynamics. ^[99,111]

A representative example is the ultrathin 2D/2D CoAl-LDHs/ZnIn₂S₄ S-scheme heterojunction ^[111] with defect-assisted Co-Sx interfacial bonding. In this system, CoAl-LDHs and ZnIn₂S₄ nanosheets were assembled into an intimate face-to-face 2D/2D interface, enabling strong interfacial electronic coupling (Fig.17 e). Oxygen vacancies in CoAl-LDHs enriched the local electron density around Co sites, thereby facilitating the formation of Co-Sx bonds at the heterointerface. The newly emerged S 2p signal assigned to Co-Sx bonding (Fig.17 d), together with FT-IR and UV-vis DRS results, verified the formation of chemical coupling rather than simple physical stacking. These Co-Sx bonds served as atomic-level charge-transfer channels, strengthening interfacial electronic interaction and accelerating S-scheme charge transfer (Fig. 17 f). As a result, the combined effects of ultrathin 2D/2D architecture, oxygen-vacancy modulation, and Co-Sx-mediated interfacial transport substantially improved the photocatalytic hydrogen evolution performance of CoAl-LDHs/ZnIn₂S₄.

Another representative molecularly bridged system is the 2D/2D NH₂-TiO₂/ReS₂ S-scheme heterojunction.^[99]In this system, APTMS was used to introduce amino groups onto the TiO₂ surface, creating a molecular bridge between TiO₂ and ReS₂ (Fig.17 a). XPS (Fig.17 c) analysis verified the formation of Ti-O-Si and N-Re interfacial linkages, and charge-density-difference calculations further revealed pronounced electron redistribution between APTMS and ReS₂. Such molecularly mediated interfacial coupling lowered the charge-transfer resistance and modulated the work function of TiO₂, thereby shifting TiO₂/ReS₂ from type-I band alignment to an S-scheme charge-transfer pathway. As a result, NH₂-TiO₂/ReS₂ delivered markedly enhanced H₂ evolution activity and favorable cycling stability.

Collectively, these examples suggest that interfacial chemical bonding plays a role beyond simple structural reinforcement. By integrating structural connection, electronic coupling, built-in electric field modulation, and directional carrier migration, interfacial bonding can substantially improve charge separation, interfacial transport kinetics, and structural robustness in 2D/2D S-scheme heterojunctions. Accordingly, interfacial bonding engineering represents an important strategy for the rational design of high-performance 2D/2D S-scheme photocatalysts.

5.3 Element doping

Elemental doping represents an effective atomic-level strategy for enhancing the photocatalytic performance of 2D/2D S-scheme heterojunctions.^[146] By incorporating metal or nonmetal dopants into the semiconductor lattice, key electronic properties, including the work function, Fermi level, band gap, defect states, and local charge distribution, can be effectively modulated. These electronic-structure changes can further influence band bending, the built-in electric field, and the directional migration of photogenerated carriers at the heterointerface.^[145] For S-scheme heterojunctions, the primary role of elemental doping is to optimize band alignment and interfacial charge transfer, while simultaneously regulating the adsorption/desorption behavior of reaction intermediates. This enables the preserved high-energy electrons and holes to participate more efficiently in surface redox reactions.^[146]

A representative case is the 2D/2D SnNb₂O₆/Ni-doped ZnIn₂S₄ (Fig.18 a) S-scheme heterojunction.^[147] DOS analysis (Fig.18 b, c) shows that pristine ZnIn₂S₄ (ZIS) has a clear band gap and limited electronic states near the Fermi level, whereas Ni-doped ZIS exhibits dopant-induced defect states close to the Fermi level. These Ni-related states reconstruct the electronic structure of ZIS, narrow the effective band gap, and provide additional pathways for photoinduced charge excitation and transfer. Meanwhile, Ni doping modulates the local electronic environment of ZIS, thereby improving its surface HER kinetics. After coupling with SnNb₂O₆ (SNO), the work-function difference between Ni-doped ZIS and SNO drives electron transfer from Ni-doped ZIS to SNO, resulting in the formation of a built-in electric field. Under illumination, this field facilitates the preferential recombination of low-energy electrons in SNO with holes in Ni-doped ZIS, while retaining highly reductive electrons in Ni-doped ZIS for H₂ evolution. This example highlights that metal doping can work in concert with S-scheme interface engineering to promote charge separation and HER kinetics.

Another representative example involves nonmetal-doped g-C₃N₄/MS₂ (M = Sn or Zr) 2D/2D S-scheme heterojunctions.^[146] DFT calculations (Fig.18 e) revealed that O or S doping lowered the work function and upshifted the Fermi level of g-C₃N₄, thereby increasing the Fermi-level offset relative to SnS₂/ZrS₂. This enlarged Fermi-level difference promoted interfacial electron transfer, as reflected by the increase in Bader charge transfer (Fig.18 f) from 0.015-0.049 e to 0.179-0.243 e. Meanwhile, the built-in electric field was enhanced from 1.32 × 10⁹-2.80 × 10⁹ V m^-1 to 4.98 × 10⁹-7.11 × 10⁹ V m^-1. Such a strengthened internal field promoted the selective recombination of low-energy carriers while retaining high-energy electrons and holes for surface redox reactions. Experimentally, the H₂ evolution rate of O-doped g-C₃N₄/SnS₂ increased from 38 to 154 μmol/g/h, further confirming that nonmetal doping can amplify the built-in-field-driven charge separation in S-scheme heterojunctions.^[146]

Collectively, these cases indicate that elemental doping goes beyond simple compositional modification and serves as an atomic-level strategy for tuning electronic structure, defect states, built-in electric fields, and surface adsorption behavior in S-scheme heterojunctions. Ni doping mainly improves the reductive capability and H₂ adsorption/desorption kinetics of ZnIn₂S₄, whereas O or S doping upshifts the Fermi level of g-C₃N₄ and strengthens interfacial electron transfer. In this regard, elemental doping provides an effective route to couple band-structure modulation, built-in electric field enhancement, charge-separation optimization, and surface reaction kinetics regulation, thereby improving the photocatalytic performance of 2D/2D S-scheme systems

5.4 Defect engineering

Defect engineering represents an important atomic-level approach for improving the photocatalytic performance of 2D/2D S-scheme heterojunctions. Rather than merely increasing the density of active sites, controlled vacancy formation can modulate the local electronic structure, thereby enhancing light absorption, interfacial charge migration, and reactant adsorption/activation.^[148] In S-scheme heterojunctions, surface vacancies can cooperate with the built-in electric field to promote the selective recombination of low-energy carriers at the interface, while preserving high-energy electrons or holes for surface redox reactions.^[149,150] This coupling enables the simultaneous optimization of charge separation, surface activation, and product selectivity.

A representative case is the Br-vacancy-engineered H₂WO₄/Cs₂AgBiBr₆ (HWO/CABB) 2D/2D S-scheme heterojunction.^[150] Zhou et al. constructed a strongly coupled HWO/CABB interface via acid cleavage of Bi₂WO₆ followed by in-situ epitaxial growth of Cs₂AgBiBr₆ (Fig.19 a). The obtained heterostructure featured surface Br vacancies together with Bi-O/Ag-O interfacial linkages. EPR (Fig.19 b) and elemental ratio analyses confirmed the presence of Br vacancies, while interfacial bonding and charge redistribution supported the formation of an S-scheme charge-transfer pathway. DFT (Fig.19 d) calculations revealed that Br vacancies strengthened CO₂ adsorption/activation, reduced the formation barriers of COOH and CHO, and promoted multi-electron CO₂ reduction toward CH₄. Notably, the barrier for CHO* formation was lower than that for CO desorption, favoring further hydrogenation toward CH₄ rather than CO release (Fig.19 e). As a result, HWO/CABB delivered a CH₄ production rate of 22.6 μmol/g/h with a CH₄ selectivity of 86.1%, demonstrating the effectiveness of Br-vacancy engineering in optimizing CO₂ activation and intermediate conversion pathways.

Another representative example is the dual-oxygen-vacancy-engineered TiO₂-Vo/BiOBr-Vo (Fig.19 f) 2D/2D S-scheme heterojunction.^[151] Tian et al. introduced oxygen vacancies into both TiO₂ and BiOBr, with abundant vacancies in TOB-0.1 confirmed by EPR (Fig.19 g). These vacancy-induced defect states broadened visible-light absorption. Upon interfacial contact, the Fermi-level difference between TiO₂-Vo and BiOBr-Vo generated a built-in electric field, driving S-scheme charge transfer while preserving high-energy electrons and holes for photocatalytic reactions (Fig. 19 h). The oxytetracycline degradation rate constant reached 0.030 min^-1, which was 3.75 and 1.76 times those of TiO₂-Vo and BiOBr-Vo, respectively. Trapping and ESR results confirmed h⁺ and •O₂^- as the main reactive species, indicating that oxygen vacancies enhance pollutant degradation by improving light absorption, charge separation, and reactive oxygen species generation.

Collectively, defect engineering provides an effective route to tailor the interfacial electronic structure and surface reaction microenvironment of 2D/2D S-scheme heterojunctions. Vacancies can serve as atomic-scale regulatory centers that bridge S-scheme interfacial charge migration with surface catalytic processes. Therefore, rational defect engineering can integrate electronic-structure modulation, interfacial charge-transfer regulation, and surface-reaction optimization, offering a versatile strategy for improving the photocatalytic performance of 2D/2D S-scheme systems.

5.5 Interfacial cocatalyst engineering

S-scheme heterojunctions enable the selective recombination of low-energy carriers through built-in electric fields and band bending, while preserving highly reducing electrons and strongly oxidizing holes for surface redox reactions. Nevertheless, efficient charge separation can be converted into enhanced catalytic performance only if these energetic carriers are effectively extracted and consumed at reactive surface sites. Therefore, incorporating cocatalysts into 2D/2D S-scheme heterojunctions represents an effective strategy to improve charge utilization, reduce reaction barriers, and modulate product selectivity. In general, cocatalysts serve as charge-carrier trapping centers, surface active sites, and interfacial charge-transfer mediators. ^[152,153]

A representative example is the Ag/AgO-modified 2D/2D g-C₃N₄/Ni₃V₂O₈ S-scheme heterojunction. ^[154] In this heterostructure, g-C₃N₄ and Ni₃V₂O₈ establish a 2D/2D p-n S-scheme interface, while Ag/AgO nanoparticles act as cocatalysts to facilitate carrier trapping/extraction, suppress charge recombination, enhance visible-light harvesting, and provide surface catalytic sites. Consequently, the optimized composite achieved ciprofloxacin and amoxicillin removal efficiencies of 58.8% and 62.1%, respectively, with •O₂^- and h⁺ identified as the dominant reactive species. This result demonstrates that Ag/AgO enhances pollutant degradation by integrating interfacial charge separation with reactive-species generation.

Non-noble-metal cocatalysts can also promote S-scheme H₂ evolution. Hua et al. ^[98] constructed a Ni₂P-modified 2D/2D SnNb₂O₆/CdS-D S-scheme heterojunction (Fig. 20 e), where SnNb₂O₆ (SNO) and CdS-D formed the 2D/2D S-scheme interface and Ni₂P served as the HER cocatalyst. ^[98] Ni₂P loading increased the H₂-evolution rate from 7808 to 11992 μmol/g/h. XPS and DFT analyses revealed a CdS-D → SNO → Ni₂P charge-transfer pathway, indicating that Ni₂P functioned as both an electron-accumulation center and a HER active site. Thus, Ni₂P accelerated electron extraction and promoted surface H₂-evolution kinetics.

For artificial photosynthesis, cocatalyst design should address both reduction and oxidation half-reactions. A representative example is the Cu₂O/Fe₂O₃@Ni_xCo_1-x(OH)₂ 2D/2D S-scheme system^[45], in which Cu₂O and Fe₂O₃ nanosheets construct the S-scheme heterojunction, while in-situ -grown Ni-Co bimetallic hydroxide acts as a bifunctional cocatalyst (Fig.20 a). The Ni_xCo_1-x(OH)₂ cocatalyst promotes both CO₂ reduction and H₂O oxidation, facilitates interfacial charge transfer, and suppresses side reactions and photo corrosion. In particular, appropriate Co incorporation modulates the band structure and d-band center, balances electron/hole extraction, and enhances CO₂ and H₂O adsorption/activation. Consequently, the optimized CF@Ni_0.75Co_0.25 delivered a CO production rate of 552.7 μmol/gcat/h, an O₂ production rate of 313.0 μmol/gcat/h, and a CO selectivity of 91.4% (Fig. 20 c). DFT calculations (Fig.20 b) further revealed reduced reaction barriers for both CO₂-to-CO reduction and H₂O-to-O₂ oxidation, highlighting the importance of bifunctional cocatalyst engineering in overall artificial photosynthesis.

Overall, cocatalyst engineering provides an effective link between S-scheme charge separation and surface reaction kinetics. In 2D/2D S-scheme heterojunctions, suitable cocatalysts can extract and accumulate energetic carriers, create reaction-specific active sites, lower kinetic barriers, and direct interfacial reactions toward targeted pathways. Therefore, rational cocatalyst design should go beyond suppressing charge recombination and should instead coordinate carrier utilization with reaction-specific requirements, including reactive-species generation, H₂ evolution, CO₂ conversion, and coupled redox processes.

5.6 Single-atom engineering

Single-atom engineering provides an atomically precise approach for improving 2D/2D S-scheme heterojunctions.^[155] Compared with conventional nanoparticle cocatalysts, atomically dispersed metal sites offer well-defined coordination structures, maximized metal utilization, and tunable local electronic environments. In S-scheme systems, such isolated sites can function as electron-trapping centers, interfacial charge-transfer nodes, and catalytic centers for surface reactions. Therefore, the significance of single-atom engineering lies not only in the incorporation of isolated metal atoms, but also incoupling these sites with the charge-migration and surface-reaction pathways of 2D/2D S-scheme heterojunctions. This coupling enables more efficient interfacial charge separation, site-directed carrier trapping, and reactant adsorption/activation.^[156]

A representative example is the single-Ni-atom-anchored Ti-MOF/BiVO₄ 2D/2D S-scheme heterojunction.^[155] In this system, atomically dispersed Ni (II) sites were immobilized within the coordination microenvironment of NTU-9 Ti-MOF, and the obtained Ni@MOF nanosheets were coupled with BiVO₄ nanosheets to construct an ultrathin 2D/2D S-scheme heterojunction (Fig.21 a). The Ti-MOF component not only contributes to S-scheme charge separation but also provides a stable coordination environment and a CO₂-enriched local microenvironment for the isolated Ni sites, whereas BiVO₄ provides strongly oxidizing holes for water oxidation. XANES (Fig.21 b) and EXAFS (Fig. 21 c) analyses confirmed that Ni was atomically dispersed in a dominant Ni-O₅ coordination configuration. In-situ DRIFTS (Fig. 21 e) combined with DFT calculations (Fig. 21 f) indicated that the isolated Ni sites facilitated CO₂ adsorption and stabilized key Ni-COO* and Ni-COOH* intermediates, thereby lowering the kinetic barrier for selective CO formation. Consequently, the optimized Ni@6MOF/BVO exhibited a CO₂-to-CO photoconversion activity approximately 66-fold higher than that of BiVO₄ nanoparticles in pure water, along with a CO selectivity of 99.2%. This example suggests that coupling atomically dispersed active sites with tailored coordination microenvironments can effectively convert the charge-separation capability of 2D/2D S-scheme heterojunctions into enhanced CO₂ activation and selective CO production.

Another representative example is the La-single-atom/dual-vacancy-engineered WS_2-x/La_1-WO_2.9 2D/2D S-scheme heterojunction.^[156] In this system, La single atoms and O/S dual vacancies were incorporated into an in-situ formed WS_2-x/WO_2.9 2D interface, integrating single-atom, defect, and S-scheme interface engineering. AC-HAADF-STEM and La L₃-edge EXAFS analyses confirmed atomically dispersed La species with La-O coordination, whereas EPR verified the presence of O/S vacancies. KPFM and in-situ XPS results suggested that La single atoms enhanced the built-in electric field and acted as interfacial charge-transfer mediators. DFT calculations and in-situ FTIR further indicated that this atomic-level regulation promoted N₂ adsorption/activation and favored an alternating hydrogenation pathway. Consequently, WS_2-x/La_1-WO_2.9 delivered an NH₃ generation rate of 124.9 μmol/g/h, highlighting the synergistic role of La single atoms, dual vacancies, and S-scheme charge transfer in photocatalytic N₂ fixation.

Collectively, single-atom engineering can be regarded as an atomically precise strategy for integrating interfacial charge-transfer regulation with surface reaction control in 2D/2D S-scheme heterojunctions. Atomically dispersed sites can function as electron-trapping centers and interfacial charge-transfer nodes, thereby facilitating the extraction and utilization of high-energy carriers retained through the S-scheme charge-transfer pathway. Future studies should further emphasize the coordinated optimization of single-atom anchoring sites, coordination structures, interfacial locations, and local reaction microenvironments. The integration of advanced in-situ /operando characterizations with theoretical calculations will be essential for establishing reliable structure-charge transfer-reactivity relationships and for guiding the rational design of highly selective 2D/2D S-scheme photocatalysts.

5.7 Interfacial strain engineering

Interfacial strain engineering has emerged as an effective approach for modulating the band structure, built-in electric field, and surface reaction kinetics of 2D/2D S-scheme heterojunctions. In atomically thin materials, tensile or compressive strain can be introduced by lattice mismatch, van der Waals interlayer coupling, substrate interactions, or external mechanical perturbations, thereby altering local atomic configurations and interfacial electronic states.^[157] Unlike doping, defect engineering, or cocatalyst modification, strain engineering allows continuous regulation of band gaps, band-edge positions, carrier transport properties, and adsorption energetics of key intermediates without substantially changing the chemical composition of the photocatalyst. Appropriate interfacial strain can therefore strengthen the built-in electric field, promote the selective recombination of low-energy carriers, and reduce the kinetic barriers of surface reactions such as HER, OER, and CO₂ reduction. ^[158,159]

The Ga₂SSe/SnS₂^[158] van der Waals heterojunction provides a representative theoretical model for strain-regulated 2D/2D S-scheme photocatalysts. As illustrated in Fig.22(g), the built-in electric field at the interface drives an S-scheme charge-transfer pathway, enabling low-energy carriers to recombine while preserving electrons and holes with strong redox potentials. More importantly, this system reveals a strain-dependent balance between surface reaction kinetics and band-edge energetics. Tensile strain markedly improves HER kinetics, as reflected by the decrease in ΔG_H* from 1.601 eV under −6% compressive strain to 0.405 eV under 6% tensile strain, approaching a more favorable hydrogen adsorption state (Fig.22 h). However, tensile strain also shifts the CBM of Ga₂SSe downward and the VBM of SnS₂ upward, meaning that excessive strain may compromise the redox driving force required for overall water splitting (Fig.22 i). This example highlights that interfacial strain engineering should be optimized within an appropriate window, rather than simply maximized, to balance light absorption, band-edge alignment, redox capability, and reaction barriers.

The strain-induced g-C₃N₄/ZnO heterojunction ^[87] further demonstrates that stress regulation can serve as an effective approach to inducing S-scheme heterojunction formation. As shown in Fig.22(j), increasing the lattice parameter of the heterostructure continuously changes the work function and band alignment, leading to an S-scheme band configuration favorable for overall water splitting at a lattice parameter of 7.13 Å. In this configuration, strongly reductive electrons are retained on g-C₃N₄, whereas strongly oxidative holes remain on ZnO. The charge-density difference and planar-averaged charge-density profiles further reveal the formation of a built-in electric field from g-C₃N₄ to ZnO (Fig.22 k), which promotes the recombination of low-energy electrons in ZnO with holes in g-C₃N₄ and establishes the S-scheme charge-transfer pathway. This example suggests that stress engineering can not only optimize existing S-scheme heterojunctions, but also induce S-scheme charge-transfer dynamics by regulating band alignment and interfacial electric fields.

In addition to these 2D/2D theoretical models, the Co₉S₈@ZnIn₂S₄ system ^[159] provides experimental evidence for interfacial stress regulation. In this system, the lattice mismatch between Co₉S₈ and ZnIn₂S₄ induces localized interfacial strain, with GPA results showing that the strain is mainly concentrated at the heterointerface (Fig.22 b-d). This interfacial stress not only regulates the charge-transfer direction and favors the formation of the S-scheme built-in electric field, but also reconstructs the surface reactive sites. Specifically, CO₂ adsorption configurations and charge-density-difference analysis show that stress-induced surface reconstruction transforms the active motif of Co₉S₈ from Co-Co to Co-S-Co. As a result, CO₂ adsorption changes from a nearly linear configuration to a distinctly bent adsorption geometry, accompanied by an increase in electron transfer from 0.06 e to 0.54 e, indicating substantially enhanced CO₂ activation (Fig.22 e). Free-energy calculations further show that Co₉S₈@ZnIn₂S₄ lowers the key barrier for CO₂ reduction and favors *CO desorption over further hydrogenation (Fig. 22 f), ultimately enabling highly selective photothermal reduction of low-concentration CO₂ (Fig. 22 a). This system demonstrates that interfacial stress can regulate not only charge dynamics but also active-site configurations and reaction selectivity.

Overall, interfacial strain engineering enables the coordinated optimization of S-scheme charge transfer and surface reactions through lattice distortion, band-structure modulation, built-in electric-field reconstruction, and reaction-barrier regulation. Future studies should focus on controllable methods for introducing and stabilizing interfacial strain, combined with GPA strain mapping, in-situ /operando spectroscopy, and theoretical calculations, to establish quantitative correlations among strain states, interfacial electric fields, charge-transfer pathways, and catalytic activity.

5.8 Facet engineering

Facet engineering has emerged as an effective strategy for atomic-level interface design in 2D/2D S-scheme heterojunctions. Different crystal facets generally possess distinct atomic arrangements, surface energies, work functions, surface potentials, and reactant adsorption behaviors, which can directly affect interfacial contact geometry, band bending, built-in electric-field strength, and interfacial charge-transfer resistance.^[160] Compared with conventional heterojunction construction, facet engineering emphasizes the precise regulation of interfacial contact planes. By selectively exposing specific facets or constructing oriented facet-to-facet coupling, the interfacial electronic structure and S-scheme charge-transfer pathway can be rationally modulated.^[161] This strategy therefore links controllable synthesis, interface-structure design, and charge-dynamics optimization, providing a rational route for developing high-performance 2D/2D S-scheme photocatalysts. ^[162,163]

The ZnIn₂S₄/BiOBr S-scheme heterojunction ^[162] provides a representative example of interfacial facet regulation. By growing ZnIn₂S₄ in-situ on BiOBr nanosheets dominated by either the (010) or (001) facet, ZIS/BOB-(010) and ZIS/BOB-(001) heterojunctions were constructed, respectively (Fig. 23 a). Compared with ZIS/BOB-(010), ZIS/BOB-(001) exhibits a larger Fermi-level difference, a stronger built-in electric field, and more pronounced band bending. These features accelerate the interfacial recombination of charge carriers with weak redox ability, while more effectively preserving electrons and holes with strong redox potentials on ZnIn₂S₄ and BiOBr, respectively, for photocatalytic H₂ evolution (Fig. 23 c). This result indicates that the selection of interfacial facets not only determines the structural contact mode of the heterojunction but also serves as a key parameter for tuning S-scheme charge migration and built-in electric-field strength.

The TiO₂/COF S-scheme heterostructure ^[163] further illustrates the role of facet effects in CO₂ photoreduction. By assembling two-dimensional Tp-Tta COF onto TiO₂ dominated by either (001) or (101) facets, T-001/COF and T-101/COF heterostructures were obtained. Compared with T-001/COF, T-101/COF exhibits more efficient interfacial charge separation and transfer. This improvement is mainly attributed to the electron-rich surface and more suitable conduction-band position of (101)-faceted TiO₂, which favor S-scheme recombination between electrons in the TiO₂ conduction band and holes in the COF valence band. As a result, electrons with strong reduction ability are retained on COF for CO₂ reduction. Consequently, T-101/COF achieves a higher CO evolution rate and selectivity, demonstrating that facet engineering can modulate not only interfacial charge dynamics but also reactant adsorption and product selectivity.

Overall, facet engineering enables the coordinated regulation of built-in electric fields, band bending, charge migration, and surface reactions in S-scheme heterojunctions by controlling exposed facets, interfacial contact orientation, and facet-dependent electronic structures. Future studies should focus on facet-controlled synthesis, oriented assembly, and atomic-level interface characterization, combined with in-situ spectroscopy, potential mapping, and theoretical calculations to clarify how specific facet contacts influence Fermi-level equilibration, S-scheme charge-transfer pathways, and reaction kinetics. Such efforts will help advance 2D/2D S-scheme heterojunctions from empirical interface construction toward rational interface design.

6. Applications

Owing to their intimate face-to-face interfaces and S-scheme charge-transfer characteristics, 2D/2D S-scheme heterojunctions have been widely investigated for solar energy conversion and environmental remediation. The 2D/2D configuration is conducive to shortening carrier-migration distances and providing abundant interfacial charge-transfer pathways, while the S-scheme pathway may help maintain relatively strong redox capability under suitable band alignment and interfacial coupling. However, their photocatalytic performance is difficult to rationalize solely from the perspective of charge separation, because different reactions involve distinct thermodynamic requirements, kinetic limitations, intermediate-evolution pathways, and stability concerns. Accordingly, interface construction, interfacial bonding, elemental doping, defect engineering, cocatalyst regulation, and single-atom engineering are better examined in a reaction-specific context.

This section summarizes representative applications of 2D/2D S-scheme heterojunctions in H₂ evolution, CO₂ reduction, pollutant degradation, and H₂O₂ production, with emphasis on electron utilization, regulation of product selectivity, reactive oxygen species generation, and two-electron pathway control.

6.1 Photocatalytic hydrogen evolution

Hydrogen is widely recognized as a promising clean-energy carrier, and solar-driven photocatalytic water splitting offers a potentially sustainable route for H₂ production. Efficient photocatalytic H₂ evolution generally requires appropriate band-edge positions, adequate light-harvesting capability, efficient charge separation, and abundant active sites for the hydrogen evolution reaction (HER).^[164,165] However, conventional single-component photocatalysts are often limited by the intrinsic trade-off between efficient charge separation and strong redox capability, making it difficult to simultaneously achieve broad light harvesting, rapid carrier transport, and highly reductive electrons. In recent years, 2D/2D S-scheme heterojunctions have emerged as promising candidates for photocatalytic H₂ production because they integrate the structural merits of ultrathin nanosheets with the unique charge-transfer characteristics of S-scheme systems.^[2]. In photocatalytic H₂ evolution, the overall efficiency is fundamentally governed by the generation, separation, transport, and consumption of photogenerated electrons. More importantly, electrons participating in proton reduction must retain sufficiently negative reduction potentials during interfacial transfer. Unlike conventional type-II heterojunctions, which commonly weaken redox ability owing to staggered band migration, S-scheme heterojunctions enable the spatial separation of charge carriers while preserving highly reductive electrons on the reduction photocatalyst under the driving force of band bending and internal electric fields. Meanwhile, the intimate face-to-face contact in 2D/2D architectures provides enlarged interfacial areas and shortened diffusion pathways, thereby facilitating directional charge transport and suppressing carrier recombination. ^[166,167] As a result, the improved H₂-evolution activity of 2D/2D S-scheme photocatalysts is generally associated with the synergistic optimization of redox capability, interfacial carrier dynamics, and surface HER kinetics. The table (Table 1.) below summarizes the hydrogen evolution performance and corresponding reaction conditions of representative 2D/2D S-scheme heterojunction systems.

Hu et al.^[174] reported a 2D/2D H₂N-Cu-MOF/TpPa-1-COF S-scheme heterojunction prepared by grinding and ultrasonication (Fig.24 e). As shown in the figure, the 2D/2D MOF/COF composite exhibits markedly enhanced photocatalytic H₂-evolution activity compared with the individual H₂N-Cu-MOF and TpPa-1-COF components (Fig.24 g). Notably, the optimized Pt-loaded composite delivers a substantially higher H₂-production rate, demonstrating the synergistic effect of intimate 2D/2D interfacial contact and Pt-promoted HER kinetics. The face-to-face MOF/COF assembly, stabilized by hydrogen-bonding and π-π interactions, is beneficial for interfacial charge transfer, while Pt nanoparticles accelerate electron utilization at proton-reduction sites. These results indicate that the photocatalytic performance of organic 2D/2D S-scheme systems can be effectively enhanced by concurrently optimizing supramolecular interfacial assembly and surface reaction kinetics.

Gao et al. ^[43] constructed a lattice- and dimension-matched Py-HOF/Py-COF 2D/2D S-scheme heterojunction (Fig.24 a). The performance results shown in Fig.24 (b) demonstrate that the HOF/COF hybrid exhibits a much higher H₂-production rate than pristine Py-HOF and Py-COF, highlighting the benefit of constructing a molecularly compatible organic-framework heterointerface. Compared with randomly stacked organic nanosheets, the lattice- and dimension-matched interface enables more extensive and continuous interfacial contact, thereby promoting charge separation and migration. The enhanced visible-light absorption and apparent quantum yield further verify the improved photocatalytic efficiency of the hybrid system. This study underscores the importance of interfacial compatibility in improving H₂-evolution performance in organic-framework-based S-scheme photocatalysts, although cocatalyst loading and reaction conditions should be considered when comparing activities among different systems.

Li et al. ^[180] further proposed an S-scheme homojunction by in-situ growing pristine ZnIn₂S₄ on oxygen-doped ZnIn₂S₄ nanosheets. The reported activity comparison shows that the homojunction photocatalyst achieves substantially improved H₂-evolution performance compared with the corresponding single-component ZnIn₂S₄ samples. Oxygen doping modulates the electronic properties of ZnIn₂S₄, while the homogeneous interface facilitates charge transfer and suppresses charge recombination. This finding demonstrates that oxygen-doping-induced electronic modulation, coupled with homojunction engineering, provides an effective strategy for improving photocatalytic H₂ evolution.

Overall, the improved photocatalytic H₂-evolution performance of 2D/2D S-scheme heterojunctions is commonly linked to more effective utilization of photogenerated electrons. The S-scheme charge-transfer pathway is generally considered favorable for preserving electrons with relatively strong reduction capability, while the 2D/2D architecture may provide shortened migration distances and enlarged interfacial contact areas for charge transport. Nevertheless, efficient H₂ evolution is also closely related to the rapid extraction and consumption of electrons at suitable HER active sites. In this context, cocatalysts or conductive components, such as Pt, Ni₂P, MoS₂, and MXene, are often introduced to facilitate electron accumulation, proton adsorption, and surface reaction kinetics. Therefore, the future development of H₂-evolution-oriented 2D/2D S-scheme photocatalysts may benefit from the coordinated optimization of band alignment, interfacial charge transport, cocatalyst spatial distribution, and surface proton-reduction sites, instead of emphasizing apparent charge-separation efficiency alone.

6.2 Photocatalytic CO₂ reduction

With rapid industrialization and massive fossil fuel consumption, excessive CO₂ emissions are a primary driver of climate change and attendant environmental issues.^[79] Consequently, CO₂ emission mitigation and valorization have attracted increasing attention in energy and environmental research. Solar-driven photocatalytic CO₂ reduction provides a green and sustainable route for converting CO₂ into value-added fuels and chemicals, including CO, CH₄, and CH₃OH.^[79,181] However, the high thermodynamic stability and kinetic inertness of CO₂, combined with the involvement of multielectron/proton-transfer steps and diverse reaction intermediates, impose substantial challenges on CO₂ activation and product-selectivity control.^[37] Photocatalytic CO₂ reduction involves a series of coupled elementary processes, including CO₂ adsorption, molecular activation, intermediate stabilization, proton-coupled electron transfer, and, in some cases, C-C coupling. Accordingly, photocatalyst design should move beyond charge-separation enhancement and place greater emphasis on active-site engineering, interfacial microenvironment modulation, and reaction-pathway regulation.^[182] The table (Table 2.) below summarizes the selectivity and yield of CO₂ reduction products over representative 2D/2D S-scheme heterojunction systems.

2D/2D S-scheme heterojunctions construct efficient interfacial charge-transfer channels for photocatalytic CO₂ reduction and enable active-site-mediated modulation of key intermediate evolution, thus regulating product selectivity. In the BP/Bi₂WO₆ system, ^[42] the intimate coupling between 2D BP and Bi₂WO₆ nanosheets forms an S-scheme heterojunction (Fig.25 a), in which the interfacial Bi-O-P channel facilitates photogenerated charge separation and promotes electron accumulation at the BP reduction sites. Coupling CO₂ reduction with benzylamine oxidation lowers the kinetic barrier of the oxidative half-reaction and improves the local CO₂/proton supply, thus promoting continuous proton-coupled electron transfer and C-C coupling. Product distribution (Fig. 25 c) and energy-barrier analyses (Fig.25 d, e) indicate that 6%BP/BWO favors further hydrogenation of *CO and subsequent C-C coupling, thereby shifting the CO₂ reduction pathway from C₁ products toward C₂H₅OH formation.

In contrast, the Zn-MOF/BiVO₄ system ^[185] exemplifies the role of MOF metal nodes in regulating CO₂ adsorption/activation and the C₁ product pathway. The porphyrin-based Zn-MOF and BiVO₄ nanosheets form an intimate 2D/2D S-scheme heterojunction through hydroxyl-induced assembly (Fig.25 f), where the porous framework and dispersed Zn₂(COO)₄ metal nodes of Zn-MOF facilitate CO₂ adsorption and activation. In-situ DRIFTS (Fig. 25 i) results reveal the formation of key intermediates, including monodentate/bidentate carbonate species, HCO₃⁻, COO⁻, and *COOH, on the 20Zn-MOF/BVON surface, suggesting that CO₂ conversion in this system mainly proceeds through CO₂ adsorption, *COOH formation, *CO generation, and CO desorption. These results indicate that product selectivity in 2D/2D S-scheme heterojunctions is not governed solely by charge separation but is collectively determined by interfacial charge transfer, active-site structure, intermediate stabilization, and the synergistic regulation of the oxidative half-reaction.

In summary, the design of 2D/2D S-scheme photocatalysts for CO₂ reduction should evolve from charge-separation optimization toward the integrated regulation of charge transfer, intermediate evolution, and local reaction environments. While rational band alignment and interfacial engineering are crucial for retaining electrons with strong reduction ability, product selectivity is largely dictated by CO₂ adsorption/activation and the stabilization or conversion of key intermediates. For C₁ products, controlling *COOH formation and *CO desorption is essential, whereas the formation of C₂ and higher-carbon products requires the enrichment of carbon-containing intermediates and adjacent active sites that favor C-C coupling. Representative Zn-MOF/BiVO₄ and BP/Bi₂WO₆ systems demonstrate that MOF metal nodes, interfacial bonds, and electron-rich low-dimensional sites can steer the *COOH/*CO pathway, *CO hydrogenation, and C-C coupling. Future studies should integrate in-situ spectroscopy, isotope-labeling experiments, and theoretical calculations to resolve the dynamic evolution of key intermediates and establish more reliable structure-activity-selectivity relationships for CO₂ photoconversion.

6.3 Pollutant degradation

Rapid economic development and the continuous discharge of refractory organic pollutants have created an increasing demand for sustainable water-remediation technologies. Semiconductor photocatalysis provides a green strategy for pollutant degradation by harvesting solar energy to generate electron-hole pairs. During photocatalytic degradation, photogenerated holes can directly oxidize adsorbed pollutants or convert surface-adsorbed H₂O/OH⁻ into , whereas photogenerated electrons reduce dissolved O₂ to , thereby driving the oxidative decomposition and mineralization of organic contaminants ^[2,195]. However, rapid charge recombination often suppresses ROS generation and limits photocatalytic efficiency.2D/2D S-scheme heterojunctions are particularly attractive for ROS-mediated degradation because their intimate interfacial contact and internal electric field promote directional charge separation while preserving carriers with strong redox potentials ^[94,196,197]. Accordingly, their degradation performance is governed not only by charge-separation efficiency but also by ROS-generation and hole-oxidation pathways, active-site accessibility, and structural stability in complex aqueous environments.^[198] The table (Table 3.) below summarizes the degradation performance of organic pollutants over representative 2D/2D S-scheme heterojunction systems.

Several representative systems demonstrate how 2D/2D S-scheme architectures regulate ROS generation during photocatalytic pollutant degradation. Huang et al.^[198] constructed a Bi₂MoO₆/BiOCl_0.7I_0.3 2D/2D S-scheme van der Waals heterojunction for tetracycline hydrochloride degradation. The optimized BMO-BOCI composite promoted interfacial charge separation and enhanced the generation of both •O₂⁻ and •OH, indicating that 2D/2D S-scheme coupling can preserve carriers with sufficient redox potentials for O₂ reduction and H₂O/OH⁻ oxidation.

The CNQDs/TCN/ZnIn₂S₄ system ^[208] illustrates how multilevel interfacial engineering can broaden ROS-generation pathways. By integrating CNQDs-induced Schottky contacts with a TCN/ZnIn₂S₄ S-scheme heterojunction (Fig.26 a), additional electron-migration channels are established beyond the binary 2D/2D interface. Radical-trapping results (Fig. 26 c, d) show that •OH, •O₂⁻, and h⁺ jointly contribute to n-tetradecane degradation. In the binary TCN/ZnIn₂S₄ system, •OH serves as the dominant oxidative species, whereas CNQDs incorporation further promotes •O₂⁻ formation by facilitating electron extraction and O₂ reduction. These results suggest that Schottky/S-scheme coupling enables both hole-driven •OH formation and electron-mediated O₂ activation, thereby supporting efficient petroleum hydrocarbon degradation in complex aqueous environments.

The PCN/BOCI system ^[145] demonstrates that heteroatom doping and 2D/2D van der Waals coupling can further tune the dominant ROS pathway. In the P-doped g-C₃N₄/BiOCl_0.75I_0.25 heterojunction, P doping modulates the electronic structure of g-C₃N₄ (Fig.26 f), while the intimate 2D/2D interface facilitates directional S-scheme charge transfer. Quenching (Fig. 23 i) and EPR results (Fig. 26 g, h) indicate that h⁺ and •O₂⁻ are the primary active species for tetracycline degradation, with a comparatively minor contribution from •OH. Accordingly, PCN/BOCI degradation mainly proceeds through a coupled h⁺/•O₂⁻ oxidation pathway, in which holes directly oxidize adsorbed pollutants and electrons reduce dissolved O₂ to •O₂⁻.

Overall, ROS generation provides a key mechanistic bridge between S-scheme charge transfer and photocatalytic pollutant degradation. For 2D/2D S-scheme photocatalysts designed for environmental remediation, efficient charge separation alone is insufficient; the spatial distribution and redox capability of the retained carriers must be precisely regulated to promote O₂ reduction, H₂O/OH⁻ oxidation, and direct hole oxidation. Interfacial coupling, heteroatom doping, defect engineering, cocatalyst loading, quantum-dot modification, and van der Waals interface construction can all modulate the relative contributions of h⁺, •O₂⁻, and •OH. Future design should therefore integrate directional charge migration with controllable ROS generation, while ensuring sufficient active-site exposure and structural stability in complex aqueous environments. Establishing clear structure-charge dynamics-ROS generation-degradation relationships will be essential for developing efficient and durable photocatalysts for environmental remediation.

6.4 Photocatalytic H₂O₂ production

Hydrogen peroxide (H₂O₂) is widely used in environmental remediation, chemical synthesis, and antibacterial treatment, making its green and efficient production highly important. The traditional anthraquinone process suffers from high energy consumption, a complicated procedure, and pollution issues, while direct synthesis is also limited by safety concerns and low selectivity.^[47,209] Photocatalytic H₂O₂ production has therefore attracted increasing attention as a mild and sustainable alternative.^[210,211] In most semiconductor photocatalytic systems, H₂O₂ is mainly generated through the two-electron oxygen reduction reaction (2e⁻ ORR), whereas the two-electron water oxidation pathway is limited by sluggish kinetics and competing oxygen evolution.^[212,213] Efficient H₂O₂ photosynthesis thus requires not only effective charge separation, but also selective O₂ activation, stabilization of oxygenated intermediates, suppression of the competing four-electron ORR, and inhibition of H₂O₂ decomposition. 2D/2D S-scheme heterojunctions provide a promising platform for this reaction because their extended interfacial contact, shortened charge-transfer distance, and built-in electric field facilitate directional carrier migration while preserving strong redox potentials.^[120] When coupled with defect engineering, heteroatom doping, or polar-interface regulation, these systems can further modulate local electronic structures and surface reaction pathways, thereby enhancing ORR selectivity and steering ROS generation toward H₂O₂. Therefore, the design of 2D/2D S-scheme photocatalysts for H₂O₂ production should move beyond charge-separation enhancement alone and emphasize the integrated regulation of interfacial charge transfer, O₂ activation, reaction selectivity, and peroxide stability.

Shi et al.^[214] developed a homogeneously distributed SCN/VS-SnS₂ 2D/2D S-scheme heterojunction through one-step CVD growth (Fig.27 a). The schematic illustrations indicate that the in-situ coupling of S-doped carbon nitride with S-vacancy-containing SnS₂ generates an intimate 2D/2D interface, which shortens the interfacial charge-transfer distance and reinforces the built-in electric field. Such an interfacial configuration favors directional S-scheme charge migration, retaining highly reducing electrons on SCN for O₂ reduction and strongly oxidizing holes on VS-SnS₂ for hole-mediated water oxidation (Fig. 27 c, d). Consequently, SCN/VS-SnS₂ exhibits substantially enhanced H₂O₂ production in pure water compared with the individual components (Fig. 27 b). The average electron-transfer number close to two further suggests a preferential 2e⁻ ORR pathway, rather than the competing four-electron ORR (Fig. 27 e). These results demonstrate that efficient H₂O₂ photosynthesis in this system arises from the cooperative optimization of S-scheme charge transfer, S-doping-induced O₂ activation, and S-vacancy-assisted water oxidation.

Zhang et al. ^[75] further emphasized the role of polar-surface regulation in a BP/BiOCl-based 2D/2D S-scheme heterojunction. Compared with pristine BiOCl, BP/BiOCl shows enhanced H₂O₂ accumulation (Fig. 27 f), indicating that 2D/2D S-scheme coupling facilitates O₂-reduction-dominated H₂O₂ formation. Upon polar-surface modification, the shortened triplet-exciton lifetime of H/BP/BiOCl evidences accelerated exciton dissociation (Fig. 27 h, i), which is associated with a shift in ROS evolution from short-lived ¹O₂ toward H₂O₂ generation. The improved bacterial inactivation efficiency under simulated sunlight further indicates that H₂O₂-mediated ROS chemistry contributes to photocatalytic water disinfection (Fig.27 g). Therefore, polar-interface engineering expands the role of 2D/2D S-scheme heterojunctions from charge-separation enhancement to pathway-selective ROS regulation for H₂O₂ production and antibacterial applications.

Overall, photocatalytic H₂O₂ production over 2D/2D S-scheme heterojunctions should be viewed as a pathway-selective redox process rather than merely a charge-separation enhancement strategy. The intimate 2D/2D interface, built-in electric field, and S-scheme charge-transfer pathway can maintain the high redox potentials of photogenerated carriers, while defect engineering, heteroatom doping, and polar-interface regulation further modulate O₂ activation, oxygenated-intermediate stabilization, exciton dissociation, and ROS generation pathways. These effects collectively favor the 2e⁻ ORR pathway, suppress competing side reactions, and inhibit H₂O₂ decomposition. Therefore, future development of 2D/2D S-scheme photocatalysts should emphasize the integrated regulation of interfacial charge transfer, surface reaction selectivity, and H₂O₂ formation, preservation, and downstream activation, particularly for water disinfection and environmental remediation applications.

7. Conclusions and outlook

2D/2D S-scheme heterojunctions have emerged as promising photocatalytic platforms for solar energy conversion and environmental remediation. Their face-to-face architectures afford large interfacial contact areas, shortened carrier migration distances, abundant exposed active sites, and favorable conditions for the formation of built-in electric fields. Driven by band bending and interfacial electric fields, low-energy electrons and holes selectively recombine at the heterointerface, while electrons with high reduction potentials and holes with high oxidation potentials are well retained for subsequent surface redox reactions. This unique charge-transfer mode enables efficient carrier separation while preserving robust redox capabilities, which is essential for photocatalytic H₂ evolution, CO₂ reduction, pollutant degradation, and H₂O₂ production.

Recent advances have deepened the fundamental understanding of interface construction, charge-transfer mechanisms, and reaction-specific optimization of 2D/2D S-scheme systems. Interface engineering has evolved from simple physical assembly to more controllable strategies, including in-situ growth, interfacial bonding, molecular bridging, defect regulation, and surface chemical modification. Meanwhile, characterization techniques such as TEM/HRTEM, AFM, XPS, KPFM, EPR, in-situ XPS, transient spectroscopy, together with DFT theoretical calculations, have provided solid and reliable evidence for verifying the S-scheme charge migration behavior. Furthermore, modification strategies involving heteroatom doping, defect engineering, cocatalyst loading, single-atom regulation, and multidimensional interface construction have significantly promoted interfacial charge transfer, improved active-site utilization, and optimized surface reaction kinetics.

Despite these remarkable achievements, several critical challenges still remain. The precise modulation of stacking orientation, interlayer spacing, interfacial bonding configuration, defect distribution, and active-site location at 2D/2D heterointerfaces remains difficult to realize. In addition, the identification of S-scheme charge transfer heavily relies on indirect experimental evidence, making it difficult to unambiguously distinguish S-scheme pathways from Type-II transfer, Z-scheme transfer, defect-mediated migration, and cocatalyst-assisted charge extraction. Moreover, quantitative correlations among work-function difference, built-in electric-field strength, interfacial coupling intensity, carrier dynamics, and photocatalytic activity have not been fully established. The practical applications of 2D/2D S-scheme photocatalysts are also restricted by inherent photo corrosion, difficult catalyst recovery, unsatisfactory long-term stability, dependence on sacrificial agents or noble metals, and the lack of unified and standardized performance evaluation protocols.

Future research should be directed toward predictive, mechanism-guided, and application-oriented photocatalyst design (Fig. 28). Machine learning and high-throughput computational screening can accelerate the rational selection of optimal 2D/2D material combinations by theoretically predicting band alignment, work-function difference, interfacial stability, charge-transfer behavior, and adsorption energies of key reaction intermediates. Such data-driven strategies can effectively expand the material library beyond conventional g-C₃N₄ and Bi-based systems to emerging candidates, including MXenes, MOFs, COFs, HOFs, black phosphorus, layered double hydroxides, and halide perovskites.

From a mechanistic perspective, advanced in-situ and operando characterization techniques should be extensively adopted to dynamically track interfacial charge migration, active-site evolution, surface reconstruction, and intermediate transformation under realistic reaction conditions. The integration of operando spectroscopy, time-resolved kinetic analysis, isotope-labeling experiments, and theoretical simulations is crucial for distinguishing genuine S-scheme charge transfer from other competing charge-transfer pathways and establishing a comprehensive structure-charge dynamics-intermediate-activity relationship.

For practical deployment, conventional powder photocatalysts require structural integration and device optimization, such as immobilized films, membranes, functional electrodes, monolithic carriers, and continuous-flow reactors. Such structural configurations can effectively address catalyst recycling issues, enhance mass transfer efficiency, improve operational stability, and enable reliable long-term performance evaluation under application-relevant conditions. The integration of data-driven material screening, operando mechanistic analysis, and reactor-level engineering will further promote the development of 2D/2D S-scheme heterojunctions from fundamental laboratory research to practical technologies for sustainable energy conversion and environmental purification.

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