Progress on 2D–2D heterostructured hybrid materials for efficient electrocatalysis

Abstract

Two-dimensional (2D) materials are widely used in many energy conversion reactions, especially 2D–2D heterostructured hybrid materials that have been extensively investigated in recent years. In this review article, we provide a comprehensive review of the recent research progress on 2D–2D hybrid materials, starting from the introduction of these emerging hybrids. Then, the fabrication strategies are discussed with their pros and cons. Furthermore, the techniques, especially those for heterointerface characterization, are highlighted. Afterward, the applications of these 2D–2D heterostructured hybrid materials in energy conversion reactions with representative examples in the last five years are discussed and summarized. Finally, a summary and perspectives for realizing the rational design and synthesis of 2D–2D hybrids as advanced electrocatalysts towards active and stable energy conversion reactions are provided.

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Xia Long

Xia Long is a tenured Associate Professor at Shanghai Jiao Tong University. She obtained a PhD degree from Peking University in 2013, followed by working at The Hong Kong University of Science and Technology, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, and Peking University Shenzhen Graduate School successively. Her current research interests focus on low carbon energy materials and devices, noble metal-free catalysts and devices for energy conversion and storage.

Lihua Zhang

Lihua Zhang is working at Shanghai Jiao Tong University as a postdoctoral researcher. She received her BS degree in Energy Materials from Southern University of Science and Technology in 2016. She received a PhD degree in chemistry from The Hong Kong University of Science and Technology in 2020. Her work focuses on the synthesis of novel and functional nanostructured materials, including perovskite oxides and high-entropy oxide nanoparticles, to explore their potential applications in electrocatalysis.

Zheng Tan

Zheng Tan received his bachelor's degree from School of Mechanical Engineering, Shanghai Jiao Tong University in 2021 and is pursuing his master's degree at China–UK Low Carbon College, Shanghai Jiao Tong University. His research focuses on the surface and interface engineering of noble metal-free catalysts for efficient hydrogen production via water splitting.

Bowei Zhou

Bowei Zhou received his bachelor's degree in New Energy Science and Technology from Northeastern University in 2022 and is pursuing his master's degree at China–UK Low Carbon college, Shanghai Jiao Tong University. His research focuses on key materials and techniques for hydrogen production via seawater electrolysis.

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Introduction

Developing renewable green energy sources is one of the most critical topics in material- and energy-related fields nowadays, due to the growing demand for energy and increasing concerns regarding environmental pollution problems.^1,2 Meanwhile, the generation and conversion efficiency of these renewable energy systems largely depend on the effectiveness of the catalysts. Among the developed functional materials for energy conversion reactions, such as hydrogen production via water splitting, carbon dioxide reduction to form valuable chemicals and nitrogen reduction to form ammonia, etc., two-dimensional (2D) materials offer unique advantages for these processes due to their large specific surface area, high charge migration rate, adjustable electronic properties, ultralight weight, assembly properties to form heterostructures, high mechanical flexibility, etc.^3–8

Actually, 2D materials have boomed since the successful fabrication of graphene from graphite in 2004 by Novoselvo and co-workers.⁹ Since then, many 2D materials have been developed such as MXenes,^10,11 layered double hydroxides (LDHs),^8,12,13 transition metal-based dichalcogenides (TMDs),^14–17 black phosphorus,^18–20 covalent-organic frameworks (COFs)^21,22 metal–organic frameworks (MOFs),^23–25 metallenes,²⁶ borophene,²⁷etc.^28,29 The unique physical, electronic and chemical properties of these 2D materials mean that they can be expected to have rich and exciting structural and physiochemical properties, making them promising for numerous applications in electronics, sensing, photonics, catalysis, bioengineering, energy harvesting, flexible electronics, etc.^3,7,8,30

Furthermore, based on the unique assembly properties of 2D materials, they can be easily combined with other functional materials to form heterostructured hybrids that would give rise to intriguing features, such as integrating the merits and overcoming the weaknesses of the individual counterparts and even generating new properties or functions.^31–33

Generally, among all the 2D heterostructured hybrids, the 2D–2D heterostructures have the largest interfacial contact area and the strongest interactions between the two distinct components (Fig. 1A), when compared with the 0D–2D and 1D–2D heterostructures. So, the coupling of the heterointerfaces in the face to face stacking of 2D–2D hybrid materials would be intense, which would further benefit the rearrangement of the electronic structures of the active sites and facilitate the charge transfer during energy conversion reactions, thus contributing to improved catalytic performance.^34,35 Owing to the above-mentioned merits (Table 1), the 2D–2D heterostructured hybrid materials exhibit vast application prospects and a large number of related works have been reported recently. Though there are many excellent review articles on 2D-based materials,^7,33,36–39 a comprehensive review on 2D–2D hybrids with representative examples reported in the last five years and the emphasis on the characterization and effects of heterointerfaces would provide a timely understanding of progress of this promising research field and also promote the development of 2D–2D heterostructured hybrid materials for efficient electrocatalysis towards energy conversion reactions.

Fig. 1 Schematic illustration of (A) the structure of 2D–2D hybrids, (B–D) 2D–2D hybrid materials made by (B) direct growth, (C) surface conversion, and (D) assembly processes, and (E) the interaction of the two components at the heterointerface.

Table 1 Summary of the merits of 2D–2D hybrids when compared with other kinds of hybrids

Hybrids	Merits
2D–2D	Large interfacial contact area
	Strong heterointerface interaction
	Intense coupling effects
	Favourable rearrangement of electronic structure
	Facilitating better charge transfer

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This review focuses on the most recently published and instructive studies based on the 2D–2D hybrid catalysts for energy conversion reactions (Fig. 1). Firstly, the fabrication methods of 2D–2D hybrid materials and their unique physicochemical properties will be introduced. Next, the techniques for the characterization of heterointerfaces and their effects will be discussed. Furthermore, the application of the 2D–2D heterostructured hybrid materials in energy conversion catalysis, especially towards hydrogen production via water splitting, will be fully demonstrated. Finally, the challenges and perspectives in this promising research area will be expounded based on the current research status.

Fabrication of 2D–2D heterostructured hybrid materials

The performance of 2D–2D heterostructured materials in energy conversion catalysis not only depends on their chemical composition, crystal phase, microstructure, and exposed facets, but also on the distribution of each component and the quality of the heterointerface. Therefore, rational design and controlled synthesis methods are critically important for achieving advanced catalytic performance of the as-prepared 2D–2D hybrid materials. Generally, the construction of 2D–2D hybrid materials with heterointerfaces could be classified as growth and conversion processes and assembly processes that will be introduced in detail in this section.

Growth and conversion processes

The growth of 2D–2D heterostructured hybrid materials can be generally described as the growth or formation of the second 2D material on the surface of the primarily synthesized 2D material (the substrate material) (Fig. 1B). Epitaxial growth is one of the most promising approaches for controlled synthesis of materials with heterointerfaces, which refers to a crystalline material grown on the well-defined surface of another crystalline material (the substrate) via chemical vapor deposition (CVD) or wet-chemical processes. The deposited crystalline material generally has the same orientation of crystal lattice as the substrate material. Graphene is one of the most widely used 2D substrates for epitaxial growth of 2D–2D hybrid materials such as graphene-TMDs^40,41 and graphene-MoC₂.⁴² In addition, h-BN has also been widely used as a 2D substrate material for epitaxial growth of graphene,⁴³ MoS₂,⁴⁴ and WS₂⁴⁵ to form the corresponding 2D–2D hybrids.

In addition to carbon materials, transition metal-based 2D materials have also been applied as the substrate for the growth of the second 2D materials. For example, by altering the growth temperature, MoS₂/WS₂ and WS₂/MoS₂ hybrids were synthesized via a two-step CVD strategy (Fig. 2A).⁴⁶ What is more, it was found that by carefully controlling the temperature (Fig. 2B), a one-step growth was enough for realizing the formation of both vertically stacked and in-plane interconnected heterostructured 2D–2D WS₂/MoS₂ hybrids (Fig. 2C).⁴⁷ This synthesis method could be further modified to have scalability and patterning characters for producing large-area 2D–2D hybrids.⁴⁸ Besides, many 2D heterostructured hybrids have been reported by using liquid phase epitaxial growth, such as CuS–TiS₂, ZnS–TiS₂, Ni₃S₂–TiS₂, etc.⁴⁹

Fig. 2 Schematic illustration of CVD processes for the formation of 2D–2D hybrids. (A) The tunable two-step CVD growth process for the fabrication of MoS₂–WS₂ and WS₂–MoS₂ 2D–2D hybrids. Reproduced with permission from ref. 46, copyright Wiley. (B) Schematic of the synthesis process for vertical and in-plane structured WS₂/MoS₂ hybrids. (C) Schematic of the vertically stacked and in-plane WS₂/MoS₂ heterostructures at 850 °C and 650 °C, respectively. Reproduced with permission from ref. 47, copyright Springer Nature.

Though many 2D–2D hybrids have been successfully fabricated by epitaxial growth, the selection of the two components is relatively limited, which should have similar lattice structures to lower the degradation of the quality of the interface resulting from the lattice mismatch. In this regard, functionalization of the surface of 2D substrate materials to make them more suitable for the growth of the second 2D material is necessary. Gao et al.⁵⁰ fabricated Ti₃C₂ nanosheets with a negatively charged surface and then grew Bi₂WO₆ nanosheets on them and successfully constructed Ti₃C₂–Bi₂WO₆ hybrid nanosheets. By similar wet-chemical processes, Xia et al.⁵¹ also fabricated a MnO₂–gC₃N₄ hybrid by adsorption of Mn²⁺ ions on the defect- and OH-rich g-C₃N₄ nanosheets.

Different from the growth of the second 2D component, the controlled topotactic conversion of the 2D substrate materials is also an efficient method for the fabrication of 2D–2D hybrids (Fig. 1C). Actually, morphology-retained topotactical conversion processes have been successfully applied to synthesize many functional materials to change their physiochemical properties and then catalytic performance.^52–55 For 2D–2D hybrid construction, Qian et al. fabricated a Bi₄Ti₃O₁₂/I-BiOCl heterostructured hybrid by adding HCl into a Bi₄Ti₃O₁₂ nanosheet suspension. The surface of the Bi₄Ti₃O₁₂ nanosheets was dissolved in the presence of HCl and the released Bi³⁺ ions near the surface hydrolyzed to BiO⁺, which was further transformed to IBiOCl, resulting in the formation of a Bi₄Ti₃O1₂/I-BiOCl 2D–2D hybrid.⁵⁶ Recently, via an in situ topotactic hydrothermal transformation process, Shi et al. constructed a Cu₂S/Zn_0.67Cd_0.33S hybrid nanosheet with a 2D–2D atomic level in-plane heterointerface by using 2D CuZnCdAl LDH as the precursor.⁵⁷

Assembly processes

Distinct from the above-mentioned growth or conversion processes, the formation of 2D–2D hybrids was achieved via assembly processes generally through the following steps: first, the two 2D components were designed and synthesized separately and then these two pre-synthesized 2D materials were combined together through assembly processes (Fig. 1D). In contrast to the epitaxial growth of 2D–2D hybrids that have a lattice matching constraint, the 2D–2D heterostructures were made by assembly processes often attracted by much weaker electrostatic or van der Waals forces, enabling much more selection space for the construction of 2D–2D hybrids (Fig. 1E).^58,59 Moreover, although the force is relatively weak, the coupling effects between the two 2D components around the interface are still strong enough to affect the re-organization of the electronic structure and then mediate the catalytic performance of the hybrid material for energy conversion catalysis.

Considering that both the CrSe₂ and Ti₃C₂ MXene are advanced energy storage materials with 2D layered structures, Raj et al. constructed a CrSe₂/Ti₃C₂ MXene 2D/2D hybrid by a facile assembly process. The as-prepared hybrids showed a promising energy storage property owing to the increased number of active sites and improved electrochemical stability arising from the synergistic interplay effect triggered by fast ionic/electronic transportation, reduced agglomeration and volume expansion.⁶⁰ Recently, Chen et al. reported an LDH-birnessite hybrid by a self-assembly of exfoliated LDH nanosheets and birnessite microplates, by taking advantage of the positively charged LDH and negatively charged birnessite and the 2D characters of these two components that readily formed a film.⁶¹ The coupling of the two components at the atomic level allows the formation of a built-in electric field in the hybrid, which contributed to lowering the overpotential of the OER catalysed by this heterostructured hybrid catalyst. Actually, a variety of advanced 2D–2D materials have been successfully synthesized by this route, such as WO₃-ZnIn₂S₄,⁶² black phosphorus-Bi₂WO₆,⁶³ Fe₂O₃/g-C₃N₄,⁶⁴ metal-free phosphorene (FP)/graphitic carbon nitride,⁶⁵etc.

In addition to the low temperature assembly process, 2D–2D hybrids were also prepared via the assembly of two components under hydrothermal conditions, to further enhance the coupling of the two materials.^66,67 More recently, Ju et al. assembled an LDH with ppy to form an organic–inorganic 2D–2D hybrid of LDH-ppy via two steps. Firstly, the negatively charged pyrrole monomer was intercalated into the interlayer of the LDH nanosheets, where they were further polymerized to form a 2D ultrathin film by ultraviolet illumination,⁶⁸ suggesting the universality of the assembly process for the fabrication of 2D–2D hybrid materials with various chemical compositions.

Besides wet-chemical assembly processes, the solid state sintering process has also been proven to be an effective approach for the construction of 2D–2D heterostructured materials. For example, Xu et al. mixed Bi₄NbO₈Cl and g-C₃N₄ nanosheets together by ball-milling and then annealed the mixture in air. The as-prepared Bi₄NbO₈C–g-C₃N₄ showed a clear heterointerface, suggesting the 2D–2D heterostructure of the hybrid.⁶⁹ Similarly, a Bi₄Ti₃O₁₂–Ni(OH)₂ 2D–2D hybrid was also reported by the solid state sintering process.⁷⁰ Though there is no lattice matching constraint for the selection of the two 2D components to form the 2D–2D hybrid via solid state sintering methods, they should be chemically and structurally stable during the high temperature annealing procedures. What is more, the interface adhesion in these 2D–2D hybrids is relatively weak, which would probably result in the deterioration of the heterointerface during the catalytic performance evaluation. Therefore, rationally modifying the surface of the 2D materials to consolidate the heterointerface with electrostatic, van der Waals force or even covalent bonding would improve the stability of the 2D–2D hybrid catalysts.

Though the above-mentioned assembly method could produce a large quantity of 2D–2D hybrid materials, they cannot realize the fabrication of large-area 2D–2D hybrids that maintain the intrinsic properties of the two components, while producing pristine interlayer heterointerfaces. Therefore, Kang et al.⁵⁹ developed a programmed vacuum stack process (PVS) that could generate wafer-scale 2D–2D hybrid films with a very high level of spatial uniformity and pristine interfaces (Fig. 3). The chemical composition and properties of these 2D–2D hybrid films could be designed at atomic scale using layer-by-layer assembly of the 2D building blocks under a vacuum. What is more, the as-prepared large-scale hybrid films were detachable, suspendable and compatible with water or other surfaces, enabling the integration of the prepared 2D–2D hybrid materials with advanced optical and mechanical systems.

Fig. 3 (A) A schematic of the programmed vacuum stack (PVS) process. (B) The cross-sectional STEM image of a MoSe₂/MoS₂/WS₂ film with the electron beam aligned with the armchair axis of MoSe₂ (top) and MoS₂ (middle). Reproduced with permission from ref. 59, copyright Springer Nature.

In summary, the growth and conversion methods for preparing 2D–2D hybrid materials start from the synthesis of the 2D substrate materials, and then the second 2D materials are in situ grown or formed via topotactic conversion. Therefore, this method would be scalable for constructing many different kinds of 2D–2D hybrid materials with relatively strong interface interaction. What is more, the 2D–2D hybrids constructed via the in situ CVD process generally have high quality heterointerfaces that are even suitable for applications in electronics, though expensive facilities are required, while the assembly process is the most universal method to fabricate 2D–2D hybrids by using the two 2D materials that have been synthesized separately as the building blocks. Therefore, the 2D–2D hybrids made by the assembly method feature relatively larger interlayer spacings and more defects, which could benefit the mass/charge transport and provide more active sites during the electrochemical processes. To further realize the uniform hybridization, and then enhance the catalytic performance of the 2D–2D hybrids via the assembly processes, the quality and the surface charge of the two 2D building blocks, and the assembly conditions such as temperature, stirring speed, dispersing agent, etc. should be well controlled.

Characterization and roles of heterointerfaces

The techniques for characterization of heterointerfaces are vital to guarantee their quality and figure out the role of the interface for catalysis and then promote the development of heterostructured hybrid materials with tailored interfacial structures and properties. Therefore, to fully understand the critical roles of the heterointerfaces and to further improve the overall performance of the hybrids, it is of great importance to obtain accurate information about the microscopic structures and properties of the heterointerfaces by applying reliable characterization techniques.

Electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are powerful and extensively used for visualization of the surface of materials at the nanometer scale. However, they could not provide the critical information of the heterointerface of hybrid materials. However, high resolution TEM (HRTEM) that could achieve atomic resolution, when coupled with the focused ion beam lithography (FIB) technique, is a widely used tool for getting the lattice alignment at the heterointerfaces of the 2D–2D hybrid materials. For example, the interface of Cu_1.94S–CdS 2D–2D nanoplates has been well observed by HRTEM, which showed well-aligned lattice fringes of Cu_1.94S (800) and CdS (100) planes.⁷¹ The boundary between TiS₂–CuS heterostructured nanoplates was also characterized by HRTEM and SAED,⁴⁹ suggesting the universality of this technique for investigating the interfaces of heterostructured materials.

X-ray photoelectron spectroscopy (XPS) is a useful surface sensitive spectroscopic technique to determine the elemental composition of materials. For the 2D–2D hybrids, the coupling effects arising from the close contact of two distinct materials at the heterointerface would change the electronic configuration of the material, which could be identified by XPS. What is more, the interaction between the two materials would change the surface potential of the hybrid, which could be well characterized by Kelvin probe force microcopy (KPFM). Specifically, KPFM gives precise information on the work function distribution of the materials and the interfacial interaction between the 2D materials. For example, MoS₂/WS₂ hetero-junction arrays have been electrically characterized by using conducting atomic force microscopy (CAFM) and the surface potentials of MoS₂ and WS₂ layers were measured by KPFM, which showed a built-in field at the interface to align the Fermi level.⁷² Recently, due to the structural compatibility of Bi₂WO₆ and Bi₂O₂S, both of which were 2D semiconductors sharing a similar structural unit of a [Bi₂O₂]²⁺, Xing et al.⁷³ fabricated an ultrathin nanosheet with a tightly bonded 2D–2D heterojunction between the two components by facilely joining the [Bi₂O₂]²⁺ and [S]²⁻ slices (Fig. 4A). The as-prepared Bi₂WO₆–Bi₂O₂S 2D–2D hybrid not only extended light absorption but also significantly enhanced photo(electro)chemical water splitting efficiencies due to the close bonding-promoted interfacial charge separation that has been observed by in situ KPFM (Fig. 4B and C).

Fig. 4 (A) Schematics of the hydrothermal synthesis process and the transformation process from the monolayered Bi₂WO₆ nanosheet to the Bi₂WO₆-Bi₂O₂S 2D–2D heterojunction nanosheet. (B) The AFM and KPFM images in the dark and under illumination of Bi₂WO₆–Bi₂O₂S nanosheets. (C) Schematics of the charge flow direction in a 2D–2D heterojunction. Reproduced with permission from ref. 73, copyright ACS.

Though HRTEM gives lattice information and KPFM provides the work function distribution of hybrid materials, they could not determine the exact elemental location around the heterointerface, which is critically important for identifying the boundary of the heterointerface and then facilitating the investigation of the critical roles of the heterointerface for energy conversion catalysis. Electron tomography (CT), which could provide the three-dimensional structure of complex materials in nanometer resolution, is powerful for the characterization of heterostructured catalytic materials, especially the scanning transmission electron microscopy (STEM). STEM is one of the most widely used imaging modes that avoids the appearance of diffraction-related artifacts and is also highly sensitive to the chemical composition of the material. Therefore, it has been rapidly developed as a powerful technique for the characterization of heterostructured catalytic systems,⁷⁴ especially the 2D–2D hybrid catalysts. For example, the heterointerface of Bi₂Te₃ with a six-fold symmetry and FeTe with a four-fold symmetry was clearly observed by spherical aberration-corrected STEM.⁷⁵ Both the high-angle annular dark field (HAADF, Fig. 5A) and annular bright field (Fig. 5B) images have high contrast with significant sensitivity to the atomic number and thus provide robust visibility of both light and heavy atoms. From the high-magnification HAADF image (Fig. 5C), the Bi₂Te₃ and FeTe were found to be separated by a van der Waals gap with their own lattices undisrupted, confirming the van der Waals epitaxial growth of this 2D–2D hybrid material. Similarly, through atomic-resolution HAADF-STEM, a continuous lattice fringe without any grain boundary or misorientation in a lateral WSe₂–MoS₂ heterohybrid was also clearly observed⁷⁶ and the distribution of Mo and W atoms around the heterointerface could be well determined due to the distinct weights of the two elements, suggesting that powerful STEM provides both the structural and chemical information of the heterointerface of 2D–2D hybrid materials.

Fig. 5 The scanning transmission electron microscopy micrographs of a Bi₂Te₃/FeTe heterostructure. (A) The HADDF image. (B) The annular bright field image. (C) The higher-magnification HADDF image showing the atomically sharp interface between Bi₂Te₃ and FeTe. Reproduced with permission from ref. 75, copyright Springer Nature.

Though there are many powerful techniques that could provide some important information on the heterointerfaces and many in situ/operando techniques that could visualize the dynamic behaviours of the catalyst surface, there are few techniques that could track the real time change of heterointerfaces during the catalytic reaction process. What is more, the complexity of the 2D–2D hybrid catalysts would further make the revelation of the effects of the heterointerfaces difficult. Therefore, to determine the critical role of the heterointerfaces in the catalytic performance of the 2D–2D hybrid materials, the coupling of many advanced in situ characterization techniques should be developed and a facile synthesis approach for controlled fabrication of well-designed 2D–2D hybrid materials that have relatively simple structures and chemical compositions would also be critically important.

Application of 2D–2D hybrid materials for energy conversion reactions

As mentioned above, 2D materials offer unique merits to energy conversion catalysis due to their large specific surface area, high charge migration rate, tuneable electronic properties, ultralight weight, assembly properties and high mechanical flexibility. What is more, the 2D–2D hybrids with heterostructures and heterointerfaces could further overcome many drawbacks of the single component 2D materials. For energy conversion reaction catalysis, the coupling of the two components at atomic level was found to enhance the activity of the catalytic material,⁷⁷ optimize the electronic structure of active ions and also accelerate the electron transport processes, all of which would contribute to the enhanced inherent catalytic activity of the 2D–2D hybrid materials.⁷⁸ Actually, the advanced 2D–2D hybrids have been widely applied in diverse energy conversion reactions that will be fully discussed with typical examples reported in the last five years in this section.

Hydrogen produced by water splitting is one of the most promising alternatives to replace the fossil fuels, owing to its high energy density (142 MJ kg⁻¹), zero-carbon combustion production and eco-friendly synthesis route.^79,80 For realizing efficient hydrogen production, effective catalysts are critically important and should possess a low overpotential, fast reaction kinetics and good long-term stability in practical applications. Early in 2014, Long et al.⁸¹ reported a reduced graphene oxide (rGO) intercalated NiFe LDH 2D–2D hybrid as an efficient OER catalyst in alkali solutions. The direct contact of the highly conductive rGO and OER active NiFe LDH at the atomic level greatly enhanced the charge transfer properties and accelerated the OER kinetics. Later, a superlattice of alternately stacked GO-LDH and rGO-LDH hybrids was further synthesized and applied as both the cathode and anode, which exhibited advanced full water splitting performance that could be driven by a commercial 1.5 V AA battery.⁸² What is more, many other 2D materials such as MXenes, TMDs, etc. coupled with 2D carbon (graphene, graphene oxides, and rGO) have also been reported, which showed much improved catalytic performance for hydrogen production via water splitting, due to the increased active sites and accelerated mass transfer arising from the support of 2D carbon that inhibited the self-stacking of the catalytic active 2D materials and the much enhanced conductivity that improved the charge transfer.^83–88

In addition to carbon materials, transition metal-based 2D nanomaterials have also been coupled with 2D black phosphorus to construct efficient 2D–2D heterostructured catalysts for water splitting.^89–91 Mei et al.⁹² reported heterostructured Ni(OH)₂-black phosphorus (BP) hybrid nanosheets (BNHNSs) by combining the liquid-exfoliated and mildly oxidized BP with wet-chemical synthesized Ni(OH)₂ nanosheets (Fig. 6A). According to the charge density difference (CDD) and electron localization function (ELF) maps (Fig. 6B), an interfacial charge transfer from the 2D Ni(OH)₂ nanosheets to the 2D BP nanosheets at the heterointerface could be clearly observed, leading to a favourable electronic structure of the as-prepared BNHNSs. What is more, the adsorption properties of BNHNSs were also found to be desirable for intermediates of alkaline water splitting reactions. Therefore, when applied as an OER catalyst, the 2D–2D BNHNS heterostructured hybrid exhibited excellent catalytic performance with a low overpotential, fast reaction kinetics (Fig. 6C) and long-term stability (Fig. 6D).

Fig. 6 (A) Schematic illustration of the synthesis of 2D–2D heterostructured nanosheets (BNHNSs) by coupling 2D Ni(OH)₂ nanosheets (NH-NSs) and 2D black phosphorus (BP) nanosheets (BP-NSs). (B) The charge density difference and electron localization function maps of 2D–2D BNHNSs. (C) Comparison of the overpotentials with those of the reported electrocatalysts at 10 mA cm⁻². (D) Time-dependent overpotential curve of the 2D–2D BNHNSs catalyst in KOH for 1200 min. Reproduced with permission from ref. 92, copyright Wiley.

The combination of two 2D materials, both of which have excellent redox characters that could work as electrocatalysts, have attracted much attention. For example, Shinde et al.⁹³ developed a MoTe₂/Ti₃C₂T_x 2D–2D hybrid that exhibited efficient HER activity in acidic solutions. The overpotential for attaining the current density of 10 mA cm⁻² was as low as 293 mV and the Tafel slope was 65 mV dec⁻¹, which was much lower than the two single component counterparts. Based on the analysis of the electronic density of states (DOS), charge transfer, and charge density plot, the authors found that an improvement in electronic density states near the Fermi energy for the hybrid system due to orbital hybridization increased the conductivity of the hybrid material. More recently, Chen et al.⁶¹ reported a LDH-birnessite hybrid that realized the direct contact of two OER active species of brucite layer of LDH and MnO_x layer of birnessite at the atomic level (Fig. 7A). They found an electron transfer from Ni sites to both the in-plane Fe site and through-plane Mn sites, which not only greatly idealized the electronic structure of the three catalytic active sites (Fe and Ni in LDH and Mn in birnessite, Fig. 7B), but also created a built-in electric field (Fig. 7A) that accelerated the charge transfer during the OER process, hence greatly increasing the reaction kinetics of the alkali OER (Fig. 7C and D). The as-prepared 2D–2D hybrid solved the instability problem of LDH and the low activity of birnessite together, leading to an excellent OER performance with both high activity and long-term stability even under industrial alkali water splitting conditions (Fig. 7E).⁶¹ Similarly, transition metal-based LDH-TMDs with different chemical compositions,⁹⁴ LDH-Mxenes,^95–102 MXene-TM sulphides,¹⁰³ and LDH-LDH¹⁰⁴ have been successfully synthesized and all of them showed improved catalytic performance towards water splitting, due to the synergistic effect arising from the coupling of two components at the heterointerface.^105,106 However, despite much progress on the fabrication of 2D–2D hybrid materials towards efficient water splitting and many of them even showing superior activity under industrial alkali water splitting conditions, the long-term stability and durability the 2D–2D hybrid catalysts should be further improved and evaluated to meet the requirements of practical applications.

Fig. 7 (A) Schematic illustration the built-in electric field in the LDH-Bir 2D–2D hybrid. (B) PDOS of LDH-Bir, (C) lsv and (D) Tafel plots of LDH-Bir and the control catalysts, and (E) long term stability of LDH-Bir under the industrial alkali water splitting conditions. Reproduced with permission from ref. 61, copyright Wiley.

In addition to the production of green hydrogen via water splitting, efficient CO₂ reduction (CO₂R) could mitigate the environmental pollution problems, while generating valuable chemicals. However, efficient CO₂R is still challenging for realizing high activity, selectivity and stability, in which, the advanced catalysts are critically important. Due to the above-mentioned structural merits, 2D–2D heterostructured hybrid materials have promising potential as advanced CO₂RR catalysts.¹⁰⁷ For example, Gu et al. reported an ultrathin conjugated metalloporphyrin COF-graphene van der Waals heterostructured hybrid material for electrocatalytic CO₂ reduction. The strong electronic coupling between the van der Waals layers accelerated the dynamics of the CO₂ conversion process, leading to advanced conversion efficiency with high CO selectivity.¹⁰⁸

However, due to the strong reductive reaction conditions of the CO₂RR, the transition metal-based active species would easily be reduced, which would result in the change of both the physiochemical properties and microstructures of the catalysts and hence deteriorate the catalytic performance of the materials. For example, Shen et al. found that the CuS–Bi₂S₃ heterojunction would be in situ reconstructed to Cu-doped bismuth (CDB) during the CO₂ reduction reaction, though the as-formed CDB also exhibited CO₂RR catalytic performance with an industrially compatible current density of −1.1 A cm⁻² and high formate selectivity.¹⁰⁹ Therefore, the stability of the 2D–2D hybrids under the CO₂RR conditions should be paid more attention.

Actually, many strategies have been developed to stabilize the CO₂RR catalysts under the highly reductive conditions such as doping and surface/interface engineering^110–114 and rational construction of 2D–2D heterostructures is one of the promising methods. For example, Cai et al.¹¹⁵ reported metallic copper–Cu_xOF 2D hybrid nanoplates for the CO₂RR (Fig. 8A–C). The authors found that the Cu/Cu_xOF 2D–2D hybrid surface showed a strong perturbation of the electronic distribution (Fig. 8D) and the Cu-3d orbitals were located at a position close to the Fermi level (Fig. 8E), suggesting a higher d-band centre and thus an improved electroactivity. When applied as the CO₂RR catalysts, the prepared Cu/Cu_xOF 2D–2D hybrid exhibited excellent CO₂RR activity with high selectivity for acetic acid formation (Fig. 8F). The authors proposed that the F-modified heterointerface could effectively tailor the exposed facets of the metallic Cu nanoparticles, stabilize the oxidized copper active species under the CO₂RR conditions, and purposely induce the s, p–d coupling between the metal and hetero-anions, leading to the selective formation of acetate.

Fig. 8 2D–2D hybrid materials for the CO₂RR. (A) Schematic atomic structures of Cu(OH)F and the transformed Cu/Cu_xOF hybrid. (B) The SEM images with high and low (inset) magnifications of Cu/Cu_xOF 2D–2D nanosheets. (C) TEM images of Cu/Cu_xOF 2D–2D nanosheets. (D) 3D contour plot for electronic distribution and (E) PDOS of Cu/Cu_xOF. (F) FEs of all the products on Cu/Cu_xOF at various applied potentials. Reproduced with permission from ref. 115, copyright RSC.

However, despite many successful applications of 2D–2D hybrids for efficient CO₂RR and other energy conversion reactions such as nitrogen fixation,^{107,116–119} this promising research area is still in an initial stage that needs more efforts. Revealing the critical roles of heterointerface by using advanced techniques is of importance for directing the rational design of advanced 2D–2D materials and developing a general and controllable construction method to realize the fabrication of targeted 2D–2D heterostructured hybrid materials with particular chemical composition and interface properties is also critical for achieving efficient energy conversion with high activity, selectivity and stability.

Summary and perspectives

The 2D–2D heterostructured hybrid materials have the largest interfacial contact area and the strongest interactions between the two 2D components, resulting in strong coupling effects that would influence the electronic structure of the active sites and the charge transfer character during energy conversion reactions. In this review article, we focus on the most recently reported and instructive studies based on the 2D–2D hybrid catalysts for energy conversion reactions, including the fabrication methods, characterization techniques and applications in energy conversion reactions, particularly in water splitting (Table 2).

Table 2 Summary of the electrocatalytic performance of the most active 2D–2D hybrid catalysts for energy conversion reactions

Catalysts	Performance	Ref.
Note: RT means room temperature.
LDH-rGO	Alkali OER: η = 210 mV@10 mA cm^−2, 1 M KOH, and RT	81
MoS2–graphene	Acidic HER: η = 137 mV@10 mA cm^−2, 0.5 M H2SO4, and RT	85
MXene/CN/rGO	Acidic HER: η = 147 mV@10 mA cm^−2, 0.5 M H2SO4, and RT	88
Black P@NG	Full water splitting, cell voltage of 1.54 V at 10 mA cm^−2, 1 M KOH, and RT	89
CoFeOx-black P	η(OER) = 266 mV@10 mA cm^−2, η(HER) = 88 mV@10 mA cm^−2, 1 M KOH, and RT	91
Black P-Ni(OH)2	Alkali OER: η = 297 V @10 mA cm^−2, 1 M KOH, and RT	92
MXene-LDH	Alkali OER: η = 298 V @10 mA cm^−2, 1 M KOH, and RT	96
LDH-LDH	Full water splitting, cell voltage of 1.59 V at 10 mA cm^−2, 1 M KOH, and RT	104
TMD-LDH	Full water splitting, cell voltage of 1.49 V at 10 mA cm^−2, 0.1 M KOH, and RT	106
MoTe2/Ti3C2Tx	Acidic HER: η = 293 mV@10 mA cm^−2, 0.5 M H2SO4, and RT	93
LDH-Bir	Industrial alkali OER: 0.6 V vs. Hg/HgO, 6 M KOH, 85 °C	61
Co-COF/graphene	CO2RR: 8.2 mA cm^−2 in H cell, FE(CO) = 97%, 191 mA cm^−2 in flow cell, FE(CO) = 99%	108
Cu-CuxOF	CO2RR: FE(acetic acid) = 27%, −0.3 V	115
MoS2/C3N4	NRR: NH3 yield = 18.5 μg h^−1 mg^−1, FE (−0.3 V) = 17.8%	117
CuSe/g-C3N4	CO2RR: FE(CO) = 85.28%, −1.2 V	118

---

Despite the considerable achievements in the design and application of 2D–2D hybrid materials for energy conversion reactions, there are still many challenges ahead in this promising research area. (1) There is a lack of a facile and economic method for the preparation of large scale and high quality 2D–2D hybrid materials with controlled chemical compositions, sizes and microstructures. (2) The synergistic effects between the two 2D components are complicated; however, they are critically important for understanding the roles of the heterointerface in catalysis. So, advanced characterization techniques, especially the in situ/operando techniques that could provide the real time information on the structural and compositional evolution of 2D–2D heterostructured hybrid materials during the catalysis processes and rationally designed and precisely controlled 2D–2D hybrid systems need to be developed. (3) Theoretical simulations such as DFT with accurate models should be carried out for understanding the reaction mechanism, which could couple with high throughput screening and machine learning to promote a better construction of 2D–2D hybrid materials to meet the requirements of targeted reactions. (4) Fabrication methods of free-standing and large scale 2D–2D heterostructured hybrid material-based electrodes need to be developed to guarantee the high activity and increase the durability of the catalysis reactions under practical application conditions, and hence close the huge gap between the laboratory work and commercialization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors appreciate financial support from Shanghai Jiao Tong University (No. WH220828001, ZXDF280001) and Shenzhen Science and Technology Innovation Commission (JCYJ20190808155413194).

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