2D/2D Heterostructures:Rational Design for Advanced Batteries and Electrocatalysis

Jun Mei ,Ting Liao,and Ziqi Sun*

Two-dimensional/two-dimensional(2D/2D)heterostructures consisting of two or more 2D building blocks possess intriguing electronic features at the nanosized interfacial regions,endowing the possibility for effectively modulating the confinement,and transport of charge carriers,excitons,photons,phonons,etc.to bring about a wide range of extraordinary physical,chemical,thermal,and/or mechanical properties.By rational design and synthesis of 2D/2D heterostructures,electrochemical properties for advanced batteries and electrocatalysis can be well regulated to meet some practical requirements.In this review,a summary on the commonly employed synthetic strategies for 2D/2D heterostructures is first given,followed by a comprehensive review on recent progress for their applications in batteries and various electrocatalysis reactions.Finally,a critical outlook on the current challenges and promising solutions is presented,which is expected to offer some insightful ideas on the design principles of advanced 2D-based nanomaterials to address the current challenges in sustainable energy storages and green fuel generations.

Keywords

2D heterostructures,2D nanomaterials,battery,electrocatalysis,energy storage

1.Introduction

Two-dimensional(2D)nanostructures feature a atomically thin geometry in thickness and a ultrahigh surface-to-volume ratio compared to other dimensional nanomaterials(Figure 1a).[1,2]Generally,a monolayer in a 2D-layered materials is composed of an atom-level-thickness,covalently bonded lattice,and these dangling bond-free nanosheets often exhibit extraordinary electronic and optical properties.Furthermore,van der Waals(vdW)interactions are usually identified in between neighboring layers of 2D layered materials.[3]Owing to the absence of direct chemical bonding capability,the assembly of 2D-based van der Waals heterostructures(vdWHs)can be beyond the constraints from lattice matching and processing compatibility.Until now,considerable efforts have been devoted to the design and synthesis of 2D vdWHs by either vertically stacking or inplane growth of distinct 2D basic building blocks(Figure 1b).[4]In contrast to bulk materials with limited interfaces,2D/2D heterostructures offer an intriguing possibility to take the full use of the unique electronic features at the interfacial region,which usually lies in the range of several to a few tens of nanometers(Figure 1c).[5,6]2D/2D heterostructures offers ideal platforms for effectively modulating the generation,confinement and transport of charge carriers,excitons,photons,phonons,etc.within these atom-level interfaces,which significantly facilitates the design and further modification on 2D/2D interfaces for regulating electronic structures for various applications.[7-12]

Figure 1.Composition and advantages of 2D/2D heterostructures.a)Summary on electro-and photorelated properties of low-dimensional nanomaterials and their heterostructures.Reproduced with permission from ref.[1]Copyright 2018,Springer Nature.b)Schematic illustration of 2D/2D vdW heterostructures.Reproduced with permission from ref.[4]Copyright 2016,Springer Nature.c)Schematic illustration of bulk and 2D/2D heterostructures.Reproduced with permission from ref.[5]Copyright 2019,the authors under a CC BY 4.0 license.d)Representative examples of 2D nanomaterials and their derived 2D/2D heterostructures.Reproduced with permission from ref.[13]Copyright 2017,Springer Nature.

To build various 2D/2D heterostructures,different 2D basic building blocks are required,which can be usually obtained via exfoliation or chemical growth method.[13-20]As illustrated in Figure 1d,there are some frequently applied 2D building blocks for 2D/2D heterostructures.Owing to its exceptional physical and chemical properties,conductive graphene is most widely unitized as an essential component of constructing various 2D/2D heterostructures,[21,22]such as graphene/hexagonal boron nitride(h-BN),[23]graphene/metal oxides (MOs),[24-27]graphene/transition metal dichalcogenides(TMDs),[28-35]graphene/layered double hydroxides(LDHs),[36,37]graphene/MXene,[38-42]graphene/black phosphorus (BP),[43,44]graphene/borophene[45]graphene/GaN heterostructures,etc.[46]As another typical class of 2D materials,TMDs in the general form of MX2(M=Mo or W;X=S,Se or Te),possesses adjustable electronic properties and tuneable bandgaps,and has been widely used as energy and catalytic materials.Compared with the surficial chemically inert pristine graphene,[47]2D TMDs prepared through solution-based methods provide many chemical pathways for constructing functional heterostructures,[48-50]for example,TMD-based heterostructures can be covalently bonded with metal sites for improved catalytic activity and selectivity,[51]and electronically coupled with 2D polymer for enhanced photoluminescence.[52]2D transition metal oxides(TMOs)with various chemical compositions and rich valence states is another common building block,which is primarily due to their relatively low costs and complex redox chemistry.[53-57]Besides the above-listed 2D materials,there are many other 2D basic building blocks that can be synthesized and modified for creating 2D/2D heterostructures,such as BP monolayer with an intrinsic direct bandgap of 1.5 eV,[58,59]LDHs with tuneable chemical compositions,[60]MXenes with a good conductivity and variable interlayer spacing,[61-64]graphene consisting of sp-and sp2-hybridized carbon atoms,[65]and insulating h-BN with a graphite-like honeycomb atomic structure.[66]These 2D monolayers could be combined to produce various 2D/2D heterostructures,in which two or more 2D materials are vertically layer-by-layer stacked or in-plane arranged to form different lateral heterostructures.[67]

For these 2D/2D heterostructures consisting of metallic,semiconducting,or insulating layers,with many extraordinary physical,chemical,thermal and/or mechanical properties,[68-85]these exceptional 2D/2D heterostructures have been widely explored for a variety of applications,mainly including electronic(e.g.,transistors),[86-88]optoelectronic(e.g.,light-emitting diodes,photovoltaics,photodetectors,optical modulator,and solar cells),[89-91]magnetic(e.g.,spintronics,magnonics,and spin-orbitronics),[92,93]and energy conversion and storage devices(e.g.,batteries,supercapacitors,photocatalysis,and electrocatalysis).[13,94-99]In terms of synthetic strategies,chemical vapor deposition(CVD)and solution-based assembly are the two most commonly used synthetic strategies.[100-104]For example,the vertical and lateral WS2/MoS2heterostructures can be easily manipulated via a one-step vaporphase growth by controlling the growth temperature or the reactive gas composition,[105,106]and the MoS2/MoSe2and WS2/WSe2lateral heterostructures can be fabricated by in situ modulation of the vaporphase reactants during the CVD growth process.[107]Despite that CVD offers controllable synthesis of various heterostructures with clean surfaces and/or interfaces,the large-scale production is likely to be very costly.Alternatively,solution-based assembly,in which pre-synthesized 2D materials are manually stacked with the assistance of weak vdW interlayer forces,is easy to be scaled.[108,109]Moreover,this method could achieve generalized integration on 2D materials with different morphologies and chemistries into multilayer heterostructures.

In this review,we primarily focus on the synthesis and applications of 2D/2D heterostructures,particularly in the advanced batteries and electrocatalysis fields.First,a detailed introduction is given on the commonly used synthetic strategies for obtaining 2D/2D heterostructures,and then a comprehensive review on recent progress is systematically provided for identifying the specific roles of these heterostructures for batteries and electrocatalysis reactions including hydrogen evolution reaction(HER),oxygen evolution reaction(OER),oxygen reduction reaction(ORR),nitrogen reduction reaction(NRR),and CO2reduction reaction(CO2RR).Finally,current challenges on the synthesis and their applications for batteries and electrocatalysis are critically outlined,and finally some promising solutions are outlooked for offering some useful references for the researchers in the fields of materials,chemistry,and chemical engineering.It is expected that this review could offer us some insightful perspectives for advanced materials design to address the current energy-related challenges.

2.Synthetic Strategies of 2D/2D Heterostructures

2D/2D heterostructures have been verified as a type of versatile nanomaterials and have attracted much interest for addressing the current energy-related issues.To achieve the wide range applications of 2D/2D heterostructures,reliable synthesis with precisely controllable chemical compositions is an essential prerequisite.Currently,three commonly used synthetic strategies,namely,high-temperature solid-state reaction,CVD,and wet-chemistry assembly.The synthesis and fabrication of 2D-based materials have been well documented in some reviews.[6,21,34,100]Here,we only give a brief summary on the representative fabrication methods but highlight some recent innovative examples in the preparation of 2D-2D heterostructures.

2.1.High-temperature Solid-state Reaction Method

The high-temperature solid-state reaction method is very simple in its setup and operation in the fabrication of 2D/2D heterostructures,including oxidation,carburization,sulphurization,nitrogenation,and phosphorization,etc.[24,110]This strategy allows the fabrication assisted with slow solid-state diffusion reactions which can only be occurred at an elevated temperature.The obtained products,however,often possess severe sintering effect,large and interconnected aggregations,widely distributed thickness,complex phase segregations,etc.,resulted by the high temperatures.[111]High-temperature solid-state reaction can also be an assisting method combining with other methods to synthesize 2D-2D heterostructures.Mei et al.reported the synthesis of lateral 2D-2D bismuth oxide(Bi2O3)heterostructured nanosheets(BOHNs)by a two-step approach including a solution-based molecular self-assembly and a following annealing step.Through this method,the BOHNs nanosheets consisting of side-by-side configured lateral α-and β-phase heterostructures were obtained.[24]Further structural optimization on 2D BOHNs to form a 2D/1D/2D heterostructure film in the presence of 1D carbon nanotubes(CNTs)and 2D graphene to form a highly conductive composite(BOHNCG)paper has been performed through a vacuum filtering method,which have evidenced to be high-performance anodes for both Li-ion batteries(LIBs)and Na-ion batteries(SIBs).[24]

2.2.One-step CVD Growth Method

CVD is the most widely used method to synthesize high-quality 2D/2D heterostructures,and in most cases,on specific substrates,with probably successive transfer steps for device assembly.[112]Recently,the direct heterostructure CVD growth without a transfer step has also been developed.Geng et al.reported a one-step synthesis of 2D Mo2C-ongraphene heterostructured film via a molten copper-catalyzed CVD method by using a high- flux methane as carbon precursor.[113]At the reaction temperature of 1100°C,a Mo-Cu alloy formed and facilitated the further growth of a large Mo2C film on graphene at a centimeter scale(Figure 2a).In the Mo2C/graphene heterostructure,graphene acted as a diffusion barrier,which greatly confined the reaction rate between the surface phase-segregated Mo atoms and the hydrocarbon precursor,and then led to the formation of the final Mo2C along the graphene plains in nanosized thickness.Scanning transmission electron microscopy annular dark- field(STEM-ADF)measurements confirmed that the graphenesupported large-sized Mo2C crystals were homogeneously stacked with a low-level defect density.Contributed by the superior graphene-mediated charge transfer kinetics,the as-grown Mo2C-on-graphene heterostructure exhibited a low onset HER voltage of 87 mV and a low HER overpotential of 236 mV at 10 mA cm-2,while these were 189 and 320 mV,respectively,for the pristine Mo2C electrodes.[113]

Figure 2.Synthesis of 2D/2D heterostructures via vapor growth.a)Schematic illustration of the Mo2C/graphene vertical heterostructures and the corresponding optical and STEM-ADF images.Reproduced with permission from ref.[113]Copyright 2017,Wiley-VCH.b)Schematic illustration of the synthesis of Janus materials and their lateral and vertical heterostructures through a selective epitaxy atomic replacement(SEAR)process.Reproduced with permission from ref.[118]Copyright 2020,Wiley-VCH.c)Schematic illustration of the synthetic system for the epitaxial growth of 2D lateral heterostructures,and the optical microscope images of the resultant(c1)WS2-WSe2,(c2)WSe2-MoS2,(c3)WS2-MoS2,and(c4)WSe2-MoSe2lateral heterostructures.Reproduced with permission from ref.[120]Copyright 2017,The American Association for the Advancement of Science.d)Schematic illustration of the growth of metallic/semiconducting transition metal dichalcogenides(m-TMD/s-TMD)van der Waals heterostructures(vdWHs)arrays.(d1)AFM image of a 3×3 VSe2/WSe2vdWH array,(d2-d5)optical and AFM images of(d2,d3)NiTe2/WSe2and(d4,d5)CoTe2/WSe2heterostructure arrays.Reproduced with permission from ref.[123]Copyright 2020,Springer Nature.

2.3.Two-step CVD Growth Method

Different to the synthesis of individual 2D materials,the fabrication of some complex 2D-2D heterostructures usually needs multiple steps and multiple fabrication techniques.The two-step synthesis method,as an instance,has been demonstrated a powerful approach for the synthesis of multiscale hierarchical nanomaterials.[114]Janus crystals featuring different atomic species on the upper and lower facets have become an exciting class of 2D materials.This atomic-level 2D-2D heterostructures with symmetry breaking in their crystal structures can induce an internal electric field within the 2D Janus layers and bring about strongly correlated electronic states across the layers or the grain boundaries in their derived heterostructures.[115]The controllable synthesis of highquality 2D Janus layers and their heterostructures at one step,unfortunately,remains a major challenge,but this challenge can be solved by a two-step approach.As a typical example,monolayer MoSSe Janus crystals were reported to be synthesized by either controlled sulfurization of MoSe2or hydrogen plasma stripping for sulfur removal of MoS2( first step)coupled by a following thermal selenization step at 350-450°C(second step).[116,117]In both steps,the required high processing temperatures may drive the formation of the thermodynamically favorable 2D alloys instead of the thermodynamically unfavorable 2D Janus layers.This difficulty can also find in the synthesis of other 2D Janus materials and their heterostructures.Recently,a room temperature selective epitaxy atomic replacement(SEAR)technique was proposed for the synthesis of a variety of 2D Janus TMDs(e.g.,WSSe and MoSSe)and their derived heterostructures(e.g.,WSSe/MoSSe and WSe2/WSSe)(Figure 2b).[118]This is also a two-step process,which involves the initial selective removal of the uppermost chalcogen layer to create chalcogen vacancies in the pre-deposited 2D TMDs(e.g.,WSe2and MoSe2)or junctions(e.g.,WSe2/MoSe2)with the assistance of hydrogen radicals produced by an inductively coupled plasma(ICP)and the subsequent replacement by sulfur atoms in the presence of low-energy reactive sulfur radicals.This substitution process was kinetically driven and easily controlled over the kinetic energy of radical species with respect to the metal-chalcogen bond energy.The room temperature reaction condition applied in this process is beneficial to the formation of high-quality 2D Janus heterostructures and avoids the thermal disturbance aroused by the high temperature.

For these 2D materials with similar crystal structures and compatible lattice constants,their derived heterostructures can be synthesized through a two-stage epitaxial growth process.In this process,a first starting 2D material can be grown on a specific substrate,and then a second material can be further grown at the edge of an existing domain of the first material.Therefore,if such sequential growth process can be repeated for several times,the block-by-block growth of 2D/2D-based multiheterostructures(with multiple distinct material blocks)or superlattices(with multiple alternating blocks)can be produced.In some practical cases for the two-stage epitaxial growth of 2D/2D heterostructures or superlattices,however,the vapor source and/or the growth conditions of different types of 2D materials are distinct and a periodic chemical supply and growth condition swaps are often required,which can pose some negative effects,such as that structural damage or component degradation can happen during these the swaps of growth steps.[119]Specifically,the pre-grown 2D structures with atomic-level ultrathin thickness are very sensitive to the change of external conditions,which can result in the failure of the first 2D structure and then the disordered growth of the second 2D structure.To achieve the controllable growth of 2D/2D multiheterostructures or superlattices,as illustrated in Figure 2c,a modified two-step thermal CVD process was rationally designed,in which a selected solid powder was vaporized under argon flowing,and the heterostructures were formed by the continued epitaxial growth at the edge of the pre-grown 2D crystals placed at the downstream end of the furnace.[120]Particularly,a reverse flow from the substrate to the source was applied to cool down the existing pregrown 2D materials to eliminate the adverse thermal degradation or the undesired homogeneous nucleation during the temperature ramping,the stabilization period,and in between the two successive growth stages(Step 1).As the desired growth temperature for the second 2D crystal was reached,a forward carrying flow from the source to the substrate for the sequential growth was applied onto the same substrate(Step 2).With the accurate regulation on the reaction conditions in each growth step,the quality of the monolayer 2D heterostructures was well preserved after the multiple sequential growth steps.This approach has been demonstrated its feasibility in the growth of diverse 2D/2D heterostructures,such as WS2-WSe2(Figure 2c1),WSe2-MoS2,(Figure 2c2),WS2-MoS2,(Figure 2c3),WSe2-MoSe2(Figure 2c4)heterostructures,and their derived multiheterostructures(e.g.,WS2-WSe2-MoS2,WS2-MoS2-WS2,and WS2-MoSe2-WSe2)and superlattices(e.g.,WS2-WSe2-WS2-WSe2).[119]Apart from the vapor-phase deposition method,solution-phase epitaxial growth has also been used for synthesizing some symmetry-mismatched heterostructures on 2D atomic crystals,driven by the dangling bond-free surface of the 2D crystals.[121]

Recently,2D/2D vdWHs integrating different types of 2D materials have attracted intensive interests,which break the strict requirement limits on lattice matching and growth compatibility existing in the fabrication of covalence bonded 2D-2D heterostructures.[4,122]In principle,2D/2D vdWHs can be formed via a two-step procedure through free combination of any 2D materials which may have different crystal structures,chemical compositions,lattice orientations,etc.,to realize required physical,chemical,mechanical properties,and functionalities for some practical applications.Although considerable progress has been made in the synthesis of 2D/2D heterostructures by modified CVD methods,the CVD growth of uniform and high-quality 2D vdWHs is yet a major challenge.Recently,Li et al.reported a general synthetic approach to obtain 2D vdWHs arrays consisting of 2D semiconducting TMDs(s-TMDs,e.g.,MoX2and WX2,X=S,Se,Te)and their metallic counterparts(m-TMDs,e.g.,VX2,NbX2,and TaX2).[123]Specifically,as demonstrated in Figure 2d,large-area 2D s-TMDs were first grown by using a CVD process and then selectively patterned by a focused laser irradiation combined with a raster scan to create periodic local defect arrays to work as exclusive nucleation sites for selective growth of m-TMDs,and finally produced 2D/2D m-TMD/s-TMD vdWH arrays via the second CVD process.By using this synthetic protocol,various 2D vdWHs,such as VSe2/WSe2(Figure 2d1),NiTe2/WSe2(Figure 2d2 and d3),and CoTe2/WSe2(Figure 2d4 and d5)heterostructure arrays,were successfully synthesized.The atomically clean interface in these metal/semiconductor vdWHs enables the promising applications in high-performance electronic devices.[123]This work presents significant advances in the controlled synthesis of 2D/2D heterostructure arrays by using a two-step CVD method and pave the way for the fabrication of advanced metal/semiconductor heterostructures.

2.4.Wet-chemistry Assembly Method

Solution-based assembly is another representative fabrication method for the preparation of 2D/2D heterostructures.[124,125]Compared to the above-mentioned CVD methods which usually need expensive facilities and precursors and delicate growth conditions,the wet-chemistry assembly strategy has the advantages of low-cost,high-yield,and easy in operation.Moreover,the overall setup is simple and does not demand the use of catalytic substrates and complicated transfer process,which are generally required for CVD growth methods.Generally speaking,CVD-produced heterostructures possess a high quality in terms of surface chemistry,interfaces,crystallinity, flatness,etc.,and suitable for applications in electronics which usually have extremely high-demand on the quality of materials,while the wet-chemistry assembled 2D/2D heterostructures feature larger interlayer spacing,multiple interfacial mass/charge transport channels,large-size active surfaces,abundant chemically active defects,high disordering of interfaces,and exhibit great advantages in chemical or electrochemical applications,such as advanced batteries and electrocatalysis.It should be noted that the resultant products using this method are relatively nonuniform with some potential residual impurities from solutions or raw materials.

It has been verified that 2D/2D MoS2/graphene heterostructure could greatly enhance the electronic conductivity and the ion diffusion kinetics,thus resulting in superior electrochemical ion storage capacity.[126]By intelligently employing the electrostatic adsorption in solutions,the MoS2/graphene heterostructures have been produced by ballmilling-driven insertion and exfoliation of commercial MoS2bulk and graphite and then heterostructure formation through electrostatic adsorption(Figure 3a).[28]When employed as the anode of SIBs,the MoS2/graphene heterostructured nanosheets exhibit remarkably stable reversible capacity of～200 mA h g-1at a current density as high as 50 A g-1)and outstanding cycling stability with a capacity retention of 95% after 250 cycles at 300 mA g-1.[28]

Figure 3.Synthesis of 2D/2D layer-by-layer heterostructures via solution-processing steps.a)Schematic illustration of the synthesis of 2D/2D MoS2/graphene hybrids.Reproduced with permission from ref.[28]Copyright 2017,Wiley-VCH.b)Schematic illustration of the presence of Ti-O-C covalent bonding in 2D/2D MXene-GO heterostructures;c)AFM image and d)an illustration of the MXene-functionalized GO platelets;e)photograph of a folded 2D/2D MXene-GO film;f)high-resolution transmission electron microscopy(HR-TEM)image of the cross-section of the film showing the co-existence of BP and GO.Reproduced with permission from ref.[38]Copyright 2020,the authors under a CC BY 4.0 license.g)Schematic illustration of the preparation of BP-functionalized graphene films,and the corresponding SAXS patterns of h)rGO and i)2D/2D rGO-BP-AD heterostructured films,accompanied by j)the intensity curves based on SAXS patterns;k)Porosity of the pure rGO and rGO-BP-AD heterostructured films,and l)HR-TEM image of the cross-section of the heterostructured films.Reproduced with permission from ref.[43]Copyright 2020,the authors under a CC BY-NC-ND 4.0 license.

As emerging 2D nanomaterials,MXenes and black phosphorus(BP)nanosheets have also been incorporated with other 2D materials,such as graphene,to form new types of 2D/2D heterostructures via solution-based assembly method.[127]The first example is graphene-MXene heterostructure.Compared to the high-cost and low-defect graphene film fabricated by CVD,the relatively low-cost 2D reduced graphene oxide(rGO)nanosheets by chemical reduction of graphene oxide(GO)in solution is regarded as one promising electrode material for energy storage devices.Unfortunately,there are two major limiting factors for the rGO nanosheets,that is,poor mechanical properties and inferior electrical conductivities,which greatly deteriorate their electrochemical performance.The formation of heterostructures therefore provides a feasible solution to address these two limiting factors.As shown in Figure 3b-d,2D/2D MXene-rGO(MrGO)layer-by-layer sheet stacks were fabricated via the solution-based assembly method by simply mixing the well-dispersed aqueous solutions with desired ratios and then vacuum filtration.[38,128]After chemically reduction of the MrGO sheets by hydroiodic acid(HI)and then immersed in an organic N,N-dimethylformamide(DMF)solution containing a long-chain conjugated molecule(1-aminopyrene-disuccinimidyl suberate,AD)to finally form free-standing MrGO-AD films(Figure 3e,f)with π-π bridging interactions between adjacent rGO sheets.This solutionassembled MrGO heterostructure had enhanced interfacial binding force achieved by the construction of interfacial Ti-O-C covalent bonds,which improve the electrical conductivities and mechanical properties of both the rGO and the heterostructure.[38,128]With the assistance of MXene sheets and AD molecules,the alignment degree of the randomly stacked graphene sheets was significantly improved,resulting in ultrahigh toughness(～42.7 MJm-3)accompanied by an electrical conductivity as high as～1329.0 S cm-1of the heterostructured films,which should have great potential for flexible energy storage devices.[38]

Similar principle has also been applied for the synthesis of ultratough 2D/2D graphene-BP(rGO-BP-AD) films.[43]The small-angle X-ray scattering(SAXS)indicated that the rGO-BP-AD film had a decreased intensity(Figure 3h,i)and a decreased porosity(Figure 3j)compared to the rGO film.In the rGO-BP-AD film,the small-sized BP nanosheets were attached on the GO surfaces via P-O-C covalent bonds and filled the interlayer voids of the lamellar graphene film,which endows the modified graphene film with much more compact density and a record toughness of～51.8 MJ m-3.In situ Raman measurements and molecular dynamics(MD)simulations evidenced that the primary reasons for the excellent mechanical property are the synergistic interactions of the lubrication of BP nanosheets,the P-O-C covalent bonding,and the π-π stacking interactions in the BP-functionalized graphene films.[43]In addition,recent study has revealed that the presence of few-layer holey graphene oxide(FHGO)in 2D-based BP/MXene heterostructure membranes can passivate undesired oxidative degradation behaviors of BP or MXene layers and meanwhile achieve the selective molecular diffusion,which will bring some new properties into the 2D-2D heterostructures and widen their applications.[129]

2.5.Liquid and Mechanical Exfoliation Methods

Besides CVD growth and solution-based assembly,some other precise and time-saving strategies have been explored for scalable production of 2D/2D heterostructures.Among them,liquid-phase exfoliation is still one major method for the scalable fabrication of both monolithic 2D materials and 2D-based heterostructures.[130,131]This direct printing method can fabricate large-area heterostructure arrays or patterns on various substates,such as plastics and papers,without the limits on strict substrates and harsh reaction conditions demanded by other fabrication methods.[132]Compared to CVD growth,the product quality of the exfoliated heterostructures is largely dependent on the states of the layered crystal precursors,such as interlayer bonding states,crystal and sheet size,weakly bonded layer thickness distribution,oxidation or types of intercalation atoms/molecules,etc.,which greatly affect the exfoliated products and the thereafter device performances.Based on this technique,water-based printing of liquid-phase exfoliated dispersions(e.g.,WS2,MoS2,BN,and graphene),achieved by either ultra-sonication or shear force exfoliation of the corresponding bulky materials,was further developed for producing all-inkjetprinted vdW heterostructures.[132]The residual solvent in the printed heterostructure films,however,may pose negative effects on the electrical properties if the fabrication is performed at ambient conditions.

Mechanical abrasion of bulk powders or what we referred as mechanical exfoliation,a method has been widely used in the fabrication of 2D materials,has been modified to synthesize various heterostructures by combining different types of vdW materials,such as graphite,MoS2,WS2,MoSe2,and BN.[133-135]In a specific case,seed layers were first deposited on SiO2or polymer substrates by the electrostatic interaction between the material and the substrate,and then the subsequent deposition of hetero-layer was conducted by employing the friction-facilitated basal cleavage of the targeting layered microcrystals,in which the vdW-bonded layered microcrystal powders were rubbed against the seed layers pre-adhered onto the substrate.The exfoliation efficiency of this method,unfortunately,is very low.One approach to enhance the efficiency is promoting the interfacial binding capability between the substrate and the vdW material,however,the adhesion at the material-substrate interface should be ideally weak enough to be adapted for the successive transfer process.The distinct requirements in exfoliation and transfer processes of the mechanical and liquid exfoliation of 2D heterostructures introduce extra complexity and cost in the practical utilization of these methods,other methods with high-yield and low-cost should be further developed.[136,137]Huang et al.proposed a dual-function polymeric film-assisted strategy to achieve independent regulation on the interfacial adhesion behavior during exfoliation and transfer steps,in which the SiO2substrate was first modified by a sub-5 nm poly(vinyl alcohol)(PVA) film to promote the exfoliation process,and the exfoliated sheets were free from the substrate accompanied by the removal of PVA during the transfer process.[138]This design can greatly increase the yield and the produce of large-size 2D film,such as ＞10 000 μm2for monolayer graphene.The transfer process is also easy without the use of additional tools(e.g.,the commonly used stamp),which can be utilized to fabricate a variety of vdW 2D structures and their heterostructures.

In summary,high-temperature solid-state reaction method is one of widely used strategy for in situ growth of 2D structures on 2D substrates,which is applicable for various types of metal-containing materials,including oxides,sulfides,selenides,phosphides,and carbide.CVD growth is mainly applied for metal chalcogenide-based heterostructures.Wet-chemistry assemble can achieve the generalized integration on 2D materials with different morphologies and chemistries into multilayer heterostructures.In spite of the achieved impressive progress,to take the full potentials of 2D/2D heterostructures for different applications requires more reliable and scalable synthetic strategies with precisely controlled interfacial adhesion,surface orientation,and chemical compositions,which are major challenges in the cutting-edge of nanoscience and nanotechnology.Several factors,including affordable production costs,simple and controllable operations,easy post-treatment procedures,high product quality,abundant and environmentally friendly raw materials,and precursors,etc.,should be carefully considered for practical engineering-scale fabrication.It is also highly desired that more novel synthetic methods will be explored to achieve customized heterostructure to meet the specified requirements or functionalities in different practical applications.

3.2D/2D Heterostructures for Advanced Batteries

Recently,2D/2D heterostructures have been verified as promising electrodes for advanced batteries.[13,139,140]The primary advantages of 2D/2D heterostructures for each type of batteries can be summarized as follows:1)improving ion storage capacity for metal-ion batteries;2)reducing active species loss for metal-sulfur batteries;3)regulating the growth of discharge products for metal-oxygen batteries;and 4)modulating ion redistribution and metal deposition behaviors for metal batteries.Furthermore,as illustrated in Figure 4a,the adjustable interlayer distance in the 2D/2D heterostructures can increase active sites,accelerate ion diffusion,and partly suppress volume change upon discharging and charging,and thus improving mechanical and electrochemical stability of the battery systems.

Figure 4.2D/2D heterostructures for metal-ion batteries.a)Schematic illustration of discharge and charge processes in a battery model with 2D/2D heterostructured electrodes.Reproduced with permission from ref.[13]Copyright 2017,Springer Nature.b)Schematic illustration of the ion diffusion and transfer pathways in 2D-TiO2-2D heterostructured electrode.Reproduced with permission from ref.[141]Copyright 2019,Elsevier.c)Schematic illustration of the intercalation behaviors of Na+,K+,Zn2+,and Al3+cations and the structural change comparison over cycling for both individual 2D VOPO4and 2D/2D multilayered VOPO4-graphene heterostructure electrodes.Reproduced with permission from ref.[151]Copyright 2020,the authors under a CC BY 4.0 license.

For metal-ion rechargeable batteries,Mei et al.proposed a novel 2DTiO2-2D vdW heterostructure for improving the electrochemical Li+storage properties.[141]In this design,as shown in Figure 4b,2D-graphene and 2D BP nanosheets were chosen as building blocks,and nanosized TiO2particles acted as an interfacial spacer and a bridging agent between the two 2D units.The TiO2bridging nanoparticles effectively connect two different 2D materials through interfacial bonds and simultaneously separate the 2D nanosheets from self-aggregation,which not only provide interlayer charge transfer channels but also enlarge the interfacial mass transfers through well-regulated 2D interlayer spaces.Furthermore,this 2D-2D heterostructure combines the merits of each components and thus provides superior electrochemical performances:1)BP possesses an electrical conductivity of～100 S m-1and an energy gap of～0.34 eV and can provide the electrode excellent conductivity;2)element phosphors possess a high theoretical capacity of 2596 mAh g-1and a low Li-ion diffusion energy barrier(0.08 eV)can contribute to high capacity;3)BP monolayer exhibits a theoretical capacity of about 430 mAh g-1toward lithium at an average potential window of 2.9 V and possesses a semiconducting-to-metallic transition during the ion intercalation process,which widen the potential window and the further conductivity of the electrode;4)TiO2delivers a theoretical intercalation-type lithium capacity of～336 mAh g-1close to that of the commercialized graphite anode(372 mAh g-1),which also contributes to the storage capacity;5)TiO2crystal presents a relatively stable structure without obvious volume expansion upon cycling.When applied as the anode of LIBs,this 2D-TiO2-2D heterostructured electrode manifested excellent electrochemical properties,such as high initial discharge capacity(～1330 mAh g-1at 0.2 A g-1),good rate capability(～270 mAh g-1at 5.0 A g-1),and good cycling stability(502 mAh g-1after 180 cycles).[141]Other 2D/2D heterostructures,such as mesoporous TiO2/rGO,[142]g-C3N4/MXene,[143]Ni7S6/graphene,[144]mesoporous-carbon/MoS2,[145]MXene/graphene,[146]MoS2-on-MXene,[147]were also designed and synthesized as the electrode materials of LIBs for improving the lithium storage capacity.In the application of LIBs,the 2D/2D heterostructures contribute to enhancing the LIB performance by effectively enhancing the ion diffusion and transfer kinetics and the conductivity of the overall electrode,combing the advantages of each constitutional material,and providing a synergic effect through specially coupled interfaces for ultrafast interlayer mass and charge transfers.[148]

Apart from LIBs,2D/2D heterostructures have been explored for various post-lithium metal-ion batteries,such as SIB,potassium-ion batteries(PIBs),aluminum-ion batteries(AIBs),and zinc-ion batteries(ZIBs).[149,150]Through interface strain engineering,multilayered 2D/2D VOPO4-graphene heterostructure consisting of 2D VOPO4nanosheets obtained by intercalation and exfoliation from the layered bulky materials and 2D graphene has been fabricated for reversible storage of Na+,K+,Zn2+,and Al3+cations.[151]It has been evidenced that the 2D/2D heterostructure electrode presented a reversible intercalation mechanism during the charging/discharging and exhibited a negligible volume change ratio,while these effects cannot be found in the individual VOPO4electrode.The strain engineering heterostructure electrodes presented good electrochemical properties for SIBs,PIBs,ZIBs,and AIBs.Particularly,when utilized as cathodes in KIBs,a capacity as high as 160 mAh g-1and an energy density up to 570 Wh kg-1were achieved.[151]2D MXene-based heterostructures are another class of promising electrode materials for superior sodium and potassium storage.By integrating the highly conductive and mechanically robust MXene materials with the high theoretical ion storage capacity of BP,2D/2D MXene/BP heterostructures have been fabricated and verified as one effective class of electrode materials for promoting the performance of SIBs and KIBs.[152-155]By constructing strong chemical interactions at the interface between the 2D MoSe2and the 2D MXene in the presence of conductive carbon to form carbon-coated MoSe2/MXene heterostructure,this 2D heterostructure anode can boost reaction kinetics and structural durability,and lead to a high reversible potassium capacity(355 mA h g-1at 0.2 A g-1after 100 cycles)and an excellent rate performance(183 mA h g-1at a rate of 10.0 A g-1).[156]It has also been reported that the 2D/2D TMD/carbon nanosheets heterostructures coupling 2D TMDs(e.g.,MoS2and MoSe2)with 2D carbon nanosheets could facilitate Na+or K+diffusion and thus improve the ion storage performance.[157,158]

In metal-sulfur batteries,2D/2D heterostructure is a promising sulfur host for rationally regulating the polysulfides by improving chemical adsorption capability and enhancing surface redox kinetics.As we have known,one of major challenges in metal-sulfur batteries significantly limiting the overall battery performance is the undesired solubility of intermediate lithium polysulfides species.[159]When the 2D/2D heterostructures are used as the sulfur host,the dual-phase interfaces in the 2D/2D heterostructures offer more possibility for optimizing physicochemical properties to suppress the loss of polysulfides.As elucidated in the 2D/2D MoN-VN heterostructure shown in Figure 5a,the theoretical calculations indicated that electronic structure of 2D MoN is favorite to incorporate with V species to form 2D/2D MoN-VN heterostructure,which is favorable for promoting the polysulfides adsorption and improving the utilization ratio of the active species.[160]As a result,the assembled lithium-sulfur batteries based on MoN-VN heterostructure delivered a reversible capacity of 708 mAh g-1at 2 C and a capacity decay of only 0.068% per cycle up to 500 cycles.Later,a sandwich-structured MoN-C-MoN trilayer architecture was designed for achieving strong adsorption of polysulfides species and meanwhile accelerated conversion reactions on the nitride polar surfaces,through the high- flux electron transfer contributed by the interlayered carbon.[161]The 3D self-assembly of these 2D/2D sandwich structures could ensure the maximized exposure of adsorptive/catalytic sites,bringing about excellent cycling stability with a low capacity decay of 0.033% per cycle at 1 C during 1000 cycles and superior rate capability with a capacity of 515 mAh g-1at a rate as high as 4.0°C.This work provides some new insights into the exploration on effective 2D/2D heterostructured sulfur hosts for simultaneously enhancing absorption and catalytic properties for lithium-sulfur batteries.

Figure 5.2D/2D heterostructures for metal-sulfur,metal-air,and lithium metal batteries.a)Schematic illustration of 2D/2D MoN-VN heterostructure with the enhanced polysulfides adsorption,and the corresponding HAADF-STEM images.Reproduced with permission from ref.[160]Copyright 2018,Wiley-VCH.b)Schematic illustration of G/Au-NP/Au-NS electrode with Au nanoparticles(Au-NP)sandwiched between graphene and Au nanosheets(Au-NS)on Ni during discharge process.Reproduced with permission from ref.[163]Copyright 2016,the authors under a CC BY 4.0 license.c)Schematic illustration of reaction mechanism of metal hydroxide/carbon nanosheets heterostructure electrode for Li-O2battery.Reproduced with permission from ref.[164]Copyright 2016,American Chemical Society.d)Photograph and SEM images showing nonuniform Li deposition on rGO and uniform plating in the presence of MXene,and the long-life Coulombic efficiencies of an MXene/graphene(MG)electrode at 0.5 mA cm-2.Reproduced with permission from ref.[165]Copyright 2019,American Chemical Society.e)Schematic illustration on electrochemical deposition behaviors of 2D/2D mesoporous polypyrrole-GO (mPPy-GO)heterostructures for uniform Li deposition,and the relative concentration of Li ions at different thickness levels.Reproduced with permission from ref.[166]Copyright 2020,Wiley-VCH.

2D/2D heterostructures have also been used in metal-oxygen batteries.One major issue in hindering the large-scale application of metal-oxygen batteries is largely associated with the discharge product.[162]Taking lithium-oxygen batteries as a typical example,lithium peroxide(Li2O2)is generally identified as the discharge product.The intrinsically low electrical conductivity of Li2O2,however,renders sluggish oxygen reduction/evolution reaction(ORR/OER),and thus more high-efficient catalysts are often required.Considering the attractive electrocatalytic activity of noble metals and structured carbon,a G/Au-NP/Au-NS heterostructure composed of Au nanoparticles (Au-NP) sandwiched between the few-layer graphene and the thin Au nanosheets(Au-NS)was designed for lithium-oxygen batteries(Figure 5b).[163]In this framework,electronically conductive graphene can provide electron transfer channels for ORR/OER,and Au NPs acts as the efficient catalyst to ensure confined growth of Li2O2on the surface of Au particles.Meanwhile,the additional Au NSs can stabilize the Au-NP layer and restrain the Li2O2-encapsulated Au NPs into the sandwich framework.This unique electrode design can effectively reduce the direct contact of Li2O2with the graphene and the electrolyte to alleviate possible carbon decomposition and undesired side reactions.Consequently,the assembled batteries by using the G/Au-NP/Au-NS cathode sustained 300 stable cycles under 400 mA g-1at a limited capacity of 500 mAh g-1,and 100 cycles under 800 mA g-1at a full charge/discharge mode in the cutoff voltage of 2-4.5 V.[163]Besides the expensive noble metals,some active low-cost metal hydroxides have been investigated to construct bifunctional catalysts toward both ORR and OER in lithium-oxygen batteries.By coupling with vertically aligned electrochemically active carbon nanosheets(CNS)and catalytic metal hydroxides,porous 2D/2D metal hydroxides/CNS(M(OH)x@CNS,M=Co or Fe)heterostructures possessing high conductivity,high surface area,high electrolyte/electrode interfaces,and connected interstitial cavities were fabricated,which allowed efficient ion/O2transportation and strongly bound CNS and M(OH)xnanoparticles.As the cathode for Li-O2batteries,the Co(OH)2@CNS and Fe(OH)3@CNS heterostructures presented high specific capacities of 5403 and 3762 mAh g-1,respectively.[164]As the scheme shown in Figure 5c,the enhanced performance is large attributed to the improved O2/ion transport properties and the spatially confined growth of Li2O2of this unique 2D/2D heterostructure.This strategy for integrating bifunctional catalysts with electrode architecture design provides us ideas in building high-performance cathodes for other metal-oxygen batteries.

Besides working as catalytic cathodes,the 2D/2D heterostructures are promising candidates for dendrite-free lithium metal anodes.It is well accepted that the uncontrollable dendrite growth is a major barrier for the utilization of Li-metal anode for high-energy-density batteries with long-life cyclability.Rationally regulating the Li-ion transport behaviors for homogeneous and dispersive metal distribution is crucial for suppressing the dendrite growth,while 2D/2D heterostructures can provide regulated Li-ion transport and homogeneous Li deposition,and thus avoid the serious growth of Li dendrites.Shi et al.reported a conductive MXene/graphene heterostructure with uniformly dispersed lithiophilic 2D MXene nanosheets as Li nucleation sites.This heterostructured anode contained an ultrahigh Li content(～92% of the theoretical capacity)and manifested exceptional capabilities in suppressing the undesired dendrite formation and accommodating the vast volume changes.[165]On a rGO-based film electrode with only half surfaces coated by MXene layer was fabricated,it is interesting that no Li dendrites growth occurred on the MXene-modified surface,while irregular Li dendrites were clearly observed on the bare rGO surface,which reveals the critical role of MXene layer on suppressing the growth of Li dendrites(Figure 5d).As a consequence,the heterostructured electrode exhibited high Coulombic efficiencies of around 99% with a lifespan as long as 2700 h at 0.5 mA cm-2and kept stable for 230 cycles at a high current density of 20 mA cm-2.This type of 2D heterostructure regulated stable metallic Li anodes is very promising to be implanted in various Li-metal-based batteries,such as Li-S,Li-O2,and solid-state batteries,to boost energy density without safety concern.[165]In a similar way,another 2D/2D heterostructure based on defective GO and mesoporous polypyrrole(mPPy)was proposed to regulate Li-ion distribution and homogenize Li deposition behaviors for achieving stable Li anodes.[166]In this design,the defective GO acted as an ion sieve for decreasing the ion shuttling speed in electrodeelectrolyte interfaces and the growth rate of metallic dendrite,and the mPPy layer with highly ordered mesoporous arrays offered abundant ion-conducting nanochannels to lead to a uniform Li-ion flux during operation(Figure 5e).Meanwhile,this heterostructured electrode provided favorable electrolyte wettability and good structural stability,which allow the formation of a uniform SEI film to protect the metallic lithium electrode from the destructive corrosion by the electrolyte.Owing to the synergic effect aroused by the 2D/2D heterostructure,the mPPy-GO heterostructured electrode showed a good cycling stability up to 1000 cycles,even at under extreme temperature conditions(＞50 cycles at 0 °C and 290 cycles at 50 °C),and a steady voltage hysteresis of 70 mV at 10.0 mA cm-2after 100 cycles.Furthermore,the assembled mPPy-GO-Li//LiCoO2full batteries by coupled with LiCoO2cathode demonstrated a remarkable capacity of 130 mAh g-1and a good capacity retention over 90% after 450 cycles.[166]Obviously,this dual-functional 2D/2D heterostructure Li-ion regulated electrode will inspire the further development of multifunctional 2D/2D heterostructures for high-energy-density metal-ion batteries.

In summary,2D/2D heterostructures with atomic-level thickness and unique interfaces can effectively shorten the ion diffusion length,promote charge transfer behaviors,improve electrical conductivity,create abundant electrochemical active surface and defects,achieve rapid interlayer charge/mass transfer,suppress the undesired loss of active species,regulate the growth states of the metallic dendrites,and accelerate the penetration of electrolyte ions,which provide many unprecedented opportunities toward next-generation high-performance and high-safety rechargeable batteries.

4.2D/2D Heterostructures for Electrocatalysis

The chemical reactions involved in the sustainable energy storages and green fuels generation reactions usually suffer from sluggish kinetics and low efficiency and high-performance catalysts are thus necessary to trigger these reactions at a lower energy input.[167,168]2D/2D heterostructures are considered as efficient catalysts for improving the kinetics and stability for various efficient and durable electrocatalysis reactions,including HER,OER,ORR,NRR,CO2RR,etc.[169]The main advantages of 2D/2D heterostructures for catalysis include:1)rich exposed active sites contributed by surface area and interfacial defects;2)high-speed interfacial charge transport;3)band bending for improved redox capability;4)potential for integrating Janus or bi-functional catalysts into single heterostructure;and 5)optimized adsorption energy of intermediates for lowered reaction barriers.The 2D/2D heterostructures thus possess vast space to modulate the electronic structure to optimize the rate-determining steps of the catalysis reactions.[170]

2D/2D heterostructures have been identified as efficient HER catalysts.One typical type of HER catalyst is 2D MoS2-based heterostructures.MoS2is a superior and low-cost HER catalyst with thermoneutral hydrogen adsorption energy and good structural stability in both acidic and alkaline media.[1,171,172]Unfortunately,the intrinsic MoS2HER activity is unsatisfactory,primarily due to the inadequate active sites at the edges and the poor electron conductivity of the material.The construction of 2D/2D heterostructures with other transition metal nanosheets or conductive carbonaceous matrix can effectively solve these issues.2D/2D MoS2/graphene heterostructure catalyst(Figure 6a)consisting of monolayer MoS2nanosheets and conductive graphene significantly facilitates the creation of active sites for efficient adsorption and desorption of hydrogen,and meanwhile promotes the charge transfer,leading to a superior HER performance in both 0.5 M H2SO4(180 mV at10 mA cm-2) and 1.0 M KOH (183 mV at 10 mA cm-2)media.[173]From this case,we can understand that the formation of 2D/2D heterostructure with conductive compounds like graphene can dramatically increase the conductivity and enhance the charge transfer and separation capability of the active materials.Furthermore,DFT investigation demonstrated that a specific interfacial coupling state can form between the two nanosheets.[35]The MoS2nanolayer has been activated by an obvious metallic state of MoS2introduced by the reduction associated by the electrons transfer to form a strong contact with the reduced graphene.By this way,the exposed sites and the rational unitization of reactive surface are maximized to enhance the HER performance.Particularly,more active edges and defects are generated with the formation of strong interfacial interactions of the 2D/2D heterostructures,which further contribute to the catalytic activity enhancement.Therefore,the formation of interfacially coupled 2D/2D heterostructure combining the catalytic active but less conductive 2D nanomaterials with the electrically conductive 2D structures is a significant design principle for the design of high-performance composite catalysts.Good electrocatalytic durability is another critical consideration in designing high-performance catalysts.For 2D/2D heterostructures,an intimate interfacial contact between the 2D components can provide significantly enhanced durability.Recently,a surface-confined lateral growth of 2D porous Pt nanolayer was achieved on 2D NiFe-LDH nanosheets to produce a highly integrated 2D/2D frame for alkaline HER.[174]This design considerably increased Ptatomic utilization efficiency and boosted the water dissociation for hydrogen generation,inducing an impressive 6.1-fold mass activity compared to the commercial 20% Pt/C,together with a high HER operational stability for 50 h with only 19 mV potential drop.[174]The present work offers some hints on improving the stability of HER catalysts by the formation of intimate interfacial contact within 2D-based heterostructures.

Figure 6.2D/2D heterostructures for electrocatalysis.a)Schematic illustration of 2D/2D MoS2/graphene vdW heterostructure for HER.Reproduced with permission from ref.[173]Copyright 2020,American Chemical Society.b)Schematic illustration of 2D/2D activated graphene(AG)-BP nanosheets heterostructure anchored by Ni3N particles for OER,and the corresponding comparison on the polarization curves and Tafel slopes.Reproduced with permission from ref.[177]Copyright 2019,Wiley-VCH.c)Schematic illustration of 2D/2D exfoliated BP-on-nitrogen-doped graphene for overall water splitting in alkaline media,and the corresponding operation voltage and a demonstration of a solar power-driven water splitting device.Reproduced with permission from ref.[179]Copyright 2019,American Chemical Society.d)Schematic illustration of 2D/2D superlattice-like Fe-N-C/MXene heterostructures for ORR.Reproduced with permission from ref.[186]Copyright 2020,American Chemical Society.e)Schematic illustration of 2D/2D MoS2/C3N4heterostructure for NRR.Reproduced with permission from ref.[187]Copyright 2020,American Chemical Society.f)Schematic illustration of 2D/2D g-C3N4/doped graphene heterostructure for CO2RR.Reproduced with permission from ref.[188]Copyright 2019,Wiley-VCH.

OER is another important reaction for oxygen evolution involved in sustainable fuel generations or energy storages.This includes a kinetically sluggish multiple electron transfers process and high-efficiency and low-cost electrocatalysts with good structural stability are yet in an urgent demand.[162]It is widely accepted that Ir/Ru-based materials are deemed as the state-of-the-art OER electrocatalysts,but the high-cost of the noble metals urges the researchers to either find abundant and nonnoble replacements or lower the dosage without detriment of the activity.The design of 2D/2D heterostructures is one promising solution on these challenges.2D/2D RuO2/graphene heterostructures were synthesized by in situ growth of defective RuO2on graphene,which exhibited the ultrathin thickness of 9 nm and high specific surface area of 125 m2g-1.Owing to enriched hydroxylated surface,notably intrinsic defective RuO2,and low Ru-O coordination number of 5 associated with the formation of 2D/2D heterostructure,excellent OER catalytic activity was achieved in all-pH-range electrolyzers.[175]The 2D/2D heterostructure thus lowers the use of noble RuO2and boosts the OER catalytic activity by the creation of defective Ru sites,which enhances the adsorption and accelerates the decomposition of hydroxyl groups in OER.

2D/2D heterostructures also provide options for exploring low-cost non-noble metal-based catalysts,which realize a compromise between the costs and the catalytic performance.It has been reported that coupling 2D non-noble metal alloy(e.g.,NiFe,NiCo,and NiCu)nanosheets with 2D layered platforms(e.g.,MXene,graphene,and MoS2)can give rise to strong interfacial coupling effects to boost the OER performance in alkaline media.[176]In this class of 2D/2D heterostructures,1)strong interfacial coupling states were identified,such as in NiFe/MoS2heterostructure,which facilitate the charge transfer and thus promote the OER activity;2)more exposed active sites contributed by suppressed aggregation and in situ reduction of metal precursors promoted the catalytic activity,3)altered electronic structures optimized the OER intermediate adsorption energy,and thus the 2D/2D heterostructures presented much improved OER performance than the individual catalysts.The catalytic activity of 2D/2D heterostructures can be further improved by incorporating multiple active materials.Wang et al.reported a composite electrocatalyst by anchoring Ni3N particles on a stable 2D/2D graphene-BP heterostructure for OER.In this heterostructure,strong P-C bonds were introduced through a mechanical ball-milling(Figure 6b).[177]It has been revealed that strong P-C bonds can optimize the adsorption energy of intermediates to the thermoneutral point and thus promote the catalytic activity.The as-synthesized hybrid catalysts delivered a OER overpotential of 233 mV at 10 mA cm-2and a Tafel slope of 42 mV dec-1,as well as about 86.4% retention after 10 000 s,demonstrating the efficiency of this design strategy for active OER catalysis.[177]

Overall water splitting is an entire reaction combining both cathodic HER and anodic OER to electrolyze water into gaseous H2and O2.Ideally,bifunctional electrocatalysts which can simultaneously catalyze HER and OER are a promising approach to simplify the system design and save the overall cost.The theoretical thermodynamic potential of water splitting is 1.23 V at an ambient environment regardless of the reaction media,while the actual overpotential to be conquered for overall water splitting also includes the intrinsic energy barriers for both cathode and anode and the contact and solution resistances(Eop=1.23 V+ ηa+ ηc+ ηother),which highly depend on the use of highly active HER and OER catalysts and the optimization of electrolytes.[178]The 2D/2D heterostructures which provide opportunities to construct two different active catalysts into one material give rise to the chance for bifunctional catalysis and overall water splitting.As illustrated in Figure 6c,2D/2D BP/N-doped graphene heterostructure with unique interfacial charge transfer was evidenced as an efficient bifunctional catalysts for overall water splitting in alkaline media.[179]In this heterostructure,the electronic structures were rationally modulated to activate their intrinsic activities of BP and graphene.Compared to N-doped graphene,the Fermi level of BP is relatively lower,which can induce directional interfacial electron transfer via dual-phase electronic interaction.This modulation enriches the electron density over BP to optimize the ad/desorption of hydrogen for HER,and meanwhile,generates abundant positively charged carbon sites in N-doped graphene framework to provide favorable formation of OER intermediates(OOH*).Based on this heterostructure,an asymmetric water splitting device assembled with the 2D/2D BP/N-doped graphene heterostructure catalyst presented a operational voltage of 1.54 V at 10 mA cm-2,which outperformed the noble metal-based Pt/C@RuO2cell(1.60 V).[179]Additionally,some 2D/2D metal-containing bifunctional heterostructure catalysts,such as metal carbide/nitride[180]and metal sulfide/graphene,[181]have also been reported to be effective for overall water splitting.[182]

Another important oxygen-involving electrocatalytic reaction,ORR,greatly limits the conversion efficiency of fuel cells and metal-oxygen batteries,which require a high-efficiency catalyst.[183]While the best performed Pt-based catalysts have been restricted by their scarcity and high-cost,the non-noble metal-based catalysts and the nitrogen-doped carbon materials have attracted increasing attention for ORR catalysis.[184,185]The construction of 2D/2D heterostructure,surprisingly,can further combine these two types of emerging ORR catalysts together and boost the catalytic activity.As shown in Figure 6d,2D/2D superlattice-like Fe-N-C/MXene heterostructures,consisting of ironcluster-directed cationic Fe-N-C nanosheets and anionic MXene,stacked at a face-to-face model by repeated 0.4 nm-thick Fe-N-C and 2.1 nmthick MXene units.[186]The as-synthesized Fe-N-C/MXene heterostructure demonstrated good electrocatalytic ORR performance via a fourelectron-transfer pathway,showing a positive onset potential of 0.92 V,a half-wave potential of 0.84 V,and long-life durability up to 20 h toward methanol crossover.[186]

Besides the above-mentioned electrocatalysis reactions,2D/2D heterostructures have been designed for NRR with the conversion of N2to NH3and CO2RR with the production of valuable hydrocarbons.During the NRR and CO2RR catalysis,the 2D/2D heterostructures can provide improved interfacial coupling behaviors,which facilitate ultrafast interlayer charge and mass transport or generate more active reaction centers through the resultant surface electronic density redistribution and surface defects formation.A 2D/2D MoS2/C3N4heterostructure with a strong interfacial coupling interaction was synthesized.This strong interfacial coupling promoted the stabilization of*N2H at the Mo edge sites of MoS2,reduced the reaction energy barrier,and prevented the NRR-active Mo edge sites from the competing HER.[187]Benefiting from the NRR activity enhancement,the 2D/2D MoS2/C3N4heterostructure offered an NH3yield of 18.5 μg h-1mg-1and a high Faradaic efficiency(FE)of 17.8% ,which are much superior to the individual MoS2and C3N4component(Figure 6e).[187]The critical role of interfacial electron transfer of 2D/2D heterostructures has also been identified in CO2RR catalysis.A series of 2D/2D g-C3N4/heteroatom-doped graphene heterostructures(C3N4/XG,X=B,N,O,or P)have been theoretically predicted for the CO2RR catalysis from CO2to methane(Figure 6f).At the interface of heterostructures,particularly,the nitrogen dopant in the graphene can lead to an increased electron transfer from the heteroatom-doped graphene to the g-C3N4nanosheets,which facilitates the protonation of CO2and decreases the free energy barrier toward CO2RR.By this way,the 2D/2D g-C3N4/NG heterostructure can catalyze the CO2RR at an overpotential as low as 450 mV.[188]

To summarize,2D/2D heterostructures with a unique quantum confinement effect and rapid interlayer mass and charge transfer characteristics could modulate electronic and physical coupling interactions to facilitate electrocatalysis reactions.The electrocatalytic activities of the 2D/2D heterostructures are largely dependent on the constituting compositions,active exposed areas,interfacial coupling induced defects,and interfacial coupling induced electron density redistribution of the heterostructures.2D materials have become a big family with rapid growing numbers which possess diverse chemical/physical properties.The combination of the different 2D materials into 2D/2D heterostructures provides us vast space to design distinct catalysts to meet different requirements to trigger the various sustainable energy storage and green fuel generation reactions,as demonstrated in Table 1.[189-192]More future work should be conducted to identify the possible permutations and combinations for efficient 2D/2D heterostructure catalysts.Moreover,the mechanism understandings at an atomic or molecular level on the above-mentioned catalytic activity originations within the 2D/2D heterostructures through advanced in situ/operando characterization techniques and theoretical calculation and simulation approaches are urgently needed.

Table 1.Summary on 2D/2D heterostructures for electrocatalytic applications.

5.Perspectives and Outlook

As a summary,plenty of plausive advances on the synthesis of 2D/2D heterostructures and their applications for advanced battery and electrocatalysis have been achieved,which demonstrated the brilliant future of this vast class of materials in promising practice applications for sustainable energy conversion and storages and green fuels generations.Some critical challenges,however,still exist,as illustrated in Figure 7.In this section,the primary barriers for impeding the engineering application of the 2D/2D heterostructures are discussed,followed by our proposals on promising solutions for addressing these issues.Finally,an outlook is provided for possible future studies of this class of materials in contributing to maintain sustainable environment and community.

Figure 7.Summary on the major challenges and possible solutions of 2D/2D heterostructures for advanced batteries and electrocatalysis.

5.1.Current Challenges on Synthesis and Applications

5.1.1.Cost-effective and Scalable Synthetic Strategies for 2D/2D Heterostructures

The lack of cost-effective synthetic strategies for achieving scalable production of 2D/2D heterostructures with satisfied quality remains a primary barrier for the engineering applications of this class of materials.Despite that the high-quality and clean heterostructures can be fabricated by CVD methods,an industrial-scale production of 2D/2D heterostructures by CVD has yet under investigation.Furthermore,the sequential CVD growth is highly effective to produce multilayered vdWHs,but it is very sensitive to the growth conditions and difficult in actual operation,owing to the frequently changed deposition precursors,parameters,and atmospheres for the deposition of different layers.As the layer number increases,besides the possible mechanical,chemical,or thermal damages and other detrimental effects to the pre-deposited layers,some more accurate control on the crystal orientation,dimensional size,surface states,etc.,are needed,which make this technology more complex and expensive.Solution-based assembly is another major fabrication method,which is very flexible for the preparation of vdWHs,as the assembly occurring in the liquid solutions without the strict requirements on precursors and surface states of the substrate layers.The controllable layer-by-layer assembly of multilayer vdWHs through this method,however,is still very challenging for the difficult in the control of the surface chemistry or surface charge of the multiple types of 2D materials or molecules.Moreover,the interfacial quality of the vdWHs prepared by solution-based method is also hard to compare with those prepared by mechanical exfoliation or CVD growth.Therefore,it is much essential to develop reliable and scalable approaches for both the synthesis of 2D building blocks and the fabrication of 2D/2D heterostructures,with capability in accurate manipulation on chemical compositions,stacking models,orientations,interfacial adhesion,etc.,of the 2D/2D heterostructures.

5.1.2.2D/2D Heterostructures for Advanced Batteries

In the application of 2D/2D heterostructures as the electrode materials for advanced batteries,the exposed high-area surface of the 2D nanostructures could ensure intimate contact with both the electrolyte and the current collector,which can improve the kinetics of desired ion transfer.The combination of different 2D building blocks can give rise to synergistic effects to maximize the storage capability of each constituent component.On the other hand,this high-area contact can consume more electrolyte accompanied by the formation of solidelectrolyte interface(SEI)layer on the surface,thus resulting in obvious irreversible capacity loss during the initial cycles.It is also possible that the abundant active sites within the 2D/2D heterostructures may ignite some undesired parasitic reactions associated with the electrolyte decomposition.

When used as electrodes,2D/2D heterostructures could be easily manufactured in the desired electrode structure by filtering,printing,rolling,spraying,or spin-coating,even in the absence of binders and current collectors.For real engineering applications,such as in the batteries for smart phones,electrical vehicles,and unmanned drones,unfortunately,it is still challenging to find a suitable method for the fabrication of 2D/2D heterostructure-based electrode,even though there is yet a long way to the engineering application of 2D/2D heterostructures.

5.1.3.2D/2D Heterostructures for Electrocatalysis

With respect to the water-splitting reactions,2D/2D heterostructures have demonstrated low overpotentials and Tafel slope values toward HER and OER catalysis.However,the performances of the 2D/2D heterostructures composed of non-noble metal-based and/or metalfree catalysts in terms of high current density catalysis and long durability exhibit a big gap compared to these of the noble metal-based catalysts.Moreover,the exploration of 2D/2D heterostructures for efficient ORR,NRR,and CO2RR is not as intensive as those for HER and OER,and more systematic investigations are required for identifying the merits of 2D/2D heterostructures in these reduction reactions.Moreover,there are plenty of permutations and combinations in choosing proper 2D building blocks with different properties or functions to trigger different electrocatalysis reactions through rational design of 2D/2D heterostructures.High throughput methods to sieve proper combinations and configurations for the 2D/2D heterostructure catalysts are urgently needed.It is expected that highly active and selective 2D/2D heterostructure catalysts can be designed for transforming lowcost raw materials even wastes into high-added-value products and contribute to neutralizing the carbon emissions.

5.1.4.High-precise and Quantitative Characterizations

The characterizations of 2D materials are always difficult,so do for the 2D/2D heterostructures.It is the fundamental to have an accurate understanding on the structural and physical/chemical properties of the 2D/2D heterostructures for their further various applications,including structural parameters and characteristics,surface chemistry,defects,interlayer bonding,and spacing features,interlayer electronic coupling behaviors,etc.,which are needed the support of sophisticated characterizations techniques.Particularly,the catalytic applications of the materials are very sensitive to the localized coordination environment and the charge distributions,such as surface oxidation,adsorption species,lattice distortions,defective degree,etc.,and the precise characterizations are critical to establish the“structures-properties”relationships.

5.2.Potential Solutions for Addressing the Challenges

5.2.1.Structure and Composition Optimization on 2D/2D Heterostructures

To better adapt the application of 2D/2D heterostructures in the practical batteries and electrocatalysis and further promote the performance,optimizations on both the compositions and structures,such as surface modifications,doping,alloying,defects and strain engineering,interlayer spacing tailoring,etc.,can be performed by either post-treatment or in situ chemical reactions.For example,pore engineering can transform the well-defined 2D structures into porous structures,which can further enlarge the surface area,create trans-plane channels to accelerate electrolyte penetration and transfer,provide active and low-coordinated edge and corner active sites to catalyze the redox chemistry,and suppress the restacking of 2D-based structures from dense packing.

5.2.2.Atomic/molecular Level Mechanism Understandings for 2D/2D Heterostructures

To identify the underlying correlations between the material characteristics and the functional properties and explore the general principles for materials design to meet the electrochemical performance requirements,a comprehensive understanding of the electrochemical storage or catalytic mechanism at an atomic or molecular level for 2D/2D heterostructures is highly desired.Thanks for the rapid development of characterization technologies,the atomic and molecular structures of the 2D/2D heterostructures can be well-parsed by some advanced characterization techniques,such as advanced electron and optical microscopy,X-ray structural analysis,Raman/infrared/ultraviolet-visible light techniques,neutron depth pro filing,X?ray absorption spectroscopy,nuclear magnetic resonance,M?ssbauer spectroscopy,[193-203]While the atomic or molecular characterizations on the electrochemical processes during the batteries and electrocatalysis applications can be reached by some advanced electrochemical monitoring methods,such as differential electrochemical mass spectrometry and scanning electrochemical microscopy.[204]Particularly,some in situ and operando technologies have been developed to reveal the underlying electrochemical mechanisms during electrochemical processes,which can provide reliable and valuable details on the contributions of the crystal structure,surface,edges,interfaces,morphologies,compositions,valence states,specific active sites,intermediate phases,to the electrochemical reactions and the associated phase formation and transformation processes.It should be noted that some advanced experimental characterizations are highly dependent on the availability of capital instrumentations,such as synchrotron facility,which are impossible to open to all users.

5.2.3.Development of Novel 2D/2D Heterostructures

In recent years,the innovations in 2D ultrathin layered materials have attended great progress.Plenty of new 2D materials,such as borophene,[205]silicene,[206]bismuthene,[207]etc.,have been discovered,[208]which offer us more choices for constructing novel 2D/2D heterostructures and provide more combinations to achieve higher electrochemical storage and catalytic performances.It is believed that the family of 2D/2D heterostructures will be continued to expand with the growth of 2D materials.The combination of computational screening and experimental verification will also be a time-saving approach to identify high-performance 2D/2D heterostructures from the numerous possible permutations of the 2D materials.

5.3.Future Outlooks

Based on the recent progress,it is very clear that the incorporation of 2D/2D heterostructures can greatly boost the current battery performance with respect to the capacity level,the rate capability,and the cycling life,and significantly promote the electrocatalytic efficiency by increasing the accessibility of active centers,lowering the overall reaction energy barriers,improving the selectivity,prolonging the durability,etc.With the development of cost-effective fabrication and the atomic/molecular level understandings on the electrochemical mechanism of the 2D/2D heterostructures,this unique family of materials will exhibit a promising future in the application in real engineering sustainable energy conversion or storage and green fuels generation devices.We hope that this work can give some inspirations into the development of 2D-based materials and the innovations of sustainable environmental and energy technologies.

Acknowledgments

This work was supported by Australian Research Council(ARC)through an ARC Discovery Project(DP200103568)and two ARC Future Fellowship projects(FT180100387 and FT160100281).J.Mei acknowledges the financial support by a QUT 2020 ECR Scheme Grant(No.2020001179)and the project mentorship provided by Prof.Godwin Ayoko.

Conflict of Interest

The authors declare no conflict of interest.