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

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.

Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

With a large number of researches being conducted on two?dimen?sional(2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that pro?vides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high?quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of di erent types of 2D heterostruc?tures, followed by the discussions on present challenges and perspectives of further investigations.

Trimetallic synergistic optimization of 0D NiCoFe-P QDs anchoring on 2D porous carbon for efficient electrocatalysis and high-energy supercapacitor

Developing multi-functional and low-cost noble-metal-free catalysts such as transition metal phosphides(TMPs)to replace noble-metal is of practical significance for energy conversion and storage.However,the low-durability and the agglomeration phenomenon during the electrochemical process limit their practical applications.Herein,using metal–organic frameworks(MOFs)as the precursor and a combined strategy of gradient temperature calcination and thermal phosphorization,a 0D/2D heterostructure of NiCoFe-P quantum dots(QDs)anchored on porous carbon was successfully developed as highly efficient electrode materials for overall water splitting and supercapacitors.Owing to this distinctive 0D/2D heterostructure and the synergistic effect of multi-metallic TMPs,the NiCoFe-P/C exhibits excellent electrocatalytic activity and durability of HER(87 mV at 10 mA cm^(-2))and OER(257 mV at 100 mA cm^(-2))in the KOH electrolyte.When NiCoFe-P/C is used as the two electrodes of electrolyzed water,only 1.55 V can drive the current density to 10 m A cm^(-2).At the same time,our NiCoFe-P/C possessed extraordinary property for charge storage.In particular,an ultra-high energy density of 100.8 Wh kg^(-1) was achieved at a power density of 900.0 W kg^(-1) for our assembled hybrid supercapacitor device NiCoFe-P/C(2:1)//activated carbon(AC).This work may open a potential way for the design of 0D/2D hybrid multifunctional nanomaterials based on TMPs QDs.

Allying interfacial engineering of 2D carbon nanosheet-,graphene-,and graphdiyne-based heterostructured electrocatalysts toward hydrogen evolution and overall water splitting

Electrochemical hydrogen evolution reaction(HER)and overall water splitting(OWS)for renewable energy generation have recently become a highly promising and sustainable strategy to tackle energy crisis and global warming arising from our overreliance on fossil fuels.Previously,tremendous research breakthroughs have been made in 2D carbon-based heterostructured electrocatalysts in this field.Such heterostructures are distinguished by their remarkable electrical conductivity,exposed active sites,and mechanical stability.Herein,with fundamental mechanisms of electrocatalytic OWS summarized,our review critically emphasized on state-of-the-art 2D carbon nanosheet-,graphene-,and graphdiyne-based heterostructured electrocatalysts in HER and OWS since 2018.Particularly,the three emerging carbonaceous substrates tend to be incorporated with metal carbides,phosphides,dichalcogenides,nitrides,oxides,nanoparticles,single atom catalysts,or layered double hydroxides.Meanwhile,fascinating structural engineering and facile synthesis strategies were also unraveled to establish the structure-activity relationship,which will enlighten future electrocatalyst developments toward ameliorated HER and OWS activities.Additionally,computational results from density functional theory simulations were highlighted as well to better comprehend the synergistic effects within the heterostructures.Finally,current stages and future recommendations of this brand-new electrocatalyst type were concluded and discussed for advanced catalyst designs and future practical applications.

Synthesis of two-dimensional/one-dimensional heterostructures with tunable width

Two-dimensional/one-dimensional(2D/1D)heterostructures as a new type of heterostructure have been studied for their unusual properties and promising applications in electronic and optoelectronic devices.However,the studies of 2D/1D heterostructures are mainly focused on vertical heterostructures,such as MoS_(2) nanosheet-carbon nanotubes.The research on lateral 2D/1D heterostructures with a tunable width of 1D material is still scarce.In this study,bidirectional flow chemical vapor deposition(CVD)was used to accurately control the width of the WS_(2)/WSe2(WS_(2)/MoS_(2))heterostructures by controlling reacting time.WSe2 and MoS_(2) with different widths were epitaxially grown at the edge of WS_(2),respectively.Optical microscope,atomic force microscope(AFM),and scanning electron microscope(SEM)images show the morphology and width of the heterostructures.These results show that the width of the heterostructures can be as low as 10 nm by using this method.The interface of the heterostructure is clear and smooth,which is suitable for application.This report offers a new method for the growth of 1D nanowires,and lays the foundation for the future study of the physical and chemical properties of 2D/1D lateral heterostructures.

Nonlinear optics of two-dimensional heterostructures

Two-dimensional (2D) materials exhibit exceptionally strong nonlinear optical responses, benefiting from their reduced dimensionality, relaxed phase-matching requirements, and enhanced light-matter interaction. With additional degrees of freedom in the modulation of the physical properties by stacking 2D layers together, nonlinear optics of 2D heterostructures becomes increasingly fascinating. In this perspective, we provide a brief overview of recent advances in the field of nonlinear optics of 2D heterostructures, with a particular focus on their remarkable capabilities in characterization and modulation. Given the recent advances and the emergence of novel heterostructures, combined with innovative tuning knobs and advanced nonlinear optical techniques, we anticipate deeper insights into the underlying mechanisms and more associated applications in this rapidly evolving field.

Recent advances in 2D organic−inorganic heterostructures for electronics and optoelectronics

Two‐dimensional(2D)materials show outstanding properties such as dangling bond‐free surfaces,strong in‐plane while weak out‐of‐plane bonding,layer‐dependent electronic structures,and tunable electronic and optoelectronic properties,making them promising for numerous applications.Integrating 2D inorganics with organic materials to make van der Waals heterostructures at the 2D thickness limit has created new platforms for fabricating on‐demand multifunctional devices.To further broaden the limited choices of 2D inorganic‐based heterostructures,a wide range of available 2D organic materials with tunable properties have opened new opportunities for designing large numbers of heterostructures with 2D inorganic materials.This review aims to attract the attention of researchers toward this emerging 2D organic−inorganic field.We first highlight recent progress in organic−inorganic heterostructures and their synthesis and then discuss their potential applications,such as field‐effect transistors,photodetectors,solar cells,and neuromorphic computing devices.In the end,we present a summary of challenges and opportunities in this field.

MoS_(2)/Si tunnel diodes based on comprehensive transfer technique

Due to the pristine interface of the 2D/3D face-tunneling heterostructure with an ultra-sharp doping profile, the 2D/3D tunneling field-effect transistor(TFET) is considered as one of the most promising low-power devices that can simultaneously obtain low off-state current(IOFF), high on-state current(ION) and steep subthreshold swing(SS). As a key element for the 2D/3D TFET, the intensive exploration of the tunnel diode based on the 2D/3D heterostructure is in urgent need.The transfer technique composed of the exfoliation and the release process is currently the most common approach to fabricating the 2D/3D heterostructures. However, the well-established transfer technique of the 2D materials is still unavailable.Only a small part of the irregular films can usually be obtained by mechanical exfoliation, while the choice of the chemical exfoliation may lead to the contamination of the 2D material films by the ions in the chemical etchants. Moreover, the deformation of the 2D material in the transfer process due to its soft nature also leads to the nonuniformity of the transferred film,which is one of the main reasons for the presence of the wrinkles and the stacks in the transferred film. Thus, the large-scale fabrication of the high-quality 2D/3D tunnel diodes is limited. In this article, a comprehensive transfer technique that can mend up the shortages mentioned above with the aid of the water and the thermal release tape(TRT) is proposed. Based on the method we proposed, the MoS_(2)/Si tunnel diode is experimentally demonstrated and the transferred monolayer MoS_(2) film with the relatively high crystal quality is confirmed by atomic force microscopy(AFM), scanning electron microscopy(SEM), and Raman characterizations. Besides, the prominent negative differential resistance(NDR) effect is observed at room temperature, which verifies the relatively high quality of the MoS_(2)/Si heterojunction. The bilayer MoS_(2)/Si tunnel diode is also experimentally fabricated by repeating the transfer process we proposed, followed by the specific analysis of the electrical characteristics. This study shows the advantages of the transfer technique we proposed and indicates the great application foreground of the fabricated 2D/3D heterostructure for ultralow-power tunneling devices.

Alternating nanolayers as lithiophilic scaffolds for Li-metal anode

Nanostructured scaffolds offer promising opportunities in enabling dendrite-free long-cycle life Li metal anode.The rational design and controllable synthesis of scaffolding architectures are imperative for development of rechargeable Li metal batteries.In this study,we explore the fabrication and application of a tin monoxide/graphene hybrid architecture as a lithiophilic host for high-performance Li metal anode.Using a polymer-assisted sonochemical synthesis route,we tuned the thickness of SnO nanolayers and the nanostructure of alternatively stacking thin SnO nanosheet/graphene(SnO-NS/G) heterostructure.Offering abundant nucleation sites,fast ion transport tunnels,and 3D-conductivity,the unique 2D-2D architecture enables stable lithium plating-stripping cycling with low nucleation overpotential and high coulombic efficiency(CE).Hosted by SnO-NS/G scaffold,the resulting Li metal anode exhibits stable cycling over 200 cycles at 0.5 mA cm^(-2)(2 mAh).Full cell pairing high-mass-loading cathode LiCoO_(2)(LCO)(12 mg cm^(-2)) with SnO-NS/G hosted Li metal anode delivers high energy density of 402 Wh kg^(-1) and stable cyclability of over 100 cycles.We elucidate the structure-property relationship between nanolayer thickness and Li-metal plating behaviors,giving new insight on structuring 2D-nanomaterials with ideal architectures for stable lithium metal batteries.

Atomically constructing a van der Waals heterostructure of CrTe_(2)/Bi_(2)Te_(3) by molecular beam epitaxy

A 2D heterostructure with proximity coupling of magnetism and topology can provide enthralling prospects for hosting new quantum states and exotic properties that are relevant to next-generation spintronic devices.Here,we synthesize a delicate van der Waals(vdW)heterostructure of CrTe_(2)/Bi_(2)Te_(3) at the atomic scale via molecular beam epitaxy.Low-temperature scanning tunneling microscopy/spectroscopy measurements are utilized to characterize the geometric and electronic properties of the CrTe_(2)/Bi_(2)Te_(3) heterostructure with a compressed vdW gap.Detailed structural analysis reveals complex interfacial structures with diversiform step heights and intriguing moirépatterns.The formation of the interface is ascribed to the embedded characteristics of CrTe_(2) and Bi_(2)Te_(3) by sharing Te atomic layer upon interfacing,showing intercoupled features of electronic structure for CrTe_(2) and Bi_(2)Te_(3).Our study demonstrates a possible approach to construct artificial heterostructures with different types of ordered states,which may be of use for achieving tunable interfacial Dzyaloshinsky–Moriya interactions and tailoring the functional building blocks in low dimensions.

Opto-valleytronics in the 2D van der Waals heterostructure

The development of information processing devices with minimum carbon emission is crucial in this information age. One of the approaches to tackle this challenge is by using valleys (local extremum points in the momentum space) to encode the information instead of charges. The valley information in some material such as monolayer transition metal dichalcogenide (TMD) can be controlled by using circularly polarized light. This opens a new field called opto-valleytronics. In this article, we first review the valley physics in monolayer TMD and two-dimensional (2D) heterostructure composed of monolayer TMD and other materials. Such 2D heterostructure has been shown to exhibit interesting phenomena such as interlayer exciton, magnetic proximity effect, and spin-orbit proximity effect, which is beneficial for opto-valleytronics application. We then review some of the optical valley control methods that have been used in the monolayer TMD and the 2D heterostructure. Finally, a summary and outlook of the 2D heterostructure opto-valleytronics are given.

A prospective future towards bio/medical technology and bioelectronics based on 2D vdWs heterostructures

Nano-biotechnology research has become extremely important due to the possibilities in manipulation and characterization of biological molecules through nanodevices.Nanomaterials exhibit exciting electrical,optoelectronic,magnetic,mechanical and chemical properties that can be exploited to develop efficient biosensors or bio-probes.Those unique properties in nanomaterials can also be used in bioimaging and cancer therapeutics,where biomolecules influence the inherent properties in nanomaterials.Effective manipulation of nanomaterial properties can lead to many breakthroughs in nanotechnology applications.Nowadays,2D nanomaterials have emerged as viable materials for nanotechnology.Large cross-section area and functional availability of 2D or 1D quantum limit in these nanomaterials allow greater flexibility and better nanodevice performance.2D nanomaterials enable advanced bioelectronics to be more easily integrated due to their atomic thickness,biocompatibility,mechanical flexibility and conformity.Furthermore,with the development of 2D material heterostructures,enhanced material properties can be obtained which can directly influence bio-nanotechnology applications.This article firstly reviews the development of various types of 2D heterostructures in a wide variety of nano-biotechnology applications.Furthermore,future 2D heterostructure scopes in bioimaging,nanomedicine,bio-markers/therapy and bioelectronics are discussed.This paper can be an avenue for providing a wide scope for 2D van der Waals(vdWs)heterostructures in bio-and medical fields.

Optoelectronics based on 2D TMDs and heterostructures

2D materials including graphene and TMDs have proven interesting physical properties and promising optoelectronic applications.We reviewed the growth,characterization and optoelectronics based on 2D TMDs and their heterostructures,and demonstrated their unique and high quality of performances.For example,we observed the large mobility,fast response and high photo-responsivity in Mo S;,WS;and WSe;phototransistors,as well as the novel performances in vd W heterostructures such as the strong interlayer coupling,am-bipolar and rectifying behaviour,and the obvious photovoltaic effect.It is being possible that 2D family materials could play an increasingly important role in the future nano- and opto-electronics,more even than traditional semiconductors such as silicon.

Two-dimensional heterostructure promoted infrared photodetection devices

It is a rapidly developed subject in expanding the fundamental properties and application of two-dimensional(2D)materials.The weak van der Waals interaction in 2D materials inspired researchers to explore 2D heterostructures(2DHs)based broadband photodetectors in the far-infrared(IR)and middle-IR regions with high response and high detectivity.This review focuses on the strategy and motivation of designing 2DHs based high-performance IR photodetectors,which provides a wide view of this field and new expectation for advanced photodetectors.First,the photocarriers'generation mechanism and frequently employed device structures are presented.Then,the 2DHs are divided into semimetal/semiconductor 2DHs,semiconductor/semiconductor 2DHs,and multidimensional semi-2DHs;the advantages,motivation,mechanism,recent progress,and outlook are discussed.Finally,the challenges for next-generation photodetectors are described for this rapidly developing field.

Multidimensional CdS nanowire/CdIn2S4 nanosheet heterostructure for photocatalytic and photoelectrochemical applications

Nanomaterial shapes can have profound effects on material properties, and therefore offer an efficient way to improve the performances of designed materials and devices. The rational fabrication of multidimensional architectures such as one dimensional （1D）-two dimensional （2D） hybrid nanomaterials can integrate the merits of individual components and provide enhanced functionality. However, it is still very challenging to fabricate 1D/2D architectures because of the different growth mechanisms of the nanostructures. Here, we present a new solvent- mediated, surface reaction-driven growth route for synthesis of CdS nanowire （NW）/CdIn2S4 nanosheet （NS） 1D/2D architectures. The as-obtained CdS NW/ CdIn2S4 NS structures exhibit much higher visible-light-responsive photocatalytic activities for water splitting than the individual components. The CdS NW/CdIn2S4 NS heterostructure was further fabricated into photoelectrodes, which achieved a considerable photocurrent density of 2.85 mA·cm^-2 at 0 V vs. the reversible hydrogen electrode （RHE） without use of any co-catalysts. This represents one of the best results from a CdS-based photoelectrochemical （PEC） cell. Both the multidimensional nature and type II band alignment of the 1D/2D CdS/CdIn2S4 heterostructure contribute to the enhanced photocatalyfic and photoelectrochemical activity. The present work not only provides a new strategy for designing multidimensional 1D/2D heterostructures, but also documents the development of highly efficient energy conversion catalysts.

Effects of interlayer coupling on the excitons and electronic structures of WS_(2)/hBN/MoS_(2) van der Waals heterostructures

Inserting hexagonal boron nitride(hBN)as barrier layers into bilayer transition metal dichalcogenides heterointerface has been proved an efficient method to improve two dimensional tunneling optoelectronic device performance.Nevertheless,the physical picture of interlayer coupling effect during incorporation of monolayer(1L-)hBN is not explicit yet.In this article,spectroscopic ellipsometry was used to experimentally obtain the broadband excitonic and critical point properties of WS_(2)/MoS_(2)and WS_(2)/hBN/MoS_(2)van der Waals heterostructures.We find that 1L-hBN can only slightly block the interlayer electron transfer from WS_(2)layer to MoS_(2)layer.Moreover,insertion of 1L-hBN weakens the interlayer coupling effect by releasing quantum confinement and reducing efficient dielectric screening.Consequently,the exciton binding energies in WS_(2)/hBN/MoS_(2)heterostructures blueshift comparing to those in WS_(2)/MoS_(2)heterostructures.In this exciton binding energies tuning process,the reducing dielectric screening effect plays a leading role.In the meantime,the quasi-particle(QP)bandgap remains unchanged before and after 1L-hBN insertion,which is attributed to released quantum confinement and decreased dielectric screening effects canceling each other.Unchanged QP bandgap as along with blueshift exciton binding energies lead to the redshift exciton transition energies in WS_(2)/hBN/MoS_(2)heterostructures.

Spatially-separated redox sites enabling selective atmospheric CO_(2)photoreduction to CH_(4)

CO_(2)photoreduction to high-valued CH_(4)is highly attractive,whereas the CH_(4)selectivity and activity,especially under atmospheric CO_(2),is still unsatisfying.Here,we design spatially-separated redox sites on two-dimensional heterostructured nanosheets with loaded metal oxides,thus achieving high reactivity and selectivity of photocatalytic atmospheric CO_(2)reduction to CH_(4).Taking the synthetic In_(2)O_(3)/In_(2)S_(3)nanosheets with loaded PdO quantum dots as a prototype,quasi in-situ X-ray photoelectron spectra reveal the Pd sites accumulate photogenerated holes for dissociating H_(2)O and the In sites accept photoexcited electrons to activate CO_(2).Moreover,the Pd-OD bond is confirmed by in-situ Fourier-transform infrared spectra during the D2O labeling experiment,indicating the PdO quantum dots participate in H_(2)O oxidation to supply hydrogen species for CO_(2)methanation.As a result,in a simulated air atmosphere,the PdO-In_(2)O_(3)/In_(2)S_(3)nanosheets enable favorable atmospheric CO_(2)-to CH_(4)photoreduction with nearly 100%selectivity and ultralong stability of 240 h as well as CO_(2)conversion of 48.2%.This study opens an approach towards designing photocatalysts with spatially-separated redox sites to achieve efficient oxidation and reduction of CO_(2)photocatalysis to CH_(4).

Zinc battery goes to anode-free

The zinc(Zn)batteries have challenges include uncontrollable dendritic growth,unreasonable negative to positive ratio and limited areal capacity.This highlight presents the latest development to resolve the uncontrollable Zn dendrite formation at high areal capacities of 200 mAh·cm^(-2) through a two-dimensional metal/metal-Zn alloy heterostructured interface.The anode-free Zn batteries with an attractive and practical pouch cell energy density of 62 Wh·kg^(-1) enlighten an arena towards their commercialization.

Improving the band alignment at PtSe_(2)grain boundaries with selective adsorption of TCNQ

Grain boundaries in two-dimensional(2D)semiconductors generally induce distorted band alignment and interfacial charge,which impair their electronic properties for device applications.Here,we report the improvement of band alignment at the grain boundaries of PtSe_(2),a 2D semiconductor,with selective adsorption of a presentative organic acceptor,tetracyanoquinodimethane(TCNQ).TCNQ molecules show selective adsorption at the PtSe_(2)grain boundary with strong interfacial charge.The adsorption of TCNQ distinctly improves the band alignment at the PtSe_(2)grain boundaries.With the charge transfer between the grain boundary and TCNQ,the local charge is inhibited,and the band bending at the grain boundary is suppressed,as revealed by the scanning tunneling microscopy and spectroscopy(STM/S)results.Our finding provides an effective method for the advancement of the band alignment at the grain boundary by functional molecules,improving the electronic properties of 2D semiconductors for their future applications.

具备大层间距、高度周期性类似手风琴的氧化石墨烯框架用于制备交替排列的二维异质结构

二维异质结构在集成材料设计中具有广阔的应用前景,但目前的合成策略用于多层异质结构构建和大规模生产时仍面临着诸多挑战.本论文报导了一种基于非剥离层状氧化石墨烯(LGO)的主客体策略,以构建由多层交替排列的石墨烯和金属氧化物纳米片组成的石墨烯基异质结构.二维排列的氧化石墨烯和开放的层内空间使LGO成为构建周期性二维宿主框架的理想平台.聚醚胺低聚物被用来共价连接相邻的氧化石墨烯.伸长的分子链构象使制得的手风琴石墨烯框架具有良好的结构稳定性、周期性和超大的层间空间.开放、排列良好的二维通道与客体材料金属离子前驱体的高度亲和性,可限域合成各种高质量的二维石墨烯基异质结构.此外,进一步通过去除石墨烯骨架,还可以制备各种剥离的超薄氧化物纳米片.本工作中提出的灵活可调的层间化学,为合成一系列高质量石墨烯-无机/有机二维异质结构提供了新思路.