Jiemin Wang,Liangzhu Zhang,Lifeng Wang,Weiwei Lei,and Zhong-Shuai Wu*

Keywords

boron nitride,electronics,energy,functionalization,nanosheets,twodimensional materials

1.Introduction

In recent years,2D materials are becoming a hot topic in various researches from physics,crystallography,and synthetic methods to various applications.[1,2]Generally,a common feature of 2D materials is that the electrons can enable mobility along the 2D plane within the nanoscale of 1-100 nm,therefore presenting unique electronic, optic, magnetic, and thermal properties.The representative 2D material graphene consists of carbon atoms with sp2-hybridized orbitals and constitutes an intact 2D π-π-conjugated plane,offering outstanding electronic effect(electron mobility μe～15000 cm2V-1s-1), phonon conduction(thermal conductivity K～5300W m-1K-1),and excellent mechanical property(Young’s modulus E～1.0TPa).However,the intrinsic carbon/carbon-bonding nature undoubtedly presents issues toward oxidation under high temperatures or strong acid/base,therefore restricting the utility in such extreme conditions.Alternatively,analogous to graphene with similar lattice parameters and bonding orbital, hexagonal boron nitride (h-BN)nanosheets(BNNSs),so-called “white graphene,”in contrast possess opposite electronic property but superior stability.Although the boron(B)and nitrogen(N)atom sp2hybrids gift the h-BN with π electron stacking,the electronegativity between B and N atoms results in confined electron movement,contributing to a wide bandgap(～5.5 eV)and insulating and inertness property.[3]For instance,BNNSs have large resistivity of～1018 Ω.cm, great oxidation resistance above 800°C,and strong chemical corrosion resistance.Moreover,the similar phonon diffusion mode on 2D honeycomb crystal layer structure also enables high thermal conductance(K～300-2000 W m-1K-1)of BNNS,exceptional thermal stability(＞3000 °C under N2),and low coefficient of thermal expansion(CTE)(basal planes:-2.7×10-6C-1).[4]In addition,the ceramic characteristics of BN validate BNNS-pronounced dielectric property with large permittivity(?r～ 3-5),breakdown strength(Eb～ 40 kV mm-1),and small dielectric loss(tg～2×10-4).Together with the 2D layered property,it further provides BNNS strong rigidity(the Mohs hardness～2)and wearing resistance,hence being applied to the lubricants or cements as effective additives.Additionally,in the environmental field,porous BNNS powders or sponges unusually serve as high-quality absorbents to trap the organics and oil molecules,therefore purifying the wastewater.[5]BN absorbents are capable of recycling after thermal treatment owing to the excellent thermal stability and chemical innerness,highlighting the prominence for addressing environmental problems.[6]Taking advantages of insulation feature of BNNS,BN is applicated as dielectric layers electronics and energy domains.BNNSs further function as fillers in the formation of nanocomposites for dielectric capacitive and thermal energy areas.Very recently,the advances of supercapacitors and batteries provide a platform for BNNS applicated as in the electrode interlayer,electrolyte additive,separator coating,etc.[7-12]For instance,both the chemical stability and functional groups of BNNS are beneficial for inhibiting lithium dendrites or eliminating polysulfide shuttle.It should be noted that BN offers unprecedented opportunities for broad applications while facing new challenges for productions.Fundamentally,the integrated properties and ultimate performances of BNNS-based electronics and energy units are dependent on the chemical and physical properties of BNNS,including crystalline,lateral size,layers,and surface functional groups.Therefore,the synthesis strategies and functionalization methodology should be systematically investigated to achieve high-quality BNNS and meet practical demand.

Here,this review begins with a summary of the synthesis and functionalization routes,where we discuss the bottom-up and top-down methods and covalent/non-covalent modifications to achieve various BNNSs.It is then structured into sections to introduce 2D BN for relevant electronics and energy applications based on BNNS from field-effect transistors(FETs),dielectric capacitive layers to thermal energy and electric energy,offering the perspective of the electronics,energy storage and conversion.Also,we elucidate the future challenges and outlooks for developing BN on the emerging applications in energy and electronics.

2.Synthesis and Functionalization

As mentioned above,the synthesis strategy and functionalization routes are crucial for the properties of BNNS,such as quality,dimension,layer thickness,and functional groups.Accordingly,the first insight into BNNS designs originates from the goals that support the proposed applications.For instance,the chemical vapor deposition(CVD)derived large size and crystalline BNNS can directly play a role in atomic tunneling layer for 2D heterojunction devices,while liquid-exfoliated BNNSs are normally used as fillers for thermal conductive and dielectric polymer composites.In addition,the second step takes the physics and chemistry knowledge into account throughout the synthesis and functionalization.These processes from simple to complex refer to mechanical shearing,thermal expansion,crystalline growth,covalent and noncovalent bonding,etc.Besides,various methods here are under discussion and comparison with the aim to provide optimal cases,some of which are quite general,and others that require specific conditions.

2.1.Synthesis

Reliable methods for producing BNNS with uniform characteristics are basically demanded for electronics and energy applications.Herein,top-down exfoliation and down-top synthesis methods are reviewed and their advantages are discussed respectively.

2.1.1.Top-down Methods

An appealing method,named as mechanical peeling,was frequently applied as top-down method to obtain atomic-thin nanosheets from bulk crystals.[13]The mechanical peeling is applying an adhesive tape to attach h-BN single crystals.[14]The BNNSs were peeled and transferred to target substrates.The single-layer BN was readily identified under optical microscope according to the optical contrast(Figure 1a,b).Notably,the exfoliated BN monolayers are of high quality without dangling bonds and charge traps,allowing them suitable for fundamental research on optoelectronics and physics.This method is not only useful for isolating the monolayers of BN and graphene but also effective for peeling MoS2,MoTe2,NiPS3,etc.However,the yield rate of BNNS by mechanical peeling is low,and this method is not able to precisely control the size and thickness of exfoliated flakes.The rapid development of flexible electronics requires the preparing methods to producing large quantities of BNNS which can be easily dispersed in water or organic solutions.The BN colloidal solutions are essential for fabrication of thin films through a spray coating,inkjet printing,or doctor blading process.Besides,the BN nanocomposites are readily achieved by mixing the BN dispersion with functional polymers.This area is attracting plenty of attention,such as BN nanocomposite dielectrics and smart thermal managing nanocomposites,which are discussed in the following parts.

Figure 1.a)Optical image of mechanical exfoliated atomically thin h-BN on SiO2/Si with different thickness.The inset photographs present their different thickness by AFM technique.b)Optical contrast photographs of h-BN sheets under different light sources.Reproduced with permission from ref.[14]Copyright 2011 Wiley-VCH.

Among the various preparation methods,solid-phase exfoliation by ball milling is one of the most attractive strategies for producing gramscale BNNS powder and high-concentration colloidal dispersion.Ballmilling exfoliation is using a planetary mill to mix and roll the balls,raw materials,and milling agents to reduce the thickness of 2D materials by shear force.For example,Lei et al.developed an effective solidphase exfoliation for BNNS by ball milling the mixture,the BN powder and urea molecules(Figure 2a,b).[5]The produced BNNS endowed a high quality which is few-layered structures with a narrow thickness distribution below 2 nm.Moreover,this method reached a high exfoliation efficiency up to 85% .The obtained BNNS powder is readily dispersed in water to form a stable colloidal solution with tunable concentration due to the existence of functionalized amine group.Especially,a record-high concentration of 30 mg mL-1BN colloidal solution was reported(Figure 2c).For an intriguing application,the BN colloidal solution was facilely processed into BN aerogel and freestanding membranes,which were important for thermal managing and molecule sieving(Figure 2d,e).This work provides a solid strategy for researchers to the solid-phase functionalizing and exfoliating two-dimensional materials.

Figure 2.a)Schematic illustration of solid-phase exfoliation of BNNS by ball mill.b)TEM image of BNNS.Scale bar is 50 nm.c)Photograph of BNNS colloidal solution with various concentrations from 0.5 mg mL-1to 20 mg mL-1.d)Photograph of BN aerogel.e)BN membrane.Reproduced with permission from ref.[5]Copyright 2015,Springer Nature.

Besides the urea as the milling agents,other kinds of milling agents,such as benzyl benzoate,sodium hydroxide,sugar,benzyl benzoate,and NH4Cl were applied.[15-19]Similarly,these milling agents also efficiently peeling off the BNNS.Particularly,few defects and large flake size of 1.5 μm BNNS were achieved by choosing concentrated sodium hydroxide solution.A porous BNNS showed a large surface area of 751 m2g-1by annealing the balled BN/NH4Cl mixture under nitrogen,demonstrating great potential for water cleaning under ambient conditions.[20]

Liquid-phase exfoliation of BN is available in proper solvents,including distilled water,organic solvents,ionic liquids,and aqueous surfactant solutions.Not only BNNSs were obtained by this method but also other layered materials were successfully exfoliated.Commonly,the raw materials were dispersed in an aqueous solution and sonicated for several hours.The 2D crystallites were exfoliated into nanosheets with several hundred nanometers by cavitation effect.After exfoliation,polymer solvents were used to stabilize the colloidal solution through electrostatic repulsion force.For instance,Coleman et al.successfully produced BNNS colloidal solution by liquid exfoliation(Figure 3a).[1]The prepared BNNS exhibits high quality with a fewlayered structure(Figure 3b),enabling further application as a thin film(Figure 3c).Although liquid-phase exfoliation demonstrated their merits of uniformity and scalability,their yield efficiency is still below 10% and the monolayered nanosheets are low rate of production.Therefore,further reducing the thickness of BNNS is critical,which can be validated by bottom-up methods.

Figure 3.a)Photograph of BN,MoS2,and WS2by liquid exfoliation.b)TEM images of BNNS.c)Photograph image of BN,MoS2,and WS2 films.Reproduced with permission from ref.[1]Copyright 2011,the American Association for the Advancement of Science.

In addition,other recently developed exfoliation methods also demonstrate the effectiveness.For instance,by employing supercritical CO2,BNNS(＜5 layers)or layered h-BN/MoS2heterostructures can be achieved with larger lateral size of～ 0.5-2μm.[21]In contrast with traditional cases,this approach substantially improved the solubilities of solutes,hence benefiting for both materials synthesis and intercalation,as well as being applicable for mass processing in industry.

2.1.2.Bottom-up Methods

Importantly,the synthesis of high-quality and large-area uniformity 2D materials is the ultimate object for practical applications in electronics.Wafer-scale BN films are important for applications,such as transparent optoelectronics,dielectric semiconductors,and flexible electronics.Recently,the CVD approach provides a platform to synthesize h-BN film with large-sized domains.Inspired by this,the researchers have applied the controllable CVD techniques to synthesize 2D h-BN singlecrystal and film on various substrates(such as metals and insulators)and realize device fabrication and applications.In this part,we summarize recent achievement of layered 2D h-BN growth,such as growth substrates(metals,alloy,and insulators),and further discuss the domain size engineering including monolayer h-BN film with largesized domains and large-area single-crystal film.

Metal substrate:In the starting of h-BN research,many groups obtained h-BN on single-crystal metal substrates via thermal decomposition processes under an ultrahigh vacuum environment,such as Cu,Ni,Pd,Pt,Rh,Ag,Ir,and Ru.[22-28]However,this requires strict growth conditions such as growth substrates and can only produce small-size 2D h-BN.Therefore,the improvement of CVD strategy is highly significant for faster h-BN growth into larger sheets.Recently,advanced progress has reached the goal of wafer-scale single-crystal BN films.

Similar to graphene growth,polycrystalline copper(Cu)has been chosen as an ideal growth substrate for h-BN synthesis as well.For example,Song et al.used ammonia borane as a source to produce large-area few-layer h-BN films via atmospheric pressure CVD(APCVD).[29]A series of characterization methods,such as optical microscopy(OM)images,SEM,and AFM,were used to confirm the quality and layer number of film(Figure 4a-d).Lee et al.utilized the electrochemical polishing technique to extend the grain boundary size of Cu foil and decrease substrate roughness to obtain a highquality h-BN film.[30]Further,Kim et al.reported the first 2D h-BN monolayer film grown on Cu foil via using a two-zone furnace.[31]In this case,triangle-shaped monolayer h-BN domain and continuous film can be obtained under their growth conditions.Their research results indicate that the h-BN crystals preferred nucleation on the high-energy surface area,such as a rolling line of the Cu surface.Wang et al.enlarged the grain boundary domain of the Cu substrate via annealing approach to produce single-crystal monolayer h-BN film with a large-sized domain,as shown in Figure 4e-g.[32]By oxidizing Cu foil in the air at 100°C,the h-BN triangle-shaped domains can be clearly identified on the Cu surface by OM(Figure 4h,i).In addition,Zhang et al.prepared wafer-scale h-BN fewlayer films by using liquid borazine as source in plasma-enhanced CVD system(PECVD)(Figure 4j,k).[33]AFM and HRTEM images indicated the synthesized h-BN film a few layers and high crystalline film(Figure 4l,m).The homogeneous h-BN few-layer film was also used for graphene plasmonic.Additionally,Song et al.[33]demonstrated a wafer-scale film consisting of～92% monolayer.[34]

Figure 4.a)Optical photograph of the h-BN film.Scale bar is 1 cm.b)SEM characterization of h-BN film.Scale bar is 10 μm.c,d)AFM photograph of h-BN film.Reproduced with permission from ref.[22-28]Copyright 2010,American Chemical Society.e-g)SEM characterizations of h-BN with different annealed times(0,3,and 6 h).h,i)Optical images of h-BN flakes on non-oxidation and oxidation Cu foil.Reproduced with permission from ref.[31]Copyright 2014,Wiley-VCH.j)Optical microscopy of a large-area h-BN film.k)Optical image of h-BN/SiO2/Si substrate.l)AFM characterization of h-BN film.m)HRTEM photograph of continuous h-BN film.Reproduced with permission from ref.[32]Copyright 2013,Wiley-VCH.

Akin to the h-BN synthesized on copper foil,the researchers also produced the large-area h-BN film on Ni foil.For instance,Shi et al.reported the large-area few-layer h-BN film synthesized on Ni foil via APCVD(Figure 5a,b).[35]h-BN film homogeneously covered the whole Ni substrate surface along the grain boundaries(Figure 5b,c).The interlayer distance of the synthesis h-BN was～0.35 nm,indicating that few-layer h-BN continuous film was successfully prepared(Figure 5d).Besides,Lee et al.revealed that h-BN film preferred nucleation and growth on Ni(100)surface.[36]Oh et al.demonstrated the synthesis of h-BN on single-crystal Ni(111)substrate via APCVD by using sublimation source and choosing proper synthesis temperature(Figure 5e).[37]Impressively,the Ni substrate can be reusable for producing the uniform h-BN film in a reusable substrate(Figure 5f).Therefore,current growth technology can controllably and efficiently produce h-BN with uniform thickness and flatness.

Figure 5.a,b)Optical photographs of the h-BN/Ni film and onto h-BN/SiO2/Si.c)AFM and thickness images of the h-BN/SiO2/Si.d)HRTEM characterization of h-BN film.Reproduced with permission from ref.[34]Copyright 2010,American Chemical Society.e)Optical photograph of h-BN film transferred onto SiO2/Si substrate.f)Optical photographs of h-BN/SiO2/Si films synthesized via reusable Ni foil.Reproduced with permission from ref.[36]Copyright 2016,Nature Publishing Group.

Similarly,2D h-BN single-crystal and film grown on Pt foil has been widely studied.As an example,Kim et al.presented the high-quality h-BN monolayer film by applying reusable Pt foil,which can be facile transferred onto target substrate via electrochemical bubbling technique(Figure 6a-c).[38]Gao et al.reported controllable growth of monoand bilayer h-BN domains on Pt foil via simply adjusting the amount of precursor(Figure 6d-g).[39]Compared with using solid ammonia borane,Park et al.used liquid borazine as the source to decrease the deposition of BN nanoparticles to obtain large-area and super-clean monolayer h-BN film.[40]

Figure 6.a)Schematic image of the transferring process,indicating the reusable of the Pt foil.b)AFM image of monolayer h-BN film.The inset optical photograph was the monolayer h-BN/SiO2/Si film.c)TEM characterization of the h-BN monolayer film.Scale bar is 1 nm.Reproduced with permission from ref.[38]Copyright 2013,American Chemical Society.d,f)The AFM images of mono-and bilayer h-BN domains.e,g)TEM characterization of h-BN with different thickness.Reproduced with permission from ref.[39]Copyright 2014,American Chemical Society.

For applications,such as dielectric layers or insulating substrates in electronic and optoelectronic devices,the desirable thickness of h-BN films as dielectric substrate is～5-15 nm.Typically,Kim et al.presented a strategy for synthesizing h-BN multilayer films on Fe substrate at 1100°C(Figure 7a).[41]The surface roughness of synthesized h-BN is comparable to the exfoliated h-BN nanosheets,and interlayer distance of film is around 0.33 nm(Figure 7b,c).Caneva et al.also exhibited that monolayer h-BN single crystal is deposited growth on Fe foil.[42]For example,they developed an approach to obtain homogeneity and controllable layer number for h-BN via pre-annealing Fe foil under NH3(Figures 7d,e).

Figure 7.a)Growth setup image of the multilayer h-BN film.b)AFM image of multilayer h-BN film.Reproduced with permission from ref.[41]c)TEM and SAED pattern images of multilayer h-BN film.Copyright 2016,Nature Publishing Group.d)Growth process image of the improved CVD technique.e)SEM characterizations of h-BN single crystal grown on Fe substrate with different growth conditions.Reproduced with permission from ref.[42]Copyright 2016,American Chemical Society.

Metal alloy substrate:Different from the pure metal substrates,the metal alloy provides us a promising route to h-BN single-crystal and film with controllable crystal size and layer number.For instance,Zhang et al.demonstrated that the h-BN film with different thickness can be produced via co-segregation technique(Figure 8a,b).[43]Additionally,Fu et al.presented the synthesis of h-BN films on Ga-Ni substrates and obtained MoS2/h-BN heterostructures.[44]Akin to the graphene grown on Cu-Ni substrate,Lu et al.[44]showed that h-BN domains with 7500 μm2were produced on alloy substrate of Cu-Ni foil(15 atom% Ni).[45]The authors investigated the influence of the amount of Ni atom on h-BN domain size and density and found that Ni atoms in the alloy enhanced the decomposition of precursor and therefore contributed to h-BN with large-sized domains.Until now,controllable growth of uniform h-BN multilayer film is still a great challenge,with consideration of uniformity,thickness,and stacking order.Shi et al.developed a vapor-liquid-solid growth approach to produce uniform h-BN multilayer film on molten Fe82B18alloy substrate under N2atmosphere(Figures 8c).[46]The thickness of obtained h-BN can be simply tuned via changing growth time and temperature(Figure 8d-f).

Figure 8.a)Growth mechanism of the co-segregation method.b)Typical AFM characterization of h-BN monolayer film.c)HRTEM characterization of the h-BN.The inset image shows the SAED pattern.d-f)Optical images of h-BN whit different thickness.The scale bars are 100 μm.Reproduced with permission from ref.[43]Copyright 2014,Wiley-VCH.g)Schematic photograph of multilayer h-BN synthesized visa Fe82B18 alloy and nitrogen as sources.h-k)TEM characterizations of different thickness of multilayer h-BN films.Reproduced with permission from ref.[46]Copyright 2020,Nature Publishing Group.

Insulator substrate:Direct growth of 2D h-BN on target substrates can avoid transferring process and impurities for device integration and applications.However,this process also places issues due to the low catalytic activity of the substrate surface,thus impeding the kinetics for growth.In this regard,many groups explored several strategies to grow h-BN on target dielectric substrates.For example,Kim et al.revealed that h-BN can be directly synthesized on the insulator substrate,such as silicon(001)and Al2O3(0001).[4]Along this line,Tay et al.produced few-layer h-BN nanocrystalline on SiO2/Si substrates via thermal CVD.[47]Recently,some significant achievements have been gained due to the rational design and growth protocols,thus excellently controlling whole CVD process.Behura et al.presented an oxygen-assisted synthesis of h-BN/SiO2/Si via LPCVD(Figure 9a).[48]This work indicated that the oxygen groups on the substrates played a vital role in synthesizing h-BN/SiO2/Si film(Figure 9b-d).Jang et al.demonstrated that wrinkle-free h-BN few-layer film was produced on a Al2O3substrate at 1400°C via LPCVD technique.[49]The large-scale h-BN monolayer film had a lower surface roughness(0.169 nm)compared with Pt(1.09 nm).Besides,Liu et al.presented that h-BN film with different thickness was synthesized on SiO2/Si,quartz,sapphire,and silicon via PECVD at 300°C(Figure 9e-i).[50]More importantly,they also demonstrated that the WSe2/h-BN transistor demonstrated carried mobility around 56-121 cm2v-1s-1.

Figure 9.a)Growth mechanism image of h-BN film.b)Optical image of h-BN film.c)Optical photograph and selective area Raman spectra.d)SAED photograph of h-BN film.Reproduced with permission from ref.[48]Copyright 2015,American Chemical Society.e)Growth setup schematic image of PECVD.f-i)AFM characterizations of different thickness of h-BN film.Reproduced with permission from ref.[50]Copyright 2019,Nature Publishing Group.

Domain size engineering:There are many efforts to synthesize h-BN film with a large-sized flake.However,the average flake size of h-BN is still limited to several micrometers,much smaller the size of the graphene domain(over 1cm).In order to gain large-sized h-BN flakes,to reduce the nucleation density is an important route to enlarge the domain size of single crystal.On account of it,numerous strategies have been explored to decrease the nucleation density.For example,Tay et al.utilized electrochemical polish technique to obtain the flatness Cu foil to produce the hexagonal-shaped h-BN domains(～35 μm2).[51]Wang et al.developed the water-assisted CVD method to synthesize monolayer h-BN triangle-shaped domain(～300 μm in edge length)on liquid Cu substrate(Figure 10a-c).[52]The Auger electron spectroscopy mappings for h-BN domains confirmed the homogeneous distribution of B and N elements(Figure 10d,e).Stehle et al.showed that the growth temperature strongly influenced the domain size of h-BN in the APCVD system,where higher temperature resulted in the large-sized domains,reaching ～ 170 μm2under 1065 °C.[53]In order to efficiently tune the precursor feeding rate and nucleation density,Song et al.utilized a folded Cu foil as a growth substrate to produce single-crystal domains on its internal surface.[34]The closed growth space and smooth metal surface effectively decreased the number of precursors and induced the large-sized h-BN domains(72 μm in edge length)(Figure 10g,h).Alloy metal substrate is also a promising substrate to obtain large-sized 2D crystals.Lu et al.reported that large-sized h-BN single crystals can be produced on metal alloy substrate,such as Cu-Ni.[45]It showed that the nucleation density could be dramatically adjusted by introducing a different amount of Ni to the Cu substrate.Caneva et al.revealed that h-BN with large-sized domain(～0.3 mm)can be synthesized on Si-doping Fe foil at high temperature(Figure 10h-m).[54]By changing the thickness of the SiO2layer,the amount of Si diffusion into Fe also can be tuned,which was vital for controlling the nucleation density.The selected area diffraction mapping on h-BN triangle flakes had identical orientation,indicating that the triangular regions were indeed single-crystalline domains(Figure 10n).

Figure 10.a-c)A series of SEM images of h-BN domains under different growth time.d and e)The images of N(KLL)and B(KLL)Auger electron maps.Reproduced with permission from ref.[52]Copyright 2017,Royal Society of Chemistry.f)SEM characterization of h-BN single flake.g)Large-area monolayer h-BN film.Reproduced with permission from ref.[34]Copyright 2015,Springer International Publishing AG.h)The photograph of growth substrate.i and m)SEM photographs of single flake and film of h-BN.n)SAED characterization on an h-BN single flake.Reproduced with permission from ref.[54]Copyright 2015,American Chemical Society.

There are also other strategies to gain the h-BN single-crystal film,which assembled the small aligned single-crystal domains with the same orientation.For instance,Wang et al.reported the synthesis of a monolayer h-BN domain and film on single-crystal Cu(110)surface by annealing a commercial copper foil(Figure 11a).[55]Advanced structure characterizations and theoretical calculations revealed that h-BN domains with zigzag edges were successfully synthesized on Cu(211)surface,hence breaking the equivalence of antiparallel h-BN flakes and enabling over 99% unidirectional domain alignment.To take a further step,Chen et al.reported the epitaxial synthesis of monolayer h-BN domains on Cu(111)surface(Figure 11b-e),which was impossible in theory in the past.[56]Based on the first-principles calculations,their results suggested that lateral syncretic of h-BN domains on Cu(111)steps was enhanced,thereby guaranteeing the uniform growth orientation of monolayer h-BN single crystal.Besides,Lee et al.explored a synthesis method of monolayer h-BN single-crystalline films on a liquid gold substrate(Figure 11f,g).[57]The h-BN flakes gradually packed and connected via electrostatic interaction to form a wafer-scale monolayer h-BN film.

Figure 11.a)Optical characterization of the annealed Cu foil and SEM photograph of h-BN triangular domains.Reproduced with permission from ref.[55]Copyright 2019,Nature Publishing Group.b)Optical photograph of single-crystal Cu(111)foil.c)Optical characterization of triangle-shaped h-BN domains.d)AFM photograph of monolayer h-BN film.Reproduced with permission from ref.[56]Copyright 2020,Nature Publishing Group.f)Schematic diagram of h-BN domains synthesized on liquid Au substrate.g)SEM images of single flakes under different growth times(0.5,10,20,30,and 60 min).Reproduced with permission from ref.[57]Copyright 2018,American Association for the Advancement of Science(AAAS).

2.2.Functionalization

It is very difficult to directly utilize pure BNNS for chemical reactivity or composite fabrication,due to the inertness,poor interface adhesion,and local conjugacy of boron and nitride bonds.Therefore,to better exploit this flatland,the functionalization and modification of BNNS are significantly proposed.The strategy can be divided into non-covalent and covalent methods(Scheme 1).For both routes,each has its own pros and cons.Typically,non-covalent methods via π-π conjugation,Lewis acid-base or hydrogen bonding,and electrostatic interaction are more facile and economic.However,weak interaction and van der Waals force behind non-covalent modification easily lead to failure especially in harsh conditions.As for covalent functionalization,despite the virtue of more effective and tailorable design,sophisticated reaction mechanisms and multifarious experimental processes are time-consuming,hence hampering the development of BNNS for further applications.In terms of that,we thereby systematically discuss and compromise both methods with the aim to provide more rational and practical approaches for BNNS functionalization and modification.

2.2.1.Non-covalent Functionalization

The non-covalent functionalization mechanism can be depicted from the internal chemical structure of BNNS.On one hand,analogue to graphene,BNNSs possess π conjugation that enables π-π stacking between BNNS and other molecules with benzene ring or other conjugated units.On the other hand,unlike the carbon allotropes,BN bond,which is similar to ionic bond,intrinsically possesses electronic polarity.That is to say,affluent charges appear around nitride atoms,while insufficient charges are left around boron atoms.Hence,the electronegativity can be additionally applied by Lewis acid-base principle through positive and negative electric attraction,therefore producing non-covalent interaction.Moreover,other non-covalent interactions such as H-bonding,electrostatic interaction,or even defect introduction are also affordable to modify the surface force of BNNS for improved solubility and compatibility.

Among those methods,π-π conjunction,particularly with molecular equipped with benzene ring affinity structure,is the most widely used to impose functionalization on BNNS.In the early stage,Zhi et al.reported a non-covalent bond induced composite film by polyaniline(PANI)-wrapped BN nanotubes(BNNTs),which revealed a strong π-π overlap interaction between BNNT and conjugated conducting polymers.[58]Interestingly,it is also suggested that this π-π force was stronger than that of carbon nanotube(CNNT)and PANI.The results highlighted the electrical polarization in broken symmetry of B-N bonds.Inspired by that,the polymers with conjugated ring structure such as poly[m-phenylenevinylene-co-(2,dioctoxy dioctoxy-p-phenylenevinylene)](PmPV),poly(p-phenylene ethynylene)s(PPEs),polythiophene(PT),and polystyrene(PS)[3,59]were employed for π conjugation functionalization of BN nanomaterials.Thanks to the conjugated effect and rule of similar phase dissolution,the modified BN nanomaterials can be dispersed in organic solvents such as chloroform,N,N-dimethylacetamide(DMF),and tetrahydrofuran.Notably,among the macromolecules,the steric configuration contributed a lot to the binding force for π-π stacking.For example,the PPE backbone unit with more planarization conjugated structures showed a closer interfacial adhesion to the BN plane than that of PT,followed by PmPV and PS.Therefore,the rational selection of molecules is the first step for effective π-π conjugation interaction of BNNS.Moreover,for 2D materials,the function of molecular chain intercalation cannot be neglected.Unlike 1D materials with rod or tube morphology where the conjugated molecules wrapped it as a shell,for the nanosheets or layers,the molecules are firstly adsorbed on the 2D plane via π-π stacking.Then,the molecular chains oriented and stretched to form a thermodynamically stable configuration.Consequently,it expanded the distance between the layers or 2D planes,therefore decreasing the van der Waals force and enabling the intercalation.Of course,beyond the modification,the dynamic process also resulted in the successful exfoliation of BNNS and further improved the miscibility in organic matrix.Besides,in some cases,the strategy by selecting functionalized agent integrating conjugated groups and other groups has proven to be more effective.For instance,the bio-organic molecules such as catechin and dopamine with both benzene ring and hydroxyl or amine group can serve as good “surfactant”to not only π-π functionalize BNNS but also form H-bonding with polymer matrices such as polyvinyl alcohol(PVA)and polyvinyl formal(PVF).[60,61]Thus,it greatly enhanced the compatibility between the inorganic and organic interfaces,hence improving the performances such as mechanical and thermal properties.Likewise,1-pyrenebutyric acid(PBA)or 1-pyrenesulfonic acid(PSA)was reported for the third additive in the BNNS/epoxy and BNNS/poly(ether ether ketone)(sPEEK)composites,respectively(Figure 12a,b).[62,63]On one hand,the polycyclic structure of PBA or PSA attached BNNS by π conjugation.On the other hand,the carboxyl or sulfuric acid group facilitated strong interaction with epoxy or sPEEK via H-bonding or proton transport,thereby benefiting the dispersibility of BNNS in the polymer matrix,followed by mechanical property,stability improvement of the composite at a relatively low BNNS fraction.

Figure 12. π-π conjunction functionalization of BNNS.a)PBA-functionalized BNNS for epoxy composite.Reproduced with permission from ref.[62,63]Copyright 2013,Wiley-VCH.b)PSA-functionalized BNNS for sPEEK composite.Reproduced with permission from ref.[62,63]Copyright 2014,American Chemical Society.

Apart from conjugated effect,it is well known that the electronegativity of nitride atoms is stronger than boron atoms,contributing to electron-deficient B atoms in the B-N bonding of BNNS.In the electronic theory of acid and alkali,the Lewis acid is defined as atoms,molecules,or ions,which plays a role as electron pair acceptor,otherwise the Lewis base.In terms of that,B atoms can be broadly treated as Lewis acid while N atoms are Lewis base.Theoretically,it can form a non-covalent interaction between B atoms and some electron affluent atoms so long as some ligands approach BNNS.Different from π-π conjunction,the Lewis base function can distinguish BN materials from carbon materials.This can be exampled by cases of polar-group end-capped poly(ethyleneglycol)(PEG)functionalized BN nanomaterials.For example,BNNSs with 3-20 layers(or a thickness of 1-7 nm)were successfully exfoliated by the intercalation of an amine-terminated PEG or octadecylamine(ODA,CH3(CH2)17NH2)(Figure 13a).[64]The NMR results suggested no existence of physisorption or hydrophobic interactions between the ODA alkyl chains and conjugated surface of BNNS,while instead of the hydrophilic repeating units,the amine of the PEG offered interactions to the h-BN surface(Figure 13b).As a result,the amine-PEG-attached BNNS showed good solubility in water and chloroform,while ODA-BNNS can be dissolved in chloroform,toluene,and methylene chloride.Therefore,the facile approach was versatile to functionalize and exfoliate mono-or few-layered h-BN and dispersed them in organic solvents.In addition,to enlarge the effect of Lewis base interaction,sometimes the introduction of defects on BNNS was feasible.In Lin et al.’s work,they ball-milled BNNS to achieve defective nanosheets firstly(Figure 13c).Then,they found that BNNSs with more defects by more milling time were able to be functionalized with more ODA moieties,and thus enhance water dispersibility(Figure 13d).[65]This is because that active B atoms at those defect sites or vacancy positions were more vulnerable,hence forming stronger Lewis acid attracted by the amino group.Moreover,as mentioned above,the synergistic effect of both π-π conjunction and Lewis acid--base interaction provides a more complicated but larger platform for BNNS utility.Especially in the biological area,BN nanomaterials can act as the carriers for drug and medicine.Owing to the conjugated and Lewis acid-base interactions between BN and biomolecules such as hemoglobin protein,amino acid,or DNA molecules,it can form strong forces and realize the targeted delivery.

Figure 13.Lewis acid-base functionalization of BNNS.a)TEM and AFM images of the PEG-BNNS.b)2D13C NMR spectra of ODA-BNNS in roomtemperature CDCl3.Reproduced with permission from ref.[64]Copyright 2010,American Chemical Society.c)The scheme for ODA-functionalized ball-milled BNNS.d)The optical image of the dispersions for the ODA-BNNS with different ball-milling time.Reproduced with permission from ref.[65]Copyright 2010,American Chemical Society.

There are other interactions such as H-bonding forces,[66]charge adsorption/repulsion,[67]cation-π interaction,[68]hydrophobic effect,[69]and defect effect,[70]which offer multiple means for BNNS non-covalent functionalization.Although the forces can attract the BNNS partially,however,the interactions are weak enough for the pristine nanosheets assembly or dispersion in liquid or solid medium.Hence,as mentioned above,defect engineering was normally introduced to offer more active sites.For example,some studies pointed out that BNNS with induced Stone-Wales defects or vacancy exhibited narrower bandgap and spontaneous magnetization upon CO adsorption and preference for H2capture,since the doped and defective graphitic-BN sheet(g-BN)exhibited much higher affinities with the probe molecule than pristine g-BN(Figure 14).[71]

Figure 14.a-c)Optimized adsorption configurations of CO molecule on Al-doped g-BN a),the Stone-Wales g-BN b)and N-vacancy g-BN c)surface.d)Total charge densities of CO interacting with pristine g-BN,Stone-Wales g-BN,and N-vacancy g-BN(from top to down).Reproduced with permission from ref.[71]Copyright 2010,American Chemical Society.

Furthermore,some metal(M)or metallic oxide(MxOy)nanoparticles were found to be easily attached on BN nanomaterials since they contributed to either electrostatic interaction or the formation of M-N bonds as well as the small-size effect.Remarkably,the metal clusters and derives also endow BNNS more special properties.For example,FeO-or Fe2O3-loaded BNNS presented magnetic c-axis orientation.[72,73]Under the magnetic field,the BNNSs were coincident with alignment along the axis,followed by the axial thermal conductivity enhancement in the polymer composite.It may therefore be expected that the multiple non-covalent functionalization can exert a significant influence on modification,intercalation,and exfoliation of BNNS,where the physical properties such as polarity,solubility,and magnetism are also changed.

2.2.2.Covalent Functionalization

The covalent functionalization imposes more influences on the surface chemistry of the BNNS since the bonding energy is greater and the process is irreversible once completed.Therefore,this strategy brings about many advantages such as stability and possibility for secondary grafting on the nanosheets.For BNNS,the covalent-functionalized small groups such as amino(NH2-)or hydroxyl groups(OH-)are fundamental but applicable for further modification.Indeed,the amino or imide(NH-)group exists in the h-BN,especially synthesized by urea,ammonia,and other reactants containing nitride.However,the amount of such groups is not sufficient for BNNS participating in reaction.Inspired by functionalization of graphene oxide(GO),to graft oxygen-containing group such as OH-in the BNNS also arouses enormous concerns.For example,the OH-groups present broaden scope of utility and compatibility with traditional resins such as epoxy,polymethyl methacrylate(PMMA),and polyvinyl alcohol(PVA)by hydrogen bonding with abundant oxygen-containing groups.From this perspective,here we mainly elaborate the basic NH2-and OH-functionalization,as well as extending the derived chemical modification based on them.As for other covalent functionalization with radical reactions such as alkane carbene,and nitrene,it will be referred as well but not in a prime category.

Scheme 1.The chart for BNNS functionalization.

From the perspective of chemistry,amino group theoretically affords to react with plenty of groups such as carboxyl group aldehyde and isocyanate.It can hence generate widely reactions such as Schiff base reaction referring to-RN=C-organic network and carbamate that can be grafted into polyurea or polyurethane(PU).Previous study has displayed NH2-GO via reaction between carboxyl groups in the GO and 1,3-diaminopropane.[74]The obtained NH2-GO was subsequently upgraded by reacting with aromatic dialdehydes,forming Schiff base--graphene for following cross-linking.However,for BNNS,to introduce amino or imide groups does not appear to be such complicated as GO.It depends on electrophilicity of boron atoms or some nitride defective sites.In addition,the amino-modified BNNSs also exhibit the merits of anti-oxidation property and stability even under acid/alkane,high temperature,and long hours,which is beneficial for other treatment.Therefore,there are a number of researches reporting the amination for BN.Among those methods,the most facile and effective way is ball milling with urea.During this procedure,high-energy ball sheared and collided the h-BN layers,resulting in a mass of defects and exfoliation(Figure 2).[5]Then,hydro-nitrogen bond from urea broke and reconnected with the defective BN to form amino group.As expected,the functionalized BNNS presented excellent aqueous dispersibility that can be vacuum- filtrated into membrane or freeze-dried into aerogel.Another promising and simple approach for NH2-functionalization is plasma irradiation.By utilizing ammonia plasma,amino was successfully grafted onto BN plane.[75]This was confirmed by acylation reacting with 3-bromopropanoyl chloride.[76]Further theory simulation revealed that NH3chemically attached on the top of boron atom,as well as transferring a charge to the BN nanomaterials.In addition,there was an electron inductive effect for amino derivatives reacting with BN.For instance,NH2CH3,NH2CH2COOH,and NH2CH2OCH3with electron-donating group CH2-/CH3-can be chemically absorbed on the sidewall of BN by forming covalent bond with B atom,whereas NH2COOH containing relatively strong electron-withdrawing carboxyl group simply exhibited physical interaction with boron.[77]Moreover,nitrogen and hydrogen mixture(N2+15% H2)plasma is also available to employ nitrogenous groups such as amide on the BN.Nevertheless,it is worth noting that plasma may eliminate other oxygen doping groups such as hydroxyl,which would restrain the integral reactive activity of BNNS.[78]Having briefed the main strategies for amino group introduction,the following chemical grafting based on NH2-is more significant to discuss for covalent modification of BNNS.It should be noted that the amino group directly connected to the boron atom contributes electrons partially to the BN ring,hence decreasing the polarity for some nucleophilic and condensation reactions such as Schiff base reaction.Therefore,so far,the BNNH2covalent modification is mainly confined to the following aspects.The first one is amidation.The NH2-in BN nanomaterials can be condensed with acyl chloride or carboxyl groups,forming amido bond.By this means,long-chain alkane and its end groups are able to be grafted into BN.For instance,the stearoyl chloride with 17 CH2units was connected to the BN-NH2via amination.[76]Similarly,the sulfhydryl was imported into the BN through the reaction between NH2-and 3-mercaptopropionic acid(MPA)(Figure 15a).[79]Another common grafting way based on NH2-is carbamate reaction,where both amino and hydroxyl afford to react with isocyanate(-N=C=O).Therefore,this strategy is widely applied for BNNS covalently bonding to PU matrix and offers more space for third grafting or cross-links,due to the symmetrical diisocyanate.As exampled,Gao et al.reported a series of chain reactions by introducing 4,4’-methylenebis(phenyl isocyanate)(MDI) firstly.During this process,NH2-and OH-in BNNS reacted with-N=C=O,forming carbamate at one end of MDI(Figure 15b).[80]Then,diamine diphenyl sulfone(DDS)was supplied to react with another side of MDI,elongating the chains in the BNNS.Finally,the as-modified BNNSs were added into bismaleimide(BD)resin for further curing.As a result,the composite with stronger interfacial compatibility between additives and matrix displayed smaller coefficient of thermal expansion(CTE)and dielectric loss.Likewise,isophorone diisocyanate(IPDI)can be applicable to bridge one isocyanate group on the surface of BN nanomaterials(Figure 15c).Hence,the isocyanate-terminated BN possessed many options for further modification such as alcoholization amination and carboxylation addition.[81]Also,the NH2-on the BNNS can undertake direct addition with some chemicals such as 4,4’-bismaleimidodiphenylmethane(BDM)where the amino was preferred rather than hydroxyl(Figure 15d).[82]In fact,the analysis by Huang et al.pointed out that amine groups contributed greater than hydroxyl groups to the overall surface chemistry of h-BN.[83]However,the post-grafting after amination sometimes remains a challenge,especially considering that some modifiers such as isocyanate are toxic.Therefore,referencing GO,more universal covalent functionalization for BNNS concentrates on the hydroxyl derivatives.In contrast with amine or other groups of modification,the-OH-functionalized BN nanomaterials are seemingly to be easier and applied in a wider range of areas.However,in fact,the oxygen functionalization situation is more intricate than expected.The density functional theory(DFT)disclosed that,in BN ribbons,edge oxidation normally resulted in peroxide-like structures,whereas-OH terminated edge displayed a polyol-like structure with fringes of-OH group stabilizing the edge oxidized plane.[84]Meanwhile,the peroxide was able to break the B-N bonds,as well as connecting hydroxyl radicals to the N or B sites.Therefore,the highly reactive sites do not merely depend on hydroxyl group in BNNS when considering the overall condition of oxygen doping configuration.In spite of it,the strategies for direct-OH or oxygen functionalization are more diversified compared with other groups such as amine and halogen.Except for ball milling and plasma as mentioned above,the peroxide and strong acid/alkaline oxidation,hydrothermal treatment,and even water sonication all afford the hydroxyl group or other oxygen-containing group functionalization.For instance,hydrogen peroxide treatment provided BN nanomaterials with negative-charged oxygen-containing surface,which effectively absorb positive-charged polydiallyldimethylammonium chloride(PDDA)for self-assembly.[85]Particularly,the mixture of OH-BNNT/PDDA and OH-BNNS/PDDA(OH-BNNT acted as the bridge to string OH-BNNS)further facilitated the thermal conductive liquid by integrating the merits of high thermal transfer and low fluid viscosity.Furthermore,strong acid such as HNO3/H2SO4or oleum not only equipped h-BN with-OH,but also enabled the BNNS exfoliation with the yield of 70% wt.of soluble BN colloid in water and DMF.[86]In fact,the Br?nsted acid such as H2SO4,H3PO4,and HClO4with protonation process was critical in the intercalation of 2D materials.Figure 16a shows the H3PO4molecules inside the h-BN superlattice using empirical molecular dynamics.Obviously,there were maximum 11 PO-H．．．N bond formation between 12 H3PO4molecules and h-BN supercell for the intercalation(Figure 16b).[87]The OH-BNNS with micron lateral size therefore can be largely exfoliated,as well as possessing good dispersibility in many organic solvents(ethanol,IPA,acetone,and DMF)(Figure 16c,d).[88]Likewise,strong base such as NaOH was employed to hydroxylate BN nanomaterials,contributing to OH-BN with better miscibility.The hydrothermal treatment was also able to largely cut the bulk BN flakes into sheets and intercalate hydroxyl on the edge of BNNS(Figure 17a).The as-obtained OH-BNNS formed hydrogen bonding with poly(N-isopropylacrylamide)(PNIAM),generating thermosensitive hydrogels.[89]In reality,hydroxylation can even be realized in aqueous solution by sonication,where the ultrasound-assisted hydrolysis offered more dissociative-OH groups.For instance,Lin et al.adopted the simple and facile way to successfully obtained smaller lateral sizes of BNNS than other methods reported(Figure 17b).[90]Similarly,OH-BNNS powders were innovatively prepared by freeze-drying after ultrasonic treatment in water bath,followed by in situ polymerization of OH-BNNS with biodegradable polybutylene succinate(PBS).[91]Interestingly,OHBNNS fully served as nucleating agent for PBS to form smaller spherulites during the polymerization,benefiting from the effect of-OH compatibility(Figure 17c).

Figure 15.Schemes of a)modification of NH2-BNNTs with MPA via amide formation.Reproduced with permission from ref.[79]Copyright 2007,American Chemical Society.b)Modification of NH2-BNNS via MDI.Reproduced with permission from ref.[80]Copyright 2013,American Chemical Society.c)Modification of NH2-BNNTs via IPDI.Reproduced with permission from ref.[81]Copyright 2013,Elsevier.d)Modification of h-BN with BDM.Reproduced with permission from ref.[82]Copyright 2012,Wiley-VCH.

Figure 16.a)Schematic illustration of H3PO4molecules in the h-BN gallery simulation.Reproduced with permission from ref.[86]Copyright 2013,American Chemical Society.b)Average number of P-OH··N bonds vs.number of molecules in the supercell.Reproduced with permission from ref.[87]Copyright 2013,American Chemical Society.c)SEM images of exfoliated BNNS with a large size.d)Stability of BNNS in isopropyl alcohol,DI water,acetone,and ethanol.Reproduced with permission from ref.[88]Copyright 2014,Royal Society of Chemistry.

Figure 17.a)Schematic illustration of water-assisted exfoliation and hydroxylation of h-BN bulks in hot water stem.Reproduced with permission from ref.[89]Copyright 2015,Wiley-VCH.b)An example scheme shown is the cutting started from a nitrogen-edged hole defect site(marked with triangle)with the arrow as the cutting direction.Reproduced with permission from ref.[90]Copyright 2011,American Chemical Society.c)Spherulitic morphologies of(I)P-0 at 90 °C and(II)P-0.03,(III)P-0.1,and(IV)P-0.3 at 100 °C.Bar length in each image=100 μm.Reproduced with permission from ref.[92]Copyright 2014,American Chemical Society.d)The calculated stable configuration and the respective adsorption energies of 20% ,60% ,and 100% OH coverage on h-BN sheet.Reproduced with permission from ref.[93]Copyright 2013,Springer-Verlag GmbH Germany,part of Springer Nature.

However,there are some limitations of hydroxylation functionalization in water.Firstly,the intrinsic hydrophobicity of h-BN crystalline confined the dispersity of h-BN in an aqueous medium,which was an issue for peroxide,and strong acid or base modification.Secondly,water treatment resulted in small-sized BNNS fragments that were not optimal for matrix reinforcement.Additionally,the production rate was low with small scalable hydroxyl addition to B atoms.Beyond water,alcohol solvents such as isopropanol alcohol(IPA)also had similar function on BNNS hydroxylation since the h-BN possessed better dispersibility in IPA than water.[92]Nevertheless,the hydroxylation efficiency was still not competitive to other chemical modified cases.DFT pointed out that the functionalized BNNS with 30% hydroxyl radicals(60% coverage)were the most thermally and dynamically stable at room temperature(Figure 17d).[93]To maximize the hydroxylation,some more complex methods were designed to create stable and a considerate quality of hydroxyls on the BNNS.Sainsbury et al.adopted organic peroxide:di-tert-butyl peroxide(TBP)to form boronate ester at the surface of BNNS at first.Then,piranha solution(H2SO4:H2O2,3:1)was inducted to react with TB radicals on the BNNS,thus producing-OH group.[15]

To take a step further,hydroxyl-tailored BNNS can be directly mixed with some polymers containing-OH or-COOH via hydrogen-bonding effect.Nevertheless,owing to different flexibility and plasticity in various polymers,the chain coil agglomeration or maldistribution easily formed with OH-BN and affected the properties of the nanocomposites.Moreover,the alkane chains of polymers did not interact well with the OH-BN.Therefore,to improve the interfacial compatibility,the next modification for hydroxyl group-functionalized BNNS shows necessity.Generally,the most common one is the grafting of silane coupling agents.It is well known that silane coupling agents have Si-O bonding that can be hydrolyzed after linkage with-OH of BNNS,and the final functionalization depends on the end groups in the silane chains.Here,we list some main cases as examples.The first one is the γ-aminopropyltriethoxysilane(APTES)(KH550)[94,95]and its counterparts 3-aminopropyltrimethoxysilane(APTS)(KBM903),[96]which have a-NH2group at the end of hydrocarbon chain.Through the hydrolysis,the terminated-NH2can be introduced onto the BNNS.Different from the direct grafting NH2-BNNS as mentioned above,the-NH2group in APTES-modified OH-BNNS presented steric effect supported by the stretching silicon/alkane chains,which optimized the distance between BNNS and polymer chains.Hence,it satisfied better interaction with hydrophobic long-chain polymers or corresponding reactions to extend the branch chains with amino end(Figure 18a).[95]Further,it is indicated that APTES modification was beneficial for not only improving the interface affinity of BNNS with epoxy,but also serving as branching agent to anchor hyperbranched aromatic polyamide(HBP)for deep curing,which the ODA non-covalent functionalization cannot afford(Figure 18b).[97]

Figure 18.a)Scheme of the APTES and further TPP functionalization of BNNS.Reproduced with permission from ref.[95]Copyright 2014,Royal Society of Chemistry.b)1H NMR spectra of ODA and BN-ODA in CDCl3and BN-HBP in DMSO-d6.The feature peaks from the aromatic protons are observed in the 1H NMR spectra of BN-HBP nanoplatelets.Reproduced with permission from ref.[97]Copyright 2012,Elsevier.

The second one is the(3-acryl-oxypropyl)trimethoxysilane(APMS)with acryl-oxypropyl as the terminated group,which was also normally used for BNNS dispersibility enhancement.In addition,two common silane coupling agents,3-glycidoxypropyltrimethoxysilane(KBM-403)and 3-chloropropyltrimethoxysilane(KBM-703),can be compared for briefing the modification effect in the epoxy matrix.Obviously,the KBM403 modified BNNS with epoxide group seemed to perform more affinity to the epoxy.Thus,the corresponding composites placed a higher thermal conductivity value(4.11 W m k-1)than KBM-703 treated one(3.88 W m k-1).[97,98]Moreover,further simulation and calculation proposed that KBM-403 modified OH-BN existed three different configurations.Within the increment of KBM-403 increment,the polar interaction and H-bonding effect became stronger,as well as producing more volumes and voids in the epoxy matrix,which led to lower cohesive and surface free energy.Accordingly,the wettability and thermal properties between BN and epoxy reversely declined.Hence,the consumption of siloxane was supposed to be well modulated.[99]Besides,to make the BNNS dispersible in oil agents,the octadecyltriethoxysilane(ODTES)was also introduced to be grafted onto the OH-BNNS.[100]In addition to the silane coupling agents,other methods to modify hydroxylated BNNS such as esterification and esterification ester reactions were reported as well,which we do not give details here but can be referenced in the above amino modification section.

Of course,except for NH2and-OH,there were other direct covalent functionalization strategies that proved to be either complicated or requiring high energy for B-N cleavage.We here classified them into inorganic heteroatoms doping and organic hydrocarbon functionalization.To dope heteroatoms such as fluorine,sulfur or carbon is regarded as an access to tuning the hierarchy of BN as well as adjusting the bandgap energy for improved semi-conductive property from insulator.For instance,F-doped BNNSs were produced by fluorinating BNNS influoboric acid(HBF4),which exhibited a current value from-15.854 to 13.663 μA at-50-50 V,superior to un-doped BNNS(-300 to 300 nA).[101]The regulation of F doping was further investigated via DFT(Figure 19).During the electron doping,heteroatoms(H or F atoms)would exclusively bond with B atom,leading to possible magnetization of the system,whereas hole prefers the adatoms to construct insulating orthodimer structures on the BN.As for the synergistic F doping,it gave rise to a highly disordered atom arrangement of the BN and released deep donor or acceptor impurity states.[102]Therefore,both BNNT and BNNS favored fluorination chemisorption(1.08 ev)than desorption(2.0 ev).And under a controllable carrier protocol,the magnetization of F-BN could be easily switched.

Figure 19.a)Atomic configurations of four adsorption structures for F atoms on a h-BN sheet.b)Energy differences in NN1and orthodimer sheets,and NN3and orthodimer sheets via the functions of carrier density,m.c)I.Calculated MEPs for a F atom diffusing from the NN1to the orthodimer sheet at various carrier densities,m.II.Calculated MEPs for a F atom diffusing from the NN1sheet to the paradimer sheet and ultimately to the NN2sheet at various carrier densities.Reproduced with permission from ref.[101]Copyright 2011,American Chemical Society.

Similarly,by incorporating C atoms into the h-BN,weak high-temperature ferromagnetism can be observed in the carbon-doped BN(BC-N)nanosheets.[103]Furthermore,although BN allotropes are chemically inert materials,the electronegativity of B-N bond maintains the basic feasibility for electrophilic or nucleophilic addition,which enables the direct organic hydrocarbon grafting on B or N atom.Unlike surface treatment,it can fundamentally divert the property of BN to some extent with bond cleavage.Hence,halide organics are highly preferable to anchor long chains to BN materials for their electron-withdrawing property.For example,an SN2 nucleophilic substitution reaction was proposed to successfully connect a long-chain alkane to the N atom of BN via N-H bond on the surface of BN nanomaterials.[68]Indeed,more bond cleavages were built on B-N bond.It is suggested from DFT simulation that,for acetylene-functionalized BN,with the electron-donating character of the functional groups increased,the functionalization energy grew from -1.03 to 3.13eV with the order:C2F2＞(OCH3)2C2＞C2H2＞(CH2F)2C2＞(CN)2C2.[104]Except for that,other articles also predicted the possibility of carbene cycloaddition on the BN.[105]For instance,by introducing bromoform precursor-based dibromocarbene(DBC),CBr2radicals can be resoundingly connected to BNNS via B-N bond cleavage from C insertion(Figure 20).Then,the obtained DBC-BNNS was available to be grafted with alkyl long chains through alkyl/halogen exchange reaction between n-butyl lithium(n-Bu Li)and Br atom.Finally,it is found that Bu-BNNS displayed enhanced compatibility with alkyl-based polymer composites such as polyethylene(PE).It is then pointed out that in the case of CR2,when R=H,F,and Cl,the halogen element carbenes tended to be a bond cleavage on the BN sidewall with an open structure,whereas C(CH3)2and C(CN)2existed between cyclopropane-like open and closed three-membered ring structure.[106]As for C(NO2)2,a double fivemembered ring with high stability could be obtained.Instead of breaking B-N bond,some radical transfer pathways can directly graft chains or even polymers to active B atom of BN.For example,hexane was covalently grafted to BN through radical activation of B atom in sodium naphthalide.[107]Of interest,excessive electrons in the B centered orbitals facilitated the reactive activity of BN and devoted to alkylation from 1-bromohexane,where it can be regarded as BN reduction reaction.Besides carbene,similar nitrene addition between BNNS and azide was also reported.In this study,methoxyphenyl carbamate(MPC)was energetically grafted to BNNS via 4-methoxybenzyloxycarbonyl azide as organo-azide precursor.[108]The MPC-BNNS could be further mixed with polycarbonate(PC)for polymer composites on account of their chemical similarity.Furthermore,co-polymer poly(bisphenol A-co-epichlorohydrin)(PBCE)chains were covalently attached on the surface of MPC-BNNS through esterification,which provided promising better compatibility than simply blending because of chemical similarity and inter-miscibility.Moreover,surface modification of BN with polymer brushes was conducted by the multifarious means of surface-initiated atom transfer radical polymerization (SI-ATRP).ATRP initiating dots were immobilized by introducing bis(4-bromomethylbenzoyl)peroxide(BBMBPO)to occupy B atom site at first,which was also the critical procedure.[109]After that,glycidyl methacrylate(GMA)or styrene(St)monomers were free-radically polymerized to form polymer-brushed nanocomposites.Not like general non-covalent polymer/BN nanocomposites,the polymer-brushed BN realized authentic chemical integration of polymer and nanomaterials.Of course,there were other sophisticated chemical modification strategies implicating organic and inorganic synthesis;however,the main design principles were based on what we have included in the above sections.

Figure 20.a)Scheme of dibromocarbene(DBC)functionalization of exfoliated BNNSs(DBC-BNNSs).b)Mechanistic processes for carbene attack on the BNNS substrate.Reproduced with permission from ref.[105]Copyright 2014,American Chemical Society.

3.Applications

The superior chemical and thermal stability alongside with some unique properties such as high thermal conductance and low dielectric loss enable the practicability of BNNS in many fields.Especially in the electronics and energy applications,for the electronics,BN layer can satisfy both high-performance dielectrics and insulator,thus widely applied for in-plane microelectronics such as FET.Besides,past decades have witnessed the prosperity in dielectric capacitive energy storage and thermal energy management.However,the utilization in electric energy is rarely reported owing to the insulation nature of BN.Until recently,the advance in Li-ion battery and supercapacitor technology also pushed the employment of BNNS in the electrode,electrolyte,interlayer,and separator for more efficient electric energy units.Hence,in this section,we will discuss the main electronic and energy applications for BNNS from traditional dielectric matrix,thermal energy conversion,to current battery and supercapacitors(Table 1).

Table 1.The summary of main electronic and energy applications of 2D BN

3.1.BN for Electronics

3.1.1.Atomic BN Layer for FET

Due to its atomically smooth and uniform surface,free surface of dangling bonds and charge traps,h-BN film has widely been chosen as a dielectric substrate in FET devices to obtain high mobility and on/off ratio.For example,Dean et al.fabricated mono-and bilayer graphene/h-BN devices via using a mechanical exfoliation and transfer technique.[14]The bilayer graphene/h-BN FET device presented electron and hole in the graphene up to 60 000 and 80 000 cm2V-1s-1(Figure 21a-d).In addition to graphene,Ross et al.reported a WS2heterostructure based on h-BN dielectric layer and two back-gate electrodes(Figure 21e,f).[110]Their device structure induced effective injection of electrons and holes and resulted in electroluminescence phenomenon.

Figure 21.a)Optical characterization of graphene/SiO2/Si.The inset is the optical photograph of the graphene FET device.The scale bar is 10 μm.b)AFM photograph of monolayer graphene/h-BN FET device.c,d)Transfer lines of graphene devices.The inset curves show the conductivity.Reproduced with permission.[14]Copyright 2010,Springer Nature.e)Schematic and microscopy images of h-BN/WSe2/SiO2/Si FET device.Reproduced with permission.[109]Copyright 2014,Springer Nature.g)SEM and AFM images of graphene and h-BN heterostructures.h)Drain current(Ids)and resistant(R)versus gate voltage(Vgate).The inset photograph shows the graphene/SiO2/Si device.The scale bar is 20 μm.Reproduced with permission ref.[110]Copyright 2015,Nature Publishing Group.i)Schematic photograph of a WSe2/h-BN FET device.k)Transfer curve of WSe2/h-BN/SiO2/Si FET.m)Mobility and saturated power density of the WSe2/h-BN FET devices.Reproduced with permission from ref.[49]Copyright 2019,Nature Publishing Group.

Beyond the exfoliated h-BN flakes used as the dielectric substrate,the h-BN films synthesized via CVD approach show the great promising opportunity to meet the practical electronic devices application requirements.Gao et al.obtained the in-plane and vertical graphene/h-BN heterostructures via using benzoic acid as source(Figure 21g,h).[111]The fabricated FET device showed a hole mobility of 15,000 cm2V-1s-1in the air at the room temperature.Likewise,Wang et al.used monolayer h-BN film synthesized via PECVD strategy at low 500°C and fabricated graphene/h-BN FET devices.[112]More importantly,the graphene/h-BN/SiO2/Si FET device showed the carried mobility of 10,500(hole)and 4,750(electron)cm2V-1s-1,respectively,revealing the high quality of the as-grown h-BN film.Apart from monolayer,uniform multilayer h-BN films are also regarded as a promising candidate in FET applications because the thicker films could guarantee excellent dielectric properties and decrease the trap charges.In terms of that,the multilayer h-BN synthesized on Fe foil was used as a dielectric substrate to fabricate graphene-based FET device.The mobility of graphene devices reached up to 24 000 cm2V-1s-1in air.[41]Besides,h-BN film synthesized on sapphire was transferred onto SiO2/Si substrate to fabricate graphene FET device.The tested values of hole and electrons mobility can be up to 14 175 and 8670 cm2V-1s-1,respectively.[49]It is noted that these results were comparable with mechanically exfoliated graphene/h-BN devices.

Apart from the mechanical transferred CVD-grown h-BN used as the dielectric layer,the direct synthesis h-BN film on target substrate is also widely utilized as method to fabricate FET devices.Recently,Wei et al.reported an attempt of utilizing PECVD to synthesize h-BN monolayer film as the dielectric layer for WSe2device(Figure 21i).[50]The backgated FET device based on h-BN/SiO2/Si demonstrated the excellent carried mobility:around 56-121 cm2V-1s-1,which was higher than that of CVD-grown WSe2/SiO2/Si device(2-21 cm2V-1s-1)(Figure 21k,m).

3.1.2.BN Nanocomposites for Dielectric Devices

The ever-increasing demand of sustainable electrical energy for reliable and efficient power systems has enabled the energy storage technologies such as dielectric capacitors,electrochemical capacitors,and batteries in commercial,civilian,and military fields.Among them,dielectric capacitors that deliver higher power density,larger operating voltage,lower loss,and long-life stability display irreplaceable merits compared with other types of capacitors such as electrolytic capacitors and supercapacitors.In the dielectric layers,the energy can be stored and released in the form of an electrostatic field through electric polarization or depolarization,resulting in the fast charge-discharge process.The maximum capacitive energy density(U)can be determined by the dielectric constant,that is,permittivity(K),vacuum permittivity(?0),and applied electric field breakdown strength(Eb),followed by the equation:

Obviously,both K and Eb,which mainly depend on the properties of dielectric materials,are critical to define the final performances of the capacitors.[112,113]For dielectric capacitors,the pristine dielectrics usually consist of ceramics and glasses materials such as Al2O3,HfO2,BaTiO3,and AlN.Although these materials possess high K,however,the ceramics and glasses are normally stiffy and brittle,hence bringing difficulty in electronics processing such as ink printing.[114]Compared with bulk ceramics and glasses,dielectric polymers own the prominent advantages of low cost,easy-processing,lightweight,and excellent flexibility,catering for the emerging technologies of flexible electronics.For instance,the biaxially oriented polypropylenes(BOPP)have been commercially applied as dielectric layer for film capacitors.However,despite the superior Ebof 700 MV m-1,BOPP-based capacitor normally delivers a limited energy density of＜5 J cm-3due to the low dielectric constant(K=2.2),which is lower than the supercapacitors and batteries on the market.To enhance the values of K,polarized polymers like poly(vinylidene fluoride)(PVDF)akin ferroelectric polymers are employed.[113,114]For these polymers,the C-F bonds with the spontaneous alignment of dipoles in the crystallite phases can induce higher K up to 10,with energy densities over 10 J cm-3.However,owing to the high permittivity of these terpolymers,their electric polarization tends to saturate at relatively low electric field(～100 MV m-1)with low energy density.Therefore,to further improve the performance,recent studies suggest that incorporating BNNS into the polymer matrix can increase the dielectric strength and energy efficiency since BNNSs not only exhibit ultrahigh Ebof～1200 MV V m-1and ultra-low dielectric loss(～2.5 × 10-4),but also enable excellent electrical insulating feature due to the wide bandgap(～5.5 eV).Moreover,BNNSs possess larger thermal conductivity within 300-2000 W m-1K-1,which can reduce the heat accumulation efficiently,thereby preventing thermal breakdown phenomenon.Pure BNNS membrane is too fragile to serve as dielectric layer directly.Therefore,the BNNS/polymer composites are proposed by integrating the good dielectric and mechanical properties.Up to now,BNNS/polymer nanocomposite-based film dielectric capacitors have exhibited superior performances of high breakdown strength,high energy density,and high-temperature stability.For instance,in Li et al.’s work,ultrathin BNNSs were added into the ferroelectric terpolymer of P(VDF-TrFE-CFE).As a result,the P(VDF-TrFE-CFE)/BNNS nanocomposite displayed an impressively high charge-discharge efficiencies with 83% at 300 MV m-1and 80% at 600 MV m-1,respectively.[115]It was found that the thinner the BNNSs were,the higher Weibull breakdown strength would be.In general,the dielectric strength increased with the decrement of the thickness in the dielectric layer.Especially when the thickness dropped to a certain value,the dielectric strength with an intrinsic number would not be determined on the dimension.Hence,the 2D materials such as BNNSs with thin layers are more promising for film dielectric capacitors than their bulk ceramics.In addition,the merit of thermal stability paves the way for BNNS composite-based dielectric to be applied in a wider temperature range.Li et al.further developed cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutene)(c-BCB)/BNNS nanocomposite films.In contrast with conventional ferroelectric polymers,the c-BCB/BNNS exhibited outstanding high-temperature dielectric energy storage ability with a Weibull breakdown strength of 403 MV m-1and a discharged energy density of 1.8 J cm-3at 250°C.[116]Also,the temperature coefficient of K for c-BCB/BNNS(65 p.p.m)was much smaller than commercial state-of-art dielectric polymer-based film capacitors such as fluorene polyester(FPE)(308 p.p.m)and Kapton(498 p.p.m)during 25-300°C.To make the dielectric layer thinner and more intact,CVD route was also introduced to directly grow h-BN layer,followed by transferring to the polyetherimide(PEI) films(Figure 22).[117]Hence,large area of BNNS/PEI layer-by-layer composite dielectric can form after hot pressing and Cu etching.The as-obtainedsandwiched configuration showed less defects and ultrahigh dielectric stability and cyclability within a straight 55,000 charge-discharge cycles,which proved to be promising for scalable integrated capacitive devices.For further improvement of the efficiency,Jiang et al.designed a novel interpenetrating gradient structure through filling PVDF with Ba(Zr0.21Ti0.79)O3nanofibers and BNNS under non-equilibrium processing.[118]The gradient configuration in the ternary system enabled high-permittivity nanofillers without compromising the Eband allowing greater electric polarization.As a result,the architecture demonstrated to be effective in enhancement of the both discharge density(23.4 J cm-3)and efficiency(～83% )at larger Ebof 678 MV m-1.Similarly,by coating BNNS with Fe3O4,it can form the dipoles at the interfacial area,hence improving the electric displacement and dielectric permittivity of the nanocomposite.[119]Consequently,the discharge energy density of the sandwiched BZT-BCT NFs-PVDF/Fe3O4@BNNS-PVDF composite reached around 8.79 J cm-3,740% higher than BOPP.Furthermore,the abovementioned polymers such as ferroelectrics containing F are neither environmentally friendly nor decomposable.On account of that,cellulose/BNNS composite dielectric was developed.The green regenerated cellulose can well stabilize and align BNNS at higher filler content.[120]Thus,the composite disclosed excellent flexibility,high energy storage density of 4.1 J cm-3,and breakdown voltage of 370 MV m-1,which was prominent for biomass-based green dielectric devices.Furthermore,it is worth noting that the processability of the dielectrics and the interfacial compatibility with electrodes are key as well for fabricating highperformance capacitor.Hence,to develop sticky and processable dielectric glues that can be pasted to the electrode substrate is feasible for micro-,portable,and integrated dielectric capacitors.In Zhu et al.’s work,they developed a h-BN/polyurethane-based composite ink,which can be adhesive to substrates such as ITO and metals(Figure 23a,b).[121]The as-printable composite dielectric film therefore showed optical transparency,uniformity,and good interfacial binding.After bending 10 times around the pen,the capacitance values of the 3 samples(1:18 pF,2:94 pF,and 3:301 pF,respectively)remained nearly no change(Figure 23c,d),demonstrative of the robustness and reliability for practicality in soft electronics.

Figure 22.a)Schematic plot of the CVD-grown and transfer h-BN films.b)The h-BN/PEI/h-BN film after Cu plate etching and HRTEM image of the nanocomposite.c)Discharged energy density and d)charge-discharge efficiency of the PEI/h-BN and pristine PEI with different cycles.Reproduced with permission from ref.[117]Copyright 2017,Wiley-VCH.

Figure 23.a)A schematic plot of the mixture coating.b)Schematic diagram and picture of the fabricated capacitor for dielectric measurement.c)Capacitance before and during bending(3 samples).d)A magnified capacitance diagram of sample 1 in c).Reproduced with permission from ref.[121]Copyright 2020,Wiley-VCH.

Besides,it is interesting that the elastic h-BN/polydimethylsiloxane(PDMS)composite foams can be employed as air dielectric substitute for multifunctional capacitive sensor applications.[122]In Tay et al.’s study,the h-BN foam was firstly synthesized by CVD method on the Ni foam(Figure 24a).After coating with PDMS and etching Ni,the flexible and elastic nanocomposite dielectric was achieved(Figure 24b).Being mechanically resilient yet highly compressible and electrically nonconductive,the dielectric properties such as dielectric capacitance were in response to the varying strains.As shown in Figure 24c,under strain,the dielectric constant relative to air(K/Kair)increased due to the decrement of the air gaps by compression,and the change kept consistent in the loading/unloading cycle.Even after 10 cycles,the response attained and demonstrated the reproducibility.Apart from the strain,the h-BN/PDMS also exhibited certain changes in capacitance(ΔC/C)when stimulus such as finger or tweezer was alternatively close to the capacitor(Figure 24d).It attributes to the reason that size and distance of the touching object,partial fringing electric field were absorbed,resulting in the variation of fringing electric field within the dielectric layer,thereby changing the ΔC/C.Therefore,beyond the dielectric energy storage,more capacitive applications based on BN composite can be exploited for future dielectric power devices.

Figure 24.a)Schematic picture of the fabrication process of BNF@PDMS.b)Pictures(top and bottom)of a capacitive sensor with ITO/PETs for conducting electrodes and BNF@PDMS as a dielectric layer,respectively.c)k/kairvs.strain plot.The inset displays the change in k/kairat 0 and 90% strains for 10 compressive cycles.d)The non-contact approach by a finger(red)and a tweezer(blue).Reproduced with permission from ref.[122]Copyright 2020,Wiley-VCH.

3.2.BN for Energy Applications

3.2.1.BN for Thermal Energy

As the rapid development of batteries,electronic chips,automobile cooling systems,and 5G communication,the heat accumulation is intensified unprecedentedly,hence easily leading to the malfunction of devices.Therefore,the research of efficient heat sink materials is necessary to cater for the emerging electronics technology.With a 2D planar configuration,BNNSs possess a high in-plane thermal conductivity(Kin-plane)up to 2000 W m-1K-1,which are widely applied as heat dissipation materials for thermal management.[123]However,due to the wide bandgap and insulation nature,the BNNS cannot directly serve as the semi-conductive conductor for photothermal and thermoelectric conversions.Therefore,more complicated design and engineering the thermal conductance of BNNS and the characteristics of other materials are proposed for efficient thermopower harvest and conversion.Here,this section will briefly describe the thermal management of BNNS-based nanocomposites at first and then discuss the photothermal application such as phase-change materials(PCMs)in solar energy battery and thermoelectric application such as Seebeck thermoelectric generator(TEG)-based devices.

As mentioned above,the pure BNNSs constituting membranes are too brittle and fragile to be directly used as robust heat sinks for thermal management.[124]Thus,BNNS-based nanocomposites associated with polymers are normally introduced.Generally,the thermo-conductive BNNS/polymer nanocomposites range from 1D fibers,2D membranes to 3D hydrogels,aerogels,and monoliths with various architectures and heat transfer concepts.For the nanocomposite fibers,wet spinning is usually employed for single fiber fabrication since the shearing,drawing,and stretching process can benefit the axial alignment of BNNS packing,thereby enhancing the K along this direction.For example,Gao et al.adapted hot-drawing procedure to treat wet-spinning BNNS/PVA fiber and achieved aligned BNNS in the filament(Figure 25a).[124]As a result,the single fiber presented larger axial K of 0.078W m-1K-1,200% larger than the case of natural cotton,which was prominent for textiles thermal regulation.Likewise,Wu et al.further suggested that higher speed(12m min-1)of spinning can facilitate the shear force to induce the orientation of BNNS in cellulose fiber matrix(Figure 25b).[125]Then,under certain drawing ratio,larger K close to 3 W m-1K-1can be obtained for a single BNNS/cellulose nanocomposite fiber.Meanwhile,the wet-spinning-produced fibers commonly possess high mechanical performance with tensile modulus over 100 MPa.Nevertheless,the diameter of wet-spinning-derived fiber is usually tens and hundreds of microns,whereas the BNNSs exhibit the hundreds of nanometers lateral size.The mismatch in order of magnitudes easily results in partial aggregation in the fibrous internal structure.To address this limitation,electro-spinning that generates finer nanofibers is utilized to reach nanoscale alignment of BNNS in the polymer filament.In Wang et al.’s study,hydroxyl-functionalized BNNSs(FBN)were mixed with poly(amic acid)(PAA)in water/ethanol solution.[126]Then,the mixture solution was electro-span and thermal cross-linked to form FBN-polyimide nanofibers.Obviously,the BNNSs were well distributed and stacked in the single nanofilament(Figure 25c).The as-obtained FBN-PI textiles hence benefited better heat dissipation and realize nearly 20°C cooling for LED bubbles(Figure 25d).Additionally,2D BNNS-based membranes are the most widely studied among the thermo-conductive nanocomposites.On one hand,the BNNSs are easy to be functionalized and dispersed in solutions(e.g.,water,ETA,IPA,and DMF).On the other hand,instead of traditional coating,the generalization of vacuum filtration(VAF)fabrication greatly promotes the quality of membranes/ films with nacre-like laminar structure and large thermal anisotropy.For example,despite the similar flexibility and intactness of membranes by coating(BNNS/PI),[127]VAF(BNNS/PVA),[128]and(BNNS/ANF),[129]respectively,from optical observation,the distinction can be observed from SEM image comparison of cross sections in Figure 26a-c.Through VAF,both BNNS/PVA and BNNS/aramid nanofiber(ANF)membranes displayed layered structures whereas the distribution of BNNS in the spin-coated PI composite was random.The laminal structure-orientated in-plane K can reach above 20 W m-1K-1at low BNNS weight fractions(≤30% ),while the coated case usually arrives as less than 20 W m-1K-1at similar contents of fillers.Moreover,beyond the macroscopical structure,to design advanced thermo-conductive nanocomposites can further rely on the perspective of molecular chains and crystal units.For instance,phonon density of states(PDOS)was adopted to calculate the phonon spectral matching degree of h-BN and ANF,followed by non-equilibrium molecular dynamics(NEMD)simulation to manifest the efficient heat transfer along ANF chains within in-plane and out-of-plane(Figure 26d).The results then can predict and determine the practical thermal performances of BNNS/ANF blends.Also,current research has brought the thermal management of BNNS-based nanocomposites from normal temperature(＜100°C)to high temperature(＞200°C).This is ascribed to the development of engineering polymers that can afford extreme thermal conditions.For the BNNS/ANF membrane,it successfully enabled 50°C of temperature decrement under the microelectrode heating,as well as cooling the microelectrode to below 100°C under non-working condition(Figure 26e),thereby facilitating high-temperature thermal regulation. Nevertheless, the planar configuration for BNNS-based membrane, film,and paper is only effective along the in-plane direction for heat dissipation.For thermal interface materials(TIMs)integrating with heat sinks,the isotropous thermal conduction is required.Therefore,constructing 3D BNNS thermo-conductive network is of great significance.In general,traditional freeze-drying method can produce BNNS/polymer aerogels with honeycomb structures(Figure 27a).[130]The hexagon cell architecture guaranteed the basic mechanical property such as elasticity of the aerogel,which can be employed directly for flexible and lightweight heat sinks.However,it is pointed out that the thermal conduction path in such network was random that impeded the vertical heat transfer.To improve it,they introduced bidirectional freezing technique together with epoxy resin infiltration(Figure 27b).[131]The as-achieved BNNS composite skeleton exhibited long-range lamella order,hence constructing an entire latticed thermal conductance network throughout the epoxy matrix,which is more effective than randomly distributed cases.Nevertheless,in contrast with honeycomb architecture,this BNNS 3D scaffold was more fragile which cannot serve as heat conductor directly.The following casting polymer resin not only increased the bulk density but also decreased the K,which was not light weight and efficient for portable electronics cooling.Hence,some work proposed micro/nano-engineering of tetrahedral structures with PDMS as the matrix and BNNS as the coatings(Figure 27c).[132]This design addressed the issues such as non-elasticity and mechanical non-robustness,as well as providing reasonable heat conduction path along horizonal and vertical directions.Thus,it can be further applied as a flexible heat sink for microelectronic devices such as metal-oxide-semiconductor field-effect transistor(MOSFET)cooling(Figure 27d).For example,with the same MOSFET loaded on structured BNNS/PDMS and Kapton film,respectively,BNNS nanocomposite-loaded MOSFET showed higher saturation drain current(Figure 27e)and mobility values of 296 cm2V-1s-1with on/off ratio improved from 107to 108(Figure 27f),highlighting the effective heat dissipation that reduced the density of phonons.

Figure 25.a)Schematic picture of the thermal regulation textile.The thermal regulation textile is made of thermally conductive composite fibers.Reproduced with permission from ref.[124]Copyright 2017,American Chemical Society.b)SEM image of the fracture surface of BNNS/cellulose fibers,indicating good axis alignment of BNNS.Reproduced with permission from ref.[125]Copyright 2019,American Chemical Society.c)SEM image of internal FBN filament structure of FBN-PI nanofiber after TGA test.d)Optical pictures and IR images of pure PI and FBN-PI fabrics for LED heat sinks.Reproduced with permission from ref.[126]Copyright 2018,Royal Society of Chemistry.

Figure 26.a)Cross-sectional SEM image of BNNS/PI membrane by spin coating.Reproduced with permission from ref.[127]Copyright 2013,American Chemical Society.b)Cross-sectional SEM image of BNNS/PVA membrane.Reproduced with permission from ref.[128]Copyright 2018,Royal Society of Chemistry.c)Cross-sectional SEM image of BNNS/ANF membrane by VAF.The inset images show the flexibility of these membranes.d)Schematic illustrations and calculations of h-BN supercell,ANF supercell,and NEMD models for axial or radical thermal conductivity of rigid ANF.e)Infrared images and corresponding temperature-time plot for BNNS/ANF membrane in microelectrodes thermal management.Reproduced with permission from ref.[129]Wiley-VCH.

Figure 27.a)SEM image of BNNS/PI aerogel.Reproduced with permission from ref.[128]2019 American Chemical Society.b)SEM image of BNNS/epoxy microstructure.Reproduced with permission from ref.[130]2019 Wiley-VCH.c)SEM image of BNNS/PDMS structured monolith.d)Cross-sectional schematic of MOSFET on BN composite substrate.Comparison of e)I-V characteristics at different applied gate voltage of 1.7-2.5 V,interval of 0.2 V,and f)transfer characteristic of transistor on Kapton substrate(thickness:120 μm)and BN composite substrate(thickness:120 μm).Reproduced with permission from ref.[131]2019 Wiley-VCH.

Moreover,the as-briefed 3D structure can be applied to thermal energy storage and release when PCMs such as wax and PEG were infiltrated into it.[133]The BNNS-based PCMs possess the merits of fast heat adsorption/desorption due to the enhanced K,and the dimensional and thermal stability.For example,under sunlight irradiation of 100 mWcm-2,the cellulose/BNNS-PEG can absorb heat quickly and reach temperature above 60°C within 2500 s,whereas the pure PEG needed 4000 s to accomplish the process.[134]However,despite the enhanced dynamic thermopower storage,it is worth noting that the BNNS fraction inevitably resulted in the decrement of the total thermal energy density since BNNSs were thermo-conductive and insulating that cannot absorb sunlight and store heat directly.The DSC curves certified that the melting enthalpy(ΔHmc)of pure PEG was 175 J g-1,while it dropped to around 130 J g-1for BNNS/PEG nanocomposites,indicating the loss of latent heat.To enhance the efficiency,PDA coating or GO blending was employed to modify the BNNS.[135]As a result,the ΔHmccan be improved to around 150 J g-1with light-toheat and energy storage efficiency of 73.1% under 1 sun.Besides,to make the harvested thermal energy better conversion,TEG was further introduced to convert the thermopower into electric energy.As shown in Figure 28a,the BNNS/PEG composite PCMs were placed on a TE module firstly.[136]Then,under sunlight,the absorbed heat led to the larger temperature increment,therefore delivering Seebeck output current via TE conversion(Figure 28b).In contrast,when the light was off,the heat was released and the current dropped to zero at last.The concept successfully integrated the photothermal and electrothermal energy harvest and conversion,which was also applicable to the utilization of dissipation heat produced by BNNS-based heat sinks.In Wang et al.’s work,both BNNS/PI aerogels and membranes were chosen as heat conductors on the cold side surface of TEG(Figure 28c,d).[130]Up heating the hot side of TEG,the heat sinks worked and generated huge temperature differences(ΔT)between both ends of TEG,hence providing output power and enabling the dissipation heat conversion.Especially at hightemperature heating(＞200°C),the vast dissipated heat can be converted to sufficient electric power to light a LED or calculator when the tri-devices in series(Figure 28e).[137]Therefore,here the BNNS-based nanocomposites validated not only thermal management,but also thermopower conversion for better thermal energy exploitation.

Figure 28.a)Schematic picture of experimental setup of light-to-electric energy conversion.b)The thermoelectric power output of PCMs at different weights under 800 mW cm-2sunlight irradiation.Reproduced with permission from ref.[136]2017 Royal Society of Chemistry.c)Schematic picture of dissipation heat-electric power conversion for BNNS/PI aerogel.Reproduced with permission from ref.[130]2019 American Chemical Society.d)Schematic picture of BNNS/PI membrane as heat sinks.e)The scheme of tandem heat dissipation units for high-temperature cooling and practical thermal energy conversion.Reproduced with permission from ref.[137]2020 Elsevier.

3.2.2.BN for Electrochemical Energy Storage

Solid-state batteries:Batteries have proved to be a solid energy storage technology that powers the portable electronics(such as mobile phones and laptops)and electric vehicles.The lithium-ion researchers were also credited with 2019 Nobel Prize in Chemistry.[138,139]Although enormous achievements have been done in developing the key materials,safety issues are still challenging.Therefore,from the view of composition-property-performances,scientists are trying to develop solid-state batteries,high-temperature-tolerable separator,and electrolyte.In this regard,BN is chemically stable in high temperature that can make contributions in this area.

In order to power the electric vehicles,ultrasafe,high energy density,long-life lithium-ion batteries are desired.To improve the safety,rechargeable solid-state lithium batteries are developing.Metallic lithium anode,benefitting excellent theoretical specific capacity(3861 mAh g-1),offers an attractive candidate as anode material compared with commercially used graphite.Moreover,the potential of-3.04 V is the lowest negative potential among the anode materials,enabling the high voltage output.However,the challenges,such as dendrites of lithium metal and poor lithium metal/electrolyte contact,limit the practical application of lithium metal anode.[140,141]For example,a dendrite-free BN-lithium metal anode was designed for all-solid-state lithium metal batteries by utilizing the lithium-philic properties of BNNS.[7]The lithium-BN composite anode was prepared by mixing few-layered BNNS with molten lithium metal and readily formed fully adhesive contact with garnet solid electrolyte,reducing the resistance from 560 Ω cm2(bare Lig-1arnet interface)to 9 Ω cm2(Li-BNg-1arnet interface)(Figure 29a).Furthermore,Li-BN anode was assembled with LiFePO4(LFP)to fabricate a full cell.The specific capacity by applying Li-BN anode was improved to 150 mAh g-1at 0.2 C compared with using pristine Li anode(117 mAh g-1)(Figure 29b,c).The Li-BN full batteries offered an excellent stability with 135 mAh g-1capacity remained after 100 cycling at 0.5 C(Figure 29d),while the pristine Li-anode full battery was broken after 100 discharging cycles.Therefore,this work provides a valuable insight to rationally design an advanced lithium composite anode by utilizing 2D materials with lithium metal to improve the wetting ability with solid-state electrolyte.

Figure 29.a)Schematic fabrication of BN-BNNS composite anode.b,c)Charge-discharge test performed at 0.2 C of b)Lig-1arnet/LFP battery and c)Li-BNNSg-1arnet/LFP battery.d)Cycling stability of Lig-1arnet/LFP and Li-BNNSg-1arnet/LFP batteries.Reproduced with permission from ref.[7]Copyright 2011,American Chemical Society.

The issue of solid-state electrolyte reduction is one of the reasons for deterioration of battery performances.To solve the incompatibility between lithium metal and solid-state electrolyte,constructing a chemically stable interface with the merits of ionically conducting but electronically insolating is needed.As a typical example,Qian et al.fabricated a long-life solid-state lithium battery by applying an BNNS as a stable interface coated on solid electrolyte surface.[8]The BN layer with thickness of 5-10 nm was coated on Li1.3Al0.3Ti1.7(PO4)3(LATP)solid electrolyte by CVD of borane-ammonia complex(Figure 30a).In this study,in situ TEM characterization verified the protective ability of precluding LATP reduction of BN.Moreover,the symmetric Li/Li cell-cycling test presented that the cells applying BN-coated LATP exhibited a small overpotential after 500 hours,while bare LATP cells were broken after 81 h,confirming the largely improved ion transport ability by BN coating on LATP(Figure 30b).The full cell of LiNi0.33Mn0.33Co0.33O2(NMC)/LATF/BN/Li demonstrated a remarkable capacity retention ability of 92.9% compared with full batteries without the protection of BN nanofilm(Figure 30c).Besides,the BN-coated LATP remained undamaged after 500 cycles compared with the fragmented bare LATP after 100 cycles(Figure 30d-g).Notably,this work provided a useful strategy to stabilize the versatile unstable solid-state electrolytes and paved the way for the industrial application of long-life and high-energy-density solid-state batteries.

Figure 30.a)Schematic illustration of BN-coated LATP for prohibiting reduction in LATP.b)Cycling test of symmetric batteries for using LATP and LATP/BN electrolytes.c)Long-term stability of full batteries using LATP and LATP/BN electrolytes.d,e)Photograph and SEM images of LATP/BN pallets after cycling.f,g)Photograph and SEM images of LATP after cycling.Reproduced with permission from ref.[8]Copyright 2019,American Chemical Society.

Zinc- flow batteries:Zinc- flow batteries(ZIBs)with the merits of cheap price,excellent energy density,and essential high safety are one of the fascinating rechargeable batteries for practical applications on stationary energy storage,including solar energy,geothermal energy,and wind power energy.[137,142,143]Nonetheless,zinc dendrite/accumulation issues threatened the safety of ZIBs by piercing the separator membrane.Typically,benefiting the excellent modulus strength of BNNS,Hu et al.designed an ultra-long life span zinc- flow batteries though suppressing the zinc dendrite by BN engineered membranes.”(Figure 31a).[9]BNNS coated the porous poly(ether sulfone)(PES)membrane enhanced the modulus to 5.84 GPa versus pristine PEM of 3.22 GPa.Therefore,the BNNS-coated membrane converted the zinc metal to a French fries-like morphology compared with the needle-like shape by applying pristine membrane(Figure 31b,c).The French fries-like zinc metal shared a uniform plating/stripping process and hence prohibited the breakdown of membrane and evenly dispersed the heat.As a result,the ZIB using BN-coated PES exhibited ultra-stable cycling stability with coulombic efficiency of 98.5% and energy efficiency of 87.6% ,while ZIBs applying pristine PES membrane was not stable and broken after 40 hours(Figure 31d).The failure of ZIBs was ascribed to the crossover of the electrolyte between negative and positive tank generated by the zinc dendrite.Impressively,the reported alkaline ZIBs were able to operate under high temperature at 50°C at a high current density of 200 mA cm-2and delivered a record-high energy efficiency of 80% .Therefore,this work enables the practical application of ZIBs and offers a valuable opinion on designing BN-based nanocomposite membranes for flow batteries.

Figure 31.a)Illustration of BNNS for regulating the morphology of zinc metal deposition.b,c)SEM images of zinc metal electrode by using P-M and BN-M after charging 200 mA cm-2at 50°C.d)The profiles of voltage-time at a current density of 200 mA cm-2for ZFB using P-M and BN-M.Reproduced with permission from ref.[9]Copyright 2020,Wiley-VCH.

Lithium-sulfur batteries:As an alternative battery,lithium-sulfur batteries are favored by researchers due to the super-high theoretical specific capacity of 1675 mA h g-1,greatly outperforming the most advanced LIBs.[138,139]Additionally,sulfur is abundant and therefore is very cheap and eco-friendly to nature.However,the shuttling of soluble polysulfides and low-active products of Li2S/Li2S2caused the capacity deterioration.For instance,Fan et al.designed a BN/graphene interlayer on separator to alleviate the shuttling of polysulfides for CNT/Sulfur cathode to improve the performances of Li-S batteries.[10]Owing to the amine group-functionalized BNNS with abundant edges and surfaces,the BNNS exhibited excellent trapping effect for polysulfides anions(Figure 32a-d).The amine-BN also demonstrated the better adsorption ability in adsorption test for polysulfides compared with commercial BN and porous BN.Besides,Li-S batteries with FBN/graphene interlayer presented outstanding cyclability with 700 mA h g-1and 558 mA h g-1remained,respectively,after 1000 cycles under current density of 1 C and 3 C,which greatly exceeded the performances of bare Li-S batteries with only 380 mAh g-1and 80 mA h g-1remained under the same condition(Figure 32f).Therefore,this study highlights the idea for designing functionalized BNNS with different surface groups for controlling the ion transportation in an energy storage system.Similarly,they also applied the CO32--functionalized BNNS to repel polysulfide crossover and therefore allowed the Li-S batteries with remarkably reversible capacity,good rate capability,and long-life duration.[144]

Figure 32.a)Schematic illustration of cathode electrode structure.b-d)SEM images of cathode electrode.e)Adsorption of NH2-BN for polysulfides.f)Cycling stability of Li-S batteries with FBN.Reproduced with permission from ref.[10]Copyright 2020,Wiley-VCH.

Separator:Separator is one of the essential components for batteries and supercapacitors to physically isolate the positive and negative electrode from short circuit and preserve the liquid electrolyte.[140,141,145-148]Commercially, polymers, including polyethylene(PE)and predominantly polypropylene(PP),are used for lithium batteries and supercapacitor separators owing to merits of mechanical robustness and shutdown properties.Nevertheless,the mechanical instability under high temperature prohibits the practical applications on extremely hot condition.Plenty of strategies have been developed by surface coating with intrinsically thermal-stable inorganic materials,such as Al2O3,SiC,and TiO2.However,the issue of separation of coating layer with polymer during long cycle test is still challenging.BNNSs are favored as additive for separator application owing to both ultra-stable thermal and chemical properties.[149-151]Muhammad et al.presented a wet chemistry method to produce a BNNS-based separator by mixing BNNS with PE in organic solution and attached the BN-PE layer with poly(vinylidene fluoridehexafluoropropylene)(PE-BN/PVDF-HFP)as a bilayer separator.[11]The BN-PE/PVDF-HFP separator exhibited excellent thermal stability with only 6.6% shrinkage after heating treatment at 140°C for 1 h,while the Celgard 2325 separator showed a 32% thermal shrinkage under the same condition(Figure 33a,b).The infrared thermal images exhibited that the trilayer Celgard 2325 began melting from 100 °C and completely melted after heating 160 °C for 300 s.In contrast,under different heating temperature,a better rate capability was obtained for lithium batteries using PE-BN/PVDF-HFP versus with using Celgard 2325(Figure 33c).Importantly,lithium batteries applied PE-BN/PVDF-HFP delivered much better electrochemical stability under high-temperature testing and offered a higher capacity of 135 mA h g-1,larger than the cell(115 mAh g-1)by using Celgard 2325(Figure 33d,e).This work contributes to a method on fabricating high-thermal-tolerable separator via BN/polymer composites and holds great promise for industrial applications.Similarly,the BNNSs were added to the gel electrolyte to improve the high electrochemical performances of lithium-ion batteries by suppressing the lithium dendrite and enhancing thermal stability,mechanical strength,and ionic conductivity.

Figure 33.a,b)Photograph of Celgard 2325 and PE-BN/PVDF-HFP films at room temperature and 140°C,respectively.c)Infrared images of films under different temperature.d)Rate performances and e)capacity test under different temperature of lithium-ion batteries applied Celgard 2325 and PE-BN/PVDFHFP Celgard 2325 and PE-BN/PVDF-HFP separator.Reproduced with permission from ref.[11]Copyright 2019,Wiley-VCH.

In addition,BNNSs are also promising as ultrathin separator for supercapacitor through wet processing methods,such as spray printing and ink printing.Typically,Zheng et al.offered a concept to replace the polymer separator with a thin-BN layer for asymmetric flexible supercapacitors.[12]The fabricating process relied on spray coating the materials of MnO2/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(MP)as positive electrode,BNNS film as separator,and electrochemically exfoliated graphene as negative electrode(Figure 34a).Notably,the BN layer thickness is merely 2.2 μm and therefore the supercapacitor device showed outstanding flexibility(Figure 34b,d).Further,the integrity and repeatability of supercapacitors by applying BN separator were verified by the electrochemical performances of serially and parallel-connected supercapacitors(Figure 34e,f).

Figure 34.a)Fabrication process of planar sandwich-like supercapacitors by using BNNS as separator.b)The optical image of the supercapacitor.c)Illustration and d)cross-sectional images of supercapacitor structure.e)CV curves of parallelly connected supercapacitors.f)GCD profiles of series-connected supercapacitors.Reproduced with permission from ref.[12]Copyright 2018,Elsevier.

4.Summary and Perspectives

Two-dimensional BN(2D BN),that is,h-BN layers and BNNS,consisting of alternating hexagonal B and N atoms via van der Waals interaction between layers,associating with partial π conjugation have aroused great interest owing to their unique properties in electrology, thermology, photology, and magnetics.Particularly,possessing the merits of electro-insulation,chemical/thermal stability,high thermo-conductance,and excellent dielectric features,2D BN has placed great values in microelectronics(e.g.,FET,dielectric capacitor)and energy applications(e.g.,heat sink,batteries,and supercapacitors)that closely correspond to the state-of-the-art technologies,as well as satisfying the demand of next-generation electronic and energy units.Moreover,similar to its counterpart,graphene,the industrialization and production of 2D BN for commercial market are also emerging in recent years.This requires more generalized,continuous,and scalable synthesis methods and fabrication strategies for 2D BN manufacture.This review has comprehensively introduced the top-down,bottom-up,and associated functionalization approaches to produce various 2D BN.Despite significant efforts on the growth,quality improvement,or surface modification,those revenues still remain space for further development.

Fundamentally,developing CVD and exfoliation methods for producing large-scale monolayer or few-layer BNNS is extremely important for their fundamental and practical applications.High-quality and large-area BN films on PET flexible substrate or copper foil are expected for roll-to-roll synthesis at low temperature below 100°C,thus enabling the industrial applications.Additionally,the concise control growth of wafer-scale and single-crystalline BN films still remains a challenge.Although the progress was achieved on copper substrate,single-crystalline BN on other substrates were still rarely reported.Furthermore,advancement in directly growth of BN-based heterojunction is another flatland to be exploited.Due to the emerging demand of huge quantities of BNNS for electronics and energy applications,scalable synthesis methods such as solid-phase exfoliation and liquid-phase exfoliation are encouraged for scalable fabrication to offer kilogram-scale products.It should be noted that converting the laboratory technology into industrial production is necessary since this transformation would fast accelerate the nanotechnology development.Additionally,exfoliating the BNNS from a solid phase by employing the high-power laser or high-pressure air will be more facile and innovative,thus reducing the consumable machines such as ball milling.Moreover,we also expect that,beyond gradient centrifugation,more efficient strategies are available for sorting the BNNS with uniform size and thickness in the future,which can guarantee the high-quality production of BNNS.

Furthermore,the functionalization of BNNS is supposed based on the monolayer.This is because that the single-layer 2D BN can present superior property such as orders of magnitude enhancement of thermo-conductance or dielectric than few layers,followed by bulks.However,to reach the functionalization on single-layer 2D BN is still challenging.For one thing,conventional CVD routes can achieve largearea monolayer 2D BN;nevertheless,the bottom-up way relies on the nucleation on the substrate,while the whole growth process can hardly introduce other chemicals for following treatment.Hence,the 2D BN from this strategy is normally applied for dielectric layer rather than fillers for composites.Indeed,recent studies on MXenes,TMDs,or other 2D materials have successfully enabled functionalization such as halogen or other elements dangling bonds through molten-salt-assisted CVD, thus providing principles and guidelines for future functionalization of monolayer 2D BN via CVD.For another,to better exploit BNNS as fillers,most studies adopted top-down methods to exfoliate and functionalize BNNS from bulk h-BN.Despite various strategies referring to non-covalent/covalent functionalization,most of them concentrated on a few layers(＞6 layers)rather than single layer.It is worth noting that the approaches such as physical exfoliation such as sonification,ball milling,or chemical expansion can reach intercalation between layers but not being effective for monolayers deriving.The following chemical modification such as grafting on such few-layered BNNS sometimes led to unavoidable aggregation and insufficient functionalization on all layers.To address this issue,future BNNS exfoliation can be referred from the functionalization concept of synthesis monolayer MoS2via ion exfoliation.For instance,monolayer BNNS can be obtained by employing tertiary butyl lithium to intercalate few-layered BNNS firstly and then followed by corresponding chemical modification.[15,105]However,the process seems to be complicated,especially for the industrialization.Therefore,for future 2D BN functionalization,more generalized yet efficient strategies are supposed to be investigated continuously with integration of quality,simplification,scale,and cost.

To take a further step,the successful synthesis and functionalization provide opportunities for multifunctional 2D BN-based hybrids,thus broadening this material to a wider scope.Beyond the heterojunctions,the van der Waals force also allows the assembly of 2D BN with other 2D materials such as graphene and MoS2by simply mixing.This contributes to applications such as comprehensive lubricants and thermal additives,and even thermo-stable sites for thermocatalysis.Also,the functionalized 2D BN exhibits great potential for printable inorganic/organic hybrid inks that is prominent for future micro-and nano-fabrication of FET devices and smart TIMs,especially with some conjugated organic fluids.

In addition,the development of 2D BN also caters for the update of microelectronics and power systems that integrate lightweight,thin,and portable properties,associated with multifunction.For electronics,in the FET components,BN as a dielectric layer provide enormous advantages on atomic smooth surface over other dielectric oxides,therefore contributing to excellent mobility and on/off ratio.The future development is moving toward fabricating millions of FETs into a chip,for example,by applying atomic-thin BN and transition metal dichalcogenide nanosheets.

In the dielectrics of capacitors,current studies still focus on materials design,configuration constitution,and performance enhancement.However,it is believed that the concepts of 2D BN adding for dielectric property improvement have being matured and saturated.More efforts now will transfer to the industrialization and potential commercial practicability.Hence,the continuous production of large-area monolayer 2D BN crystallites or other formations is significant for the future BN-based dielectric devices.Also,in addition to dielectric energy storage,other correlative utilizations such as capacitive sensors and actuators will be exploited to enrich this domain.

For the energy applications,the thermal energy management is a hot topic with the demand of cooling electronics.In contrast with acting as composite heat sinks for simple heat dissipating,the future design of 2D BN-based thermo-cells will integrate PCMs,TEGs,and other thermal translators for multifunctional power conversion.That is to say,the dissipated waste heat from 2D BN is expected to be harvested and exploited.Also,2D BN-based high-performance thermally conductive units are promising to replace traditional metals and carbon materials for use in extreme conditions such as high temperatures and strong acid/alkaline environments.

Finally,the transformation from thermal energy to electro-energy is successful with the rise of batteries and supercapacitors.It has reversed common sense that BN is insulating and not being directly affordable to electrodes.However,the complexity in electrodes,electrolytes,and separators has provided 2D BN roles for various types of batteries with restrain of Li dendrites,cyclability enhancement,polysulfide suppression,separator,and so on.Taking merits of the layered structure,innerness,strength,and thermal stability of BNNS,it would become of great importance for the design of high-level safe batteries and supercapacitors.However,the mechanisms of superior electrochemical performances of 2D BN together with anode,separator,and electrolyte are rarely studied.Therefore,revealing the mechanisms by advanced in situ and operando characterizations is proposed in the following research.

Acknowledgments

J.W.and L.Z.contributed equally to this work.Dr.Jiemin Wang sincerely thanks Prof.Xu Wang’s group supporting his work at School of New Energy and Materials,Southwest Petroleum University,during COVID-19.This work was financially supported by the National Key R@D Program of China(Grants 2016YBF0100100 and 2016YFA0200200),National Natural Science Foundation of China(Grants 51872283,and 21805273),Liaoning BaiQianWan Talents Program,LiaoNing Revitalization Talents Program(Grant XLYC1807153),Natural Science Foundation of Liaoning Province,Joint Research Fund Liaoning-Shenyang National Laboratory for Materials Science(Grant 20180510038),DICP(DICP ZZBS201708,DICP ZZBS201802,and DICP I202032),Dalian National Laboratory For Clean Energy(DNL),CAS,DNL Cooperation Fund,CAS(DNL180310,DNL180308,DNL201912,and DNL201915),the Australian Research Council Discovery Program(DP190103290),and Australian Research Council Discovery Early Career Researcher Award scheme(DE150101617).

Conflict of Interests

The authors declare no conflict of interest.