Haitao Li ,Liang Chen ,Xiaomin Li ,Daoguang Sun ,Haijiao Zhang

ABSTRACT Carbon? and silica?based nanomaterials possess a set of merits including large surface area,good structural stability,diversified morphology,adjustable structure,and biocompatibility.These outstanding features make them widely applied in different fields.However,limited by the surface free energy effect,the current studies mainly focus on the symmetric structures,such as nano?spheres,nanoflowers,nanowires,nanosheets,and core—shell structured compos?ites.By comparison,the asymmetric structure with ingenious adjustability not only exhibits a larger effective surface area accompanied with more active sites,but also enables each component to work independently or corporately to har?ness their own merits,thus showing the unusual performances in some specific applications.The current review mainly focuses on the recent progress of design principles and synthesis methods of asymmetric carbon? and silica?based nano?materials,and their applications in energy storage,catalysis,and biomedicine.Particularly,we provide some deep insights into their unique advantages in related fields from the perspective of materials’ structure—performance relationship.Fur?thermore,the challenges and development prospects on the synthesis and applications of asymmetric carbon? and silica?based nanomaterials are also presented and highlighted.

KEYWORDS Carbon? and silica?based nanoparticles;Asymmetric structure;Synthetic strategies;Energy storage and conversion;Biomedicine

1 Introduction

As two of the most important branches in material science,carbon? and silica?based nanomaterials have shown fasci?nating application prospects in energy,catalysis,adsorp?tion,and biology fields due to their outstanding advantages such as structural adjustability,excellent biocompatibility,and easy to functionalize [1—6].Over the past few decades,carbon? and silica?based nanomaterials with all kinds of structures have been well developed,and their structure—per?formance relationship has also been deeply explored to a certain extent [3,7—9].Despite some of the achievements made,most of them were mainly synthesized and employed in the form of conventional symmetric structures,e.g.,uni?form nanospheres and some core—shell nanocomposites.

Since Gennes et al.proposed the concept of Janus in 1991 for the first time [10],the asymmetric structure has gradu?ally attracted increasing attention.In the past more than two decades,different asymmetric nanomaterials have been massively designed,and the definition of asymmetric struc?ture was far beyond the original Janus structure (Fig.1).It includes not only the Janus structure with different surface properties,but also the asymmetry in topography,such as bowl?shaped [11],snowman?shaped [12],disk?shaped [13],and raspberry?shaped structures [14].Compared with the conventional symmetric structures,asymmetric structures possess some unique advantages.As shown in Fig.2,mul?tiple functions are one of the main advantages of asymmet?ric structures.Due to the different surface physicochemical properties or different components,asymmetric nanoparti?cles can contain several distinct properties simultaneously,such as hydrophilicity and hydrophobicity,optical,and mag?netic properties.Thus,it is an ideal choice for designing“nano?intelligent systems”based on a single asymmetric particle and shows a great application potential in the fields of electrochemistry,interfacial stabilizer,and biomedicine.The stronger synergistic effect is another advantage of asymmetric structure.The distinct domains of asymmetric structure can work independently without interfering with each other and even can cooperate with each other to real?ize significantly improved properties in comparison with traditional core—shell symmetric structures.On the other hand,the asymmetric structure with ingenious adjustability exhibits a larger effective surface area accompanied with more active sites,which is critical to the properties of the material.For example,when used as the electrode materials,the larger effective specific surface area can maximize the electrolyte permeation and enhance the contact area between electrode and electrolyte interfaces,which can effectively boost the electrochemical performances.Owing to those attractive features of asymmetric structure,the develop?ment of carbon? and silica?based asymmetric nanomateri?als has flourished in the past ten years.With the exploration of advanced synthetic methods,the carbon and silica can be married with diversified functional nanomaterials in the form of asymmetric structure,thereby significantly extend?ing their application fields.

Fig.1 Historical development of asymmetric carbon? and silica?based nanomaterials

Fig.2 Comparisons of advantages and disadvantages of symmetric and asymmetrical structures

Although several previous reviews have demonstrated the design and fabrication of asymmetric nanomaterials [15—17],the underlying synthetic mechanism still needs to be discussed in detail.Besides,the systematic reviews on the asymmetric carbon? and silica?based nanomaterials have rarely been reported.Moreover,the advantages of asym?metric structured carbon? and silica?based nanomaterials in specific fields should be elaborated from the perspective of structure—performance relationship.Therefore,it is urgent to summarize the preparation methods and design mecha?nisms of asymmetric carbon? and silica? materials,as well as the relationship between their structure and properties to better understand and widely apply the kind of promising nanomaterials.

Here,we mainly focus on the lasted developments of asymmetric carbon? and silica?based nanomaterials in this review.First,the synthetic strategies and fundamental mechanisms for various asymmetric carbon? and silica?based nanomaterials are systematically summarized.Then,some representative applications of asymmetric carbon? and silica?based nanomaterials are elaborated to clarify the advantages of asymmetric structure on their performance.At last,the future development prospects of asymmetric structure car?bon? and silica?based nanomaterials have been also proposed and discussed.

2 Design Principles and Synthesis Strategies of Asymmetric Carbon? and Silica?Based Nanomaterials

As mentioned above,a diversity of carbon? and silica?based asymmetric nanomaterials have been developed in recent years with the rapid development of nanotechnology.However,the synthesis of asymmetric carbon nanomateri?als differs slightly from the asymmetric silica nanomateri?als.In general,carbon nanomaterials can be obtained by the carbonization of polymer?based nanomaterials,result?ing in the formation of geometrically asymmetric struc?tures.With regard to asymmetric silica nanomaterials,the well?developed“silica chemistry”greatly facilitates the preparation of asymmetric silica nanomaterials,realizing pure silica nanostructures with geometrical asymmetry or nanocomposites with two or more districts that differ in chemical compositions or properties.In this section,the design principles and synthesis strategies will be scientifi?cally discussed according to the compositions of asymmetric nanomaterials,ranging from pure carbon and silica to their composite counterparts.

2.1 Asymmetric Pure Carbon Nanoparticles

Different geometrically asymmetric carbon nanoparticles,such as bowl?,red blood cell (RBC)?,and vase?shaped structures,have been reported.The synthesis methods of asymmetric carbon nanomaterials are similar to those tra?ditional symmetric nanomaterials,which mainly include hard?templating,soft?templating,self?templating techniques [11,18—26].Nevertheless,their formation mechanisms are essentially different.Here,the mainly synthesis strategies and fundamental mechanisms for asymmetric carbon nano?particles will be presented.

The low?surface?rigidity?induced deformation of the hol?low sphere is the most facile and fundamental mechanism to prepare asymmetric carbon nanoparticles.The rigidity of the hollow polymer sphere can be precisely adjusted by various means.Selective removal of nonessential compo?nents from sphere shells is one of the most common meth?ods to alter the rigid structure of nanoparticles.Shi’s group prepared RBC?like carbon nanoparticles by using SiO2as the hard template and mesoporous organosilica as the carbon and silica precursor,respectively (Fig.3a).In this case,the SiO2core was preferentially etched and the hollow SiO2/C nanospheres were obtained after carbonization.After fur?ther removing the SiO2in shell,the rigidity of remaining carbon shell was not enough to support the original hollow structure,thus giving rise to the unique RBC?shaped car?bon nanoparticles [27].By using similar strategy,a series of bowl? and RBC?shaped carbon nanoparticles have been prepared [19,22,27—30].Evenly,the bowl?like mesoporous carbon can also be prepared [31].However,the above syn?thesis processes are usually complicated and uncontrolla?ble.Recently,activation pore?making strategy was used to adjust the rigid structure of the original materials (Fig.3c) [32].In this strategy,the hollow carbon spheres were first prepared by a typical one?pot in situ template route.Then,the obtained hollow carbon spheres were mixed with differ?ent dosages of KOH for high?temperature activation pore?making.In this process,low KOH dosage can etch the shell to enhance the porosity of the spherical shell,while under high KOH dosage,the seriously etched shell would collapse due to inward depression to form a bowl?like morphology.Notably,this synthetic strategy showed a higher application value.Besides,some organic solvents also can be used to remove the nonessential components of polymer sphere.For example,ethanol was skillfully selected as organic solvent to remove incomplete oligomers from polymer spheres and pre?pared bowl?shaped carbon nanoparticles after carbonization (Fig.3b) [33].Specifically,the hollow polymer nanospheres were first prepared by the hydrothermal carbonization of glucose monomers using poly(ethylene glycol) (P123) and sodium dodecyl sulfate (SDS) as soft templates.Then,the oligomer remained in the polymer shell can be preferentially dissolved by ethanol,which leads to the decreased rigidity of the nanospheres,and finally induced the buckling of the shells to form the bowl?like morphology.

Fig.3 Various synthetic strategies for preparing asymmetric pure carbon nanoparticles.a Alkali etching destroys the rigid structure to obtain a bowl?shaped carbon material.Reproduced with permission from Ref.[27].Copyright 2013,Wiley—VCH.b Preparation of asymmetric carbon nanoparticles by subsequent activation pore?making.Reproduced with permission from Ref.[32].Copyright 2018,American Chemical Society.c Selectively dissolving oligomers by solvents to obtain bowl?shaped carbon nanoparticles.Reproduced with permission from Ref.[33].Copy?right 2018,Wiley—VCH.d Preparation of asymmetric carbon nanoparticles by adjusting the thickness of the precursor.Reproduced with permis?sion from Ref.[20].Copyright 2015,Elsevier B.V

In addition,the surface rigidity of hollow polymer sphere can be adjusted by controlling the thickness of polymer shell during synthesis.For example,our group prepared carbon nanoparticles with diverse structures by using P123 and sodium dodecylbenzene sulfonate (SDBS) as the soft templates and glucose as the carbon source (Fig.3d) [20].It was found that the SDBS played a crucial role on the morphology of prepared carbon nanospheres.Specifically,the structure of carbon nanoparticles changed from solid to hollow sphere when the SDBS was added.And the shell thickness of the hollow carbon sphere was significantly reduced as the amount of SDBS further increased.When the thickness was reduced to a certain extent,the rigidity of the carbon shell cannot support the original spherical structure.Finally,the bowl?shaped structure was formed.Moreover,other synthesis parameters,such as reaction temperature and precursor concentration,also can adjust the thickness of the polymer shell.Zhang and co?workers systematically stud?ied the influence of synthetic parameters on the structure of carbon nanoparticles in a recent work,in which the mixed micelles P123 and sodium oleate (SO) were used as soft templates and glucose was used as carbon source [34].They found that the concentration of precursor,surfactant,and reaction time had a decisive effect on the thickness of the carbon shell.Thus,the structure can be changed from sphere to bowl?like by adjusting the above synthetic parameters.Evenly,the open structured carbon spheres can be fabricated when the polymer shells were cracked.For example,open structured carbon nanoparticles were fabricated using phe?nolic resin (PR) as precursor and polystyrene (PS) spheres as template [35].The inner pressure of the hollow PR shell was gradually increased as the decomposition of PS core at high temperature carbonization.Accordingly,the carbon shells were cracked and the open structured carbon spheres were formed once the pressure exceeded the endurance of the carbon shell.Analogously,Lou’s group also fabricated single?hole cobalt?/N?doped carbon (Co/NC) hollow par?ticles by using PS spheres as template [36].Although the above strategies can simply prepare various asymmetric carbon nanoparticles,most of them were mainly bowl?like structures,while mesoporous asymmetric carbon materials are difficult to be well prepared.

The interfacial assembly,for example emulsion inter?face,provides an effective way for designing the asym?metric carbon nanoparticles.Most importantly,some mesoporous or more interesting asymmetric structure can be achieved based on the facile strategy.For instance,asymmetric mesoporous carbon nanoparticles were fabri?cated by using poly(propylene glycol)?block?poly(ethylene glycol)?block?poly(propylene glycol) (F127) and 1,3,5?tri?methylbenzene (TMB) mixed emulsion as soft template and dopamine hydrochloride as carbon precursor.Specifi?cally,the oil—water interface of emulsion droplet formed by F127/TMB/polydopamine can direct the island nucleation and anisotropic growth of polydopamine nanoparticles.With the increased amount of TMB,the morphology of obtained polydopamine nanoparticles can evolve from large?pore mesoporous structure to bowl?like nanoparticles with radial meso?channel (Fig.4a) [37].However,the assembly mechanism of the above asymmetric mesoporous carbon lacked in?depth research.Recently,Mai’s group prepared mushroom?shaped mesoporous carbon nanoparticles and investigated the assembly mechanism through the interfa?cial interactions [38].They found that the average size of oil droplets has a significant influence on the morphology of the consequent carbon nanoparticles.When the size of oil droplets was relatively small,the mushroom?shaped mesoporous carbon nanoparticles were obtained under the action of interfacial tension.The structure of carbon nano?particles changed from wheel?shaped to acorn?like as the size of oil droplets further increased.Moreover,Lee et al.designed mesoporous carbon bowls based on the polymer blend?directed anisotropic self?assembly strategy [39].The spreading coefficients (λijk) in poly(ethylene oxide)?b?poly(styrene) (BCP),homopoly(methylmethacrylate) (hPMMA),and homopoly(styrene) (hPS) ternary immis?cible blends were introduced to understand the assembly behavior.According to the Harkin’s theory,the tendency for BCP phase to spread over hPS was slightly more favored due to the high value ofλPMMA/BCP/PS,leading to bowl?like mesoporous structures.In addition,the as?made products were microtomed at a thickness of 100 nm for transmission electron microscopy to elucidate the morphology formation and further justified the phase behaviors of blends.How?ever,the precursors used in the above methods were usually limited to those that easily polymerized at low temperatures due to the weak interaction between emulsion templates and precursors at high temperature.Recently,asymmet?ric flask?like carbon nanoparticles were fabricated using the high?temperature hydrothermal approach at the emul?sion interface formed by mixing micelles of P123 and SO (Fig.4b) [40].Here,the ribose was first polymerized at the emulsion interface formed by mixing micelles of P123 and SO as the temperature increased.Meanwhile,the hydration level of PEO block in P123 decreased and penetrated into SO;thereby,the tensile stress generated by volume expan?sion of nano?emulsions and finally caused the polymer shell cracked when the pressure exceeded the critical pressure.As the reaction progressed,the nano?emulsion template flowed out to form a fresh template surface on which the precursors would continue to be polymerized,causing the asymmetric flask?like hollow carbonaceous nanoparticles.Moreover,the length of the neck and the diameter of the inner cavity can also be adjusted by controlling the reaction time.In addition,the interactions between the templates and precursors can also be used to regulate the structure during the preparation of hollow carbon spheres using hard templates.Stucky’s group prepared concave?structured carbon nanoparticles by regulating the interface adhesion between template core and carbon shell.They used polystyrene@carbon (PS@C) nanoparticles to prove a concept of“swelling?induced plastic deformation.”The adhesive force between the PS core and the carbon shell was completely changed by a heating—cool?ing treatment.Concretely,the PS core was significantly swelled under the alcohol thermal treatment,which rapidly generates high tensile stress to induce the elastic expan?sion of the carbon shell.After cooling,the shrinkage of PS core turned to pull the enlarged shell under the effect of the adhesive force,thereby resulting in buckling of the carbon shells to form the concave?structured carbon nanoparticles [41].Later,Zhao and co?workers coated a layer of silica on the surface of the polymer nanospheres to form a poly?mer—silica interface.Similarly,the steam repulsion,interface tension,and constriction force were generated at high tem?perature carbonization.Thus,the polymer precursor would be shrunk asymmetrically under the combined effect of the above force,and the hemispherical carbon nanoparticles were produced after further removing the silica shell [42].

Fig.4 a Fabrication of asymmetric mesoporous carbon nanoparticles through interfacial assistance.Reproduced with permission from Ref.[37].Copyright 2016,American Chemical Society.b Asymmetric flask?like carbon nanoparticles were fabricated using the high?temperature hydrothermal approach at the emulsion interface.Reproduced with permission from Ref.[40].Copyright 2017,American Chemical Society.c Fabrication of bowl?like structures through the asymmetric template.Reproduced with permission from Ref.[21].Copyright 2016,Royal Society of Chemistry.d Fabrication of asymmetric carbon through other method.Reproduced with permission from Ref.[43].Copyright 2018,Wiley?VCH

Directly coating carbon precursors onto an asymmetric template or selectively coating on a symmetric template is another available route for fabricating asymmetric carbon structures [44].For example,bowl?shaped carbon nanopar?ticles were directly prepared by using kippah?vesicle?shaped polystyrene?block?poly(ethylene oxide) (PS?b?PEO) as asymmetric template (Fig.4c) [21].And carbon nanocups also were prepared by selectively coated carbon precursor onto ZnO nanorods through chemical vapor deposition [45].

Apart from the above?mentioned synthesis strategies,asymmetric carbon nanoparticles can also be directly syn?thesized.For example,bowl?shaped carbon nanoparticles were directly prepared by processing worm shells [46].And Guo’s group prepared pistachio?shuck?like MoSe2/C nano?materials by using sodium molybdate,dibenzyl diselenide,and oleylamine as precursors.Meanwhile,the structures can be easily adjusted through controlling the high?temperature treatment time (Fig.4d) [43].

2.2 Asymmetric Carbon?Based Nanomaterials

Although different asymmetric carbon nanoparticles with unique advantages have been designed,their applications are severely limited because of the individual carbon com?ponents.In principle,reasonable design and preparation of asymmetric carbon?based composites can finely adjust their electronic,magnetic,and mechanical properties to extend their applications.

Generally,it requires the combination of polymer with other functional nanoparticles to realize the carbon?based composites.However,the anisotropic growth of polymer onto different substrates remains a huge challenge.Wang’s group proposed a facile strategy to prepare asymmetric metal—polymer nanocomposites [47].Interestingly,they found that Au—polyacrylic acid (PAA) Janus nanoparti?cles (JNPs) were generated by adding the isopropyl alco?hol (IPA) into the aqueous solution of citrate—Au NPs and PAA under the alkaline condition.This method was also applicable when the seeds are replaced with other materials (Ag nanocubes [48],gold nanorods [49],Pd nanosheets [50],etc.).Furthermore,more complex asymmetric nanostruc?tures can also be synthesized in a controlled manner by the metal—polymer JNPs as seeds.As an example,the group pre?pared Au/Fe3O4@C JNPs by carbonization and etching the Au/Fe(OH)3?PAA@SiO2JNPs (Fig.5a) [51].The Au—PAA JNPs were first prepared by controlling the surface interfa?cial energy.Subsequently,Fe2+was selectively deposited on the surface of PAA and hydrolyzed to Fe(OH)3.Then,the whole JNPs were coated by SiO2to protect from unde?sirable agglomeration during the calcination process.Last,the Au/Fe3O4@C JNPs were obtained by carbonization and etching.Similar to this report,other polymers? and metal organic framework (MOF)?based asymmetric nanoparticles have also been designed [49,50,52,53].Besides,the asym?metric MOF?based materials have also been synthesized by selectively modifying the surface of seeds.Li’s group pre?pared Janus?structured MOF?up conversion nanoparticles (UCNPs) by a convenient solvothermal method (Fig.5b) [54].In this study,hydrophobic UCNPs were converted into hydrophilic nanocrystals by coating polyvinylpyrrolidone (PVP),and then,the MOF?UCNPs JNPs were obtained through anisotropic growth of MOFs on UCNPs.

Fig.5 Preparation of various asymmetric polymer? and MOF?based nanoparticles.a Preparation of Au/Fe3O4@C Janus nanoparticles by con?trolling the surface interfacial energy.Reproduced with permission from Ref.[51].Copyright 2017,Wiley—VCH.b Fabrication of Janus struc?tured MOF?UCNPs by selectively modifying the surface.Reproduced with permission from Ref.[54].Copyright 2017,American Chemical Society

Apart from the nanoparticles mentioned above,graphene?based Janus films with heterogeneous layers were another type of promising nanomaterials because of their unique two?dimensional structure,large surface area,and superior performance.The asymmetric graphene?based material was mainly synthesized by selective modification,which involved exposing one face of graphene to atmosphere or solutions while protecting the other side for the prepara?tion of graphene?based Janus structure.According to this template?assisted strategy,a variety of graphene?based Janus films with two distinct functionalized surfaces have been prepared.In 2010,Robinson’s group first reported that graphene films grown on Cu foils were fluorinated with xenon difluoride (XeF2) gas on one or both sides [55].After that,diverse graphene?based Janus structural nanomateri?als have been designed via different modifying reagents,including small organic molecules [56—58] and polymers [59—62] [63,64].Nonetheless,the functionality of above?mentioned Janus graphene nanomaterials was too simple.In 2013,Zhang’s group prepared Janus?structured graphene by co?grafting of halogen and aryl/oxygen?functional groups on each side for the first time [65].In the experiment,they first functionalized one side of graphene by grafting halogen groups and then coated a PMMA film on the single?sided functionalized graphene.Subsequently,the PMMA?coated graphene was peeled off from the substrate and the other side of the graphene was asymmetrically grafted by aryl or oxygen?functional groups.Finally,the Janus graphene was obtained after removing the PMMA film from the gra?phene.Inspired by this work,Janus graphene functionalized with nitrobenzene diazonium (NBD) and methoxybenzene diazonium (MBD) on the opposite sides of graphene was prepared using aqueous solutions of aryl diazonium mol?ecules [66].However,the yield obtained was limited by using this method.The above shortcomings can be effec?tively solved using the Pickering emulsion template [67].For instance,GO nanosheets were first assembled on the surface of wax particles via a Pickering?type emulsion;then,PMMA was grafted selectively from the exposed face.At last,Janus PMMA?GO?X was collected after removing the wax substrate [68].Although the synthesis technology of graphene?based Janus structure has matured,the synthesis process was relatively complicated,and the asymmetry was based on opposite sheet faces.Recently,Zhang’s group pro?posed a modified method to fabricate Janus?like GO (JGO) with a novel asymmetric structure along the faces of the sheets (Fig.6a) [69].In this study,the oxidation process preferentially occurs at the edges and defect locations,and the middle part of graphene was protected,which is similar to the inert template protection.

It should be noted that the selective deposition of met?als or metal oxides on the surface of graphene can endow graphene with specific electrical and chemical proper?ties.For example,ZnO and Au nanoparticles were suc?cessfully deposited on the different sides of graphene (Fig.6b) [70].The whole synthetic process was mainly divided into three steps:first,selectively deposit ZnO on the exposed graphene layer;second,deposit a PMMA layer to protect the ZnO and remove the silicon matrix;and finally the other side of the graphene was modified and then gold nanoparticles are deposited.In addition,different metals or metal oxides,such as Au [71,72],Ag [73,74],ZnO [75],and Cu2O [76],have been selectively deposited on the one side of graphene surface by chemical vapor deposition,electrochemical deposition,and other methods.

Fig.6 Preparation of various asymmetric carbon?based nanomaterials.a Preparation of novel asymmetric structure along the faces of the sheets.Reproduced with permission from Ref.[69].Copyright 2020,Elsevier B.V.b Fabrication of Janus structured ZnO?graphene?Au compos?ite materials by selectively modifying the surface.Reproduced with permission from Ref.[70].Copyright 2011,Wiley?VCH

2.3 Asymmetric Pure Silica Nanoparticles

Silica nanoparticles are commonly synthesized by the well?developed“sol—gel”chemistry.By regulating the interfa?cial interaction in reaction system,pure asymmetric silica nanoparticles can be prepared via the wet?chemistry method,but anisotropic nucleation and growth of silica onto different seeds can be realized to prepare hybrid silica?based asym?metric nanoparticles.

Similarly,asymmetric silica nanoparticles can also be got via changing the rigidity of the silica shell.In 2005,Zoldesi’s group prepared silica capsules with hemispheri?cal cap and microballoon structure by using liquid core and TEOS as the template silica source,respectively [77],where the rigidity of the silica shell was adjusted by the size of liquid core.Notably,selective etching is also used to adjust the rigidity of silica nanoparticles to produce asym?metric structure.For instance,organosilica nanobowls were fabricated through a preferential etching approach (Fig.7a) [78].Because of the highly cross?linking of Si—OH groups on the surfaces,the interiors of mesoporous organosilica spheres were preferentially etched.After etching the cores completely and selectively etching the inorganic silicon in the shell,the frameworks were mainly dominated by the flexible Si—R—Si chains and non?cross?linked free Si?OX groups and were insufficient to support the original spheri?cal structure,forming a bowl?like shape.Similarly,Zhang’s group also used this method to prepare mesoporous orga?nosilica nanobowls [79].More interesting silica nanopar?ticles with asymmetric structures also can be prepared via this etching strategy.The eccentric single?hole mesoporous nanocages have been designed by Zhao’s group.The SiO2@PMO composites were first prepared by anisotropically encapsulating the PMO on dense SiO2nanoparticles.After removing the SiO2nanoparticles,the eccentric hollow PMO nanoparticles were formed.Finally,a single?hole structured silica nanoparticles were formed after further etching by HF (Fig.7b) [80].

Fig.7 Various synthetic strategies for preparation of asymmetric pure silica nanoparticles.a Alkali etching destroys the rigid structure of the silica and results in a bowl?shaped silica material.Reproduced with permission from Ref.[78].Copyright 2017,American Chemical Society.b Fabrication of open?structured silica nanoparticles through chemical etching.Reproduced with permission from Ref.[80].Copyright 2015,American Chemical Society.c Preparation of bottles?like silica nanoparticles through two?phase interface control.Reproduced with permission from Ref.[81].Copyright 2016,American Chemical Society.d Fabrication of badminton?like structure through two?phase interface control.Reproduced with permission from Ref.[82].Copyright 2017,American Chemical Society

The interfacial assembly,especially oil—water interface,is another effective strategy to prepare asymmetric silica nanoparticles.In 2011,Kuijk et al.prepared bullet?shaped asymmetric silica nanomaterial with tunable length by using an mixed solution of pentanol—water as solvent,and the growth process was recorded by transmission electron cryomicroscopy (cryo?TEM) at different reaction times [83].It was found that the hydrolyzed TEOS only presented in the water?rich emulsion droplets,the nucleus grown in one direction after nucleation at the oil—water interface.And the bullet?shaped asymmetric silica nanoparticles were obtained due to the surface tension.Inspired by this work,Wang et al.used a similar method designed for the tadpole?like nanow?ires by introducing the surfactant trimethoxy(octadecyl)silane [84].Later,bottle?like silica nanoparticles were also successfully prepared using two precursors,(3?chloropro?pyl)trimethoxysilane (CPTMS) and tetrabutylorthosilicate (TBOS) in the water?n?pentanol system (Fig.7c) [81].In this study,a silica film was first formed on the surface of the water droplets by the polymerization of —Si(—O?)3,which was produced by the hydrolysis of CPTMS.Then,the bowl?shaped silica shell was formed when TBOS was condensated on the silica film formed above.As the precursor contin?ues to polymerize on the surface of the water droplet,the thickness of the shell increased and the water droplet was squeezed to form a new oil—water interface;then,the precur?sors continue to polymerize at the newly formed oil—water interface,obtaining a flask?like structure with an opening.Nonetheless,constructing asymmetric silica nanoparticles with a highly controllable dendritic silica compartment with large pores is still a great challenge.Inspired by the hydrolysis and condensation principle of TEOS in water/oil biphasic systems,Yu and co?workers successfully designed the badminton?like mesoporous silica (Fig.7d) [82].In a typical synthesis,monodispersed silicas as head particles were dispersed in a water—chlorobenzene system,TEOS as silica source,triethanolamine (TEA) as a catalyst,and cetyltrimethylammonium chloride (CTAC) was used as a structure?directing agent.Owing to the electrostatic attrac?tion,the surface of head particles changed to positively charged after CTA+modification.The negatively charged silicate species produced by hydrolysis and condensation of TEOS preferentially nucleated and grown on the surface of positively charged head particles near the water phase.Thus,the head—tail mesoporous silica nanoparticles were fabricated.The morphology,particle size,and tail length can be facilely tuned by varying the volume of TEOS,the amounts of head particles,and reaction time.

2.4 Asymmetric Silica?Based Nanoparticles

In order to achieve more complex functionalities and extend their applications,different silica?based composite nanopar?ticles have also attracted widespread attention.Compared with conventional core—shell silica?based nanoparticles,a remarkable advantage of asymmetric silica nanostructure is that it can combine individual components in different spaces of single particle without interfering their optical,magnetic,and electronic properties.Therefore,different ingredients can work together to achieve multiple functions.According to the synthetic mechanism,the methods for sil?ica?based asymmetric structures roughly include two strate?gies,namely interface modification and nucleation control.

The construction of the Pickering emulsion interface has been regarded as one of the most basic methods to prepare asymmetric silica?based nanoparticles [85—98].The paraffin/water interface is one of the most commonly used Picker?ing emulsion interfaces [99].He et al.used the emulsion of paraffin and water to fix the hydroxylated silica parti?cles,leaving half encapsulated in paraffin and another half exposed outside.Then,the exposed surface of silica was functionalized with amino groups.Finally,paraffin was removed to obtain Janus silica particles modified by amino and hydroxyl on both sides,respectively (Fig.8a) [100].In addition,other functional materials can be selectively grown on the silica surface following this method [101—104].For example,gold—mesoporous silica Janus nanoparticles were successfully fabricated.The mesoporous silica nanoparticles were firstly fixed onto the emulsion interface of paraffin and water [105].By partially functionalized with thiol groups by the addition of (3?mercaptopropyl)trimethoxysilane on the exposed surface,gold nanoparticles were selectively grown on it by the formation Au—S bonds to fabricate the Janus gold—mesoporous silica nanoparticles after removing paraffin.

Fig.8 Preparation of various asymmetric silica?based nanoparticles.a Preparation of amphiphilic Janus silica particles by Pickering emulsion.Reproduced with permission from Ref.[100].Copyright 2015,American Chemical Society.b Preparation of Janus Au?SiO2 nanoparticles by selective modification.Reproduced with permission from Ref.[106].Copyright 2010,American Chemical Society

Selective modification is also a direct way to endow the seeds with different surface properties for the growth of Janus structure [107—109].For instance,Chen et al.simul?taneously modified Au nanoparticles with 4?mercaptophe?nylacetic acid (4?MPAA) and poly(acrylic acid) (PAA86) [110].Because of the different properties of the modifiers,the silica species preferentially grew onto the 4?MPAA mod?ified side of gold seeds rather than the PAA?modified side,thereby obtaining Janus silica—Au nanoparticles (Fig.8b).Kane et al.simultaneously modified the Au nanoparticles with PAA and 4?mercaptobenzoic acid (4?MBA) to render the surface of the gold seeds with different properties based on the similar strategy [106].Owing to the high interfa?cial energy of the gold—ligand—solution interface,only the 4?MBA?modified part of Au nanoparticles can promote het?erogeneous nucleation of silica to produce asymmetrically coated Au nanoparticles.The researchers also demonstrated that silica—gold nanoparticles with the core—shell structure can be achieved when only 4?MBA modification was per?formed,and further confirmed the critical role of selective modification for generating Janus structure.

Apart from these,some unique asymmetric silica?based nanoparticles can be fabricated by surface?controlled nuclea?tion and growth.Generally,the nucleation and growth method are determined by the overall Gibbs free energy,which can be represented as Δσ=σ2?s?σ1?s+σ1?2,where σ2?sis the interfacial energy between component 2 and sol?vent,σ1?sis the interfacial energy between substrate and solvent,σ1?2is the interfacial energy between substrate and component 2.When Δσ>0,it is difficult for component 2 to diffuse on the surface of substrate,thus resulting in an asymmetric structure.The choice of the reaction solvent was an effective method to control the overall Gibbs energy for preparing the Janus structure.In 2011,Sun’s group pre?pared the Fe3O4—SiO2Janus particles with an accurately adjustable aspect ratio through the control of the reaction solvent for the first time [111].In the synthesis,the tradi?tional core—shell structure of Fe3O4@silica was synthesized when ethanol was added,further illustrating the importance of the reaction solvent.Unfortunately,the work did not clar?ify the growth mechanism of this unique Janus structure from a deep insight.Driven by this work,Ag—SiO2[112,113],Fe3O4—SiO2[114],Au—SiO2[115—117],and other [118] silica?based Janus composites have been synthesized.Later,Chen’s group prepared Janus Ag—mesoporous silica nanoparticles and explained the formation mechanism using the theory of surface free energy (Fig.9a) [119].In this system,the total surface free energy can be represented as Δσ=σmsio2?water?σAg?water+σAg?msio2,and the surface free energy of mSiO2σmsio2?waterin the pure water system was significantly higher than that in alcohol—water sys?tem.Therefore,the total surface free energy Δσ>0,which would hinder the diffusion of CTAB/silicate micelles on Ag cores.After the nucleation site was formed,since the energy barrier of heterogeneous growth is much greater than that of homogeneous growth,mSiO2will grow asymmetrically on the nucleation site instead of gradually covering the Ag core to form a symmetric structure.Finally,the core—shell structured Ag—mSiO2Janus nanocomposites were real?ized.In addition to those simple silica?based asymmetric structures mentioned above,it has been a long?term goal to achieve more complex and multifunctional silica?based asymmetric structures by adjusting the interfacial tension.Surface?controlled nucleation and growth was the most effective strategy to fabricate complex asymmetric silica?based nanoparticles [47,120,121].For instance,the multi?functional dual?compartment Janus silica nanocomposites UCNP@SiO2@mSiO2&PMO (UCNP,=NaGdF4:Yb,Tm@NaGdF4,) were prepared by anisotropic island nucleation and growth (Fig.9b) [122].In this case,the ratio of water to ethanol played an important role to the total surface energy of the system.Moreover,when the volume ratio of H2O:ethanol was reached to 15:1,causing Δσ>0,it is dif?ficult for CTAB/silicate micelles to diffuse on the surface of UCNP@SiO2@mSiO2,Thus,the asymmetric structured UCNP@SiO2@mSiO2&PMO nanoparticles were formed.Inversely,the concentric core@shell@shell@shell struc?tured UCNP@SiO2@mSiO2@PMO nanocomposites were obtained when the volume ratio between H2O and ethanol was decreased to 2:1 due to the Δσ<0.After that,other complex and multifunctional asymmetric silica?based nano?particles had been designed by the same group [123,124].For example,dual?mesoporous Fe3O4@mC&mSiO2Janus magnetic nanoparticles with tunable hydrophilic/hydropho?bic ratio were designed,recently (Fig.9c) [125].First,the Fe3O4—SiO2Janus nanoparticles were fabricated through the regulation of interface energy.Subsequently,mesoporous polydopamine was selectively modified on Fe3O4segment through electrostatic interaction.Finally,dual?mesoporous Fe3O4@mC&mSiO2Janus nanoparticles were fabricated by high?temperature carbonization treatment.More recently,the same group reported an interesting strategy called“surface?kinetic?mediated multi?site nucleation”for syn?thesizing asymmetric mesoporous multipods composed of a centering core@shell Fe3O4@SiO2@RF nanoparticle and four surrounding PMO nanocubes as pods [124].The results demonstrated that the heterogeneous nucleation kinetics of oligomers on the substrate can be precisely regulated by changing the surface functional groups of RF layers.Con?sequently,the number of nucleation sites at the beginning of reaction significantly increased with the increase in the nucleation kinetics,thereby enabling the uniform growth of one pod,two plane?distributed pods,tripods,tetrapods,or even multi?pods on the centering substrate.

Fig.9 Preparation of various asymmetric silica?based nanoparticles.a Fabrication of Janus Ag?silica nanoparticles through nucleation con?trol by the regulation of the reaction solvent.Reproduced with permission from Ref.[119].Copyright 2016,American Chemical Society.b Preparation of the multifunctional dual?compartment Janus silica nanocomposites through nucleation control.Reproduced with permission from Ref.[122].Copyright 2014,American Chemical Society.c Fabrication of Janus structure through interface modification and nucleation control.Reproduced with permission from Ref.[125].Copyright 2018,American Chemical Society.d Preparation of Janus structure by regulating the concentration of surfactants.Reproduced with permission from Ref.[120].Copyright 2017,Wiley?VCH

In addition to the solvent,the selection of surfactants also can regulate the total interfacial energy in reaction system.Liu’s group used resorcinol—formaldehyde (RF) nanospheres as the seeds and 1,4?bis(triethoxysilyl)benzene (BTEB) as the silica source to prepare silica—RF nanocomposites (Fig.9d) [120].The structure of the nanocomposites can be changed by regulating the amount of surfactant CTAB.Spe?cially,only core—shell structure was formed in the absence of CTAB,while a dumbbell?shaped Janus structure can be prepared with the increased amount of CTAB.

Some new synthetic techniques can also be used to pre?pare some emerging asymmetric materials [126—128].For instance,zinc?doped indium oxide has been successfully printed on the silica surface to obtain asymmetric 2D silica nanocomposite [129].

3 Applications of Asymmetric Structure of Carbon? and Silica?Based Nanoparticles

By virtue of the unique advantages of asymmetric structure,such as larger effective surface area,controllable structure and composition,the asymmetric carbon? and silica?based nanoparticles have attracted increased interest in various application fields in the past decade (Table 1).In this sec?tion,we will focus on the applications of asymmetric car?bon? and silica?based nanoparticles in energy,catalysis,bio?medicine,sensor,and other related directions,in particular highlighting the structural advantages from the viewpoint of structure—performance relationship.

Table 1 A summary of synthetic methods,morphology and structure,and their different applications of asymmetric carbon? and silica?based nanoparticles

3.1 Electrochemical Energy Storage Applications

In the contemporary environment of pursuing low?carbon life,the use of new renewable clean energy instead of the fossil fuels has emerged.The electrochemical energy storage devices have been widely deemed as one of the best types of energy storage in various new energy sources due to its good stability,recyclability,and no memory effect.According to the energy storage mechanism,the electrochemical energy storage device is mainly divided into reversible second?ary batteries (e.g.,known lithium?ion batteries,LIBs) and supercapacitors [152—158].As one of the main components of energy storage device,it has been demonstrated that the structure and composition of electrode materials including cathode and anode seriously affect their electrochemical performances.

Carbon? and silica?based nanoparticles,especially carbon nanoparticles,have become the main electrode materials due to the good conductivity,large surface area,and excel?lent electrochemical stability [159—165].Also,the rational design of materials structure can effectively improve its energy storage performance [166—168].The traditional hol?low structure is one of the most ideal structures because of its tunable inner cavity,which can effectively alleviate the large?volume expansions during charge—discharge pro?cesses.However,it also suffers from a limited volumetric energy density owing to a larger internal cavity,resulting ina smaller bulk density.Additionally,a larger internal cavity will reduce the structural stability of the electrode materials,thereby leading to poor cycling stability and low capability.In contrast,the asymmetric structure can effectively alleviate the above shortcomings and exhibit some unique new func?tions benefiting from its unique structural merits.

For the electrode materials,the larger effective spe?cific surface area provided by asymmetric structures can maximize the electrolyte permeation and enhance the con?tact area between electrode and electrolyte interfaces,which can effectively boost the electrochemical performances.For example,Zheng et al.designed porous bowl?like N?doped carbon with an extremely high BET surface area of 2161 m2g?1[11].When as cathodes for Li—S batteries,the obtained S/N?HPCB electrode showed a high reversible capacity of 894 mAh g?1at 0.1 C,and it still retained 706 mAh g?1after 400 cycles at 1.0 C.The superior electro?chemical performance was attributed to efficiently immo?bilize sulfur and polysulfide provided by the high surface area of bowl?like structure,leading to maximum interaction between sulfur and carbon host and minimal dissolution of lithium polysulfide intermediates.Other asymmetric carbon nanoparticles have also been designed as electrode materials for reversible secondary batteries [32,45,46].For instance,Jian et al.prepared carbon nanocups through a facile cata?lytic chemical vapor deposition (CCVD) technique [45].Benefiting from the enhanced spacings for ionic storage and mass transport of unique opening structure,the resulting carbon cups delivered a high reversible capacity of 953 mAh g?1after 100 cycles at 50 mA g?1and 468 mAh g?1after 100 cycles at 25 mA g?1,respectively,for LIBs and sodium?ion batteries (SIBs).Compared with LIBs and SIBs,more severe volumetric changes on electrode materials during the intercalation of K ions have become the main obstacle for obtaining the stable electrode materials capable of under?going long?term potassiation/depotassiation,caused by the larger radius of K ions (1.38 ?).To address the issue,bowl?like asymmetric hollow multi?hole carbon nanoparticles (CHMBs) were prepared and systematically explored the stress response by simulating the von Mises stress distribu?tions (Fig.10a) [130].The result showed that the maximum stress of hollow multi?hole bowl was 0.254 MPa,consider?ably smaller than that of the solid sphere (1.154 MPa) and a little higher than that of the hollow sphere (0.184 MPa).However,CHMBs had a higher tap density in comparison with the hollow sphere.Consistent with the above simulation results,the CHMBs electrode exhibits an excellent durabil?ity and a high reversible capacity of 304 mAh g?1at 0.1 A g?1due to the shortened electron/ion transport distance.Especially,the volumetric specific capacity of CHMBs was 56% higher than that of hollow carbon spheres due to the higher pack density.Similar to other symmetrical nanoparti?cles,the electrochemical performance of asymmetric carbon nanoparticles can also be further improved by introducing heteroatoms or compounding with other high?capacity materials [169—175].For example,bowl?like carbon hybrid?ized with SnO2nanosheets (SnO2@C) has been designed as electrode materials for lithium storage (Fig.10b) [132].Thanks to the unique structure and composition advantages,the as?made SnO2@C electrode demonstrated a high revers?ible capacity of 963 mAh g?1at 0.4 A g?1after 100 cycles.Analogously,various asymmetric carbon?based composite nanoparticles,including C?NiO [131,176],C?Fe3O4[133],C?CoS [134],C?MoS2[135,177—179] and C?FePO4?RGO [136],have been designed as electrode materials for revers?ible secondary batteries.Nevertheless,most of them were compounded in two steps,and the synthesis process is rela?tively complicated and time?consuming.Recently,Guo’s group prepared pistachio?shuck?like asymmetric MoSe2/C composite (PMC) based on a simple one?step process (Fig.10c) [43].The unique structure can not only improve the diffusion of K?ions and increase the electrons and ions transfer,but also effectively enhance the volumetric energy density.The resulted PMC exhibited a high capacity of 322 mAh g?1at 0.2 A g?1over 100 cycles and excellent cycle stability.

Fig.10 Application of asymmetric structure in secondary batteries.a Application of asymmetric pure carbon in secondary batteries.Repro?duced with permission from Ref.[130].Copyright 2019,American Chemical Society.b Application of asymmetric SnO2@C composite mate?rial in Li?ion batteries.Reproduced with permission from Ref.[132].Copyright 2014,Wiley—VCH.c Application of asymmetric MoSe2/C com?posite material in K ion batteries.Reproduced with permission from Ref.[43].Copyright 2019,American Chemical Society

Supercapacitors are another common electrochemical energy storage device.However,achieving high power and large energy capacity at a high rate is still difficult in the development of electrochemical capacitors owing to the pri?mary kinetic limits of electrochemical processes in electrode materials.Benefiting from the large surface area,higher bulk density,and better structural stability,asymmetric carbon?based nanoparticles demonstrated a fascinating application prospect in supercapacitors.For instance,bowl?shaped carbon nanosheets have been used as an electrode material for supercapacitors (Fig.11a) [29].In virtue of the merits including higher bulk density,interconnected channel,and hierarchical porosity of thin?walled bowl?like sheet,the pre?pared carbon sheet had a high specific capacitance of 151 F g?1at 0.5 A g?1.Similarly,a series of asymmetric bowl?like carbon nanoparticles have also been designed as elec?trode materials with excellent electrochemical performance for supercapacitors [21,22,32,34,44].Besides,a unique flask?like hollow carbonaceous electrode material was also synthesized,exhibiting a high specific capacitance of 263 F g?1at 0.1 A g?1and 222 F g?1at 1.0 A g?1.Importantly,the obtained carbon electrode material had no decrease in capacitance after 10,000 cycles at 20 A g?1.The superior electrochemical performance can be attributed to the larger surface area (2335 m2g?1),pore volume (1.34 cm3g?1),and better structural stability provided by the unusual hol?low and open structure,which can effectively improve the charge storage and accelerate ion transport (Fig.11b) [40].However,pure carbon nanoparticles still exhibited a low the?oretical capacity.Similar to the secondary batteries,it was also one of the most effective ways to improve the energy storage capacity of supercapacitors by coupling with other high?capacity materials [180—183].Recently,Ma’s group got a hemispherical N?doped porous carbon/NiCo2O4(NPC/NiCo2O4) composite (Fig.11c) [137],where the hemispheri?cal N?doped porous carbon as buttress to restrain NiCo2O4nanosheets from aggregation and NiCo2O4as covering layer to enhance the capacitance of carbon.When as the electrode material for supercapacitors,NPC/NiCo2O4reached specific capacitance of 948.30 F g?1at 1.0 A g?1,and the capacitance retention of 87.4% at 10 A g?1after 2000 cycles,manifesting an excellent durability.

Fig.11 Application of asymmetric structure in capacitors.a Application of asymmetric pure carbon in capacitor.Reproduced with permission from Ref.[29].Copyright 2014,Elsevier B.V.b Application of flask?like pure carbon in capacitor.Reproduced with permission from Ref.[40].Copyright 2017,American Chemical Society.c Application of asymmetric hemispherical N?doped porous carbon/NiCo2O4 composite material in capacitor.Reproduced with permission from Ref.[137].Copyright 2019,Elsevier B.V

3.2 Catalytic Applications

As we known,the catalytic reaction generally takes place at the interface between reactants and catalyst.This means that only the active ingredients that are exposed to reac?tants are highly active.Thus,the performance of the catalyst material is closely related to their surface morphology and internal structure [184—189].Traditional hollow structures have showed a good application prospect in the catalytic fields in virtue of their large surface area and low density [190—194].Nonetheless,the larger internal space usually results in a lower effective surface area and a smaller bulk density.In contrast,the unique advantages of asymmetric structures,e.g.,larger effective surface area,can signifi?cantly provide more active sites for substrates,thereby boost?ing their catalytic performance [19,28,138].For example,bowl?shaped carbon nanoparticles (BLCs) with large surface area (1108.3 m2g?1) and large pore volume (2.7 cm3g?1) have been applied as supporting materials of noble Pt for oxygen reduction reaction (ORR) in acidic media [28].Owing to the synergistic effect between the uniform bowl?like structure and uniform and stable loading of smaller Pt particles,the Pt/BLC electrocatalyst exhibits much higher electrocatalytic activity and stability.The mass current densities were 1.6 times for ORR as high as that of com?mercial Pt/C.In addition,the catalytic performance can be further improved by controlling the chemical compositions of catalysts [195—198].Lou’s group designed single?hole cobalt?/N?doped carbon hollow particles for oxygen reduc?tion reaction (Fig.12a) [36],which exhibited a superior elec?trocatalytic performance.Similarly,N or N and S co?doped bowl?like carbon nanoparticles also have been designed as electrocatalysts for oxygen reduction reaction [37,139].However,the above catalysts have a lower half?wave poten?tial compared with commercial Pt/C catalysts.Recently,Fe?embedded porous nanobowls were prepared for ORR (Fig.12b) [140].The obtained Fe3O4@PCN?800 catalyst had the higher half?wave potential (0.911 V vs.RHE) than Pt/C (0.845 V vs.RHE).On the other hand,the asymmetric structures also exhibit a higher electrocatalytic activity for the methanol electro?oxidation.Hollow carbon hemispheres were prepared as the support material for designing Pt nan?oparticle catalysts (Pt/HCHS) [19].Thanks to the unique asymmetric structure,which were more favorable for the dispersion of Pt nanoparticles,the obtained Pt/HCHS cata?lysts showed a superiority electrocatalytic activity for the methanol electro?oxidation in terms of the onset potential,current density,and stability in alkaline solution.Besides,benefiting from the unique structure advantages,the asym?metric structure as supporting materials can significantly improve the reactivity of single?metal electrocatalyst for the formic acid oxidation reaction.The high?quality carbon nanobowls (CNBs) with high surface area were designed as advanced supporting material to anchor Pd nanocrys?tals for formic acid oxidation reaction [138].The prepared Pd/CNBs nanohybrids exhibited much higher formic acid oxidation reaction activity and durability than commercial Pd/C electrocatalyst due to the uniform dispersion of the Pd nanocrystals,which were beneficial from the high surface area and unique structure advantages of bowl?like structure.

In addition to the above electrocatalysis,biphasic cataly?sis is another important catalytic reaction [199—203].The reactions are carried out in a biphasic mixture of two immis?cible solvents for biphasic catalysis.As a result,the emul?sion stability,efficient reaction interface,and recycling of catalysts are the major objectives of biphasic catalysis [86,204—211].The asymmetric structure exhibits irreplaceable application potential in biphasic catalytic because of tunable amphiphilicity,which can not only stabilize the emulsion and increase catalytic efficiency,but also effectively simplify the reaction process [212—214].In 2010,Crossley’s group reported the application of Janus?structured recoverable catalyst in two?phase interfacial catalysis for the first time [215].In this work,the emulsions’ stabilization and biphasic hydrodeoxygenation and condensation can be simultane?ously achieved by selectively depositing Pd onto either the hydrophilic silica oxide or the hydrophobic nanotubes of the carbon nanotube—silica.Since then,Janus nanoparticles with different structures were well developed as catalysts for two?phase interfacial catalysis [141].For instance,Liu’s group prepared snowman?like SiO2@PDVB/PS Janus cata?lysts with Au nanoparticles selectively modified on one or both heads to systematically investigate the catalytic per?formance through the reduction of 4?nitroanisole (Fig.12c) [142].Owing to the larger reaction area and good emulsi?fication property,the as?prepared Janus?type catalysts not only exhibited excellent catalytic activity in homogeneous reaction system,but also more efficient catalytic activity at water?in?oil emulsion interface.Among these asymmetric morphologies,the dumbbell?shaped Janus particles exhibit some unique advantages in exquisite controlling the chemi?cal reaction of the two?phase mixture because they tend to assemble in a single direction perpendicular to the oil—water interface [216].Pt?loaded dumbbell?shaped mesoporous car?bon—PMO Janus nanoparticles were prepared by the regula?tion of interface energy and used as catalysts for nitroarene reduction in water and toluene mixture solvents [120].The nitrobenzene was fully converted to aniline under the stir?free conditions after 6 h.The superior catalytic efficiency can be ascribed to the short diffusion distances,controlled reaction location,and large reaction interfacial area pro?vided by the unique dumbbell?shaped structure.Recently,dual?mesoporous Fe3O4@mC&mSiO2Janus magnetic solid catalysts with absolute spatial isolation of carbon and silica were designed by Zhao’s group (Fig.12d) [125].The result?ing Janus nanoparticles showed outstanding performances in biphasic reduction of 4?nitroanisole with 100% conversion efficiency after electively anchoring catalytic active sites into different domains.Importantly,the catalysts can be easily recycled due to the magnetic functionality.

Fig.12 Application of asymmetric structure in catalysis.a Single?hole cobalt?/N?doped carbon hollow particles for oxygen reduction reaction.Reproduced with permission from Ref.[36].Copyright 2017,Wiley—VCH.b Fe?embedded porous nanobowls for ORR.Reproduced with per?mission from Ref.[140].Copyright 2019,Elsevier B.V.c Application of the SiO2@PDVB/PS Janus particles in two?phase catalytic reaction.Reproduced with permission from Ref.[142].Copyright 2017,Elsevier B.V.d Application of the spatial isolation silica—carbon Janus structure complex in two?phase catalytic reaction.Reproduced with permission from Ref.[125].Copyright 2018,American Chemical Society

3.3 Biomedical Applications

With regard to biomedicine,the advantages of asymmetric nanoparticles lie in their possibility of integrating different functional components,structures,and even properties.The remarkable achievements of symmetric carbon? and silica?based nanostructures have greatly impelled the researchers to explore the biomedical applications of their asymmetric counterpart.Chen et al.prepared a bullet?like nanoparticle with a head of Fe3O4and a body of mesoporous silica for drug delivery [143].The magnetic Fe3O4enables the mag?netic field?guided tumor accumulation and enhanced the cel?lular uptake of Janus nanostructure,while mesoporous silica allows the efficient loading of antitumor drugs.Photosensi?tizers can also be loaded into the Janus Fe3O4—mesoporous silica,and Fe3O4was utilized for combining magnetic hyper?thermia with photodynamic therapy to potentiate the anti?metastatic immunotherapy [147].By replacing the Fe3O4segment with gold or silver nanoparticles,the resultant noble metal—silica asymmetric nanostructures are endowed with photothermal or antibacterial effect for synergistic photothermal/chemotherapy or antibacterial applications,respectively [113,144,145].Moreover,the excellent radio?sensitization and computerized tomography (CT) imaging capacity of gold are successfully imparted into the asym?metric nanostructure for multifunctional theranostics of tumor [115,116].To improve the delivery efficiency,lac?tobionic acid (LA) was also selectively conjugated onto the silica segments of octopus?type gold nanostar?mesoporous silica asymmetric nanoparticles.The obtained products not only possess high drug loading content,but also exhibit pH and near?infrared (NIR) dual?responsive release properties,which was used for actively targeted chemo?photothermal therapy (Fig.13a) [47].In comparison with silica?based nanoparticles,the biomedical applications of asymmet?ric carbon?based nanoparticles have rarely been reported.Wang’s group prepared a snowman?shaped nanostructure with ternary functional components of gold,Fe3O4,and car?bon by controlling interfacial energy and selective modifi?cation [51].Therefore,the multi?modality of CT/magnetic resonance (MR) imaging and chemo?photothermal syner?gistic therapy can be implemented in a single asymmetric nanoplatform.

These asymmetric nanoparticles can also exert its advan?tages in cargo delivery.Traditional mesoporous silica nanoparticles always show inferior loading efficiency for large?sized biomolecules like proteins and nucleic acids.To solve this problem,Qiao et al.developed bowl?shaped silica nanoparticles by nucleation control [146],which exhibited high loading capacity for plasmid DNA and great poten?tial for DNA delivery applications.The cellular uptake of nanocarriers is also essential for efficient drug delivery.In a recent work,Li et al.proposed a asymmetric nanotruck with rough silica as“engine”and periodic mesoporous orga?nosilica (PMO) rod as“trailer”for enhanced drug delivery (Fig.13b) [217].The silica head with rough surface sig?nificantly improves the intracellular uptake of the nanotruck compared with mesoporous silica nanoparticles with smooth surface.To realize precise therapy,upconverting nanoparti?cles (UCNPs) were also encapsulated into rough silica head for bio?imaging and on?demand drug release triggered by the NIR light.This ingenious design provides an inspiring paradigm of integrating the structural advantages in inde?pendent domains of asymmetric silica?based nanostructure.

In addition,the properties of asymmetric nanoparti?cles can be tailored to meet some specific requirements in biomedicine.Zhao et al.synthesized dual?compartment asymmetric silica nanocomposites with two segments of mesoporous silica and PMO for dual?drug delivery (Fig.13c) [122].More specifically,the hydrophilic prop?erty of mesoporous silica domains and hydrophobic property of PMO can accommodate two distinctly different spaces for separately loading hydrophilic and hydrophobic drugs without interfering each other.As a consequence,the bi?modal of heat and NIR light?triggered independent release of dual drug remarkably improves the cancer cell killing effect in comparison with single?triggered delivery system.Apart from the hydrophilicity/hydrophobicity,the surface of asymmetric nanostructure can provide different districts for specific functionalization.As an example,Vallet?Regí et al.reported the asymmetrically modification of MSNs with two targeted ligands,in which one side of folic acid binds with the receptors on cell membrane and another side of triph?enylphosphine aim to the mitochondria [96].This type of asymmetric modification ensures more specific and sequen?tial targeting process of nanocarriers from cell to organelle,showing noteworthy advantages over traditional symmetric nanocarrier with homogeneous surface.

Fig.13 Application of asymmetric structure in biomedicine.a PEG?Au?PAA/mSiO2?LA Janus NPs for synergistic actively targeted and chemo?photothermal therapy.Reproduced with permission from Ref.[47].Copyright 2016,Wiley?VCH.b Engine?trailer?structured nanotrucks for enhanced drug delivery.Reproduced with permission from Ref.[217].Copyright 2020,Elsevier B.V.c Dual?compartment asymmetric silica nanocomposites for dual?drug delivery.Reproduced with permission from Ref.[122].Copyright 2014,American Chemical Society.d MSNs?based nanomotor for thermomechanically percolating cell.Reproduced with permission from Ref.[148].Copyright 2018,Wiley—VCH

Another potential application of asymmetric nanoparticles was the exploration of nanomotors for active drug delivery.The asymmetric structure or selective modification ena?bled the self?propelled motion of nanomotors to fulfil the delivery task under complex biological conditions.MSN?based nanomotor was designed by selectively coated gold nanoshell onto the one side of silica surface (Fig.13d) [148].Upon NIR light irradiation,the half gold nanoshell converts the light into heat energy,generating a heat gradient on the Janus gold?mesoporous silica nanoparticle.Accordingly,the self?thermophoretic forces induced by anisotropic heating efficiently accelerate the internalization of Janus nanopar?ticles.Moreover,nanomotors powered by catalytic reaction have also been fabricated through selective deposition or modification of Pt or enzymes on MSNs [81,218,219].Several studies claim that the autonomous motion of asym?metric silica?based motors can significantly improve their performance on penetrating cell membrane or even tumor tissue [105,220,221].Based on these preliminary achieve?ments,asymmetric nanoparticles are expected to fully exert their structural advantages in the emerging field of nanomo?tor,which urgently requires the development of new type of asymmetric nanoparticles with diversified composition,morphology,and functionality.

3.4 Other Applications

Apart from the above?mentioned directions,asymmetric nanostructures have also shown great potential for other inter?esting applications,like logic gate and sensors.Logic gates,presenting specific output signals in response to input stimuli,have drawn intensive interest in the area of molecular?scale electronics,and chemical and biological computers [222].Generally,the fabrication of complex logic gates involves the integration of several gating systems and effectors in sin?gle nanoplatform.Asymmetric nanoparticles can perfectly achieve this goal,considering that Janus structures provide two different surfaces and enable conflicting functionali?zation process.Following this general concept,Pingarrón et al.prepared Janus gold?mesoporous silica nanoparticles as a“l(fā)ogic gate”controlled drug delivery system (Fig.14a) [149].The mesoporous silica segment can be served as a nanocarrier for cargo loading,and β?cyclodextrin (β?CD) was conjugated onto it as an acid?cleavable gatekeeper by supramolecular chemistry.On the side of the Janus struc?ture,glucose oxidase (GOx) and esterase were attached onto gold surface as two effectors of“l(fā)ogic gate.”Therefore,the GOx and esterase can generate acid microenvironment under the catalytic decomposition reaction of glucose and ester,respectively,which subsequently induced the dissolution of supramolecular gatekeeper and open the mesochannel for drug release.This design utilizes an“OR”logic gate to govern the drug release in the presence of either stimuli in the system.In comparison,the“AND”logic gate can ensure more precise drug release behavior,which only occurs when two inputs are both delivered.However,the application of Janus structure for an“AND”logic gate?based system has not yet been developed.Beyond the logic gate,Martínez?Má?ez and co?workers used the Janus gold?mesoporous silica to establish an interesting communication system based on the chemical reactions occurred on different compartments of the Janus structure [150].Two nanomachines were prepared for this interactive model of communication:for the first nanomachine (S1Gal),the mesoporous silica was loaded with (Ru(bpy)3)2+and modified with β?CD via disulfide bonds,and the gold side was immobilized with β?galactosidase;for the second nanomachine (S2GOx),the mesoporous silica segment was loaded with N?acetyl?L?cysteine and covered with β?CD through acid?sensitive bond,while the gold was immobilized with GOx.When the above two nanoma?chines encounter each other in solution,the communica?tion process can be initiated by the addition of lactose.The β?galactosidase on S1Galcatalyzes the hydrolysis of lactose into galactose and glucose.Subsequently,the resultant glu?cose diffused in the solution can be further decomposed to gluconic acid under the catalytic effect of GOx on S2GOx.The produced gluconic acid then makes the decrease in pH of the solution,causing the disassembly of supramolecular valves betweenβ?CD and benzimidazole and further result?ing in the diffusion out ofN?acetyl?l?cysteine from the pores of mesoporous silica.Afterward,the releasedN?acetyl? l?cysteine acted as a feedback signal to induce the cleav?age of the disulfide bonds on mesoporous silica segment of S1Gal.Finally,the uncapping ofβ?CD on S1Galinduced the release of loaded (Ru(bpy)3)2+into the system,which was considered as the only“output”signal for the communica?tion of two nanomachines.This work forcefully demonstrated the advantages of asymmetric silica nanoparticles on diverse functionalization.Nevertheless,those communications were all based on the chemical reactions in solution system.Many other stimuli,including light,heat,magnetic field or even ultrasound,can also be used to construct remotely controlled“l(fā)ogic gate”or communication system for more intelligent applications,which encouraged the development of novel asymmetric silica?based nanoparticles combined with dif?ferent functional materials.

It should be mentioned that symmetric nanoparticles often encounter difficulties on surface functionalization for sens?ing applications.For instance,nanoparticle?based sensors for capturing some biomarkers from cancer cells always required the conjugation of efficient targeting molecules to endow them highly specific binding capacity toward the certain cell lines.Additionally,attaching ligand or biomac?romolecules onto the surface of nanoparticles would impair the detecting efficiency of the sensors to some degree.As mentioned above,asymmetric nanoparticles perfectly solve this issue by offering different districts for conflicting modi?fications [223].For instance,Lu et al.recently developed a Janus?structured sensors for colorimetric detection of glu?cose (Fig.14b) [151].The Janus nanoparticles comprised two sides ofγ?Fe2O3and silica.The GOx was selectively modified onto silica surface by covalent bonds,while theγ?Fe2O3segment acted as a catalyst for the chromogenic reaction.The detection mainly involves two steps:the GOx on silica side catalyzed the decomposition of glucose into gluconic acid and H2O2,and the Fenton reaction between generated H2O2with theγ?Fe2O3to generate products of distinct colors.The Fenton reaction can produce hydroxyl radical and further oxidize the tetramethylbenzidine (TMB) into a blue product.Therefore,the developed asymmetricγ?Fe2O3—SiO2nanoparticles can not only accomplish the analytic mission,but also enable easy separation and recy?cling of the nanoparticles due to the magnetic response of Fe2O3hemisphere.

Fig.14 Application of asymmetric structure in others.a Application of Janus gold?mesoporous silica nanoparticles in logic gate.Reproduced with permission from Ref.[149].Copyright 2014,American Chemical Society.b Application of Janus particles in sensors.Reproduced with permission from Ref.[151].Copyright 2015,American Chemical Society

4 Summary and Outlook

In recent years,asymmetric carbon? and silica?based nano?materials have aroused more and more attention due to their attractive structure and composition advantages.The asymmetric structures with ingenious adjustability not only exhibit larger effective surface area accompanied with more active sites,but also are an ideal choice for designing“nano?intelligent systems”based on a single asymmetric particle.After decades of efforts,some preliminary results have been achieved in the design of asymmetric structures.Here,we mainly review the recent advances about the basic design principles and synthesis methods of asymmetric carbon? and silica?based nanomaterials.Meanwhile,the applications of different asymmetric structures in energy storage,cataly?sis,and biomedicine are discussed,and the performance advantages brought by their structures are also presented.Although some significant progress has been made in the design and application of asymmetric carbon? and silica?based nanomaterials,their development is still in the ini?tial stage,and many challenges still need to be overcome (Fig.15).

Fig.15 The main challenges and the future development prospects of asymmetric carbon? and silica?based nanomaterials

The first one is about the exploration of simple,efficient and universal synthesis strategies.Although some meth?ods have been used to prepare different asymmetric struc?tures,the current preparation techniques are relatively con?ventional and still have some inherent disadvantages.For instance,although the soft?templating route is relatively simple in experimental process,the resulting particles are usually large in size,often in the order of microns,while the hard?template method generally contains the tedious preparation process,resulting in high costs.Additionally,the current asymmetric structures,especially the carbon?based nanoparticles,are mainly limited to the simple bowl?shaped structure.How to easily and efficiently prepare the asym?metric structures with more complex morphology and func?tionalities is highly desirable.Furthermore,in most common methods for preparing asymmetric structures,only a small number of products can be got at a time.Meanwhile,the cost is another major problem need to be considered in the large?scale production.Thus,it remains a huge challenge for the preparation of asymmetric structures with uniform mor?phology and high yields.As a result,it is very meaningful to develop some new,facile,high?efficient,low?cost,large?scale routes to fabricate the high?performance asymmetric nanomaterials.

The second one is to further understand the growth pro?cess and formation mechanism of asymmetric nanomateri?als.Limited by the current characterization methods and synthesis techniques,although some consensus has been reached,many assembly mechanisms remain unclear.There?fore,it is necessary to comprehensively observe the growth process of materials by means of in situ characterization techniques,such as in situ electron microscopy,cryo?elec?tron microscopy,synchrotron radiation,and other advanced characterization methods.In turn,a deep understanding of the formation mechanism will help us better regulate the structures of materials in a controlled manner.Additionally,with the rapid development of computer science,some new structures or new methods can be predicted through theo?retical simulations and calculations,which can provide a theoretical guidance for designing of some new and unique asymmetric structures.

Third,the function?oriented preparation should be the focus of future research to maximize the performances of asymmetric nanomaterials.Through the establishment of theoretical models,the internal relation between the struc?ture and performance should be further explored by system?atically investigating the influence of particle size,composi?tion,configuration and other factors on their performance,which would better serve the design of the asymmetric structures.On the other hand,designing customized asym?metric structures to meet specific application requirements is of importance.For example,for the application of energy storage and conversion,it usually requires that the structure possesses larger effective surface area,higher packing den?sity,better electrical conductivity,and excellent mechanical stability.In terms of biomedical application,it is necessary to fabricate asymmetric structure with small size,good biocompatibility,and easily functional inside and outside surfaces.The last but not least is to design asymmetric structures with more complex structures and components,thereby extending their application prospects.That needs the researchers in various fields to merge their expertise and cooperate together to explore new application opportunities.

AcknowledgementsWe appreciate the financial support from the Shuguang Program supported by Shanghai Education Develop?ment Foundation and Shanghai Municipal Education Commis?sion (18SG035),and Shanghai Engineering Research Center of Advanced Thermal Functional Materials (Shanghai Polytechnic University).

FundingOpen access funding provided by Shanghai Jiao Tong University.

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