Progresses in Template Synthesis and Applications of Hollow Materials

YANG Jiye，SUN Dayin，WANG Yan，GU Anqi，YE Yilan，DING Shujiang，YANG Zhenzhong*

（1.Institute of Polymer Science and Engineering，Department of Chemical Engineering，Tsinghua University，Beijing 100084，China；2.Engineering Research Center of Energy Storage Materials and Devices，Ministry of Education，Department of Applied Chemistry，School of Chemistry，Xi’an Jiaotong University，Xi’an 710049，China）

Abstract Hollow materials are emerging as functional materials with unique characteristics in tunable cavity，high specific active surface area and enhanced mass transfer.Janus hollow materials are readily derived upon spatial compartmentalization of multiple components thus functions.Template synthesis is highly efficient to synthesize the hollow materials with tunable microstructure and composition.In the first part of this review，recent advances in the template synthesis of the representative hollow materials are concisely summarized.In the second part，typical applications of the hollow materials in catalysis，energy storage，oil/water separation and drug delivery are introduced.This review will end with a perspective to call for more efforts in the development of new methods toward high performance hollow materials with tailored properties.

Keywords Hollow material；Janus；Template synthesis；Catalysis；Oil/water separation；Drug delivery

1 Introduction

In recent years，hollow materials have attracted extensive attentions owing to their unique structures and performances［1—5］.Hollow materials are advantageous in adjustable pore structure，pore size and surface chemistry，which are conducive to controlled release and enhanced mass transfer.Janus features are acquired when the hollow materials become asymmetric with spatial compartmentalization of multiple compounds［6］.It is key to develop methods to synthesize the Janus hollow materials with tunable composition and topology.Among the various synthetic strategies［7］，template methods are easy yet highly effective to synthesize the hollow materials.Hard template method involves protection and deprotection for precise control of the hollow materials.Soft template method is flexible to achieve the hollow materials with tunable shapes.In the selftemplate approach，many strategies including the Kirkendall effect［8，9］，primary electron substitution reaction and chemical etching are feasible to fabricate the hollow materials especially metal systems［10］.Core-shell templates are usually used to derive hollow materials by chemical corrosion［8］，ion exchange，self-assembly，and thermal decomposition.Many materials including zeolites，mesoporous silicates，colloidal particles，and metal oxides serve as the candidate templates.

In this mini-review，the template synthetic strategies to prepare various hollow materials and the representative applications are concisely summarized.We wish to provide some guidelines to design and generate Janus hollow materials，and call for more efforts to develop new methods to large scale fabricate the functional materials with tailored performances.

2 Template Synthesis of Hollow Materials

2.1 Hard Template

Hard template method is straightforward and effective to prepare the hollow materials with tunable composition and shape.Four main steps are involved：（1）preparation of the hard template framework；（2）surface modification to introduce functional groups；（3）deposition of desired materials at the hard template；（4）removal of the template to achieve the hollow material.

2.1.1 Silicon Dioxide Template Silicon dioxides possess excellent mechanical and thermal stabilities，which are widely used as templates to achieve hollow materials.The abundant hydroxyl groups（Si—OH）at the template surface are conducive to favorable growth of a variety of materials due to specific interactions，providing a versatile template synthesis of the hollow materials.A hollow sphere of poly（3，4-ethylenedioxythiophene）（PEDOT）was synthesized by using monodisperse silica sphere template［11］.Ti（OH）x@SiO2coreshell sphere was also achieved by deposition of a layer of Ti（OH）xat the silica sphere surface，and a sea urchin-like hollow sphere of Na2Ti3O7was derived upon the one-step hydrothermal reaction in the presence of H2O2and NaOH［12］.Similarly，a representative pH-responsive Janus hollow sphere was synthesized against the silica sphere template［Fig.1（A）］［13］.Amino groups were introduced at the template sphere surface by modification with 3-aminopropyl dimethylethoxysilane to assist the grafting of polycaprolactone at the sphere surface by ring-opening polymerization（ROP）in the presence of glycidol intermediate.Bromoisobutyryl bromide groups were grafted at polycaprolactone for further grafting of poly［2-（diethylamino）ethyl methacrylate］（PDEAEMA，a pH responsive polymer）by atom transfer radical polymerization（ATRP）.The Janus hollow sphere with hydrophobic poly（ε-caprolactone）（PCL）at the interior surface and pH responsive PDEAEMA at the exterior surface of the shell was achieved after the selective etching of the silica template with ammonium fluoride.The hydroxyl groups at the silica sphere surface could be used to terminate cationic polymer chains，providing a simple way to grow polystyrene（PS）coating at the sphere by cationic polymerization［Fig.1（B）］［14］.Especially，a hairy hollow sphere was achieved when the cationic polymerization was performed in a PS poor solvent.When the exterior surface was sulfonated to introduce sulfonic acid groups，the PS layer at the interior surface was intact to support the sulfonated polystyrene gel layer.After removal of the silica template，the Janus hollow sphere was derived.The shell is composed of a hydrophilic sulfonated polystyrene at the exterior surface and a hydrophobic polystyrene surface at the interior surface.In addition，titania could be preferentially grownviasol-gel process by specific interactions.A series of hollow titania spheres with varied crystallinity was achieved upon thermal treatment at varied temperature.

Fig.1 Synthesis of the Janus hollow spheres of PCL-PDEAEMA(A)[13]and polystyrene-sulfonated polystyrene gel(B)[14]

2.1.2 Polymer Template Polymer templates are attractive due to the easiness to introduce functional groups for favorable growth of materials and remove either by etching or calcination at high temperature.Yanget al.［15］firstly proposed a template synthesis of monodisperse hollow titania spheres with continuously tunable shell thickness and cavity size by using sulfonated polystyrene（sPS）gel sphere template［Fig.2（A）］.It is key that the thickness of the sPS gel shell could be readily controlled within the whole particle size by simply control of sulfonation time and temperature.Desired materials such as titania could be preferentially grown within the sPS gel shell by specific interactions.No byproducts were found in the serum，meaning that the synthesis was facile and highly efficient without further purifications.The titania hollow spheres were achieved upon removal of the template by either dissolution or calcination.The titania/gel shell could become coarsening while the sol-gel process was performed under an electric field，and a porous Janus hollow sphere was derived after removal of the polymer template.This approach is versatile to allow a broad adjustment of composition and thickness of the shell.Similarly，other functional hollow spheres of polyaniline（PANi），silica and iron oxides were achieved by the specific interactions［Fig.2（B）］［16］.In the case of PANi hollow sphere，acid-base interaction was responsible for the favorable growth of the composite shell，while the ion exchange effect was responsible for the favorable growth of iron oxides within the sPS gel.In addition，the example hollow composite sphere of PANi and iron oxides was derived which was capable to screen electromagnetic waves.The commercially available polystyrene hollow sphere could serve as an interesting template owing to the presence of interior gel surface and transverse channels through the shell.Therefore，functional materials could be sequentially grown in the order from the interior surface，within the channels and at the exterior surface of the shell.Accordingly，the double-shelled functional hollow spheres were obtained after removal of the polymer template.The two shells are bridgedviathe pillars formed within the transverse channels.It is noted that the exterior surface became coarsening with the presence of the pillar protrusions.The unique structure would be interesting to mimic biostructures for enhanced interactions with targets.Hollow double-shelled or multishelled structure exhibits unique properties such as enhanced mechanical strength and preserved permeability.Electromagnetic wave or light could reflect between shells to achieve frequency tuning or sensing.Thus，it is highly demanded to develop facile approach to synthesize hollow double-shelled or multishelled structure.Upon sulfonation of the polystyrene hollow sphere，the shell was converted into sandwich-structured sPS-PS-sPS［Fig.2（C）］［17］.Functional materials including TiO2，F(xiàn)e3O4and PANi could be preferentially grown at the two sPS gel surfaces in one step.As a result，double-shelled functional hollow spheres were obtained in one step after removal of the polymer template.Shell thickness，gap distance between the shells and the cavity size of the double-shelled hollow spheres are greatly tunable，which is significant to improve light harvest efficiency by multiple scattering.

Fig.2 SEM and TEM images of the representative titania hollow spheres with tunable shell thickness and cavity size(A)[15],preparation of the composite capsules and the corresponding hollow spheres of PANi and silica(B)[16] and formation of the double-shelled hollow sphere against sPS-PS-sPS sandwiched hollow sphere template(C)[17]

It is interesting to control microstructure of the Janus hollow spheres.Against a polymer sphere of polybenzoxazine（PB），the negatively charged sol of tetraethoxysilane（TEOS）and cetyltrimethylammonium bromide（CTAB）was prone to deposition onto the sphere to form a core-shell sphere［Fig.3（A）］［18］.The Janus NxC@mSiO2hollow sphere with a porous silica shell was achieved by further pyrolysis in Argon.The shell is composed of a hydrophilic exterior surface and a hydrophobic interior surface.By using a PS hollow sphere template，divinylbenzene（DVB）and vinylbenzenechloride（VBC）were polymerized within the PS shell by swelling emulsion polymerization to form a crosslinked shell of PDVB/PVBC［Fig.3（B）］［19］.The amineterminated polyethylene oxide（PEO）could be conjugated at the exterior surface of the PDVB/PVBC hollow sphereviathe reaction between amine and benzyl chloride.The Janus hollow sphere of PDVB/PVBC-PEO was achieved upon dissolution of the linear PS.Meanwhile，the shell became porous to facilitate mass transfer.Functional polymers for example poly（N-isopropylarylamide）（PNIPAM，which is a thermal responsive polymer）were readily grafted at the interior surface of the shell by ATRP to derive a responsive Janus hollow sphere of PNIPAM-PDVB/PVBC-PEO.PNIPAM and PEO are responsible for the thermally triggered loading/release and good dispersion in water，respectively.Besides the spatial compartmentalization of varied compositions at the two surfaces of the shell，snowman-like Janus hollow spheres constructed with an asymmetric shape are of great significance with an additional compartmentalization of compositions at the contour surface.Against a highly crosslinked hollow sphere of polyacrylonitrile（PAN），a snowmanlike sphere was synthesized by swelling emulsion polymerization of styrene/DVB［Fig.3（C）］［20］.The internal elastic stress in the PAN shell arisen from the crosslinking was conducive to the protrusion of crosslinking PS thus formation of the lobe while the stress was released.It should be noticed that the snowman-like sphere is not strictly Janus although the contour is asymmetric in shape.The crosslinked PAN portion should contain some PS due to the incomplete phase separation although the PS lobe contains no PAN.It is key to selectively modify the PAN portion to introduce functional groups such as poly（acrylic acid），which could further induce favorable growth of functional materials by specific interactions to form a coating.As a result，the effect of residual PS within PAN was screened，meanwhile the composition was broadly tunable.The snowman-like sphere became strictly Janus both in shape and wettability.What’s more，shell of the hollow portion is composed of inorganic materials at the exterior surface and PAN/PS at the interior surface，implying the hollow portion is Janus as well.Thanks to the presence of the hollow portion，density of the snowman-like Janus spheres could be greatly tunable thereby.The seeded emulsion swelling polymerization strategy was powerful to obtain other Janus spheres with tunable contour such as acorn-like and elephant trunk-like［21］.

Fig.3 Schematic preparation of a multifunctional Janus hollow sphere of NxC@mSiO2(A)[18],illustrative synthesis of the thermally responsive Janus porous hollow sphere of PNIPAM-cPVBC-PEO(B)[19],and schematic synthesis of the snowman-like hollow sphere with dually Janus features(C)[20]

2.1.3 Metal Template In general，solid metal particle template synthesis is advantageous in coating diversify materials onto the particles and easy removal by various etching methods to tune composition and microstructure of the derived hollow spheres.A hollow metal nanoparticle of Ru was achieved by using monodisperse Ni nanoparticle template，in which oleic amine（or oleic acid）and aqueous HNO3were used as the capping agent and etching agent respectively［22］.Similarly，a hollow nanostructure of Pt was achieved against the Ag nanowireviagalvanization［23］.By using Fe3O4magnetic particle template，a Janus hollow sphere was achieved containing PNIPAM at the interior surface of the shell and Cl-based ionic liquid（IL）moiety at the exterior surface［Fig.4（A）］［24］.A magnetic particle core with tunable size was thus encapsulated inside the Janus hollow sphere after stepwise etching the nanoparticle of Fe3O4with hydrochloric acid.The example AlSi10alloy particles could be used as hard templates to construct coral-like Janus hollow spheres by stepwise etching Al followed by the orthogonal modification to introduce functional materials［Fig.4（B）］［25］.It is key that the silicon oxide coated silicon skeleton was derived while selective etching Al from the alloy matrix，providing the main bone of the coral-like Janus hollow spheres.

Fig.4 Illustrative synthesis of the IL/PNIPAM magnetic Janus hollow sphere(A)[24] and illustrative synthesis of the Janus coral-like porous sphere by stepwise de-alloying of the AlSi10 alloy sphere and subsequential modification(B)[25]

2.1.4 Carbonaceous Template Carbonaceous templates are easily achieved by carbonization of a number of organic substances such as sugars and polymers，which contain diversify functional groups including carbonyl groups，carboxyl groups，hydroxyl groups，etc.These functional groups could assist the construction of more complex structuresviaelectrostatic adsorption of metal cations.The example Al-stabilized CaO-based hollow microspheres were thus prepared against carbon microspheres［26］.Starting from carbon spheres with a smooth surface，a series of mixed metal oxide porous hollow spheres（ZnCo2O4，NiFe2O4and ZnSnO3）was synthesized by a favorable adsorption and calcination［27］.A hollow sphere of MgO was prepared against a mesoporous carbon template upon combustion in air［28］.An interesting flower-shaped hollow sphere of NiCo2O4/carbon was prepared with a high specific surface area［29］.A flaky Janus hollow structure was obtained by sequential coating titania and mesostructured silica onto the graphene template by the corresponding sol-gel processes［30］.The intermediate titania is significant for the homogeneous coating of silica.In comparison，silica was found in the continuous phase after the sol-gel process in the absence of the intermediate titania layer onto graphene.Porous silica coating was easily generated upon feeding porogens such as cetyltrimethylammonium bromide（CTAB）during the sol-gel process.PEO was further conjugated at the exterior surface of mesostructured silica coating by using the corresponding silane coupling agent.A porous flaky Janus hollow material was obtained after removal of CTAB and titania by dissolution.

2.2 Soft Template

The soft templates including microemulsions，micelles，vesicles，bubbles could be used for the synthesis of hollow materials.In comparison with the hard templates，the soft templates could be much easily removed.The amphiphilic interfaces between oil and water are naturally Janus，serving as temporal soft templates to construct hollow Janus materials.Especially，the compartmentalized moieties at the interfaces are conducive to effective generation of Janus membranes by materialization.It is noted that the materialization processes against Janus interfaces are highly dynamic under various conditions（ionic strength，stirring，pH，solvent，etc.）［31］.

2.2.1 O/W Microemulsion Template Microemulsions are composed of two immiscible liquids at the characteristic length scale of 10—100 nm［32］.Specific interactions including electrostatic attraction and hydrogen bonding with the precursors assist the favorable growth of inorganic hollow nanostructures by the microemulsion template synthesis.A hollow sphere ofα-Fe2O3was generated upon hydrothermal treatment of the glycerol/water microemulsion［33］.The strong coordination between glycerol and metal ions（Fe2+）at the droplet surface was conducive to the formation of primary Fe2O3nanoparticles，which were further reassembled and crystallized to form the hollow spheres.Against the O/W emulsions stabilized with hydrolyzed styrene-maleic anhydride alternating copolymer［34］，a self-organized sol-gel process occurred at the interface to form a silica based Janus hollow sphere［Fig.5（A）］.The hydrophilic and hydrophobic groups from the silane mixture in the oil phase are self-organized at the interface.Especially，the self-organization of the silane with amine group is directed by the acrylic acid groups at the interface.In the presence of poly（ethylene glycol）-b-poly（L-lysine）-b-poly（styrene）（PEG-b-PLL-b-PS）triblock terpolymer，a stable emulsion was formed with a well-defined amphiphilic interface［Fig.5（B）］［35］.A Janus hollow sphere was generated by the favorable growth of newly hydrolyzed tetramethoxysilane with the negatively charged PLL moiety at the interface.The Janus hollow sphere（JHS）contains hydrophilic PEG at the exterior surface and hydrophobic PS at the interior surface.Besides molecular surfactants，those solid particles of properly hydrophilic or hydrophobic performance are highly active to form so-called Pickering emulsions.A paraffin-in-water Pickering emulsion was formed in the presence of amine modified silica sphere［Fig.5（C）］［36］.And a Janus hollow sphere was prepared by interfacial copolymerization of acrylamide（AM）in the water phase and divinylbenzene（DVB）in the paraffin phase.After etching the silica spheres from the composite shell，transverse channels within the Janus shell were generated.A more complex Jellyfish-like Janus hollow structure was prepared by the O/W emulsion template synthesis［Fig.5（D）］［37］.In the first stage，a hemisphere was formed at the emulsion interface by polymerization of styrene in the paraffin emulsion droplet.The individual PS primary particles were prone to coalescence by Oswald ripening in the paraffin phase and diffusion toward the interface due to the Pickering effect.The convex side of the hemisphere was immersed in the paraffin phase，while the planner side was exposed toward the aqueous phase at the interface.A Janus hemisphere of PS@PAM was generated by further grafting PAM at the planner side when the monomer of AM was fed in the aqueous phase.The belly part was formed when the excess monomers of styrene and DVB in the paraffin droplet were further copolymerized at the interface with additional hydrophilic monomers such as acrylic acid（AA）in the continuous aqueous phase.The jellyfish-like hollow sphere was Janus both at the contour surface and within the hollow structure.

Fig.5 Schematic synthesis of the Janus hollow sphere by emulsion interfacial self-organized sol-gel process(A)[34],schematic illustration of the biomimetic synthesis of JHS(B)[35],schematic synthesis of the Janus macroporous hollow sphere by using Pickering emulsion template(C)[36]and schematic synthesis of the jellyfish-like Janus hollow sphere by two-step emulsion interfacial polymerization(D)[37]

2.2.2 W/O Microemulsion Template W/O microemulsions are known as inverse emulsions，which are also promising as templates to achieve Janus hollow spheres.An example silica porous hollow sphere was synthesized by using the W/O microemulsion template［Fig.6（A）］［38］.The W/O emulsion was achieved by emulsifying the mixture of tetraethoxysilane(TEOS)/cyclohexane solution and ammonia contained water in the presence of nonionic surfactants alkylphenol polyoxyethylene ether(TX-4).The silica porous hollow structure was generated through the diffusion and reaction of TEOS at the W/O interface.The pore size and specific surface area could be further adjusted by calcination.A tadpole-like Janus hollow material was prepared by using the W/O microe-mulsion template［Fig.6（B）］［39］.An inversion emulsion was formed by emulsifying 1，2-bis（triethoxysilyl）ethane contained 1-pentanol in water in the presence of polyvinylpyrrolidone（PVP）.An organosilane hybrid film was formed at the interface by the sol-gel process.While the silica became matured progressively along with the sol-gel process，a kind of internal stress was arisen and accumulated thereby.At a certain maturation degree，the internal stress reached a critical value and started to release resulting in the formation of a tail at one side from the droplet interface.The tadpole-like hollow structure could be adjusted by controlling the hydrolysis rate and droplet size.In the case of a mixture ofn-octyltriethoxysilane and 1，2-bis（triethoxysilyl）ethane，a Janus shell was formed by the sol-gel process.Functional groups were further introduced to the exterior surface of the shell by selective modifications.On the other hand，functional materials such as Pb and Fe3O4nanoparticles could be preferentially encapsulated inside the cavity of the hollow spheres.

Fig.6 Schematic synthesis of the silica hollow sphere by W/O inverse emulsion template(A)[38] and schematic formation of the tadpole-like nanotube and the corresponding Janus one(B)[39]

2.2.3 Micelle/Vesicle Template The self-assembled micelles or vesicles by amphiphilic molecules serve as efficient templates to induce formation of hollow spheres by favorable adsorption of ionsviaelectrostatic attraction and hydrogen bonding［40］.A mesoporous hollow carbon sphere with Pt nanoparticles（NPs）incorporated at the interior wall（Pt@HCmeso）was synthesized by a dual-template method［41］.Pt NPs were firstly deposited onto a monodisperse silica sphere（SiO2）to form Pt/SiO2.In the following step，the precursor of micelle-polydopamine（micelle-PDA）was coated onto the Pt/SiO2to achieve the core-shell structure of Pt/SiO2@micelle-PDA.The subsequent carbonization and etching with KOH allowed the conversion of the micelle-PDA shell into mesoporous carbon and removal of the silica core toward the composite hollow sphere.Bubbles are also effective as soft templates to synthesize hollow spheres.The general route involves deposition of precursors onto the bubble template and further aggregation to form a shell［42］.Mn3O4-incoporated carbon hollow nanospheres were prepared on elastic titanium foil substrates by using the unique template of sulfur-based bubbles in an ethanol flame［43］.The effective process requires simple burning in a flame and annealing in air without complicated steps and sophisticated apparatus.

2.3 Self-template

In contrast with the core-shell approach toward the synthesis of hollow spheres，self-template synthesis is advantageous without the requirement of removal of the templates.The synthesis is simple for large scale production of hollow spheres［44］.Many strategies including Ostwald ripening，Kirkendall effect，electrochemical replacement，microgel and selective etching are feasible for the self-template synthesis.

2.3.1 Ostwald Ripening Ostwald ripening is a classical physical phenomenon that describes dissolution of smaller crystalline or solute particles and re-organization to form larger crystals or particles.Accordingly，small crystals in the central region are readily dissolved and re-organized outwardly to form a hollow structure.No removal of template is required which differs from the conventional template method［44］.The as-prepared hollow structures could preserve the original contour.The example hollow sphere of TiO2was achieved by fluoride mediated Ostwald ripening［Fig.7（A）］［45］.Morphology of the hollow sphere was varied by using different fluorides whose specific surface area was different as well.Similarly，a multi-shelled hollow sphere of Cu2O was prepared by multi-step Oswald ripening method［46］.In addition，continuous Oswald ripening could be used to achieve multi-shelled hollow sphere of Cu2O［47］.The Cu2O core could be dissolved and deposited continuously as a crystalline species to form a multiple-shelled layer.What’s more，eccentric multi-shelled hollow spheres could be achieved by the continuous Ostwald ripening process.

Fig.7 Schematic fluoride-induced self-transformation of TiO2 toward hollow structures in different ion environments(A)[45]and formation of Janus microgels and microshells(B)[56]

2.3.2 Kirkendall Effect The Kirkendall effect is related with diffusion variance between different metallic solids.As a result，hollow structural defects are generated inside the solid due to excessive loss of atoms.In 1947，Kirkendall reported the experiment by plating copper on top of brass and placing two rows of Mo wires at the interface.After annealing at high temperature，some small holes were found in the brass while the interface layer became mesoporous.The effect was attributed to different diffusion rates of Cu and Zn atoms.In 2004，a hollow sphere of Co was firstly achieved by using the Kirkendall effect［48］.Treatment the crystalline Co nanoparticles with S in liquid phase led to formation of the hollow structures.The similar hollow structures could be obtained by treatment with mixed gas of O2and Ar and Se.Owing to the varied diffusion coefficients of oxygen and sulfur to cobalt，diffusion of Co toward the exterior oxide layer from the crystalline nanoparticle was much faster at a higher temperature，and a large number of cavities were generated inside the nanoparticle.The Kirkendall approach is general.A unique hollow nano-octahedra of NiO was synthesized against NiSe2nano-octahedra by the hydrothermal process［49］.During the oxidation at high temperature，the outward diffusion of nickel cation and selenium from NiSe2was faster than the inward diffusion of oxygen to form the hollow NiO nano-octahedra.Solid palladium nanocrystals were easily converted into hollow palladium nanocrystals by insertion and extraction of phosphorus［50］.Efficient extraction of phosphorus through the oxidation is key to drive the outward diffusion of phosphorus from the palladium phosphide crystals thus the inward diffusion of vacancies and further agglomeration to form the large voids.The repeated Kirkendall cavitation process facilitates progressive expansion of cavities into hollow nanocrystals.A hollow nanoparticle of cobalt oxide（Co3O4）with abundant oxygen vacancies was achieved while concentration of the oxygen vacancy defects could be increased progressively along with the Kirkendall progress［51］.

2.3.3 Galvanic Replacement Galvanic replacement is a simple and easy way to prepare hollow structures based on the electronegativity difference of metal atoms toin situgenerate redox reactions in the systems.The process could be controlled as a function of metal ion concentration，time and temperature of the Galvanic replacement.A hollow structure of Au was achieved by treating Ag with Au3+for the redox reaction.In the presence of Au3+，metal Ag was converted into Ag+upon losing an electron and became readily dissolved，while Au3+was reduced into gold attaching to the outer layer of Ag［52］.The reaction ended when all Ag was completely converted into Ag+to achieve the hollow structure of Au.A hollow nanoparticle of Ag/Au was prepared by the similar Galvanic replacement［53］.A hollow nanoframe was achieved by the Galvanic replacement reaction with SnCl4·5H2O in ethylene glycol against the PtNi3nanoparticle with a thin and incomplete tin layer covering the platinum-rich edges［54］.The Galvanic replacement rate was controlled by varying the concentration of solvent and tin ion，and structure of the hollow nanoframe was greatly tunable.A hollow gold-based nanoalloy decorating with platinum or palladium was synthesized by ambient electrochemical substitution against a hollow nanoparticle of gold by using the intermediate of mercury.Hollow structure of the gold nanoparticle was essential to increase the number of surface active sites of the obtained polymetallic nanoalloys，while introduction of mercury was conducive to elimination of the effect of platinum/palladium and gold in the electrochemical reaction to further improve alloying and preserve the hollow nanostructure［55］.

2.3.4 Microgel Template Gels with diversify functional groups are sufficiently strong to support the contour shape，which is important in the template synthesis of hollow materials.A Janus hollow structure of functional microgel was synthesized from crosslinkable oligomers by using the microfluidic technology［Fig.7（B）］［56］.The resultant microgel particles are compartmentalized into two distinguishable regions，which could be controlled with flow rate during the reaction process.Moreover，the hollow microcapsules with two different sides（Janus shell）were formed by using the twin-screw method.The Janus microgel contained ferromagnetic additives is capable to drive by remote stimuli.

2.3.5 Selective Etching Selective etching interior part of a nanoparticle provides a way to generate hollow structures，in which the original particle size and crystallographic structure could be preserved during the carving process.In principle，the selective etching process is alike to etching template in the hard template synthesis.The selective etching is a self-template synthesis.Starting from a textured homogeneous material，the etching process usually occurs inwardly.Therefore，it is important to generate differences in solubility or chemical stability within the target materials to accelerate etching in the internal region.A mesoporous hollow sphere of SiO2was generated by selective etching with sodium hydroxide in the presence of a protective agent of PVP［57］.It is explained that the internal region of the silica particle was less compact thus prone to etching.A nanocage with a pyramidal surface was derived from nickel-cobalt Prussian blue（Ni-Co PBA）［58］.The Ni-Co PBA solid cube was enriched with more lattice defects along the diagonal direction of the volume.As a result，this direction was preferentially etched with alkaline agents to form the cubic hollow cage.

3 Applications of the Hollow Materials

3.1 In Catalysis

Hollow structural materials are advantageous in large specific surface area，fast mass and charge transfer，low density and high structural stability，which are regarded as ideal electrocatalysts in energy storage and transformation［59］.As the oxygen reduction reaction（ORR）catalysts，the hollow materials are highly stable and easy to capture oxygen.They can be either directly used as active catalysts，or combined with other active components to achieve a higher catalytic activity.

3.1.1 Direct Applications Pt has been most widely studied as an ORR catalyst.The hollow structure can significantly improve the utilization efficiency and accelerate the mass transfer to enhance ORR performance.A hollow sphere of Pt was achieved after acid leaching the pyrolysed product of Pt-Fe doped resorcinol-formaldehyde resin［Fig.8（A）—（C）］［60］.By properly adjusting the ratio of Pt and Fe，the sample shows an excellent ORR performance（E1/2：0.903 Vvs.RHE）and the mass activity（MA，0.69 A/mgPt）outperforms the commercial Pt/C catalyst（E1/2：0.876 Vvs.RHE，MA：0.3 A/mgPt）［Fig.8（D）and（E）］.One-dimensional hollow nanochain of PtFe was preparedviathe galvanic replacement reaction between the nanochain of Fe and H2PtCl6［Fig.8（F）—（H）］［61］.The chains were assembled by the nanocages with a wholly porous and open architecture to provide highly exposed active sites.The resultant structure shows a higher mass activity and specific activity of 1.49 A/mgPtand 4.85 mA/cmPt2，which are 7.45 and 12.44 folds higher than the commercial Pt/C catalyst.After 30000 cycles in the durability test，the new catalyst remained highly stable with a negligible activity decay［Fig.8（I）and（J）］.

Fig.8 TEM image(A),STEM image(B)and line scanning of the PtFe(0.9)-C(C),LSV polarization curves of the sample(D),mass activity,specific activity,and electrochemical surface area measured at 0.9 V and half-wave potential versus commercial Pt/C(E)[60],TEM and EDS images of the PtFe-HNC(F—H),LSV curves(I)and MA and SA results before and after durability test of the PtFe-HNC/C(J)[61]

Transition metal-based materials with a low cost have proved as highly efficient ORR catalysts.The metal oxides are particularly promising owing to their inherent high activity，excellent durability and abundant natural resources.However，drawbacks in the poor electrocatalytic activity，low conductivity and active area are the obstacles in the way for their potential applications.A hollow nanoparticle of NiCo2O4was prepared by a facile template-free solvothermal approach followed by the annealing treatment［Fig.9（A）—（C）］［62］.The presence of interconnected hierarchical porous structure and rich oxygen vacancy renders the hollow NiCo2O4nanoparticle outstanding electrocatalytic activity for ORR in alkali［Fig.9（D）and（E）］.

Fig.9 SEM images of the NiCo2O4-450-Vo at two magnifications(A,B),TEM image of the NiCo2O4-450-Vo(C),LSVs of different catalysts in O2-saturated 0.1 mol/L KOH(D),chronoamperometric curves(normalized to initial current)of the NiCo2O4-450-Vo and Pt/C at 0.6 V vs.RHE(E)[62]

At present，non-metal carbon-based nanomaterials with low cost，high conductivity and various structure have shown great potentials as efficient ORR catalysts in alkaline media.For carbon-based materials，heteroatom doping such as N is necessary to break neutrality of the carbon matrix and generate active sites.An example nitrogen-doped hollow carbon polyhedron（NHCP）was synthesized by using NaCl as the template［Fig.10（A）—（C）］［63］.The hollow structure with a carbon shell provides larger surface area to form more active sites.LSV shows thatE1/2of the NHCP is 0.86 Vvs.RHE（equivalent to 20%Pt/C：0.88 Vvs.RHE），which are above most of the reported non-metal carbon based electrocatalysts［Fig.10（D）and（E）］.A hollow nitrogen-doped carbon nanoflower（h-N-CF）containing a nanoplatelet was synthesized by using 3，5-diaminobenzoic acid-1，3，5-benzenetricarboxaldehyde（DABA-BTCO）complex nanoflower as the self-template and nitrogen source for the doping［Fig.10（F）—（H）］［64］.As a result，both electrochemical catalytic activity and stability of this catalyst are higher than the standard Pt/C catalyst［Fig.10（I）and（J）］.

Fig.10 TEM image(A)and HR-TEM images(B,C)of the NHCP-1000,ORR polarization curves of the NC,NHCP-1000 and commercial 20%(mass fraction) Pt/C in O2 saturated 0.1 mol/L KOH solution at 1600 r/min with a scanning rate of 10 mV/s(D), Jk at 0.75 V versus the RHE of NHCP at varied pyrolysis temperature(E)[63],SEM(F),TEM(G) and AFM(H) images of the h-N-CF-800,durability of the catalysts before and after 5000 cycles(I),chronoamperometric test of the h-N-CF-800 in O2 purged 0.1 mol/L KOH electrolyte(J)[64]

3.1.2 Hollow Material Supported Catalysts The active catalytic nanoparticles could be supported by dispersion within the pores to preserve the high catalytic performance［59］.Impregnation reduction strategy is usually used to load catalytic nanoparticles within the porous voids with the solution of metal precursors.A hollow catalyst of Fe-N/C was synthesized by hard template strategy in one step，in which FeNxsites were well dispersed within the carbon sphere.In comparison with the solid Fe-N/C，Brunauer-Emmett-Teller（BET）surface area of the hollow Fe-N/C was significantly increased［65］.Thanks to the high specific surface area and abundant electrically active sites，the modified catalyst was conducive to adsorption of the oxygen intermediates with an excellent ORR activity in alkaline solutions［Fig.11（A）—（C）］.By combination of the stressinduced shrinkage strategy with impregnation reduction method，a Pt@Fe-NC electrocatalyst was achieved by incorporation Pt nanoparticles within the hollow Fe-NC dodecahedral nanomaterial［66］.Wealthy active sites were acquired from the hollow porous structure［Fig.11（D）—（F）］.The ORR activity of Pt@Fe-NC is rather high with a half-wave potential at 0.936 V.The new catalyst demonstrates a mass activity of 1.34 A/mgPt，which is 6.77 folds higher than the commercial Pt/C one（0.198 A/mgPt）.The Pt@Fe-NC catalyst is much durable than the commercial Pt/C catalyst.

Fig.11 Schematic synthesis(A),SEM(B) and TEM(C) images of the hollow Fe-N/C-800[65],synthesis of the Pt@Fe-NC catalyst(D),LSV plots of the Pt/C and Pt@Fe-NC(E),mass activity and specific activities of the Pt/C and Pt@Fe-NC at 0.9 V versus RHE(F)[66]

Besides the role of dispersion nanoparticles，confinement effect of the active ingredients within the cavity could be achieved to construct nanoscale reactors.In comparison with individual free catalysts，the catalytic activity could be better maintained by this new design especially under harsh conditions.A heterostructure of Cu/Co93Cu7nanoparticles confined inside a N-doped carbon nanocage（Cu/Co93Cu7@NC NC）was synthesized and used in the ORR catalysis［Fig.12（A）］［67］.The unique hollow structure and optimal interface make the Cu/Co93Cu7@NC NC catalyst more attractive in ORR catalysis.Monodisperse Pt nanoparticles were studded in a graphene nanobox（Pt@GB）by a simplein situconfined growth［Fig.12（B）］.The resultant Pt@GB catalyst exhibits an excellent electrocatalytic activity for ORR with a higher current density，higher electron transfer number and better durability［68］.Fe3O4nanoparticles were grown on the interior surface of the N-doped hollow carbon sphere（Fex@N/HCS）by the capillary effect of the mesoporous carbon shell［Fig.12（C）—（L）］［69］.The mesoporous carbon sphere could effectively prevent the Fe3O4nanoparticles from falling off and agglomeration，thus to ensure full contact between active sites and oxygen molecules.Each core-void-shell structure serves as an independent ORR active ecosystem.Such unique properties endow the Fex@N/HCS high electrocatalytic activity for ORR with a high current density，low overpotential and excellent durability.

Fig.12 Synthesis of the Cu/Co93Cu7@NC NCs catalyst(A) [67],TEM image of the Pt@GB(B) [68],synthesis of the Fex@N/HCS(C,D),TEM images(E,F),HR-TEM image(G) and elemental mapping(H—I)of the Fe20@N/HCS catalyst[69]

3.2 Energy Storage

The unique features of the hollow structural materials make them promising candidates for energy storage applications such as lithium-sulfur batteries，lithium-ion batteries，supercapacitors，etc［70，71］.Porous carbon nanospheres（PCNSs）with hollow structure were synthesized［Fig.13（A）and（B）］.As electrode material for lithium-sulfur batteries，the internal cavity of hollow structural material provided buffers against the volume change during cycling，which greatly improved the stability.PCNSs with sulfur content of 70% could retain nearly 90% of initial capacity after 100 cycles at current rate of 1C［Fig.13（C）］［72］.Beyond volume change，shuttle effect of lithium-polysulfides（LiPSs）is another issue needed to be considered for lithium-sulfur batteries.Hollow oxygen deficient titania（TiO2-x）arrays could efficiently solve the problem［Fig.13（D）and（E）］.The hollow TiO2-xelectrode could realize both physical encapsulation and chemical binding of polysulfides，which promoted the safety and stability of the lithium-sulfur batteries.In optimized condition，capacity of 890 mAh/g could retain after 200 cycles at C/5 rate［Fig.13（F）］［73］.Multishelled hollow spheres have superior performance as electrode materials for lithium-ion batteries compared to single-shelled hollow spheres［74］.Multishelled TiO2hollow microspheres（MS-TiO2-HMS）were prepared by facile template method［Fig.13（G）and（H）］.Due to larger interfacial surface and shorter lithium-ion diffusion pathway，high specific capacity and cycling performance could be guaranteed.After 100 cycles，the specific capacity of MS-TiO2-HMS with triple-shell（3S-TiO2-HMS）could reach 237 mA·h/g［Fig.13（I）］［75］.Similar synthesis method was applied to fabricate hollow multishelled structures（HoMSs）Co3O4.As electrode material for supercapacitors，the specific capacitance could reach 688.2 F/g at current density of 0.5 A/g［76］.Excellent rate performance and cycling performance were also exhibited，which showed the structural advantages of the multishelled hollow structures.

Fig.13 SEM(A)and STEM(B)images of PCNSs,rate capability of p-PCNS-M-70 electrode over 100 cycles at various current rates(C)[72],TEM (D) and zoom-in images(E) of the reduced hollow TiO2 nanospheres,cycling performance of TiO2-x/sulfur composite cathode at a current rate of C/5(F)[73],SEM(G) and TEM(H) micrographs of 3S-TiO2-HMS,cycling performance at the current rate of 1 C between 1.0 and 3.0 V(I)[75]

3.3 Separation of Oil/Water Mixtures

Effect separation of oil/water mixtures is important in many fields such as chemical engineering and environmental remedy.Janus hollow structural materials with varied wettability at the interior and exterior surfaces of the shell provide new tools for the effective separation of oil/water mixtures including the emulsified ones.Upon grafting responsive polymers（pH or thermal responsive polymers）at interior surface of the shell，the Janus porous sphere could selectively capture oil from the aqueous surroundings which could be released under the corresponding stimuli［25］.A single-hole Janus hollow sphere was achieved by the crosslinking induced phase separation of emulsion droplets followed by the selective modification［77］.The aqueous phase（water）and the oil phase（epoxy resin solution in toluene and paraffin）were emulsified to obtain an oil-inwater emulsion.After feeding the crosslinking agent of 2-ethyl-4-methylimidazole in the aqueous phase，a crosslinked epoxy resin（EP）polymer shell was achieved at a high temperature.Upon lowering temperature，the paraffin phase was frozen，serving as a hard template to protect the interior surface of the crosslinked shell.The exterior surface of the shell became hydrophilic upon modification with the amine terminated PEO.After removal of paraffin，the Janus hollow sphere was derived.What’s more，PDEAEMA could be grafted at the interior surface by ATRP after removal of paraffin.At a high pH above 7.2，the PDEAEMA grafted interior surface demonstrated hydrophobicity，and the Janus hollow sphere could capture oil inside the cavity.At a low pH below 7.2，the interior surface demonstrated hydrophilicity and was positively charged，and the captured oil could be released outwardly［Fig.14（A）］.A responsive Janus porous hollow sphere with mesoporous channels at the shell was synthesized，which could selectively capture oil from the oil/water emulsions independent on surfactant type［Fig.14（B）］［19］.Loading/release of the oil could be manipulated by temperature change since thermo-responsive PNIPAM were readily grafted at the interior surface［19］.When the pores are large，oily solid substances could be captured inside the cavity［Fig.14（C）］［36］.Besides as a container，the Janus hollow sphere could serve as a reactor to decompose the captured organic pollutants which are confined inside the cavity［78］.The example catalytic TiO2nanoparticle（P25）was dispersed in an acidic aqueous solution of hydroxyethyl chitosan and benzaldehyde terminated poly（ethylene glycol）（OHC-PEGCHO）.Upon spraying the dispersion in an ammonia atmosphere，the droplets became crosslinked upon forming the dynamic Schiff base bonding between chitosan and the benzaldehyde terminated poly（ethylene glycol）（OHC-PEG-CHO）.Silica was further coated at the exterior surface of the crosslinked sphere by the sol-gel process of tetraethoxysilane.After PEG was conjugated at exterior surface of the silica shell，the crosslinked sphere was dissolved with acidic solvents to generate transverse channels.The interior surface of the silica shell was further modified withn-octyltriethoxysilane to introduce a hydrophobic moiety.Oily substances could be selectively captured inside the Janus hollow sphere，which could be easily decomposed under UV irradiation to trigger the photocatalytic reaction by P25.This performance is promising in environmental remedy and heterogeneous catalysis.

Fig.14 Fluorescence microscopy images of the PDEAEMA-EP@silica-PEO Janus hollow sphere in the n-hexane/water mixture at varied pHs(A)[77],separation of n-hexane/water emulsion by thermoresponsive PNIPAM-cPVBC-PEO Janus cage column(B)[19] and adsorption of paraffin/toluene and modified lipophilic silica particles by PAM/PDVB Janus cage(C)[36]

3.4 Drug Delivery

Hollow structural materials possess wealthy internal cavities，providing the space to load drugs for transport toward the targets.A Janus flaky hollow material was synthesized consisting of a flaky graphene encapsulated with a mesoporous silica coating.An anti-tumor drug of doxorubicin hydrochloride could be loaded，which could be released under near-infrared（NIR）irradiation to kill cancer cells［Fig.15（A）—（C）］［30］.The asymmetric flaky shape allows the directional interaction with the targeting cells，and the mesoporous coating provides larger area to adhere to the tumor cells.Multi-shelled hollow spheres were interesting for programmable drug delivery［Fig.15（D）and（E）］［79］.Drugs usually preferentially adsorb in the micropores of the shell.Thus，more drugs are expected to be loaded by multi-shelled hollow spheres compared to the single-shelled hollow spheres.The results in this study showed that the loading efficiency of drugs was increased with the shell number.In the case of deliver drugs with the titania hollow sphere［80］，zetapotential of the hollow sphere was dramatically changed from -23 mV to +8.67 mV after the positively charged drug of gentamicin was loaded in the negatively charged titania hollow sphere.A Janus-type micromotor（JPM）was fabricated to deliver drugs.Erythrocyte membrane（EM）was used to coat the JPM in order to improve the biocompatibility.Movement of the EM-JPM was guided by NIR irradiation of the incorporated Au nanoparticles to realize a precise drug delivery［81］.

Fig.15 CLSM images of the cells after uptaking the FITC-labeled Janus nanocage(A),CLSM images of the HeLa cells after uptaking the DOX@RGO@mSiO2-PEG Janus nanocage for 12 h(B),viabilities of the HeLa cells under different treatments(C)[30],CLSM images(D) and cell viabilities(E) after treating the cancer cells with DOX-loaded multi-shelled hollow spheres[79]

4 Summary and Outlook

In this review，we briefly survey the representative recent progresses in the template synthesis and applications of hollow materials.Shell number and composition，and asymmetric characteristic of the hollow materials are greatly tunable，and the materials are promising in catalysis，oil-water separation and drug delivery.Especially，those Janus hollow spheres with asymmetric compartmentalization of compositions and functions are more attractive in controlled release against targets by external guidance.

High performance hollow materials are awaiting the breakthrough in new methods toward efficient synthesis of the hollow materials with tailored properties.Although template synthesis is effective，great restrictions remain in fine control of composition of the hollow materials with many steps to remove templates by either dissolution or calcination.In the case of soft template synthesis，the hollow materials are usually less uniform in size and morphology.

The hollow material exhibits ORR performance comparable to that of Pt/C catalysts.However，there are still a lot of active sites buried in the multi-level hole structure，which cannot exert the best performance.Meanwhile，the stability of the present hollow material is still not comparable to that of the Pt/C catalyst.Thus，improving the active site activity and reducing the dissolution of active metals during the use are significant.As for the applications in energy storage，the large specific surface area of hollow material may lead to the increase in side reactions.Careful design of structure and surface modification are important to solve the problem and promote the applications of hollow materials.