Recent Advances in Metal-organic Framework Derived Hollow Superstructures：Synthesis and Applications

ZOU Yingying，ZHANG Chaoqi，YUAN Ling，LIU Chao*，YU Chengzhong，2*

（1.School of Chemistry and Molecular Engineering，East China Normal University，Shanghai 200241，China；2.Australian Institute for Bioengineering and Nanotechnology，The University of Queensland，Brisbane 4072，Australia）

Abstract Rational design of metal-organic framework（MOF）derived functional materials with elaborate structures is of great significance for diverse applications.Assembly of simple MOF derivatives as building blocks into hollow superstructures（HSSs）represents a promising strategy for creating higher-order structures with improved performance.Even many excellent reviews on MOF derivatives have been reported，a dedicated review from the angle of synthesis and applications of HSSs is still lacking.Here we provide a timely and systematic review on the recent advances of MOF derived HSSs.Firstly，five types of MOF derived HSSs are classified according to their architectural features.Then，the synthesis strategies for creating HSSs from MOF derivatives are summarized，with an emphasis on how to design MOF precursors and select conversion conditions.Afterwards，the promising applications of MOF derived HSSs in energy and catalysis related fields are highlighted.Finally，our perspectives on challenges and future opportunities in MOF-derived HSSs are presented，aiming to provide guidance for the design of advanced MOF derivatives with intricate structures and enhanced properties.

Keywords Metal-organic framework；Functional material；Hollow superstructure；Synthesis；Application

1 Introduction

Metal-organic frameworks（MOFs），assembled by metal ions/clusters and organic ligands through coordination bonds，are an emerging class of crystalline organic-inorganic hybrid porous materials［1—12］.The excellent properties of MOFs，such as high specific surface area，regular pore structures，tunable compositions and morphologies，and multifunctionality，make them promising candidates for a variety of applications［13—22］.Moreover，MOFs have been utilized as precursors for the synthesis of inorganic functional materials including carbon［23—29］，metal-based compounds［30—34］and carbon/metal composites［35—37］by controlled chemical/thermal treatment.These derivatives can inherit the features of MOF templates（e.g.，high specific surface area and porosity，controllable morphologies and tunable compositions）to some extent，and even offer additional advantages such as higher chemical stability and electronic conductivity，enhanced redox ability and light absorption capacity［38—44］.

Over the past decades，tremendous efforts have been dedicated to the structural design of MOF derivatives to regulate their properties.The significant advances achieved in the field give rise to innovative hollow superstructures（HSSs）［45，46］of MOF derivatives，which are formed by assembly of simple building blocks（BBs，e.g.，nanocage［47］，nanorod［48，49］，nanosheet［50，51］，nanotube［52，53］）with and/or around hollow cavity.For the synthesis of MOF derived HSSs，the selection of appropriate MOF precursors and conversion routes is the key.The highly sophisticated architectures of MOF derived HSSs can integrate multiple functions and even generate new properties.For example，the nano-BBs in superstructures are connected with each other，contributing to enhanced structural robustness and facilitated electron transfer［54］.The hollow structures facilitate the utilization of active sites，cargo loading，light harvesting，etc［55，56］.In some cases，additional porous structures can be created by assembling subunits with specific spatial arrangement toward enhanced surface exposure and mass transfer［57，58］.By virtue of these fascinating characters，MOF derived HSSs have been widely applied in many fields（e.g.，catalysis［59，60］，energy storage and conversion［61—63］）with excellent performances，outperforming their counterparts with traditional bulk or isolated particle forms.

As an emerging but fast-growing field，many breakthroughs have been made in recent years.Despite of many excellent reviews on MOF derived functional materials［28，39，42，64—66］，a dedicated review on the engineering of MOF derived HSSs and their applications is rare.Herein，we provide a timely summary on the latest advances of MOF derived HSSs.Starting from a brief introduction on the five types of architectures in MOF derived HSSs，then their synthetic strategies and corresponding formation mechanisms will be elucidated to provide understandings on how to design MOF precursors and select conversion conditions.Afterwards，the promising applications of MOF derived HSSs in selected areas will be summarized to demonstrate the structural superiorities.Finally，our perspectives on the challenges and future directions in this exciting field will be provided.

2 Architectures of MOF Derived HSSs

The architectures of MOF derived HSSs are highly complicated，which are mainly determined by the structures of BBs and their various arrangements.To gain insights into the synthesis-structure-function relationship of different MOF derived HSSs，a classification from architectural perspective is firstly introduced.The architectures of reported MOF derived HSSs can be mainly divided into the following five types（Ⅰ—Ⅴ，Scheme 1）.Type I of MOF derived HSSs is formed by the self-assembly of solid BBs around a hollow cavity.For Type II，the solid BBs are grown on a pre-formed hollow shell as a supporter，thus the overall morphology of Type II is dependent on the supporter.The shapes of BBs in Type I and II can be nanospheres［67］，nanorods［48］，nanosheets［50］，polyhedrons［68］，etc.，and their compositions range from layered double hydroxide（LDH）［69］，metallic compounds［70］to carbon materials［71］.Besides，the compositions of the preformed hollow shell could be different from the BBs in type II MOF derived HSSs.For example，carbon and metal sulfides based nanocages have been reported as the hollow shells［72，73］.The third type of MOF derived HSSs（Type Ⅲ）is created by self-organization of hollow BBs，mainly hollow nanotubes［58，74］.As a more common type made by hollow BBs，Type Ⅳis constructed by arranging hollow particles（e.g.，nanotubes［75，76］，hollow polyhedrons［77，78］and nanoplates［79，80］）on a solid support at nano or even macroscopical scales（e.g.，metal particle/foam［81，82］，carbon nanotube/fiber［55，76］，MOF［83］or polymer［77，78］，etc.）.Different from Type Ⅰto Ⅳwith one level of hollow structure（e.g.，hollow cavity in Ⅰ，hollow shell in Ⅱ，and hollow BBs in Ⅲand Ⅳ），Type Ⅴpossesses hollow-on-hollow architectures，which are comprised of hollow BBs that are interconnected with each other around a hollow cavity.The BBs could be hollow spheres［84］，hollow polyhedrons［85］，and nanotubes［86］，etc.Although these five types of MOF derived HSSs are different from each other in shape and morphology，they all exhibit some common characteristics，for instance high specific surface area，abundant pore channels，high-degree exposure of each component.

Scheme 1 Architectural types of MOF derived HSSs

3 Synthesis of MOF Derived HSSs

The synthetic process of MOF derived HSSs generally involves two steps，the fabrication of MOF precursors and further post treatment.The reported MOF precursors include not only MOF-based composites with pre-formed superstructure but also single MOFs with simple architecture［53，87］.Using these MOF precursors assisted with customized treatment methods［30，38］，MOF derived HSSs with tunable architectures and composition can be obtained.In this section，the synthesis of MOF-derived HSSs will be reviewed from the angle of MOF precursors with different architectures，including hollow MOF superstructures［Scheme 2（A）］［88］，solid MOF superstructures［Scheme 2（B）］［76］and simple MOFs［Scheme 2（C）］［89］.The post-treatment routes and mechanisms will also be elucidated in each category.

Scheme 2 Three categories of precursors and post-treatment routes in the synthesis of MOF derived HSSs

3.1 Hollow MOF Superstructures

One straightforward pathway for constructing MOF derived HSSs is conversion of corresponding MOF HSSs as precursors［Scheme 2（A）］.Typically，MOF HSSs built by self-assembled hollow BBs（Route 1）or organization of solid BBs surrounding a hollow cavity（Route 2）can be directly transformed into MOF-derived HSSs with inherited architectures.Another way（Route 3）is hollowing the solid BBs toward type ⅤMOFderived HSSs.For these routes，elaborate arrangements of hollow MOF units or assembly of solid MOF units with a hollow cavity are the key.One efficient strategy is to use sacrificial templates with pre-formed superstructures for MOF growth.In Jiang′s work［58］，urchin-like superstructures consisted by cobalt carbonate hydroxide nanorods were employed as templates.By direct reaction with 2-methylimidazole（2-MeIM）ligands，urchin-like ZIF-67（ZIF=zeolitic imidazolate framework）nanotube HSSs were prepared.During this process，cobalt carbonate hydroxide acted as both Co2+sources for ZIF-67 growth and template for introducing hollow cavity.By further carbonization，the multi-tunnel HSSs（Type III）were well preserved for the resultant Co single atoms incorporated carbon materials［Fig.1（A）］.Guided by the similar design principle，cobalt acetate hydroxide with prism-like structure was reported to induce the formation of ZIF-67 hollow prisms assembled by interconnected solid nanopolyhedra［88］.Furthermore，these ZIF-67 hollow prims were converted into hollow-on-hollow superstructures composed of hollow CoS4nanobubbles through sulfidation and subsequent thermal treatment.

Fig.1 Schematic illustration of the fabrication process of Co atom embedded carbon micro-urchins(A)[58],illustration of the formation process of spherical HSSs of carbon nanorods(B)[49],schematic representation of the synthesis of frame-like Co-Fe oxide(C)[87]

Apart from the non-MOFs templates，a MOF engaged self-transformation strategy was developed by Xu and co-workers for the fabrication of spherical HSSs of MOF nanorods［49］［Fig.1（B）］.Firstly，Zn-MOF（Zn-MOF-74）nanoparticles were prepared by reacting zinc acetate dihydrate with 2，5-dihydroxyterephthalic acid in methanol solution.Through a hydrothermal process in water with urea as a modulator，the Zn-MOF-74 nanoparticles were transformed into nanorods assembled HSSs（Type I）with chestnut-shell-like morphologyviaa dissolution-regrowth mechanism.Similar HSSs could also be synthesized by replacing the urea with hexadecyltrimethylammonium bromide or polyvinylpyrrolidone as the modulator.Carbonization of Zn-MOF HSSs in argon at 1000 ℃produced well-defined carbon spheres with 1D porous nanorods assembled around a hollow cavity.

Compared to the two-step templating route，a facile one-pot synthesis of MOFs HSSs can be achieved by regulating the nucleation and growth manners.For instance，Louet al.［87］reported a one-step synthesis of prussian blue analog HSSsviaanin situselective growth-etching strategy［Fig.1（C）］.By reaction of trisodium citrate，K3［Fe（CN）6］，Co（C2H3O2）2and H2O at 35 ℃，anisotropic KCoFe-2（CoII—N≡C—FeIII）nanocubes with an FCC structure were quickly formed in 5 s.Because of the selective adsorption of citrate ion on｛100｝facets and proton-coupled electron transfer from CoIIto FeIII，small nanoparticles with a new KCoFe-1（CoIII—N≡C—FeII）phase were preferentially deposited on the corners and edges by epitaxial growth as reaction time reached 2 h.By prolonging the reaction time to 9 h，the further growth and fusion of KCoFe-1viaan oriented attachment process produced a cubic frame-like shell on KCoFe-2 core.With the subsequent decomposition of interior KCoFe-2 with more defects，open frame-like KCoFe-1 HSSs were obtained.After calcination in air，the resultant KCoFe-1 was transformed into Co-Fe oxide with a well inherited architecture（Type I）.

3.2 Solid MOF Superstructures

Similar to aforementioned MOF HSSs，the solid MOF superstructures are also constructed as precursors by organization of MOF BBs on selected substrates［Scheme 2（B）］.The difference is that both MOF BBs and substrate are solid.When used to synthesize derived HSSs，the pre-formed composite materials may go through the following three different conversion pathways for creating hollow cavity：（1）removing the inner substrates（Route 1），（2）transforming MOF BBs into hollow structures（Route 2），and（3）simultaneously hollowing the MOF BBs and substrates（Route 3）.

For route 1，the substrates for MOF growth are removable during or after MOF conversion.For instance，Wanget al.［81］synthesized hollow spherical superstructure of carbon nanosheets using core-shell metal@MOF nanosheet composites as precursors followed by carbonization［Fig.2（A）］.Commercial Zn powder composed of spherical Zn particles was used as the starting materials with subsequent surface deposition of FeCoNi-based hydroxides nanosheets，forming FeCoNi-OH/Zn with a sheet-on-sphere superstructure.Through further substitution of OH-by 2-MeIM，the FeCoNi-OH was converted into ZIF materials with the nanosheet morphology well kept.Eventually，F(xiàn)eCoNi-ZIF/Zn composites were pyrolyzed at 950 ℃under inert atmosphere.In this process，the interior Zn particles were evaporated，leaving a large hollow cavity.Together with the carbonization of ZIF nanosheets into Fe，Co，Ni nanoparticles decorated carbon nanosheets，the carbon based HSSs with type I architecture were obtained.

Except for Zn particles，electrospun polyacrylonitrile（PAN）nanofibers were also employed by Louet al.［90］to synthesize hollow tubular MOF derived HSSs built by metal oxides nanorods［Fig.2（B）］.They firstly fabricated PAN nanofibers containing Co（Ac）2and Cu（NO3）2as the hard templates.By the strong coordination between the metal ions and ligands（2，5-dihydroxyterephthalic acid），MOF-74 nanorod arrays were grown on the surface of PAN nanofibers.Through a thermal annealing treatment in air，the MOF-74 array was transformed to Co/Cu mixed oxides with the PAN core removed.Consequently，Co—Cu—O hierarchical tubular heterostructures（HTHSs）with the morphology of type I HSSs were obtained.

Fig.2 Illustration of the fabrication of FeCoNi decorated carbon HSSs(A)[81],a scheme of the construction of HSSs composed of Co/Cu mixed oxide nanorods(B)[90],a scheme of the formation process of hollow Co,N-doped carbon nanotube arrays(C)[75]

Different from the relatively inert metal and polymer materials，metal oxides as active substrates can not only serve as the hard templates for introducing hollow cavity during MOFs conversion but also provide metal sources for MOFs growth.For example，by immersing the pre-formed ZnO nanorod arrays into the 2-MeIM/Co2+solution，a bimetallic Zn/Co-ZIF shell was generated on ZnO template by the coordination of released Zn2+from ZnO and Co2+in solution with 2-MeIM［75］.The core-shell ZnO@Zn/Co-ZIF nanorods were subsequently pyrolyzed under Ar with the ZnO corein situremoved，forming hollow Co，N-doped carbon nanotube arrays with type IV configuration［Fig.2（C）］.

Besides removing sacrificial substrates，some MOF particles can be directly converted into hollow derivatives by utilizing the distinct conversion behaviors between the external and internal regions of MOFs（Route 2）.This provides another alternative for constructing MOF derived HSSs using solid MOF superstructures.For example，uniform 2D cobalt-based MOF（Co-MOF）solid nanowall arrays were facilely grown on clean carbon clothviaimmersion in 2-MeIM and Co2+mixed aqueous solution［Fig.3（A）］［79］.By further reacting Co-MOF/carbon cloth with Ni（NO3）2ethanol solution，the external region of Co-MOF nanowall was converted into Ni-Co layered double-hydroxide（LDH）shell by ion-exchange with internal part etched，forming hollow cavity.After annealing in air，the Ni-Co LDH was decomposed，leading to the formation of highly porous and crystallized NiCo2O4hollow nanowall arrays on carbon cloth，assigned to type IV architecture.When changing the etchant from Ni（NO3）2into Na2MoO4，hollow Mo-Co LDH shell arrays were synthesized［80］.Through a phosphidation treatment，similar HSSs composed of hollow Mo-doped CoP shell arrays were obtained.

Fig.3 Schematic illustration of the synthetic process of NiCo2O4 hollow nanowall arrays(A)[79],illustration of the fabrication process of 1D carbon HSSs(B)[78]

In addition to conversion-etching process，thermal treatment of superstructured MOF composites can also give rise to HSSs derivatives.For example，Wanget al.［78］reported ZIF-8@PAN composite fibers by integrating pre-prepared ZIF-8 nanocubes with electrospun PAN fibers［Fig.3（B）］.Followed by a carbonization reaction，ZIF-8 nanocubes were transformed into a hollow structure through an outward contraction mechanism，leading to 1D HSSs（Type IV）of interconnected hollow carbon nanocubes with PAN converted carbon fiber as binder.Such a strategy has also been extended to ZIF-8 with smaller sizes［77］，ZIF with different metal centers and even other MOFs for the fabrication of similar 1D HSSs with tunable compositions［91—93］.

Beyond individually hollowing the substrates or MOF BBs，the concurrence of these two processes can create more complicated HSSs with a hollow-on-hollow architecture（Type V，Route 3）.To this end，conjugation of different MOF units into MOF-on-MOF hybrids with preorganized superstructures as precursors is currently a predominate strategy based on the different conversion manners of MOF units.In 2022，Yu and co-workers reported the MOF-on-MOF derived metal sulfide HSSs with a hollow-on-hollow superstructure（Fig.4）［83］.Typically，using cake-like NH2-MIL-125 as host MOFs，NH2-MIL-125@ZIF-67 MOF-on-MOF hybrids were prepared by growing densely packed ZIF-67 polyhedrons on the surface of NH2-MIL-125.During the subsequent sulfidation process，ZIF-67 nanocrystals were first selectively transformed into interconnected hollow CoSxnanocages by the anion exchange between S2-and 2-MeIM.Afterwards，owing to the interruption of coordination bonds between Ti—O clusters and aminoterephthalate，the NH2-MIL-125 cakes were selectively etched with decomposed Ti species partially doped into the framework of CoSx，resulting in the pacman-like bimetal Ti-CoSxHSSs with a cage-on-cake morphology.Using similar synthetic strategy，the same group fabricated hollow-on-hollow TiZnCo trimetal sulfide HSSs by growth of ZnCo-ZIF nanocrystals on NH2-MIL-125 with further sulfidation treatment［85］.

Fig.4 Scheme of the fabrication of pacman-like Ti-CoSx HSSs[83]

3.3 Simple MOF Precursors

Compared to the construction of pre-organized superstructured MOF precursors，controllable synthesis of MOF micro-/nanoparticles with different shapes（e.g.，octahedron，plate，cube，dodecahedron，spindle and rod）can be relatively easier.Therefore，conversion of simple MOFs micro-/nanoparticles without pre-formed superstructures into HSSs derivatives is highly attractive.Currently，such synthesis can be achieved by two routes：（1）sequential conversion-growth；（2）direct conversionviahydrolysis or carbonization.For route 1，the MOFs particles are firstly converted into hollow derivatives as substrates for further growth of other materials as subunits on their shell with controlled spatial arrangement.For example，using a ZIF-67 dodecahedron as the precursor，a Co9S8hollow cage was producedviaa sulfidation treatment［Fig.5（A）］［94］.The further growth of interconnected ZnIn2S4nanosheets on the shell of Co9S8cage through a solvothermal process resulted in a hierarchical Co9S8@ZnIn2S4composite HSSs（type II）.The similar synthetic route can be applied for the fabrication of HSSs with various architectures and components，such as tubular In2O3-ZnInS4［95］，tubular In2S3-CdIn2S4［96］and polyhedral Co3S4@MoS2［72］.

Fig.5 Synthetic route of hierarchical Co9S8@ZnIn2S4 composite HSSs(A)[94],illustration of the synthetic process of HSSs comprised of Ni-Co sulfide nanosheets(B)[97]

When choosing appropriate reaction conditions，the simple MOFs can be directly converted into HSSs derivatives（Route 2）.Also starting with a ZIF-67 dodecahedron，a thermal treatment was first performed to enhance the cross-linking of the organic ligand and enable the formation of amorphous carbon in the final product.A carbon decorated Ni-Co LDH（C/LDH）assembled hollow cage was then formed by reacting with Ni（NO3）2ethanol solution［Fig.5（B）］［97］.During this process，protons from the hydrolysis of Ni2+ions can gradually decompose ZIF-67.The released Co2+ions may be partially oxidized into Co3+by dissolved O2and NO3-ions in the solution.Then，the coprecipitation of Co2+/Co3+and Ni2+ions resulted in the C/LDH HSSs.By further sulfurization，carbon doped Ni-Co sulfides with well-retained structure were synthesized（Type I）.

In addition to the single shelled HSSs，double shelled HSSs have also been synthesized by controlled hydrolysis reactions.Lou and colleagues reported the synthesis of Ni-Fe LDH HSSs using MIL-88A nanorods as templates［69］.Through the hydrolysis reaction of urea and Ni（NO3）2in a mixed solution of ethanol/water，the MIL-88A was etched with released Fe species coprecipitating with Ni2+and OH-，resulting in Ni-Fe LDH shell.Notably，the shell numbers of Ni-Fe LDH could be facilely tuned by changing the etching and coprecipitation rates.Specifically，more ethanol in the mixed solvent slowed down the etching of MIL-88A，leading to the formation of double shelled Ni-Fe LDH HSSs（Type Ⅰ）.In contrast，more water in the mixed solvent caused faster hydrolysis and etching of the template，only producing single-shelled counterparts［Fig.6（A）］.

Fig.6 Schematic illustration of the synthesis of single and double shelled Ni-Fe LDH HSSs(A)[69],SEM images of ZIF-67 and derived HSSs of hierarchical CNTs frameworks(B)[52]

Aside from hydrolysis treatment，pyrolysis is another route for converting simple MOFs into HSSs derivatives.By direct carbonization of ZIF-67 in an Ar-H2atmosphere at 700 ℃，many tiny carbon nanotubes（CNTs）were generated on the hollow shell of carbon substrates，resulting in type IV HSSs［Fig.6（B）］［52］.The H2atmosphere was demonstrated to play a vital role in the formation of HSSs with hierarchical CNTs frameworks.The existence of H2induced the fast formation of metallic Co nanoparticles，which catalyzed the growth of CNTs.Besides，the co-occurrence of quick pyrolysis of imidazolate and gradual consumption of carbon for growing CNTs generated the inner hollow cavity.In contrast，only solid carbon polyhedrons are obtained in the absence of H2，even when increasing the pyrolysis temperature.

4 Applications of MOF Derived HSSs

To date，MOF derived HSSs have been explored for a variety of applications due to the distinctive structural features from integration of MOF derivative BBs and hollow structure，such as high specific surface area，tunable morphologies and components，hierarchical porous structures，highly exposed active centers，good electron conductivity and stability.Their applications together with architectures and compositions are summarized in Table 1.In this section，the applications of MOF derived HSSs in supercapacitors，batteries，electrocatalysis，photocatalysis and heterogeneous catalysis will be selectively discussed to highlight their structural merits（Scheme 3）.

Scheme 3 Applications of MOF derived HSSs

Table 1 Architectural types,composition and performance of MOF derived HSSs

Continued

4.1 Supercapacitors

In view of the fast growing energy demand and increasing environmental crisis caused by over-consumption of fossil fuels，the search for green and renewable energy solutions has been a central research subject over the past decades［98—101］.Supercapacitors，as a promising type of energy storage device，have received extensive attention due to their advantages of fast charge-discharge rates，high power densities and long cycle life［102—106］.The reported supercapacitors are mainly divided into two types：electrostatic double-layer capacitors（EDLCs）［107］and pseudo-capacitors［108］，the performances of which relied on the structure and property of electrode materials［109］.Recent advances have demonstrated the superiorities of HSSs as electrode materials for supercapacitors，compared to the isolated particles.For instance，Chen and coworkers reported 1D fiber based HSSs of interconnected hollow carbon cages（denoted as HPCNFs-N，Type Ⅳ）viaa carbonization process of electrospun ZIF-8/PAN composite fiber at 900 ℃［Fig.7（A）and（B）］［78］.The resultant HPCNFs-N as EDLCs electrode materials delivered a specific capacitance of 307.2 F/g at a current density of 1 A/g，higher than that of carbon particles（ca.189 F/g）obtained by directly carbonizing ZIF-8.Even at a current density high up to 50.0 A/g，the specific capacitance of HPCNFs-N reached a high value of 193.4 F/g，outperforming other two 1D HSSs obtained at 800 and 1000 ℃［denoted as HPCNFs-N-800 and 1000 respectively，F(xiàn)ig.7（C）］.Moreover，the specific capacitance of HPCNFs-N after 10000 cycles maintained 98.2%of the initial value［Fig.7（D）］，showing remarkable electrochemical robustness.The enhanced performances of HPCNFs-N may be ascribed to the sufficient electrochemically active sites，improved electrochemical kinetics，and enhanced electron transfer and structural stability，thanks to the hierarchical porous structure composed of numerous interconnected carbon hollow nanoparticles，1D fiber-like architecture and high N-doping level.

Fig.7 SEM(A) and TEM(B) images of HPCNFs-N,specific capacitance of HPCNFs-N and control samples(C),cycling stability(D)[78]

Compared to EDLCs，the pseudo-capacitors arisen from fast and reversible redox reactions often offer higher specific capacitance and energy density.The most commonly used pseudocapacitive materials are transition metal sulfides and oxides，and conductive polymers［110，111］.MOF derived HSSs of transition metal sulfides and oxides have been synthesized；and their applications in pseudo-capacitors have also been reported recently.For example，Luet al.［90］constructed the CoO/Co-Cu-S HTHSs［Fig.8（A）and（B），Type Ⅰ］by further vulcanization treatment of Co-Cu-O HTHSs.CoO/Co-Cu-S HTHSs with optimized Co2+/Cu2+molar ratio of 2 exhibited a specific capacity of 320 mA·h/g（2300 F/g）at 2.0 A/g and excellent rate capability with 74% of the capacity retained by increasing the current density from 2.0 A/g to 30 A/g，outperforming single CoO/Co-Cu-S nanoneedles and HTHSs with other Co2+/Cu2+molar ratios［Fig.8（C）］.The CoO/Co-Cu-S-2 HTHSs electrode also showed superior cycling stability among different samples with only 3.5 % of capacity decline after 5000 cycles at 10 A/g［Fig.8（D）］.Moreover，an all-solid-state hybrid supercapacitor was constructed by using CoO/Co-Cu-S-2 HTHSs as the positive electrode and active carbon as negative，showing a high energy density of 90.7 W·h/kg at a power density of 800 W/kg［Fig.8（E）］.

Fig.8 SEM(A) and TEM(B) images of CoO/Co-Cu-S-2 HTHSs,specific capacitances and cycling stability of various samples(C),specific capacities and coulombic efficiencies of CoO/Co-Cu-S-2 HTHSs//AC hybrid supercapacitor(D)[90]

4.2 Batteries

As another excellent candidate to resolve energy problems，battery technologies have commercial applications in many areas such as portable electronic products［112—114］.Among various battery technologies，lithium-ion batteries（LIBs）and sodium-ion batteries（SIBs）have attracted the most attention due to their high theoretical energy density and specific capacity，long cycle life and light weight［115—117］.Nevertheless，it still remains a great challenge to design suitable electrode materials for LIBs and SIBs［118，119］.The applications of simple or bulk MOF derivatives as electrode materials in LIBs and SIBs have been widely studied，however there exist some problems such as low conductivity and significant volume variation during cycling［120，121］.Recently，construction of MOF derived HSSs was demonstrated as an efficient way for overcoming these problems.For instance，Chen et al.synthesized hierarchical tubular HSSs composed of Co3O4hollow nanoparticles and CNTs derived from tubular-like ZIF-67 through a two-step annealing process（Type V）［84］.As an anode material for lithium-ion batteries，the CNT/Co3O4HSSs［Fig.9（A）］displayed remarkable performances with a high reversible capacity of 1281 mA·h/g at 0.1 A/g［Fig.9（B）］and exceptional rate capability［Fig.9（C）］.Moreover，the coulomb efficiency was measured to be close to 100% after 200 cycles with negligible capacity reduction at the current density of 1 and 4 A/g，showing excellent cyclability［Fig.9（D）］.The authors attributed the high performance to the unique structural and compositional features.First，the hollow Co3O4nanoparticles and CNT subunits provided a short diffusion pathway for fast diffusion of Li+ions and enabled sufficient contact between active materials and electrolyte.The tubular architecture and hollow cavity within Co3O4could significantly buffer the volume expansion and thus preserve the structural integrity.Further combination with the enhanced electronic conductivity by the assembly of CNTs and Co3O4nanoparticles within the skeleton of 1D tubulars，the overall electrochemical properties were reinforced.

Fig.9 Schematic illustration for the synthesis process(A),charge/discharge profiles(B),rate capability(C)and cycling performance of CNT/Co3O4 HSSs(D)[84]

Compared to LIBs，SIBs have advantages of abundant resources，low price，and high system safety［122，123］.Considering the similar working mechanism of SIBs with LIBs，boosted performances can also be achieved by using MOF derived HSSs as SIBs electrode materials.For example，CoP nanoparticle-embedded hierarchical hollow carbon superstructure（CoP@N-HP/CT，Type V）was synthesized by treating core-shell ZIF-8@ZIF-67 precursor with subsequent carbonization-oxidation-phosphorization strategy［124］.Owing to the distinctive features of HSSs［Fig.10（A）］，the CoP@N-HP/CT composites exhibited superior SIBs performances of excellent rate capability［176 mA·h/g at 0.1 A/g，F(xiàn)ig.10（B）］and stable cyclability［a capacity loss of 0.02%per cycle after 2500 cycles，F(xiàn)ig.10（C）］，obviously outperforming bulk CoP and Co3O4.

Fig.10 Scheme of the function of CoP@N-HP/CT during sodium storage process(A),rate performance of different samples at various current densities(B),long-term cycling performance of CoP@NHP/CT anodes(C)[124]

4.3 Electrocatalysis

Electrocatalysis reactions such as oxygen evolution reaction（OER），oxygen reduction reaction（ORR）are the key reactions for several clean energy conversion technologies including fuel cell and zinc-air batteries［125—129］.To improve the performance，MOF derived electrocatalysts with hollow superstructures have been investigated.Meng and coworkers reported the synthesis of N-doped carbon nanotubes-assembled hollow dodecahedra（Type V）derived from ZIF-67 as an ORR electrocatalyst［Fig.11（A）］［53］.The optimized carbon HSSs carbonized at 650 ℃exhibited large specific surface area，appropriate N doping，interior voids and robust frameworks.As a result，superior ORR activity in O2saturated 0.5 mol/L KOH solution was achieved with a half-wave potential（E1/2）of 0.85 Vvs.RHE，more positive than commercial Pt/C and samples treated at other temperatures［Fig.11（B）］.The electron transfer numbers（n）was calculated to beca.3.93，revealing an apparent 4e-ORR pathway with high selectivity［Fig.11（C）］.In addition，excellent long-term stability with 95% current retention after 40000 s was demonstrated，exceeding commercial Pt/C catalyst（78% current retention）［Fig.11（D）］.

Fig.11 Schematic illustrations of preparation route of N-CNTs HSSs(A),LSV profiles of different catalysts(B),K-L plots(C)and chronoamperometric responses N-CNTs-650(D)[53]

Also using ZIF-67 as precursors，nanocomposite nanoboxes by assembly of Ni-Co mixed metal phosphide nanosheets on amorphous carbon（NiCoP/C，Type Ⅰ）were fabricated as electrocatalysts for OER［54］［Fig.12（A）］.By integrating the structural advantages of metal phosphides，carbon materials and HSSs，the synthesized NiCoP/C nanoboxes exhibited enhanced OER catalytic performance in comparison with NiCoP and Ni-Co LDH nanoboxes.Specifically，the NiCoP/C HSSs required a much lower overpotential of 330 mV to reach a current density of 10 mA/cm2［Fig.12（B）］.The Tafel slope was measured to be 96 mV/dec［Fig.12（C）］，indicating a favorable OER kinetics.

Fig.12 Illustration of the formation process of NiCoP/C HSSs(A),OER polarization curves(B) and Tafel slopes of different samples(C)[54]

4.4 Photocatalysis

Solar energy is an important sustainable energy resource，and semiconducting materials with elaborate properties are essential for efficient solar energy-driven applications in photocatalysis［130—134］.Compared to traditional MOF derivatives in the form of bulk materials or simple particles，the unique HSSs can efficiently enhance light harvest，expedite the separation and transfer of light-induced charges，possess large specific surface area and rich reactive sites for photocatalytic redox reactions，thus have enhanced photocatalytic performances in various reactions［55，73，95］.For instance，hierarchical In2S3-CdIn2S4heterostructured nanotubes（type ⅠI）derived from In-MIL-68 were employed as photocatalysts for visible light CO2reduction（CO2RR）by Wanget al.［96］in 2017.Thanks to the advantages of HSSs，a high CO generation rate（825 μmol·h-1·g-1）［Fig.13（A）］and outstanding stability［Fig.13（B）］were achieved under visible light irradiation.Besides，the photocatalytic hydrogen evolution performance of Co9S8@ZnIn2S4HSSs（Type ⅠⅠ）synthesized by growing ZnIn2S4nanosheets（NSs）on the surface of ZIF-67 derived Co9S8dodecahedral cages was investigated［94］，showing an outstanding activity with a hydrogen-producing rate of 6250 μmol·h-1·g-1［Fig.13（C）and（D）］.

Fig.13 CO2 photoreduction activities of different samples(A),cycle performance of In2S3-CdIn2S4-10(B)[96],photocatalytic H2 evolution activities of different samples(C),H2 evolution rate of Co9S8@ZnIn2S4 in stability tests(D)[94]

4.5 Heterogeneous Catalysis

Apart from electrocatalysis and photocatalysis，MOF derived HSSs have also shown great potential as active species or catalyst supports for many other heterogeneous catalysis reactions.For example，heterometal doped Co3O4hollow nanocages composed of interlaced nanosheets（type Ⅰ）were fabricated for organic pollutant degradation by using bimetallic ZIFs as precursors［Fig.14（A）］［135］.Among all heterometal（e.g.，Ni，Mn，Cu，Zn）doped samples，the Cu doped Co3O4HSSs displayed best performances as peroxymonosulfate（PMS）activator for rhodamine degradation with a degradation efficiency of 93.41%after 60 min reaction［Fig.14（B）］and a degradation kinetic constant of 0.2257 min-1when the PMS concentration was increased to 0.15 g/L［Fig.14（C）］.Moreover，the activity of optimized sample outperformed most reported catalysts.

Fig.14 Schematic representation for the design of C-CoM-HNCs(A),c/c0 vs.time of RhB degradation over C-Co-HNC(B),catalytic dynamics of C-Cu-HNC with different PMS concentrations(C)[135]

When used as supporters，MOF derived HSSs can also reinforce the catalytic performances of other active species.One typical example is that a hollow spherical superstructure of carbon nanorods［SS-CNR，F(xiàn)ig.15（A）and（B），Type Ⅰ］was used to load highly dispersed Pd nanoparticles as nanoreactors for formic acid dehydrogenation［49］.For comparison，other two samples of Pd@CNP and Pd@XC72 were prepared by using carbon nanoparticles（CNP）derived from Zn-MOF nanoparticles and commercial XC-72 as supports for loading Pd.The catalytic results showed that Pd@SS-CNR with a hollow superstructure offered an excellent catalytic activity at 25 ℃，which is about 1.8 and 6.3 times higher than Pd@CNP and Pd@XC72［Fig.15（C）］.A high turnover frequency of 7200 h-1with 100% H2selectivity was achieved when the temperature increased to 60 ℃［Fig.15（D）］.The superiorities of Pd@SS-CNR for FA dehydrogenation may be ascribed to following aspects：（1）SS-CNR assembled by 1D porous carbon nanorod exhibited a higher specific surface area and larger pore volume for immobilizing Pd NPs with high dispersity；（2）the space between the nanorods promoted the access of formic acid and the diffusion of released gas.

Fig.15 TEM(A) and HAADF-STEM images of Pd@SS-CNR(B),catalytic activities of Pd loaded carbon materials for FA dehydrogenation(C),temperature-dependent gas evolution over Pd@SS-CNR(D)[49]

5 Summary and Outlook

In this review，we have provided an overview of the recent advances of MOF derived HSSs，including their architectural diversities，synthetic strategies and applications.Specifically，five architectural types of MOF derived HSSs have been classified.Their corresponding synthetic strategies are summarized from the angle of different MOF precursors，with an emphasis on how to design and convert these MOFs toward HSSs derivatives.We also highlight their structural superiorities in several catalysis and energy related application fields.Even significant progresses have been achieved，it is noted that challenges still exist.Substantial efforts are suggested to be devoted to the following issues：

The architecture of MOF derived HSSs is highly dependent on the structure and organization of basic BBs.Except for the solid particles，the reported hollow BBs are mainly single shell.In fact，MOF derived hollow particles with higher complexity such as yolk-shell［136］，multi-shell［137］and open-frame structures［138］have been synthesized.However，the arrangement of these particles as BBs into HSSs is rarely reported，which is expected to greatly enrich the architectural diversity of MOF derived HSSs.Besides，either solid or hollow BBs were predominately assembled into 1D and 3D structures with 2D HSSs overlooked.Aside from architecture，the composition of MOF derived HSSs is mainly limited in metal oxides，metal hydroxides，metal sulfides and carbon materials.Other compositions such as metal phosphides［139］，metal carbides［140］and metal nitrides［141］are rarely prepared in the form of HSSs，but useful for energy storage and conversion，or catalysis applications.Integration of multi-components into MOF derived HSSs may generate additional synergistic effects with enhanced properties.

To overcome the limitations in MOF derived HSSs with respect to architecture and composition，more efforts should be devoted to the exploration of synthetic methodologies.Site-selective etching and/or conversion strategies［142］by utilizing the stability/reactivity differences at different regions in MOF precursors can be used to produce yolk-shell or open frame-like morphology.Sequential templating approach［143］is suggested for the construction of hollow multi-shell structures in MOF derived HSSs by precise control of the conversion conditions，especially temperature and heating rate during pyrolysis.To fulfill the gap of 2D HSSs，2D materials（e.g.，graphene［144］，Mxene［145］）are suggested to be used as substrates for MOF growth.In addition，interfacial assembly route［146，147］may be another alternative for guiding 2D assembly of MOF BBs.Through further conversion processes，corresponding 2D HSSs can be obtained.To enrich the composition，P，N，S containing reactive atmosphere（e.g.，PH3，NH3，H2S）may be used instead of traditional N2and air during pyrolysis for the preparation of diverse metal compounds.

Even simple MOF particles can be employed for the synthesis of HSSs derivatives，their hollowing mechanisms need further investigation.For example，different mechanisms have been proposed for the conversion processes from MOF particles to hollow LDH derivatives，but direct evidences are needed.In situtechniques such as TEM［148，149］，XRD［150］and FTIR［151，152］are recommended to monitor the structural and compositional changes of MOFs during conversion to better understand the conversion mechanisms.

From the application side，the MOF derived HSSs have been applied in applications including supercapacitors，batteries，electrocatalysis，photocatalysis and heterogeneous catalysis.The investigation of MOF derived HSSs in other important fields such as sensors［153］，drug delivery［154］and separation［155］has just started.Understanding the fundamental structure-property relationship still has a long way to go，especially in these relatively new application fields.Besides，large-scale production of HSSs is highly important for practical applications.New synthetic methodologies based on cheap MOF BBs and economically friendly conversion routes are desired.

Despite some existing challenges in the synthesis and application of MOF derived HSSs，this emerging field is expected to receive increasing attention in the future.We hope this review can shed some light on the design of novel MOF derived HSSs materials with improved properties.