Characterization and Application of Nanomaterials (2022) Volume 5 Issue 2 doi:10.24294/can.v5i2.1700 83 Original Research Article Preparation and application of carbon-based hollow structured na- nomaterials Xingmiao Zhang, Wei Zhang, Wei Li* Department of Chemistry, Fudan University, Shanghai 200433, China. E-mail: weilichen@fudan.edu.cn ABSTRACT Carbon-based hollow structured nanomaterials have become one of the hot areas for research and development of hollow structured nanomaterials due to their unique structure, excellent physicochemical properties and promising ap- plications. The design and synthesis of novel carbon-based hollow structured nanomaterials are of great scientific sig- nificance and wide application value. The recent research on the synthesis, structure and functionalization of car- bon-based hollow structured nanomaterials and their related applications are reviewed. The basic synthetic strategies of carbon-based hollow structure nanomaterials are briefly introduced, and the structural design, material functionalization and main applications of carbon-based hollow structure nanomaterials are described in detail. Finally, the current chal- lenges and opportunities in the synthesis and application of carbon-based hollow structured nanomaterials are discussed. Keywords: Hollow Structure; Carbon-Based Nanomaterials; Preparation Methods ARTICLE INFO Received: 25 July 2022 Accepted: 15 September 2022 Available online: 3 October 2022 COPYRIGHT Copyright © 2022 Xingmiao Zhang, et al. EnPress Publisher LLC. This work is li- censed under the Creative Commons At- tribution-NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by-nc/ 4.0/ 1. Introduction Hollow-structured nanomaterials have been one of the most popu- lar materials among advanced nanomaterials due to their low mass den- sity, high porosity and large surface area. The unique properties of hol- low-structured nanomaterials have led to their use in a wide range of applications, such as energy storage and conversion, catalytic, water and air purification and adsorption, etc.[1–4]. In recent years, researchers have focused on the synthesis, characterization and application of hol- low structures. Many studies have provided novel insights into the structural and functional design and synthesis of hollow structured na- nomaterials, which have contributed to the development of hollow structured nanomaterials[5–8]. Among the many hollow structured nano- materials, carbon-based hollow structured nanomaterials have attracted much attention due to their great potential applications in energy and catalysis[9]. Carbon-based hollow structured nanomaterials are favored by re- searchers due to their tunable specific surface area, cavity size, mor- phological structure and composition. Based on the needs of different applications, the research on carbon-based hollow structure nano- materials has focused on the design and synthesis of cavity structure, shell structure and functionality. Researchers have focused on obtaining carbon-based hollow structure nanomaterials with the following proper- ties: (1) large specific surface area and effective dispersion and loading of metal nanoparticles or other active substances on their surface, func- tionalized surface structure; (2) excellent electrical conductivity; (3) 84 tunable porosity and particle size; and (4) high me- chanical stability[10]. In order to impart these excel- lent properties to carbon materials, a series of syn- thetic strategies have been successfully developed, such as: Stöber method, template method, micro- emulsion method, etc.[11–13]. These works have con- tributed to the development of carbon-based hollow structured nanomaterials and have played an im- portant role in understanding their synthesis mech- anisms and expanding their applications. In this paper, we summarize the basic synthe- sis strategies of carbon-based hollow structure na- nomaterials in terms of synthesis, structure, func- tionalization design and their related applications in recent years. The structural design and functionali- zation of carbon-based hollow structured nano- materials and their main applications are described in detail. The current challenges and opportunities in the synthesis and application of carbon-based hollow structured nanomaterials are discussed. 2. Synthesis and application of carbon-based hollow structured nanomaterials 2.1 Synthesis of carbon-based hollow struc- tured nanomaterials The synthesis of carbon-based hollow structure nanomaterials is basically the same as the prepara- tion of other hollow structures (Figure 1)[9]. It can be divided into hard template method, soft tem- plate method and self-template method. The most distinctive feature is that the shell layer is a carbon material. Therefore, the successful coating of car- bon precursors on the synthesized templates is the key to the preparation of carbon-based hollow structure nanomaterials. The soft and self-templating methods are relatively limited in the preparation of carbon-based hollow structure na- nomaterials. The soft template method usually uses stable surfactant micelles or emulsions as templates, which can only be used for the coating of carbon precursors in the liquid phase system, and the mor- phology and pore size are relatively difficult to reg- ulate[14]. The self-templating method refers to a template-free strategy, and the preparation of car- bon-based hollow structure nanomaterials using this method is generally carbonized with some hollow carbon precursors. In addition, in recent years, some new metal-organic skeletal materials can also be obtained through appropriate control of calcination conditions of carbon-based hollow structure, greatly enriching the scope of self-templating method for the preparation of carbon-based hollow structure nanomaterials[15,16]. Figure 1. Synthesis of carbon-based hollow nanomaterials[9]. Compared with soft and self-templating meth- ods, hard template method has more advantages in the design and preparation of carbon-based hollow structure nanomaterials, and in consideration of the price and ease of operation, hard templates such as silica, metal oxides and polymers are often chosen. Methane, furfuryl alcohol, glucose, asphalt, and phenolic resins are available as carbon sources. In addition, some nitrogen-containing compounds (e.g., aniline, pyrrole, dopamine, and ionic liquids) have also been used to synthesize heteroatom-doped carbon[17]. 85 The coating strategies of carbon precursors on templates are generally classified into physical and chemical coating. The physical coating is usually performed by vapor deposition (CVD), where the carbon source is usually styrene, acetonitrile, ben- zene and methane. The main advantage of CVD method is that the thickness of carbon layer can be precisely controlled, especially for depositing atomic layer thickness. However, the size of carbon spheres prepared by this method is mostly limited to larger sizes (>500 nm), and it is difficult to obtain small size hollow carbon spheres by this method. Chen et al.[18] used SiO2@m-SiO2 with core-shell structure as hard template and ethane as carbon source, and deposited the carbon source into the mesoporous silicon oxide pore channel by CVD method, and then etched the silicon oxide tem- plate by hydrofluoric acid to obtain hollow meso- porous carbon material. By controlling the size of the template, the shell thickness and cavity size of the hollow mesoporous carbon spheres were con- trolled. In addition, the hollow mesoporous carbon spheres were obtained with a high degree of graphi- tization and a specific surface area of 771 m2·g−1 due to the use of ethane as the carbon source. The chemical cladding method uses a chemical reaction to clad the chemical precursors on the outer surface of the template to form a template@carbon precursor core-shell structure complex. Finally, the hollow structure is obtained by etching the template. The development of carbon precursor chemistry is the key to this approach. Considering that the hy- drolytic polymerization process and the structure of phenolic resin are similar to the formation of SiO2. The reaction mechanism is shown in Figure 2. Resorcinol and formaldehyde monomers can form emulsion droplets through the interaction of hydro- gen bonds in the mixture system of ethanol and wa- ter, and then undergo a polymerization reaction cat- alyzed by ammonia water to form phenolic resin colloidal microspheres. Subsequent studies have shown that this Stöber method can be extended to the hydrolytic polymerization of other carbon pre- cursors, such as phenolic resin derivatives, polydo- pamine and polybenzoxazine[20,21]. Figure 2. Synthesis of phenolic resin microspheres[19]. Inspired by the successful application of the silicate sol-gel process for the preparation of hollow structured materials, the Stöber method system was also used for the synthesis of carbon-based hollow structured nanomaterials. Using the Stöber system to combine the synthesis of SiO2 and RF, Fuertes et al.[22] synthesized core-shell structured SiO2@RF microspheres and hollow porous carbon spheres by a one-pot method with TEOS, resorcinol and for- maldehyde in a mixture of water, ethanol and am- monia. Yin, Zheng and Huo[23–25] have inde- pendently investigated the synthesis of hollow or Yolk-shell structured carbon-based materials by depositing phenolic resin and cationic surfactant complexes on the surface of silicon oxide or silicon oxide core-shell structured nanoparticles. The 86 co-assembly of positively charged cationic surfac- tants with phenolic resins (RF) on the negatively charged silicon oxide surface is the key to the RF coating process. The introduction of the cationic surfactant enhances the interaction between the two negatively charged materials, RF and the silicon oxide core, allowing the RF to be successfully en- capsulated on the surface of the silicon oxide core. In order to further increase the porosity of hollow carbon spheres, Zheng’s group[26] developed a mul- ti-step coating process to synthesize a sandwich structure of SiO2@RF/CTAB@SiO2 complexes. The hollow carbon spheres obtained with this structure have a foam-like shell, which is similar to that of SiO2@RF/CTAB-derived hollow carbon spheres (specific surface area and porosity of 639 m2·g–1 and 0.56 cm3·g–1, respectively). In compari- son, it has a higher specific surface area (1,286 m2·g–1) and a larger pore volume (2.25 cm3·g–1). In addition, in the modified Stöber method system, the mechanism of co-assembly of silica-based oligo- mers, cetyltrimethylammonium chloride (CTAC) and RF, Qiao et al. developed a silica-assisted pro- cess to regulate the structure and size of pores (Figure 3)[27]. The SiO2@RF/CTAC/SiO2 compo- site microspheres were prepared by a one-step reac- tion using TEOS, formaldehyde and resorcinol in the ethanol/water/ammonia/CTAC system. In the RF/CTAC/SiO2 composite shell layer, SiO2 can be used as a pore-forming agent, and the pore size of the hollow carbon sphere shell layer can be adjusted from 3.2 to 3.6 nm when the template is removed. In addition, the size of the hollow spheres (180–800 nm in diameter) can also be adjusted by the amount of TEOS and the water/ethanol ratio. Figure 3. Synthesis of hollow Carbon Nanostructures via a “Silica-Assisted” Strategy[27]. As researchers gained a better understanding of carbon precursor chemistry, they began to work on simplifying the Stöber method for the controlled synthesis of hollow carbon spheres. In recent years, a series of general synthetic methods have been de- veloped for the preparation of hollow carbon spheres, ranging from surfactant-free to silicon ox- ide free. Wang et al.[28] reported a facile synthesis of nitrogen-doped polybenzoxazine microspheres and nitrogen-doped carbon microspheres. The polymer microspheres were obtained by condensation of resorcinol, formaldehyde and 1,6-diaminohexane, and the size could be adjusted by adjusting the temperature parameters. Considering that the posi- tively charged polybenzoxazine can be adsorbed on the negatively charged SiO2 surface by electrostatic interaction, the nitrogen-doped hollow carbon spheres were further synthesized by the surfac- tant-free Stöber system designed by Nanfeng Zheng’s group. The synthetic route and the resulting products are shown in Figure 4. When resor- cinol/formaldehyde/ethylenediamine is used as the polybenzoxazine (PB) precursor and TEOS as the SiO2 precursor, the PB/SiO2 composite shell layer can be directly coated on the core surface of SiO2 spheres in a solvent mixture of water and ethanol. In this system, ethylenediamine acts as a base cata- lyst to promote the hydrolysis of TEOS. The posi- tively charged polybenzoxazine and negatively charged silicon oxide oligomers are coassembled 87 and deposited on the negatively charged SiO2 core surface to form the SiO2@PB/SiO2 core-shell structure. This core-shell structure can be further transformed into nitrogen-doped hollow carbon materials. The structure of hollow carbon spheres can also be changed from a spherical shape to a bowl-shaped hollow structure by changing the ratio of TEOS and PB[29]. Figure 4. Silica-assisted polybenzoxazine coating strategy for the synthesis of N doped hollow carbon spheres (a); SEM images of as-obtained products (b–f)[29]. Figure 5. Fabrication of multi-shells hollow carbon spheres using 3-aminophenol/formaldehyde resin[30]. In addition, the hollow carbon spheres were recently prepared by the group of Lijun Wan by se- lective dissolution of 3-aminophen/formaldehyde resin microspheres (3-AF), which discarded the surfactant and silicon oxide template used in the previous method (Figure5)[30]. The polymerization of 3-aminophen and formaldehyde in a mixed sys- tem of water and NH4OH produced 3-aminophen/formaldehyde resin microspheres with a heterogeneous chemical composition. The micro- spheres are composed of 3-AF oligomers and high molecular weight 3-AF with high polymerization degree, and the overall core-shell structure of oli- gomers@high polymers. Since the internal oligo- mer 3-AF can be dissolved by acetone, the prepared 3-aminophenol/formaldehyde resin microspheres can be treated with acetone to obtain hollow 3-AF microspheres. When the hollow 3-AF microspheres were used as a template, the growth and dissolution process was repeated to obtain multishell 3-AF spheres. The corresponding multishell hollow car- bon spheres can be obtained by carbonization. This work provides a controlled pathway for the synthe- sis of multishell hollow carbon spheres. In addition, the Stöber-based strategy for the synthesis of poly- meric microspheres has been extended to other 88 polymers in recent years, for example, the success- ful application of polydopamine provides an effec- tive way to directly prepare nitrogen-doped car- bon-based materials[31]. 2.2 Morphology and structural design of carbon-based hollow structure nanomateri- als The morphology and structure design of car- bon-based hollow nanomaterials are similar to other hollow materials, and the morphology and structure of the templates can be controlled to obtain hollow carbon structures with different morphologies. More importantly, the morphological and functional diversity of carbon-based hollow structure nano- materials can be enriched through the rational de- sign of template@carbon precursor complexes. When a core/shell material is used as a tem- plate,two typical sandwich-like intermediates (e.g., sacrificial layer@functional layer@carbon precur- sor structure and functional layer@sacrificial lay- er@carbon precursor structure) can be obtained by coating the template with carbon precursors, which can be converted into hollow core/shell car- bon based material and Yolk@Shell hollow car- bon based structure after removing the template, respectively. This strategy of modifying hard tem- plates provides an effective way to directly design multifunctional carbon-based functional materials, which greatly facilitates the application of car- bon-based hollow structure nanomaterials[32]. Carbon-based hollow structure nanomaterials have great potential for applications in catalysis, drug delivery, and batteries. For these applications, the design of the shell structure is an important as- pect to further improve the performance of hollow nanomaterials. 2.2.1 Ultra-thin shell layer design In carbon-based hollow nanomaterials, the shell structure plays a decisive role in their performance. The thickness of the shell layer directly affects the physicochemical properties of the material, such as electrical conductivity, specific surface area, poros- ity, density, and the number of active sites available. In energy storage and catalysis applications, nano- materials with ultra-thin hollow shell layers are preferred by researchers. Liu et al. first synthesized SiO2@PDA core-shell structures under alkaline conditions using dopamine as the green carbon source, and then carbonized and etched the core template to obtain ultra-thin hollow carbon spheres with a shell layer thickness of only 4 nm (Figure 6). In addition, Au@C Yolk⁃Shell structure was pre- pared by using Au@SiO2 core-shell structure as a template and used in the reduction of nitrobenzene phenol, which showed high catalytic activity and stability[33]. Han et al. also developed a tem- plate-assisted thermal decomposition method to prepare ultrathin hollow nitrogen-doped carbon spheres loaded with single-atom Co active sites (Figure 7)[34], with a shell thickness of 5 nm. The ultrathin carbon shell layer loaded with single-atom Co is beneficial for improving the utilization of ac- tive sites. In addition, the ultra-thin shell layer facil- itates the transport of the reacting species. The ma terial showed high catalytic activity and stability for oxygen reduction catalytic reactions under acidic conditions. Figure 6. TEM image of super-thin hollow carbon spheres[33]. 89 Figure 7. Schematic illustration of the fabrication (a) and TEM of images of super-thin hollow carbon spheres (b,c)[34]. 2.2.2 The porous shell layer design Nanomaterials with porous shell layer struc- ture can further increase the specific surface area of hollow structured materials compared to hollow micro/nano structured materials with solid shell layer. Most of the research in this area is based on a combination of soft and hard template synthesis methods. In addition to the hard template method for cavity fabrication, porogenic agents are intro- duced during the coating process of the shell layer material, and hollow micro/nano-structured materi- als with porous shell layer structure can be obtained after the removal of the cavity template and the shell layer porogenic agents[35]. For example, hol- low mesoporous carbon microspheres were also successfully synthesized by the co-hydrolysis of organic polymers and inorganic silica-derived pre- cursors by Yu’s group at the University of Queens- land, Australia[36]. A core-shell structure of phenolic resin/silicon composite shell layer with a silicon oxide core was successfully prepared by co-hydrolysis/cross-linking of resorcinol, formal- dehyde and silica in an ethanol/water system, fol- lowed by carbonization to remove the template to obtain hollow mesoporous carbon microspheres (Figure 8). The final hollow structure and mesopo- rous shell structure can also be controlled by con- trolling the amount of silicon source and the type of silicon source in the system. The hollow mesopo- rous carbon was prepared using a combination of soft and hard templates by Lou from Nanyang Technological University. In the alcohol/water sys- tem, the carbon source dopamine, the porogenic agent F127, and the pore expander trimethylben- zene were co-assembled and coated on different substrates catalyzed by ammonia water. After high temperature carbonization, core-shell nanocompo- sites with mesoporous carbon shell layer were suc- cessfully obtained (Figure 9). The removal of the substrate template resulted in hollow mesoporous carbon spheres with different morphologies[37]. 2.3 Functionalization of carbon-based hollow structured nanomaterials 2.3.1 Heteroatom doping effects In order to create more active sites and further improve the catalytic activity as well as the interac- tion between carbon composites and reactants, doping the carbon skeleton with other heteroatoms such as nitrogen (N), boron (B), sulfur (S), phos- phorus (P) or a combination has been widely adopted[38]. Among them, the chemical reactivity, electrical conductivity, affinity for CO2 adsorption and specific capacity for supercapacitors are much better than pure carbon materials due to the introduction of N elements into the carbon skele- ton[39,40]. Nitrogen-doped carbon materials can be prepared by: (1) direct carbonization of precursors containing heteroatoms to obtain heteroatom-doped carbon materials; (2) post-processing methods, which refers to the heteroatom post-merging in the pure carbon material skeleton. Usually, the carbon material is calcined together with precursors con- taining heteroatoms such as ammonia or thiourea. The biggest disadvantage of the post-treatment doping method is that it is difficult for the heteroa- toms to enter the carbon skeleton, only a certain modification of the surface carbon atoms is possible. Therefore, most of the efforts have been devoted to the synthesis of homogeneous heteroatom-doped carbon materials using the first approach. To date, significant progress has been made in the design of heteroatom-doped carbon materials based on the molecular level, especially using phenolic resin 90 polymer precursors. A series of nitrogen-containing compounds have been used to prepare nitro- gen-doped carbon materials, such as: melamine, aminophenol, 4⁃amino⁃3⁃nitrophenol, 4⁃aminophenol, hexamethylenetetramine, and 1,6⁃diaminohexane. In addition, some nitro- gen-containing monomers, such as aniline, pyrrole and dopamine, were also used to prepare nitro- gen-doped carbon materials. The content of nitro- gen in nitrogen-doped carbon materials can be ad- justed by regulating the addition of nitro- gen-containing precursors and calcination condi- tions. Similarly, in the preparation of sulfur-doped carbon and boron-doped carbon materials, the cor- responding heteroatom-doped carbon materials can be obtained by introducing the corresponding monomers. Polyatomic doped carbon materials can be prepared by introducing monomers contain- ing different elements, or by combining various methods to introduce different elements separately. Figure 8. Fabrication of mesoporous carbon structures via a carbon/silica composite shell method[36]. Figure 9. Fabrication of mesoporous carbon spheres via a micelle assembly strategy[37]. 2.3.2 Surface modification of carbon materi- als Carbon materials are often used as catalyst carriers to enhance the performance of catalytically active materials. Two main approaches can be used to functionalize the surface of carbon: (1) deposi- tion of nanoparticles; (2) surface grafting of func- tional groups. The loading of nanoparticles or single atoms on carbon surfaces is often considered an effective way to tune the interfacial properties and improve the molecular interactions to achieve desirable cat- alytic and electrochemical properties. For nanopar- ticles highly dispersed on the surface of carbon ma- 91 terials, the size can be adjusted from nanoscale to single atoms, and the smaller the size, the more cat- alytic sites are exposed, and the corresponding electrochemical and catalytic properties are more desirable. For example, Li’s group has successfully loaded highly dispersed cobalt atoms on the surface of hollow nitrogen-doped carbon spheres using a thermal decomposition method. Using this material for oxygen reduction reactions, the atomically dis- persed cobalt sites expose the maximum number of catalytic sites, and the hollow structure facilitates the rapid transport of oxygen reduction-related spe- cies. This combination of structures gives the mate- rial a high catalytic performance[34]. In addition, a similar structure was used to load cobalt atoms in Co-N5 coordination in nitrogen-doped hollow mes- oporous carbon spheres for the CO2 reduction reac- tion, which exhibited nearly 100% CO selectivity and high stability[41]. Hollow carbon spheres and carbon-based Yolk-shell structures were considered as ideal carriers to prevent the precipitation and ag- gregation of noble metals on the surface of carbon carriers at high temperatures in catalytic reactions. Liu et al. prepared Ag/AgBr@SiO2@RF core-shell structure by one-step Stöber method, and obtained Ag@C Yolk⁃shell nanostructured microspheres by carbon etching the template[42]. In addition, Galeano et al. confined small size Pt nanoparticles in meso- porous hollow carbon materials and used them in electrochemical oxygen reduction reactions with much better stability than commercial Pt/C[43,44]. The functional groups on the surface of carbon materials can be used as anchor sites for metal cat- alysts and contribute to the adsorption of metals on the carbon surface. This usually results in a high dispersion of the metal species on the carbon sur- face and enhances the chemical properties. Com- monly used methods such as oxidation, KOH acti- vation and sulfonation have been widely used for the grafting of functionalized functional groups on the surface of carbon materials[45]. In addition, the polarity of carbon materials can also be changed by the grafting of functional groups, such as the intro- duction of various polar groups such as carboxyl, nitrate, sulfonic acid and hydroxyl groups on the surface of carbon materials so that its own non-polar surface is transformed into a polar sur- face. For example, Wang et al. obtained sulfonated hollow mesoporous carbon spheres (Ar- SO3H-HMCS) by etching the core-shell structure of SiO2@C with hydrofluoric acid and further treat- ment with chlorosulfonic acid[46]. Song et al. successfully synthesized hollow mesoporous carbon spheres functionalized with aryl sulfonic acid by diazo coupling using furfuryl al- cohol as the carbon precursor by the hard template method, and the synthetic route is shown in Figure 10[47]. The amount of ArSO3H grafted on the surface of the carbon material could be controlled by the amount of sulfonate. The results showed that the prepared sulfonate-functionalized hollow mesopo- rous carbon spheres were used as solid acid cata- lysts with high catalytic activity and stability for the alcoholysis reaction of ethyl levulinate or furfuryl alcohol by esterification of levulinic acid. The strong Brönsted acidity of ArSO3H⁃HMCSs plays a crucial role in the catalytic activity. The thin carbon shell layer and its hollow structure shorten the dif- fusion distance of the reacting species and facilitate the rapid transfer between reactants and products. 2.4 Applications of carbon-based hollow structured nanomaterials 2.4.1 Applications in energy storage For chemical batteries, high cycle stability and multiplicative performance is a goal to be pursued in the design of any type of battery. Fast ion and electron transport requires electrode materials with high conductivity and open structure. High cycling stability requires relatively stable active materials and host electrode materials that can mitigate the large volume expansion of active materials during charging and discharging, or effectively inhibit the dissolution of specific active materials in the elec- trolyte. Due to their unique structural properties, such as high conductivity, open structure and large cavities, carbon-based hollow structure nanomateri- als are widely used to load various battery electrode active materials and relieve the volume expansion of active materials. Hollow carbon spheres can be used as host materials for loading high-capacity but unstable electrode active materials. For example, when silicon is used as the anode material for ba- 92 Figure 10. Fabrication of ArSO3H-mesoporous hollow carbon spheres[47]. Figure 11. (a) Fabrication of silicon nanoparticles confined in double-shell carbon nanoparticles; (b) corresponding TEM image of the material, with the selected area electron diffraction (SAED) pattern in the inset; (c) structural change of the material during the charge-discharge process[49]. teries, the capacity decays rapidly due to the rupture of the material structure, and the cycling stability is poor[48]. Chen et al. confined Si nanoparticles in hollow carbon spheres with a bilayer shell struc- ture by a multi-step coating and etching strategy. The inner carbon shell provides voids to buffer the large volume expansion changes of the Si nanopar- ticles, while the outer shell helps to form a stable SEI film (Figure 11)[49]. Similar strategies have been applied to protect other less stable active materials, such as Sn, SnO2, MoS2, etc.[50–52]. Li-S batteries are considered to be one of the most promising secondary batteries due to their high energy density. The cathode active material is non-metallic S. Despite its high theoretical capacity, S has a lot of shortcomings as a battery electrode material. The low electrical conductivity of S, the shuttle effect of lithium polysulfide, and the high-volume expansion limit the practical applica- tion of Li-S batteries[53,54]. To alleviate these defects, hollow carbon spheres are widely used as S host materials for Li-S cells. Carbon-based hollow structured nanomaterials loaded with polar inor- ganic nanoparticles are the most effective strategy to solve the defects of Li-S cells[55–58]. Wu et al. used hollow carbon spheres loaded with cobalt ox- ide nanoparticles (HPCS/CoO) for Li-S batteries, which showed excellent cycling stability[59]. Due to the strong interaction between polar CoO nanopar- ticles and lithium polysulfide, the shuttle effect phenomenon can be suppressed. Compared with the HPCS/S electrode material, the cycling stability of the HPCS/CoO/S composite electrode material is significantly enhanced, and the capacity can still reach 629 mAh·g−1 after 1,000 cycles at a current density of 100 C. In contrast, the specific capacity of the HPCS/S electrode after 1,000 cycles is only 302 mAh·g−1 (Figure 12). The researchers also de- veloped C/Co9S8, C/TiO, Fe3O4@C, MnO2@HCF and other composites for Li-S batteries, which ef- 93 fectively solved the defect problem of S anode and achieved excellent battery performance[60–63]. Figure 12. Schematic illustration of the fabrication of HPCS/CoO/S and cycling performanceof HPCS/CoO/S for Li-S battery[59]. Unlike the storage of lithium ions in the bat- tery anode material, lithium metal batteries directly utilize lithium metal as the anode material. Lithium metal has a higher specific capacity (3,860 mAh·g−1) than the conventional graphite anode material used in Li-ion batteries[64]. Very attractive energy densi- ties (e.g., 3,500 Wh·kg−1 for Li-O2 cells and 2,600 Wh·kg−1 S cells for Li-S cells) can be obtained when lithium metal cathodes are coupled with other high-capacity cathode materials, such as oxygen and sulfur cathodes[65]. Despite these advantages, lithium metal has inherent defects, including high reactivity, large volume changes during cycling, and formation of lithium dendrites[66]. To solve this problem, surface protection of lithium metal is es- sential, and hollow carbon microspheres are ideal for solving this problem. The consistent deposition of Li metal in hollow carbon spheres is a great challenge due to the difficulty in controlling its morphology and deposition location during lithium metal deposition. Until recently, Au nanoparticles were first deposited in hollow carbon spheres, and then controlled deposition of Li metal was success- fully achieved in hollow carbon spheres using Au as the seed (Figure 13). By avoiding direct con- tact between the lithium metal and the electrolyte, Figure 13. (a) TEM images of gold nanoparticles trapped inside hollow carbon nanospheres;(b) Lithium deposition process inside the hollow carbon structure;(c) Corresponding TEM images of the lithium disposition at different stages[67]. 94 this restricted space can effectively improve the stability of the electrochemical cycle of the lithium metal and minimize the undesirable side ef- fects between the lithium metal and the electrolyte. The cycle performance of the lithium metal cell is improved with a coulombic efficiency of 98% in the carbonate electrolyte and excellent cycle stability over 300 cycles[67]. 2.4.2 Applications in electrocatalysis Electrocatalysis is an important process in the field of clean energy conversion (fuel cells, water splitting and metal-air cells). However, the sluggish kinetics of hydrogen precipitation reactions (HER), oxygen precipitation reactions (OER), and oxygen reduction reactions (ORR) limit the practical appli- cations in these fields. In addition, carbon dioxide reduction, methanol oxidation and nitrogen reduc- tion are also key factors in the conversion of re- newable energy sources. Carbon based hollow structured nanomaterials with high electrical con- ductivity can improve the charge transfer efficiency. In addition, hollow structure and porous structure can improve the mass transfer efficiency. Based on the unique structural characteristics of carbon-based hollow structure nanomaterials mentioned above, researchers generally consider carbon-based hollow structure nanomaterials as one of the best candi- dates for electrocatalysts with high activity and sta- bility. The oxygen reduction reaction (ORR) plays a very important role in metal-air cells and fuel cells. Pt electrocatalysts exhibit excellent ORR activity under acidic and alkaline conditions, but their high cost and poor stability have limited the mass pro- duction and further practical application of noble metal Pt. In recent years, researchers have focused on the synthesis of low-cost and stable electrocata- lysts. Carbon based hollow structured nanomaterials are gaining more and more attention due to their low cost, high activity and stability. Non-metallic carbon-based materials can be doped with various heteroatoms such as N, S and P to modify their electronic structure and chemical activity. The elec- tronegativity and atomic radius differences between heteroatoms and carbon atoms lead to changes in the electronic structure of C atoms adjacent to the heteroatoms, which facilitate the adsorption and electron transfer of oxygen species and enhance their ORR activity. For example, Lou’s group de- veloped a strategy to synthesize ultra-thin hollow nitrogen-doped carbon spheres by self-assembling polymer block copolymer F127 and polydopamine composite micelles. The ultrathin shell layer with monolayer mesoporous structure ensured the rapid mass transfer process and high catalytic activity[37]. Although the heteroatom-doped carbon materials exhibited stable and efficient ORR properties in alkaline solutions, their ORR activity under acidic conditions was not satisfactory[68]. To solve this problem, carbon-based materials modified with non-precious metals (Fe, Co, and Ni) have been extensively investigated. 2.4.3 Carbon-based hollow structured na- nomaterials as nanoreactors for catalytic reactions The design of a suitable reactor is essential in chemical reaction engineering. In order to obtain optimal reaction conditions, fluid dynamics, mass transfer, heat transfer, and reaction kinetics must be taken into account. The development of nanotech- nology has transformed the reactor vessel from macroscopic to microscopic structures and opened up the concept of new nano-reactors. From the chemical engineering point of view, the designed and prepared catalysts are considered as nano-reactors. Compared to conventional reactors, nanoreactors offer the advantage of parallel chemi- cal reactions, avoiding the formation of undesirable products and enhancing catalytic performance. For example, in hollow carbon-based catalysts, multiple catalytic sites can be located on the inner and outer surfaces of the carbon shell, in the voids of the car- bon particles and in anisotropic locations to achieve the best catalytic performance. Compared with solid carbon sphere catalysts, hollow carbon sphere cata- lytic nano-reactors have the following advantages: (1) catalytically active species can be loaded inside the cavity to prevent the migration and agglomera- tion of active particles during the catalytic reaction, which leads to the reduction of catalytic activity; (2) selective catalytic activity can be further en- 95 hanced by the regulation of shell structure; (3) mass transfer process can be further enhanced during the catalytic reaction compared with the same mass of bulk catalyst; (4) the catalytic reaction rate can be improved by regulating the hydrophilici- ty/hydrophobicity in the cavity to enrich the reac- tion products in the cavity[69]. Figure 14. (a) Schematic illustration of the synthesis of PtCo@C Yolk-shell micro-spheres; (b) TEM image; (c) EDX mapping images of PtCo@C Yolk-shell micro-spheres[70]. Selective hydrogenation is essential in organic synthesis, such as the synthesis of epoxy com- pounds. Most of the catalysis has to be carried out at high temperatures, which makes the active nano- particles susceptible to sintering and loss of activity during the reaction. Schüth and co-workers devel- oped a simple method to confine 4 nm PtCo nano- particles inside a hollow carbon sphere cavity (Fig- ure 14)[2]. The catalytic conversion of 5-hydroxymethylfurfural to 2,5-dimethylfuran within 10 min was achieved with an efficiency of 100% using this material as a catalyst. The yield of 2,5-furan after 2 h was 98%. In another work, Wang et al. successfully prepared Pd@C Yolk⁃Shell structured catalysts by hydrothermal treatment of emulsions containing P123/sodium oleate/PdCl42- and polymer precursors of 2,4-dihydroxybenzoic acid and hexamethylenetetramine[70]. Recently, Tian et al. also reported a Yolk⁃Shell structured sub-micron reactor loaded with metal nanoparticles in the structure of Pd@Zn/C Yolk⁃Shell[71]. When used as a catalyst for the hydrogenation of styrene from phenylacetylene, the catalyst achieved 99% selectivity, high conversion and excellent catalytic stability. 3. Conclusions and prospects In the past decade, carbon-based hollow struc- tured nanomaterials have been developed by leaps and bounds, and fruitful results have been achieved in synthesis, characterization and applications. In this paper, we summarize the basic synthesis strat- egy, structural design, material functionalization and main applications of carbon-based hollow structured nanomaterials. The unique structure of carbon-based hollow structured nanomaterials has shown great potential applications in many fields. It is this promising application that the re- search, development and application of such mate- rials remain a hot topic in the field of hollow struc- tural materials. 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