The controlled synthesis of complex hollow nanostructures and prospective applications

Abstract

Functionality in nanoengineered materials has been usually explored on structural and chemical compositional aspects of matter that exist in such solid materials. It is well known that the absence of solid matter is also relevant and the existence of voids confined in the nanostructure of certain particles is no exception. Indeed, over the past decades, there has been great interest in exploring hollow nanostructured materials that besides the properties recognized in the dense particles also provide empty spaces, in the sense of condensed matter absence, as an additional functionality to be explored. As such, the chemical synthesis of hollow nanostructures has been driven not only for tailoring the size and shape of particles with well-defined chemical composition, but also to achieve control on the type of hollowness that characterize such materials. This review describes the state of the art on late developments concerning the chemical synthesis of hollow nanostructures, providing a number of examples of materials obtained by distinct strategies. It will be apparent by reading this progress report that the absence of solid matter determines the functionality of hollow nanomaterials for several technological applications.

1. Introduction

Hollow nanostructures with controlled chemical composition and morphology have emerged as an important class of materials [1–4]. These nanostructures can be regarded as particulate materials with interior cavities and uniform morphologies, displaying well-defined boundaries [1,3–6]. Because of the existence of cavities, the surface area in these particles is significantly larger while the density is much lower than that of their dense solid counterparts, for equal volumes of material. The voids created in these particles can be used for different purposes, such as the storage of different cargoes. Indeed, hollow nanostructures are regarded as functional nanomaterials with attractive properties such as high specific surface areas, large pore volume and high loading capacity, which endow them with potential applications, namely in lithium-ion batteries [7–12], catalysis [13–17], drug delivery [6,18–21], sensors [22–25], water treatment [26–30], nanoreactors [31,32] and dye-sensitized solar cells [33–37].

Generally, hollow structures can exist in different configurations and diverse architectures have been fabricated in the form of spheres [38–43], tubes [44–49], fibres [50–54], boxes [55–61], among many others. Although these materials share some features with other common porous materials, such as zeolites, they are distinct on the type of porosity and mainly because the voids result from the formation of well-defined hollows confined by an inorganic compound that in other forms can also exist as a denser material. In fact, the most used synthetic routes for the preparation of such hollow structures have been template-based strategies. In these chemical routes, surface-mediated processes are crucial in determining the final morphology, firstly in the selection of hard- or soft templates, and also in the preparation of the template, the surface coating of the template and, finally, the removal of the template by a post-treatment step [6,13,62,63]. In addition, free template routes based on chemical etching, Ostwald ripening driven processes, Kirkendall effect and galvanic replacement have also been applied for the fabrication of hollow materials [6,13,62,63]. Free template methods will not be reviewed here.

An interesting possibility in fabricating hollow nanostructures is the use of nano-objects that are shaped into well-defined and hierarchically more complex particles. In these cases, besides the functionality that arise due to the presence of the empty spaces, the nanostructured walls themselves can be exploited for specific functions, depending on the type of functional nanoparticles employed in the building-up process. An example of such methods involves colloidal templating as described by Caruso and co-workers for the fabrication of hollow SiO₂ and inorganic/polymer hybrid spheres [64]. In these syntheses, the hollow spheres are prepared by combining colloidal templating and shell-by-shell deposition, followed by removal of the sacrificial core. Previously, Matijeć and co-workers have reported a variety of colloidal core/shell particles with different morphologies [65–67]. The combination of both approaches provides a number of possibilities for the fabrication of hollow structures based on hard-templating methods, thus allowing a wider range of hollow materials available for applications.

This review focuses on recent developments in the controlled synthesis of complex hollow structures by the following strategies: (i) hard-templating; (ii) soft-templating and (iii) self-templating. For each method, we highlight the main aspects to take into consideration envisaging the control of the structure and morphology of the final hollow particles. Finally, a brief outlook about their main applications will be presented.

2. Classification of hollow nanostructured particulates

Hollow nanostructures can be divided into simple or complex by taking into account their structural complexity (figure 1). Based on the number of the outer shells, these particles can be termed as single-, double- and multi-shell hollow structures. Single-shell hollow structures have a single-layer encapsulating a central cavity. On the other hand, multi-shell hollow structures have multiple interior cavities and multiple boundaries, as such there is an increase in the number of interfaces [1,3,63]. Compared with simple hollow structures, complex hollow materials are composed of multiple shells that in principle provide additional functionalities for several applications. Because of their complexity, the synthesis and manipulation of complex hollow structures are more challenging when compared with the single-shell counterparts.

Figure 1.

Classification of complex hollow structures based on structural and chemical composition. (Online version in colour.)

In addition, different nomenclatures can be found in the literature according to the number of shells and type arrangement. Figure 2 illustrates the most common configurations for a spherical hollow structure. In the simplest form, the structure is a single-shell hollow particle, but the architecture complexity can be increased by adding shells that lead to multi-shell or onion-like hollow particles. The latter definition is usually reserved for cases where the number of shells is superior to two or three. Either single or multi-shell hollow particles can contain a discrete solid particle in the central cavity, resulting in the so-called yolk–(multi)shell particles. When the cargo material comprises multiple particles, hollow materials are typically denoted as rattle-type particles. More complex multi-shell hollow structures may combine a single-yolk particle in the central cavity and a rattle-type cargo within the inter-shell spacing.

Figure 2.

Schematic of different configurations of hollow structures. For convenience, a general spherical morphology is adopted in all cases. Spheres represent metal (oxide) nanoparticles. (a) Single-shell, (b) multi-shell, (c) yolk-shell, (d) rattle-type and (e) yolk multi-shell hollow particles. (Adapted with permission from Prieto et al. [13] Copyright © 2016, American Chemical Society.) (Online version in colour.)

3. Strategies for the controlled synthesis of complex hollow structures

A wide diversity of methods have been proposed to obtain complex hollow structures, which in a number of situations use commonly available precursors [1,3,4,6,18,62,63]. Here, the fundamentals of such approaches will be discussed by selecting illustrative cases from the literature. For a comprehensive discussion, the strategies for the controlled synthesis of complex hollow structures have been grouped into three categories considering the types of templates employed for the creation of the hollow core. In a typical process, the templates are firstly prepared, followed by coating the outer surface with a layer of a shell material, thus providing a precursor composite type (shell)@(template). Then, the hollow structures are obtained after target removal of the sacrificial templates. Most of the templating syntheses of hollow structures could be classified as ‘bottom-up’ techniques [68]. The cavity size and shape are determined by the size and shape of the template, while the shell thickness is generally adjusted during the coating process.

(a). Hard-templating strategy

Hard-templating methods for the fabrication of hollow structures are generally simple, effective and straightforward [5]. Typically, the process involves four major steps: (i) formation of the hard template material, typically an inorganic compound or polymer that can be chemically etched in a subsequent step; (ii) chemical modification of the template's surface; (iii) coating the template by growing a distinct inorganic phase and (iv) selective removal of the internal template in order to obtain the hollow structures. The modification of the template's surface might involve chemical functionalization, which is generally a challenging step because it requires robust methods to efficiently precipitate the shell materials on substrates with sizes in the micro/nanometre range. The chemical functionalization of the hard template surface can improve the compatibility with the shells, for example, by conferring specific functional groups for the subsequent coating/deposition step. In order to achieve a successful coating on the surface of the template, a modification step is applied that changes for instances the charge distribution and polarity at the surfaces. A number of methods, such as sol–gel process or hydrothermal reactions, have been used to deposit the shell materials on the template's surface. Thus, the shells can be grown onto the hard template by employing distinct strategies, that rely on well-known methods, such as sol–gel processes, hydrothermal reactions, electrostatic assembly and chimie douce routes. For the removal of the template, three main approaches have been used such as chemical etching, thermal treatment or by dissolving the template in an appropriate solvent, which is very common for the case of a polymer template. Using either approach, a judicious choice of the experimental conditions is required to prevent collapse of the shells during template removal, namely by taking into consideration the chemical nature of the hard template. Hence, commonly employed hard templates include amorphous nanosilicas, metal carbonates and polymer beads (latex). These templates are normally used in the form of spherical particulates with a narrow particle size distribution and can be prepared in large amounts using well-known procedures. The synthesis of non-spherical hollow structures remains quite challenging due to lack of suitable non-spherical hard template materials as well as difficulties in achieving uniform surface coating on templates with sharp edges and corners [1,13,63]. Nevertheless, hollow structures with cubic, ellipsoidal and octahedral morphologies, among others, have been reported [69–72]. In the following sections, we will discuss the use of distinct hard templates that are commonly used to synthesize complex hollow structures.

(i). Polymer template-based methods

Although the definition of a polymer as a hard template is debatable, in this context, it is commonly defined as such because the main criterion considers the type of chemical bonding at the template/shell interface, which is assumed to involve covalent linkages [73]. Also, this approach shares some similarities with methods that use inorganic templates, such as the application of a chemical transformation in which the polymer plays the role of a sacrificial template [74]. Hence, common polymers used in this context include poly(methyl methacrylate) (PMMA) and polystyrene (PS), which can be removed by dissolving in an organic solvent or following thermal treatment. For example, Li et al. have reported the preparation of nearly monodispersed PMMA double-shelled hollow spheres and SiO₂ core-in-double-shell hollow spheres via hard-templating [75]. Alternate SiO₂/polymer tetra- and penta-layered hybrid spheres were firstly prepared via combined sol–gel process and distillation–precipitation polymerization. PMMA hollow spheres with asymmetric double shells were then produced after HF etching of the silica layers in the SiO₂@PMMA@SiO₂@PMMA tetra-layer, generating in turn double-shelled PMMA hollow spheres. On the other hand, the calcination of the PMMA layers in the SiO₂@PMMA@SiO₂@PMMA@SiO₂ penta-layered spheres produces SiO₂ core-in-double-shell hollow spheres.

Tu et al. have reported the synthesis of robust hollow spheres comprising alternate titania (Ti_0.91O₂) and graphene nanosheets fabricated by a layer-by-layer assembly technique and using PMMA beads as sacrificial templates [76]. The PMMA beads were first modified with positively charged polyethylenimine (PEI), negatively charged titania nanosheets, another layer of PEI and then treated with graphene oxide (GO) suspensions, that at the working pH were negatively charged. The process was repeated sequentially to ensure enough titania and GO loading. After microwave irradiation in Ar atmosphere, the GO was transformed into reduced graphene. The PEI moiety was then removed, and the PMMA spheres as sacrificial templates were decomposed into an exhaust gas. The remaining PMMA residue was removed with tetrahydrofuran. Su and co-workers have prepared hollow polypyrrole nanospheres by in situ polymerization of pyrrole monomer on PMMA surfaces, but in this case the PMMA cores were removed by acetone treatment [77]. Recently, Xu et al. have reported the synthesis of hollow mesoporous SiO₂ nanoparticles via a dual template method that used poly(styrene-co-methyl methacrylate-co-methacrylic acid) (PS-PMMA-PMAA) as hard template and cetyltrimethylammonium bromide (CTAB) as a mesostructure directing agent [78]. The hollow mesoporous SiO₂ nanoparticles were obtained after calcination of the template. Many other hollow structures have been prepared using polystyrene (PS) as a sacrificial hard template. For example, Yang et al. reported a synthetic approach to produce double-shelled hollow spheres using latex spheres as hard template [79]. First, the synthesis involved the sulfonation of PS hollow spheres followed by the immersion of these particulates into tetrabutyl titanate. This process generates sandwich type TiO₂@PS@TiO₂ hollow spheres. Finally, selective removal of PS by a suitable solvent or calcination leads to the formation of TiO₂ double-shelled hollow structures. Liang et al. have used the bowl-like sulfonated PS hollow particles for the growth of Ni-precursor nanosheets [80]. The drying process compresses the sulfonated PS hollow spheres into bowl-like hollow particles due to the capillary force during solvent evaporation. Recently, Yang and co-workers have reported the fabrication of hollow periodic mesoporous organosilica particles with perpendicularly ordered mesopores, using PS as sacrificial hard template for the generation of the hollow cavities [81]. The PS template was then removed by organic solvent treatment.

Zinc oxide has been a prototype material in the design of morphological distinct nano structures, including the synthesis of ZnO hollow nanostructures [82]. Thus, hollow urchin-like ZnO thin films [83], ZnO hollow hemispheres [84] and inter-connected hollow carbon nanospheres [85], were prepared by templating against PS crystalline arrays. In all these cases, PS multilayer arrays were first self-assembled on a substrate, followed by an electrodeposition or oblique angle deposition process that is used to synthesize the shell layer on PS surface. Finally, the hollow structure arrays were obtained after removing the PS templates via calcination or organic solvent dissolution. Trindade et al. have shown that ZnO-based hollow particles can be obtained by using commercial available templates such as PS beads normally used as a cationic exchange resin. In this process, hollow ZnO particles were obtained after calcination of the resin beads, which were previously coated with a basic zinc carbonate generated in situ by homogeneous hydrolysis of urea [86]. These hollow structures were then investigated as substrates for the deposition of BiVO₄, a technological relevant but expensive inorganic pigment [87]. In this case, the hollow ZnO particles were firstly prepared and dispersed in a chemical bath containing Bi(III) sequestered by the ligand EDTA and vanadate species. An increase of the reacting mixture induces the release of cationic bismuth that will precipitate in the form of monoclinic BiVO₄ onto the hollow ZnO structures.

(ii). Silica template-based methods

Amorphous silica particles have been widely used as hard templates namely because they are available with high morphological uniformity and tunable particle size distribution at low cost [74]. Typically, monodispersed SiO₂ spheres in the micrometre size range are obtained through the classical sol–gel method (aka Stöber method) that involves the hydrolysis and condensation of silicon alkoxides in a water/alcohol mixture and in the presence of a catalyst (e.g. ammonia) [74]. For example, Jang et al. have reported the hard-templating synthesis of TiO₂ hollow spheres with controllable shell numbers [88]. In this approach, SiO₂ spheres prepared by the Stöber method were employed as the hard templates. TiO₂ and SiO₂ layers have been alternately coated on the surface of SiO₂ spheres to obtain multi-layered SiO₂@TiO₂ core–shell spheres. TiO₂ double-shelled hollow spheres and triple-shelled hollow spheres were obtained after calcination and NaOH etching. Lou et al. prepared tin oxide (SnO₂) multi-shelled hollow colloids with nano-architectured walls via a shell-by-shell method [72]. Briefly, hydrothermally generated polycrystalline SnO₂ was deposited on SiO₂ spheres forming a uniform single-shell, then by repeating the deposition process the formation of double shells could be promoted. It was found that the second shell did not attach closely to the first shell. After removal of the inner silica template, the multi-shell SnO₂ structure was obtained.

Yu and co-workers extended the Stöber method to the synthesis of various hollow carbon colloids, including double-shelled and triple-shelled hollow spheres (figure 3) [89]. The synthesis was carried out under sol–gel conditions using a mixture of TEOS, resorcinol and formaldehyde. For the synthesis of double-shelled hollow spheres, TEOS was re-introduced in the reacting mixture at a given reaction time. By controlling the polymerization kinetics after mixing both systems, sequential heterogeneous nucleation could be induced leading to monodispersed and mesostructured hollow carbon nanoparticles with large mesopores, controllable mesostructures (bi- and triple-layered) and rich morphologies (intact, endo-invaginated and invaginated spheres). The hollow carbon spheres were obtained after thermal treatment of the precursor composites, at 700°C under N₂ atmosphere, and subsequent removal of silica by hydrofluoric acid etching. Li et al. have reported the synthesis of double-layer hollow silica shells by templating against mesoporous hollow SiO₂ shells [90]. The second hollow shell grew inside the hollow template, in which the key parameter was the surface modification of organosilane on the template surface.

Figure 3.

TEM images of carbon hollow particles: (a) intact double-shelled hollow spheres, (b) endo-invaginated double-shelled hollow spheres, (c) intact triple-shelled hollow spheres and (d) invaginated triple-shelled hollow spheres. (Adapted with permission from Zhang et al. [89] Copyright © 2015, American Chemical Society.)

The hard-templating strategy has been applied to obtain non-spherical hollow structures, using SiO₂ as sacrificial template. For example, Lou and co-workers have reported the synthesis of hematite (α-Fe₂O₃) spindles with a SiO₂ layer using the Stöber process, which produces ellipsoidal α-Fe₂O₃@SiO₂ core–shell particles [91]. The ensuing α-Fe₂O₃@SiO₂ particles were then employed as the hard templates and subjected to hydrothermal shell-by-shell deposition of polycrystalline SnO₂. After SiO₂ removal, double-shelled hollow SnO₂ nanococoons with movable α-Fe₂O₃ spindles were obtained via repeated hydrothermal deposition or via single-step hydrothermal deposition, in case a lower amount of template was used. Similarly, Yu and co-workers have reported the preparation of metal sulfides (M = Ni, Cu, Mn) box-in-box hollow structures using SiO₂ nanoboxes as templates [92]. The SiO₂ nanobox templates were firstly synthesized by templating against cubic Fe₂O₃ nanostructures. These materials were then submitted to a hydrothermal treatment leading to metal silicate box-in-box hollow structures. Finally, the metal silicates have been converted to the corresponding metal sulfides by sulfidation in the presence of Na₂S, retaining the particle morphology of the template used. Metal sulfide hollow structures can also be obtained by employing a single-molecule approach that provides the metal and sulfide in a single precursor compound. Hence, Trindade et al. have explored the liquid phase thermolysis of metal dialkyldithiocarbamates in the presence of several types of heterogeneous substrates in order to obtain metal sulfides deposited onto amorphous SiO₂ nanoparticles [93], TiO₂ particles [94], carbon nanotubes [95] and GO sheets [96]. This method can be extended to coat sacrificial templates (e.g. MnCO₃) with a metal sulfide, leading to composite particles that dissolve in moderate acidic conditions, resulting in hollow metal sulfide nanostructures as illustrated in figure 4 for a sample obtained in our laboratory. Although this method does not involve an assembly process into hierarchically complex hollow structures, on the other hand, it is one of the few examples on the use of a single-source for hollow metal sulfides.

Figure 4.

Metal sulfide coating of MnCO₃ particles using a single-source approach envisaging the production of precursor particles for hollow structures after acidic treatment (unpublished results, courtesy of Dr M. C. Neves, University of Aveiro, 2017).

(iii). Carbon-based template methods

Carbon spheres have been used as the hard templates for preparing complex hollow structures because of their low cost and feasible complete removal. Moreover, carbon spheres with porous surfaces can sorb the chemical precursors used which might facilitate the shell formation process. Thus, hard templates such as carbonaceous microspheres (CMS) obtained from hydrothermal methods are porous enough to allow deep penetration of highly concentrated metal cations, which provides a route to construct multi-shelled hollow spheres [97–99]. For instance, Lai et al. have reported a sequential templating approach to multi-shelled hollow metal oxide microspheres by using CMS as templates to adsorb metal ions followed by a one-step heating process [98]. A series of multi-shelled hollow microspheres of metal oxides, such as α-Fe₂O₃, Co₃O₄, NiO, CuO and ZnO, have been successfully synthesized. The number of shells could be adjusted by the feeding rate of the metal precursors.

Zhang et al. have reported a strategy for the synthesis of various mixed metal oxide multi-shelled hollow spheres, including CoMn₂O₄, Co_1.5Mn_1.5O₄, MnCo₂O₄, ZnMn₂O₄, ZnCo₂O₄ and NiCo₂O [99]. First, CMS were obtained by a hydrothermal method and then were dispersed in an ethylene glycol solution of metal acetate precursors. In order to load the metal ions into the CMS, the reacting mixture was heated up to 120°C and, in a second step, the temperature was further increased to 170°C. During this process, the metal glycolate was formed in both the interior and outer surface of the CMS. Finally, multi-shelled hollow spheres were produced through annealing in air. This method has been applied to various transition metal oxides by employing different transition metal ions in the polyol process, which is especially advantageous for the synthesis of mixed metal oxide hollow spheres (figure 5). In addition, the number of shells and the interior cavities can be accurately controlled by adjusting synthetic parameters such as concentration of metal cations, diffusion rate of hydrated metal cations, heating conditions and sorption capability of the CMS used as templates. Moreover, this method can be used to synthesize a series of multi-shelled metal oxide spheres, such as MnO₂, MoO₃, Cr₂O₃ V₂O₅ and WO₃ using CMS templates, confirming the versatility of this hard-templating route [100].

Figure 5.

FESEM images of (a) CoMn₂O₄, (b) Co_1.5Mn_1.5O₄, (c) MnCo₂O₄, (g) ZnMn₂O₄, (h) ZnCo₂O₄ and (i) NiCo₂O₄. TEM images of complex hollow spheres of mixed metal oxides: (d) CoMn₂O₄, (e) Co_1.5Mn_1.5O₄, (f) MnCo₂O₄, (j) ZnMn₂O₄, (k) ZnCo₂O₄ and (l) NiCo₂O₄. The scale bars are 500 nm. (Adapted with permission from Zhang et al. [99] Copyright © 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Zong et al. have reported the fabrication of multi-shell hollow spheres of BiVO₄ using CMS as sacrificial template [101]. The complexity of the resulting hollow spheres was tuned by controlling several deposition parameters such as the solute precursors concentration and the heating rate applied. Xu and co-workers have reported the synthesis of multi-shelled copper oxide hollow composite microspheres whose number of shells was controlled by using CMS templates [102]. The carbohydrate source used to obtain the CMS influenced the composition of the final copper oxides spheres. For example, by changing the source from sucrose to glucose, hollow spheres of cupric oxide (CuO) and cuprous oxide (Cu₂O) were synthesized instead of pure CuO hollow spheres. In another report, Wu et al. described the synthesis of uniform multi-shelled NiO hollow spheres using glucose as the carbon source [103]. Noteworthy, quintuple-shelled NiO hollow spheres were reported for the first time by a simple shell-by-shell self-assembly process, that allowed to control the number of shells by adjusting the thermal treatment conditions. The shell-by-shell self-assembly process in this method is an important feature because it promotes the formation of more shells within the CMS templates, making the method extensive to other metal oxides [103].

(b). Soft-templating methods

Several strategies have been proposed for the preparation of complex hollow structures based on soft-templating approaches. The illustrative methods explored in the next sections allow the synthesis of a diversity of hollow materials with variable chemical composition, including SiO₂, metal oxides, carbon-based or polymer-based materials. In general, these soft methods relate to the use of templates that are either in fluid or in gas form (e.g. emulsion droplets, vesicles/micelles and gas bubbles). These methods make it easier to refill the hollow interior with dispersed functional species or to encapsulate in situ guest molecules during shell formation. However, when compared with other approaches, the control of particle shape can be more difficult using soft methods. Among these methods the following will be discussed next: (i) emulsion templating, (ii) vesicle/micelle templating; (iii) gas bubble templating and (iv) electrospray.

(i). Emulsion template-based methods

Systems comprising two or more immiscible liquids may be explored as templates for the preparation of complex hollow structures of variable sizes. The dispersion of such immiscible liquids by an emulsification step result in a dispersed phase and a continuous phase, where the boundary of the two phases is defined by an interface. Because of the low thermodynamic stability of these dispersions, amphiphilic molecules (surfactants) are employed to reduce the interfacial tension. Depending on the composition of both phases, an emulsion can be defined as direct (oil-in-water) or reverse (water-in-oil) [5]. Many emulsion-based methods involve metastable conditions and are highly sensitive to templating conditions, such as temperature, pH, ionic strength, solvent or organic/inorganic additives. While this sensitivity can be exploited for the preparation of highly complex hollow nanostructures, it also presents limitations under harsh chemical conditions. As a result, a tight control on the reaction parameters is important to prepare hollow materials through these methods [3].

Direct emulsion templates have been used for the preparation of polymer-based complex hollow nanospheres. Hence, Wichaita et al. have reported a one-pot synthesis of hollow latex core–shell particles by seeded emulsion polymerization of methyl methacrylate/divinyl benzene/acrylic acid (MMA/DVB/AA) on natural rubber seeds (NR) [104]. The use of tert-butyl hydroperoxide/tetraethylene pentamine (t-BuHP/TEPA) as redox initiators resulted in localized polymerization at the surface of the NR-seeds. After swelling with monomers, DVB in NR-seeds moved outwards to compensate the copolymerization. This resulted in phase separation between the seed and the polymeric shell, which was then exploited to form a void space using MMA/DVB at a molar ratio of 2.7/1. By varying the monomer to seed ratio it was possible to control the size of the cavity. This strategy allowed the preparation of a double-layered NRP(MMA/DVB/AA) nanocomposite with an average size of 298 ± 58 nm.

Pan et al. have reported the preparation of magnetic/hollow double-shelled imprinted polymers (MH-MIPs) by Pickering emulsion polymerization [105]. In a Pickering emulsion, particulates are used to achieve stabilization instead of common surfactants or block copolymers. In this work, a magnesium aluminium phyllosilicate [(Mg,Al)₂Si₄O₁₀(OH)·4(H₂O), attapulgite (ATP)] was used as stabilizer to form oil-in-water emulsions along with hydrophilic Fe₃O₄ nanoparticles, which allow magnetic separation. While ATP and Fe₃O₄ were dispersed in the continuous phase (water), the oil phase was composed by styrene (ST), 4-vinylpyridine (VPy), methylacrylic acid (MA), λ-cyhalothrin and 2,2′-azobis(2-methylpropionate) dissolved in hexadecane. The imprinted system was tailored by using 4VPy and MA as functional monomers for the formation of self-assembled complexes with λ-cyhalothrin via π-π stacking interactions and hydrogen bonding. The ensuing double-shelled particles present good stability, high affinity and magnetic properties, thus offering an efficient way for the removal of λ-cyhalothrin from waters. An interesting strategy for the preparation of multi-ocular hollow spheres involved an ultrasound-assisted emulsion droplet template method. In this case, the self-assembly of diphenylalanine (FF) was promoted by using emulsion droplets as templates and glutaraldehyde as crosslinker. By using ultrasound and exploiting the relative miscibility of two liquids (i.e. hexafluoro-2-propanol to hexane/toluene), the authors were able to obtain different emulsion droplet templates with no requirement of additional surfactants [106].

Unsurprisingly, just taking into consideration the use of porosity in zeolites and mesoporous materials, hollow structures have also great potential in catalysis. The preparation of Au and Pt hollow capsules with single holes for catalytic applications has been reported [107]. In order to obtain these complex structures, the authors started with the preparation of Cu₂O multifaced particles on wax, resulting on the formation of a Pickering emulsion. These particles were then used as sacrificial templates for the subsequent asymmetric galvanic reaction at the oil–water interfaces (Cu₂O stabilized wax beads), which resulted in the formation of open-mouthed Au and Pt hollow capsules. Recently, Zha et al. have reported structurally complex hollow α-Fe₂O₃ chestnut buds and nests by a solvothermal process using water/glycerol and water/2-propanol as mixed solvents, respectively (figure 6) [108]. This quasi-reverse emulsion soft-templating approach can be used to tailor the monodispersity and structure of the final particulates. These particles were then used to capture Cd(II) species with a maximum adsorption capacity of 175.8 (chestnut buds) and 126.3 mg.g⁻¹ (nests).

Figure 6.

TEM and SEM micrographs of αFe₂O₃ chestnut nets (a–c) and buds (d–f) prepared by a quasi-reverseemulsion soft-templating method. (Adapted with permission from Zha et al. [108]. Published by The Royal Society of Chemistry under the Creative Commons (CC-BY) license.) (Online version in colour.)

Multiple emulsions (e.g. W/O/W or O/W/O) can be exploited for the preparation of hollow materials. Recently, Nabavi et al. have described the formation of multicore double emulsion droplets by microfluidic generation [109]. Dual-core double emulsion drops were then used as soft templates for the preparation of polymeric capsules by ‘on the fly’ photopolymerization. Adjusting the concentration of the lipophilic surfactants present in the medium allowed to control the morphology of the capsules. These surfactants have a hydrophilic–lipophilic balance inferior to 10, thus tend to originate W/O emulsions. Zhang et al. have prepared hollow particles of variable complexity by using a double emulsion method (W/O/W). Basically, glycerol was added into a solution of PLLA in water/THF resulting in the formation of glycerol microdroplets. This was followed by a phase inversion process, which resulted in the formation of glycerol-in-polymer/solution-in-glycerol double emulsions. By immersing these double emulsion structures in liquid nitrogen and subsequent extraction of the solvent and glycerol, nanofibrous hollow microspheres were obtained. Interestingly, the authors also investigated the effect of a glycerol derivative (diacetin) at variable concentrations, resulting in the formation hollow particles ranging from hollow spheres to discs [110].

(iii). Micelle or vesicle-based soft-template methods

Micelles can be formed by the self-assembly of amphiphilic molecules in a single-phase solvent. The self-assembly process is observed when the concentration of such molecules exceeds the critical micelle concentration. Through this strategy, hollow materials can be obtained either by the direct assembly of the precursors or via chemical interactions between the molecular precursors and the surface of the template. Similar to emulsion methods, and depending on the type of solvent, micelle templates can also be divided into direct or reverse methods [5]. Many reaction parameters can be exploited to prepare micelles/vesicles of variable shapes, such as the concentration of surfactant, ionic strength, temperature, pH or chemical additives. Hence, Qiu et al. have described the preparation of patchy and hollow rectangular platelet micelles for the seeded growth of highly complex hollow structures. In this case, diverse hollow structures were achieved by adding blends of crystalline coil block copolymers and the corresponding crystalline homopolymer to the cylindrical micelle seed. By varying several reaction conditions, concentric rectangular patches with distinct coronal chemistries were achieved [111]. The use of non-conventional surfactants can be also explored together with self-assembly methods. Wang et al. described the synthesis of multi-layered single crystalline Cu₂O hollow spheres, using CTAB as vesicle-template. By varying the concentration of CTAB, it was possible to control the number of Cu₂O layers [112]. Thus, Landsmann et al. proposed the use of bolaform surfactants, consisting of 11 tungsten atoms, for the synthesis of nanosized polyoxometalate (POM) hollow structures [113]. These surfactants were composed of negatively charged [PW₁₁O₃₉]³⁻ head groups and positively charged $\text{N}{{\text{R}}_4}^{+}$ endgroups with long alkyl chains as spacers. By varying the reaction temperature, it was possible to control the self-assembly process and prepare bigger particles, resembling layer-by-layer assembly methods.

Hollow structures made of biocompatible materials are particularly useful for drug delivery applications. In this respect, amorphous SiO₂ and carbon are particularly useful materials for these purposes. Meka et al. have proposed the synthesis of complex amine functionalized hollow dendritic mesoporous SiO₂ nanospheres for protein delivery. Basically, tetraethylorthosilicate (TEOS) and 3-aminopropyltriethoxysilane were introduced into an oil/water mixture of chlorobenzene-water, resulting in the formation of composite vesicles. These preformed vesicles were then exploited for the assembly of the final product [114]. Recently, Chen et al. proposed the synthesis of asymmetric flask-like hollow carbonaceous structures with uniform morphology and narrow size distribution by using mixed micelles. Basically, the reacting conditions provided by the several precursors employed, contributed for the selective breaking of the carbonaceous shells, which promoted further growth of the shell into a flask-like structure. By exploring variable reaction parameters (e.g. temperature and time), particles of different sizes were obtained (figure 7) [115].

Figure 7.

SEM (a–d) and TEM (e–h) images of flask-like hollow carbonaceous nanostructures prepared under different hydrothermal times. (Adapted with permission from Chen et al. [115] Copyright © 2017, American Chemical Society.) (Online version in colour.)

(iii). Gas bubble-based soft templates

Gas bubbles dispersed in a liquid phase can be exploited as soft templates for the preparation of complex hollow materials. The process involves the formation of bubble emulsions with the subsequent deposition/adsorption of the precursor at the surface of the gas bubbles. The templating effect of the bubbles is affected by several parameters such as surface charge, particle size or hydrophilicity. Gas bubbles emulsion systems can be obtained by several strategies such as, sonication, gas blowing or chemical reactions [5]. For example, Zuo et al. described the bubble template-assisted synthesis of inorganic fullerene-like MoS₂ nanocages (MoS₂, approx. 100 nm), forming ‘close edge’ nanocages by employing an ammonia cation bubble template. These nanocages were then investigated regarding their performance as a novel material for lithium-ion batteries. It was shown that the MoS₂ hollow nanocages have improved lithium storage performance with high reversible capacity, good cycling behaviour and high rate capability [116].

Chen et al. proposed a bubble template reaction for the synthesis of complex honeycomb-like hollow microspheres for a Li_1.2Mn_0.52Ni_0.2Co_0.08O₂ cathode material. In this case, a bubble-bath reaction was achieved by the thermal decomposition of hexamethylenetetramine (HMT). By varying multiple reaction parameters, such as the nature of the precursor or decomposition temperature, particles with variable complex morphologies were achieved. In this reaction, the CO₂ bubbles produced by the HMT decomposition acted as soft template for the deposition of the transition metal ions, leading to the formation of seeds for further crystal growth. Preliminary electrochemical experiments indicated a high capacity and superior retention after 100 cycles, appearing as a promising platform for high-performance cathode materials [117]. Yec and Zeng have described a surface catalysed dual templating method for the preparation of manganese silicate nanobubble constructed hollow microspheres, mainly based on braunite-1Q (Mn²⁺Mn³⁺₆[O₈|SiO₄]), but other manganese silicates were observed. This synthesis involved the hydrothermal treatment of amorphous SiO₂ spheres in the presence of Mn(CH₃CO₂)₂. In this case, the Mn(II) species served as catalyst for the decomposition of carboxylate anions, by decreasing the activation energy of the reaction. This resulted in the in situ generation of CO₂ which served as template for the formation of manganese silicate nanobubbles. On the other hand, the SiO₂ beads served as hard templates, for the direct assembly of tiny nanobubbles into microbubbles (figure 8). These hollow nanostructures were then investigated as effective heterogenous catalysts for the degradation of organic dyes and for the preparation of other hollow materials [118].

Figure 8.

The preparation of complex Mn silicate hollow spheres using CO₂ bubbles as soft template. (Adapted with permission from Yec et al. [118] Copyright © 2014, American Chemical Society.) (Online version in colour.)

(iv). Electrospray techniques

Electrospray techniques have been used to prepare a diversity of hollow nanostructured materials. In these methods, a liquid is pumped through a stainless-steel capillary tube and an electric field is then applied in order to compete with the surface tension of the liquid and promote the formation of a sharp cone. The applied electric field is also important to break the cone into finely divided liquid droplets. The formation of the hollow materials is usually attributed to a series of events mainly due to evaporation of enclosed solvent and ripening process. The morphology of the final material can be finely controlled by changing several conditions such as, the applied voltage magnitude, type and concentration of precursor, flow rate or temperature of the collecting substrate [5].

Chen et al. have described a one-step preparation of TiO₂ microparticles with multiple compartments using a compound fluidic electrospray method (i.e. non-contact). In this technique, two liquid components were enveloped by using a Ti(OBu)₄ solution (containing PVP, ethanol, acetic acid and rhodamine B for labelling) as the shell precursor and several stained paraffin/glycerol oils as filling agents (figure 9). Given the versatility of the set-up (figure 9a), the authors suggested that a simple adjustment of the capillary system allows the production of a multicomponent system with the possibility to incorporate diverse loading agents. As a result, this strategy provides a way to prepare hollow structures designed for multicomponent drug delivery or as multireactors [119].

Figure 9.

Scheme of the electrospray system for the preparation of the capsules (a); SEM images of TiO₂-based capsules (b); LSCM overlay image of TiO₂ composite (stained in red) showing two compartments (one stained in blue) containing paraffin (c); TEM image of TiO₂ particles after removal of the contents through calcination (d) and the organic-based multicomponent microcapsules where the compartments were filled with glycerol containing different stains (e) (Note: Scale bar 1 µm for (a)–(d) and 20 µm (e)). (Adapted with permission from Chen et al. [119] Copyright © 2008, American Chemical Society.) (Online version in colour.)

The electrospray approaches have been explored for the fabrication of hollow nanostructures of hybrid chemical nature, i.e. with inorganic and organic components. Hence, Yunoki et al. have described the preparation of inorganic/organic polymer hybrid microcapsules with high encapsulation efficiency using such techniques. An aqueous solution containing CaCl₂ and chitosan was electrosprayed into a phosphate solution to form a calcium phosphate shell on the sprayed droplets. By varying the electrospray conditions, capsules of complex shapes where obtained. The ensuing capsules were then explored for the encapsulation of cargo, including fluorescent microspheres, calcein or living cells [120]. Suhendi et al. have explored the self-assembly of colloidal nanoparticles inside charged droplets produced by electrospray [121]. The unique self-assembly profile observed during spray-drying allowed fabrication of several complex nanostructured particles. In this case, SiO₂ nanoparticles and poly(styrene) beads were used as models. It was proposed that the self-assembly process was driven by a series of repulsive and attractive interactions between the charged colloidal nanoparticles and the droplet surface.

(c). Self-templating methods

Hollow materials can be directly prepared by taking advantage of self-templating methods that are not dependent on an external templating agent and thus require a less number of steps for their preparation. This strategy is usually followed to reduce the production cost and facilitate the large-scale synthesis of hollow materials. Several approaches can be used for the direct synthesis of hollow structures, such as (i) etching; (ii) Ostwald ripening; (iii) Kirkendall effect and (iv) galvanic replacement. In general terms, most of these methods rely in a two-step approach: (1) the synthesis of the non-hollow material followed by (2) the conversion of this material into hollow structures. The direct synthesis offers several advantages over the template-based ones, such as reproducibility and superior control over the shell thickness and particle size distribution [5].

(i). Etching

Several etching strategies are available for the preparation of complex hollow materials. For example, surface-protected etching relies on the pre-coating of particulates with a protecting layer followed by the etching of the interior. The protecting layers keep the size of the original particles, while the etching agent produces hollow interior. In this strategy, sol–gel derived colloids are typically used thanks to their porous structure which allows the inner migration of an etching agent [5,122].

Complex TiO₂ hollow structures have been fabricated by using etching routes (figure 10) [124]. By employing an ionic liquid-assisted etching step, TiO₂ nanocubes and nanocube assemblies have been produced via surface fluorination/nitridation and high energy {001} crystal facet exposure. The hollow TiO₂ spheres were firstly synthesized by a sol–gel method and then hydrothermally etched with fluorine and nitrogen-containing ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate without further additives. The resulting TiO₂ nanocubes were then assembled into hollow assemblies of variable complexity by the tight control of the reaction parameters. This strategy allowed the well-controlled preparation of highly complex hollow materials with improved photocatalytic performance [124]. In another study, Cu₂O@Fe(OH)_x nanorattles and Fe(OH)_x cages of variable shapes and dimensions were prepared by template engaged redox etching of shape-controlled Cu₂O crystals. Hollow materials were prepared by the redox etching of Cu₂O to form soluble Cu(II) ions that deposited at the surface. Structures of variable complexity were obtained by extended reaction times along with the presence of Fe(OH)_x as a secondary precursor. At room temperature, the standard reduction potential of Fe³⁺/Fe²⁺ pair is higher than that of Cu²⁺/Cu₂O, resulting in the oxidation of the Cu₂O substrates in the presence of ferric ions. The Fe(II) ions produced during the reaction were confined to the ‘template’ surface, resulting in the formation of iron hydroxides. Once the concentration of iron precursor reached a critical value, iron hydroxides (Fe(OH)_x) nucleated and grown into conformal shells around the Cu₂O template. In addition, the diffusion of Cu(II) and Fe(II) ions resulted in the formation of porous structures while the Cu₂O core was depleted during the reaction. Consequently, by varying the reaction time, hollow nanostructure with variable morphology were obtained [125].

Figure 10.

The self-etching approach for the preparation of complex non-spherical polydopamine capsules by varying the reaction time. (Adapted with permission from Ye et al. [123] Copyright © 2017, American Chemical Society.) (Online version in colour.)

Li et al. have reported a new class of low-dimensional nanocrystal (NC) superlattices. Their method relied on the epitaxial growth via assembly of colloidal NCs in a porous anodized aluminium oxide template, which was then followed by in situ ligand carbonization and selective etching. In this work, Mn₃O₄ NCs were used mainly due to their relevance for chemical energy conversion and storage applications. To this end, the authors produced hierarchical carbon-coated tubular monolayer superlattices based on etched hollow Mn₃O₄ NCs (h-Mn₃O₄-TMSLs). The obtained material displayed a well-defined mesoscale tubular geometry, which delivers several advantages for fast mass transport [126]. The self-etching of metalorganic framework (MOF) templates for the synthesis of complex non-spherical polydopamine capsules have been reported, envisaging intracellular vectors for metal ions (figure 10). The coating of well-defined MOF crystalline particles with polydopamine (PDA) by a pH-induced self-polymerization of dopamine, results in the self-etching of MOF. By varying the reaction time, hollow structures of variable complexity were achieved. This self-etching of MOF templates is observed due to the chelation of the metal nodes of the MOFs by the catechol units of the PDA layer [123].

(ii). Ostwald ripening

Ostwald ripening in colloidal systems relates to the change of an inhomogeneous structure over time. Briefly, it refers to a series of events related to small crystals or sol particles that dissolve and redeposit onto larger particles. This thermodynamically driven process is observed due to the fact that larger particles are energetically favoured when compared with smaller ones due to surface energy decrease [127,128]. The Ostwald ripening mechanism has been explored for the preparation of complex hollow materials. Zhang and Wang reported a multistep Ostwald ripening approach for the preparation of multi-layered Cu₂O nanoparticles. In a first approach, Cu₂O hollow spheres were prepared by symmetric hollowing at room temperature due to Ostwald ripening mechanism, in which the spherical particles were converted into thick or thin nanoshells. Upon the introduction of additional reactants, more Cu₂O nanocrystalline were obtained, which were further assembled into the first nanoshell layer. This sequential Ostwald ripening process was then exploited under variable conditions to prepare hollow materials with variable shell thickness (figure 11) [129].

Figure 11.

Representation of the sequential Ostwald ripening process to prepare multi-layered Cu₂O hollow particles (a); TEM images of the particles at variable reaction time (b) and (c). (Adapted with permission from Zhang et al. [129] Copyright © 2011, American Chemical Society.) (Online version in colour.)

Gao et al. have described the synthesis of uniform bundle shaped NaYF₄ hollow microtubes composed of halfpipes through an Ostwald ripening mechanism caused by Mn²⁺ doping. The Mn²⁺ and Y³⁺ ions were carefully selected in order to control the phase transition and create interior density gradient of a solid aggregate. This profile allows the preparation of hollow materials, by taking advantage of the Ostwald ripening process driven by the particle density gradient. The obtained materials were then investigated for their tunable up-conversion emission via back energy transfer between Er³⁺ and Mn²⁺ dopant ions [130]. Xie et al. have described the fabrication of amorphous double-shelled zinc-cobalt citrate hollow microspheres and crystalline doubled-shelled ZnCo₂O₄ hollow microspheres through an Ostwald ripening process. These materials were prepared by a facile route based on an ageing process at 70°C. The inward and outward Ostwald ripening mechanisms resulted in the formation of the complex hollow structures. The resulting crystalline materials exhibited a large reversible capacity, superior cycling stability and good rate capability [131]. The synthesis of rambutan-like FeCO₃ hollow microspheres through a facile one-step hydrothermal method. The symmetric inside/out Ostwald ripening resulted in the formation of microporous/nanoporous construction based on nanofibres building blocks. These materials were then explored as anode materials for Li-ion batteries showing superior reversible capacity and cycling stability [132]. Qin et al. reported the synthesis of hollow CuO materials with hierarchical structure through a one-pot template-free method. A series of mechanisms involving an ‘oriented attachment’ growth step followed by an Ostwald ripening process resulted in a hierarchical structure. The hierarchical structure of these hollow CuO spheres could be defined by the assembly of primary CuO nanograins resulting in the formation of quasi-single crystal nanosheets. The final assembly of these nanosheets resulted in complex hollow CuO particles. The tight control in the assembly process results also in hierarchical pores, ranging from quasi-micropores (1.0–2.2 nm) to macropores (2–4 µm). These particles were then investigated as sensing materials for detection of H₂S. Thanks to the unique hierarchical structure, the detection limit for H₂S was as low as 2 ppb, with a fast response- and recovery-time [133].

(iii). Other self-templating routes

The Kirkendall effect is a classic phenomenon that is used to describe the motion of atoms at the interface between two metals that possess different diffusion rates. The net flow of mass in a specific direction is balanced by a series of vacancies. When the proportion of vacancies exceeds the saturation value, they are more likely to coalesce into hollow structures. Despite the fact that the formation of Kirkendall voids is not a desirable process in metallurgical manufacturing, it has been explored as an alternative route for the preparation of several hollow nanostructures in materials chemistry [5,134]. Another route that is concomitantly used with the Kirkendall effect is the galvanic replacement reaction. This replacement process is achieved by the force promoted from the electrochemical potential difference between two metals, in which one of the metals behave as reducing agent (cathode) and the salt of another metal as oxidizing agent (anode). The anodic nanostructures are obtained in a first phase. Upon contact with the metal ions with higher reduction potential, oxidation and redissolution of the anode atoms are observed, while the other metal precursor is reduced and plated onto the surface of the cathode template [5].

By taking advantage of both effects, i.e. galvanic replacement and Kirkendall effect, porous dendritic Pt nanotubes have been prepared in aqueous solution at room temperature and in a large scale [135]. The obtained structures were porous, hollow, hierarchical and single crystalline, which deliver a high surface area for catalytic applications. The Pt dendritic structures were prepared by a two-step approach. Initially, Ag dendrites were obtained by galvanic replacement reactions and then used as templates for the preparation of Pt dendritic tubes through a second galvanic replacement. The formation of hollow structures observed in the second replacement reaction was due to a Kirkendall effect, which resulted in the outward growth of the initial Ag hollow ‘templates’ [135]. Recently, Liu et al. have prepared biomimetic yolk-shell Sb@C structures via nanoconfined galvanic replacement route. Firstly, uniform hollow SnO₂ nanospheres were prepared by hydrothermal synthesis, in which hollow structures were obtained via Ostwald ripening. In a second step, these hollow SnO₂ spheres were hydrothermally coated and annealed in a H₂ atmosphere, resulting in the reduction of SnO₂ into Sn cores. In parallel, carbon-rich polysaccharide derived from hydrothermal carbonization of glycose was pyrolysed into a carbon shell. In the following galvanic replacement reaction/Kirkendall effect (by adding SbCl₃), encapsulated solid Sn yolks were chemically transformed back into hollow Sb, resulting in uniform hollow Sb@C yolk-shell spheres. The resulting materials were then explored as anode materials for Li/Na-ion batteries [136].

Acapulco Jr. et al. have prepared multi-shelled bimetallic Ag/Au hollow structures by a sequence of galvanic replacement reactions of the solid Ag ‘core’ coated with Au(III) species resulting in bimetallic Ag/Au hollow materials. Multiple nanoshells were obtained by simply repeating the galvanic replacement reactions. Also, by taking advantage of the Kirkendall effect, a tight control in the shell thickness could be achieved. This was possible due to epitaxial growth of Au onto the Ag layers, in which the Ag was continuously oxidized from the inner layers, dissolved and reduced/redeposited on the outer layer [137]. A series of polymetallic hollow nanoparticles of high complexity by sequential galvanic exchange and Kirkendall growth have been prepared by González et al. [138]. In order to achieve these materials, Ag was used as the ‘template’, PVP as the surfactant, CTAB as surfactant and complexing agent, Au, Pd and Pt precursors as oxidizing agents and ascorbic acid as reducing agent. With minor modifications in the chemical environment, particles with variable morphology and composition were obtained by this strategy [138]. Other hollow materials have been obtained by employing a galvanic replacement etching process, such as cubic PtCu₃ nanocages obtained via one-pot solvothermal process and in the presence of CTAB, by reacting oleylamine-coated Cu nanocrystals with Pt species [139].

4. Recent advances in complex hollow nanostructured materials for twenty-first century technologies

The properties of hollow nanostructures, such as large surface area per volume, low density, abundant inner void space, multiple shells and potential multi-functionality (e.g. magnetic, optical and catalytic properties), make these materials good candidates for several applications (figure 12). For instance, the cavity inside the hollow shell could be used as micro- or nanocontainers for chemical reactions. Chemical species loaded either inside the hollow cavity or the porous shell, can serve as anodes for lithium-ion batteries or carriers for drug delivery. In comparison to other nanomaterials, hollow structures offer the additional possibility to use voids for specific functions, with or without chemical functionalization of the surfaces. Although this review focuses on the preparative strategies for hollow structures, this section presents a brief reference to illustrative applications involving such materials and how they are strictly dependent on the preparative method.

Figure 12.

Examples of domains of applications for hollow nanostructured materials. (Online version in colour.)

In the field of energy storage and conversion, complex hollow structures are playing an important role, because the void space of complex hollow structures can effectively accommodate the volume change of high-capacity lithium-ion battery anode materials, such as transition metal oxides, Sn and Si, boosting the cycling stability. If nanopores are created in the shell, complex hollow structures can provide high specific surface area for charge storage in supercapacitors.

Rambutan-like FeCO₃ hollow microspheres obtained through a facile one-step hydrothermal method were explored as anode materials for lithium batteries (figure 13). It was observed that the unique morphology of these microspheres, along with its hollow interior, provided ideal conditions for electrolyte contact, low Li⁺ diffusion path and sufficient void space to accommodate large volume variation [132]. Recently, multi-shelled Co₃O₄@Co₃V₂O₈ hollow nanoboxes prepared by a metalorganic-framework-engaged strategy were investigated regarding Li⁺ storage properties. It was found that by tuning the concentration of vanadium precursor during the reaction, a fine control of the number of shells was achieved. Moreover, the triple-shelled Co₃O₄@Co₃V₂O₈ nanoboxes displayed high Li⁺ storage capacity and superior cycling capacity (948 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹) [141]. Recently, onion-like nanoporous CuCo₂O₄ hollow spheres containing several shells (at least six) were prepared using bimetal-organic frameworks which were simultaneously used as self-sacrificial templates and precursors in order to achieve high-performance asymmetric supercapacitors. These onion-like hollow spheres showed a specific capacitance of 1700 F g⁻¹ at 2 A g⁻¹ with excellent rate capability. The authors supported that the superior electrochemical performance observed in these materials was possible due to the nanoporous nature of the shells, along with the high surface area available. In addition, the multi-shelled structure with nanometric width delivered an important synergistic effect between Cu and Co species in the spinel structure of the final material [142].

Figure 13.

The rambutan-like FeCO₃ hollow particles and their use as anode for Li-ion batteries. (Adapted with permission from Zhong et al. [132] Copyright © 2013, American Chemical Society.) (Online version in colour.)

In dye-sensitized solar cells and photocatalysis, complex hollow structures enable multiple light reflection and scattering, leading to enhanced light-harvesting capability and superior power conversion efficiency or photocatalytic activity. Hollow box-structured CoFeMoS_x nanomaterials with a unique complex surface chemistry was recently reported as an high-performance counter electrode catalyst for Pt-free dye-sensitized solar cells. By incremental addition of ${\text{MoS}}_4^{2-}$ to the system, the surface of Co-Fe-MoS_x was precisely controlled, becoming rough with bushy embedded nanosheets. By taking advantage of the unique three-dimensional structure of the final materials, and with the appropriate doping ratio, the Co-Fe-MoS_x−2 hollow nanoboxes exhibited well-defined interior voids, large specific surface area and superior catalytic activity. The resulting material showed high power conversion efficiency (9.63%) at AM 1.5 G irradiation [143]. Multi-shelled ZnS-CdS rhombic dodecahedral cages with tunable composition and shell number were recently synthesized by a sequential chemical etching, sulfidation and cation exchange strategy. The resulting materials were then investigated as anodes for photoelectrochemical cells. The authors concluded that the heterojunction formed between the UV-light-responsive ZnS and the visible light-responsive CdS, along with the complex multi-shelled structure of the hollow materials, resulted on enhanced photoelectrochemical performance [144].

There are several advantages in using complex hollow structures in catalysis, including effective isolation of catalytic species, ability for cascade reactions by placing catalytic functionality in sequentially localized compartments and, if the pore sizes in the shells are precisely controlled, complex hollow structures can be used to improve selectivity of catalytic reactions by molecular sieving or strong differences of diffusivities. Recently, it was reported a stable and magnetic recyclable yolk-shell nanocatalyst composed of Au nanoparticles encapsulated in hollow mesoporous carbon shells containing FeCo/graphitic carbon nanoparticles [140]. The resulting material displayed superparamagnetism, high saturation magnetization at room temperature and uniform meso-channels (3.5 nm) that allow the diffusion/separation of small molecules to the interior. The incorporation of FeCo/graphitic carbon in the hollow mesoporous carbon shell provided enough stability to avoid coalescence of the hollow structure. Moreover, it was observed that the material could be used for the reduction of nitroaromatics and be easily collected from the reaction medium through the application of an external magnetic gradient (figure 14).

Figure 14.

Chemical route for the preparation of yolk-shell magnetic nanocatalysts and their use in the reduction of nitroaromatics along with its recycling capability using magnetic separation technologies. (Adapted with permission from Hong et al. [140]. Published by Nature, Scientific Reports, under the Creative Commons (CC-BY) license.) (Online version in colour.)

Complex hollow structures have also been investigated as efficient electrochemical catalysts for water-splitting technologies to achieve clean renewable energy. For example, a novel hybrid nanostructure comprising CoP nanoparticles embedded in an N-doped carbon nanotube hollow polyhedron was prepared through a pyrolysis−oxidation–phosphidation strategy derived from two core–shell zeolitic imidazolate frameworks (ZIF-8@ZIF-67). The resulting material was then investigated regarding its water splitting efficiency. Thanks to the synergistic effect between CoP nanoparticles and the N-doped carbon nanotube hollow polyhedrons, outstanding electrolytic performances were observed. When the final material was used as both anode and cathode during the water splitting process, a relatively low potential was required to achieve the current density of 10 mA cm⁻². Moreover, the material exhibited sustained performance after 36 working hours. The authors supported that the improved electrocatalytic performance of these materials could be attributed to electron transfer from the N-doped carbon nanotube hollow polyhedrons to the CoP nanoparticles, resulting in an increased electronic state of Co d-orbital and the consequent superior binding strength with H. Interestingly, the strong stability of the material was attributed to its unique morphology which provided high resistance of CoP nanoparticles against oxidation [145].

Complex hollow nanostructures are also advantageous for drug delivery due to their distinct thermal/chemical stability, high drug loading capability, sustained drug release from the supports and rich surface chemical functionalities for molecular recognition and targeted delivery. Hence, complex hollow silica materials have been used as drug delivery vehicles for their many advantages such as biocompatibility, excellent chemical stability and ability for bioconjugation via surface chemistry methods [146]. In addition, CaCO₃ nanoparticles have also been used as sacrificial template substrates for depositing starch via a gelation process, in order to obtain pH-responsive hollow starch structures as nanocarriers for drug delivery [21]. Doxorubicin hydrochloride was readily encased in the nanocarriers with a high loading efficiency (97%) and a high loading content (37%). Adhikari et al., have successfully fabricated multi-shelled hollow SiO₂ nanospheres via selective etching of iron oxide that were used to deliver doxorubicin under external stimuli. The authors studied the release of the drug from the multi-shelled hollow silica spheres under acidic conditions. The system is able to release the drug for a period of 4 h [147]. Recently, peptide-based hollow capsules of variable lengths have been reported as containers for the encapsulation of the model anticancer drug, cisplatin (figure 15). It was observed that the unique shape and high aspect ratio of the capsules provided a fast-cellular uptake and higher accumulation at the tumour site when compared with spherical particles [148].

Figure 15.

The use of peptide-based torpedo-shaped hollow capsules for the delivery of the model anticancer drug: cisplatin. (Adapted with permission from Ueda et al. [148] Copyright © 2019, American Chemical Society.) (Online version in colour.)

The physicochemical properties of complex hollow materials have also been explored for water purification applications. Recently, urchin-like Fe₃O₄@polydopamine (PDA)-Ag hollow microspheres were explored regarding their adsorption/catalytic performance for the removal of organic dyes (methylene blue and rhodamine B) from water under several operational conditions. It was concluded that the surface chemistry of these particles delivered an enhanced adsorption capacity towards the dye molecules. Moreover, the urchin-like Fe₃O₄@PDA-Ag hollow microspheres exhibited high reusability, easy separability and fast regeneration without loss of performance after five cycles [149]. Hierarchical hollow superparamagnetic Fe₃O₄ nanospheres were recently reported for the facile removal of a cyanobacteria produced toxin (Mycrocystin-LR) from water [150]. A tetragonal carbon nitride hollow tube was recently synthesized and explored regarding their photocatalytic and adsorption performance. It was found that the prepared hollow tetragonal carbon nitride tubes exhibited high photocatalytic performance towards the degradation of phenol and superior adsorption capacity of methylene blue [151].

Hollow nanostructured materials have been investigated as sensing devices. Hence, SnO₂–TiO₂ hollow materials comprising double shells were investigated for the detection of ethanol gas. The multi-shelled structure and abundant hetero-interface delivered a rapid response rate and superior reproducibility [152]. NiO hierarchical hollow microspheres doped with Fe were proposed for the detection of triethylamine (TEA). The NiO microspheres exhibited a high sensing response (10 ppm TEA) given the large specific surface area of the hierarchical structure [153]. Another example is the preparation of a complex hollow CuFe₂O₄/α-Fe₂O₃ composite containing an ultrathin porous shell for the detection of acetone with a response as low as 100 ppm. It was suggested that the enhanced sensing performance could be attributed to hollow and porous architecture of CuFe₂O₄/α-Fe₂O₃, the resistance modulation effects of heterojunction as well as the catalytic properties of CuFe₂O₄ [154]. Another interesting approach was the fabrication of a hollow Pt functionalized SnO₂ hemipill (HPN) network for the detection of acetone, which can be used as a biomarker for diabetes. It was demonstrated that the HPN exhibited an impressive detecting capability (200 ppb) for acetone under high humidity (RH 80%). The detection limit was found to be 3.6 ppb which is low enough for effective diagnosis of diabetes through breath [155]. Triple-shelled ZnO/ZnFe₂O₄ hollow microspheres were also recently prepared and explored regarding their sensing performance towards acetone vapour. The unique architecture of the resulting microspheres (i.e. high ZnO and ZnFe₂O₄ heterojunctions and superior specific surface area) delivered superior sensing performance towards acetone vapour [156].

5. Conclusion and perspectives

This review has presented a number of relevant synthetic methods for the preparation of complex hollow nanostructures. This topic has an increasing relevance namely due to the envisaged applications for technologies that are required to meet criteria and global challenges such as those put forward in the 2030 agenda for sustainable development. Table 1 provides an overview of hollow structures obtained by distinct synthetic routes envisaging specific applications. This allows the reader to get a first impression of how the performance of such hollow materials are strongly dependent on the chemical strategies applied in their synthesis. As such, we though this work as a timely review of methods reported mainly in the last decade and that have contributed for the design of complex hollow micro- and nanomaterials. We also suggest the further reading of recent review articles such as [157,158] which provide an interesting overview regarding the innovative methods for the synthesis of complex hollow materials and their potential in future applications. The ability to prepare such complex structures have contributed to improve the performance of certain materials for several applications when compared with more simple configurations. Despite the recent progress observed in this domain, there are still important challenges that need to be surpassed in order to facilitate the industrial production and large-scale application of hollow nanostructures in several technologies. Among them, we highlight a few that are particularly relevant for the materials chemist. The tight control and manipulation of hollow structures of high complexity are still challenging, where only a few systems can be finely tuned in a facile manner regarding their structure and chemical composition. The integration of distinct synthetic routes to produce well-defined hollow materials, i.e. that allows a precise control of their structure and composition, it will be a prevalent trend in the future. The available synthetic routes for the preparation of complex hollow materials, are in a number of cases, costly and difficult to scale-up. Consequently, many template-free methods have been explored in the last years in order to reduce the operational costs associated with the preparation of hollow materials and facilitate their production at large scale. Such template-free methods are usually simpler and involve less synthetic steps when compared with template-based ones. Moreover, most of the complex hollow materials that have been investigated are focused on native elements, and their oxides and sulfides. However, other equally important hollow materials based on carbides, nitrides, phosphides and selenides should be thoroughly investigated.

Table 1.

Overview regarding the several strategies for the preparation of complex hollow materials of variable composition and the corresponding potential applications.

synthetic route	template	final material	potential application	ref.
Hard-templating	polymers	Sol–gel process and polymerization reactions of PMMA resulting in hollow microspheres with several layers.	Controlled drug delivery.	[75]
		Deposition of titania (Ti0.91O2) nanosheets and GO onto PMMA beads through layerbylayer technique. Microwave irradiation is used to remove the sacrificial template and reduce GO.	Photocatalysis.	[76]
		Hollow mesoporous silica nanoparticles prepared by a dual template method using PSPMMAPMAA particles as template and CTAB as directing agent.	Deposition of Ag nanoparticles provide antibacterial activity.	[78]
		Deposition of Ni-precursor nanosheets on sulfonated bowl-like PS particles followed by thermal treatment.	Unique shape provides superior packing density resulting in superior lithium storage capacity.	[80]
		Highly-ordered hollow urchin-like singlecrystal ZnO nanowires deposited on PS particles.	The shape of the hollow material delivers improved light scattering with high potential for nanostructured solar cells.	[83]
	Silica	TiO2 hollow spheres with multiple layers obtained by calcination and NaOH etching.	Energy material applications, such as lithium-ion batteries, photocatalysts, water-splitting and supercapacitors.	[88]
		Hollow carbon colloids with multiple mesoporous shells.	High loading capacity with the potential for drug/biomolecule delivery.	[89]
		SnO2 nano-cocoons with movable α-Fe2O3 spindles. The ferrite cores can then be reduced into Fe3O4.	Possible introduction of photocatalysts coupled with magnetic properties.	[91]
		Box-in-box hollow structures based on several metal sulfides (M=Ni, Cu, Mn).	Electrochemical energy storage, exhibiting high specific capacitance, excellent rate capability and good cycling stability.	[92]
	Carbonaceous	Several multi-shelled metal oxides (e.g. α-Fe2O3, Co3O4, NiO, CuO, and ZnO)	Gas sensing.	[98]
		Multi-shelled hollow spheres based on CoMn2O4, Co1.5Mn1.5O4, MnCo2O4, ZnMn2O4, ZnCo2O4, and NiCo2O.	Excellent lithium storage capacity.	[99]
Soft-templating	Emulsions	Methyl methacrylate/divinyl benzene/acrylic acid (MMA/DVB/AA) double-shelled spheres.	Substances encapsulation or nanofiller in composites.	[104]
		Magnetic/hollow double-shelled imprinted polymers (MH-MIPs).	Removal of λ-cyhalothrin from waters.	[105]
		Diphenylalanine (FF) unilocular and multilocular hollow spheres.	Fabrication of peptide-based materials with novel structure.	[106]
		Au/Pt hollow capsules.	Catalytic reduction of 4-nitrophenol	[107]
		α-Fe2O3 chestnut buds and nests.	Removal of Cd(II).	[108]
		Multi-core double emulsion drops and silicon microcapsules.	Encapsulation of CO2 absorbents.	[109]
		Nanofibrous hollow microspheres using polyacrylonitrile and Nylon.	Nano- and microfabrication technologies.	[110]
	micelle or vesicle	Rectangular platelet micelles.	Fluorescent imaging, sensing, electronics, and catalysis.	[111]
		Multi-layered single crystalline Cu2O hollow spheres.	Potential applications in the fields of solar energy conversion, catalysis and gas sensors.	[112]
		Polyoxometalate (POM) clusters.	Nanocontainers.	[113]
		Hollow dendritic mesoporous SiO2 nanospheres.	Protein delivery.	[114]
		Asymmetric flask-like hollow carbonaceous particles.	Supercapacitors.	[115]
	gas-bubble	Fullerene-like MoS2 nanocages.	Lithium storage.	[116]
		Honeycomb-like hollow Li1.2Mn0.52Ni0.2Co0.08O2 microsphere.	High-performance lithium-rich layered oxide cathode material.	[117]
		Manganese silicate nanobubble constructed hollow microspheres.	Heterogenous catalysts for the degradation of organic dyes.	[118]
	electrospray	Multicomponent TiO2 microcapsules.	Drug delivery or multireactors.	[119]
		Microcapsules composed of calcium phosphate and chitosan.	Encapsulation of cargo.	[120]
		Pure silica nanoparticles and surfactant-free polystyrene (PS) spheres.	Aerosol-assisted spray-drying process to production of functional nanostructured particles.	[121]
self-templating	etching	Cu2O2@Fe(OH)x nanorattles of several shapes and sizes by redox etching of Cu2O crystals. Other structures based of Au or MnOx can be prepared through a similar strategy.	Multi-shelled structures with the ability to act as nanocontainers.	[125]
		pH-induced self-etching of MOF crystalline particles coated with polydopamine resulting in non-spherical capsules.	Intracellular delivery of metal ions with the ability to be used for cancer therapy.	[123]
	Ostwald ripening	Multi-layered Cu2O hollow nanospheres with variable shell thickness.	Nanocontainers.	[129]
		Uniform bundle shaped β-NaYF2 hollow microtubes composed of halfpipes induced by Mn2+ doping.	Tuneable optical output with several applications such as lasers, displays, imaging or biosensing.	[130]
	other strategies	Dendritic Pt nanotubes by taking advantage of galvanic replacement and Kirkendall effect.	High-performance catalyst with low Pt loading.	[135]
		Biomimetic yolk-shell Sb@C structures via nanoconfined galvanic replacement route.	Anode materials for Li/Na-ion batteries with large specific capacity, high rate capability and stable cycling performance.	[136]
		Multi-shelled bimetallic Ag/Au hollow structures by a series of galvanic replacement reactions. By taking advantage of Kirkendall effect the shell thickness can also be controlled.	Surface-enhanced Raman spectroscopy (SERS) probe.	[137]

Also from a fundamental point of view, it is clear that detailed mechanisms regarding the formation of hollow materials have been in most of the cases elusive. Therefore, it is increasingly important to systematically explore the mechanisms involved in preparation of such structures. This strategy will provide new tools for the development of novel and powerful methodologies required for the further preparation of complex hollow materials. The use of hollow materials has been extensively explored for many different areas such as catalysis, sensors or energy-related applications. The continuous advancement of the field will also contribute for the development of new materials which hopefully will have a positive impact for the establishment of a sustainable society.

Data accessibility

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Authors' contributions

All authors contributed equally to the article.

Competing interests

We declare we have no competing interests.

Funding

This work was developed in the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by National funds through the FCT/MEC and when appropriate cofinanced by the European Regional Development Fund (FEDER) under the PT2020 Partnership Agreement. S.F.S. thanks the Fundação para a Ciência e Tecnologia (FCT) for the PhD grant SFRH/BD/121366/2016. T.F. thanks FCT for the PhD grant SFRH/BD/130934/2017.