Review pubs.acs.org/CR
Synthesis, Properties, and Applications of Hollow Micro-/ Nanostructures Xiaojing Wang,†,‡ Ji Feng,† Yaocai Bai,†,‡ Qiao Zhang,*,†,§ and Yadong Yin*,†,‡ †
Department of Chemistry, and ‡Materials Science and Engineering Program, University of California, Riverside, California 92521, United States § Institute of Functional Nano and Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, People’s Republic of China ABSTRACT: In this Review, we aim to provide an updated summary of the research related to hollow micro- and nanostructures, covering both their synthesis and their applications. After a brief introduction to the definition and classification of the hollow micro-/nanostructures, we discuss various synthetic strategies that can be grouped into three major categories, including hard templating, soft templating, and self-templating synthesis. For both hard and soft templating strategies, we focus on how different types of templates are generated and then used for creating hollow structures. At the end of each section, the structural and morphological control over the product is discussed. For the self-templating strategy, we survey a number of unconventional synthetic methods, such as surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement. We then discuss the unique properties and niche applications of the hollow structures in diverse fields, including micro-/nanocontainers and reactors, optical properties and applications, magnetic properties, energy storage, catalysis, biomedical applications, environmental remediation, and sensors. Finally, we provide a perspective on future development in the research relevant to hollow micro-/nanostructures.
CONTENTS 1. Introduction 1.1. Hollow Micro-/Nanostructures 1.2. Scope of This Review 2. Hard Templating Synthesis 2.1. Polymer-Based Hard Templates 2.1.1. Polystyrene Templates 2.1.2. Formaldehyde Resin Templates 2.1.3. Other Polymer Templates 2.2. Silica-Based Hard Templates 2.2.1. Solid Silica Templates 2.2.2. Mesoporous Silica Templates 2.2.3. Silica Shell as Hard Templates 2.3. Other Hard Templates 2.3.1. Carbon-Based Templates 2.3.2. Metal-Based Templates 2.3.3. Ceramic Templates 2.3.4. Hard Templates Based on Inorganic and Complex Salts 2.3.5. Natural Materials as Hard Templates 2.4. Morphology and Structure Design 2.4.1. Design of Shape 2.4.2. Design of Interior Structure 2.4.3. Design of Shell Structure 3. Soft Templating Synthesis 3.1. Emulsion-Based Soft Templates 3.1.1. Direct Emulsion Templates 3.1.2. Reverse Emulsion Templates 3.1.3. Double Emulsion Templates © 2016 American Chemical Society
3.2. Micelle/Vesicle-Based Soft Templates 3.3. Gas Bubble-Based Soft Templates 3.4. Electrospray Method 3.5. Morphology and Structure Design 3.5.1. Design of Shape 3.5.2. Design of Interior Structure 3.5.3. Design of Shell Structure 3.5.4. Other Hierarchical Hollow Structures 4. Self-Templating Synthesis 4.1. Surface-Protected Etching 4.2. Ostwald Ripening 4.3. The Kirkendall Effect 4.4. Galvanic Replacement 5. Properties and Applications 5.1. Micro-/Nanocontainers and Reactors 5.1.1. Micro-/Nanocontainers 5.1.2. Formation of Micro-/Nanostructures in Hollow Reactors 5.1.3. Catalytic Reactions Inside Hollow Reactors 5.2. Optical Properties and Applications 5.3. Magnetic Properties 5.4. Energy Storage 5.4.1. Lithium-Ion Batteries 5.4.2. Lithium Sulfur Batteries
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Special Issue: Nanoparticle Chemistry Received: December 15, 2015 Published: May 9, 2016 10983
DOI: 10.1021/acs.chemrev.5b00731 Chem. Rev. 2016, 116, 10983−11060
Chemical Reviews 5.4.3. Supercapacitors 5.5. Catalysis 5.5.1. Catalytic Applications in Organic Transformation 5.5.2. Electrocatalysis 5.6. Photocatalysis 5.6.1. Single Component Hollow Structures 5.6.2. Multicomponent Hollow Structures 5.7. Biomedical Applications 5.7.1. Bioimaging Contrast Agents 5.7.2. Drug Delivery Carriers 5.7.3. Multifunctional Diagnostic and Therapeutic Applications 5.8. Environmental Remediation 5.9. Sensor 5.9.1. Gas Sensor 5.9.2. Biosensor 5.9.3. Metal Ion Sensor 5.9.4. Pressure Sensor 6. Summary and Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments References
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Figure 1. Schematic illustration showing various hollow structures: (left) hollow spheres/boxes/tubes; (middle) multi-shelled hollow spheres/boxes/tubes; (right) yolk−shell, cube-in-box, and wire-in-tube structures.
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many hollow micro-/nanostructures have been developed on the basis of the similar templating concept.1 The templates have evolved from the classic silica colloidal particles to polymer micro-/nanospheres, various inorganic and organic micro-/ nanostructures, and many unconventional ones such as air bubbles and liquid droplets, while their shapes have been extended from the simple spheres to ellipsoids,2−7 cubes,8−17 rods,18−27 wires,28−33 rings,34,35 and many others. The shell composition has also evolved from silica and polymer to many more functional materials such as metals, metal oxides, metal chalcogenides, and complex compounds. More importantly, the research community has quickly realized the fascinating properties associated with the unique hollow structures, such as large surface area, low density, and high loading capacity, and demonstrated a large variety of applications, including micro-/ nanoreactors,36−44 catalysis,45−47 energy storage,48−61 biomedicine,25,41,62−68 sensors,33,69−71 environmental remediation,53,72−80 and so forth. Because of the existence of a hollow cavity, the surface area of hollow structures is significantly larger while the density is much lower than that of their solid counterparts with the same composition and size. These properties significantly boost the application of hollow micro-/ nanostructures for catalysis either as support materials or as active catalysts. Because the void space can be used as the storage for different cargoes, they could serve as imaging contrast agents, drug delivery carriers, and anodes or cathodes for lithium ion batteries. Determined by the intrinsic properties of different compositions, such as magnetic property and catalytic property, hollow micro-/nanostructures could have many other applications, such as sensing, water treatment, and environmental remediation. In short, as compared to their solid counterparts, hollow micro-/nanostructures offer additional possibilities for structural and compositional tuning that can be well utilized for rational design of novel functional materials toward many desired applications. From a simple point of view, hollow micro-/nanostructures can be easily prepared by creating a void space inside a solid precursor. Yet such an operation is indeed difficult to achieve at the nano- or microscale. Additionally, to meet the requirement for practical applications, hollow micro-/nanostructures should be prepared with high uniformity and well-controlled morphology in a reproducible, scalable, and cost-effective way. There are three major approaches for the synthesis of hollow structures: (1) hard templating method; (2) soft templating method; and (3) self-templating method. Among all three commonly used strategies, hard templating strategy is conceptually the simplest one.81,82 In a typical process, the templates were prepared first, followed by coating the outer surface with a layer of shell material. Hollow structures were then obtained after selectively removing the templates. Hard
1. INTRODUCTION 1.1. Hollow Micro-/Nanostructures
Hollow micro-/nanostructures are a class of special micro-/ nanomaterials being named on the basis of their morphologies. When serving as an adjective, the word “hollow” means “having a hole or empty space inside”. Although any structures with empty space inside, based on the definition, could be classified as “hollow structures”, the term usually refers to those containing considerably a large fraction of empty spaces. In this Review, we define a “hollow micro-/nanostructure” as a solid structure with void space inside a distinct shell, and its dimensions are in the nanometer or micrometer range. Hollow micro-/nanostructures can be categorized into various groups from different perspectives. For example, there could be hollow spheres, tubes, fibers, boxes, etc., reflecting different overall shapes. On the basis of the number of outer shells, they can be termed as single-, double-, and multi-shelled (or walled) hollow structures. Considering the different compositions of the shell structures, they can be divided into organic and inorganic hollow micro-/nanostructures, or more specifically into polymer, ceramics, metal, and composite hollow structures. Often, the terms from different categories are combined to describe the morphology more precisely. For example, hollow structures with an overall spherical morphology could be named as hollow spheres, multi-shelled spheres, yolk−shell, or rattle-type spheres, while tubular and cubic structures could be termed as tubes or boxes, multi-walled tubes or boxes, wire-in-tube or cube-in-box structures, etc. (Figure 1). It is well-known that the properties of materials are mainly determined by their structures, while the applications of materials are largely determined by their properties. Since the seminal report by Caruso and co-workers on the fabrication of hollow silica and inorganic−polymer hybrid spheres in 1998, 10984
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categories: (1) hard templating synthesis, (2) soft templating synthesis, and (3) self-templating synthesis. We first discuss hard and soft templating strategies as they share some general and relatively simple working principles, with emphasis on the production and chemical characteristics. At the end of each section, we point out the general methods in controlling the structure and morphology of the synthesized nanostructures. We then introduce the self-templating strategies with important examples including surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement method. In the second main part, we discuss the properties of the hollow structures together with their applications in different fields such as micro-/nanocontainers and reactors, optical properties and applications, magnetic properties, energy storage, catalysts, biomedical applications, environmental remediation, and sensors. We discuss the properties and applications together as they are closely related and they are associated with specific hollow structures.
templates are composed of rigid micro- or nanoscale objects, such as polymer beads, silica, carbon, ceramics, or metal nanoparticles. Although in principle objects created by microfabrication techniques could be used as templates, largescale and high-throughput production of hollow micro-/ nanostructures mostly relies on templates obtained by solution or gas-phase synthesis. From this point of view, most of the templating syntheses of hollow structures could be classified as “bottom-up” techniques.83 The void size and shape are determined by the size and shape of the template, while the shell thickness is mainly determined by the coating process. In most cases, the templates are removed completely without any contribution to the composition of the final products, while in some other cases, part of the template is intentionally left and contributes both composition and properties to the final product. In most soft templating approaches, only the coating process is needed, because it is unnecessary to remove the soft templates, which are usually in the fluid form, such as emulsion droplets, vesicles/micelles, and gas bubbles. As compared to the hard templating strategy, the soft templating method offers less control over the uniformity of the products, but provides more possibilities in tuning both the internal and the external structures, producing more complicated hierarchical structures. Self-templating strategy refers to the direct synthesis of hollow structures without the need of additional templates. It is preferred in practical applications because of the significantly reduced production cost, simplified synthesis procedures, and the ease of scaling up. The drawback is that it is only suitable for the synthesis of hollow structures with specific compositions, limiting its application field. With the rapid development of nanotechnology, the boundaries among the three strategies become ambiguous. For instance, for the surface protected etching method,84 because the template itself is partially converted to a hollow shell, it could be classified as a selftemplating process. On the other hand, one could also call this a hard templating process, because the removed core indeed acted as sacrificial solid templates. In this context, we will keep the classification flexible and focus on some key questions, such as the formation mechanism, structure design, etc. To prepare hollow materials with desired structures and properties, the three synthetic methods can be used separately or together. For example, although porous hollow structures could be achieved by using a single templating approach, hollow particles with ordered mesoporous shells are usually achieved by combining both hard templating and soft templating methods,85−88 in which the hard templating approach was used to create the void space inside the hollow shell, and the soft templating approach was explored to make the mesoporous shell.
2. HARD TEMPLATING SYNTHESIS Hard templating method for the synthesis of hollow structures is very straightforward.96 Briefly, hard templates with specific shapes were synthesized first, followed by coating the outer surface with a layer of desired materials. The core materials were then selectively removed to obtain the hollow structure (see scheme in Figure 2). To achieve a successful coating on
Figure 2. Schematic illustration showing the hard templating synthesis process.
the surface of template, a surface modification step that can change the surface functionality, such as surface charge and polarity, is usually applied. A number of methods, such as sol− gel process or hydrothermal reactions, could be used to deposit the shell materials on the template surface. The selective removal of the hard template could be achieved through chemical etching, thermal treatment or calcination, or simply by dissolving in particular solvents. The choice of template removal method is mainly determined by the composition of hard templates. In some cases, post treatment such as reduction or calcination is required to improve certain properties of the resulting shells. In the following sections, we will introduce several typical hard templates on the basis of their different compositions, followed by detailed discussions on the control over the morphology of the hollow structures.
1.2. Scope of This Review
Several excellent reviews have been made available in summarizing the synthesis and applications of hollow micro-/ nanostructures, but with focuses either on a specific category of materials or on specific applications.46,47,67,81,89−95 It is thus the right time to present a comprehensive and up-to-date review on the synthesis and applications of hollow micro-/nanostructures. Considering the rapid development of nanotechnology, we believe that this Review would serve as both a scientific introduction for newcomers to the relevant fields and also a comprehensive reference for experienced researchers. This Review aims to focus on both the synthesis and the applications of hollow micro-/nanostructures. In the synthesis part, we divide the synthesis strategies into three main
2.1. Polymer-Based Hard Templates
Templating against polymer nanoparticles to synthesize hollow micro-/nanostructures might be the most popular method mainly due to the relative ease in selectively removing the templates.82 The commonly used polymers include polystyrene (PS) and its derivatives, formaldehyde resin, and poly(methyl methacrylate) (PMMA). 2.1.1. Polystyrene Templates. 2.1.1.1. Polystyrene Colloidal Templates. The first report on the fabrication of hollow 10985
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comonomer. Hydrolysis and condensation of tetraethoxysilane (TEOS) at the SiOH-PS particle surface resulted in a silica layer coating. Hollow silica nanoparticles were obtained by a thermal degradation process to remove the PS cores. Yang et al. reported the synthesis of hollow silica and titania particles using amine-functionalized polystyrene beads as the templates.101 Hydrolyzed tetramethoxysilane (TMOS) was used as the precursor for the silica shell, while titanium tert-butoxide (TBOT) was used as the precursor for the titania shell. The PS cores were then removed by either calcining at high temperature or dissolving in a mixture solvent of ethanol and dichloromethane. Song et al. prepared nitrogen-doped hollow SiO2/TiO2 hybrid spheres using triethylamine as the nitrogen source.100 In their synthesis, the elimination of the PS core, the nitrogen doping process, and the crystallization of TiO2 were simultaneously conducted in the one-step calcination process. In situ polymerization on the surface of PS and then removing the PS template could be used to synthesize hollow polymer or polymer-containing composite spheres.104,105 Yang et al. prepared uniform hollow conductive polyaniline (PANI) and hollow polypyrrole (PPy) microspheres by in situ polymerizing monomers on sulfonated PS microspheres.104 The PS spheres were first treated by sulfuric acid to produce sulfone groups on the surface, then modified by aniline or pyrrole monomer through electrostatic forces. Introducing an initiator to the system triggered the in situ polymerization, leading to the formation of core−shell structured microspheres. Uniform hollow polymer microspheres could be obtained by dissolving the PS cores in dimethylformamide (DMF). Further pyrolysis of the polymer hollow spheres could result in hollow carbon spheres. By taking a step further, Han et al. prepared porous nitrogen-doped hollow carbon spheres and explored their application for high performance supercapacitors.106 White and Tang et al. reported the synthesis of hollow carbon nanospheres by using PS spheres as the template.107,108 An aqueous dispersion of PS latex with desired size was mixed with D-glucose, the carbon precursor, which was then heated at 180 °C for 20 h. To remove the polymer template and to graphitize the carbonaceous shell, the composite material was heated above the decomposition temperature (e.g., >500 °C) of the template. The resulting carbon shell has a thickness of ∼12 nm, which could be easily adjusted by altering the sugar/PS ratio. Other hollow structures with designed shell structures, such as mesoporous silica hollow microcapsules,109 doubleshelled hollow silica microspheres,110 mesopore-free hollow silica particles,111 hierarchically structured hollow silica spheres,112 double-shelled TiO2/SnO2 composite hollow spheres,113 etc., could also be synthesized by using PS particles as the templates. The detailed sample synthesis and design strategy will be summarized in section 2.4.3. On the other hand, macroporous polystyrene containing uniform and periodic voids could also be used as hard template to grow hollow structure inside their void space. One good example was reported by Jiang et al.,5 who used a kind of “lostwax method” that can create molds for casting sculptures. First, a macroporous PS template was prepared by replicating the convective silica colloidal assembly. A key feature of the resulted macroporous PS is the interconnected pores, through which the voids could be filled with precursor solutions in the second templating process. As a result, new colloidal particles could be grown within the voids of the porous polymer. For example, in the synthesis of hollow ceramic colloids, the PS film was first immersed into an alcoholic solution of metal alkoxide.
nanostructures was in 1998, when Caruso and co-workers prepared hollow inorganic silica and inorganic−polymer hybrid spheres through the electrostatic layer-by-layer (LBL) selfassembly of silica nanoparticles and polymer multi-layers on colloidal templates.1 Figure 3 illustrates the schematic
Figure 3. Illustration of procedures for preparing inorganic and hybrid hollow spheres by templating against PS latex particles. Reproduced with permission from ref 1. Copyright 1998 The American Association for the Advancement of Science.
procedures. Polystyrene latex particles with a diameter of 640 nm were used as templates. Three-layer cationic poly(diallyldimethylammonium chloride) (PDADMAC) film was then deposited onto the negatively charged PS particles. The positively charged surface could be utilized for the adsorption of SiO2 particles with ∼25 nm in diameter. Through an additional calcination or tetrahydrofuran (THF) dissolution process, PS templates could be removed, forming hollow silica or inorganic−polymer hybrid spheres. The shell thickness of the hollow spheres could be adjusted from tens to hundreds of nanometers by simply tuning the number of SiO2-PDADMAC layer deposition cycles. A similar strategy could be applied to different colloidal cores, such as weakly cross-linked melamineformaldehyde (MF) particles. The MF core could be removed by exposing the coated particles to either acidic solutions (pH100 nm) and microemulsions (or nanoemulsions
