空心球合成2

5472 Chem. Soc. Rev., 2011, 40, 5472–5491 This journal is c The Royal Society of Chemistry 2011 Cite this: Chem. Soc. Rev., 2011, 40, 5472–5491 Fabrication and application of inorganic hollow spheres Jing Hu,ab Min Chen,a Xiaosheng Fanga and Limin Wu*a Received 25th April 2011 DOI: 10.1039/c1cs15103g Inorganic hollow spheres have attracted considerable interest due to their singular properties and wide range of potential applications. In this critical review, we provide a comprehensive overview of the preparation and applications of inorganic hollow spheres. We first discuss the syntheses of inorganic hollow spheres by use of polymers, inorganic nonmetals, metal-based hard templates, small-molecule emulsion, surfactant micelle-based soft-templates, and the template-free approach. For each method, a critical comment is given based on our knowledge and related research experience. We go on to discuss some important applications of inorganic hollow spheres in 0D, 2D, and 3D arrays. We conclude this review with some perspectives on the future research and development of inorganic hollow spheres (235 references). 1. Introduction Monodisperse hollow spheres have attracted considerable interest in the past few decades due to their well-defined morphology, uniform size, low density, large surface area, and wide range of potential applications. For instance, the large fraction of void space in hollow structures has been used to load and control releasing systems for special materials, such as drugs, genes, peptides, spiceries, and biological mole- cules.1 They can also be used to modulate refractive index, lower density, increase the active area for catalysis and adsorption, improve particles’ ability to withstand cyclic changes in volume, and expand the array of imaging markers suitable for early detection of cancer.2,3 Inorganic hollow spheres have special optical, optoelectronic, magnetic, electrical, thermal, electrochemical, photoelectro- chemical, mechanical, and catalytic properties, suggesting that they comprise a more common, more diverse, and probably richer class of materials than organic hollow spheres.4 Beginning with the pioneering work carried out by Kowalski and colleagues at Rohm and Haas,5,6 a variety of chemical and physicochemical strategies, including heterophase polymerization/ combined with a sol–gel process,7 emulsion/interfacial polymerization methods,8–10 self-assembly techniques,11,12 and surface living polymerization process13–15 have been aDepartment of Materials Science and the Key Laboratory of Molecular Engineering of Polymers of MOE, Fudan University, Shanghai 200433, P. R. China. E-mail: lmw@fudan.edu.cn b School of Perfume and Aroma Technology, Shanghai Institute of Technology, Shanghai 200235, China Jing Hu Jing Hu received her PhD degree from Shaanxi Univer- sity of Science and Techno- logy in July, 2009. Then, she joined the School of Perfume and Aroma Technology of Shanghai Institute of Techno- logy as a lecturer in perfume and aroma technology. In 2010, she was awarded ‘‘Shanghai Chenguang Scholar’’ by Shanghai municipality. Her current research interests include preparation and assem- bly of nanocapsules, function mechanism of sustained fra- grance and propagation fibers. Min Chen Min Chen received her PhD degree from Fudan University under the supervision of Professor Limin Wu in 2006. Her dissertation was chosen as one of the ‘‘Top 100 National Excellent Doctoral Disserta- tions’’ in 2008. Just after finishing the doctorate work, she joined the Department of Materials Science, Fudan University, and was succes- sively awarded ‘‘Shanghai Chenguang Scholar’’ and ‘‘Rising Star’’ by Shanghai municipality. She is currently an Associate Professor. Her research interests include the synthesis, characterization, assembly and properties of novel organic–inorganic hybrid nanostructured materials and inorganic hollow spheres. Chem Soc Rev Dynamic Article Links www.rsc.org/csr CRITICAL REVIEW D ow nl oa de d by T ia nji n U niv ers ity on 22 N ov em be r 2 01 1 Pu bl ish ed o n 29 Ju ly 2 01 1 on h ttp :// pu bs .rs c.o rg | d oi: 10. 10 39/ C1 CS 151 03G View Online / Journal Homepage / Table of Contents for this issue This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5472–5491 5473 employed to prepare inorganic hollow spheres. In particular, the template method is the most common. In this method, at least two steps are usually indispensable. First, the templates must be modified to give them the ability to coax inorganic precursors (salts or alkoxides) onto the surface of the template core. Then, after the inorganic shell is decorated outside the scaffold, the templates must be eliminated in some way, leaving behind a hollow shell. Generally, templates can be divided into hard and soft templates. When hard templates (e.g., SiO2, 16–18 C spheres,19 polymers,20–24 metal particles25) are employed, the structure of the hollow product is similar to that of the template, with a well-defined and monodisperse morphology. However, the removal of the templates by either thermal (sintering) or chemical (etching) means is very com- plicated and energy-consuming. As for soft templates (bacteria,26,27 droplets,28 vesicles29,30 and more), although it is relatively easier to remove the templates, the morphology and monodispersity of the as-prepared hollow products are usually poor due to the deformability of the soft template. Although the drawbacks in these template strategies seem to be inherent and insurmountable, some novel techniques that seem to overcome them, such as sacrificial templates, modified soft templates, etc., are emerging. Another important devel- opment in the preparation of inorganic hollow spheres is called the template-free method, such as the Ostwald ripening process, which not only combines the advantages of hard- and soft-template methods but also avoids their pitfalls. Recent reviews have provided a comprehensive description of these methods for fabrication of inorganic hollow spheres, from multilevel hollow spheres to non-spherical, even one- dimensional (1D) hollow structures.4,31–34 In order to avoid overlapping reviews, this article will mainly focus on the syntheses and applications of inorganic hollow spheres rather than any non-spherical and 1D hollow structures. We will first discuss the syntheses of inorganic hollow spheres with polymer, inorganic nonmetal, and metal-based hard templates and small-molecule or oligomer emulsion, surfactant micelle-based soft-templates, and template- free approaches. For each method, we provide critical comments based on our knowledge and related research experience. Then we will introduce some important applica- tions of the inorganic hollow spheres (0D) and two- dimensional (2D) and three-dimensional (3D) arrays of inorganic hollow spheres. Considering the rapidly expanding body of literature in the field, the list of examples provided in this review is by no means exhaustive, some excellent papers reporting novel approaches and applications are even omitted. Representative works were selected from the most recent literature available, with exceptions made only for special cases. The intent is to give the readers a critical discussion of the syntheses and applications of inorganic hollow spheres. Finally, we conclude this review with some perspectives on the future research and development of inorganic hollow spheres. 2. Hard template strategy Hard templates are widely used to fabricate inorganic hollow spheres. Many compounds, such as polymeric, inorganic nonmetallic, and metallic particles, can be used as hard templates. The final shape and size of the inorganic hollow sphere are essentially dependent upon the templates. 2.1 Polymer template-based methods Templating against polymer colloids is probably the most common approach to produce hollow spheres. Two methods in particular are used to fabricate hollow spheres with homo- geneous, dense layers. One is templating against colloid poly- styrene (PS) and its derivatives as the particles to fabricate SiO2, 35 SnO2, 36 magnet (ccp-Co, hcp-Co, Co3O4, a-Fe, Fe3O4 and a-Fe2O3), 37,38 metal–metalloid Ni–B,39 Ni(OH)2, 40 and others.41,42 In its typical procedure, the PS template particles are coated in solution either by controlled surface precipita- tion of inorganic molecule precursors (SiO2, TiO2, etc.) or by Xiaosheng Fang Xiaosheng Fang received his PhD degree from the Institute of Solid State Physics, Chinese Academy of Sciences in 2006, under the supervision of Professor Lide Zhang. He joined the National Institute for Materials Science (NIMS), Japan, as a JSPS postdoctoral fellow and then the International Center for Young Scientists (ICYS)— International Center for Materials Nanoarchitectonics (MANA) as a researcher. Currently, he is professor at the Department of Materials Science, Fudan University, China. His current research topic is the controlled fabrication, novel properties and optoelectronic applications of semiconductor nanostructures, with a focus on II–VI inorganic semiconductor nanostructures-based optoelectronic devices. Limin Wu Limin Wu received his PhD degree from Zhejiang Univer- sity in 1991. He worked as a lecturer then an associate pro- fessor from 1991 to 1994. He worked as a visiting professor at Pennsylvania State Univer- sity and Eastern Michigan University from 1994 to 1999. He joined Fudan Uni- versity in 1999, where he currently is ‘‘Changjiang Scholar’’ Professor awarded by the Ministry of Education of China. His current research interests include synthesis, assembly and photoelectric properties of organic–inorganic nanoparticles, hollow inorganic particles, development of functional coatings and films. D ow nl oa de d by T ia nji n U niv ers ity on 22 N ov em be r 2 01 1 Pu bl ish ed o n 29 Ju ly 2 01 1 on h ttp :// pu bs .rs c.o rg | d oi: 10. 10 39/ C1 CS 151 03G View Online 5474 Chem. Soc. Rev., 2011, 40, 5472–5491 This journal is c The Royal Society of Chemistry 2011 direct surface reactions utilizing specific functional groups on the cores to create core-shell composites. The PS template particles are then removed by selective dissolution in an appropriate solvent or by calcination at elevated temperature in air, leaving behind hollow spheres. Bourgeat-Lami et al. synthesized PS latex particles bearing silanol groups on the surface via emulsion polymerization using 3-(trimethoxysily) propyl methacrylate as a functional co-monomer.43 These PS colloids were then transferred into aqueous ethanol solution by solvent exchange, wherein the co-condensation of the silanol groups with tetraethoxysilane (TEOS) was carried out via an ammonia-catalyzed sol–gel process, causing composite particles with PS cores and SiO2 shells. Hollow SiO2 spheres were obtained by thermal degradation of the PS cores at 600 1C. Yang and Lu et al. first described a new approach to the generation of inorganic hollow spheres using a template of the core-shell PS gel particles synthesized by an inward sulfonation with concentrated sulfuric acid.44 They also successfully prepared double-shelled TiO2 spheres with a kind of special hollow spheres polymer as a template.45,46 This special hollow template composite was composed of hollow PS spheres containing a thin hydrophilic inner layer and trans- verse channels of poly (methyl methacrylate)–poly (methacrylic acid) (PMMA–PMA). As shown in Fig. 1, when the hollow sphere template was first treated with sulfuric acid, the sulfonation took place in the exterior shell surface, the interior shell surface, and the transverse channels. This sulfo- nation of the hollow spheres enhanced the hydrophilicity of the spheres and provided a suitable graft surface for adsorp- tion or forming complexes with a large variety of functional components, such as metal ions, metal oxide precursors, and organic precursors. Using TiO2 as an example, the sulfonated hollow spheres were immersed into a Ti(OBu)4 sol to coat a layer onto both the inner and outer interfaces of the hollow spheres. The existence of PMMA–PMA transverse channels on the PS shell acted as the entrance for the TiO2 sol. The double-shelled TiO2 hollow spheres were obtained after the intermediate PS layer was removed by a solvent. Following a similar procedure, the same authors successfully prepared carbon,47 TiO2, BaTiO3 and SrTiO3 hollow spheres. 48 Ma et al. used porous polystyrene–divinyl-benzene (PS–DVB) spheres as templates to synthesize multi-shelled spheres and sphere-in-sphere structures by modifying the post-calcination process.49 The temperature during the preheating process directly affects the final structures of TiO2 spheres. Except for using PS and its derivatives as templates, melamine formaldehyde can also be used to prepare hollow spheres based on noble metal oxides and magnetic oxides.50 Another method, termed the layer-by-layer (LbL) self- assembly technique, has become an attractive topic of investi- gation ever since it was first developed by Caruso et al.51,52 The principle of this process is based on the electrostatic association between alternately deposited, oppositely charged species. Multilayered shells are assembled onto submicrometer- sized colloidal PS particles by the sequential adsorption of polyelectrolytes and oppositely charged nanoparticles. Upon calcination of the obtained core-shell particles, uniform-sized hollow spheres of various diameters and wall thicknesses can be generated from a variety of inorganic materials, including SiO2, 51,52 TiO2, 53 Mn2O3, 54 zeolite,55 and other materials.56 In polymer template-based methods for preparation of inorganic hollow spheres, the biggest advantage is that the polymer templates are easily prepared with controllable sizes and surface functional groups, thus many hollow spheres of nonmetallic oxides, metallic oxides, and even metals can be fabricated through this approach. However, the preparation processes can require a lot of energy and time. First, multi-step processes are required for the synthesis of core-shell composite particles, e.g., the surface-functionalization of templating particles and the exchange of solvent/coating reaction in the templating particle approach and repeated adsorption/ centrifugation/washing/redispersion cycles in the LbL method. Second, in order to obtain hollow spheres from core-shell composite particles, removing the core particles by selective dissolution in an appropriate solvent or by calcination at elevated temperature in air is indispensable. Recently, Wu et al. reported a one-step process of fabricat- ing monodisperse hollow SiO2 and TiO2 spheres. This means the formation of the inorganic shells and dissolution of core polymer particles occurs in the same medium (Fig. 2).57–60 In this method, monodisperse, positively charged PS beads were prepared by dispersion polymerization using cationic monomer 2-(methacryloyl) ethyltrimethylammonium chloride as co-monomer, which ensures the resulting silica or titania nanoparticles from the hydrolysis and condensation of TEOS or tetra-n-butyl titanate could be rapidly captured by PS beads Fig. 1 Illustration of the formation of double-shelled hollow spheres. (a) The sulfonated polymer hollow sphere templates; (b) titania composite hollow spheres; (c) doubled-shelled titania hollow spheres. Reprinted with permission from ref. 45. Copyright 2005 Wiley-VCH. Fig. 2 Schematic illustrations of SiO2 (a), TiO2 (b) and ZnO (c) hollow spheres prepared via one-step process. D ow nl oa de d by T ia nji n U niv ers ity on 22 N ov em be r 2 01 1 Pu bl ish ed o n 29 Ju ly 2 01 1 on h ttp :// pu bs .rs c.o rg | d oi: 10. 10 39/ C1 CS 151 03G View Online This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5472–5491 5475 via electrostatic interaction in aqueous ammoniacal alcohol medium at 50 1C. Very interestingly, the PS beads are ‘‘dissolved’’ into PS macromolecule chains or their aggregates subsequently, even synchronously, in the same medium, and are further diffused out gradually through the silica or titania shells since the silica or titania shells prepared by the Sto¨ber method are usually porous, directly forming hollow SiO2 or TiO2 spheres. Neither additional dissolution nor calcination processes are needed to remove the PS cores. If negatively charged PS beads are used as templates, then ZnO or even Ag/SiO2 double-shelled hollow spheres could also be prepared based on the one-step method.61,62 If this coating process occurs under acidic circumstances, PS/SiO2 hybrid hollow spheres and PS/rare-earth-doped nanocrystals (LaF3: Eu 3+, LaF3: Ce 3+–Tb3+, and YVO4: Dy 3+) hybrid hollow spheres could be directly obtained via a one-pot synthesis, in which the PS macromolecular chains diffused from core into the voids between inorganic nanoparticles driven by the strong capillary force to form hybrid shells. This organic–inorganic hybrid shell is expected to improve the mechanical properties of inorganic hollow spheres.63,64 2.2 Inorganic nonmetallic template-based methods Inorganic nonmetallic templates mainly include carbon and silica particles. Carbon spheres appear to be particularly suitable for templating due to their rich reactive groups and ease of removal. Many uniform micro/nano-sized hollow spheres of metallic oxides such as VO2, 65 Gd2O3: Ln (Ln = Eu3+, Sm3+), Ga2O3, NiO, MnO2 66 and so on,67 have been fabricated using carbonaceous polysaccharide spheres as the templates. The surfaces of the carbonaceous microspheres, prepared from saccharide starting materials by dehydration under hydrothermal conditions, are hydrophilic and functiona- lized with –OH and CQO groups. Upon dispersal of the carbonaceous microspheres in metal salt solutions, the func- tional groups on the surface layer are able to bind metal cations through coordination or electrostatic interactions. In the subsequent calcination process, the surface layers incorporating the cationic metal ions are condensed and cross-linked to form oxide hollow spheres. For example, Yang and co-workers reported that hollow Gd2O3: Ln (Ln = Eu 3+, Sm3+) micro- spheres with diameters of about 300 nm were successfully fabricated by using carbon spheres as templates.68 As shown in Fig. 3a, when the precipitation agent, urea, was dissolved in water, it decomposed into CO2 and OH �, coupled with a large number of –OH bonds on the surfaces of the carbon spheres. In the coating process, Gd3+ and Ln3+ were easily precipitated on the surfaces of carbon spheres. Then the high crystallization of Gd2O3: Ln (Fig. 3b) was formed and the carbon spheres were removed at a calcination temperature of 700 1C. This fabrica- tion process involves neither organic compounds nor etching agents. Li and co-workers prepared Ga2O3 and GaN semiconductor hollow spheres, ranging from 100 nm to 1.5 mm in size, by adsorption of metal cations to the surface layer of hydrophilic carbon (carbonaceous polysaccharide) spheres with copious –OH groups, followed by calcination in air.69,70 They then extended this method to prepare hollow spheres from a wide range of metal oxides, including main group metal oxides (Al2O3, SnO2), transition metal oxides (ZrO2, TiO2, CoO, NiO, Cr2O3, Mn3O4), and rare earth oxides (La2O3, Y2O3, Lu2O3, CeO2). 71 Suslick et al. reported a sonochemical fabri- cation of crystalline hollow hematite (R-Fe2O3) using carbon nanoparticles as a spontaneously removable template for nanosized hollow core formation.72 In order to simplify the preparation of inorganic hollow spher