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Matsuhama, J. Mitzuguchi, Acta Cryst. C 1999, 55, 131. Preparation of Mesoscale Hollow Spheres of TiO 2 and SnO 2 by Templating Against Crystalline Arrays of Polystyrene Beads** By Ziyi Zhong, Yadong Yin, Byron Gates , and Younan Xia* This paper describes a new method that generates meso- scale hollow spheres of TiO 2 and SnO 2 by templating their sol±gel precursor solutions against crystalline arrays of monodisperse polystyrene beads. The void size of these hollow spheres was determined by the diameter of the polystyrene template, and the thickness of the ceramic wall could be easily changed in the range of 30±100 nm by using sol±gel precursor solutions with different concentrations. Mesoscale hollow spheres of ceramic materials are useful in many areas. For example, they can serve as extremely small containers for encapsulationÐa process that has been extensively explored for applications in catalysis; delivery of drugs; development of artificial cells; and protection of biologically active agents such as proteins, enzymes, or DNAs. [1] When used as fillers (or pigments, coatings), hol- low spheres also provide some advantages over their solid counterparts because hollow spheres usually have lower densities. [2] Furthermore, hollow spheres usually evolve from their precursor formsÐcomposite particles consisting of cores covered with thin shells of different chemical com- positions. These core±shell structures may exhibit proper- ties that are substantially different from those of the core particles. [3] It has been shown that the structure, size, and composition of these hybrid particles could be readily alter- nated in a controllable way to tailor their optical, electrical, thermal, mechanical, electro-optical, magnetic, and catalyt- ic properties over a broad range. [4] Templating against colloidal particles is probably the most effective approach to the formation of hollow spheres of ceramic materials. The colloidal particles that have been used include nanoscale gold, silver, or CdS particles; and mesoscale silica or polymer beads. [5] In a typical procedure, a thin coating of the ceramic material (or its precursor) is formed on the template to create a core±shell composite; subsequent removal of the template (by calcination at ele- vated temperatures in air or selective etching in an appro- priate solution) generates ceramic hollow spheres whose inner diameter is determined by the size of the template. A number of methods have been demonstrated for coating the template with a thin shell of the desired material. One of the simplest approaches involves the use of controlled adsorption and/or reactions (e.g., precipitation, grafted po- lymerization, or sol±gel condensation) on the surfaces of template particles. [6] With this method, however, it may be difficult to control the homogeneity and thickness of the coating, and sometimes it may lead to clumping and het- erocoagulation. More recently, two elegant approaches were demonstrated by several groups, which have allowed formation of homogeneous, dense, thin coatings of silica on various types of templates. In the first method, the surface of the template (e.g., a gold or silver colloidal particle) was modified with an appropriate primer that could greatly en- hance the coupling (and thus deposition) of silica mono- mers or oligomers to this surface. [7] In the second method, layer-by-layer adsorption of polyelectrolytes and charged nanoparticles was used to build a shell structure around the template particle whose surface had been derivatized with appropriate functional groups. [8] Both of these methods have been successfully applied to the formation of homoge- neous and dense coatings of ceramic materials on the sur- faces of a variety of colloidal particles. Subsequent removal of the core particles yielded hollow spheres of ceramic ma- terials with a range of dimensions. [8b] Here we wish to re- port another effective method that is capable of forming ceramic hollow spheres with a well-defined void size and homogeneous wall thickness. Figure 1 shows the schematic procedure. The crystalline array of polystyrene beads was fabricated between two glass substrates using a previously published method. [9] After the water had been removed by evaporation at room temperature, the hydrodynamic size of the polymer beads shrank by ~20 %, and the magnitude of this shrinkage is mainly determined by the electrostatic interactions among the polymer beads. The strength of such interactions strongly depends on the density of charges on the surface of each polymer bead and the total concentration of free electrolytes in the water. [10] Although the polystyrene beads were in physical contact in the dried sample, they be- came separated from each other when the packing cell was infiltrated with a sol±gel precursor solution by capillary ac- ± [*] Prof. Y. Xia, Dr. Z. Zhong,Y. Yin, B. Gates Department of Chemistry University of Washington Seattle, WA 98195-1700 (USA) [**] This work was supported in part by a New Faculty Award from the Dreyfus Foundation, a subcontract from the AFOSR MURI Center at the University of Southern California, and start-up funds from the UW. B.G. thanks the Center for Nanotechnology at the UW for a fel- lowship.