A Facile Method to Synthesis Hollow Silica Ellipsoids and Their Derivatives
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A Facile Method to Synthesis Hollow Silica Ellipsoids and Their Derivatives Henan Zhang and Daniel L. Akins The City College of The City University of New York 160 Convent Avenue, New York, NY U.S.A. ABSTRACT We have utilized wet-chemical etching of ellipsoidal silica nanoparticles to form silica nanoshells of a range of elliptical morphologies. The thicknesses of the silica ellipsoidal nanoshells are controlled through variation of synthesis conditions. A mechanism has been proposed to explain how the nanoshells are formed, and we have demonstrated that the porosity of the silica ellipsoid plays a role in the dissolution-regrowth process. We have also, via appropriate functionalization of the silica surface, coated the ellipsoidal nanoshells with Au nanoparticles. INTRODUCTION Silica nanostructures (because of the ease of surface functionalization) have proven to be useful absorbent materials in composite structures for enabling controlled porosity, solvent compatibility, and ease of processability.1-3 Although chemical etching was first reported for the fabrication of hollow silica spheres, recently a broad spectrum of other morphologies has been created using this fundamental approach.4,5 It is of note, that although the literature is replete with reports on the formation of silica shells of a range of diameters larger than 100 nm using the chemical etching approaches, preparation of silica nanoshells is not widely discussed. This omission is principally due to the challenge of controlling the strong etching additives. Chemical etching allows the control of surface porosity as well as minimizes coalescence of nanoparticles, and because of the porosity the reactivity of encapsulated catalytic agents are maintained.6 A controllable structural difference-based selective etching strategy has been used to produce nanosized hollow/rattle-type mesoporous silica structures;7 and, recently, with the assistance of cationic surfactants, a cationic surface-assisted selective etching strategy (CSASE) has produced hollow spherical silica particles with either wormhole-like or oriented mesoporous shells.8 It is generally found that chemical etching of silica particles leads to the formation of silicates, and the redeposition of silicates results in the regeneration of silica with the retention of overall initial shape. In the present paper, we utilize a chemical etching strategy on porous ellipsoidal silica nanoparticles, resulting in the formation of hollow ellipsoidal silica nanostructures. We demonstrate that the porous structures of silica precursors facilitate the chemical etching, which results in silica nanoshells. We also report the preparation of Au-nanoparticle coated, ellipsoidal silica shell nanocomposites through functionalization (i.e., silylation) of the surface of the ellipsoidal silica shells with (3-aminopropyl)trimethoxysilane (APTS). EXPERIMENT The detailed scheme concerning the synthesis of ellipsoidal silica nanoparticles has been discussed previously.9 We used ca. 0.03 g of prior prepared ellipsoidal sili
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