Langmuir 2007, 23, 9455-9462
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Tailored Core-Shell-Shell Nanostructures: Sandwiching Gold Nanoparticles between Silica Cores and Tunable Silica Shells Yan-Li Shi and Tewodros Asefa* Department of Chemistry, Syracuse UniVersity, Syracuse, New York 13244 ReceiVed March 24, 2007. In Final Form: June 19, 2007 Size tunable and structure tailored core-shell-shell nanospheres containing silica cores, gold nanoparticle shells, and controlled thicknesses of smooth, corrugated, or porous silica shells over the gold nanoparticles have been synthesized. The synthesis involved the deposition of gold nanoparticles on silica cores, followed by sol-gel processing of tetraethoxysilane (TEOS) or sodium silicate to form dense or porous silica shells, respectively, over the gold nanoparticles. The structures and sizes of the resulting core-shell-shell nanospheres were found to heavily depend on the sizes of the core nanoparticles, the relative population of the gold nanoparticles on each core, and the concentration of TEOS. While a higher TEOS concentration resulted in thicker and more uniform silica shells around individual larger silica cores (approximately g250 nm in diameter), the same TEOS concentration resulted in aggregated and twin coreshell-shell nanostructures for smaller silica cores (approximately e110 nm in diameter). The thinner silica shells were synthesized by using a lower TEOS concentration. By using sodium silicate (Ung et al. J. Phys. Chem. B 1999, 103, 6770), the porous silica shells were synthesized. Controlled chemical etching of the core-shell-shell nanoparticles with an aqueous KCN solution resulted in corrugated silica shells around the gold nanoparticles or corrugated silica nanospheres with few or no gold nanoparticles. This has allowed synthesis of new types of core-shell-shell nanoparticles with tailored corrugated shells. The nanoporous silica shells provided accessible structures to the embedded metal nanoparticles as observed from the electrochemical response of the gold nanoparticles.
Introduction The development of synthetic methods for making coreshell nanostructures consisting of noble metals and metal oxides has emerged as an attractive research area in material chemistry.1 By using metal nanoparticles as a core and metal oxide as the shell (or alternatively, by using the dielectric metal oxide as the core and the metal as the shell), a broad range of hybrid materials with novel properties may result.2 Among several possible nanostructures resulting from these approaches, core-shell nanoparticles made from noble metal nanoparticle cores and metal oxide shells are well-reported.3 In particular, silver and gold nanoparticle cores with silica shells are the most widely studied due to the potential applications of gold and silver nanoparticles in areas ranging from catalysis and optical devices to immunoassay labeling and surface enhanced Raman spectroscopy (SERS).4 Some attention has also been given to the synthesis of core-shell nanoparticles consisting of metal oxide cores and metallic shells. The thrust of the synthesis of the latter type of materials, particularly with silica nanosphere cores g100 * Corresponding author. E-mail:
[email protected]. (1) (a) Teng, X.; Black, D.; Watkins, N. J.; Gao, Y.; Yang, H. Nano Lett. 2003, 3, 261-264. (b) Carotenuto, G.; Pepe, G. P.; Nicolais, L. Eur. Phys. J. B 2000, 16, 11. (c) Wei, A.; Kim, B.; Sadtler, B.; Tripp, S. L. Chem. Phys. Chem. 2001, 2, 743. (d) Fornasiero, D.; Grieser, F. J. Colloid Interface Sci. 1991, 141, 168. (e) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (f) Liu, S.; Zhang, Z.; Wang, Y.; Wang, F.; Han, M.-Y. Talanta 2005, 456-461. (2) Kobayashi, Y.; Correa-Duarte, M. A.; Liz-Marza´n, L. M. Langmuir 2001, 17, 6375-6379. (3) (a) Tunc, I.; Suzer, S.; Correa-Duarte, M. A.; Liz-Marzan, L. M. J. Phys. Chem. B 2005, 109, 7597-7600. (b) Salgueirin˜o-Maceira, V.; Correa-Duarte, M. A.; Farle, M.; Lo´pez-Quintela, A.; Sieradzki, K.; Diaz, R. Chem. Mater. 2006, 18, 2701-2706. (c) Tunc, I.; Demirok, U. K.; Suzer, S.; Correa-Duatre, M. A.; Liz-Marzan, L. M. J. Phys. Chem. B 2005, 109, 24182-24184. (d) Botella, P.; Corma, A.; Navarro, M. T. Chem. Mater. 2007, 19, 1979-1983. (e) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 1999, 103, 6770-6773. (f) Suryanarayanan, V.; Nair, A. S.; Tom, R. T.; Pradeep, T. J. Mater. Chem. 2004, 14, 2661-2666. (4) (a) Liu, S.; Zhang, Z.; Wang, Y.; Wang, F.; Han, M.-Y. Talanta 2005, 67, 456-461. (b) Salgueirin˜o-Maceira, V.; Caruso, F.; Liz-Marza´n, L. M. J. Phys. Chem. B 2003, 107, 10990-10994. (c) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 1999, 103, 6770-6773.
nm in size and a few nanometer metal shells, has been to create core-shell nanoparticles having optical properties similar to the pure metal nanospheres g100 nm in diameter.5 Further, a metal oxide/metal core-shell structure can result in monodisperse naked metal nanoparticles, which exhibit superior activity in heterogeneous catalysis for reactions such as oxidation of CO and NO.6 Additional advantages of the synthesis of core-shell nanospheres with silica microsphere cores and metallic shells are that (1) the silica cores can be grown with low polydispersities (e.g.,
