Tree-like Ag nanostructures based on monolithic mesoporous silica
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Herbert Hofmeister Max Planck Institute of Microstructure Physics, D-06120 Halle, Germany (Received 25 November 2003; acepted 20 January 2004)
A novel tree-like nanostructured Ag crystal, with stems, branches, and leaves, has been synthesized by pre-forming Au seeds, soaking, and annealing, based on monolithic mesoporous silica. The obtained Ag nanotrees are of single-crystal nature and statistically symmetrical in geometry. Further experiments revealed that the interconnected channels of the porous silica, heating at low temperature, and the pre-formed Au seeds are crucial to form such structure. Its formation can be attributed to the low nucleation rate and preferentially unidirectional diffusion of Ag atoms to the Au seeds along interconnected channels. This nanostructured material is of great potential to be building blocks for assembling some mini-functional devices of the next generation. The current study is also of importance in studying the diffusion mechanism of single-crystal formation, and especially in improving our understanding of the underlying physical structure of both natural and synthetic porous materials.
I. INTRODUCTION
Metal nanostructures are of great importance in nanotechnology due to their potential applications as building blocks in optoelectronic devices, catalysis, chemical sensors, and other areas.1–5 Many synthesis methods have been developed for the production of nanomaterials with controlled sizes and shapes. Recently, the preparation of metal nanowires or nanorods has attracted significant attention because of their novel optical properties. Silver, as the metal with high electrical and thermal conductivity, has many technological applications, and thus many efforts have been made to prepare rod- or wire-like Ag nanostructures. For example, various template (including hard templates and soft templates) assisted electrochemical and chemical deposition have been used to fabricate uniform Ag nanowires thanks to their attractive aspects on the assembling structures.6–9 Some groups adopt wetchemical method to prepare Ag nanorods with different aspect ratios through introduction of reducing agent or pre-formed “seeds.”10–13 Meanwhile, fractal (randomly ramified) and dendritic (symmetrically branched) Ag nanostructures have also attracted more and more a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0173 1328
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 5, May 2004 Downloaded: 03 Apr 2015
attention nonequilibrium growth process.14–16 In general, fractal growth is expected when randomness dominates, whereas dendritic growth is caused by the anisotropy.17,18 Accordingly, either random or ordered crystalline structures depends on the exact growth conditions. For instance, Xiao et al.19 prepared Pd and Ag dendritic nanostructures in Raney Ni template with the assistance of ultrasonic waves. Brune and co-workers20,21 investigated the crossover from fractal to dendritic Ag patterns on Pt(111) surface through varying the electroche
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