pubs.acs.org/cm r XXXX American Chemical Society Chem. Mater. XXXX, XXX, 000–000 A DOI:10.1021/cm100418w Synthesis of Vertically Aligned Hollow Platinum Nanotubes with Single Crystalline Nanoflakes Lichun Liu, † Sang-Hoon Yoo, † and Sungho Park* ,†,‡,§ † Department of Chemistry, Sungkyunkwan University, Suwon 440-746, South Korea, ‡ Department of Energy Science, Sungkyunkwan University, Suwon 440-746, South Korea, and § SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South Korea Received February 9, 2010 This paper reports on a methodology for synthesizing vertical arrays of hollow platinum nanotubes with [111] single-crystalline nanoflakes. Initially, single-component nickel nanorods were fabricated with the aid of AAO templates and electrochemical deposition. When the resulting nickel nanorods were immersed in a Pt-ion-containing aqueous solution, the nickel metal dissolved into Ni 2þ ions through spontaneous galvanic replacement with Pt ions. However, the direct replacement between nickel nanorods and Pt ions led to an irregular architecture in the resulting deposition of platinum. Instead, a pitting corrosion pretreatment of the nickel nanorods produced nucleation sites for replacement with the Pt ions. This step was critical for accelerating the interfacial replacement reaction rate and the formation of the regular platinum nanotubes with ultrathin superficial nanoflakes. We found that the Kirkendall effect was operative in the formation of platinum nanotubes. Introduction Hollow nanomaterials covering porous nanomaterials are of critical importance in advanced applications in modern science and technology, such as catalysis, photo- nics, and drug delivery, 1 principally because of their high surface area, low density, and usable nanoscale inner space. Since platinum-based nanomaterials play key roles in many catalysis processes, because of their distinct chemical and physical properties, 2 a wide variety of research has focused on the synthesis of novel hollow platinum nanostructures to maximize the use of plati- num, compared to its solid counterparts. A large number of alloyed platinum hollow nanostruc- tures with definitely higher surface-area:mass ratios have readily been fabricated by controlling the concentration of reduction agent, using direct thermolytic reduction, an in situ sacrificial template, and colloidal templating. 3 For example, using cobalt nanoparticles as sacrificial tem- plates, Liang and co-workers have synthesized colloidal platinum hollow nanospheres, exhibiting enhanced electro- catalytic activity for the oxidation of methanol. 4 Given the advantages of interconnected nanopores and nano- channels in porous gold substrates via the dealloying of Au-Ag alloys, 5 a large surface area of subnanometer- thick platinum coating can be generated using a under- potential deposition (UPD) process. 6 More-diverse recipes for synthesizing porous platinum nanostructures have also been reported. 7 Porous-like platinum film with all-over nanorod arrays has a highly accessible effective surface area, because of the generation of a large side surface of vertically aligned length-tunable nanorods. However, the surface-area:mass ratio of platinum nanorods is confined by their solid interiors and smooth surfaces. An effective and straightforward method to enhance the surface area: mass ratio is to hollow the interior and roughen the surfaces of these nanorod materials. Herein, we demon- strate a facile approach for fabricating novel vertical hollow platinum nanotube arrays with epitaxial single- crystalline superficial thin nanoflakes. Experimental Section AAO membrane templates (diameter ≈ 25 mm, pore size ≈ 200 nm, thickness ≈ 60 μm) were purchased from Whatman *Author to whom correspondence should be addressed. Fax: 82-31-290- 7075. E-mail: spark72@skku.edu. (1) (a) Lim, B.; Jiang, M. J.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. A. Science 2009, 324(5932), 1302–1305. (b) Cregan, R. F.; Mangan, B. J.; Knight, J. C.; Birks, T. A.; Russell, P. S. J.; Roberts, P. J.; Allan, D. C. Science 1999, 285(5433), 1537– 1539. (c) Im, S. H.; Jeong, U.; Xia, Y. 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