Three-Dimensional Interstitial Nanovoid of Nanoparticulate Pt Film Electroplated from Reverse Micelle Solution Sejin Park, † Sun Young Lee, ‡,§ Hankil Boo, † Hyun-Mi Kim, | Ki-Bum Kim, | Hee Chan Kim, ⊥ Youn Joo Song, ‡ and Taek Dong Chung* ,‡ Nomadien Corporation, 249-1 Dongseon-dong, Sungbuk-gu, Seoul 136-742, Korea, School of Materials Science and Engineering, Seoul National UniVersity, Seoul 151-742, Korea, Department of Biomedical Eingineering, Seoul National UniVersity, Seoul 110-744, Korea, and School of Chemistry, Seoul National UniVersity, Seoul 151-747, Korea ReceiVed April 20, 2007 ReVised Manuscript ReceiVed May 30, 2007 So far, metal thin films with nanopores on the scale of a few nanometers have been fabricated by electrochemical deposition only in the limited types of surfactant-based templates, such as hexagonal (H 1 ) lyotropic liquid crystalline (LLC) phase 1 or potential-controlled micelle assembly. 2 Both kinds of templates produce hexagonally ordered (one- dimensional, 1D) nanopores, the mass transport along which could be easily inhibited by pore blocking. In addition to the LLC phase and the micelle-type aggregation, the reverse micelle (L 2 ) solution is another phase to be potentially exploited as templates for nanostructured materials of higher dimension. However, there rarely has been studies on the nanoporous metal films electroplated from L 2 solution. In this study, we propose a new method to fabricate three- dimensional (3D) nanoporous Pt film via electrochemical deposition in L 2 solution of a nonionic surfactant. Metallic thin films with nanoporous structure are of great use and importance in the fields of energy storage materials, 3 catalysts, 4,5 and sensors. 6-9 There have been only a few methods introduced to fabricate such nanoporous metal films, for example, electroless plating of Au inside the pores of the membrane filter 10 and dissolution of one component of alloys (dealloying). 11,12 Moreover, to obtain a free-standing nanoporous film with pore size of a few nanometers, the only method has been to electroplate by the aid of templates of self-assembled surfactants, which utilizes either LLC template 1 or potential-controlled surfactant assembly. 2 Attard et al. reported the electrodeposition of nanoporous Pt (symbolized as H 1 -ePt) in the LLC template, 1,13 and the H 1 -ePt showed hexagonally arranged pores of approximately 2.5 nm in diameter with a pore-pore distance of ap- proximately 5 nm. 1 Stucky et al. 2 showed that cylindrical/ hemicylindrical micelle assembly of surfactant was formed at the electrode/solution interface where electric field was applied and functioned as a template for nanoporous Pt film with pore diameter of approximately 4 nm and wall thickness of approximately 4 nm. Relevant works were also reported for lamellar LLC 14 or LLC of block copolymer. 15 Although the nanoporous Pt films electroplated from both LLC template and potential-controlled surfactant assembly allow us to take novel advantage of the nanoporous structures in the various applications, there are the issues to be addressed for practical uses; for example, sluggish diffusion along the 1D pore and pore clogging resulting in deactivation of the entire surface of the inner wall. In these respects, the nanoporous structures of higher dimension are expected to alleviate the problems. Although there is a report on 3D nanoporous Pt, it has rarely been fabricated in the form of thin film by electroplating. 16 In the present study, we propose the L 2 phase as another electroplating environment for nanoporous Pt films. Seem- ingly the Pt deposition might not readily take place in the L 2 solution, because the aqueous regions containing Pt precursors are individually enclosed by nonpolar moieties of surfactant molecules. However, the proton self-diffusion is reportedly active among the neighboring aqueous domains as the water content exceeds 13 wt % in the L 2 phase of the binary mixture of Triton X-100 and water. 17 Thus it is conceivable that the L 2 phase with relatively high water content may work as an alternative template for electroplating metals. Furthermore, the encounter of micelles and Pt nanoparticles can result in the growth of network of Pt nanoparticles, in the dynamic equilibrium phase of L 2 . As Pt nanoparticles grow in the aqueous region of L 2 phase, it is expected that the interstitial voids gradually form the structure of 3D nanopores. 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