Preparation and Characterization of Rhodium Nanostructures through the Evolution of Microgalvanic Cells and Their Enhanced Electrocatalytic Activity for Formaldehyde Oxidation Bhaskar R. Sathe, Dhanraj B. Shinde, and Vijayamohanan K. Pillai* Physical and Materials Chemistry DiVision, National Chemical Laboratory, Pune 411 008, India ReceiVed: February 5, 2009; ReVised Manuscript ReceiVed: April 9, 2009 Shape-controlled morphological evolution of nanostructured Rh has been demonstrated with the help of a galvanic displacement reaction using Al in 1 mM aqueous Rh(III) chloride at an open circuit potential 0.99 V and at a temperature of 273 K (room temperature). Nanospheres composed of small nanoparticles of size around 2.9 ( 0.4 nm having uniform distribution with a FCC pattern have been evolved during the course of the reaction. Electrochemical results coupled with structural and morphological characterization data from transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and cyclic voltammetry (CV) suggested the formation of Rh nanostructures. Considering the role of the potential of substrate Al and Rh and diffusion of reactant and product species toward and from the surface of the Al, we proposed the tentative mechanism for the formation of microgalvanic cell. Significantly, these rhodium nanostructures exhibit enhanced electrocatalytic activity toward many fuel cell reactions as demonstrated by formaldehyde oxidation in 0.5 M NaOH. The present strategy is expected to be valid for preparing many other similar electrocatalysts (Pt, Au, and Pd) capable of exhibiting such a remarkable size- and shape- dependent reactivity. 1. Introduction Synthesis of metal nanostructures followed by their assembly on a desired surface in large scale is essential for many successful bottom-up approaches toward the applications of many nanomaterials with profound implications on understand- ing their behavior even when they are used for assembly/pattern- ing. 1,2 Several recent developments have enabled important applications in electronics, sensing, catalysis, and electrochem- istry due to their fascinating properties, primarily determined by their size, shape, composition, and structure. 3-6 Indeed, this has been successfully accomplished to achieve different size- controlled shapes and their self-organization on nanolevel for metals such as Ag, Au, Pd, and Pt using ingenious solution chemistry. However, these colloidal nanostructures dispersed in liquids cannot be used directly for many applications including heterogeneous catalysis and nanodevice fabrication, especially because of the challenges in separation and recycling aspects; furthermore, it is very difficult to control the size, composition, and their self-organization. 7-9 One way to over- come these problems is by using electrochemical tools to precisely control the crystal growth and design, as elegantly illustrated by the formation of ZnO nanostructures, on indium tin oxide (ITO)-coated glass. 10 Consequently, the growth of nanostructures by electrochemical methods has received con- siderable attention and partly also because of the ease of controlling even complex morphology such as fractal geometry and dendritic growth. This method is particularly advantageous because the crucial steps in nucleation and growth can be controlled by modulating appropriate physical parameters such as the electric field. In most of these cases, however, growth occurs under nonequilibrium conditions and, interestingly, a dramatic variation in morphology hence arises under low and high overpotentials, respectively. For example, Pt and RuO 2 mesostructures recently prepared using a porous alumina template show interesting potential dependent morphological evolution. 11 Such a slow evolution of Rh structures under equilibrium conditions could be effected by the approach of galvanic exchange, offering a unique way by choosing a suitable redox couple. For example, this has been applied successfully to achieve different shapes (viz., cubes, rods, wires, and core-shells) of Au, Ag, Pd, and Pt and also of some alloys. 12,13 This approach is particularly attractive for preparing nanostruc- tures with well-controlled dimensions because of their simplicity (lack of sophisticated equipment), cost effectiveness, and high throughput although some of these nanostructures are contami- nated by impurities of in situ-generated side products and there are also concerns on their variable composition and morphologi- cal dispersion. 12 Despite its expensive nature, Rh is an especially promising material for many technological uses, and stable nanostructures are desired in order to realize their complete potential in electronic, optical, and catalytic applications. More significantly, morphologies with well-defined sizes and shapes have been recently prepared for the other metals to reveal their unique catalytic properties, and hence these could be used as a bench mark to characterize properties of Rh nanostructures prepared by galvanic displacement. 12,13 Here we employ this approach for the shape-selective synthesis of nanostructured Rh (∼25 nm) in such a way that their evolution could be precisely controlled by the interplay between the growth and mass transport rate with the help of key parameters including pH, concentration, and temperature. In addition, a tentative mechanism using microgalvanic cell formation is proposed based on the results from cyclic voltammetry, SEM, EDS, and XRD. * Author to whom correspondence should be addressed. E-mail: vk.pillai@ncl.res.in, fax: +91-20-25902636, tel: +91-20-202590258. J. Phys. Chem. C 2009, 113, 9616–9622 9616 10.1021/jp901055v CCC: $40.75  2009 American Chemical Society Published on Web 05/08/2009