Preparation of Ultrathin Thiolate-Covered Bimetallic Systems: From Extended Planar to Nanoparticle Surfaces Mariano H. Fonticelli, Gasto ´ n Corthey, Guillermo A. Benitez, and Roberto C. Salvarezza* Instituto de InVestigaciones Fisicoquı ´micas Teo ´ ricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, CONICET, Sucursal 4 Casilla de Correo 16, 1900 La Plata, Argentina Lisandro J. Giovanetti and Fe ´ lix G. Requejo Instituto de InVestigaciones Fisicoquı ´micas Teo ´ ricas y Aplicadas (INIFTA) and Instituto de Fı ´sica La Plata (IFLP), Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, CONICET, Sucursal 4 Casilla de Correo 16, 1900 La Plata, Argentina Young S. Shon Department of Chemistry and Biochemistry, California State, UniVersity, Long Beach, California 90840 ReceiVed: March 3, 2007; In Final Form: April 28, 2007 A simple method entirely based on solution chemistry to deposit metals on foreign substrates at underpotential conditions is described. The method allows deposition of extremely small amounts of metals under equilibrium conditions. Thiolate-covered bimetallic extended planar and nanoparticle surfaces can be easily prepared by this route using redox couples and solution pH to control the interfacial potential of the system. Introduction Ultrathin metallic films, one atomic layer in thickness, on surfaces are of special interest in heterogeneous catalysis and electrocatalysis because of their unique catalytic and chemi- sorptive properties. 1-3 In particular, the formation of bimetallic surfaces on metal nanoparticles (NPs) has attracted considerable attention because their small size and large surface to volume ratio largely improves chemical and electrochemical reactivity 4 minimizing material costs. In fact, bimetallic Ag-AuNPs have been used to transform poisonous carbon monoxide into less harmful carbon dioxide at room temperature. 5 Furthermore, Pd- AuNPs are the most effective catalysts yet identified for remediation of groundwater pollutants such as trichloroethene. 6 Tiny Pt-RuNPs break down intermediate products that normally would keep the Pt itself from working properly in fuel cells that convert alcohol into electricity. 7 In addition, bimetallic nanoparticles also show fascinating magnetic properties and capping effects stemming from the electronic localization and the symmetry breaking at the surface by functionalization of metallic core with an ultrathin metal shell deposition. 8 For these reasons, the precise control of the size, shape, composition, and crystal structure of bimetallic nanoparticles is one of the goals for the scientists of this field. 4 Electrochemistry has been a simple, fine, and precise control route to prepare ultrathin bimetallic planar surfaces. In fact, two- dimensional structures consisting of a metal on a foreign metallic substrate can easily be prepared by underpotential deposition (UPD) from electrolyte solutions. 9 In this method, the applied potential (E) controls the surface coverage (θ) that can be varied from submonolayer coverages to a complete monolayer under well-defined equilibrium conditions. However, this procedure involves the use of electrochemical cells and an external power source to precisely control the potential or the current applied to the metal/electrolyte interface. Electrochemical preparation of bimetallic NPs is more complex because, in principle, they should be supported in a conducting substrate. Following this idea, Weaver and co-workers proposed a method that starts with the incorporation of the NPs to an inert electrode surface. 10 Then, UPD of a metal such as Cu or Ag is made on the NP surface forming a monolayer or submonolayer structure using conven- tional electrochemical equipment. 11 Finally, the UPD metal- covered NPs are left in contact with a solution containing metal cations of a more noble metal. Thus, a spontaneous redox reaction takes place: the UPD layer is oxidized to metal cations while the more noble metal species coming from the solution are spontaneously reduced at the NP surface. Using this method, Pd-Au and Pt-Au core-shell NPs have been prepared. 10 The main disadvantage of this interesting strategy is that it requires a first step to support the NPs on an electrode surface and accordingly is limited to a relatively small amount of NPs. On the other hand, it requires an intermediate step: UPD deposition of a less noble metal. 12 A method that overcomes the needing of a conducting support has been recently proposed. 13 In this method, a monolayer of a redox organic molecule (tyrosine) is self-assembled onto the AuNPs in solution. Afterward, the tyrosine-covered AuNPs are left in contact with a solution containing a less noble metal. The organic molecule is spon- taneously oxidized while the metallic cation is reduced on the AuNP. In this way Ag-AuNP has been prepared directly by simple solution chemistry. However, the amount of deposited Ag is restricted to the amount of tyrosine present on the NP surface that is determined by the self-assembly process. Therefore, it is difficult to control the amount of Ag from the submonolayer to the multilayer range. In addition, the method could result in complex Ag-tyrosine-AuNP because prepara- * To whom correspondence should be addressed. Phone: 54-221- 4257291 / 4257430. Fax: 54-221-4254642. E-mail: robsalva@inifta.unlp.edu.ar. 9359 J. Phys. Chem. C 2007, 111, 9359-9364 10.1021/jp071747f CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007