Nanostructural Formation of Pd-Co Bimetallic Complex on HOPG Surfaces: XPS and AFM Studies

A new single source approach was developed to synthesize Pd-Co nanoparticles using a bimetallic compound, [ E t 3 N H ] 2 [ C o P d 2 ( ？？ - 4 - I - 3 , 5 - M e 2 p z ) 4 C l 4 ] ( C o P d 2 ) , as a molecular precursor to obtain dispersed catalyst on highly ordered pyrolytic graphite (HOPG) surface, in view of preparing oxygen reduction catalysts for low temperature fuel cells. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques were employed to characterize the nanostructure formations and to determine the composition and morphology of the complex on the HOPG. Results of high resolution XPS analysis (HR-XPS) revealed the binding energies corresponding to the atomic constituents of the precursor. When the precursor solution was placed on the surface of the HOPG, the bimetallic complex assumes a tubular structure and it appears that the surface of the HOPG offers a ground for the self-organization of nanostructural formations. 1. Introduction Nanoarchitecture is an emerging area that fuels the interests of scientists and engineers alike, largely due to the novel material properties that can be engineered and tuned at molecular levels. These nanomaterials are mainly used for heterogeneous catalysis. Platinum is used extensively as a catalyst in both anodes and cathodes of low-temperature polymer electrolyte fuel cells. Low-temperature fuel cells are considered alternate power sources for portable and transportation applications [1]. In spite of its excellent electrocatalytic activity, the use of monometallic platinum is disadvantageous due to its intermediate poisoning and high cost. Sophisticated bimetallic, cost-effective Pd-Co catalysts were recently proposed for oxygen reduction reaction in low-temperature fuel cells, especially for direct methanol fuel cell applications due to their methanol tolerance ability and appreciable oxygen reduction efficiency [2]. When bimetallic catalysts are prepared by using multiple precursors, it is difficult to attain homogeneous catalytic system characterized by a uniform particles size distribution. In order to achieve such objective, in this paper, a single organometallic precursor characterized by a well-defined Pd-Co ratio has been used. This approach would allow the formation of nanostructures and nanoparticles with defined size and composition distribution. This paper reports on the characterization of a CoPd2 precursor and deposition for the nanoarchitectural formation on highly ordered pyrolytic graphite surface through surfaces analysis techniques such as X-ray photoelectron