Engineering anisotropic nanostructures of isotropic FCC metals (Au, Pt, or Pd) requires a symmetry-breaking mechanism. In wet chemical synthesis methods, a judicious selection of capping agents can help achieve the desired morphology. However, understanding the mechanism of formation is challenging due to the complex chemical environment and the limitation of time resolution to investigate the fast reactions. Here, we present the formation mechanism of ultrathin singlecrystalline Pt nanowires using detailed electron microscopy investigations. Various control experiments performed suggest that the binding of ligand to the Pt surface (primary vs secondary amine) plays a crucial role not only in dictating the morphology but also in controlling the nanowire growth direction. Further, a ligand-exchange mechanism is adopted to separate these as-synthesized Pt nanowire bundles into well-separated wires. This is found to be a crucial step in controlling the surface accessibility and hence improving the electrocatalytic activity of the nanowires.

The efficiency of heterogeneous catalysts critically depends on the nature of the surface. We present results on controlling the composition in ultrathin bimetallic AuPd. AuPd wires were grown using Au nanowire templates; the surface composition could be tuned by increasing the amount of Pd. Further, segregation of Pd to the surface could be induced in alloyed nanowires by annealing under a controlled CO atmosphere. Electrocatalytic activity of these bimetallic systems is assessed for the methanol oxidation reaction (MOR). While the MOR potential shows a monotonic increase with Pd content, the specific activity displays a typical volcano-type behavior. The CO-annealed nanowires show a lowering of potential owing to a higher Pd content on the surface while still maintaining the specific activity. These findings provide clear strategies to independently control the reaction potential and the activities of nanocatalysts. The experimental findings are well supported by the theoretical investigations using density functional theory (DFT) calculations.

The reaction kinetics and efficiency in any catalytic process is controlled by the adsorption and desorption of reactant and intermediates on the surface of catalyst. As a consequence, the surface properties, composition and geometry of the catalyst dictates the mechanism of reactions. The surface electronic structure is modulated by variations in surface and subsurface atomic configurations, e.g., ordering and surface segregation, which in turn can vividly affects catalytic properties.1,2 A great deal of efforts are being devoted to develop efficient heterogenous catalyst by critically controlling the surface structure and chemistry. Here, I would like to present results on two different strategies for controlling the surface composition in ultrathin bimetallic AuPd nanowires. Au nanowire template-based method was used to grow AuPd wires by gradually increasing the amount of Pd.3 Percentage of composition of Au and Pd in AuPd nanowires gave either alloy nanowires with Au-rich compositions or core-shell structure at higher Pd compositions with Au-rich core and Pd-rich shell. Surface morphology of alloyed nanowires were induced through segregation of Pd to the surface by annealing under controlled CO atmosphere. High resolution microscopic imaging was possible due to the fact of substantial difference in the atomic number between Au and Pd. Electrocatalytic methanol oxidation reaction (MOR) in alkaline medium is important for energy conversion and was selected to study the catalytic activity of AuPd nanowires. A nearly monotonic increase of MOR potential was observed with increase in Pd content. Interestingly, a typical volcano type behavior of the specific activity was observed with different composition and with 40 and 50% Pd samples showed the optimum values in terms of current, potential and If/Ib values. Owing to a higher Pd content of CO-annealed nanowires, a lowering of potential was observed though the specific activity was maintained. These findings provide clear strategies to independently control the reaction potential and the activities of nanocatalysts with smaller amounts of the expensive active metal. The compositional effects on the electronic structure of the AuPd nanowires (catalyst surface) as well as binding energy of intermediates involved in the MOR was investigated using DFT calculations. d-band center analysis was employed to check interaction of CO* which a major intermediate of MOR with the catalyst surface. The theoretical investigation align well with the experimental results where it was found that the MOR potential increases with increasing amount of Pd and for the Au rich case the reaction does not reach up to CO2 but yields HCHO as a product at lower potentials. From our experimental finding it was evident that it is possible to lower the MOR potential to attain comparable or better activity through enrichment with the precious metal on the surface while essentially dropping the real quantity used. References: (1) Zhang, N.; Shao, Q.; Xiao, X.; Huang, X. Advanced Catalysts Derived from Composition-Segregated Platinum–Nickel Nanostructures: New Opportunities and Challenges. Adv. Funct. Mater. 2019, 29 (13), 1–28. (2) Ruban, A. V.; Skriver, H. L.; Nørskov, J. K. Surface Segregation Energies in Transition-Metal Alloys. Phys. Rev. B - Condens. Matter Mater. Phys. 1999, 59 (24), 15990–16000. (3) Chatterjee, D.; Shetty, S.; Müller-Caspary, K.; Grieb, T.; Krause, F. F.; Schowalter, M.; Rosenauer, A.; Ravishankar, N. Ultrathin Au-Alloy Nanowires at the Liquid–Liquid Interface. Nano Letters 2018, 18 (3), 1903–1907. Figure 1