Fabrication and surface characterization of single crystal PtBi and PtPb (100) and (001) surfaces

Abstract

High quality single crystal PtM (M = Bi or Pb) (100) and (001) surfaces have been generated from solid state bulk materials. The specific orientation was determined via X-ray Laue back-reflection and the miscut angles of the single crystal surfaces were ∼0.3° which was limited by instrumental resolution. The PtM (001) electrode had a Pt termination based on cyclic voltammetric (CV) profiles. The surface structure and composition of single crystal PtM surfaces have been studied by synchrotron-based in situ X-ray grazing incidence diffraction (GID) under active electrochemical control. Cycling of the potential to increasingly high values resulted in dramatic changes to the surface crystalline structure and composition of these single crystal electrodes. Well-defined Pt nano-domains in a hexagonal pattern with a 23° offset angle to the substrate were formed on the PtM (001) surface after electrochemical pretreatment in supporting electrolyte (0.1M H₂SO₄), especially for E_ulp (E_ulp = upper limit potential) of +0.80 V or beyond. From an analysis of the diffraction peaks, the size of the Pt domains was estimated to be ∼15 nm. The Pt domain formation on the single crystal surfaces, similar to results on polycrystalline intermetallic phases, was due to leaching of the less-noble elements (Bi or Pb) from the intermetallic matrix and sintering of the Pt atoms on the surfaces. On the other hand, Pt domains with a preferential direction but no offset angle to the substrate were formed on PtM (100) surface after similar electrochemical pretreatment. PtBi and PtPb single crystal surfaces exhibited different anisotropic electrocatalytic activities towards the electrooxidation of formic acid and other potential fuels for fuel cell applications. The reactivities of these single crystal electrodes towards the oxidation of small organic molecules were a function of E_ulp values and maximal activities were around +0.60 V for PtBi(001) surface which might be due to formation of partially oxidized surfaces but around +1.20 V for PtPb(100) and (001) surfaces which might be due to the increasing boundary lines of Pt and PtPb grains.