Metastable Au_xRh_(100-x) Thin Films Prepared by Pulsed Laser Deposition for the Electrooxidation of Methanol

摘要

Metastable Au_xRh_(100-x). thin films were prepared by crossed beam pulsed laser deposition on two different substrates, High kinetic-energy deposition conditions were used (0.1 Torr He, 5.5 cm target-to-substrate distance), The films were characterized by X-ray diffraction and X-ray photoelectron spectroscopy, and their electrocatalytic activities for the electrooxidation of methanol were assessed by linear sweep voltammetry in 1 M NaOH. All films are made of a single fcc phase compound. For 40 < x < 85 at %, the lattice parameter deviates slightly from that expected from Vegard's law. The XRD patterns of Au_xRh_(100-x)/C and Au_xRh_(100-x)/Si did not evolve over a period of 1 year, indicating that the solid solution is kinetically stable. However, as expected from the Au—Rh binary phase diagram, heat treatment of Au_(50)Rh_(50)/C at 500 °C in Ar during 24 h leads to decomposition of the metastable fcc phase and the formation of Rh and Au. X-ray photoelectron spectroscopy revealed that the Au surface composition closely follows the Au bulk content. The binding energy difference between the Rh 3d_(5/2) and Au 4f_(7/2) core level peaks shifts from 223.3 to 222.4 eV as x is increased from 5 to 95, indicating that a surface alloy is formed between Au and Rh. In the presence of methanol, Au_xRh_(100-x)/C with x - 25, 50, and 75 shows a methanol oxidation peak (a2) in a potential region (ca. -0.12 V vs Hg/HgO), where pure Au and Rh do not exhibit any electrocatalytic activity for methanol oxidation. The current of peak a2 increases with the methanol concentration. The formation of a surface alloy significantly improves the electrocatalytic activity for methanol oxidation by providing OH~- species at lower potential (by 400 mV) compared with pure Au.