Infrared absorption enhancement for CO adsorbed on Au films in perchloric acid solutions and effects of surface structure studied by cyclic voltammetry, scanning tunneling microscopy, and surface-enhanced IR spectroscopy
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The adsorption of CO on Au thin film electrodes vacuum-deposited on silicon substrate has been studied in 0.1 M perchloric acid solution by surface-enhanced infrared absorption spectroscopy (SEIRAS) with the Kretschmann attenuated-total-reflection configuration. Scanning tunneling microscopy (STM) was used to investigate the surface structures of the Au films subjected to different treatments. The IR absorption of CO adsorbed on the Au film electrodes is 20 times larger than that of CO adsorbed on bulk Au electrodes measured by reflection-absorption spectroscopy. When the Au film is subjected to flame annealing, which reorients the crystallites of the Au film and results in a highly ordered (1 I I) surface, the enhancement factor is increased further to ca. 40. Cyclic voltammetric studies demonstrated that the highly ordered Au(lll) films possess a higher electrocatalytic activity toward CO oxidation than polycrystalline Au films without flame annealing. Two different linearly bonded CO species, COL and COL(s), were identified on the polycrystal Au surface without flame annealing. The COL is the predominant adsorbate that yields the IR absorption at 2110-2136 cm(-1) The COL(s) species is a minor adsorbate and gives rise to the IR absorption at 2020-2045 cm(-1) cm which is assigned to CO adsorbed at specific surface sites located at boundaries of Au crystallites and very close to the surface of the silicon substrate. On the highly ordered Au(1 1 1) surface prepared by flame annealing, an additional weak band assigned to bridge-bonded CO species (COB) was detected at 1925-1975 cm(-1) in a narrow potential range (0.0-0.4 V vs SCE). The present study puts emphasis on effects of surface structures of Au films for IR absorption enhancement and demonstrates also that the Au films prepared by vacuum evaporation is of great importance in fundamental studies as well as in electrocatalysis applications.