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

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-en hanced infrared absorption spectroscopy (SEIRAS) with the Kretschmann atten uated-total-reflection configuration. Scanning tunneling microscopy (STM) w as used to investigate the surface structures of the Au films subjected to different treatments. The IR absorption of CO adsorbed on the Au film elect rodes is 20 times larger than that of CO adsorbed on bulk Au electrodes mea sured by reflection-absorption spectroscopy. When the Au film is subjected to flame annealing, which reorients the crystallites of the Au film and res ults in a highly ordered (1 I I) surface, the enhancement factor is increas ed further to ca. 40. Cyclic voltammetric studies demonstrated that the hig hly ordered Au(lll) films possess a higher electrocatalytic activity toward CO oxidation than polycrystalline Au films without flame annealing. Two di fferent 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) spec ies is a minor adsorbate and gives rise to the IR absorption at 2020-2045 c m(-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 sili con substrate. On the highly ordered Au(1 1 1) surface prepared by flame an nealing, 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 fi lms prepared by vacuum evaporation is of great importance in fundamental st udies as well as in electrocatalysis applications.