IN-SITU SCANNING-TUNNELING-MICROSCOPY STUDIES OF THIOCYANATE ADLAYERSON PT(111) SINGLE-CRYSTAL ELECTRODE SURFACES

In situ STM experiments examining the spatial structure and electroche mical behavior of irreversibly adsorbed thiocyanate adlayers on a Pt(l ll) electrode surface are reported. The adlayer structure observed for this adsorbate!electrode system in NaClO4 and Ca(ClO4)(2) electrolyte s at potentials around -200 mV vs Ag/AgCl is Pt(111) (2 x 2)-SCN, thet a(SCN) = 0.5 in agreement with that initially reported by Hubbard et a l. (Langmuir, 1985, 1, 66). The images reveal the adlayer is composed of SCN molecules all protruding to approximately the same extent away from the surface consistent with residence in 3-fold coordinate sites on the electrode. Other imaging experiments following the adsorption o f Zn(SCN)2 complexes show that cycling the electrode mixes the orienta tion of the SCN adsorbates. These findings are discussed with previous structural information garnered from in situ vibrational spectroscopy measurements and ex situ ultra high vacuum (UHV) electrochemical expe riments. (C) 1998 Elsevier Science Ltd. All rights reserved.