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.