CARBON-MONOXIDE ADLAYER STRUCTURES ON PLATINUM(111) ELECTRODES - A SYNERGY BETWEEN IN-SITU SCANNING-TUNNELING-MICROSCOPY AND INFRARED-SPECTROSCOPY

The spatial structure of compressed carbon monoxide adlayers on Pt(111 ) in aqueous acidic solution has been explored by means of in-situ sca nning tunneling microscopy (STM) along with infrared reflection-absorp tion spectroscopy (IRAS). Besides offering a detailed structural pictu re of this electrochemical interface in comparison with the well-studi ed Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real- space structural information provided by STM allows an assessment of t he obfuscating influence of dynamic dipole coupling upon IRAS binding- site assignments. In turn, the latter data provide an important crossc heck on the validity of binding-site assignments deduced from the STM images. Emphasis is placed on the structures formed from near-saturate d CO solutions, encouraged by the electrode potential-induced adlayer phase transition at ca. O V vs SCE observed previously under these con ditions by IRAS. At potentials below O V, a hexagonal close-packed (2X 2)-3CO adlayer is observed, with a CO coverage, theta(CO), of 0.75. Th e z-corrugation pattern evident in the STM images indicates the presen ce of two threefold hollow and one atop CO per unit cell. This binding -site assignment is supported by the corresponding IRAS data which yie ld C-O vibrational bands at ca. 2065 and 1775 cm(-1). The relative int ensities of these two v(CO) bands, ca. 2:1, differs markedly from the 1:2 binding site occupancy deduced from STM. This apparent disparity, however, can be accounted for by dynamic dipole coupling effects betwe en the atop and multifold CO oscillators. At potentials above O V (up to the onset of CO electrooxidation at ca. 0.25 V), a markedly differe nt adlayer arrangement is formed, having a (root 19X root 19)R23.4 deg rees-13CO unit cell, with theta(CO)=13/19. This hexagonal structure fe atures CO binding in predominantly asymmetric sites inbetween atop and bridging geometries. A distinction between several alternate adlayer arrangements sharing (root 19X root 19) symmetry was achieved on the b asis of the z-corrugation pattern along with the corresponding IRAS da ta upon consideration of dipole-coupling effects. Another CO adlayer s tructure, having a (root 7X root 7)R19.1 degrees-4CO unit cell (theta( CO)=4/7), was commonly observed al potentials below 0.2 V after the re moval of solution-phase CO. These adlayer arrangements are distinctly different to the compressed Pt(111)/CO structures found in uhv. The in creased accommodation of CO in multifold sites observed for the former can be understood chiefly from the markedly (ca. 1 V) lower surface p otentials (and excess electronic surface charges) characterizing the e lectrochemical interface.