Structure of ordered electrified interfaces: EC-STM of hydroquinone sulfonate at well-defined Pd(111) electrodes

Atom-resolved electrochemical scanning tunneling microscopy (EC-STM) has be en employed to study the chemisorption of 2,4-dihydroxybenzenesulfonate (hy droquinone sulfonate, HQS) from dilute aqueous solutions, onto a well-defin ed Pd(111) electrode surface. Previous work based upon high-resolution elec tron energy loss spectroscopy had showed that HQS is oxidatively chemisorbe d as benzoquinone sulfonate (BQS). Results from the present investigation i ndicate that a well-ordered Pd(111)-(3 x 3)-BQS adlayer is formed in which the close-packed BQS molecules are oriented parallel to the surface but wit h a minimal tilt enforced by the bulky SO3- group. The flat orientation imp lies strong organic-metal interactions via the it-electron system of the qu inonoid ring. It was not possible to image the counter cations of the anion ic sulfonate group, presumably because they are not held rigidly within the BQS adlattice but are located on top of the organic adlayer where they can be easily moved aside by the STM tip. Possible structural models of the Pd (111)-(3 x 3)-BQS adlattice are presented.