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

Abstract

Atom-resolved electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the chemisorption of 2,5-dihydroxybenzenesulfonate (hydroquinone sulfonate, HQS) from dilute aqueous solutions, onto a well-defined Pd(111) electrode surface. Previous work based upon high-resolution electron energy loss spectroscopy had showed that HQS is oxidatively chemisorbed as benzoquinone sulfonate (BQS). Results from the present investigation indicate that a well-ordered Pd(111)-(3 × 3)-BQS adlayer is formed in which the close-packed BQS molecules are oriented parallel to the surface but with a minimal tilt enforced by the bulky SO₃⁻ group. The flat orientation implies strong organic–metal interactions ia the π-electron system of the quinonoid ring. It was not possible to image the counter cations of the anionic 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 × 3)-BQS adlattice are presented.