Adlayer structures of naphthalene and biphenyl on well-defined Rh(111) and Pt(111) electrodes were investigated in HF solution using in situ scanning tunneling microscopy (STM). A highly ordered adlayer of naphthalene was found to form on Rh(111) with a (3 root 3 x 3 root 3)R30 degrees structure, whereas disordered adlayers were observed on Pt(111). The longer molecular axis of naphthalene was aligned along the close-packed atomic rows of both Rh and Pt. On the other hand, the molecular axis of biphenyl was aligned along the root 3 direction on Rh(111). The influence of molecular structure on the arrangement of adsorbates is discussed, based on the adlayer structures of naphthalene and biphenyl. The desorption process of naphthalene was carefully investigated at potentials near the hydrogen evolution reaction. On Rh(111), it was found that the desorption of naphthalene occurred preferentially near defects at terraces in the early stages, followed by the formation of ordered islands on terraces at more cathodic potentials. Naphthalene adsorbed at step edges or holes on terraces was more stable against replacement by hydrogen. On Pt(111), desorption occurred more randomly, resulting in the formation of an adlayer with isolated molecules. These isolated molecules were clearly discerned by in situ STM, indicating that naphthalene is strongly attached on Pt(111). (C) 1998 Elsevier Science B.V.
COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS