Scanning tunneling spectroscopic study of bare and iodine modified Au(111) and Pt(111) in aqueous solution an in vacuum

Tunneling current ( I ) versus tip height ( s ) profiles ( I - s curves) have been measured on bare and iodine-modified surfaces of Au(111) and Pt(111) both in aqueous acid solutions and under ultrahigh vacuum (UHV) conditions to investigate the influence of these two environments. A mechanically stable electrochemical scanning tunneling microscope combined with a bi-potentiostat was constructed to measure the I - s curves accurately in aqueous solution under the N 2 atmosphere. The tunneling barrier height energy was derived in each case from the I - s curve. Relatively large values for the tunneling barrier height were obtained on well-defined clean Au(111) (about 0.7 eV) and Pt(111) (about 1 eV) in solutions. It was found that the barrier heights obtained on the bare Au(111) and Pt(111) in solutions did not show significant dependence on the electrode potential, even when their surfaces were electrochemically oxidized. The iodine adlayers on Au(111) and Pt(111) were found to reduce the barrier heights compared with those observed on the bare surfaces in solution and in UHV. The barrier height on the iodine-modified Au(111) in 0.1 M HClO 4 was far larger than that in UHV.