SCANNING TUNNELING SPECTROSCOPIC STUDY OF BARE AND IODINE MODIFIED AU(111) AND PT(111) IN AQUEOUS-SOLUTION AND 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) c onditions to investigate the influence of these two environments. A me chanically stable electrochemical scanning tunneling microscope combin ed with a bi-potentiostat was constructed to measure the I-s curves ac curately in aqueous solution under the N-2 atmosphere. The tunneling b arrier height energy was derived in each case from the I-s curve. Rela tively 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 o n the electrode potential, even when their surfaces were electrochemic ally oxidized. The iodine adlayers on Au(111) and Pt(111) were found t o reduce the barrier heights compared with those observed on the bare surfaces in solution and in UHV. The barrier height on the iodine-modi fied Au(111) in 0.1 M HClO4 was far larger than that in UHV.