Electrochemical Scanning Tunneling Microscopy Observation of Ordered Surface Layers on an Anionic Clay-Modified Electrode

Electrochemical scanning tunneling microscopy (STM) observations have been performed on a highly ordered pyrolytic graphite (HOPG) electrode modified with a cast film of hydrotalcite (HT) crystal ([Mg 6 Al 2 (OH) 16 ] 3 / 4 CO 3 · 1 / 2 Cl·2H 2 O). The electrode was in contact with an aqueous solution of 5 mM Na 3 [Fe(CN) 6 ] and 0.1 M Na 2 SO 4 at 600 mV (vs Ag/AgCl). The STM image of the electrode showed a two-dimensional lattice with a = 1.40 ± 0.04 nm, b = 1.85 ± 0.04 nm, and α = 63 ± 3°. One bright spot in the image was present at every 3.4 [Mg 6 Al 2 (OH) 16 ] 2+ units, leading to the conclusion that the spot represented a [Fe(CN) 6 ] 3 - ion adsorbed on a crystal surface. At 81 mV (vs Ag/AgCl), the STM image exhibited a two-dimensional lattice with a = 1.74 ± 0.04 nm, b = 1.83 ± 0.04 nm and α = 60 ± 3°. This was ascribed to the lattice made by a mixture of [Fe(CN) 6 ] 3 - and [Fe(CN) 6 ] 4 - on a HT surface. At −200 mV (vs Ag/AgCl), the STM image showed a two-dimensional lattice with a = 1.94 ± 0.04 nm, b = 1.90 ± 0.04 nm and α = 60 ± 3°, which was ascribed to the lattice made by reduced [Fe(CN) 6 ]. 4 - These results confirmed that an ordered molecular adsorption layer changed dynamically during the electrochemical processes.