Unique adsorbed state of 4,4′-BiPy and -BiPyH22+ on Au(111) electrode

We found using attenuated total reflection infrared (ATR-IR) spectroscopy that 4,4'-diprotonated bipyridine (BiPyH22+) adsorbs at the second layer outside the sulfate ions on a Au(1 1 1) surface at pH < 2.65 above ca. 0.0 V (vs. Ag/AgCl). Based on the IR spectral change, the surface coverage of BiPyH22+ increases from -0.3 to -0.1 V at pH x 2.65 close to pK(al) of BiPyH22+, while the BiPy coverage increases to +1.10 V as in the neutral Na(2)SaO(4) solution (pH = 7.1). The same BiPy adspecies were observed in 0.1 M HClO4 solution. Noteworthy is that BiPyH22+ adsorbs at the first layer and ClO4- at the second layer in this case. Furthermore, the surface coverage of BiPyH22+ and ClO4- increases up to ca. +0.8 V in Cl- containing solution, because the chemisorbed Cl- at the first layer stabilizes the BiPyH22+ and ClO4- adsorption at the second and third layer, respectively. (C) 2000 Elsevier Science B.V. All rights reserved.