Unique adsorbed state of 4,4′-BiPy and -BiPyH22+ on Au(1 1 1) 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≈2.65 close to pKa1 of BiPyH22+, while the BiPy coverage increases to +1.10 V as in the neutral Na2SO4 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.