Electrochemical reduction of p-nitrothiophenol-self-assembled monolayer films on Au(1 1 1) surface and coadsorption of anions and water molecules

Electrochemical reduction of p-nitrothiophenol (PNTP)-self-assembled monolayer (SAM) films on Au(1 1 1) surface was studied using attenuated total reflection-IR spectroscopy. The reduction potential was evaluated from the spectral changes, which depended on pH in electrolyte solutions, e.g. +0.05 V (pH 0.45), −0.55 V (pH 5.5) or −0.55 V (pH 13) (vs. Ag/AgCl) in accordance with voltammetric results. We found that the IR absorption bands of anions and water increases upon reduction in acidic solution at pH<pKa of p-aminothiophenol (PATP), while those for hydroxide anions decreases in alkaline solution. The observations result from the electrostatic interaction of these anions with partial charge at the PNTP- and PATP-SAM films on Au(1 1 1) surface. It suggests that the pKa of the PNTP-SAM film on the Au electrode can be evaluated from the IR spectral changes with the potential. Pronounced IR background shift observed during the PNTP reduction was discussed based on the superimposed changes from molecular structure, orientation of the SAM film and coadsorption of the anions.