Adsorption and Oxidation Dynamics of Disperse Orange 3 on a Polycrystalline Pt Electrode Studied by In Situ Second Harmonic Generation

Abstract

Adsorption mechanisms and electrochemical reaction mechanisms at the molecular level are fundamental issues in electrochemistry. Here, we used in situ second harmonic generation (SHG) combined with in situ UV–vis spectra and potential step techniques to investigate the dynamics of the adsorption and oxidation of 4-amino-4′-nitroazobenzene (disperse orange 3, DO3) molecules on a polycrystalline Pt electrode. Time-dependent SHG spectra with different polarization combinations under certain potential steps confirmed that the SHG intensity change in the DO3 molecules resulted from the number density change rather than the orientation angle change. A double potential step experiment and UV–vis spectra verified that the number density change arose from the cooperative effects of adsorption and oxidation of DO3 at the interfaces. Under an applied potential, the adsorption rate of DO3 was greater than the electrooxidation rate in the first hundred seconds until full coverage was achieved and the maximum SHG intensity was reached. Subsequently, the oxidation of DO3 dominated until equilibrium was established, while the SHG intensity stabilized. In addition, the time-dependent SHG spectra showed that the rate constants of the adsorption process and the oxidation reaction were strongly dependent on the potential. This work provides in-depth insights into the adsorption and electrochemical reaction at electrode/solution interfaces.