A comprehensive study of Indole adsorption on highly oriented pyrolytic graphite

Using electrochemical impedance spectroscopy (EIS), a change in capacitive response of the highly oriented pyrolytic graphite (HOPG)/electrolyte interfase was detected, due to the presence of a non-compact indole film adsorbed on the electrode surface. To understand the adsorption phenomenon at an atomic scale, first principles atomistic simulations were performed in HOPG/solution interface. Indole adsorption on terraces, surface defects, and functionalized steps was studied. Adsorption begins at sites located on functionalized armchair-edge steps by hydrogen bond (H-bond) interaction and then continues over terraces and surface defects where it forms ordered film due to repulsive interactions between molecules. According to the adsorption isotherm, the film grows driven by the chemical potential up to a maximum coverage of 0.61 with a periodic distribution of pores. Furthermore, the presence of the film on surface modifies the surface electronic structure due to partial charge transfer related to the adsorption process. Knowledge of indole adsorption on HOPG is of technological importance to improve the design of new and more efficient sensors.