Anchoring of porphyrins on atomically defined cobalt oxide: In-situ infrared spectroscopy at the electrified solid/liquid interface

Abstract

In this work, we studied the interaction of 5-mono(4-phosphonatophenyl)-10–15–20-(triphenyl) porphyrin (MPTPP) with an atomically-defined Co3O4(111) surface in electrochemical environments. We prepared clean and well-defined Co3O4(111) films by physical vapor deposition in ultra-high vacuum (UHV) and transferred the samples into liquid environment without exposure to ambient conditions. We anchored MPTPP to Co3O4(111) in the liquid phase using two different solvents, namely ethanol and dichloromethane. After a second transfer step into an aqueous electrolyte (0.15 M ammonia buffer, pH 10), we investigated the interaction of the porphyrin with the oxide surface in a potential window from 0.3 to 1.3 VRHE using electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We observed that MPTPP binds via the phosphonate group to the Co2+ surface ions in the form of chelating tridentate. The binding motif remains unchanged in the entire potential region studied. While the binding motif was independent of the solvent used for anchoring, a higher coverage of anchored porphyrins was observed when adsorbing from ethanol.