Self-Assembly of a Rh(I) Complex on Au(111) Surfaces and Its\nElectrocatalytic Activity toward the Hydrogen Evolution Reaction

Abstract

The self-assembly of a Wilkinson type of catalyst molecule, <i>trans</i>-RhCl(CO)(PPh₃)₂, on Au(111) surfaces and its\nelectrocatalytic properties toward the hydrogen evolution reaction (HER) are investigated by employing scanning\ntunneling microscopy (STM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). The self-assembled monolayers of RhCl(CO)(PPh₃)₂ are prepared from either dichloromethane or aqueous solutions, but the\nordered structures are observed only in atmospheric conditions after solvents evaporate. In the electrolyte solutions,\ndisordered yet uniformly sized spherical clusters of individual molecules are observed as a result of the conformational\nchange of the molecule by the solvation effect of water. The immobilized Rh(I) molecular clusters are electrochemically\nstable in a wide potential window and exhibit remarkable electrocatalytic activity toward HER in perchloric acid\nsolutions. Several comparative experiments involving similar types of immobilized complexes containing Ru(I) and\nIr(I) centers and solution species of RhCl(CO)(PPh₃)₂ are performed. However, none of them are found to be electroactive\nto HER. The Tafel slope of HER on the Rh(I) complex modified Au(111) electrode in 0.1 M HClO₄ is determined\nto be −0.061 V, which is almost in the middle of those on bare Au(111) (−0.093 V) and Rh covered (θ_Rh ≈ 0.3)\nAu(111) (−0.034 V) electrodes. XPS measurements reveal a valence change of Rh(I) to Rh(0), and an oxidative\naddition and reductive elimination mechanism is suggested for the enhancement of HER.