A Mechanistic Study of the Oxidative Reaction of Hydrogen-Terminated Si(111) Surfaces with Liquid Methanol

摘要

H-Si(111) surfaces have been reacted with liquid methanol (CH3OH) in the absence or presence of a series of oxidants and/or illumination. Oxidant-activated methoxylation of H-Si(111) surfaces was observed in the dark after exposure to CH3OH solutions that contained the one-electron oxidants acetylferrocenium, ferrocenium, or 1,1'dimethylferrocenium. The oxidant-activated reactivity toward CH3OH of intrinsic and n-type H-Si(111) surfaces increased upon exposure to ambient light. The results suggest that oxidant-activated methoxylation requires that two conditions be met: (1) the position of the quasi-Fermi levels must energetically favor oxidation of the H-Si(111) surface and (2) the position of the quasi-Fermi levels must energetically favor reduction of an oxidant in solution. Consistently, illuminated retype H-Si(111) surfaces underwent methoxylation under applied external bias more rapidly and at more negative potentials than p-type H-Si(111) surfaces. The results under potentiostatic control indicate that only conditions that favor oxidation of the H-Si(111) surface need be met, with charge balance at the surface maintained by current flow at the back of the electrode. The results are described by a mechanistic framework that analyzes the positions of the quasi-Fermi levels relative to the energy levels relevant for each system.