Comparison of the Photoelectrochemical Behavior of H‑Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, Outer-Sphere Redox Couples in CH3CN

The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A–), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, V oc, on E(A/A–) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A–) value, CH3-terminated p-Si(111) electrodes showed lower V oc values than H-terminated p-Si(111) electrodes, whereas CH3-terminated n-Si(111) electrodes showed higher V oc values than H-terminated n-Si(111) electrodes. Under 100 mW cm–2 of ELH-simulated Air Mass 1.5 illumination, n-type H–Si(111) and CH3–Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A–) and produced limiting V oc values of >0.5 V at very positive values of E(A/A–). Illuminated p-type H–Si(111) and CH3–Si(111) electrodes produced no photovoltage at positive values of E(A/A–) and produced limiting V oc values in excess of 0.5 V at very negative values of E(A/A–). In contact with CH3CN-octamethylferrocene+/0, differential capacitance vs potential experiments yielded a −0.40 V shift in flat-band potential for CH3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH3CN-1,1′-dicarbomethoxycobaltocene+/0, the differential capacitance vs potential data indicated a −0.25 V shift in the flat-band potential for CH3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in V oc vs E(A/A–), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH3–Si(111) surfaces vs H–Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current–potential behavior of Si electrodes in contact with aqueous electrolytes.