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 behivior of Hg/Si contacts, and the pH dependence of the current-potential behavior of Si electrodes in contact with aqueous electrolytes.