The surface chemistry of n-type Si electrodes that had been etched, exposed to electrolyte, and electrochemically cycled has been probed using high-resolution X-ray photoelectron spectroscopy (XPS). n-Si surfaces etched in hydrofluoric acid-ethanol solutions (in air or N2 ambients) displayed spectra in the Si 2p region that were free of detectable substrate oxide signals (≤5 × 10-11 mol cm-2 SiO2; equivalent to ≤4% of a monolayer). Exposure of HF-C2H5OH etched or of 49% HF(aq) etched n-Si surfaces to an electrolyte solution containing CH3OH, dimethylferrocene (Me2Fc), and dimethylferricenium (Me2Fc+) generated very low levels, ≤(2 ± 1) × 10-10 mol cm-2 of silicon suboxides. Only sub-monolayer levels of SiOx, (4 ± 2) × 10-10 mol cm-2, were detected after electrochemical cycling of illuminated n-Si anodes in contact with CH3OH-Me2Fc+/0 electrolytes. Even n-Si photoanodes maintained at short circuit with the CH3OH-Me2Fc+/0 electrolyte for substantial periods (> 1000 C cm-2 anodic charge passed) formed less than a single monolayer of strained SiO2 at the silicon surface. Deliberate anodization of the Si surface in these electrolyte solutions yielded controlled amounts of thicker (8-10 Å) SiO2 overlayers; these overlayers provided a useful oxide for the formation of high-performance metal-insulator-semiconductor device structures. These studies demonstrate that HF-C2H5OH- or HF(aq)-etched n-Si surfaces are remarkably resistant to oxide formation during photoelectrochemical cycling in CH3OH-based electrolytes, and that the outstanding photoelectrochemical I-V properties of the n-Si/CH3OH-Me2Fc+/0 junction are not a result of formation of passivating oxide overlayers on the Si surface.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry