Surface methoxylation as the key factor for the good performance of n-Si/methanol photoelectrochemical cells

Abstract

We have tried to elucidate the mechanism for the beneficial effect of using the solvent methanol in n-Si-based photoelectrochemical cells. Silicon surfaces were treated by exposure to methanol vapour, then tested in indifferent acetonitrile electrolyte using impedance and electromodulated infrared spectroscopic measurements, and also in a vacuum using Kelvin probe measurements. Similar experiments were also done for silicon surfaces treated with hexamethyldisilazane vapour. Treated surfaces exhibited a decrease in electron affinity and a corresponding negative shift of the flatband potential in acetonitrile electrolyte (−0.4 and −0.25 V for methanol and hexamethyl disilazane treatments, respectively). Stability of the surfaces with respect to oxidation was markedly improved. Finally, it was shown from the electromodulated infrared vibrational spectra that methoxy and trimethylsiloxy groups are attached to the surface upon methanol and hexamethyldisilazane treatment, respectively. The shifts in electron affinity and flatband potential can be simply understood as electric dipole effects. Implications regarding other semiconductor/electrolyte systems are suggested.