Natively Oxidized Silicon: Identifying Unexpected Redox Signals

Silica-terminated silicon electrodes are an underutilized material in electrochemistry, largely due to the perception that the native oxide layer passivates the surface, blocking the current in electrochemical measurements. However, it has been recently demonstrated that these electrodes are electrochemically viable because electrons can tunnel through the native silica layer, enabling both adsorbed and outer-sphere charge transfer reactions. The oxide terminating the silicon surface is useful since it stabilizes the silicon electrode, preventing further oxidation, which is a common issue with the oxide-free (Si–H) silicon electrodes. Before widespread adoption of this electrode material, it is important to understand the origin of the background redox signals from these electrodes. This is critical so that experimentalists do not misinterpret their intended redox signals with surface redox processes of the electrode itself or capacitive currents due to adsorption/desorption of ions on the electrode surface, as is the case for many electrode materials. In this study, we demonstrate that silica-terminated silicon exhibits a persistent and reversible redox signal on both n- and p-type silicon electrodes. The origin of this signal is attributed to a reversible silicon to silica redox conversion occurring at highly conducting intersections between different crystal planes. The intersection is likely between Si(111) and Si(110) planes, such as those observed recently on partially oxidized Si–H surfaces ( JACS, 2021, 143, 1267). These background signals are observed only on Si(111), Si(211), Si(311), and Si(411) due to the ability of these crystal faces to expose both Si(111) and Si(110) due to the shared atomic rows and step-terrace structures. These redox signals are reversible and dominate voltammetry in the absence of redox species in solution and therefore can be misinterpreted. Silica-terminated silicon electrodes will continue to grow in popularity since they require no etching or pre-treatment.