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Herein we show that faradaic electrochemistry can be confined to a discrete region of a monolithic semiconducting electrode by means of locally addressing the kinetics of electron transfer with a focused light-pointer. A Si(100) electrode is modified with a self-assembled monolayer to which ferrocene is attached. The extent to which a model heterogeneous electrochemical reaction can be addressed in two-dimensions was explored at Si(100) photoanodes using scanning electrochemical microscopy operating in surface-generation/tip-collection mode. The link between the experimental radial profile of the catalytic current around the site of illumination and the substrate thickness indicates an inverse relationship between thickness and spatial resolution. We show to which extent the diffusion of charge carriers in the substrate governs the light-addressability. A spatial resolutions for our electrocatalytic system can be as low as ca . 385 μm ( ca . 5-times the size of the light pointer) using a non-structured Si(100) photoanode configured as a single-wire device. [ABSTRACT FROM AUTHOR]

Due to its high sensitivity to corrosion, the use of Si in direct photoelectrochemical (PEC) water-splitting systems that convert solar energy into chemical fuels has been greatly limited. Therefore, the development of low-cost materials resistant to corrosion under oxidizing conditions is an important goal toward a suitable protection of otherwise unstable semiconductors used in PEC cells. Here, we report on the development of a protective coating based on thin and electrically conductive nanocrystalline boron-doped diamond (BDD) layers. We found that  BDD layers protect the underlying Si photoelectrodes over a wide pH range (1-14) in aqueous electrolyte solutions. A BDD layer maintains an efficient charge carrier transfer from the underlying silicon to the electrolyte solution. Si|BDD photoelectrodes show no sign of performance degradation after a continuous PEC treatment in neutral, acidic, and basic electrolytes. The deposition of a cobalt phosphate (CoPi) oxygen evolution catalyst onto the BDD layer significantly reduces the overpotential for water oxidation, demonstrating the ability of  BDD layers to substitute the transparent conductive oxide coatings, such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), frequently used as protective layers in Si photoelectrodes.