Si-based photocathodes for the photoelectrochemical purification of water

Si-based photocathodes have been investigated for use as counterelectrodes in oxide-based photoelectrochemical water purification cells. Bare p-type silicon (either crystalline silicon or amorphous hydrogenated silicon a-Si:H obtained by plasma deposition) is found to exhibit poor catalytic properties for the photoelectrochemical reduction of dioxygen. Improvement in photocathode performance has been sought through the deposition of small amounts of platinum on the Si surface, and improvement in electrode stability has been sought through the formation of a thin organic layer covalently anchored to the Si surface, or through the deposition of a thin conductive-oxide film. Combining both treatments allows to obtain photocathodes with performances just slightly better than those of a bare Pt cathode. Much larger improvements in photocathode performance (+0.9 V shift in the voltammetric curves) have been obtained by using a p⁺/i/ n⁺ a-Si:H structure with similar coatings. The concept is readily scalable to large-area electrodes, which appears promising for practical sunlight powered water purification reactors.