Sustained water oxidation by direct electrosynthesis of ultrathin organic protection films on silicon

Abstract

Artificial photosynthesis allows exceeding the efficiency and stability limits of natural photosynthesis. Based on the use of semiconducting absorbers, high efficiency in water photolysis has been achieved in various photoelectrode configurations. However, integrated systems are limited in their stability, and more stable half-cell electrodes use protection films prepared by laborious methods. Herein, the facile low-temperature preparation of ultrathin organic protection coatings is demonstrated. The formation is based on the catalytic properties of water oxidation catalysts toward alcohol-polymerization reactions, which results in the formation of hitherto unknown protection layers on silicon. The interfacial layers are generated via iodine-mediated electro-reductive polymerization of ethanol, concomitantly forming during electrophoretic transport of RuO2 onto silicon supports. Reaction chemistry analyses show that the RuO2-induced catalysis introduces E2-elimination reactions which result in a carbon sp3-sp2 transformation of the film. For the two modes of photoelectrochemical operation, the photovoltaic and the photoelectrocatalytic mode, 20 and 15 mA cm-2 photocurrent densities, respectively, are obtained with unaltered output for 8 and 24 h. The interfacial layer enables Si photovoltages of 500 mV, demonstrating extraordinary electronic interface quality. Since only hydrogen termination of the surface is a prerequisite for growth of the organic protection layer, the method is applicable to a wide range of semiconductors. Organic protection layers are electrochemically synthesized on silicon by RuO2-supported polymerization of alcohols. The resulting Si/SiO2/protection layer/RuO2 photoanode proves an unprecedented high photovoltage and efficient activity upon sustained light-induced oxidation of water. On the atomistic level, a detailed reaction scheme is provided for understanding the formation principles of the organic protection layer.