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PUBLISHED ONLINE: 1 DECEMBER 2014 | http://www.nature.com/doifinder/10.1038/nnano.2014.277
Web End =DOI: 10.1038/NNANO.2014.277
A silicon-based photocathode for water reduction with an epitaxial SrTiO3 protection layerand a nanostructured catalyst
Li Ji1,2*, Martin D. McDaniel3, Shijun Wang2, Agham B. Posadas4, Xiaohan Li1, Haiyu Huang1, Jack C. Lee1, Alexander A. Demkov4, Allen J. Bard2, John G. Ekerdt3 and Edward T. Yu1
The rapidly increasing global demand for energy combined with the environmental impact of fossil fuels has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efciencies and/or stabilities. Here we show that a silicon-based photocathode with a capping epitaxial oxide layer can provide efcient and stable hydrogen production from water. In particular, a thin epitaxial layer of strontium titanate (SrTiO3) was grown directly on Si(001) by molecular beam epitaxy. Photogenerated electrons can be transported easily through this layer because of the conduction-band alignment and lattice match between single-crystalline SrTiO3 and silicon. The approach was used to create a metalinsulatorsemiconductor photocathode that, under a broad-spectrum illumination at 100 mW cm2, exhibits a maximum photocurrent density of 35 mA cm2 and an open circuit potential of 450 mV; there was no observable decrease in performance after 35 hours of operation in 0.5 M H2SO4. The performance of the photocathode was also found to be highly dependent on the size and spacing of the structured metal catalyst. Therefore, mesh-like Ti/Pt nanostructured catalysts were created using a nanosphere lithography lift-off process and an applied-bias photon-to-current efciency of 4.9% was achieved.
Since Honda and Fujishimas pioneering work on electrochemical photolysis of water using TiO2 in 19721, much effort has been put into this area as it provides a way to convert solar energy
into a storable clean fuel. In designing photoelectrochemical (PEC) cells, a fast charge transfer at the semiconductor/aqueous interface, long-term stability and efcient harvesting of a large portion of the solar spectrum are three key priorities2. Based on this, many material systems have been studied, including Si313, various metal oxides1420, IIIV semiconductors21 and others2224. High efciencies have been achieved in triple-junction amorphous Si or IIIV semiconductors, but at high cost and device complexity10,21. For metal oxides,...