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 efficiencies and/or stabilities. Here we show that a silicon-based photocathode with a capping epitaxial oxide layer can provide efficient 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 metal-insulator-semiconductor photocathode that, under a broad-spectrum illumination at 100 mW cm(-2), exhibits a maximum photocurrent density of 35 mA cm(-2) 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 efficiency of 4.9% was achieved.

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One of the outstanding advancements in electronic-structure density-functional methods is the Sankey-Niklewski (SN) approach [Sankey and Niklewski, Phys. Rev. B 40, 3979 (1989)]; a method for computing total energies and forces, within an ab initio tight-binding formalism. Over the past two decades, several improvements to the method have been proposed and utilized to calculate materials ranging from biomolecules to semiconductors. In particular, the improved method (called FIREBALL) uses separable pseudopotentials and goes beyond the minimal sp 3 basis set of the SN method, allowing for double numerical (DN) basis sets with the addition of polarization orbitals and d-orbitals to the basis set. Herein, we report a review of the method, some improved theoretical developments, and some recent application to a variety of systems.ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction With the increase in computational power, greater efforts have been made by the electronicstructure community to optimize the performance of quantum mechanical methods. Quantum mechanical methods have become increasingly reliable as a complementary tool to experimental research. A variety of methods exist ranging in complexity from semi-empirical methods to density-functional theory (DFT) methods to highly-accurate methods going beyond the one-electron picture. Judicious approximations enable the computational materials science community to more efficiently examine a wider range of materials questions.Otto F. Sankey was one of the early visionaries by, firstly, demonstrating that molecular-dynamics (MD) simulations can be coupled efficiently with electronic-structure methods to optimize structures and evaluate energetics of materials [1]. Secondly, his judicious approximations in the

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We report a first-principles study of ͑LaAlO 3 ͒ m / ͑SrTiO 3 ͒ n heterostructures using density-functional theory at the LDA+ U level. Our results support the original explanation of Ohtomo and Hwang ͓Nature ͑London͒ 427, 423 ͑2004͔͒ that the charge at the n-type interface may be due to electrostatic doping. The internal electric field in the LaAlO 3 layer is calculated to be 0.24 V / Å. Though it is not sufficient to cause the dielectric breakdown in a wide band-gap La aluminate, it causes charge transfer into the adjacent narrower gap SrTiO 3 layer. The quasi-two-dimensional nature of the charge distribution is caused by a combination of the crystal-field effect, pseudo-Jahn-Teller distortion, and interface chemistry. Our theoretical estimate suggests that the interfacial carrier density of about 2 ϫ 10 13 cm −2 can be easily achieved.