Core-Shell Nanorods for Efficient Photoelectrochemical Hydrogen Production
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Core-Shell Nanorods for Efficient Photoelectrochemical Hydrogen Production Z. G. Yu1, C. E. Pryor2, W. H. Lau3, M. A. Berding1, and D. B. MacQueen1 1
SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025, U.S.A. Optical Science and Technology Center and Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, U.S.A. 3 Center for Spintronics and Quantum Computation and Department of Physics, University of California, Santa Barbara, California 93106, U.S.A. 2
ABSTRACT We propose core-shell InP-CdS and InP-ZnTe nanorods as photoelectrodes in the efficient photoelectrochemical hydrogen production. Based on our systematic study using strain-dependent k.p theory, we find that in these heterostructures both energies and wave-function distributions of electrons and holes can be favorably tailored to a considerable extent by exploiting the interplay between quantum confinement and strain. Consequently, these core-shell nanorods with proper dimensions (height, core radius, and shell thickness) may simultaneously satisfy all criteria for effective photoelectrodes in solar-based hydrogen production. INTRODUCTION Splitting water using a photoelectrochemical (PEC) process for efficient hydrogen production is a promising approach that can ultimately solve the energy problem facing humanity [1]. In the PEC process a semiconductor material absorbs light, resulting in the generation of electron-hole pairs, and thus serves as a photoelectrode. These electrons and holes are consumed at the semiconductor-electrolyte interface to effect the splitting of water into hydrogen and oxygen. After several decades' effort, however, the solar-tohydrogen efficiency remains too low for PEC hydrogen production to be cost-effective for large-scale applications [2]. The major obstacle is to find a suitable semiconductor material to serve as a photoelectrode that simultaneously satisfies four stringent requirements: (1) The semiconductor material must have a band gap in the range 1.5 - 2.0 eV to capture most of the photons in the solar spectrum; (2) The conduction and valence band edges, or the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), of the materials must overlap the H2/H2O and O2/H2O redox potentials to provide the free energy needed under H2/O2 evolution conditions; (3) Charge transfer across the semiconductor-liquid interface must be fast compared to the electronhole recombination; and (4) The semiconductor surface must be chemically stable in the aqueous medium [3]. For a bulk material, the band edges and their relative positions with respect to H2O redox potentials are intrinsic material properties and can be modified externally only to a limited extent [4]. Hence, considerable efforts in the field of PEC hydrogen production have been devoted to searching for a material that fortuitously meets these four criteria. To date, semiconductor photoelectrodes with a demonstrated high efficiency in splitting water have large band gaps and only make u
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