Communications of the ACM
sing Sunlight to Activate the Flow of Electrical Current in a New Material
Detail of a structural diagram showing electron hopping from an Fe2+ion at an octahedral site to an Fe3+ion at a tetrahedral site in Fe2CrO4.
Mined to make the first compass needles, the mineral magnetite is also made by migratory birds and other animals to allow them to sense north and south and thus navigate in cloudy or dark atmospheric conditions or under water. A team of scientists has compositionally modified magnetite to capture visible sunlight and convert this light energy into electrical current. This current may be useful to drive the decomposition of water into hydrogen and oxygen. The team generated this material by replacing one third of the iron atoms with chromium atoms.
The team is from Pacific Northwest National Laboratory and includes researchers from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility, and Argonne National Laboratory.
Generating materials that can harness the power of the sun to make a combustible fuel such as hydrogen, which would have no carbon footprint, represents an extremely attractive pathway to new clean energy sources. Without alternatives to fossil fuel, energy sources steadily increase the concentration of carbon dioxide in the atmosphere and the oceans, with the attendant deleterious effects on greenhouse gas accumulation in the atmosphere and ocean acidification.
By taking advantage of the compositional precision, purity, and low defect densities found in oxide films prepared by molecular beam epitaxy, the team showed that an unusual semiconducting phase, which is ferrimagnetic well above room temperature and absorbs light in the visible portion of the solar electromagnetic spectrum, can be stabilized on magnesium oxide (MgO(001)) substrates.
This phase results when precisely one third of the iron (Fe) in magnetite (Fe3O4) is replaced with chromium (Cr). The investigation revealed that chromium ions, Cr3+, substitute for iron exclusively at octahedral sites in the spinel lattice, occupying half of these sites. As a result, the charge transport mechanism involves electron hopping between iron cations at octahedral and tetrahedral sites in the lattice.
The researchers describe their work in "Electronic and Optical Properties of a Semiconducting Spinel (Fe2CrO4)," published in the journal Advanced Functional Materials.
Having shown that chemically modified magnetite (Fe2CrO4) meets the basic criteria required for an air stable, visible light photocatalyst, the investigators plan to carry out experiments in which they will transfer freshly grown Fe2CrO4surfaces to a photoelectrochemical cell under a dry nitrogen atmosphere to avoid picking up surface carbon contamination. There they will measure the photocatalytic activity for the oxygen evolution and hydrogen evolution reactions, as occur when light energy is successfully used to break water down into useable fuel.
The Advanced Functional Materials is authored by Scott A. Chambers, Tiffany C. Kaspar, Iffat H. Nayyar, Martin E. McBriarty, Peter Sushko, and Mark E. Bowden of Pacific Northwest National Laboratory, and by Steve M. Heald and Dave J. Keavney of Argonne National Laboratory.
The research is sponsored by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering.
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