Pacific Northwest National Laboratory scientists have discovered a new material, an alloy of magnetite, that absorbs visible light and then generates electricity. The team of scientists has compositionally modified magnetite to capture visible sunlight and convert this light energy into electrical current.

Structural diagram showing electron hopping from an Fe2+ ion at an octahedral (B) site to an Fe3+ ion at a tetrahedral (A) site in Fe2CrO4. Image Credit: Pacific Northwest National Laboratory. Click image for the largest view.

The mineral magnetite was mined to make the first compass needles and 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.

The mineral’s current may be useful to drive the decomposition of water into hydrogen and oxygen and so might be useful for splitting water to produce a combustible fuel, hydrogen. The team generated this material by replacing one third of the iron atoms with chromium atoms.

The team is made up from scientists at Pacific Northwest National Laboratory (PNNL) and includes researchers from EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility, and the Argonne National Laboratory.¬† The team’s study paper has been published in the journal Advanced Functional Materials.

The team took advantage of the compositional precision, purity, and low defect densities found in oxide films prepared by molecular beam epitaxy. The team was then able to show 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.

The semiconducting 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.

And yet, in contrast, replacing 2/3 of the Fe cations with Cr results in an insulator. Interesting stuff.

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 Fe2CrO4 surfaces 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.

Electricity 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. The urgent thinking is that without alternatives to fossil fuel, we are committed to steadily increasing 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.

Nary a word on the amount of light or the current level attained. This is to be expected, this is a startling new discovery. One does wonder, in the rush to publish there might have been some time to take a few measurements.

The magnetite alloy is quite interesting. Low cost as far as we can tell, operates at ambient temperatures and seems to react in most all of the visible spectrum. This discovery might be something of great note in the future.


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