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December 2015

Creating a Super Lattice: Zipping Electrons, Jumping Holes, and the Quest for Solar Fuels

Better control of a chromium sidekick makes earth-abundant iron work better in solar panels

Superlattice
Designing a superlattice of chromium and iron oxides produces a material that allows electrons excited by sunshine to freely move away from their "holes," answering a fundamental question for material scientists working to create the materials that produce fuels from sunlight.

Imagine having solar panels turn out fuel, essentially storing the sun's energy for a rainy day. Scientists are searching for a material that can handle the job. The material must excite electrons when struck by sunlight, easily transport the electrons to where they are needed, and use those electrons to create fuel—and, it must be a material that isn't in short supply. Rare metals such as platinum need not apply. Hematite, an oxide of readily available iron, is a popular choice. It meets all the requirements but one—it doesn't let the electrons zip along. Dr. Tiffany Kaspar at Pacific Northwest National Laboratory and her colleagues may have found a way to let the electrons flow—by layering on the oxide of another abundant metal: chromium.

Why It Matters: Solar energy must be used when it is generated, or it is lost. Storing the energy as fuel could allow solar power to play a larger role on the nation's energy stage. In the simplest case, solar energy would split water, H2O, to generate hydrogen, H2, fuel. This work shows how one of the challenges to solar fuels could be overcome with earth-abundant minerals. This work also shows how abrupt interfaces between hematite and chromium oxide can be controlled in such a way as to move the electricity without requiring added energy.

Methods: When you shine light on hematite, electrons are excited, leaving behind "holes," which act as the positive charge to the electron's negative charge. Unfortunately, in hematite the electrons tend to fall back into their "holes." If the electrons and holes could be quickly separated after the electron was excited, both could move on. Ideally, the holes would migrate to the material's surface, where they can catalyze the production of fuel.

To create a material where the electrons and holes are forced to separate, the team produced an artificial crystal structure called a superlattice. The team built a thin layer of hematite and then added a layer, three atoms deep, of chromium oxide. They added another layer of hematite, and then chromium oxide, like stacking up the layers on a cake. The abrupt interface between each distinct layer is key to separating the electrons and holes: the electrons prefer to remain in the hematite, while the holes are driven to the chromium oxide layers. The layers were created using the molecular beam epitaxy instrument at EMSL, a DOE scientific user facility.

Now, when light strikes the surface of the superlattice, the interfaces are such that they drive the excited electrons to the hematite and the holes to the chromium oxide. As an added benefit, the superlattice stack generates an internal voltage that is expected to drive holes to the material's surface, where they can react to create fuels.

The ability of these superlattice stacks to separate electrons and holes was first predicted in 2000 by Kaspar's colleague Dr. Scott Chambers, but no practical applications were envisioned at the time. Further study led to an understanding of the interfacial properties between the hematite and chromium oxide layers. This work proved relevant to the recent interest in using hematite to produce solar fuels, prompting Kaspar and colleagues to create and test the superlattice stacks. 

Solar panel
Solar panels don't produce electricity on overcast days, so the energy they produce when the sun shines needs to be stored. Scientists are making progress in the quest for materials that are both readily available and efficient. Stock photo: Dollar Photo Club

What's Next? Kaspar and her team are now conducting photoelectrochemical studies to take the next step: split water to produce fuel.

Acknowledgments

Sponsors: TCK, DKS, SRS, MEM, and SAC were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Financial support from the U.S. National Science Foundation (to DRG) is gratefully acknowledged.

Research Area: Materials Science

User Facility: A portion of the research was performed using EMSL, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory

Research Team: Tiffany C. Kaspar, Daniel K. Schreiber, Steven R. Spurgeon, Martin E. McBriarty, Scott A. Chambers, PNNL; Gerard M. Carroll and Daniel R. Gamelin, University of Washington

Reference: Kaspar TC, DK Schreiber, SR Spurgeon, ME McBriarty, GM Carroll, DR Gamelin, and SA Chambers. 2015. "Built-in Potential in Fe2O3-Cr2O3 Superlattices for Improved Photoexcited Carrier Separation." Advanced Materials 28:1616-1622. DOI: 10.1002/adma.201504545


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In one sentence: Scientists at Pacific Northwest National Laboratory and the University of Washington created a new material that quickly separates electrons and their holes, providing a key insight for solar fuel production

In 100 characters: Holes migrate and electrons move in designer material of interest for solar fuels

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