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Materials

Improving Energy Conversion

Microscopy: Method shows best crystal orientation in nanomaterials

by Mitch Jacoby
July 15, 2013 | APPEARED IN VOLUME 91, ISSUE 28

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Credit: Nat. Mater.
A TEM method identifies regions of similarly oriented crystallites (colors) in an aggregate, a key step in determining structure-function relations in nanoparticle materials.
09128-notw3-TEMcxd.jpg
Credit: Nat. Mater.
A TEM method identifies regions of similarly oriented crystallites (colors) in an aggregate, a key step in determining structure-function relations in nanoparticle materials.

An obstacle to developing low-cost energy conversion devices is understanding why small differences in materials can strongly affect performance. Figuring out the relationship between the structure and function of an electrode material, for example, is especially challenging for nanoparticle aggregates.

Researchers have now developed an analytical procedure to scrutinize material structure and function with nanometer resolution across micrometer distances in such aggregates. The technique enables them to design a better electrode for solar energy conversion.

The group used transmission electron microscopy (TEM) to image the spatial distribution and orientation of nanocrystals within aggregates. Then, by using a conducting atomic force microscopy method, they correlated the TEM information with the pathways that electrons follow as they move through the material. They used the combo method to probe electron transport in various nanoparticle-based iron oxide electrodes. The electron-transport proficiency of this cheap material makes it useful for generating hydrogen via light-driven water splitting. Although the electrodes they studied were similar, some worked well, while others did not.

The key finding of the team, which includes Scott C. Warren of the University of North Carolina, Chapel Hill, is that the relative orientation of adjacent nanoparticles greatly affects charge transport. A small orientation mismatch is alright, but a large mismatch results in electrical barriers that block current flow between adjacent grains. The upshot is that by identifying “winning” crystal orientations and tailoring the preparation method to favor them throughout the electrode, the group made a device that achieves a record-setting photocurrent for this class of materials (Nat. Mater. 2013, DOI: 10.1038/nmat3684).

“These results represent an important step forward in developing nanostructured materials for next-generation energy conversion devices,” says solar-fuel specialist Roel van de Krol of Technical University of Berlin.

Boston College chemist Dunwei Wang adds that the study helps explain why some nanostructures function better than others. “This work will be of value to photoelectrochemistry studies in general,” he adds.

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