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ELECTRICITY CONTROLS NANOCRYSTAL SHAPE
Researchers at Argonne have figured out the basics of using electrochemistry to control the architecture of nanocrystals, small structures with dimensions in billionths of meters. Their findings, published in the March 3 edition of the Journal of the American Chemical Society, provide a practical method of generating large quantities of architecture-controlled nanocrystals, such as superconductors, ferromagnets and noble metals. "The architectures of the nanocrystals are mainly controlled by applied voltages," said lead scientist Zhili Xiao of Argonne's Materials Science Division and Northern Illinois University's Physics Department. "This gives us much greater control over the growth conditions of the nanocrystals. We were able to create a great variety of structures with greater convenience and predictability compared with more traditional methods." Traditional methods of fabricating nanocrystals involve rapidly injecting chemicals into a heated solution at high temperatures. The downside to this approach, however, is the difficulty of controlling the solution concentration, which changes as the reaction proceeds. This change in concentration leads to changes in the electrochemical potential, the measure of a compound's ability to react in solution. Since a stable electrochemical potential is crucial for forming well-shaped nanocrystals, scientists using this method often found themselves struggling to control solution concentrations and to time the right moment to stop the reaction. In contrast, Xiao and his colleagues found that they could easily control the electrochemical potential by using electric voltage. The scientists used a technique called electrodeposition, which uses electricity passing through an electrode to reduce ions from solution on a given surface. By changing the applied voltage value and the type of chemicals in the solution, the Argonne researchers were able to synthesize large quantities of nearly 30 different nanostructures, including nanoparticles of various shapes, nanowires, nanobrushes and nanoscale tripods. "We found, for example, that shaped nanoparticles tend to form at lower voltages while higher voltages tend to produce structures such as nanowires and nanobrushes," explained Xiao. With large quantities of these nanocrystals in hand, scientists are exploring their unique physical and chemical properties. These structures can lead to discoveries of new phenomena and applications, such as the use of ferromagnetic nanocrystals as components in ultra high-density storage media and the use of certain metal nanocrystals as catalysts for hydrogen production and sensing. "When you alter the shape of a nanocrystal, you're basically setting new boundaries to the space in which its electrons can move," said Wai-Kwong Kwok, leader of the Superconductivity and Magnetism group in the Materials Science Division. "This, in turn, affects its physical properties, which explains why a triangle and a sphere made of lead can have completely different superconducting properties."
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