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After oxidizing the thin films of graphene using the established Hummers method, the researchers examined their samples using X-ray photo-emission spectroscopy (XPS). Over about 35 days, they noticed the number of epoxide functional groups declining while the number of hydroxyl groups increased slightly. After about three months, the ratio of the two groups finally reached equilibrium.
"We found that the material changed by itself at room temperature without any external stimulation," said Suenne Kim, a postdoctoral fellow in Riedo's laboratory. "The degree to which it was unstable at room temperature was surprising."
Curious about what might be causing the changes, Riedo and Kim took their measurements to Angelo Bongiorno, an assistant professor who studies computational materials chemistry in Georgia Tech's School of Chemistry and Biochemistry. Bongiorno and graduate student Si Zhou studied the changes using density functional theory, which suggested that hydrogen could be combining with oxygen in the functional groups to form water. That would favor a reduction in the epoxide groups, which is what Riedo and Kim were seeing experimentally.
"Elisa's group was doing experimental measurements, while we were doing theoretical calculations," Bongiorno said. "We combined our information to come up with the idea that maybe there was hydrogen involved."
The suspicions were confirmed experimentally, both by the Georgia Tech group and by a research team at the University of Texas at Dallas. This information about the role of hydrogen in determining the structure of graphene oxide suggests a new way to control its properties, Bongiorno noted.
"During synthesis of the m
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| Contact: John Toon jtoon@gatech.edu 404-894-6986 Georgia Institute of Technology Research News Source:Eurekalert |