CCB Colloquium presentation by Dr. YuHuang Wang, University of Maryland
Host: KiBum Lee
Title: Propagation Chemistry on a Graphene Lattice
Abstract: Covalent chemical reactions typically occur randomly on sp2-bonded carbon networks such as single-walled carbon nanotubes and graphene because electrons are delocalized over thousands of atomic sites. The random covalent modifications quickly destroy the electrical and optical properties that make these carbon nanostructures desirable materials. In this talk, I will discuss a new chemical strategy that addresses this fundamental challenge. We found that Billups-Birch reductive alkylcarboxylation, a variant of the nearly century-old Birch reduction chemistry, occurs on nanotubes exclusively by defect activated reaction propagation.  The reaction propagates outwards from "sp3 defect sites" with a probability more than 1,300 times higher than otherwise random bonding to the "π-electron sea." This unexpected mechanism allows, for the first time, growth of "functional bands" on a graphene lattice in a way reminiscent of crystal growth from a "seed".
Applications may be anticipated with this new control in carbon nanochemistry. Utilizing these strategically introduced functional defects, for example, we have synthesized CNT @α-Si heterostructures with controlled morphologies and chemically tailored C-Si interfaces. These heterostructures combine the electrical and mechanical properties of carbon nanotubes with the ultrahigh Li+ storage capacity of silicon. In situ TEM studies reveal new Li+ diffusion pathways in the synthesized heterostructures and suggest the possibility of synthesizing electrodes that are mechanically and electrochemically reversible in lithium ion batteries. Electrodes with this type of self-healing property are expected to provide a dramatic extension of the lifetime of Si-based high energy density lithium ion batteries by avoiding silicon pulverization as lithium inserts and extracts.
 "Confined propagation of covalent chemical reactions on single-walled carbon nanotubes." Deng, S.; Zhang, Y.; Brozena, A. H.; Mayes, M. L.; Banerjee, P.; Chiou, W. A.; Rubloff, G. W.; Schatz, G. C.; Wang Y. Nature Communications 2011 2:382 doi: 10.1038/ncomms1384.
 "Interfacial Mechanics of Carbon Nanotube@α-Silicon Co-axial Nanostructures." Liao, H. -W.; Karki, K.; Zhang, Y.; Cumings, J.; Wang, Y. Advanced Materials 2011, 23, 4318-4322.