Published: March 01, 2018
Pin A. Lin, Bharath NMN Natarajan, Michael P. Zwolak, Renu Sharma
During the catalytic synthesis of graphene, nanotubes, fibers, and other nanostructures, many intriguing phenomena occur, such as phase separation, precipitation, and processes similar to capillary action. The underlying mechanism of these processes and their role in determining the catalytic products remain largely unknown. Here, we demonstrate using in situ, real-time transmission electron microscope imaging and modeling that the catalytic nanoparticles are driven through a nonequilibrium thermodynamic cycle of elongation and retraction, where the large elongation observed is a manifestation of metastability. As a tubular structure grows, the particle elongates due to a favorable metal-carbon interaction that overrides the increased surface energy of the metal. The formation of subsequent nested tubes, however, drives up the particles free energy, but the particle remains trapped until an accessible free energy surface allows the particle to exit the tube and the cycle repeats. The degree of tapering determines whether such a free energy surface exists and, ultimately, the final product and functional catalyst lifetime. Since the particle reshaping is universal for different metals, particle sizes, carbon structures, etc., our results including predictive expressions unravel the different observations and suggest routes to the practical optimization of these processes for desired end products.
Pub Type: Journals
Carbon Nanostructures, thermodynamic cycle, environmental transmission electron microscope
Created March 01, 2018, Updated November 10, 2018