(600d) Analyzing Local Composition and Structural Changes Occurring in a Nanocatalyst during Reaction
Understanding the steps involved in a catalytic process is crucial for materials design. The ability to discern the phenomena occurring at the catalyst surface, subsurface and interface with the support in heterogeneous catalysts provides key information to control the structure and properties of materials for the synthesis. Single-walled carbon nanotubes (SWCNTs) are materials typically synthesized by catalytic chemical vapor deposition (CCVD) and exhibit desirable properties for a variety of applications including electronics with smaller and faster transistors, sensors, biomedical, composite materials, energy-related applications and others. SWCNTsâ?? structural features such as chirality, diameter and defects determine their properties and their suitability for a particular application. Therefore, current research efforts are focused on finding a way to grow SWCNTs with specific structures tailored by using the catalyst structure as a template. The current work combines computational techniques such as reactive molecular dynamics, density functional theory, and ab initio molecular dynamics with in situ real-time atomic-resolution transmission electron microscopy observations to discern the interactions of Co nanocatalyst, supported on MgO, with the growing nanotube throughout the nucleation and growth process. Evidence of the coexistence of a Co metal phase and carbide-like phases in the nanoparticle is presented along with a description of C dissolution, surface and bulk diffusion in the catalyst, and evolution of the C gradient in the nanoparticle. A mathematical model of the growth process considering the contributions of each of these phenomena from simulated data suggests that fluctuations in the growth rate can be attributed to fluctuations in the C composition of the nanoparticle. A similar correlation is found between changes in nanotube length and fluctuations in the size of the carbide-like regions in the nanoparticle. These results provide insight about the nature of the catalyst structure during growth and the role that metastable carbide phases may have in the process, which are both important factors in catalyst design.