Date of Award

5-13-2011

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemical Engineering

First Advisor

Dimitrios Maroudas

Second Advisor

David M. Ford

Third Advisor

Scott M. Auerbach

Subject Categories

Chemical Engineering

Abstract

Carbon nanomaterials, such as carbon nanotubes and graphene, have attracted significant interest over the past several years due to their outstanding and unusual combination of physical properties. These properties can be modified in a controllable way by chemical functionalization in order to enable specific technological applications. One example is hydrogenation, achieved by the exposure of these materials to a source of atomic hydrogen. This process has been considered for hydrogen storage purposes and for the control of the band gap of these materials for applications in carbon-based electronics. Hydrogen atoms are chemisorbed onto the surface of these materials, introducing sp3-hybridized C–C bonds in a structure originally formed by delocalized sp2 C–C bonding. This bonding transition causes severe structural and morphological changes to the graphene layers/walls. Also, it has been demonstrated that the exposure of multi-walled carbon nanotubes (MWCNTs) to a H2 plasma leads to the formation of diamond nanocrystals embedded within the nanotube walls.

This thesis presents a computational analysis of the effects of hydrogen chemisorption on the structure and morphology of graphene and single-walled carbon nanotubes (SWCNTs), as well as of the different nanostructures that can be generated upon formation of inter-shell and inter-layer sp3 C–C bonds in MWCNTs and few-layer graphene (FLG), respectively. The analysis is based on a synergistic combination of atomic-scale modeling tools, including first-principles density functional theory (DFT) calculations and classical molecular-dynamics (MD) and Monte Carlo (MC) simulations.

The results demonstrate that SWCNTs and graphene swell upon hydrogenation and provide interpretations to experiments reported in the literature; this swelling depends strongly on the hydrogen surface coverage. A MC/MD-based compositional relaxation procedure generates configurations whose arrangements of H atoms are in excellent agreement with experimental observations. Detailed structural analysis of the hydrogenated surfaces is carried out, providing information which cannot be extracted easily from conventional experimental techniques. The findings of the analysis are used to explain the limitations on the maximum H storage capacity of SWCNT bundles upon their exposure to an atomic H flux. Furthermore, it is demonstrated that the structures resulting from formation of inter-shell or inter-layer C-C bonds are stable and provide seeds for the nucleation of crystalline carbon phases embedded into the shells and layers of the MWCNT and FLG structures, respectively. The key parameter that determines the type and size of the generated nanocrystals is the chiral-angle difference between adjacent layers/walls in the original structure. A novel type of carbon structure, consisting of fullerene-like caged configurations embedded within adjacent graphene layers, has been discovered for the case where the graphene layers are rotated with respect to each other; interestingly, one class of these structures retains the unique and desired electronic properties of single-layer graphene.

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