With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO(2) and CH(4) to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH(2), H(2)PO(3), NO(2), and CH(3). Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO(2) to exceed that of CH(4) by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH(2), H(2)PO(3), NO(2), and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO(2)/CH(4) selectivity are discussed.

Using van-der-Waals-corrected density functional theory calculations, we explore the possibility of engineering the local structure and morphology of highsurface-area graphene-derived materials to improve the uptake of methane and carbon dioxide for gas storage and sensing. We test the sensitivity of the gas adsorption energy to the introduction of native point defects, curvature, and the application of strain. The binding energy at topological point defect sites is inversely correlated with the number of missing carbon atoms, causing Stone−Wales defects to show the largest enhancement with respect to pristine graphene (∼20%). Improvements of similar magnitude are observed at concavely curved surfaces in buckled graphene sheets under compressive strain, whereas tensile strain tends to weaken gas binding. Trends for CO 2 and CH 4 are similar, although CO 2 binding is generally stronger by ∼4 to 5 kJ mol −1 . However, the differential between the adsorption of CO 2 and CH 4 is much higher on folded graphene sheets and at concave curvatures; this could possibly be leveraged for CH 4 /CO 2 flow separation and gasselective sensors.

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Previous modeling and experimental studies have suggested that Cu-based metal catalysts can preferentially produce metallic carbon nanotubes. Here, we explain this selectivity by separately modeling nanotube cap nucleation and nanotube growth on Cu and Ni x Cu 1−x surfaces. Cap nucleation is modeled by invoking an epitaxial lattice matching criterion and comparing the binding strengths of different cap chiralities. Nanotube growth on various catalyst surfaces was studied by calculating differences in armchair and zigzag dangling bond energies, relative chemical activity ratios, and nanotube growth rates of different nanotube chiralities. All energies associated with nanotube cap nucleation and armchair and zigzag dangling bond energies were obtained using density functional theory (DFT). We find that certain armchair and zigzag nanotube caps exhibit higher binding strengths than chiral caps, and the stability of the caps on the various surfaces decreases as Ni > Ni 0.5 Cu 0.5 > Cu, in accordance with the respective carbon−metal adhesion strengths. Both the relative chemical activity ratios and the nanotube growth rates suggest that Ni x Cu 1−x bimetallic nanoparticles with increased bond length or lattice-strained surfaces are excellent candidates for preferentially producing metallic nanotubes.

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