Effects of nitrogenation on single-walled carbon nanotubes within density functional theory

The effects of nitrogenation on single-walled carbon nanotubes are investigated within the ab initio density functional theory. Four different types of nitrogenation have been considered: (i) direct substitution of nitrogen atoms, (ii) substitution with a formation of vacancy (pyridinelike doping), (iii) exohedral chemisorption of N adatoms, and (iv) sidewall covalent -$\mathrm{N}{\mathrm{H}}_2$ functionalization. Structural deformations, electronic band structures, density of states, and ionization potential energies are calculated and compared among the different types of nitrogenated nanotubes. Magnetism is observed for chemisorbed single-walled carbon nanotubes (SWNTs) with magnetic moment of $0.7{\mu }_B$. In addition, the relaxed structures of SWNTs with two neighboring chemisorbed N adatoms are generally more complex than those of singly chemisorbed N adatom. The barrier energies needed to coalesce two N adatoms chemisorbed on SWNTs to form a free ${\mathrm{N}}_2$ molecule are higher than those for a graphene sheet.

Figure 1

(Color online) (a) Geometrically optimized structures, HOMO, and bond lengths (in Å) of a pure zigzag (10,0) nanotube. (b) Direct substitution of two nitrogen atoms into the carbon framework without the formation of vacancies. Here, the two N substitution atoms in ${\mathrm{C}}_{78}{\mathrm{N}}_2$ are in the opposite positions. (c) N substitution into the carbon framework with the formation of vacancy: pyridinelike doping $\left({\mathrm{C}}_{72}{\mathrm{N}}_6\right)$ with two vacancies formed in opposite positions. (d) Chemisorption of a N adatom in parallel position. (e) Chemisorption of a N adatom in perpendicular position. (f) -$\mathrm{N}{\mathrm{H}}_2$ functionalization. Gray ball denotes C atom, blue ball denotes N atom, and white ball denotes H atom. A fragment of the supercell is taken out to elucidate the bond lengths at the vicinity of the N impurities.

Figure 2

(Color online) (a) Geometrically optimized structures, HOMO, and bond lengths (in Å) of a pure armchair (5,5) nanotube. (b) Direct substitution of two nitrogen atoms into the carbon framework without the formation of vacancies. Here, the two N substitution atoms in ${\mathrm{C}}_{58}{\mathrm{N}}_2$ are in the opposite positions. (c) N substitution into the carbon framework with the formation of vacancy: pyridinelike doping $\left({\mathrm{C}}_{52}{\mathrm{N}}_6\right)$ with two vacancies formed in opposite positions. (d) Chemisorption of a N adatom in parallel position. (e) Chemisorption of a N adatom in perpendicular position. (f) -$\mathrm{N}{\mathrm{H}}_2$ functionalization. Gray ball denotes C atom, blue ball denotes N atom, and white ball denotes H atom. A fragment of the supercell is taken out to elucidate the bond lengths at the vicinity of the N impurities.

Figure 3

Band structures of (10,0) and (5,5) nanotubes: [(a) and (g)] pure, [(b) and (h)] N substitution, [(c), (i), and (j)] pyridinelike doping, [(d) and (k)] chemisorption at parallel position, [(e) and (l)] chemisorption at perpendicular position, and [(f) and (m)] -$\mathrm{N}{\mathrm{H}}_2$ functionalization. (a)–(f) and (g)–(m) are for (10,0) and (5,5) nanotubes, respectively. The Fermi level is represented by the dotted horizontal lines.

Figure 4

(Color online) Total density of states (TDOS) of (a) pristine (10,0) nanotube, [(b)–(d)] nitrogen substitution, [(e) and (f)] pyridinelike doping, [(g) and (h)] chemisorption of N adatoms, and (i) -$\mathrm{N}{\mathrm{H}}_2$ functionalization. Projected DOS of nitrogen impurities and TDOS of undoped (10,0) nanotube with monovacancy are indicated as red (light gray) line and blue (dark gray) line, respectively. The Fermi level is at $0\mathrm{eV}$. A 120 atoms/cell supercell is used to compute case (b) substitution.

Figure 5

(Color online) TDOS of (a) pristine (5,5) nanotube, (b) nitrogen substitution, [(c) and (d)] pyridinelike doping, [(e) and (f)] chemisorption of N adatoms, and (g) -$\mathrm{N}{\mathrm{H}}_2$ functionalization. Projected DOS of nitrogen impurities and TDOS of undoped (5,5) nanotube with monovacancy are indicated as red (light gray) line and blue (dark gray) line, respectively. The Fermi level is at $0\mathrm{eV}$.

Figure 6

Spin-polarized band structures of singly N-chemisorbed SWNTs in [(a) and (c)] parallel and [(b) and (d)] perpendicular positions. Majority spin and minority spin electrons are denoted by solid and dotted lines, respectively.

Figure 7

(Color online) Spin-polarized local density of states of a single N adatom chemisorbed on (10,0) and (5,5) nanotubes. The pink isosurface of the spin density is set at the value of $0.05e∕{\mathrm{\AA }}^3$. The gray and blue spheres represent C and N atoms, respectively.

Figure 8

Possible configurations of two neighboring N adatoms chemisorbed on a graphene sheet. $Z_1$ and $Z_2$ directions represent the tubular axes of zigzag (10,0) and armchair (5,5) nanotubes, respectively.

Figure 9

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