Iron porphyrin attached to single-walled carbon nanotubes: Electronic and dynamical properties from ab initio calculations

Covalent and noncovalent attachment of an iron porphyrin (FeP) on the surface of single-walled carbon nanotubes (CNTs) are addressed by density-functional-theory calculations and molecular-dynamic simulations. We investigate the stability and electronic properties of several CNT-FeP assemblies in order to shed light in the experimentally reported electrocatalytic activity of carbon-supported Fe macrocycles and the role of the linking structure. Two mechanisms for the FeP attachment on metallic and semiconducting CNTs were considered: by physisorption through $\pi \text{−}\pi$ stacking interaction and by chemisorption through $s{p}^2$ and $s{p}^3$ bonding configurations. Our results suggest that FeP covalently linked to metallic CNTs would be the best electrocatalytic systems due to its metallic character at room temperature, suggesting that they may work as an electrode with the ability to transport charge to the macrocycle. Semiconducting CNTs would be unlikely because the FeP-CNT assembly preserves the semiconducting character. Noncovalent attachment of FeP onto both CNTs is also unlikely due to the absence of physical contact and the unsuccessful FeP fixation.

Figure 1

(Color online) Equilibrium geometries of CNTs with a single vacancy: (a) $\left(8,8\right)+\text{V}$ and (b) $\left(14,0\right)+\text{V}$. (c) The FeP radical with a missing H atom $\left({\text{FeP}}^{\ast }\right)$.

Figure 2

(Color online) Equilibrium geometries of FeP and ${\text{FeP}}^{\ast }$ adsorbed on the metallic (8,8) CNT. (a) FeP physisorption on the perfect CNT. (b) ${\text{FeP}}^{\ast }$ chemisorption on the defective $\left(8,8\right)+\text{V}$ CNT by $s{p}^2$ bonding. (c) ${\text{FeP}}^{\ast }$ chemisorption on the perfect CNT by $s{p}^3$ bonding.

Figure 3

(Color online) Spin-resolved band structure of FeP adsorbed on the metallic (8,8) CNT. (a) $\pi \text{−}\pi$ stacking, (b) $s{p}^2$ bonding, and (c) $s{p}^3$ bonding. Blue (red) lines indicate majority (minority) spin subbands and shaded areas the exchange splitting.

Figure 4

(Color online) Spin-resolved band structure of FeP adsorbed on the semiconducting (14,0) CNT. (a) $\pi \text{−}\pi$ stacking, (b) $s{p}^2$ bonding, and (c) $s{p}^3$ bonding. Blue (red) lines indicate majority (minority) spin subbands and shaded areas the exchange splitting.

Figure 5

(Color online) Charge density isosurfaces for the states that cross the Fermi level of the metallic systems: (a) (8,8)-FeP $s{p}^2$, (b) (8,8)-FeP $s{p}^3$, and (c) (14,0)-FeP $s{p}^2$. (d) Charge density for the states at top of the valence band of the semiconducting system (14,0)-FeP $s{p}^3$. The isosurfaces correspond to a charge of $0.005e/{\text{Å}}^3$.

Figure 6

(Color online) Spin-resolved band structure of FeP adsorbed on the semiconducting (14,0) CNT at 300 K. (a) $\pi \text{−}\pi$ stacking, (b) $s{p}^2$ bonding, and (c) $s{p}^3$ bonding. Blue (red) lines indicate majority (minority) spin subbands.