Oxygenated Surface of Carbon Nanotube Sponges: Electroactivity and Magnetic Studies

Abstract

We report the synthesis of nitrogen-doped carbon nanotube sponges (N-CNSs) by pyrolysis of solutions of benzylamine, ferrocene, thiophene, and isopropanol-based mixture at 1020 °C for 4 h using an aerosol-assisted chemical vapor deposition system. The precursors were transported through a quartz tube using a dynamic flow of H₂/Ar. We characterized the N-CNSs by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetric analysis. We found that isopropanol, isopropanol-ethanol, and isopropanol-acetone as precursors promote the formation of complex-entangled carbon fibers making knots and junctions. The N-CNSs displayed an outstanding oxygen concentration reaching a value of 9.2% for those synthesized with only isopropanol. We identified oxygen and nitrogen functional groups; in particular, the carbon fibers produced using only isopropanol exhibited a high concentration of ether groups (C-O bonds). This fact suggests the presence of phenols, carboxyl, methoxy, ethoxy, epoxy, and more complex functional groups. Usually, the functionalization of graphitic materials is carried out through aggressive acid treatments; here, we offer an alternative route to produce a superoxygenated surface. The understanding of the chemical surface of these novel materials represents a huge challenge and offers an opportunity to study complex oxygen functional groups different from the conventional quinone, carboxyl, phenols, carbonyl, methoxy, ethoxy, among others. The cyclic voltammetry measurements confirmed the importance of oxygen in N-CNSs, showing that with high oxygen concentration, the highest anodic and cathodic currents are displayed. N-CNSs displayed ferromagnetic behavior with an outstanding saturation magnetization. We envisage that our sponges are promising for anodes in lithium-ion batteries and magnetic sensor devices.

Figure 1

SEM images of N-CNS. (a,b) Sample S1, (c,d) Sample S2, and (e,f) Sample S3. The N-CNS contains predominantly entangled carbon nanofibers.

Figure 2

TEM images of N-CNS from sample S1. (a,b) Bamboo-type carbon nanotube with metallic Fe-based nanoparticles inside, (c) straight carbon nanotube, and (d) HRTEM images showing low and high ordered graphitic layers.

Figure 3

TEM images of N-CNS from sample S2. (a,b) Carbon fibers, (c) carbon fiber with Fe-based nanoparticles, and (d) HRTEM images showing the graphitic aspect of carbon fibers.

Figure 4

TEM images of N-CNS from sample S3. (a) Carbon nanotube with an irregular bamboo-shaped morphology and (b) high-magnification image from the square in (a) showing the graphitic layers. (c,d) Carbon nanotube showing the graphitic layers in different zones.

Figure 5

XRD plots for N-CNS. (a–c) Deconvolution analysis of the C(002) signal corresponding to the graphitic material. The deconvolution analysis was performed using two pseudo-Voigt curves; the γ- and π-curves refer to different graphitic materials. Table SI-1 displays the data derived from the deconvolution analysis. (d) XRD plots for 2θ = 31–90°, revealing the presence of iron-based nanoparticles, mainly Fe₃C (PDF 00-034-0001) and graphite (PDF 03-065-6212).

Figure 6

TGA curves for N-CNS. Results for samples S1 (isopropanol), S2 (isopropanol–ethanol), and S3 (isopropanol–acetone). Sample S1 showed the lowest oxidation temperature likely because of the small diameter of the carbon fiber.

Figure 7

(a) Raman spectrum for N-CNS from samples S1, S2, and S3. All samples exhibited the typical D- and G-peaks of graphitic materials. The vertical lines in (a) refer to the vibration modes of graphite. (b–d) Deconvolution analysis of D- and G-band Raman peaks using four curves (D1, D, D2, and G). Table SI-2 displays the data from the deconvolution analysis.

Figure 8

(a–c) Deconvoluted high-resolution C 1s XPS spectra for samples S1, S2, and S3. (d) Relative percentage for each chemical species calculated from the area under its corresponding curve. The deconvolution analysis shows that sp² (C=O) and sp³ (C–O) dominate.

Figure 9

(a–c) Deconvoluted high-resolution O 1s XPS spectra for samples S1, S2, and S3. (d) Relative atomic ratio (%) for each chemical species. C=O is attributed to quinone, C–O ether groups, O–C=O ester groups, and COOH carboxylic. Sample S1 synthesized with only isopropanol showed a high percentage of ether group, which could be assigned to epoxy, methoxy, and ethoxy functional groups. Also, sample S1 exhibited the highest values of C=O and COOH attributed to quinone and carboxylic groups.

Figure 10

(a–c) Deconvoluted high-resolution N 1s XPS spectra for samples S1, S2, and S3. (d) Relative atomic ratio (%) for each chemical species. The main ways of incorporating the nitrogen in graphitic layers were identified. Sample S3 displayed a high relative atomic ratio for N-pyridinic, N-pyrrolic, and N-quaternary doping.

Figure 11

FTIR spectra of N-CNS. Results for samples (a) S1 (isopropanol), (b) S2 (isopropanol–ethanol), and (c) S3 (isopropanol–acetone). The signal of C–O–C at 1055 cm^–1 is identified only for S1 and S2 samples.

Figure 12

(a) Photography of N-CNS. (b,c) Sponge immersed in gasoline for 5 min and is subsequently burnt to remove the solvent. The burnt sponge is reusable. The absorption capacity (Q) was obtained from Q = Q_f/Q₀. (e–g) Values of Q as a function of solvent density.

Figure 13

Cycle voltammetry of N-CNS electrode in 0.5 M H₂SO₄ aqueous solution. Results for SI, S2, and S3 samples in different scan rates: (a) 50 mV/s, (b) 200 mV/s, and (c) 600 mV/s. (d) Potential differences between anodic and cathodic peaks (ΔEp). Sample S1 synthesized with only isopropanol displayed the highest anodic and cathodic currents. The lowest values of ΔEp were obtained for sample S3 synthesized with isopropanol–acetone.

Figure 14

Functional groups that could participate in the redox process. For the reduction process, these groups are protonated, whereas in the oxidation process, these are deprotonated.

Figure 15

Magnetic hysteresis loops of N-CNS measured at 300 K. (a) S1 (isopropanol), (b) S2 (isopropanol–ethanol), and (c) S3 (isopropanol–acetone). The magnetic measurements were carried out at 300 K. The coercive field was 0.25, 0.18, and 0.29 kOe for samples S1, S2, and S3, respectively.