The Synthesis of Sponge-like V2O5/CNT Hybrid Nanostructures Using Vertically Aligned CNTs as Templates

Abstract

The fabrication of sponge-like vanadium pentoxide (V₂O₅) nanostructures using vertically aligned carbon nanotubes (VACNTs) as a template is presented. The VACNTs were grown on silicon substrates by chemical vapor deposition using the Fe/Al bilayer catalyst approach. The V₂O₅ nanostructures were obtained from the thermal oxidation of metallic vanadium deposited on the VACNTs. Different oxidation temperatures and vanadium thicknesses were used to study the influence of these parameters on the stability of the carbon template and the formation of the V₂O₅ nanostructures. The morphology of the samples was analyzed by scanning electron microscopy, and the structural characterization was performed by Raman, energy-dispersive X-ray, and X-ray photoelectron spectroscopies. Due to the catalytic properties of V₂O₅ in the decomposition of carbonaceous materials, it was possible to obtain supported sponge-like structures based on V₂O₅/CNT composites, in which the CNTs exhibit an increase in their graphitization. The VACNTs can be removed or preserved by modulating the thermal oxidation process and the vanadium thickness.

Keywords: chemical vapor deposition; electron beam deposition; thermal oxidation; vanadium pentoxide; vertically aligned carbon nanotubes.

Figure 1

Scheme of the synthesis process of VACNTs and V₂O₅/VACNTs.

Figure 2

SEM micrographs. Top view of samples with (a) 50 nm and (b) 200 nm of V, oxidized at 400 °C. Top views of samples with (c) 50 nm and (d) 200 nm of V, oxidized at 500 °C. Lateral view of samples with (e) 50 nm and (f) 200 nm of V, oxidized at 400 °C. Lateral view of samples with (g) 50 nm and (h) 200 nm of V, oxidized at 500 °C. The scales of 500 nm and 2 µm correspond to (a–d) and (e–h), respectively.

Figure 3

EDX spectra of VACNT samples, loaded with 0, 50, 100, 150, and 200 nm V, oxidized at (a) 400 °C and (b) 500 °C. Weight percentages of (c) V and (d) C as a function of vanadium thickness in samples oxidized at 400 °C and 500 °C.

Figure 4

Raman spectra of V-coated VACNT samples oxidized in an O₂ atmosphere at (a) 400 °C and (b) 500 °C.

Figure 5

(a) Carbon region Raman spectra of the VACNT/V₂O₅ sample compared with VACNTs as-grown and VACNTs with different thermal annealing processes. (b) The ratio of the intensities of the D and G peaks, (c) FWHM of the D and G peaks, (d) Raman shift position of the D and G peaks, and (e) intensity ratio between the 7A₁, 5A₁, and 2D peaks over the D peak.

Figure 6

High-resolution XPS spectra of the C1s signal of: (a) As grown VACNTs, (b) VACNTs oxidized at 400 °C, (c) VACNTs with 50 nm V oxidized at 400 °C, (d) VACNTs oxidized at 500 °C, (e) VACNTs with 50 nm V oxidized at 500 °C.

Figure 7

(a) and (b) correspond to the high-resolution XPS of the V2p signal for samples VACNT_50V_400 and VACNT_50V_400, respectively. The red doublet represents the V⁵⁺ contribution, while the blue doublet represents the V⁴⁺ contribution.