Metal/Carbon Hybrid Nanostructures Produced from Plasma-Enhanced Chemical Vapor Deposition over Nafion-Supported Electrochemically Deposited Cobalt Nanoparticles

Abstract

In this work, we report development of hybrid nanostructures of metal nanoparticles (NP) and carbon nanostructures with strong potential for catalysis, sensing, and energy applications. First, the etched silicon wafer substrates were passivated for subsequent electrochemical (EC) processing through grafting of nitro phenyl groups using para-nitrobenzene diazonium (PNBT). The X-ray photoelectron spectroscope (XPS) and atomic force microscope (AFM) studies confirmed presence of few layers. Cobalt-based nanoparticles were produced over dip or spin coated Nafion films under different EC reduction conditions, namely CoSO₄ salt concentration (0.1 M, 1 mM), reduction time (5, 20 s), and indirect or direct EC reduction route. Extensive AFM examination revealed NP formation with different attributes (size, distribution) depending on electrochemistry conditions. While relatively large NP with >100 nm size and bimodal distribution were obtained after 20 s EC reduction in H₃BO₃ following Co²⁺ ion uptake, ultrafine NP (<10 nm) could be produced from EC reduction in CoSO₄ and H₃BO₃ mixed solution with some tendency to form oxides. Different carbon nanostructures including few-walled or multiwalled carbon nanotubes (CNT) and carbon nanosheets were grown in a C₂H₂/NH₃ plasma using the plasma-enhanced chemical vapor deposition technique. The devised processing routes enable size controlled synthesis of cobalt nanoparticles and metal/carbon hybrid nanostructures with unique microstructural features.

Keywords: AFM; Nafion membranes; carbon nanotubes; cobalt nanoparticles; electrochemical process; nanocomposite films.

Figure 1

(a) XPS survey spectrum of the electrochemically grafted para-nitrobenzene diazonium (PNBD) and high resolution valence band regions with component peaks for (b) the C 1s and (c) the N 1s envelopes.

Figure 2

Atomic force microscope (AFM) results of the aryl grafted silicon surface after electrochemical reduction in the diazonium salt and sulfuric acid mixed solution (S₁, Table 1): (a) Two-dimensional area scan and (b) the line profile showing vertical displacement versus distance.

Figure 3

2-D and 3-D AFM scans showcasing the effect of Nafion membrane deposition process on the film morphology and the NP size and distribution after indirect EC reduction for the (a,b) dip coating (sample S₂) and (c,d) spin coating (sample S₃) processes.

Figure 4

The 20 µm area scans showing AFM images and the 3-D topography of the Nafion supported Co-based nanoparticles after 20 s EC reduction (S₄ vs. S₅, Table 1): (a,b) Co²⁺ immersion followed by EC reduction (IR, S₄) and (c,d) direct EC reduction in the CoSO₄+H₃BO₃ solution (DR, S₅).

Figure 5

TEM microstructures of the samples S₆ and S₇ (Table 1) showing the NP size dependence on the EC reduction time in 1 mM CoSO₄ 0.5 M H₃BO₃ solution: (a) 5 s and (b) 20 s.

Figure 6

Low and high magnification SEM micrographs of the carbon nanostructures after PECVD growth over Si substrates supporting NP with different sizes: (a,b) Sample S₄, (c,d) Sample S₅, and (e,f) Sample S₇.

Figure 7

High-resolution TEM microstructures of the carbon nanostructures after PECVD at 900 °C using C₂H₂:NH₃ mixture of 1:5.3 (sccm): (a) The near-tip area of an individual carbon nanofiber, (Sample S₅) and (b) top view showing few-walled CNT with open ends and carbon nanosheets (Sample S₇).