Abstract
This study intends to reveal the significance of the catalyst to substrate ratio (C/S) on the structural and electrical features of the carbon nanotubes and their polymeric nanocomposites. Here, nitrogen-doped carbon nanotube (N-MWNT) was synthesized via a chemical vapor deposition (CVD) method using three ratios (by weight) of iron (Fe) catalyst to aluminum oxide (Al2O3) substrate, i.e.,1/9, 1/4, and 2/3, by changing the Fe concentration, i.e., 10, 20, and 40 wt.% Fe. Therefore, the synthesized N-MWNT are labelled as (N-MWNTs)10, (N-MWNTs)20, and (N-MWNTs)40. TEM, XPS, Raman spectroscopy, and TGA characterizations revealed that C/S ratio has a significant impact on the physical and chemical properties of the nanotubes. For instance, by increasing the Fe catalyst from 10 to 40 wt.%, carbon purity increased from 60 to 90 wt.% and the length of the nanotubes increased from 1.2 to 2.6 µm. Interestingly, regarding nanotube morphology, at the highest C/S ratio, the N-MWNTs displayed an open-channel structure, while at the lowest catalyst concentration the nanotubes featured a bamboo-like structure. Afterwards, the network characteristics of the N-MWNTs in a polyvinylidene fluoride (PVDF) matrix were studied using imaging techniques, AC electrical conductivity, and linear and nonlinear rheological measurements. The nanocomposites were prepared via a melt-mixing method at various loadings of the synthesized N-MWNTs. The rheological results confirmed that (N-MWNTs)10, at 0.5–2.0 wt.%, did not form any substantial network through the PVDF matrix, thereby exhibiting an electrically insulative behavior, even at a higher concentration of 3.0 wt.%. Although the optical microscopy, TEM, and rheological results confirmed that both (N-MWNTs)20 and (N-MWNTs)40 established a continuous 3D network within the PVDF matrix, (N-MWNTs)40/PVDF nanocomposites exhibited approximately one order of magnitude higher electrical conductivity. The higher electrical conductivity of (N-MWNTs)40/PVDF nanocomposites is attributed to the intrinsic chemical features of (N-MWNTs)40, such as nitrogen content and nitrogen bonding types.
Highlights
In the boom of the electronics industry, multifunctional materials have gained a great deal of interest from academia and industry
In this work, we showcased the remarkable effect of iron (Fe) catalyst to substrate ratio on the physical and chemical properties of nitrogen-doped carbon nanotubes (N-MWNTs)
N-MWNT are labelled as (N-MWNTs) were synthesized at three catalyst to substrate ratios, i.e., 10, 20, and 40 wt.% iron
Summary
In the boom of the electronics industry, multifunctional materials have gained a great deal of interest from academia and industry. The introduction of N atoms into carbonaceous materials can modulate electronic structures, which significantly impacts the electrical conductivity [21,22,23] In this method, due to the different nitrogen content and various types of nitrogen bonding, i.e., graphitic, pyridinic, pyrrolic, and quaternary, physical and chemical features of the nitrogen-doped CNTs (NMWNTs) can be controlled [20,24,25,26,27,28,29,30,31]. Taken together, considering the reports mentioning the effect of synthesis parameters on CNT properties, rather than changing the temperature, type of catalyst, synthesis time, or incorporating a secondary material, we demonstrated that, with a simple factor (i.e., catalyst to substrate ratio), all the physical (e.g., length, diameter, and morphology) and chemical (e.g., N-doping content and type) features can be tuned. This study completes the understanding of the N-MWNTs synthesis and connects the broken link between various N-MWNT synthesis parameters and the resulting structural and electrical properties of N-MWNTs and their polymer-based nanocomposites in the literature
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