Determination of parameters for the synthesis of carbon nanomaterials by ultrasonic spray-chemical vapor deposition

Abstract

Nanotechnology is a field that keeps growing every year, especially carbon nanomaterials like graphene or carbon nanotubes, which are the main ones employed in developing patents [1]. These nanomaterials exhibit outstanding properties like young modules around 1 TPa [2], experimental surface area above 1,300 m2g−1 [3], among many other thermal and conductivity properties. Although a lot of research is carried out, this is not reflected in the number of products on the market for daily use. The main boundaries that delay the adoption of these materials are cost/price, production methods scaling and material quality/consistency [4]. Chemical vapor deposition (CVD) is a methodology capable of synthesizing different carbon allotropes by varying the processing parameters [5]. Also, it is a very versatile methodology because it can use precursors in different physical states and chemical natures [6]. When the carbon precursors are in a liquid state with the catalyst dissolved, an activation mode is necessary to nebulize the solution to facilitate pyrolysis [7]. The ultrasonic spray is an activation mode that assists CVD by isolating the precursor solution through the acoustic cavitation that expands and compresses the droplets until it implodes into micron-sized droplets that facilitate the pyrolysis of the reactants [8]. In collaboration with the author of this thesis, Ceron designed and manufactured a CVD system assisted by ultrasonic spray to later automate it with routines to synthesize specific carbon nanomaterials [9]. Unlike other systems, this one includes a polytetrafluoroethylene (PTFE) membrane that separates the precursor solution from the transducer to increase the dispositive’s lifetime while allowing the user to change the nebulization chamber (useful for post-processing steps). The first trials of the system provide exciting results because they synthesize different morphologies by changing the processing parameters. However, a structured design of experiments (DOE) is needed to improve the synthesized product. For this reason, this thesis aimed to determine the conditions to synthesize a specific morphology with fewer defects and maximize mass production. To do so, 23 DOE was performed by duplicating. The independent variables were the concentration (molarity) of the catalyst in the precursor solution, the Ar flow (L/min) and the temperature. Meanwhile, the dependent variables were ID/IG rate and mass production (grams). Each experiment was characterized by Raman spectroscopy and Scanning Electron Microscopy (SEM). Raman spectra and SEM micrographs validate that the system designed can synthesize different kinds of carbon allotropes like carbon spheres or CNT. Current results suggest that the system can synthesize in a homogeneous way CNT at 800ºC. Nevertheless, future TEM characterization will allow us to validate the obtaining of MWCNT (as suggested by Raman analysis). A 23 design of experiments was done to minimize the number of defects in the synthesized carbon nanostructures, where it was found that at 800ºC, 0.15M and 1.5 L/min, the smallest value of ID/IG ratio was obtained. A similar study was performed to maximize mass production, where the highest mass is obtained at 800ºC, 0.1M and 1.0 L/min. Since the conditions of each study were different, it was looked for the parameters that balanced the two properties better, being 800ºC-0.1M-1.0L/min the conditions that maximized the mass production without affecting too much the quality of the graphene sheet because this condition was the second condition with fewer defects.