Single-step flame synthesis of carbon nanoparticles with tunable structure and chemical reactivity

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Abstract
A novel method for the flame synthesis of carbon nanoparticles with controllable fraction of amorphous, graphitic-like and diamond-like phases is reported. The structure of nanoparticles was tailored using a conical chimney with an adjustable air-inlet opening. The opening was used to manipulate the combustion of an inflamed wick soaked in rapeseed oil, establishing three distinct combustion regimes at fully-open, half-open and fully-closed opening. Each regime led to the formation of carbon coatings with diverse structure and chemical reactivity through a facile, single-step process. In particular, the fully-closed opening suppressed most of the inlet air, causing an increased fuel/oxygen ratio and decreased flame temperature. In turn, the nucleation rate of soot nanoparticles was enhanced, triggering the precipitation of some of them as diamond-like carbon (DLC). Surface characterization analyses using Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron spectroscopy confirmed this hypothesis, indicating a short-range ordered nanocrystalline structure and ∼80% sp3 bonds in the coatings deposited at fully-closed opening. Furthermore, three groups of 5 MHz Quartz Crystal Microbalances (QCMs) coated with soot and DLC, corresponding to each of the three combustion regimes, showed different frequency responses to aqueous ethanol and isopropanol solutions in the concentration range of 0–12.5 wt%. The DLC coated QCMs exhibited relatively constant frequency shift of ∼2250 Hz regardless of the chemical, while the response of soot coated counterparts was influenced by the quantity of heteroatoms in the film. Our method can be applied in chemical sensing for the development of piezoresonance liquid sensors with tunable sensitivity.

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