Fluorination-Induced Changes in Hydrophobicity of Silicon Carbide-Derived Nanoporous Carbon

We report the effect of fluorine doping on hydrophobicity of microporous silicon carbide-derived carbon (SiCDC), exploring Water vapor adsorption in virgin and fluorinated samples experimentally and using a semiempirical isotherm models. The surface chemistry of the virgin carbon and samples fluorinated to three different levels is characterized by X-ray photoelectron spectroscopy and F-19 nuclear magnetic resonance analysis techniques. Besides having different textural features such as surface area and pore size distribution, the virgin and fluorinated SiCDCs show different surface chemistries in terms of the nature and quantity of the C-F bonds. The comparison of the characterization results of the samples and model parameters is used as the methodology to understand the water adsorption mechanism in virgin and fluorinated SiCDCs. We demonstrate that with increasing fluorination level the hydrophobic character of the low and medium fluorinated SiCDC samples remains almost constant while the highly fluorine-doped SiCDC is less hydrophobic. We identify a pore accessibility problem for argon in the fluorine-doped SiCDCs, demonstrating that fluorine functionalization blocks the carbon pores for the nonpolar argon while allowing polar H2O molecules to form clusters that enter the internal volume of the micropores. These findings provide important new understanding of the mechanism of water adsorption in microporous carbons and their fluorinated counterparts.