Surface Energy of Sol Gel-Derived Silicon Oxycarbide Glasses

The surface properties of different SiOC glasses before and after HF etching have been analyzed by means of nitrogen adsorption and inverse gas chromatography (IGC) at infinite dilution. Raman and Drift spectroscopies have been also used for structural characterization. The dispersive surface energy (γsd), acid–base (kA/kB) character and nanorugosity index (IM) have been compared with those found for vitreous silica (VS) glass. The γsd value of the SiOC glass is higher than VS (γsd = 58.8 mJ·m−2) and decreases from 151.02 to 104.39 mJ·m−2 with the increase of the carbon content. Raman spectroscopy corroborates the previous results in which the higher ordered graphite is attributed to the higher dispersive surface energy. After HF etching the γsd values of the SiOC glasses increase up to 200 mJ·m−2, being these results in accordance with the elimination of silica phase and therefore the presence of a high concentration of carbon mainly as ordered graphite. Raman and Drift spectroscopies suggest that HF etching not only removes silica but poorly ordered graphite and probably β-SiC nanocrystals as well. The acid–base properties, i.e., the kA/kB ratio, of the analyzed SiOC glasses are very close to those of VS or natural graphites, depending on their free carbon content. In general the acid constant is higher than the base even after HF etching, indicating that the free carbon phase contains surface active acidic groups. Finally, the surface nanorugosity of the SiOC glasses has been found to be dependent also on the graphite order. For VS the surface is completely smooth (IM = −0.76), and for the SiOC glasses having highly ordered graphite the nanorugosity is similar to graphite-like materials (IM values between −11 and −13), reaching an average value close to IM = −6.35 in the case of poorly ordered graphite carbon.