Study On Wettability Regulation Of Graphite

Studying the wettability of graphite is of great significance for the application of carbon related products.The reference water contact angle(WCA)values of graphite have been reported to be 35–126°.Here,the variations in WCA of graphite are studied from surface physiochemical perspective.With humidity control,the intrinsic wettability of graphite surface was investigated by combining macroscopic and microscopic wettability characterization.We found that the reliability of the WCA measurements of the exfoliated graphite was significantly influenced by the surface morphology and the experimental protocols.Scanning electron microscope(SEM)and WCA characterization showed that the exfoliated graphite surface is complexly fractured and partial cleavage creates plenty of wrinkles and ridges across the substrate,which contributed to observed measurement error.We developed a new protocol for more reliable WCA measurements of graphite by firstly flattening the exfoliated surface,then 2)adjusting the size of water droplets,and 3)choosing the central areas of exfoliated HOPG surface for WCA tests.SEM,atomic force microscopy(AFM),X ray photoelectron spectroscopy(XPS),infrared spectroscopy(FTIR)and Raman spectra results showed that the intrinsic wettability of freshly exfoliated graphite is mildly hydrophilic(64°),whereas becomes slight hydrophobic(~90°)after being exposed to the ambient air for 1 h.It was found that the transition from hydrophilic to hydrophobic state is due to adsorption of airborne hydrocarbons and water.UV and low-pressure air plasma treatment were used to modify the graphite surface and control the wettability.The WCAs showed that modifications caused the retarded transition from hydrophilic to hydrophobic state.The underlying mechanism is creation of many defects and active sites as active sites and interaction between water and hydrocarbons.Many oxygen functional groups were generated at the grain boundary for enhanced adsorption of surface water,and then leading to increased adsorption of surface contaminants.Microscopic adhesion force tests were carried out using AFM colloid probe technique under the controlled humidity conditions and compared to the macroscopic WCA results.Adhesion force tests confirmed that the freshly exfoliated graphite is mildly hydrophilic,whereas it will become hydrophobic after exposing to ambient air.It is found that humidity has little effect on the macroscopic WCA.The significant effect of humidity on the adhesion force can be explained by the capillary force and van der Wall's force model considering a constant microscopic WCA between the colloidal sphere and surface.