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Wettability of ultrananocrystalline diamond and graphite nanowalls films: a comparison with their single crystal analogs.

Dramatic changes in wettability of diamond and graphite are observed when these materials are prepared in nanostructured forms--undoped and nitrogen-doped ultrananocrystalline diamond (UNCD) films, and graphite nanowalls (GNW), respectively. The nanostructured carbon films were deposited on Si by microwave plasma CVD processes. The advancing contact angle theta for water on hydrogenated undoped UNCD films increases to 106 +/- 3 degrees compared to hydrogenated single crystal diamond (theta = 92 degrees). Nitrogen doping (N2 addition to plasma) during UNCD growth makes the film more hydrophilic. The GNW films exhibited superhydrophobic behavior with theta = 144 +/- 3 degrees for water, which is higher than the contact angle of monocrystalline graphite (the basal plane) by a factor of 1.8. No chemical surface treatment is necessary to achieve such high hydrophobicity, it is accomplished solely by a specific (nanoporous, high aspect ratio) surface morphology with very low free surface energy inherent in it. The wetting behaviour of nanostructured films can be described with the Cassie-Baxter equation for heterophase nanoporous surfaces. Oxidation and hydrogenation of UNCD films make it possible to control theta over a much wider range as compared to a single crystal diamond. The influence of diamond grain size on wetting is considered taking into account the surface treatment. The corresponding variation in surface energy has been determined by the modified Young's equation.