Influence of Surface Termination and Electronic Structure on the Photochemical Grafting of Alkenes to Carbon Surfaces

The ultraviolet-initiated photochemical grafting of n-alkenes to surfaces of amorphous carbon, diamond, and carbon nanofibers has recently emerged as a way to impart new surface properties to these materials. Recent studies have shown that grafting on these materials is initiated by a novel photoelectron ejection process, evidenced by a strong dependence of reaction efficiency on the electron affinity of the reactant molecules. Yet, the role of different surface functional groups and the resulting changes in valence band density of states and surface work function have not been determined previously. Understanding the influence of surface carbonyl groups is particularly significant because CO groups increase the surface work function but are also known to catalyze certain electron-transfer processes at carbon surfaces. Here, we use infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) to investigate how changes in surface termination (H-termination vs O-termination) and surface annealing impact the valence electronic structure and work function of the samples, and how these changes influence the grafting of alkenes to the surfaces. Four different n-alkenes bearing terminal groups were used in order to identify the role of the molecular electron affinity, using alkenes terminated with -NHCOCF₃, -NHCOO(tert-butyl), -COOMe, and -CH₃ groups. By using narrowly spaced interdigitated electrodes of amorphous carbon, we are able to directly detect the UV-induced photoemission. Our results show that O-termination of amorphous carbon surfaces enhances the photochemical grafting yield compared with H-termination of surfaces, contrary to what has been found on diamond surfaces. Surfaces annealed to increase the amount of sp²-hybridized carbon, and therefore being most metallic in character, have the highest reactivity. The changes in reactivity are explained in terms of the changes in valence electronic structure of the samples and the influence of oxygen on the photoelectron emission process that initiates the grafting reaction.