Exploring the role of edges in surface functionalization and stability of plasma-modified carbon materials: Experimental and DFT insights

Effective surface functionalization of carbon nanomaterials plays a crucial role in various applications. We investigated the impact of edges on surface functionalization and stability of oxygen-modified carbon materials using a combination of experimental techniques and Density Functional Theory (DFT) insights. Graphenic paper, highly oriented pyrolytic graphite (HOPG), and graphenic flakes were employed as model systems, with oxygen plasma treatment (generator power 100 W, oxygen pressure 0.2 mbar, exposure time 6 – 300 s) serving as the modification method. Surface morphology and chemical composition were characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The results revealed the introduction of oxygen functional groups on the investigated carbon surfaces (up to 20 at % by XPS) whereas; the structural integrity of the materials remained intact upon plasma modification (SEM, Raman). Work function was used as a sensitive parameter for monitoring the surface changes (increase by ∼1.4 eV, 1.3 eV, and 1 eV for graphenic paper, HOPG, and graphenic flakes, respectively) while time-dependent measurements revealed distinct kinetic processes governing the decay of functionalization, highlighting the role of surface defects in post-plasma processes. DFT calculations provided molecular-level insights into the surface processes, elucidating the mechanisms underlying the diffusion of hydroxyls, their recombination, and water desorption. Since the calculated activation barrier for recombination on basal graphenic planes (∼1.0 eV) and edges (∼5.5 eV) are distinctly different, it can be thus concluded that the persistent functionalization is due to the surface edges. Our findings contribute to a deeper understanding of surface modification processes of carbon materials and offer rationales for the design of advanced functional nanomaterials with tailored surface properties.