The chemistry and topography of stabilized and functionalized graphene oxide coatings

Graphene oxide (GO) thin films and coatings are regarded as superior in quality to other materials especially for biomedical applications. However, the lack of stability and understanding of their structure and defects hinder their use in value added applications. Here, we describe our successful attempt at stabilizing, reducing and functionalizing GO through multiple plasma treatments with polymerizing (to deposit a crosslinking and compressing layer of diamond like carbon, DLC) and non-polymerizing precursors (H₂, O₂, and N₂). The hybrid GO and DLC coatings on semi crystalline PEEK were evaluated using AFM, SEM, and XPS. The GO deposited layer showed roughness around 70 nm and, despite care, resulted in several wrinkles and particle aggregations. The hybrid coatings conformed to the roughness and crystalline features of PEEK. XPS showed that the DLC layer cross-linked the GO nano-flakes while not completely masking which enable the partial exposure of GO. The GO-DLC hybrid interface is higher in thickness than the PEEK-GO and is dominating the overall thickness of the hybrid structure ≈13 ± 1 μm. XPS measurements showed that the often unstable CO functional groups on the surface of the hybrid coating can be reduced by effective plasma treatment. Plasma treatments also generated CO functional groups that probably originated from the decomposed carboxyl groups. The plasma treatment also contributed to the reduction of GO. Treatment with H₂ was more effective in oxygen reduction than with the N₂, however, treatment with N₂ increased the reactants on GO as N₂ is heavier tending to deposit more on a surface. Plasma treatment with O₂ increased the surface oxygen content further and hence more defects on the hybrid surface.