Reduction and covalent modification of graphene-oxide by nitrogen in glow discharge plasma

In this work, we performed plasma treatment of thin layers of graphene oxide samples in various nitrogen containing gases (mainly NH3 and N-2). Experiments were performed in the preparation chamber of the X-ray photoelectron spectrometer, allowing "in situ" characterization of the treated surface. Introduction of nitrogen into the top surface was intensified by applying negative voltage on the sample between 0 and 300 V accelerating the positive plasma ions towards the sample. Significant amount of nitrogen (approximate to 10 atomic %) was built into the top atomic layers of the graphene oxide samples at application of the 2 types of plasma gases for 10-minute reaction time. When comparing the NH3 and N-2 plasma treatments, more complete reduction and closely similar amount of nitrogen was found at applying NH3 plasma. When increasing the bias, the N-content increased, together with decrease of the O content. The high-resolution C1s, O1s, and N1s spectra are broad, representing different chemical states. The peak envelopes of the O1s and N1s lines could be decomposed essentially to 3, while the C1s spectrum to 5 different, well-separated peaks, being identical for all samples. The component peaks were assigned to specific chemical bonding states (N1s: 398.3 eV sp(2) pyridine N, 399.7 eV sp(2) pyrrole, diazine or triazine N, 401.0 eV N in graphite plane; O1s: 530.8 eV carbonyl, 532.2 eV ether, epoxy, alcohol, ester C=O, 533.6 eV ester C-O-C, carboxyl OH). The relative amounts of C-O and C-N bonding states changed significantly with advancement of the treatment performed at increasing biases.