N-doped Graphene on Ni: growth, structure and reactivity

The experimental work presented in this PhD thesis fits into the fields of gas sensors, gas storage and catalysis under-cover. In particular, this study focuses on the characterization of the growth, structure and reactivity upon carbon monoxide exposure of pristine and nitrogen-doped graphene (Gr) on Ni(111), by means of Scanning Tunneling Microscopy (STM), X-ray Photoemission Spectroscopy (XPS), supported by Low-Energy Electron Diffraction (LEED). The experimental results were corroborated by theoretical calculations in collaboration with the University of Milano-Bicocca and the University of Trieste. Gr is widely studied for a variety of applications and its already fascinating properties can be tuned by introducing doping centers in the honeycomb network. For this aim, nitrogen is one of the most promising dopants, being predicted to improve Gr performances. However, the production of nitrogen-doped graphene (N-Gr) is not trivial. Several approaches were reported in literature which, however, many times do not result in reproducible and high-quality Gr layers. In the first part of this thesis, we present an alternative and reproducible growth method, which ensures the formation of N-Gr layers of high morphological quality. For this purpose, we exploited a Ni substrate, a low-cost and widely available metal. Its high catalytic activity is particularly suitable for production of high-quality Gr layers via standard chemical vapor deposition (CVD) which, combined with its capability to dissolve/segregate N into the bulk/surface, allows to easily obtain N-Gr sheets. Morphological and chemical characterization by STM and XPS shows that the process yields a flat and continuous N-Gr layer. Experimental results are complemented by a Density Functional Theory (DFT) investigation of possible structural models, to identify at the atomic scale the various N configurations in the Gr mesh. This joint approach allowed unveiling the structural, morphological and chemical properties of N dopants trapped in the network, found mainly in two configurations: graphitic N defects, where an N atom substitutes a C atom in the mesh and bonds to three neighboring C atoms, and 3N pyridinic defects, where three N are placed at the edge of a C vacancy and each of them bonds to two C atoms as part of a six-membered ring. The second part of this PhD thesis is dedicated to the investigation of the reactivity of N-Gr on Ni in comparison to pristine Gr. In particular, we focus on the reactivity towards carbon monoxide (CO), one of the simplest molecules in nature, but also potentially dangerous and lethal. We confirmed that, in the near-ambient pressure regime and at room temperature, CO intercalates at the interface between Gr and Ni, detaching the Gr layer from its substrate. We demonstrated that the same behavior occurs for N-Gr exposed to CO, but at a pressure one order of magnitude lower with respect to the pristine case, thus pointing out an enhancement of the reactivity of the layer in presence of N dopants. By means of LEED, XPS and STM measurements, we present a full chemical and morphological characterization of the pristine and N doped Gr surfaces after CO exposure. We finally rationalize the intercalation mechanism itself, not trivial for impermeable materials like Gr. Combining an experimental (STM, XPS, LEED) and theoretical (DFT) study, we described how CO molecules permeate the Gr layer, getting into the confined zone between Gr and Ni. Suitably large defects allow CO reaching the interface and the presence of N pyridinic dopants at their edges is found to stabilize the multiatomic vacancy and facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude. Summarizing, the alternative N-Gr growth method we developed is potentially scalable and suitable for the production of high-performance nano-devices, with crucial implications for Gr-based gas sensors and storage devices.