• funzionalizzazione

In this thesis, we investigate the functionalization of graphene by Lithium (Li) for Hydrogen Storage applications. Such work has been done by Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED) techniques in Ultra High Vacuum (UHV) conditions.
By now, the interest in new kinds of alternative energies is increasingly wide-spread. One of the possible and most promising candidates for green energies development is hydrogen. It is not an energy source but a secondary energy carrier. This means that hydrogen must be produced, and to do this, it is necessary to provide energy. The same amount of energy that we provide to produce hydrogen is released during its use in fuel cells. Hydrogen is the most abundant element on Earth, but less than 1% is present as molecular hydrogen gas H2. Mostly, it is chemically bound as H2O in water, and this is an advantage, because it is essentially an unlimited resource. Once produced, hydrogen is a clean, carbon-free, non toxic, synthetic fuel. However, to store hydrogen is not so easy. It is a gas at room temperature (RT) and it occupies a large amount of space at atmospheric pressure. The gas must therefore be compressed in some way to make it compact enough for mobile applications. During the last 30 years, the attempts have been a lot: to store hydrogen as a gas in high pressure gas cylinders, or in liquid form in cryogenic tanks, or by metal hydrides, or via chemical reaction, or adsorbing on solids of large surface area.
The turning point came when, in 2004, graphene was produced in the laboratory. Graphene is a carbon atom sheet, one-atom thick, arranged in a honeycomb structure. Its extraordinary properties, including a large specific surface area (2630 m2/g), make it one of the most interesting and studied materials. Its applications range over the most varied fields, and between these there is also the hydrogen storage application, where it is used to create devices to store hydrogen. However, it is found that the storage capacity in graphene significantly decreases at room temperature and ambient pressure.
Theoretical studies suggest that one method to improve the interaction between H2 molecules and graphene is to decorate the latter with elements of the periodic table that can enhance the binding energy of the H2 molecules on graphene. One approach is to use the induced polarization of H2 molecules by an electric field. Alkali metals (Li, Na, and K)-decorated graphene and alkali earth metals can bind H2 molecules with a binding energy of 0.1-0.2 eV. It has been theoretically predicted that, between these, Li enhances the gravimetric density of graphene up to 12.8 wt.%. This has produced an increasing interest in Li-functionalized graphene devices.
We investigated the Li interaction with epitaxial graphene on SiC(0001) under UHV conditions by LEED and STM. In order to perform the Li depositions, we designed a Li evaporator using a commercial Li dispenser. This has been calibrated on a Si(111)-7×7 reconstructed sample, since Li induces a 3×1 reconstruction of the Si(111)-7×7 surface. Following the theoretical missing-top-layer (MTL) model, we have been able to determine the Li amount deposited on the Si(111)-7×7 surface in a defined deposition time, calibrating the Li dispenser.
Once done this, we started the investigation of Li deposition on the epitaxial graphene surface, observing each variation for a defined Li deposited amount. This systematical study has been performed on epitaxial monolayer graphene and on buffer layer surfaces.
We observed that Li intercalates below the graphene surface probably through the SiC steps sides or graphene defects. Once intercalated, Li places itself at the interface, breaking the Si-C bonds between substrate and buffer layer, transforming the buffer layer in a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. In the LEED diffraction pattern, the 6√3 reconstruction, which is the SiC surface reconstruction due to the interaction between the buffer layer and the substrate caused by covalent bonds, disappears. Also by STM, the 6√3 periodicity is no longer visible. Furthermore, along the SiC step edges we start to observe small steps which expand inward the terraces. Increasing the amount of Li evaporated, by LEED and STM we observe that the Moiré pattern totally disappears. By this experimental evidence, we demonstrate that Li is able to detach the buffer layer from the SiC substrate, converting it in a quasi-free-standing graphene. The further deposited Li now intercalates in between the two graphene layers, and a √3×√3 reconstruction becomes visible. The latter modifies the order and periodicity of the graphene surface.
Providing thermal energy to the system, Li is shown to desorb from the surface, and this occurs in two steps: 1) after annealing at 180 °C, Li in the region between the two graphene layers moves to the interface, forming a double Li layer there. 2) After annealing at 300 °C, the Li desorption process from the interface gradually starts, reaching a peak after annealing at 500 °C. An increase in temperature up to 1000 °C results in the almost complete Li desorption from the surface, allowing the restoration of the Si-C covalent bonds. Just some small islands remains on the surface, which are probably due to Li pinned to graphene defects.
Finally, we propose a simple model which describes the Li deposition-intercalation-desorption process. By STM, for the first time, the Li intercalation process was observed, and each experimental step has been documented, such as the √3×√3 graphene surface reconstruction. Furthermore, our STM experimental results allow us to measure the interlayer distance between the graphene layers and the substrate imposed by the intercalated Li atoms. These distances were never before experimentally measured.
The obtained values provide a starting point for a hypothetical hydrogen storage application: the spacing up to 6 Å between the two graphene layers induced by Li intercalation could promote hydrogen intercalation which should bind with Li atoms, enhancing the hydrogen storage capacity of the device. In such a way, Li-functionalized graphene could represent an interesting material for hydrogen storage.