C-H bonds in Carbon Nanotubes as an Energy Carrier

The main goal of this work was to study the hydrogen interactions with carbon nanostructures (graphene grown on Pt(111), and carbon nanotube (CNT)/Pt composites) in lieu towards finding answers related to two bigger questions – (a) hydrogen storage possibility in carbon nanostructures and (b) band gap opening in graphene (for device applications). X-ray spectroscopy was used as the main experimental probe since it provides for an atom specific probe of the electronic structure pertaining to the carbon atoms. We utilize the following x-ray spectroscopic techniques – X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES) along with other typical surface science techniques to probe the chemical environment and the electronic structure of the carbon atoms while they interact with hydrogen. For hydrogen storage applications, one would need a material that would undergo loading/unloading at conditions close to ambient conditions. In other words, the material would have the adsorption and desorption energies of hydrogen to be energetically neutral (i.e. close to zero) such that no energy loss would occur during the loading/unloading process. In this work, we have successfully demonstrated two routes towards reducing the kinetic barrier involved in the process of (de-)hydrogenation. One involved hydrogenation of CNT/Pt composites at near ambient conditions (~8.25 atm of hydrogen gas at room temperature) exploiting the concept of “spillover” of H atoms from H2 molecules dissociated on the Pt catalyst. The other experiment involved the reduction in the C-H desorption barriers for graphene grown on Pt substrate, through modulation of the electronic structure of the Pt substrate through a subsurface layer of Co atoms forming a sandwich-type Pt-Co-Pt structure. Another important discovery that we made during our work is that hydrogenation of a graphene layer on a metal substrate does not result in the opening of a band gap. It is generally believed in the graphene community that functionalization of graphene (free standing or supported) through hydrogen adsorption would lead to opening of a band gap either through sublattice symmetry breaking or through patterned hydrogenation leading to quantum dot formation. We show that hydrogenation is accompanied by a symmetry change in the local carbon bonding environment, which causes strong hybridization between the C and Pt atoms. This hybridization leads to pinning of the Fermi level and delocalization of the C π band. Our results shed important light into the possible role played by the substrate in the electronic structure of hydrogenated (or functionalized) graphene, which had not been taken into consideration before. Our work also indicates that it is possible to reversibly form strong local carbon-metal bonds through hydrogenation. This discovery of a possible “welding” method could be important in the design of future graphene devices. Introduction and Background Previous studies have indicated that it is possible to approach hydrogen storage capacities of 5-6 wt.% in single walled CNTs (SWNTs) through treatment with an atomic hydrogen source through formation of C–H bonds [1]. However, practical implementation of hydrogen storage in SWNTs requires the development of low-barrier pathways for hydrogen dissociation. One potential pathway could be the “spillover” mechanism, in which molecular hydrogen spontaneously dissociates on a transition metal catalyst attached to the SWNTs, producing mobile H atoms that spill over onto the SWNTs [2]. However, the validity of the spill-over mechanism and the possibility of significant hydrogen storage in SWNTs remain controversial. Only few studies exist including inelastic neutron-scattering (INS) experiments [3] and isotope exchange TPD experiments [4] performed on metal-carbon composites which favorably suggest the existence of a spillover mechanism. Therefore, it is important to obtain a proof of principle for the existence of the spillover mechanism, which could then be exploited for hydrogen storage applications. Surface science techniques such as temperature programmed desorption (TPD) can provide information about the activation barriers involved in the hydrogen adsorptiondesorption process. Since these techniques require well-defined high-quality samples, graphene samples grown on metal substrates at ultra-high vacuum (UHV) conditions were used. Furthermore, another problem of interest to a large community can be studied at the same time, which is the control of the electronic structure through hydrogenation of graphene for electronic applications. Recently, several approaches have been explored to alter the conductance of graphene and substrate-graphene composites through modulation of the electronic structure of graphene [5–8]. While band gap opening through hydrogenation of single layer was reported for graphene/Ir(111) [5] and Au intercalated Ni(111) [8] substrates, it has also been suggested that hydrogenation leads to the formation of mid-gap impurity states for quasi-freestanding graphene [9]. Hence it is important to address the question of whether or not there is a band gap opening resulting from hydrogenation of graphene/metal systems. The experiments performed on hydrogenated graphene/Pt system indicate that strong grapheme-metal hybridization stabilizes the hydrogenation-induced symmetry change of the local carbon bonding structure. This implies that the hydrogen adsorption-desorption energetics could be controlled through tuning the strength of the resulting hybridization between the graphene overlayer and the metal substrate. Such a tuning could be achieved by modifying the valence d-electron structure of the substrate. The d-band model [10] describes how chemisorption energies of adsorbates correlate with the average energy of the d-electrons with respect to the Fermi level. The Pt-3d-Pt(111) bimetallic sandwich alloys are possible candidates to control ligand environment in the subsurface region, which varies the energy position of the dband center of the topmost layer [11]. Hence we addressed the following sub-tasks in this work: 1. Demonstrate proof-of-principle for the “spillover mechanism” in CNT/Pt composites. 2. Understand the electronic structure changes in graphene/Pt(111) due to hydrogen adsorption. 3. Desorption barrier for hydrogen on graphene/Pt-Co-Pt(111), where a decrease of the H desorption barrier in comparison to graphene/Pt(111) system is expected according to the d band model.