| Abstract Scope |
Graphene hydride, or better known as graphane is as thermodynamically stable as comparable hydrocarbons, more stable than metal hydrides and more stable than graphene by ~0.15eV. This along with its large hydrogen storage capacity 7.7 wt%, which exceeds the DOE 2010 goal, also makes it an ideal candidate for hydrogen storage. The difficulty with forming graphane is the need for atomic hydrogen. Techniques implemented by other groups involve in situ development of atomic hydrogen by hydrogen plasma or pumping explosive gases into the furnace. These techniques are costly, dangerous and lack controllability needed for possible device applications. In this paper, we demonstrate an alternative electrochemical means to generate atomic hydrogen, simplifying the synthesis and controllability of graphane formation. On-axis, semi-insulating, 6H-SiC substrates were used to form epitaxial graphene at ~1400°C in vacuum as starting material for graphane production. The ratio of the graphene Raman G-peak to the disorder D-peak was >10, showing the high quality of the starting material. Atomic hydrogen was generated using a home-built electrochemical setup with current applied though a 10% sulfuric acid solution, with a Pt wire and exposed graphene as the anode and cathode, respectively. With this setup, H+ ions are attracted to the exposed graphene. Cyclic voltammetry, with a Hg<SUB>2</SUB>SO<SUB>4</SUB> reference (0.67V vs. NHE) revealed that this conversion occurs at ~0.2V below the hydrogen evolution potential in water. Using the potential thus determined, conversion to graphane was performed until the conductivity of the graphane decreased to unmeasurable levels. This conversion was confirmed using graphane’s known Raman peaks. As expected, there was a sharp increase in the D peak as well as a red shift in the peak from 1340cm<SUP>-1</SUP> to 1330cm<SUP>-1</SUP>, likely caused by the formation of sp<SUP>3</SUP> bonds. Another peak at ~2930cm<SUP>-1</SUP> was an indication of C-H bonds. A fluorescence background, along with increased SiC substrate signal was also observed in the working area, suggesting the presence of a bandgap in the material. This conversion was distinguished from lattice damage by reversal back to graphene by annealing in argon for 4 hours at 1000°C. The Raman spectra of the area after reversal clearly shows disappearance of the C-H peak at ~2930cm<SUP>-1</SUP> showing desorption of hydrogen in the material. The D peak shifted back to pre-conversion state at 1340cm<SUP>-1</SUP> and fluorescence background was no longer present. While the D and G peaks shifted back to pre-conversion positions, their intensities show that there was some residual damage most likely caused by the strain of the hydrogenation. The conductivity of the graphane samples increased by ~100-1000x from pre-conversion graphene levels, without significant degradation of surface morphology, increase in roughness from 1.04nm to 1.62nm. Further optimization is expected to reduce this residual damage. |