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Register a new userThe Influence of Graphene Curvature on Hydrogen Adsorption: Towards Hydrogen Storage Devices
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The ability of atomic hydrogen to chemisorb on graphene makes the latter a promising material for hydrogen storage. Based on scanning tunneling microscopy techniques, we report on site-selective adsorption of atomic hydrogen on convexly curved regions of monolayer graphene grown on SiC(0001). This system exhibits an intrinsic curvature owing to the interaction with the substrate. We show that at low coverage hydrogen is found on convex areas of the graphene lattice. No hydrogen is detected on concave regions. These findings are in agreement with theoretical models which suggest that both binding energy and adsorption barrier can be tuned by controlling the local curvature of the graphene lattice. This curvature-dependence combined with the known graphene flexibility may be exploited for storage and controlled release of hydrogen at room temperature making it a valuable candidate for the implementation of hydrogen-storage devices.
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We report on hydrogen adsorption and desorption on titanium-covered graphene in order to test theoretical proposals to use of graphene functionalized with metal atoms for hydrogen storage. At room temperature titanium islands grow with an average diameter of about 10 nm. Samples were then loaded with hydrogen, and its desorption kinetics was studied by thermal desorption spectroscopy. We observe the desorption of hydrogen in the temperature range between 400K and 700 K. Our results demonstrate the stability of hydrogen binding at room temperature and show that hydrogen desorbs at moderate temperatures in line with what required for practical hydrogen-storage applications.
Hydrogen adsorption on graphene can be increased by functionalization with Ti. This requires dispersing Ti islands on graphene as small and dense as possible, in order to increase the number of hydrogen adsorption sites per Ti atom. In this report, we investigate the morphology of Ti on nanocrystalline graphene and its hydrogen adsorption by scanning tunneling microscopy and thermal desorption spectroscopy, and compare the results with equivalent measurements on single-crystalline graphene. Nanocrystalline graphene consists of extremely small crystal grains of < 5 nm size. Ti atoms preferentially adsorb at the grain boundaries of nanocrystalline graphene and form smaller and denser islands compared to single-crystalline graphene. Surprisingly, however, hydrogen adsorbs less to Ti on nanocrystalline graphene than to Ti on single-crystalline graphene. In particular, hydrogen hardly chemisorbs to 1 ML of Ti on nanocrystalline graphene. This may be attributed to strong bonds between Ti and defects located along the grain boundaries in nanocrystalline graphene. This mechanism might apply to other metals, as well, and therefore our results suggest that when functionalizing graphene by metal atoms for the purpose of hydrogen storage or other chemical reactions, it is important to consider not only the morphology of the resulting surface, but also the influence of graphene on the electronic states of the metal.
Hydrogen adsorption on graphene-supported metal clusters has brought much controversy due to the complex nature of the bonding between hydrogen and metal clusters. The bond types of hydrogen and graphene-supported Ti clusters are experimentally and theoretically investigated. Transmission electron microscopy shows that Ti clusters of nanometer-size are formed on graphene. Thermal desorption spectroscopy captures three hydrogen desorption peaks from hydrogenated graphene-supported Ti clusters. First principle calculations also found three types of interaction: Two types of bonds with different partial ionic character and physisorption. The physical origin for this rests on the charge state of the Ti clusters: when Ti clusters are neutral, H2 is dissociated, and H forms bonds with the Ti cluster. On the other hand, H2 is adsorbed in molecular form on positively charged Ti clusters, resulting in physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on graphene-supported Ti clusters.
We have used scanning tunneling microscopy (STM) to investigate two types of hydrogen defect structures on monolayer graphene supported by hexagonal boron nitride (h-BN) in a gated field-effect transistor configuration. The first H-defect type is created by bombarding graphene with 1-keV ionized hydrogen and is identified as two hydrogen atoms bonded to a graphene vacancy via comparison of experimental data to first-principles calculations. The second type of H defect is identified as dimerized hydrogen and is created by depositing atomic hydrogen having only thermal energy onto a graphene surface. Scanning tunneling spectroscopy (STS) measurements reveal that hydrogen dimers formed in this way open a new elastic channel in the tunneling conductance between an STM tip and graphene.
In order to realize applications of hydrogen-adsorbed graphene, a main issue is how to control hydrogen adsorption/desorption at room temperature. In this study, we demonstrate the possibility to tune hydrogen adsorption on graphene by applying a gate voltage. The influence of the gate voltage on graphene and its hydrogen adsorption properties was investigated by electrical transport measurements, scanning tunneling microscopy, and density functional theory calculations. We show that more hydrogen adsorbs on graphene with negative gate voltage (p-type doping), compared to that without gate voltage or positive gate voltage (n-type doping). Theoretical calculations explain the gate voltage dependence of hydrogen adsorption as modifications of the adsorption energy and diffusion barrier of hydrogen on graphene by charge doping.
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