Morphology of Ti on Monolayer Nanocrystalline Graphene and its Unexpectedly Low Hydrogen Adsorption

Abstract: 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.