  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.
