Figure S1. Calculated IR spectrum and intensities for Pd 4 H 2 /C 53 H 18 cluster model. Figure S2. a) Relaxed structures for the H 2 absorption in the gas phase Pd atom, Pd(H 2) x with x=1-4 cluster.

Synthesis of TiO2 anatase nanocrystals by hydrothermal methods often results in the formation of small quantities of brookite, which is difficult to eliminate by tuning the reaction conditions and is usually present in the final nanomaterial. The effect of this impurity on the Raman spectrum in anatase nanomaterials has not been fully explored. In this work, a study on the effect of reaction temperature on the position and line shape of the low-wavenumber Raman peak is presented. A comparison of the spectra of nanomaterials of pure anatase and anatase with brookite impurity (4–13 nm), synthesized by hydrothermal microwave heating, is made. It is shown that the low-wavenumber Raman peak (100–300 cm–1) for pure anatase nanocrystals is strongly dependent on the nanocrystal size and that the peak position is well described by the phonon confinement model (PCM). For anatase nanocrystals with 15% brookite impurity, the spectrum shows an asymmetric band, which is formed mainly by the contributions of the anatase Eg and brookite A1g modes, with the brookite B1g and B3g peaks further broadening the band. In addition, the PCM no longer describes the peak position. These results show that even a small amount of brookite can have a strong influence on the Raman spectra of anatase/brookite-impurity samples.

We report a detailed theoretical investigation of the structural and electronic properties of titanium-and nickel-doped defective graphene nanoplatelets, which are shown to be efficient materials for hydrogen storage. We found that H 2 bond cleavage is favored by Ti 4 -doped defective graphene nanoplatelets because of the strong interaction between the hydrogen 1s and titanium 3d levels that leads to the formation of metal hydrides, while H 2 adsorption on Ni 4 -doped defective graphene favors the formation of Kubas complexes as hydrogen 1s levels only interact with the nickel 4s levels. A comparison between adsorption energies, number of H 2 adsorbed molecules, and hydrogen gravimetric content shows that Ti 4 -doped graphene has a better performance for hydrogen storage with a notably high hydrogen gravimetric content of 3.4 wt %; than Ni 4 -doped graphene with a 10-fold lower gravimetric content of only 0.30 wt %. This observation can be explained by three factors: Ti is a lighter transition metal, it absorbs a larger amount H 2 per metallic atom, and it presents a planar geometry that increases the coverage of the graphene layer and makes possible that all atoms in the cluster participate in the H 2 adsorption. Our results support the hypothesis that a controlled introduction of defects in graphene followed by the anchoring of small metallic clusters is a feasible way to enhance the hydrogen gravimetric content of graphene nanoplatelets and to fine-tune hydrogen absorption energies to achieve a reversible operation at ambient temperature and moderates pressures, addressing one of the main challenges of a sustainable hydrogen-based economy.

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