The Evolution of Geometric and Electronic Structures for the Hydrogen Storage on Small Tin (n = 2-7) Clusters

摘要

We perform first principles density functional theory calculations to investigate the geometric and electronic structures of Tin-mH2 (n=2-7, and m=1 -22). By optimizing geometric structures, we obtain the saturated configurations for hydrogen storage on small Tin (n=2-7) clusters. Interestingly, we find that with an increase in the size of the Tin cluster, the effective space for each titanium atom to adsorb hydrogen molecules decreases, as does the maximum amount of hydrogen molecules adsorbed on each titanium atom. When the size of the Tin cluster goes beyond n=5, the maximum number of hydrogen molecules adsorbed on each Ti atom keeps a constant of 3. For Ti7-mH2 clusters, the average Mulliken charges increase at first and decrease afterward, the binding energy EHb per atom of H increases when the hydrogen molecule number m changes from 1 to 3, and then, it shows a slow decrease with m increasing from 3 to 21. Furthermore, we suggest/propose that the hybridization of atomic orbitals in different atoms could be used to estimate the type of the bonds between the different atoms in clusters. As the TinmH2 clusters get bigger, due to the charge density redistribution, the interaction between titanium atoms becomes weaker and the bond length of Ti-Ti increases gradually. Meanwhile, the H-H bonds are elongated or even broken. The geometry of the host cluster is distorted from D5h into C3v when 21 H2 molecules are chemisorbed.