A first-principles study of the thermodynamic and electronic properties of Mg and MgH2 nanowires

Abstract

In this article, we studied the thermodynamic and electronic properties of Mg and MgH₂ nanowires with different diameters, and elucidated why MgH₂ nanowires are good hydrogen storage materials through first-principles calculations. Previous experiments have shown that the orientation relationship between Mg and MgH₂ nanowires is the Mg[0001] direction parallel to the MgH₂[110] direction. In our calculations, Mg nanowires oriented along the [0001] direction and MgH₂ nanowires oriented along the [110] direction were built from bulk Mg and MgH₂ crystals, respectively. We found that as the diameters of Mg and MgH₂ nanowires decrease, Mg and MgH₂ nanowires become more unstable, and the hydrogen desorption energies and temperatures of MgH₂ nanowires decrease. That is, the thinner the MgH₂ nanowires get, the more dramatically hydrogen desorption temperatures (T_d) will decrease. Meanwhile, we also found that when the diameters of MgH₂ nanowires are larger than 1.94 nm, the T_d almost maintain the same value at about 440 K, only about 40 K lower than that of bulk MgH₂ crystal; if the diameters are less than 1.94 nm, the T_d reduce very quickly. In particular, compared with bulk MgH₂ crystal, the T_d of the thinnest MgH₂ nanowire with a diameter of 0.63 nm can be reduced by 164 K. In addition, the electronic structure calculations showed that Mg nanowires are metals, while MgH₂ nanowires are semiconductors. In particular, our results showed that the electronic structures of MgH₂ nanowires are influenced by the surface effect and quantum size effect. That is to say, the band gaps of MgH₂ nanowires are controlled by surface electronic states and the size of MgH₂ nanowires.