Effects of reduced dimensionality on the properties of magnesium hydroxide and calcium hydroxide nanostructures

Metal hydroxides are a class of layered materials that contain two-dimensional metal hydroxide layers that can be isolated to form layered nanostructures. In this work, density functional theory (DFT) and self-consistent-charge density-functional tight-binding (SCC-DFTB) methods have been used to investigate the properties of magnesium hydroxide and calcium hydroxide nanostructures. The properties of single layer and multi layer structures with up to 10 metal hydroxide sheets and nanoparticles containing more than 2000 atoms have been calculated and compared with the bulk properties of these systems. The accuracy of the DFT methods employed and SCC-DFTB parameters developed in this study were validated against available experimental data. The results of the calculations indicate that significant differences exist between the properties of the nanostructures and the corresponding bulk values. In particular, the interlayer binding energies, electronic band gaps, and spectroscopic features are size-dependent and tend to converge to the bulk values as the size of the nanosystem is increased. The calculated binding energies and shear moduli show that all nanostructures are mechanically stable, in agreement with the experimental reports; although, their stabilities may be affected by the presence of intercalated species. Energy decomposition analyses reveal that the intralayer interactions in the investigated systems are predominantly electrostatic in nature, while the interlayer interactions are dominated by dispersion and polarization components. The results presented here quantify various properties of magnesium hydroxide and calcium hydroxide nanostructures, and could be used to understand the properties of other nanosystems containing layers of metal hydroxides in their structure.