Structures and Stabilities of (CaO)n Nanoclusters
journal contributionposted on 2017-09-26, 00:00 authored by Mingyang Chen, K. Sahan Thanthiriwatte, David A. Dixon
The global energy minima structures for (CaO)n for n ≤ 40 were predicted using density functional theory. The cubic structures are found to be the lowest energy isomers for most (CaO)n, n ≥ 4. A fragment-based structure-energy relationship model gave an excellent fit for the calculated total energy. Based on the fitting results, the bulk limit for the normalized clustering energy for (CaO)n particles was predicted to be 157.8 kcal/mol for the enthalpy at 298 K, in good agreement with the experimental/computational bulk value of 156.5 kcal/mol. A (CaO)n nanoparticle with a size of 10 nm is predicted to have a CaO binding energy close to that of the bulk crystal. The infinite chain limit for normalized clustering energy for various one-dimensional (1-D) cubic nanoparticle series was also obtained. The surface energy densities were predicted to be 62 kcal/mol per CaO for the 3-coordinate corner fragment, 30 kcal/mol per CaO for the 4-coordinate edge fragment, and 11 kcal/mol per CaO for the 5-coordinate face fragment. On the basis of the values of the parity sum for the atom counts for the cube’s edges in three dimensions, cubic (CaO)n nanoclusters can be classified into three types with different geometries. Although no significant difference in stability was found for different types of cubic (CaO)n, several electronic properties of the cubic (CaO)n are related to the parity sum at small n. The type-2 (CaO)n clusters and ultrasmall particles in the shape of the odd × odd × even cube, with a parity sum of 2, exhibit unique electronic properties. The type-2 3 × 3 × m 1-D nanoparticle series has the lowest HOMO–LUMO excitation energy among all of the 1-D nanoparticle series at all particle sizes: 2.51 eV for the 3 × 3 × 4 cube and 0.70 eV for the 3 × 3 × 18 cube. The patterns for the variation of Eg during the 1-D layer-wise growth of (CaO)n were analyzed.