Exchange Functionals and Basis Set Comparisons for Theoretical Studies of ZnO Nonoclusters

Catalysts made of nano-scaled metal oxide clusters can push the limits of chemical reactions in the manufacture of paints, cosmetics, and pharmaceuticals. The ZnO clusters can also act as semiconductors with a wide band gap of 3.4 eV at 300 K, and are prospective phoocatalysts in many reactions including H2 production in water splitting reactions. In this project, we studied the structural (geometry) and electronic properties (vertical detachment energy and electron affinity) of ZnO monomers and dimers that form model ZnO clusters, using density functional theory (DFT) with many different exchange functionals and 29 basis sets to optimize their choice. We compared the singlet-triplet energy gaps of small ZnO clusters to find the optimal ZnO cluster size and the best theoretical method to investigate their photocatalytic water splitting activity. Our results show that B3LYP/DGDZVP2 level of exchange functional/basis set theory is the most efficient and fastest of the ones considered. Comparison of the singlet-triplet energy gaps shows that the trimer (ZnO)3, with an energy gap of 58.66 k cal/mol, is approximately equal to the energy of a visible photon at 555 nm and a HOMO-LUMO gap of 4.4 eV, and is the best choice amongst the (ZnO)n clusters of different sizes when the number of monomers n in the clusters ranges from 1 to 6. We used the Gaussian16 software package for all the calculations.