Computational prosess design, optimization and growth of zinc oxide nanostructures on graphene by ultrasonic spray pyrolysis

Abstract

Computational process design, optimization, growth, characterization and computational analysis of zinc oxide (ZnO) nanostructures grown on graphene using zinc acetylacetonate (Zn(acac)2) in the presence of hydrogen by ultrasonic spray pyrolysis were performed systematically. The dissociation of Zn ions from vapour-phase Zn(acac)2 and its adsorption onto graphene oxide were studied using quantum mechanics approach involving the use of Density Functional Theory (DFT). The reaction energies were calculated, and the proposed reaction mechanism was well supported by a simulation of infrared properties. Next, Response Surface Methodology (RSM) was used to model and optimize the pyrolysis parameters by evaluating the nanostructure density, size and shape factor. The evolution of ZnO structures was well explained confirming that RSM is a reliable tool for the modelling and optimization of the pyrolysis parameters and prediction of nanostructure sizes and shapes. Finally, a computational analysis of the measured optical and charge transport properties of the grown nanostructures, i.e. Nanosphere Clusters (NSCs), Nanorods (NRs) and Nanowires (NWs) were developed. The calculated absorbance spectra based on the time-dependent DFT showed very close similarity with the measured behaviours. The atomic models and energy level diagrams were developed and discussed to explain the structural defects and band gap. As a conclusion it was found that the induced stress in the ZnO NSCs is the cause of gap narrowing between the energy levels. ZnO NWs and NRs showed homogeneous distribution of the Lowest Unoccupied Molecular Orbitals (LUMO) and Highest Occupied Molecular Orbitals (HOMO) orbitals all over the entire heterostructure which results to the reduction of the band gap. The calculated band gaps are confirmed to be in a good agreement with the experimental results. The electrical models and electrostatic potential maps were able to calculate the electron life time and to explain the mobility and diffusion behaviours of the grown nanostructures, respectively.