Authors: Klinton Davis
Source: NC Digital Online Collection of Knowledge and Scholarship (The University of North Carolina at Greensboro) (article, 2021-01-01)
Field: Materials Science — Materials Chemistry

Sol-gel synthesis and band gap engineering of Zinc Oxide nanostructures Zinc Oxide (ZnO) belongs to the (II-VI) wurtzite semiconductor that has found practical applications across multiple disciplines. One of its first main uses was in the application of ultraviolet protection (sunscreen) since it has a bandgap that absorbs in the UV. In more recent years, researchers have been manipulating ZnO properties, specifically its bandgap, to use it as an alternative for applications that require hazardous and toxic materials. This dissertation focuses on developing sol-gel based synthesis methods to tailor electronic properties of ZnO. The sol-gel methods developed in this dissertation provides understanding of how size, shape, and composition of ZnO can be controlled by nucleation growth followed by solvent-driven self-assembly processes. Through this modified sol-gel synthesis approach, the use of dopant materials can be introduced using mostly first row transition metals and showed morphology changes as well as mixed crystal structures, leading to changes in optical and electronic properties. From the first project, the modified sol-gel synthesis made use of six organic solvents showed that solvent choice plays a role in the morphology of zinc oxide but also affects its electronic structure in terms of optical bandgap. The solvents act as a surfactant and through the promotion of Oriented Attachment allowing nanorods, nanospheres, and nanoslates to be formed. The optical bandgap showed that there was a wide range between the solvents but after annealing showed similar values of 3.37 eV showing that the solvents were on the surface of the nanostructures as a surfactant. TEM and X-ray diffraction showed all morphologies were highly crystalline except DMSO having a wide peak at the [101] due to the grain size of the structure. Overall, DMF is the preferred solvent due to its bandgap being similar to the others, high crystallinity, and solubility in the system. This shows that solvent choice can play a crucial role for modifying morphology and optical bandgap of ZnO. The modified sol-gel synthesis was further tested with the incorporation of dopants into the system at different concentrations. Choosing DMF as the solvent due to the results of Aim 1, six dopants were tested having similar ionic radii or valency. The dopants were incorporated with the dopant metal ions ranging from 1-5 % across multiply trials to dope the ZnO nanorods. Results showed that morphology was not maintained by all but two, which were Copper and Erbium. All others had either changed morphology or had mixed morphology showing incompatibility for maintaining ZnO nanorod formation. It was also observed that Copper at higher concentrations of dopant was able to produce hierarchical structures leading to the idea that solvent choice with a dopant precursor could lead to more exotic structures in the future. Optical bandgap measurements were made and while all gave a blue shift from the original ZnO nanorods spectra, all were within tenths of an eV leading to no immediate favorite. X-ray diffraction was done and showed that the largest peak shifts were from copper and erbium, both of which formed nanorods. All other dopants caused a shift but was either minimal or had added peaks in the system further showing that the multiple morphologies were separate from the ZnO crystal lattice. In short, using the modified sol-gel method in the presence of DMF show promise for certain dopant metals. The final project took the modified sol-gel synthesis and furthered the process by incorporating it as a core-shell system. Taking ZnO nanorods made in DMF, they were suspended in different concentration solutions of cobalt(II)nitrate under heat. The use of SEM showed nanoparticle formation occurred on the surface of the ZnO nanorods. Verification of the particles was done with (S)TEM/EDS showing the lattice spacing and elemental composition to be cobalt oxide. Using X-ray photoelectron spectroscopy, the cobalt oxide was determined to be in the Co2+ state. Band gap ; Materials science ; Wurtzite crystal structure ; Sol-gel ; Dopant ; Nanotechnology ; Nanorod ; Nucleation ; Zinc ; Nanostructure ; Chemical engineering ; Doping ; Chemistry ; Optoelectronics ; Organic chemistry ; Metallurgy ZnO doping and properties ; Quantum Dots Synthesis And Properties ; Gas Sensing Nanomaterials and Sensors