Study of the structure, particle morphology and optical properties of mixed metal oxides
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Date
2017-01
Authors
Mokoena, Pulane
Journal Title
Journal ISSN
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Publisher
University of the Free State
Abstract
The structure, morphology and optical properties of metal oxides (ZnO, MgO and SrO), their composites (MgO-ZnO, SrO-ZnO) and systems with different x molar concentration values (0.2, 0.4, 0.5, 0.6, 0.7, 0.8) of MgxZn1-xO and SrxZn1-xO, were synthesized via solution combustion method at initial reaction temperature of 600 ˚C for 15 minutes. These properties of the synthesized nanostructures were investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High resolution transmission electron microscope (HR-TEM) and Photoluminescence (PL) spectroscopy.
The ZnO, MgO and SrO phosphors were successfully synthesized via solution combustion method and their crystallization was confirmed by XRD analysis. The ZnO powder crystallized in the hexagonal phase. The diffraction patterns of the ZnO samples became sharper and more intense when synthesis temperature was increased from 600 ˚C to 700 ˚C indicating improvement of crystallinity and an increase in crystallite sizes from 23.3 nm to 30.06 nm of the as-prepared undoped ZnO phosphor powder. The MgO powder had cubic crystal structure with Fm-3m space group and crystallized in rocksalt/sodium chloride (NaCl) type cubic structure and the SrO sample indicated the presence of three well-defined crystalline phases which are SrO, Sr(OH)2 and Sr(CO3)2, with Sr(OH)2 appearing as the most prominent phase.
With respect to the following systems: MgxZn1-xO and SrxZn1-,xO and their composites, their XRD patterns revealed the presence of two well-defined crystalline phases, namely MgO or SrO and ZnO, the most prominent phase being ZnO.
The SEM images of ZnO showed agglomeration of small particles and flower-like morphology. The HR-TEM images showed that the nanoparticles (NPs) were hexagonally shaped and aggregated into clusters. The SEM images of MgO showed spherical cube-like morphology with the appearance of closely-packed or attached particles in all the SEM micrographs. The HR-TEM images show that the NPs were cubic-spherically shaped and aggregated into clusters.
For the SrO sample small and coagulated particles of irregular shapes and different sizes were observed. Pores of different sizes were also observed from the solution combustion synthesis. This is due to the outgassing of the gaseous products, namely N2 and CO2, of this synthesis method. The HR-TEM images showed that the NPs were spherically shaped and aggregated into clusters. The selected area electron diffraction pattern confirmed the observation of a large number of nanoparticles and hence there were many spots within each ring.
In the case of the MgxZn1-xO system SEM observations revealed different kinds of particle morphologies such as pyramids clustered together to form flowers with spherical particles grouped together on the sides, triangles grouped together in the shape of a cauliflower, tetragonally shaped particles with some degree of faceting and for the SrxZn1-xO system, flower-like structures, oval-shaped particles and elongated rod-like structures.
The photoluminescence results of ZnO exhibits two characteristic peaks: one narrow in the ultraviolet (UV) region at 380 nm which comes from recombination of free excitons, and one broad in the visible region at 639 nm for ZnO synthesized at 600 ˚C and 626 nm for ZnO synthesized at 700 ˚C, which were attributed to electron mediated defect levels in the bandgap.
The MgO sample showed three PL emission peaks at approximately 419, 432 and 465 nm and a minute emission peak at 663 nm. The SrO PL spectrum exhibited UV and deep level emission peaks. In addition, there was a narrow peak in the UV region at 397 nm and a broad peak in the visible region at 750 nm.
With regards to the MgxZn1-xO system with x ranging from 0.2, 0.4, 0.5, 0.6 and 0.7, a red shift in the emission peaks from 602 to 610 nm was observed for the 0.2 and 0.4 molar concentrations while their luminescence intensity decreased. For a molar concentration 0.5 there was a blue shift in the emission peak from 610 to 551 nm together with luminescence quenching. From molar concentration 0.5 to 0.6 there was a blue shift in the emission peaks from 551 to 539 nm with a luminescence enhancement, but when the molar concentration was 0.7 there was a slight red shift in the emission peak located from 539 to 549 nm together with a luminescence enhancement. With regards to the MgO-ZnO composite sample there was only one broad emission peak at 559 nm in the visible region and luminescence intensity increased significantly.
For molar concentrations 0.2 and 0.4 there were emission peaks at 383, 540 and 760 nm. For molar concentration 0.5 there were emission peaks at 383, 514 and 760 nm. For molar concentration 0.6 there were emission peaks at 383 nm, minor humps at 413, 435 and 760 nm and a broad peak at 514 nm. For molar concentration 0.7 there were emission peaks at 383, 514 and 760 nm and for molar concentration 0.8 there were emission peaks at 383, 514 and 760 nm. The emission peak in the UV region (383 nm) was narrow and this was ascribed to recombination of free excitons, while the broad emission peaks at 514 and 540 nm were attributed to electron mediated defect levels in the bandgap.
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Keywords
Metal oxides, Solution combustion method, Structure, Morphology, Luminescence, Dissertation (M.Sc. (Physics))--University of the Free State, 2017