The dissertation is devoted to the development of the method of obtaining powdered nanosized ZnO and ZnS crystals by the method of electrolytic synthesis and the study of their structural, optical, and morphological properties. The influence of the synthesis temperature on the obtained materials from the room temperature up to 100 °C, electrolyte concentrations, and current density in the process of their synthesis was studied in detail.
X-ray diffraction study has confirmed that the developed electrolytic method produces highly dispersed semiconductor material of ZnO, ZnS, and hydrozincite. Based on X-ray diffraction analysis (XRD), it was found that the synthesized ZnO has a hexagonal structure of wurtzite, ZnS is in cubic structure of sphalerite and a monoclinic structure was determined for hydrozincite.
Scanning electron microscopy (SEM) analysis of ZnO nanoparticle (NP) formed at different technological parameters confirmed the influence of synthesis parameters on NP morphology and size. In particular, it is shown that depending on the synthesis conditions the NPs can acquire the form of hexagonal prisms, needles, tetrapods, star-shaped (flower-like) structures. Decreasing the temperature of NP
synthesis leads to a decrease in NP size while increasing the growth temperature results in the prevalence of one of the forms of NP, in particular hexagonal prisms.
It was established by the Williamson – Hall method that tensile mechanical stresses with an average value of ~ 3 • 108 Pa are characteristic of ZnO NPs, and compressive stresses with an average value of ~ 3.5 • 108 Pa are characteristic of ZnS. From the absorption spectra of the colloidal solution of the synthesized zinc sulfide NPs, the value of the optical band gap was determined to be 3.72 eV. Based on this value the average NP size of ~ 4.6 nm was determined. The obtained value correlates with the sizes of ZnS NPs obtained from the analysis of XRD reflexes methods of Scherrer (D ~ 1.2 nm) and Williamson – Hall (D ~ 2.1 nm).
The effect of increasing the oxygen concentration in the electrolyte on the photoluminescence (PL) spectra of ZnO NCs is established. In particular, the intensity increase for the PL band with a maximum of about 440 nm is observed, which is explained by the increase in the number of oxygen-related defects and hydrozincite formation. It is shown that annealing of synthesized NC ZnO at 550 C leads to an increase of the PL intensity by order of magnitude, which is attributed due to thermal decomposition of hydrozincite into ZnO with a well-developed (porous) surface, as well as to narrowing of the UV PL band and its shift to values characteristic for ZnO, ~ 392-395 nm. The method of differential thermal analysis confirmed the changes in the phase composition of the obtained material during thermal annealing, in particular, the decomposition of hydrozincite into ZnO, carbon dioxide, and water was registered.
X-ray diffraction method showed that the use of sodium sulfide solution as an electrolyte in the process of electrolytic synthesis leads to the formation of a mixture of sulfide and zinc oxide with average sizes of 8.9 and 24.1 nm for ZnS and ZnO NPs, respectively. It was found that annealing of the powder with formed NPs leads to an increase in the size of the ZnS NPs to 20.9 nm, while the size of ZnO NPs decreases to 15.3 nm. Such changes are explained by the relaxation of mechanical compressive and tensile stresses in the initial ZnS and ZnO, respectively.
The Raman scattering spectra of synthesized NCs were analyzed, which also confirmed the formation of ZnO NCs and their high crystalline perfection after additional thermal annealing at 550° C.
As for the practical use of the results obtained in the dissertation, we can summarize the following. It is shown that the intensity of both UV and visible components of PL in ZnO NC can be significantly tuned by controlling the concentration of defects associated with oxygen. For this purpose, it is necessary to vary the intensity of air (oxygen) flux through the electrolyte in the course of synthesis. The correlations between the technological parameters of the synthesis and the physical properties of the formed NPs have been established, which can be used by technologists in the synthesis of materials with the required properties. A phosphor-based on ZnO NPs was obtained, the PL range of which covers almost the entire optical range
