Pylypko V. Chemical interactions in the system Mn2+ - S2- - stabilizer-solvent as prerequisites for the synthesis of semiconductor nanomaterials

This dissertation is devoted to the study of chemical interaction processes in the Mn2+ - S2- - stabilizer-solvent system and their influence on the optical properties, size, and morphology of the final product of the semiconductor nanomaterial MnS, the possibility of obtaining NP sulfur in the L-cysteine - sodium citrate - Na2S system and the regularities of chemical interaction in it. The first chapter analyzes the literature on the topic of the dissertation. The features of semiconductor MnS as a promising nanomaterial for use in various fields of science and technology are described. A review of synthesis methods using the selected stabilizing agents L-cysteine (L-cys), sodium citrate (citrate ion), and thioglycolic acid (TGA) is carried out. Literary sources on the preparation of non-metallic sulfur nanoparticles have been reviewed. The possibility of obtaining sulfur nanoparticles of different sizes and properties depending on the synthesis method and the selected precursors is noted. The prospect of studying the selected object and the possibilities for wide application are mentioned. The second chapter describes the methods of experimental studies used in this thesis for the synthesis of MnS NPs in aqueous and ethylene glycol solvents. The synthesis methods for studying the processes of complexation between the Mn2+ cation and the selected ligands L-cysteine, TGA, and citrate-ion are presented. Methods for the synthesis of sulfur nanoparticles in aqueous solution and the study of the influence of various factors on their size, shape, and photoluminescent properties are described. The third chapter is devoted to the description and discussion of the results obtained during the study of chemical interaction in the MnCl2 - Na2S - stabilizer - solvent system. The processes occurring in the system and the characteristic properties of the resulting products are described and analyzed. The influence of the precursor content and pH of the medium on the nature of the interaction in the system MnCl2 - Na2S - stabilizer (L-cysteine, thioglycolic acid, sodium citrate) was studied. The conditions for the formation of MnS NPs stabilized by their molecules were established and the limit of the stabilizing effect of L-cysteine was estimated with increasing content of crystal-forming ions. The possibility of forming reaction products of different natures depending on the pH of the medium was evaluated. It was found that an alkaline environment in the synthesis of L-cysteine-stabilized MnS NPs increases the photoluminescence intensity of their colloidal solutions. The composition-property relationship for aqueous solutions of mixtures of manganese(II) ions and potential ligands of L-cysteine, thioglycolic acid, and citrate-ions at pH values close to irrelevant ones (7.6 ÷ 5.5) was studied. It was found that an increase in the coordination number of the studied stabilizing agents from 2 to 6 does not affect the appearance of the spectral absorption curves at the ratio [Mn2+]: [S2-] = 1:1, but at the ratio [Mn2+]: [S2-] = 1:2, a noticeable spectral shift to the long-wavelength region was observed, which confirms the dominant effect of the S2- anion concentration on the nucleation rate and growth of MnS NPs. At the stage of formation of the [Mn(Ligand)n]n+ complex at pH ≥ 9 with SH-containing stabilizers, in contrast to citrates, new peaks in the absorption spectra at 320 nm (for L-cys) and 295 nm (for TGA) are observed. It has been found that the synthesis of MnS nanoparticles in ethylene glycol at temperatures above 353 K promotes the formation of MnS NPs with a small size dispersion and high PL properties. Post-synthesis heat treatment at temperatures ≥ 373 K for MnS NPs synthesized at lower temperatures can have a similar effect. Chapter 4 is devoted to the description and discussion of the results obtained during the synthesis of sulfur nanoparticles. It is shown that, depending on the composition of the system, the chemical interaction between the components occurs in different ways, and only the presence of all components ensures the formation of luminescent sulfur NPs. The synthesized NP sulfur is stable for at least seven months and has good PL properties with controlled emission depending on the excitation energy. An increase in the heat treatment time to 8 h contributes to a significant increase in the PL intensity of the obtained nanoparticles. The composition of the system affects the optical properties of the obtained sulfur NPs. An increase in the concentration of L-cysteine promotes an increase in the absorption intensity at a wavelength of 300 nm, while an increase in the concentration of sodium sulfide promotes the appearance of absorption maxima in the long-wavelength region of the spectrum in the range of 300-700 nm. Providing the system with a sufficient oxygen concentration is a prerequisite for obtaining low-frequency sulfur with good PL properties.