Zinc oxide (ZnO) is a material with great thermal and chemical stability and low toxicity, which makes it very suitable for use in technology. The optimal combination of optical, electrical and microstructural properties, a wide energy gap (3.37 eV) and a large exciton binding free energy (60 meV) make ZnO an attractive candidate for the production of optoelectronic devices that work in the blue and ultraviolet regions of the spectrum. The development of synthesis methods of ZnO nanoparticles of controlled size and morphology is of great importance for their technological application. In order to improve the desired properties of a certain material and its functionality, the so-called bottleneck of any research is finding the optimal combination of mechanical, physical and chemical properties of materials. In the field of ZnO nanoparticle research, there is a significant number of scientific works in which the focus is on the preparation process of this material, with the aim of achieving optimal control of the size, shape and dispersibility of the particles. However, there is a very small number of studies so far that explain in detail the coupling of microstructural parameters (particle size, stress, deformation) under non-standard pressure and temperature conditions, which are one of the decisive parameters when it comes to the direct application of this material. From a chemical point of view, a simple compound, ZnO, is morphologically very rich, as evidenced by an extremely large number of scientific studies aimed at understanding the connection between the choice of ZnO preparation method and structural/microstructural parameters (particle size, stress, and elastic deformation). An important role in the synthesis of ZnO nanoparticles, and especially in the clarification of the coupling between the size and the geometric shape of the ZnO particles, in addition to the preparation method itself, is played by other parameters of the synthesis, such as the initial zinc salt, solvent, pH-value, temperature, and ageing time. Carrying out X-ray diffraction experiments in non-standard (ambient) conditions (high pressure, high temperature) allows a detailed insight into the correlation between the structural (phase transition mechanism) and mechanical properties of the material, since the mentioned features are largely related to the appearance of stress and deformation due to a change in the structure of the material.

The scientific goal of the proposed project is to use the synchrotron X-ray radiation method at high pressure in situ to study the direct influence of pressure changes on microstructural (particle size, stress, deformation) and structural parameters resulting from the expected phase transition of ZnO samples prepared by different synthesis methods: (a) solvothermal, using of zinc acetylacetonate monohydrate, Zn(C5H7O2)2∙H2O, in the presence of an organic additive, triethanolamine (TEA), and/or different solvents at different temperatures, (b) thermal decomposition of Zn(C5H7O2)2∙H2O with direct heating at temperatures higher than 200 °C. The samples will additionally be characterized by Raman spectroscopy at high pressure. It is to be expected that systematic research, combined with advanced structural and microstructural characterization of the obtained ZnO structures (hexagonal and cubic) and theoretical studies, will enable a deeper insight into the mechanism of surface interaction between ZnO and solvent/TEA molecules. It is assumed that the surface energy with the influence of surface stabilizing groups plays a key role in determining the stability during the phase transition from the hexagonal to the cubic structure of nanocrystalline ZnO (and vice versa) under high pressure conditions. In situ X-ray powder diffraction measurements with increasing/decreasing pressure will be performed at the BL12B2 measurement station (synchrotron SPring 8, Hoygo, Japan), while high-pressure Raman spectroscopy of samples will be performed at the Centre for High Pressure Science and Technology Advanced Research , HPSTAR (Beijing, N.R. China).