The demanding task of tuning the functional properties of a material is governed by the idea of finding the correct combination of its mechanical, physical, and chemical properties in order to optimise the desired functionalities and operating conditions of the final product. Many efforts have been made to prepare the zinc oxide (ZnO) nanoparticles,, providing excellent control over crystallite size, shape, and dispersibility, but still, the relationship among microstructural parameters (crystallite size and strain) under non-ambient conditions needs to be more thoroughly explored and clarified.

Zinc oxide is a multifunctional material with useful optical, electrical, and microstructural properties. Morphologically, ZnO is a very rich compound that possesses thermal and chemical stability, which allows a wide area of application as a new low-priced material. Namely, the particle size and crystal morphology play important roles in these applications, which have driven researchers to focus on the synthesis of nanocrystalline ZnO in recent years. The starting zinc compound, synthesis procedure, chemical composition of the solvent, nature of the precipitating agent, pH, temperature, and time of ageing, all play an important role in understanding the mechanism of ZnO nanoparticles formation, with the pivotal emphasis put on the size and geometrical shape of ZnO particles. Particularly, in several recent papers Šarić et al. reported a strong influence of the synthesis route on the formation of ZnO particles, their size, and their geometrical shape. More precisely, Šarić et al. proposed the nucleation and growth mechanisms of ZnO nanoparticles based on the results obtained from quantum chemical calculations at the density functional theory. It was established that the kinetics of nucleation and ZnO nanoparticle growth in the presence of particles growth modifiers (i.e triethanolamine, TEA) and alcoholic solvent of different size and polarity are strongly dependent on the properties of alcohol. The observed results pointed to a different formation mechanism of ZnO-TEA intermediates, which initiated the nucleation processes of ZnO nanoparticles in the presence of alcohols of different size and polarity and at different molar ratio [TEA]/[Zn(acac)2·H2O]. Also, the solid-state thermal decomposition of [Zn(acac)2·H2O] was already utilized in the production of ZnO nanoparticles [4]. Notwithstanding the simplicity of this method, the mechanism of thermal decomposition of zinc acetylacetonate obtained by the sublimation process of [Zn(acac)2·H2O] powder is complex. This method can be developed to prepare various ZnO morphologies, which also have a significant influence on the optical properties of ZnO. Furthermore, despite the applied annealing treatment, one cannot completely exclude the presence of residual organic groups from zinc acetate on the surface of the ZnO samples - the role of the surface and especially surface passivating groups is expected to considerably affect the nanocrystallinity and the kinetic stabilisation of the metastable rs-ZnO phase.

Powder X-ray diffraction (PXRD) is one of the most straightforward material characterization tools. It is often the method of choice as it enables the study of materials under non-ambient pressure, temperature, or atmospheric conditions. Powder X-ray diffraction using diamond anvil cells (DACs) allows the extraction of information on p-T phase stability, bulk elasticity, as well as plastic deformation (e.g deformation mechanisms) and material strength. The possibility of performing the non-ambient PXRD experiments, enables a deeper insight into the correlation between structural features (e.g phase transition mechanism) and mechanical properties of materials, as both are largely associated with transformation induced stress-strain relations. Recently, Yang et al. using in situ synchrotron X-ray diffraction measurements under quasi-hydrostatic conditions, reported unusual compression behaviours in ZnO during the w-to-rs phase transition, indicating the internal elastic strain on selected lattice planes. Namely, combined with the established phase transition model of ZnO, the observed elastic strain is believed to be caused by the lattice misfit between the w-ZnO phase and the rs-ZnO phase during phase transition. In particular, the internal elastic strain was supposed to give rise to the strength softening of each phase of ZnO during the phase transition observed at non-hydrostatic conditions. The above results supplied a strong evidence for the existence of phase transition induced elastic lattice strain under pressure and indicated that such strain could have a significant effect on the physical property of material.

Many efforts have been made to prepare the ZnO nanoparticles, which provide excellent control over crystallite size, shape, and dispersibility, but still, the relationship among microstructural parameters (crystallite size and strain) under high pressure conditions and their photocatalytic activity need to be more thoroughly explored and systematically clarified.

By integrating synthetic chemistry and materials physics, we will target on the following research objectives:

(i) design ZnO nanoparticles via tuned synthetic procedures: (a) using zinc acetylacetonate monohydrate [Zn(acac)2·H2O] in the presence of triethanolamine (TEA) and various alcoholic solvents, ethanol or octanol, at 170 °C, (b) by thermal decomposition of [Zn(acac)2·H2O] (direct heating at ≥200°C).

(ii) improve the predictability of aggregation behavior of ZnO nanoparticles of different size and morphology to ZnO microspheres with well-defined morphologies and physico-chemical properties (optical, structural) for the adsorption and

removal of various environmental pollutants.

(iii) deliver ZnO nanoparticles having the optimal photocatalytic performance estimated by the photodegradation of Rhodamine B, methyl orange and methylen blue solutions as model organic pollutants.

(iv) develop the ZnO nanoparticles as functional materials with enhanced functionalities through the synergy of unveiled structural and microstructural features at non-ambent conditions (i.e high pressure and high temperature).