REPOSITORY > RESULTS

Abstract

In the view of designing new nanostructured materials and tailoring their functional properties, understanding the nucleation and crystal growth mechanisms is becoming increasingly important.
Nowadays, one of the most addressed scientific and social problems are those facing energy shortage and global warming. People started to realize the value and importance of a clean and healthy environment. To become independent from fossil fuels and to achieve the EU goal related to zero emissions of CO2 by 2050, intensive efforts are going on in searching for alternative energy sources. One of the sustainable approaches is to use abundant free energy from the environment (sun, wind, waste vibrations, etc.) to produce renewable fuels through various catalytic or electrochemical processes.
An enormous number of studies have been done on the topic of photocatalytic and photoelectrochemical hydrogen evolution from water splitting in the past years. Still, the hydrogen fuel is not in everyday use yet. Mainly due to the low efficiency of photocatalytic reactions and insufficient solar energy utilization. The main challenges are related to photon absorption, photoinduced charge carrier separation, and their transport to the surface, where redox reactions take place.
Size, shape, and type of exposed facets can significantly affect a material’s electrical, optical, and photocatalytic properties. Compared to large 3D particles, 2D structures with appropriate low thicknesses are more advantageous for lowering photoinduced charge carriers’ recombination. Additionally, the formation of heterostructures is known to be beneficial for the improvement of photocatalytic performance in many aspects, such as enhancing charge carrier separation, widening of utilized spectral range, and increasing stability. The photocatalysts in my thesis were designed including the above-mentioned considerations.
In my research, I focused on the preparation of H2-evolution photocatalysts based on heterostructural SrTiO3/Bi4Ti3O12 nanoplatelets and I studied the mechanism for their formation. The SrTiO3 and Bi4Ti3O12 were selected based on similar structural elements that allow epitaxial growth and enable the formation of the heterostructure or pseudomorphic transformation. Additionally, Bi4Ti3O12 spontaneously grows in a two-dimensional shape, which is not typical for SrTiO3. Both materials also have favorable relative positions of the conduction and valence bands for the formation of hydrogen and oxygen, respectively.
My studies were performed in several steps. In the first step, the process of Bi4Ti3O12 template preparation was optimized. In the second step, Bi4Ti3O12 platelets were used as a template for the topochemical conversion into different SrTiO3/Bi4Ti3O12 heterostructures and pure SrTiO3. An in-depth study of the transformation mechanism was performed through atomic scale analysis of the platelets at different transformation stages and Rietveld structural refinements of the SrTiO3 and Bi4Ti3O12 phases. Based on the latter we examined the misfits in relevant orientation relationships and at actual temperatures.
A combination of various techniques (X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy) enabled me to perform very thorough research on how to steer the topochemical conversion by balancing the lattice mismatch, template quality, and supersaturation as well as how to tailor the functional properties of the product platelets by varying experimental conditions. Moreover, the principles inferred from the study of the Bi4Ti3O12-to-SrTiO3 conversion were translated for the transformation from Bi4Ti3O12 to BaTiO3 and CaTiO3. Finally, the photocatalytic hydrogen evolution of prepared heterostructural SrTiO3/Bi4Ti3O12 and SrTiO3 platelets was evaluated.
This comprehensive investigation of the topochemical conversion mechanism from Bi4Ti3O12 to SrTiO3 with the intermediate formation of 2D epitaxial heterostructures not only contributes to a refined understanding of this system but also paves the way for the engineering of other new intricate epitaxial heterostructures in the future. Furthermore, insights gleaned from this dissertation offer a fertile ground for the development of advanced functional materials, characterized by heightened stability, efficiency, and eco-friendliness. These materials hold promise in diverse catalytic realms, spanning hydrogen evolution, CO2 reduction as well as pollutant decompositions, among others.