Okrushko O. Formation and dynamics of oxygen vacancies in CeO2-x, CeO2-ZrO2, CeO2-Y2O3 nanocrystals

Thesis for a scientific degree of candidate of science in physics and mathematics by specialty 01.04.10 - Physics of Semiconductors and Dielectrics. - Institute for Single Crystals, Kharkiv, 2019. The thesis is devoted to the study of the processes of formation of oxygen vacancies and luminescent centers with their participation (F-centers, Се3+-V-Ce3+ complexes) in CeO2-x, CeO2-ZrO2, and CeO2-Y2O3 nanocrystals. CeO2-x nanocrystals (nanoceria) are widely investigated nowadays due to their strong oxygen storage and antioxidant properties determined by high oxygen capacity and easy reduction of Ce4+ ions to Ce3+ ones. These properties of nanoceria are in turn determined by both concentration and location of oxygen vacancies in ceria lattice which content can be controlled by variation of treatment atmosphere, nanocrystal size and additional doping of nanoceria by cations with different size or valence. In the thesis for CeO2-x nanocrystals several types of luminescent centers were revealed. For nanocrystals treated in oxidation atmosphere two possible pathways of relaxation of the charge transfer excitation led to intrinsic charge transfer (CT) luminescence and luminescence of F0-centers. Treatment in reducing atmosphere leads to formation of additional luminescent centers (Ce3+-V-Ce3+ complexes). Shallow defects related to F+-centers present near edge of 4f0 band modify sufficiently the processes of excitation relaxation forming excitation traps that provide a series of trapping-retrapping acts during excitation lifetime. The content of oxygen vacancies increases significantly at incorporation of both Y3+ and Zr4+ ions into ceria lattice, and oxygen vacancies are involved in the formation of optical centers of the two types: Ce3+-V-Ce3+ complexes and F0 centers. The ratio between the optical centers of various types can be controlled by varying a concentration of impurity ions and atmosphere of high temperature treatment of nanocrystal. At low concentrations of impurity ions luminescent centers are formed preferably by F0 - centers, and at high concentrations of impurity ions by Ce3+-V-Ce3+ complexes. Ratio between intensities of 5D0-7F1 and 5D0-7F2 spectral lines of Eu3+ ions was used for determination of the content of oxygen vacancies and their location within ceria-zirconia nanocrystal. It was shown that while high-temperature treatment of 50 nm ceria nanocrystal in reducing atmosphere leads only to slight change of the content of oxygen vacancies which are formed preferably near its surface, incorporation of 20% of zirconium ions is manifested in almost tenfold increase of the content of oxygen vacancies as compared to CeO2-x nanocrystal, and these vacancies are formed within whole nanoparticle. The dynamics of nanoceria-oxidant interaction accompanied by reversible Ce3+ - Ce4+ transitions of cerium ions was studied for nanoceria water colloidal solutions using spectroscopic techniques. Interaction of nanoceria with hydrogen peroxide (HP) leads to Ce3+ - Ce4+ oxidation accompanied by quenching of Ce3+ luminescence of nanoceria, and recovery of initial Ce3+ luminescence intensity occurred with sufficient time delay (up to few days). The decisive role of oxygen diffusion within nanoceria volume both on the stages of oxidation of Ce3+ ions (Ce3+ -Ce4+) and their subsequent recovery (Ce4+ - Ce3+) slows down these processes. Both size reduction and temperature increase facilitate recovery of initial Ce3+ content in nanoceria. Luminescence of Ce3+ ions in nanoceria was used to provide a new application of nanoceria for detection and quantification of hydrogen peroxide content in water solutions. HP sensing was based on the reversible shift of Ce3+- Ce4+ balance in ceria nanoparticles as a result of nanoceria-oxidant interaction. Intensity of 5d-4f luminescence of Ce3+ ions in nanoceria decreases at increase of hydrogen peroxide concentration in water solutions and, the dependence of Ce3+ luminescence intensity on HP concentration is linear in semilogarithmic coordinates. The intensity of Ce3+ luminescence 10 min after HP addition is independent on the temperature of the solution up to 52 C. In this way, nanoceria can be applied as an efficient hydrogen peroxide sensor in water solutions with temperature stable response. Key words: nanocrystals, oxygen vacancies, cerium oxide, luminescence.