Optically Detected Effect of Size on the Oxygen Vacancies Concentration in Cerium Oxide Nanocrystals

In this work effect of the size on the oxygen vacancies concentration in cerium oxide nanocrystals have been investigated by means of luminescence spectroscopy techniques. For determination of changes of oxygen stoichiometry intensity of 5d-4f luminescence of Ce3+ ions were used. It was shown that for CeO2 nanocrystals decrease of the size from 50 nm to 2 nm manifests itself in 8 times increase of the band intensity associated with vacancy-stabilized Ce3+ ions. The same effects have been observed at atmosphere variation from oxidizing to reducing and are connected with significant increase of concentration of oxygen vacancies. Obtained results allow to determine that decrease of the size stimulate formation of oxygen vacancies in cerium oxide nanocrystals

Spectroscopically Detected Formation of Oxygen Vacancies in Nano-Crystalline CeO2 – x

In this work the peculiarities of oxygen vacancies formation in cerium oxide nanoparticles for different external influences have been investigated by spectroscopic methods. The features of oxygen vacancies and therefore non-stoichiometric cerium oxide formation in CeO2 nanocrystals depending on the atmosphere of high temperature treatment were investigated. Stimulation of oxygen vacancies formation in reducing and neutral atmospheres was revealed. Occurrence of two different luminescence centers (viz. the charge-transfer complexes formed by Ce4 + and O2 – ions, and Ce3 + ions stabilized by vacancies) after cerium oxide nanoparticles annealing in a neutral atmosphere has been observed. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3094

Optically Detected Effect of Size on the Oxygen Vacancies Concentration in Cerium Oxide Nanocrystals

In this work effect of the size on the oxygen vacancies concentration in cerium oxide nanocrystals have been investigated by means of luminescence spectroscopy techniques. For determination of changes of oxygen stoichiometry intensity of 5d-4f luminescence of Ce3+ ions were used. It was shown that for CeO2 nanocrystals decrease of the size from 50 nm to 2 nm manifests itself in 8 times increase of the band intensity associated with vacancy-stabilized Ce3+ ions. The same effects have been observed at atmosphere variation from oxidizing to reducing and are connected with significant increase of concentration of oxygen vacancies. Obtained results allow to determine that decrease of the size stimulate formation of oxygen vacancies in cerium oxide nanocrystals

Structural rearrangement and change of cerium valence in cerium dioxide (CeO₂) nanocrystals

For CeO₂ nanocrystals a large number of cerium ions undergo Ce⁴⁺→ Ce³⁺ transition leading to formation of oxygen-deficient CeO₂₋x phase in the subsurface layer of nanocrystal. Such structural rearrangement leads to density gradient in ceria nanocrystals changing thereby the elastic constants of material. The analysis of free energy balance has shown that formation of highly deficient CeO₂₋x phase occurs inevitably for the nanocrystals with sizes d ≤ 6 nm

Influence of CeO₂ nanocrystals size on the vacancies formation processes determined by spectroscopic techniques

In the paper influence of the size on the processes of oxygen vacancies formation in CeO₂ nanocrystals has been investigated. Changes in concentration of oxygen vacancies were determined by two independent indicators: intensity of 5d → 4f luminescence of Ce³⁺ ions and ratio between ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ luminescence bands of Eu³⁺ ions incorporated into ceria nanocrystals as structural probes. It was shown that for CeO₂ nanocrystals decrease of the size (from 75 nm to 20 nm) manifests itself in 1.5 times increase of the band intensity associated with vacancy-stabilized Ce³⁺ ions, while for CeO₂:Eu³⁺ nanocrystals it leads to lowering of symmetry for Eu³⁺ centre and correspondent decrease of ⁵D₀ → ⁷F₁/⁵D₀ → ⁷F₂ intensity ratio. It was shown that decrease of the size stimulates formation of the oxygen vacancies in ceria nanoparticles

Role of shallow electronic traps formed by oxygen vacancies in formation of luminescent properties of CeO₂₋x nanocrystals

In the paper methods of thermoluminescence (TSL) and time-resolved spectroscopy were used for investigation of shallow electron traps near edge of 4f⁰ band in CeO₂ and nonstoichiometric CeO₂₋x nanocrystals. It was shown that presence of the electronic traps located about 0.2 eV lower than the bottom of 4f⁰ band leads to sufficient modification of O2p-Ce4f excitation relaxation processes due to excitation retrapping. Strong dependence of TSL signal on the stoichiometry of nanocrystal allows to suppose that electronic defects are associated with oxygen vacancies and are formed by F⁺ centers

Formation of oxygen vacancies in ceria-zirconia nanocrystals studied by spectroscopic techniques

Incorporation of doped ions with different ionic radius (like Zr⁴⁺) or valence state (like Eu³⁺) into CeO₂ structure leads to sufficient modification of the processes of oxygen transport due to formation of additional oxygen vacancies (Ov). These vacancies can form complexes with doped ions (RE-Ov-RE) or cerium ions (Ce³⁺-Ov-Ce³⁺) determining the oxygen mobility in these structures. In the paper the formation of oxygen vacancies in ceria (CeO₂-x) and ceria-zirconia (CeO₂-ZrO₂) nanocrystals was studied by conventional spectroscopic techniques. 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

Energy migration processes in phosphate nanocrystals: size and dimensionality dependence

Peculiarities of electronic excitation energy migration in phosphate nanocrystals with three-dimensional (EuPO₄) and one-dimensional (EuP₃O₉) arrangement of regular ions under variation of concentration of both energy traps (Nd³⁺ ions) and scattering centres (La³⁺ ions) are discussed in the paper. Processes of energy migration in both EuPO₄ and EuP₃O₉ nanocrystals are phonon-assisted ones due to absence of resonance between energy levels of adjacent Eu³⁺ ions. The distance of energy migration for both EuPO₄ and EuP₃O₉ nanocrystalswas found to be equal to tens of nanometres, however, for EuPO₄ nanocrystals energy migration leads to stronger quenching of Eu³⁺ luminescence. For 10 nm EuPO₄ nanocrystals sufficient hampering of energy migration isobserved due to the depletion of the low-energy phonon spectrum as a result of the phonon confinement effect

Features of energy transport in EuMgB₅O₁₀ and EuP₃O₉ quasi-one-dimensional lattices

Processes of excitation energy migration in EuMgB₅O₁₀ and EuP₃O₉ quasi-one-dimensional matrices have been investigated by means of steady-state and time-resolved luminescence spectroscopy. It was shown that the patterns of energy migration in these materials are sufficiently different - while for EuMgB₅O₁₀ the most effective energy transport takes place at room temperature via ⁵D₀ →⁷F₁ transitions of Eu³⁺ ion, for EuP₃O₉ energy transfer is more effective at low temperatures and mediated by Eu³⁺-O²⁻ charge transfer states. The difference in energy transport processes can be explained taking into account the peculiarities of phonon subsystem for borate and phosphate matrices

X-ray and photo-excited luminescence of ZnWO₄ nanoparticles with different size and morphology

X-ray and photoluminescence of ZnWO₄ nanocrystals with controlled size and morphology: grains with diameter of 10-20 nm; rods with diameter of 10-20 nm and length of 200-250 nm produced by microwave hydrothermal method were studied. Red luminescence with λmax = 700 nm was found in the samples. The intensity of the red luminescence increases with decreasing of the ZnWO₄ nanoparticles size due to increase of concentration of oxygen vacancies and creation of distorted WO₆ centers. It was shown that reduction in the X-ray luminescence intensity with decrease of the ZnWO₄ nanoparticles size is due to creation of the distorted WO₆ centers which are the nonradiative relaxation channel competing with the self-trapped excitons