Defect and intrinsic luminescence of CeO₂nanocrystals

Abstract

In the paper, the luminescence processes in CeO2 nanocrystals have been investigated and several types of luminescent centers were revealed. For CeO2 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 formed by Ce3+ ions which charge is compensated by oxygen vacancies in first coordination sphere of Ce3+ ion. 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.

Enhanced oxygen diffusion in nano-structured ceria

“…3). This small value of the activation energy in the case of a nanostructured material is comparable to the energy of formation of oxygen vacancies (≈ 0.25 eV) in СеО 2 nano-crystals, when there are one-dimensional hollow channels present that are formed by the vacancies in the oxygen sublattice [18].…”

“…This is supported by the following experimental facts. Firstly, spectral studies have shown that in crystals of this size, the valence of the cation changes, Се 4+ → Се 3+ , i.e., there is a transformation of СеО 2 to the sesquioxide Се 2 О 3 [18]. Secondly, due to a change in the valence of the cation, additional ("nonstoichiometric") vacancies are formed in the anion sublattice of the crystal.…”

“…air atmosphere). The results indicate that polycrystalline nano structured СеО 2 with a grain size of ≈ (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) nm is characterized by more than two orders larger conductivity coefficient σ in comparison to the conductivity of a conventional polycrystal with a grain size ≈ (50-100) nm.…”

Wavelength‐Selective Photoreduction of Colloidal CeO2–x Nanocrystals

“…In the general case, when oxygen atoms leave nanoceria, two electrons remain within and can either be localized on two adjacent Ce 4þ cations, generating Ce 3þ -V O -Ce 3þ defects (which can, in turn, form complex defects including vacancy dimers, trimers, and linear vacancy clusters), [15] or can occupy V O , resulting in the appearance of F-centers. [16] These defects are unevenly distributed over the entire volume of the nanoceria, including its surface. [17][18][19] But just due to the surface V O , nanoceria can repeatedly pass through redox cycles by means of the uptake and release of oxygen (e.g., the Mars-van Krevelen mechanism) [20,21] or the surface cerium ions can also directly participate in Ce 3þ ↔ Ce 4þ redox cycle.…”

Estudo da influência dos íons terras raras Er3+, Eu3+ e Pr3+ nas propriedades fotoluminescentes e eletroquímicas do CeO2

“…Os valores de Egap calculados para as amostras estão próximos ao valor encontrado de Egap téorico do CeO2 (3,20 eV) (ANSARI et al, 2014;GONG et al, 2016 A amostra de CeO2 puro apresentou uma banda larga de emissão centrada em aproximadamente 560 nm e um ombro em torno de 610 nm (emissão no verde), atribuída às transferências de carga entre os íons O 2e Ce 4+ . A emissão fotoluminescente do óxido de cério puro ocorre a partir da recombinação direta dos elétrons na banda de condução do Ce (4f) com os buracos existentes na banda de valência do O 2-(2p) (DEUS et al, 2015;MASALOV et al, 2014;SEMINKO et al, 2017). A emissão fotoluminescente observada está relacionada a processos de relaxação multifônicos e estados localizados dentro do band gap dos materiais gerados por defeitos ou distorções presentes na estrutura do material, de acordo com o Modelo de Banda Larga (CASALI et al, 2014).…”