Stokes and Anti-Stokes Luminescent Rare-Earth-Doped Tantalum Oxide Nanoparticles

Doping of nano- and microparticles of oxides with rare earth elements (REEs) is used to fine-tune their structural, optical, and electrochemical properties. On the way to establish the structure-property relationship, we dope tantalum oxide (Ta₂O₅) particles with REEs to study their effect on the oxide structure and luminescence. Ta₂O₅ is highly perspective in medicine, catalysis, and optics, but its crystal structure is insufficiently studied. Two synthesis approaches (sol-gel and solvothermal) were used to obtain powders with different textures. Experimental and theoretical studies of amorphous and crystallized tantalum oxide NPs by means of X-ray powder diffraction, Rietveld analysis, EXAFS/XANES spectroscopy, and density functional theory calculations were performed. All samples (doped and undoped) crystallized in orthorhombic phase with no admixtures. It was demonstrated that Ta₂O₅ is a promising wide-spectrum luminescent material: by combining REEs, both Stokes and anti-Stokes luminescence in the visible region were obtained. By means of optical absorption spectroscopy, it was shown that the prepared samples could be classified as wide band gap semiconductors.

Structure and Magnetic Properties of a Nanosized Iron-Doped Bismuth Titanate Pyrochlore

The nanosized (50-70 nm) pyrochlore Bi_1.5Fe_0.5Ti₂O_7-δ was prepared by a coprecipitation technique. Characterization of Bi_1.5Fe_0.5Ti₂O_7-δ was carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Raman spectroscopy, Mössbauer spectroscopy, and magnetic susceptibility measurements. The study of Fe doping in Bi₂Ti₂O₇ was performed by means of density functional theory (DFT) calculations. The nanosized Bi_1.5Fe_0.5Ti₂O_7-δ sample crystallizes in the structural type of pyrochlore (Fd3̅m). The distribution of Fe atoms over the sites of Bi and Ti was studied from DFT simulations and then confirmed by the XRD analysis and Mössbauer method. The local distribution, electronic structure, and magnetic behavior of nanosized Bi_1.5Fe_0.5Ti₂O_7-δ are determined by the local microstructure of the metastable nanosized sample. Based on the examination of the Mössbauer spectrum of the Bi_1.5Fe_0.5Ti₂O_7-δ nanopowder, the following states of oxidation were revealed for iron atoms: Fe⁴⁺ in the titanium sites with a fraction of ∼5.7% and two states of Fe³⁺ (in the Bi and Ti sites) with different geometries of the oxygen surrounding. The ratio of Fe³⁺ distributed over the sites correlates well with the distribution in the ceramic sample. The presence of Fe⁴⁺ was found only in the nanosized Bi_1.5Fe_0.5Ti₂O_7-δ. The experimental effective magnetic moment of Fe atoms in the nanosized Bi_1.5Fe_0.5Ti₂O_7-δ appeared noticeably lower than that in the ceramic sample. The temperature dependence of μ_eff within the temperature range of 50-300 K is adequately described by the model of coexistence of Fe³⁺ and Fe⁴⁺ and the existence of clusters.

Structural, Optical, Luminescence, and Electrical Properties of Eu/Li- and Eu/Na-Codoped Magnesium Bismuth Niobate Pyrochlores

Eu-doped bismuth-based Bi_1.5M_0.4Mg_0.5Nb_1.5O_7-δ (M = Li and Na) pyrochlores were synthesized by the organic-inorganic precursor combustion technique. The study examined the effect of rare earth element Eu³⁺ doping on the structural, dielectric, optical, and luminescence properties of synthesized materials. The analysis showed that the substitution of Bi³⁺ cations with Eu³⁺ leads to dielectric permittivity decreasing due to the structural distortion for the Eu-concentrated compositions and low polarizability of Eu³⁺. The band gap values predicted by electronic band structure calculation using DFT-HSE03 are in line with the experimental ones and tended to increase with the decrease in the unit cell parameters with Eu concentration changing. By the optical and luminescence measurements, the specific roles of Li- and Na-containing host types, additional phases, and dopant concentration in bismuth niobate pyrochlores are shown concerning the dielectric, structural, and Eu³⁺ emission properties. All Eu-doped bismuth-based pyrochlore ceramics behave as high-frequency dielectrics up to 200 °C and have mixed conductivity (electronic, proton, and oxygen) at T > 200 °C. The obtained dielectric parameters make them suitable for high-frequency ceramic capacitors.

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study

The photocatalytic properties of Bi_2-xTi₂O_7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi₂Ti₂O₇) and 550 °C (Bi_1.5Ti₂O_6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi_1.5Ti₂O_6.25 phase in comparison with the Bi₂Ti₂O₇ phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi₂Ti₂O₇) and -27 mV (Bi_1.5Ti₂O_6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m²/g (Bi₂Ti₂O₇) and 12.00 m²/g (Bi_1.5Ti₂O_6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical E_g values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi₂Ti₂O₇ shows the highest activity in comparison with Bi_1.5Ti₂O_6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.