Synthesis,structural and optical studies of sol–gel Gd2O3:Eu3+, Tb3+ films

Eu³⁺ and Tb³⁺ co-doped Gd₂O₃ films were elaborated by sol–gel process and dip-coating technique. The films were synthesized by hydrolysis of gadolinium pentanedionate. A homogeneous and stable sol was obtained by the reaction with acetylacetone. Gd₂O₃:Eu³⁺, Tb³⁺ films were crystallized around 500 °C; at an increase of temperature up to 700 °C, oriented growth of (4 0 0) face was observed. The obtained transparent Gd₂O₃: 2.5 at.% Eu³+, 0.005 at.% Tb³⁺ waveguide films at 700 °C display significant optical properties. Different crystallographic properties can be obtained in Gd₂O₃:Eu³⁺, Tb³⁺ films with varying sintering temperatures.     

Optical properties of sol–gel derived Sr2SiO4:Dy3+ – Photo and thermally stimulated luminescence

Trivalent dysprosium-doped strontium silicate (Sr₂SiO₄) phosphors were prepared by sol–gel synthesis using tetra ethyl orthosilicate (TEOS) as precursor. The synthesis temperature could be brought down to 600 °C for formation of a single phase sample. The material was characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), photoluminescence (PL), and thermally stimulated luminescence (TSL). The luminescence study revealed strong ⁴F_9/2 → ⁶H_13/2 transition at 577 nm (yellow), strong ⁴F_9/2 → ⁶H_15/2 transition at 482 nm (blue) and weak ⁴F_9/2 → ⁶H_11/2 transition at 677 nm (red), when excited by 250 nm (Charge transfer band, CTB) or 352 nm (f–f band). The concentration of the dopant ion and the temperature of annealing were optimized for maximum PL intensity. The critical energy-transfer distance for the Dy³⁺ ions was evaluated based on which, the quenching mechanism was verified to be a multipole–multipole interaction. The thermally stimulated luminescence studies of Sr₂SiO₄:Dy³⁺ sample showed main TSL glow peak at 413 K. The trap parameters namely activation energy (E), order of kinetics (b), and frequency factor (s) for this peak were determined using glow curve shape method.     

Optical properties of Tb and Eu doped cubic YAlO3 phosphors synthesized by sol–gel method

In this work, terbium and europium doped YAlO₃ phosphors were synthesized by sol–gel method. It has been shown that after annealing at 1000 °C these phosphors crystallize in the cubic garnet phase, similar to Y₃Al₅O₁₂. As evidenced by the time-resolved photoluminescence spectroscopy and Maximum Entropy Method, the photoluminescence decay of both europium ⁵D₀ and terbium ⁵D₄ states has two lifetime components: short and long, both of the order of milliseconds. Moreover, for both phosphors the short photoluminescence lifetime was ascribed to the ions occupying low-symmetry sites in the vicinity of the nanocrystal surface, while the long decay component was related to the ions present in the bulk of YAlO₃ nanocrystal. For both dopants, the long lifetime component of Tb³⁺ and Eu³⁺ emission is longer than the equivalent lifetimes in bulk Y₃Al₅O₁₂ crystal.     

Enhanced photocatalytic activity of Fe-doped ZnO nanoparticles synthesized via a two-step sol–gel method

A series of Zn_1–x Fe _x O (x = 0, 1, 2, 3, 4 %) powders via a two-step sol–gel method in open system were successfully fabricated. Influence of Fe doping concentration on the structure, morphology, optical properties and photo catalysis properties were investigated by means of X-ray diffraction, scanning electron microscopy, UV–Vis spectrophotometer and photochemical reaction instrument. The results showed that the ZnO powders were hexagonal wurtzite structures and their crystalline sizes and particle diameters decreased with the increase of Fe doping concentration. An increase of visible light absorption value and a decrease in band gap from 3.219 to 3.167 eV were found with the increase of Fe doping concentration, which enable the sample harvest more photons to excite the electron from the valence. Enhanced visible light induced photocatalytic activity has been found in Fe doped ZnO and the ultraviolet light induced photocatalytic properties of the Fe-doped ZnO have been improved greatly compared with undoped ZnO and commercially available TiO₂ (P25). The photocatalytic activities were not significantly affected by the particle size, and the best Fe doping concentration is 1 %.     

Crystal structure and luminescent properties of nanocrystalline YAG and YAG:Nd synthesized by sol–gel method

The work describes results of synthesis of undoped and Nd-doped YAG nanopowders by sol–gel method using different complexing agents (ethylene glycol and citric acid) and characterization of the material by X-ray powder diffraction, scanning electron microscopy, photoluminescence and thermoluminescence techniques. Utilization of citrate sol–gel procedure using yttrium and aluminum nitrate nonahydrates as starting substances allowed to obtain highly stoichiometric and non-defected YAG and YAG:Nd nanocrystalline samples with good luminescence performance and low radiation storage efficiency.     

Structural and electrochemical characterization of SnO2 and SnO2–Co3O4 mixed oxides synthesized by Pechini method

Tin oxide SnO₂ and SnO₂–Co₃O₄ were synthesized by the Pechini method in presence of stannous and cobalt precursors, citric acid and ethylene glycol. The X-ray diffraction study revealed that the SnO₂ prepared exhibited tetragonal crystal structure and the binary system SnO₂–Co₃O₄ showed a biphasic behavior with a tetragonal structure and the cubic structure of Co₃O₄. The thermal stability of materials was characterized by thermogravimetric and differential thermal analysis and the morphology by scanning electron microscope. It was found that the presence of octahedral crystals of the mixed oxide increased the surface area with a maximum centered around 15 mol% of Co₃O₄. The presence of increasing ratio of Co₃O₄ in the binary system did not produce the expected increase of specific capacitance but the cycling behavior at the electrochemical test of galvanostatic charge–discharge for 15 mol% of Co₃O₄ showed 98 % of the initial capacitance after 1000 cycles making this material promising as an electrochemical capacitor with structural stability.     

Synthesis and luminescence properties of highly uniform SiO2@LaPO4:Eu3+ core–shell phosphors

SiO₂@LaPO₄:Eu³⁺ core–shell phosphors have been successfully synthesized by a one-step and economical wet-chemical route at low temperature. The as-obtained products were characterized by means of photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). The SEM, EDS and XPS analysis indicate that SiO₂@LaPO₄:Eu³⁺ core–shell phosphors can only be synthesized in a pH range of 8–11 and the possible mechanism has been proposed. The XRD results demonstrate that the structure of LaPO₄:Eu³⁺ layers is transferred into monoclinic phase from hexagonal phase after annealing at 800 °C for 2 h. The SiO₂@LaPO₄:Eu³⁺ phosphors show strong orange–red luminescence under ultraviolet excitation. The relative emission intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles, and the optimum concentration for Eu³⁺ was determined to be 5 mol% of La³⁺ in SiO₂@LaPO₄ phosphors.     

Sol–gel synthesis of long-lasting phosphors CdSiO3: Mn2+, RE3+ (RE = Tb,Eu, Nd) and luminescence mechanism research

Mn²⁺ and RE³⁺ (RE = Tb, Eu, Nd) co-doped CdSiO₃ orange phosphors were prepared at 1050 °C by a sol–gel method. The phase and crystallinity of the synthesized materials were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The luminescence characteristics were analyzed using photoluminescence (PL) spectra, afterglow decay curves, long-lasting phosphorescence spectra, and thermoluminescence (TL) spectra. Due to the difference in co-doped rare earth ionic radii, it varied greatly in trap density and trap depth caused by the different defects deriving from RE³⁺ ions co-doping into the CdSiO₃: Mn²⁺ host. The afterglow intensity and time for these samples increased as follows: CdSiO₃: Mn²⁺0.2%, Nd³⁺0.8% < CdSiO₃: Mn²⁺0.4%, Tb³⁺0.8% < CdSiO₃: Mn²⁺0.4%, Eu³⁺0.3%. CdSiO₃: Mn²⁺0.4%, Eu³⁺0.3% had the best afterglow properties, which could be due to the proper traps formed by Eu³⁺ ions co-doping into the host. The role of RE³⁺ co-doped into the CdSiO₃: Mn²⁺ matrix and the possible long-lasting phosphorescence process was also discussed in this paper.     

Facile synthesis and characterization of CdTiO<Subscript>3</Subscript> nanoparticles by Pechini sol–gel method

In this work, CdTiO₃ nanoparticles were synthesized through reaction between Cd(CH₃COO)₂.2H₂O, Ti(OC₄H₉)₄, trimesic acid as a new chelating agent and ethanol as solvent by Pechini sol–gel method. X-ray diffraction (XRD) patterns showed that CdTiO₃ nanostructures have rhombohedral structure with diameter of about 35.61 nm. The structure, morphology and size of CdTiO₃ nanoparticles were characterized by FT-IR, XRD, SEM and EDAX. The optical properties of the products were studied by DRS. Based on the results of experiments, it was found that temperature and time of calcination, pH and the solvent of reaction are important parameters for formation of CdTiO₃ nanoparticles. Utilizing trimesic acid (benzene-1,3,5-tricarboxylic acid) as a new chelating agent for preparation of CdTiO₃ nanostructures was initiative of this work.     

Local structure and photocatalytic property of sol–gel synthesized ZnO doped with transition metal oxides

ZnO nanoparticles doped with up to 5 at% of Co and Mn were prepared using a co-precipitation method. The location of dopant ions and the effect of doping on the photocatalytic activity were investigated. The crystal structure of nanoparticles and local atomic arrangements around dopant ions were analyzed by X-ray absorption spectroscopy. The results showed that the Co ions substituted the Zn ions in the ZnO wurtzite phase structure and induced lattice shrinkage, while Mn ions were not completely incorporated in the crystal lattice. The photocatalytic activity under simulated sunlight was characterized by the decomposition of Rhodamine B dye molecules. It was revealed that Co-doping strongly reduced the photocatalytic activity but Mn-doping showed a weaker effect on the reduction of the photoactivity.     

Eu3+ probe ion for rare-earth dopant site structure in sol–gel derived LiYF4 oxyfluoride glass–ceramic

Sol–gel route using metal alkoxides and trifluoroacetic acid as precursors has been used to prepare oxyfluoride glass–ceramic containing Eu³⁺-doped LiYF₄ nanocrystals of about tens of nm size embedded in a silica matrix through controlled crystallization at higher temperatures of the xerogel. Photoluminescence spectra and decay curves recorded in the Eu³⁺-doped LiYF₄ polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the glass–ceramic Eu³⁺ ions are embedded dominantly inside the LiYF₄ nanocrystals most probably as Eu–O center and/or dimer centers in low symmetry (C_2v) sites; oxygen ions were incorporated in their neighborhood during the glass ceramization.     

Comparative dielectric studies of nanostructured BaTiO3, CaCu3Ti4O12 and 0.5BaTiO3⋅ 0.5CaCu3Ti4O12 nano-composites synthesized by modified sol–gel and solid state methods

BaTiO₃ (BTO), CaCu₃Ti₄O₁₂ (CCTO) and 0.5BaTiO₃·0.5CaCu₃Ti₄O₁₂ (BTO–CCTO), as a new nano-composite ceramic, were successfully designed and fabricated by a semi-wet gel route and a modified solid state method. The dielectric properties of the BTO–CCTO ceramic were compared to those of the BTO and CCTO ceramics at lower sintering temperatures and durations. The X-ray diffraction analysis revealed that the BTO and CCTO ceramics form a single crystalline phase and the average crystalline sizes calculated from X-ray diffraction data were in the range of 40–65 nm. The particle sizes of the BTO, CCTO, and BTO–CCTO ceramics obtained from transmission electron microscopy images were in the ranges of 40–65 nm, 80–110 nm, and 70–95 nm, respectively. The phase composition and microstructure were studied by X-ray diffraction and scanning electron microscopy. The energy dispersive X-ray results demonstrated the purity and stoichiometry of the BTO–CCTO nano-composite. The grain sizes of the BTO, CCTO and BTO–CCTO ceramics were found to be in the ranges of 500 nm–1 μm, 4–24 μm, and 250 nm–4 μm, respectively. The AC conductivity as a function of frequency confirmed the semiconducting nature of all of the ceramics and obeyed the Jonscher's power law. The impedance spectrum measurement result showed that the CCTO ceramic possessed an exceptional grain boundary resistance, which supports the internal barrier layer capacitance (IBLC) mechanism present in this ceramic and is responsible for the high ε_r values.     

Comparative study of the Mn4+ 2E → 4A2 luminescence in isostructural RE2Sn2O7:Mn4+ pyrochlores (RE3+ = Y3+, Lu3+ or Gd3+)

Red emitting Mn⁴⁺-doped crystalline materials have potential for application in light emitting devices and therefore it is important to understand how the optical properties of Mn⁴⁺ are influenced by the host lattice the Mn⁴⁺ ions are situated in. In this work we investigate the effect of the host cations in the second coordination sphere on the Mn⁴⁺ emission by studying the luminescence of Mn⁴⁺ ions doped into three isostructural rare earth (RE) stannate RE₂Sn₂O₇ pyrochlores (RE³⁺ = Y³⁺, Lu³⁺ or Gd³⁺). It is found that the energies of the Mn^{4+ 4}T₁ and ⁴T₂ states significantly increase with decreasing Mn⁴⁺-O^2- distance, whereas the energy of the ²E level shows a small shift to higher energies from RE³⁺ = Gd³⁺ to Lu³⁺ to Y³⁺. The observed trend for the ²E level energy is not related to the size of the RE³⁺ ion and is not in line with theoretical calculations reported previously. Low temperature emission spectra of the RE₂Sn₂O₇:Mn⁴⁺ phosphors reveal that only asymmetrical vibronic modes couple to the ²E → ⁴A₂ transition and furthermore show there is significant and unexpected local disorder for Mn⁴⁺ in Gd₂Sn₂O₇ that is not observed for Mn⁴⁺ in the other hosts. Photoluminescence decay measurements demonstrate that the luminescence of RE₂Sn₂O₇:Mn⁴⁺ is strongly quenched below room temperature which is assigned to non-radiative relaxation via a low-lying O²⁻ → Mn⁴⁺ charge-transfer state.     

Synthesis, structure, and dielectric properties of a novel perovskite-based nanopowders via sol–gel method: (1–x)BaTiO3–xDyScO3

A sol–gel method was adopted to synthesize novel perovskite-based nanopowders: (1–x)BaTiO₃–xDyScO₃ (0 ≤ x ≤ 0.06), which exhibited a relatively pure pseudo-cubic perovskite structure when xerogel was calcined at 750 °C. Through the employment of PEG 400 as dispersant, narrow size distributed particles of ~15–20 nm were achieved. Pellets pressed from the nanopowders can be densified at a lower sintering temperature of 1150 °C, compared with 1475 °C by solid-state reaction method. The phase formation, microstructure, dielectric properties, and relaxor behavior of (1–x)BaTiO₃–xDyScO₃ were investigated systematically. With an increasing DyScO₃ doping concentration in BaTiO₃, a tetragonal to pseudo-cubic phase transition appeared at x = 0.03, and two different doping behaviors (donor or acceptor-type) of Dy³⁺ in (1–x)BaTiO₃–xDyScO₃ were discussed. The grain growth of BaTiO₃ ceramics was inhibited, and the grain size was decreased to 200 nm for x = 0.06. The dielectric peak was broadened and the curie temperature dropped gradually, accompanied by an increased room-temperature permittivity. Furthermore, a typical relaxor behavior was observed at x = 0.05 and 0.06, according to the modified Curie–Weiss law.