Anatase, rutile, and especially brookite nanocrystals have been selectively synthesized in this work via a redox route under mild hydrothermal conditions (180 °C, 3 h), employing trichloride as the titanium source and ammonium peroxodisulfate (APS), hydrogen peroxide, nitric acid, or perchloric acid as the oxidant. Characterizations of the three pure phases were achieved by XRD, Raman spectroscopy, FTIR, TG, HRTEM, UV−vis, and BET. The use of APS consistently yields anatase, but the particle morphology can be tuned from wormhole-structured agglomerates to more dispersed nanocrystallites. The use of other oxidants yields almost identical results, and phase selection can be attained in this case by controlling the reactant concentration and solution pH. The three phases show their distinctive crystal shapes: rounded nanocrystals for anatase, nanoplates for brookite, and nanorods for rutile. Both the optical band gap (3.11 eV) and the indirect band gap (2.85 eV) of brookite were found to lie in between those of anatase and rutile. Under the same surface area of loaded TiO2, the brookite nanoplates exhibit the highest efficiency in the beaching of methyl orange solution under UV irradiation. The mechanism of phase selection was discussed based upon a systematic investigation into the effects of synthetic parameters on phase constituents of the hydrothermal products.
Uniform red-phosphor spheres (∼60-300 nm in diameter) of Y 2 O 3 :Eu 3+ binary and (Y,Gd) 2 O 3 :Eu 3+ ternary systems exhibiting excellent emission at 610 nm have been converted from their colloidal precursor spheres synthesized via homogeneous precipitation. The precursor spheres (approximate composition: [(Y 1-x Gd x ) 1-y -Eu y ](OH)CO 3 • 1.3H 2 O, x ) 0-0.5 and y ) 0-0.11) are directly solid solutions, but arising from sequential nucleation each of the spheres has more Gd and especially Eu while having less Y going from the particle surface to the core. Eu 3+ is more effective than Gd 3+ in raising nucleation density, leading to rapidly decreased average size of the precursor particles at a higher Eu 3+ addition. Diminishing the concentration gradients through adequate annealing is identified to be crucial to high luminous intensity of the oxide particles. At the optimal annealing temperature of 1000 °C, cation homogenization is achieved and the oxide particles largely retain their precursor morphologies, yielding dispersed uniform spheres of excellent luminescence. The (Y 1-x -Eu x )O 1.5 phosphor particles exhibit typical red emissions at 610 nm upon UV excitation into the charge transfer band at ∼255 nm, and the quenching concentration of Eu 3+ is found to be ∼5 at. %. Partially replacing Y 3+ with Gd 3+ (up to 50 at. %) while keeping Eu 3+ at the optimal content of 5 at. % linearly improves the 610 nm emission, and the phosphor particles of [(Y 0.5 Gd 0.5 ) 0.95 Eu 0.05 ]O 1.5 exhibit an luminous intensity ∼103% of that of a commercially available Y 2 O 3 :Eu red phosphor. The uniform phosphor spheres obtained in this work are expected to have wide applications in high-resolution display technologies of contemporary interest.
In this work, blue-emitting CexSi6−zAlz−xnormalOz+1.5xnormalN8−z−x (0.3≤z≤2.5,0.5≤x≤2.5) phosphors were synthesized by firing powder mixtures of α-Si3normalN4 , AlN, Al2normalO3 , and CeO2 at 1950°C for 2 h under 1.0 MPa normalN2 . The resultant phosphors were characterized by phase identification, diffuse reflectance spectra, photoluminescence spectra, quantum efficiency, and temperature-dependent luminescence. The samples showed high purity at the overall range of the varied x and z values. A single broad emission band centered at about 486 nm at 410 nm excitation was observed. Moreover, the CexSi6−zAlz−xnormalOz+1.5xnormalN8−z−x phosphors showed high thermal stability, which could sustain 79–88% emission intensity measured at room temperature. These results indicate that the CexSi6−zAlz−xnormalOz+1.5xnormalN8−z−x phosphors are promising wavelength-conversion materials for white light emitting diodes (LEDs) using near-UV LED chips as the primary light source.
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