Yujun Liang

Highly efficient Sr3Y2(Si3O9)2:Ce3+,Tb3+/Mn2+/Eu2+ phosphors for white LEDs: structure refinement, color tuning and energy transfer

A series of highly efficient and emission-tunable Sr3Y2(Si3O9)2(SYSO):Ce3+,Tb3+/Mn2+/Eu2+ phosphors have been prepared via a solid-state reaction. The structure refinement indicates that the as-prepared phosphors crystallized in a monoclinic phase with a space group of C2/c (no. 15), and there are three kinds of cation sites in the host lattice for the doped ions to occupy. The Ce3+ emission at different lattice sites in the SYSO host has been firstly identified and discussed. When introducing other doping ions into these cation sites, there exist efficient energy transfers from Ce3+ ions to these doping ions (Tb3+, Mn2+, and Eu2+) under UV excitation. The corresponding energy transfer mechanisms from Ce3+ to Tb3+/Mn2+/Eu2+ in SYSO:Ce3+,Tb3+/Mn2+/Eu2+ systems have been studied systematically. These energy transfers not only can enhance the luminescent efficiencies and broaden the width of emission spectra of SYSO:Ce3+,Tb3+/Mn2+/Eu2+ phosphors but also can modulate their emission colors from blue to green, orange or cyan, respectively. For example, the maximum quantum yields (QYs) of as-prepared SYSO:0.15Ce3+,xTb3+/yMn2+ phosphors can reach 90.4% and 74% at x = 0.70, y = 0.01, respectively. Based on these experiment results, the as-prepared SYSO:Ce3+,Tb3+/Mn2+/Eu2+ phosphors can act as potential color-tunable and emission band-widened phosphors for possible applications in ultraviolet light based white LEDs.

Structure, luminescence properties and energy transfer behavior of color-adjustable Sr3Gd2(Si3O9)2:Ce3+, Tb3+/Mn2+ phosphors

A series of single-phased and emission-tunable Sr3Gd2(Si3O9)2 (SGSO):Ce3+, Tb3+/Mn2+ phosphors have been successfully prepared via the solid-state reaction, and the crystal structures, luminescence properties, energy transfer of Ce3+ → Tb3+ and Ce3+ → Mn2+, color tuning and thermally stability were systematically investigated respectively. The energy transfer from Ce3+ to Tb3+ or Mn2+ ions were deduced from the spectral overlap between the Ce3+ emission and Tb3+/Mn2+ excitation spectra. The energy transfer mechanisms of Ce3+ → Tb3+ and Ce3+ → Mn2+ in the host were verified to be a dipole–quadrupole interaction and dipole–dipole interaction, respectively, which made the emission color shift from blue to green and near white with the corresponding Commission Internationale deLEclairage (CIE) chromaticity coordinates from (0.174, 0.060) to (0.286, 0.615) and (0.319, 0.367), respectively. The good thermal stability of SGSO:0.26Ce3+, 0.60Tb3+ and SGSO:0.26Ce3+, 0.45Mn2+ samples showed about 72.3% and 86.1% at 150 °C of its initial emission intensity at room temperature due to the different energy transfer efficiency through Ce3+ → Tb3+ and Ce3+ → Mn2+. The maximum quantum yields (QYs) of as-prepared SGSO:0.26Ce3+, 0.10Tb3+ and SGSO:0.26Ce3+, 0.57Mn2+ phosphors could reach 80.2% and 62.4%, respectively. All these properties indicate that the SGSO:Ce3+, Tb3+/Mn2+ phosphors have potential applications as ultraviolet-convertible phosphors.

Investigation of luminescence properties and the energy transfer mechanism of tunable emitting Sr3Y2(Si3O9)2:Eu2+,Tb3+ phosphors

A series of Eu2+ and Tb3+ singly-doped and co-doped Sr3Y2(Si3O9)2 (SYSO) phosphors have been synthesized via a conventional high-temperature solid-state reaction. The crystal structures, photoluminescence properties, fluorescence lifetimes, thermal properties and energy transfer of SYSO:Eu2+,Tb3+ were systematically investigated in detail. Rietveld structure refinement of the obtained phosphors indicated that the SYSO host crystallized in a monoclinic system with the space group C2/c (15) and there are three kinds of cation sites for the doped ions to occupy forming emission centers. The photoluminescence (PL) emission bands of SYSO:xEu2+ show a red-shift tendency with increasing Eu2+ content which should be attributed to more Eu2+ ions entering into Sr2/Y2 and Sr3/Y3 sites from the Sr1/Y1 site. For the co-doped SYSO:Eu2+,Tb3+ samples, tunable colors from cyan to green can be realized by varying the doping concentration of the Tb3+ ions. The intense green emission was realized in the SYSO:Eu2+,Tb3+ phosphors on the basis of the highly efficient energy transfer from Eu2+ to Tb3+ with an efficiency of over 89%. As a result, the emission intensity of SYSO:0.01Eu2+,0.21Tb3+ is about 2.5 times higher than that of SYSO:0.21Tb3+ under 340 nm UV excitation. The energy transfer mechanism from Eu2+ to Tb3+ in the SYSO host was ascribed to the quadrupole–quadrupole interactions. These results indicated that the SYSO:Eu2+,Tb3+ phosphors can act as single-phase green emitting phosphors for possible applications in ultraviolet light-based white light-emitting diodes (w-LEDs).

Structural evolution induced preferential occupancy of designated cation sites by Eu2+ in M5(Si3O9)2 (M = Sr, Ba, Y, Mn) phosphors

In this paper, we present new insight into a changing Eu2+ crystallographic site preference in Eu-doped M5(Si3O9)2 (M = Sr, Ba, Y, Mn), which is related to the structural variation induced by M cation substitutions. The effect of the local structural geometry on the luminescence properties of Eu2+ is revealed. By substitution of Ba2+ for Sr2+, the lattice expansion is restricted to specific cation sites, resulting in the abrupt blue shifted emission of Eu2+ ions. The abnormal blue shift on replacing Sr2+ with Mn2+ is attributed to the preferential 6-fold coordination for Mn2+ that moves the Eu2+ ions to other sites. The results elucidate the mechanisms of emission band adjustment by local site coordination change and it can be potentially extended to crystals which properties are sensitive to local lattice variations.

Size-tunable synthesis and luminescent properties of Gd(OH)3:Eu3+ and Gd2O3:Eu3+ hexagonal nano-/microprisms

Uniform and monodisperse Gd(OH)3:Eu3+ hexagonal prisms were successfully synthesized at mild conditions via a large-scale and facile homogeneous coprecipitation process without using any catalysts, surfactants or templates. The size of the as-formed Gd(OH)3:Eu3+ precursor prisms could be modulated from the micro- to nanoscale by the use of urea and changing the pH values of the initial solutions. A possible formation mechanism for the Gd(OH)3:Eu3+ hexagonal nano-/microprisms was proposed. After a postannealing process, Gd2O3:Eu3+ hexagonal nano-/microprism phosphors with a slight shrinking in size can be transformed from Gd(OH)3:Eu3+. Both the Gd2O3:Eu3+ nanoprisms and microprisms exhibit the same strong red emission corresponding to the 5D0 → 7F2 transition (610 nm) of Eu3+ under UV light excitation (243 nm) and low-voltage electron beam excitation (1–6 kV). Furthermore, the experimental results indicate that the luminescence properties of the as-obtained phosphors are dependent on their morphologies and sizes. As a result of the controllable morphology and size, and excellent luminescence properties, these Gd2O3:Eu3+ nano-/microprism phosphors may find potential applications in optoelectronic devices (fluorescent lamps and field emission displays), bioanalysis and biomedical areas and so on.

Enhanced visible light photocatalytic performances of self-assembled hierarchically structured BiVO4/Bi2WO6 heterojunction composites with different morphologies

Novel self-assembled hierarchical BiVO4/Bi2WO6 heterostructured composites with different morphologies were controllably synthesized via a facile and template-free solvothermal method. The effects of the molar ratio of V to W on the structure, morphology, photocatalytic activity and photoelectrochemical properties of the BiVO4/Bi2WO6 composite photocatalysts were investigated in detail. The results showed that all the BiVO4/Bi2WO6 composites exhibited much better photocatalytic activities for methylene blue (MB) degradation and photoelectrochemical performances than pure BiVO4 and Bi2WO6. Among the composites, the BiVO4/Bi2WO6 microsphere with the molar ratio of V to W of 2u2006:u20061 exhibited the best photocatalytic performance. The mechanism of enhanced activity was systematically investigated by UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and photo-electrochemical methods including transient photocurrent responses, electrochemical impedance spectroscopy (EIS) and Mott–Schottky plots. The enhanced photocatalytic activity could be ascribed to the tetragonal–monoclinic heterophase structure, high surface oxygen vacancy concentration, narrow band gap energy and mainly the formed heterojunction structure which could effectively promote the separation of photogenerated electron–hole pairs. In addition, a possible photocatalytic mechanism for the enhanced photocatalytic activity was proposed on the basis of the calculated energy band positions of BiVO4 and Bi2WO6.

Synthesis and tunable blue–green color emission and energy transfer of Ce3+, Tb3+ co-doped BaZrSi3O9 phosphors

Utilizing energy transfer process, Ce3+, Tb3+ co-doped BaZrSi3O9 (BZS) phosphors have been successfully prepared via solid state reaction process. The phase structure, photoluminescence properties, decay curves, thermal stability, and the energy transfer mechanism were investigated in this paper. The critical distance calculated by quenching concentration method was 8.84xa0Å, and the energy transfer process from Ce3+ to Tb3+ in BZS:Ce3+/Tb3+ phosphors had been demonstrated to be a resonant type via a dipole–quadrupole mechanism. The emission colors of the phosphors could be tuned from blue (0.1625, 0.1261) to green (0.2979, 0.5473) through the corresponding Ce3+ to Tb3+ energy transfer. Moreover, thermal quenching luminescence results revealed that BZS:Ce3+, Tb3+ exhibited good thermal stability. The Tb3+ intensity of BZS:0.04Ce3+, 0.02Tb3+ sample showed about 60.88xa0% of its initial emission intensity at room temperature. The above results indicate that the phosphors may be potentially used as single-component blue–green emission phosphors for application in light emitting device.

Uniform KCaY(PO4)2:Eu3+ phosphors: sol–gel method, morphology and luminescence properties

Eu3+ doped KCaY(PO4)2 (KCYP) phosphors with irregular, hemispherical and spherical morphologies are successfully synthesized via a sol–gel method. The crystalline structure, morphology and luminescence properties of these phosphors are characterized by powder X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy. Under the excitation wavelength of near-ultravioletxa0(n-UV) 392xa0nm, the as-prepared KCYP:0.03Eu3+ phosphor exhibits an intense orange–red emission at 588xa0nm corresponding to 5D0xa0→xa07F1 transition of Eu3+. The effect of synthesis conditions such as the dose of citric acid and pH of the precursor solution on the morphology and luminescence intensity of the phosphors is discussed in detail. The morphologies of KCYP:0.03Eu3+ phosphors vary from a small amount of spheres to aggregated hemispheres and then to smooth spheres by increasing the molar ratio of citric acid to metal ions in the host. The highest emission intensity can be obtained at the molar ratio of citric acid to metal ions 2:1 and pHxa0=xa08.0, respectively.

High thermal stability and quantum yields of green-emitting Sr 3 Gd 2 (Si 3 O 9 ) 2 :Tb 3+ phosphor by co-doping Ce 3+

Abstract A series of Tb3+ mono-doped and Ce3+-Tb3+ co-doped Sr3Gd2(Si3O9)2 phosphors with high thermal stability and quantum yields were successfully prepared via the solid state reaction. The as-prepared Sr3Gd2(Si3O9)2:Tb3+ samples showed broad excitation spectrum from 250 to 400 nm and presented characteristic emission transitions 5D4→7FJ (J=6, 5, 4, 3) of Tb3+ under 313 nm excitation, which were located at about 488, 541, 584 and 620 nm. The emission intensities of Tb3+ rose steadily in Sr3Gd2(Si3O9)2 host with the increase of Tb3+ concentration even though Gd3+ ions were completely replaced by Tb3+ ions. The Ce3+ ion as a sensitizer could efficiently improve the performance of Tb3+ ion. First, with Ce3+ co-doping, the excitation spectrum of Tb3+ monitored at 541 nm showed a similar band that responds to the violet emission of Ce3+ monitored at 416 nm. Second, the quantum yields of Sr3Gd2(Si3O9)2:Tb3+ phosphors could be enhanced from 26.6% to 80.2% by co-doping Ce3+. Finally, the co-doping of Ce3+ was also effective to improve the thermal stability of Sr3Gd2(Si3O9)2:Tb3+. As the temperature rose to 150 °C, the emission intensity of Tb3+ remained at about 83.6% of that measured at room temperature, which was better than the commercial YAG:Ce phosphor in terms of their thermal quenching properties. These results indicated that the as-prepared Sr3Gd2(Si3O9)2:Tb3+,Ce3+ samples could be used as green emission phosphors for possible applications in near ultraviolet based WLEDs.

Synthesis and characterization of micro/nano-structured BaHPO4/Ba3(PO4)2/Ba5(PO4)3OH phases and their luminescence

Microspheres covered with microcuboids/nanorods and nanoparticles of BaHPO4/Ba3(PO4)2/Ba5(PO4)3OH phases have been successfully synthesized by a facile hydrothermal (HT) method using the citric acid as a surfactant at different pH values. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and fluorescence spectrometry were used to characterize the samples. It was found that the pH value was a crucial factor for the phase formation and shape determination of the final products, which were discussed in detail. Attractively, the as-prepared BaHPO4/Ba3(PO4)2/Ba5(PO4)3OH samples emitted an intense blue light in a broad band from 380 to 625 nm, for which the mechanism was complex ions luminescence originating from the transition of 3T1 → 1A1 in PO43−. Meanwhile, an obvious red shift for the emission band was observed between nano- and bulk-Ba3(PO4)2 synthesized by HT and conventional solid-state (CSS) reactions, respectively, which was due to the effect of the product being nanosized. The same effect was also revealed by the fact that the decay time of the latter was about 2.5 times that of the former. Moreover, the decay mode of Ba5(PO4)3OH was different from those of BaHPO4 and Ba3(PO4)2, which was ascribed to the effect of the substitution of three OH− for one PO43− on their electronic structures.