Hyeon Mi Noh

Chemical bond properties and charge transfer bands of O2−–Eu3+, O2−–Mo6+ and O2−–W6+ in Eu3+-doped garnet hosts Ln3M5O12 and ABO4 molybdate and tungstate phosphors

Charge transfer (CT) energy from the ligand to the central ions is an important factor in luminescence properties for rare earth doped inorganic phosphors. The dielectric theory of complex crystals was used to calculate chemical bond properties. Combining the photoluminescence and the dielectric theory of complex crystals, the CT bands of O(2-)-Eu(3+), O(2-)-Mo(6+) and O(2-)-W(6+) for Eu(3+)-doped inorganic phosphors have been investigated experimentally and theoretically. Taking Eu(3+)-doped Ln3M5O12 (Ln = Y, Lu and M = Al, Ga), Gd3Ga5O12, MMoO4 (M = Ca, Sr, Ba) and MWO4 (M = Ca, Sr, Ba) as typical phosphors, we investigated the effects of the cation size on the CT bands and chemical bond properties including the bond length (d), the covalency (fc), the bond polarizability (αb) and the environmental factor (he) of O(2-)-Eu(3+), O(2-)-Mo(6+) and O(2-)-W(6+), respectively. For systematic isostructural Ln3M5O12 (Ln = Y, Lu and M = Al, Ga) phosphors, with the increasing M ion radius, the bond length of Ln-O decreases, but fc and αb increase, which is the main reason that the environmental factor increased. For the isostructural MMoO4:Eu, with the increasing M ion radius, the Mo-O bond length increases, but fc and αb decrease, and thus he decreases. However, in the compound system MWO4:Eu (M = Ca, Ba) with the increasing M ion radius, the O-W bond length increases, but fc and αb increase, and thus he increases and the O-W CT energy decreases. Their O(2-)-Eu(3+), O(2-)-Mo(6+) and O(2-)-W(6+) CT bands as well as their full width at half maximum (FWHM) were directly influenced by he. And with the increasing he, CT bands of O-Eu or O-Mo or O-W decrease and their FWHM increases. These results indicate a promising approach for changing the material properties, searching for new Eu(3+) doped molybdate, tungstate or other oxide phosphors and analyzing the experimental result.

Wide-Band Excited Y6(WMo)0.5O12:Eu Red Phosphor for White Light Emitting Diode: Structure Evolution, Photoluminescence Properties, and Energy Transfer Mechanisms Involved

Y6(WMo)(0.5)O12 activated with Eu(3+) ions was investigated as a red-emitting conversion phosphor for white light emitting diodes (WLEDs). The phosphors were synthesized by calcining a citrate-complexation precursor at different temperatures. The photoluminescence properties of the phosphors and the energy transfer mechanisms involved were studied as a function of structure evolution. It was found that the host lattices were crystallized in a cubic or a hexagonal phase depending on the synthesis conditions. Although all the phosphors showed intensive red emission under an excitation of near-UV or blue light due to energy transfer from the host lattices to Eu(3+) ions, the photoluminescence spectra and temporal decay features were found to vary significantly with the structure and crystallinity of the host lattice. The mechanisms of the energy transfer from the host lattices to Eu(3+) ions and energy quenching among Eu(3+) ions were discussed on the basis of structure evolution of the host lattice. Phosphors calcined at 800 and 1300 °C were suggested to be promising candidates for blue and near-UV light excited WLEDs, respectively.

Controllable synthesis of uniform CaMoO4:Eu3+,M+ (M = Li, Na, K) microspheres and optimum luminescence properties

High-quality and monodisperse CaMoO4:Eu3+,M+ (M = Li, Na, K) microspheres have been synthesized with the assistance of poly-(diallyldimethylammonium chloride) (PDDA) via a facile coprecipitation hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), as well as photoluminescence (PL) spectroscopy are used to characterize the resulting samples. The results show that the CaMoO4:Eu3+,M+ (M = Li, Na, K) can be directly indexed to a tetragonal CaMoO4 phase with high purity. A series of controlled experiments indicate that PDDA as a shape modifier introduced into the reaction system plays a critical role in the morphology of the final products. Furthermore, the shape and size of the products can be further manipulated by adjusting the concentration of PDDA and pH values in the initial solution. The prepared microspheres are stable at a suitable annealing temperature. The possible formation mechanism for these microspheres is presented. Additionally, the PL properties of CaMoO4:Eu3+,M+ (M = Li, Na, K) were investigated in detail. The results reveal that the red emission peak intensities of CaMoO4:Eu3+,Li+ and CaMoO4:Eu3+,Na+ are higher than that of CaMoO4:Eu3+,K+. The particle size and shape have a remarkable effect on the photoluminescence properties of the phosphor. The luminescence intensity is observably enhanced with increasing of the annealing temperature due to eliminating PDDA and/or H2O present in the samples and to the improved crystal quality.

Preparation of 3D cerium-based coordination polymer microstructures and their conversion to ceria

A simple and practicable hydrothermal process was engaged for the synthesis of three-dimensional (3D) flowerlike cerium-based coordination polymer (CP) microstructures employing 1,2,4,5-benzenetetracarboxylic acid (BTTA) as the organic linker. Ceria with similar flowerlike microstructures was obtained after thermolysis of the cerium-based CP microstructures at 600 °C for 4 h. The products were characterized using X-ray diffraction (XRD), infrared spectroscopy (IR), thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses and scanning electron microscopy (SEM). Barrett–Emmett–Teller (BET) nitrogen sorption was also carried out. SEM images show that the ceria is porous, with diameters varying from 5 to 10 μm. The measured specific surface area of the ceria flowers is 48.31 m2 g−1. Other lanthanide-based CP microstructures were also prepared using this method and characterized.

Tunable white-light emission in single-phase Ca 9 Gd(VO 4 ) 7 :Tm 3+ , Eu 3+

A series of Tm3+ and/or Eu3+ doped Ca9Gd(VO4)7 single composition phosphors were synthesized by a solid state reaction method, and their luminescence properties were investigated. Tm3+ and Eu3+ co-doped Ca9Gd(VO4)7 phosphors showed a blue with the peak at 477 nm and red with the stronger peak at 620 nm dual emission bands under the UV excitation, which originates from the f-f transitions of Tm3+ and Eu3+ ions, respectively. The energy transfer from O2--V5+ CT (charge transfer) energy to Tm3+ and Eu3+ ions as well as the energy transfer from Tm3+ to Eu3+ ions were investigated. The photoluminescence intensity ratio of blue and red emission could be tuned by adjusting the concentration of Tm3+ and Eu3+ ions and as a result the emission color varies from blue to white to red. The white-light emission is realized in single phased phosphor of Ca9Gd(VO4)7:Tm3+, Eu3+ by combining the Tm3+-emission and the Eu3+-emission.

Luminescence and energy transfer process in Cu + ,Sm 3+ co-doped sodium silicate glasses

Highly transparent Cu+, Sm3+ co-doped sodium silicate glasses were prepared by melt-quenching technique. The optical and luminescent properties of Cu+, Sm3+ single-doped and co-doped glasses, and energy transfer process from Cu+ to Sm3+ were systemically investigated through absorption, excitation, emission spectra and lifetime measurement. Tuning the concentration of Sm3+ can generate the varied hues from yellowish green to orange. Our results suggest that Cu+, Sm3+ co-doped glasses may be used as converting phosphors for UV-excitation white LEDs.

Preferential occupancy of Eu3+ and energy transfer in Eu3+ doped Sr2V2O7, Sr9Gd(VO4)7 and Sr2V2O7/Sr9Gd(VO4)7 phosphors

The vanadate-based phosphors Sr2V2O7:Eu3+ (SV:Eu3+), Sr9Gd(VO4)7:Eu3+ (SGV:Eu3+) and Sr9Gd(VO4)7/Sr2V2O7:Eu3+ (SGV/SV:Eu3+) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu3+ based on the bond valence model. By comparing the change of bond energy when the Eu3+ ions are incorporated into the different Sr, V or Gd sites, we observed that Eu3+ doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, i.e., Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu3+, SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu3+ can be obtained through changing the concentration of Eu3+ or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O2− → V5+ charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f–f transitions of Eu3+. The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O2− → V5+ CT of SV, and a sharp characteristic spectrum (5D0–7F2) at about 615 and 621 nm.

Luminescence and thermal-quenching properties of Dy3+-doped Ba2CaWO6 phosphors

A series of new double perovskite tungstate Ba2CaWO6:xDy(3+) (0.01⩽x⩽0.15) phosphors were synthesized via solid state reaction process. XRD analysis confirmed the phase formation of Ba2CaWO6:Dy(3+) materials. The photoluminescence excitation and emission spectra, concentration effect, thermal-quenching, and decay property were investigated. The phosphor could be excited by the UV light region from 250 to 400 nm, and it exhibits blue (493 nm) and yellow (584 nm) emission corresponding to (4)F(9/2)-(6)H(15/2) transitions and (4)F9/2-(6)H13/2 transitions, respectively. The optimum dopant concentration of Dy(3+) ions in Ba2CaWO6:xDy(3+) is around 5 mol% and the critical transfer distance of Dy(3+) is calculated as 14 Å. The thermal-quenching temperature is 436 K for Ba2CaWO6:0.05Dy(3+). The fluorescence lifetime is also determined in Ba2CaWO6:0.05Dy(3+).

Butterfly-like BiVO4: Synthesis and Visible Light Photocatalytic Activity

Monoclinic-structured BiVO4 microcrystals with butterfly-like morphology were synthesized with alcoho-hydrothermal method. Compared with irregular morphologies BiVO4, the butterfly-like BiVO4 possessed higher surface area and lower energy band gap, which is conducive to promote the rapid adsorption of organic pollutants on the surface of BiVO4 and absorb the light with lower energy. Photocatalysis experiment results showed that the degradation efficiency of the butterfly-like BiVO4 for degradation MO solution is far higher than irregular morphologies BiVO4. It indicates the excellent photocatalytic activity of the butterfly-like BiVO4 may be related to its special morphology, high surface area, and low band gap energy.

Effect of Yb3+ Concentrations on the Upconversion Luminescence Properties of ZrO2:Er3+,Yb3+ Phosphors

ZrO2:Er3+ and ZrO2:Er3+,Yb3+ phosphors were synthesized via a solvothermal reaction method. For low concentrations of Yb3+, the crystalline structure changed from the tetragonal to monoclinic phase as sintering temperature increased. As the Yb3+ concentration increased to a value above 0.05 mol, ZrO2 phosphors displayed a very stable tetragonal phase. The green and red upconversion emissions of ZrO2:Er3+,Yb3+ phosphors were measured under the excitation with a 975 nm continuous wave diode laser, and the pump power dependence of upconversion intensity was investigated. As the Yb3+ concentration increased from 0 to 0.05 mol, the red upconversion emission intensity increased more rapidly than the green emission intensity. This is attributed to the energy transfer (4I11/2→4I15/24I13/2→4F9/2) between Er3+ ions and the energy back transfer [4S3/24I13/2(Er3+)→2F7/22F5/2(Yb3+)] between the Er3+ and Yb3+ ions. In this case, the pump power dependence of red emission intensity changed from quadratic to linear as Yb3+ concentration increased.