Xuejiao Wang

Breaking the strong 1D growth habit to yield quasi-equiaxed REPO4 nanocrystals (RE = La–Dy) via solvothermal reaction and investigation of photoluminescence

Quasi-equiaxed REPO4 (RE = La–Dy) nanocrystals of either monoclinic (m-) or hexagonal (h-) structure, depending on the type of RE3+, have been successfully synthesized through solvothermal reaction in the presence of ethylene glycol (EG). Detailed characterization of the products was achieved by the combined XRD, FE-SEM, TEM, FTIR, TG/DTA, PLE/PL, and fluorescence decay techniques. The intrinsically strong 1D growth of the phosphates was gradually suppressed through increasing the solution pH and EG dosage, and the formation mechanism was elucidated with h-GdPO4 as a representative. The differences in the crystal structure, IR response and crystal size of the as-synthesized REPO4 crystals were interpreted in detail. It was shown that the phase purity and crystal morphology of h-GdPO4 can be well retained up to 700 °C, followed by a complete transition from the hexagonal to monoclinic phase at 900 °C. The photoluminescence properties of Dy3+-doped h-GdPO4 nanocrystals, including excitation, emission and fluorescence decay, were thoroughly investigated, and almost pure white-light luminescence was attained with similarly strong yellow (573 nm, 4F9/2 → 6H13/2) and blue (478 nm, 4F9/2 → 6H15/2) emissions of Dy3+ under excitation of the host Gd3+ ions at ∼274 nm.

Controlled Hydrothermal Crystallization of Anhydrous Ln2(OH)4SO4 as a New Family of Layered Rare-Earth Hydroxide (Ln=Eu-Lu and Y)

Anhydrous hydroxide sulfates Ln2 (OH)4 SO4 (Ln=Eu-Lu, Y) were hydrothermally synthesized as a new family of layered rare earth metal hydroxides (LRHs). They crystallize in the monoclinic system (space group C2/m) with structures built up by alternate stacking of interlayer SO42- and the two-dimensional host layer composed of tricapped [LnO9 ] trigonal prisms along the a axis. In distinct contrast to the recently discovered hydrated LRHs Ln2 (OH)4 SO4 ⋅2u2009H2 O, which only exist for Ln=La-Dy, the host layers of the anhydrous phase are linked together by sharing edges instead of an O node of the SO42- tetrahedron. Rietveld refinement showed that the cell dimension tends to decrease for smaller Ln3+ , while the axis angle (β=98.78-100.31°) behaves oppositely. Comparative thermogravimetric/differential thermal analysis in air revealed that the dehydroxylation and desulfurization temperatures become gradually higher and lower, respectively, for smaller Ln3+ , and thus the temperature range of Ln2 O2 SO4 existence is narrowed. The newly discovered Ln2 (OH)4 SO4 , together with their hydrated counterparts, allow for the first time green synthesis of Ln2 O2 SO4 with water as the only exhaust for the full spectrum of lanthanides. Calcining Ln2 (OH)4 SO4 in H2 yielded phase-pure Ln2 O2 S for Eu and Gd and a mixture of Ln2 O2 S and Ln2 O3 for the other Ln. The effects of the lanthanide contraction were clearly revealed, and photoluminescence was found for the anhydrous LRHs of Eu and Tb.

(La 0.97 RE 0.01 Yb 0.02 ) 2 O 2 S Nanophosphors Converted from Layered Hydroxyl Sulfate and Investigation of Upconversion Photoluminescence (RE=Ho, Er)

Phase-pure (La0.97RE0.01Yb0.02)2O2S upconversion (UC) nanophosphors (average crystallite size ~u200945xa0nm; RE=Ho, Er) were annealed from their hydrothermally crystallized layered hydroxyl sulfate precursors in flowing hydrogen at 1200xa0°C for 1xa0h, with water vapor as the only exhaust. Under 978-nm laser excitation (up to 2.0xa0W), the Ho3+-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~u2009546 (5F4xa0→xa05I8), 658 (5F7xa0→xa05I8), and 763xa0nm (5F4xa0→xa05I7), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400–700xa0nm). The Er3+-doped UC phosphor, on the other hand, showed weak green (~u2009527/549xa0nm, 2H11/2,4S3/2xa0→xa04I15/2), weak red (~668/672xa0nm, 4F9/2xa0→xa04I15/2), and strong near-infrared (~u2009807/58xa0nm, 4I9/2xa0→xa04I15/2) luminescence, whose emission color in the visible region drifted from yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] with increasing excitation power. Analysis of the power-dependent UC luminescence found three- and two-photon processes for RE=Ho and Er, respectively, and the possible UC mechanisms were proposed.

Recent progress in layered rare-earth hydroxide (LRH) and its application in luminescence

This review article compiles the recent achievements made in layered rare-earth (RE) hydroxide (LRH), including controlled crystallization, structural and morphological features, anion exchange, nanosheet exfoliation, and application in the field of luminescence for both the Ln2(OH)5(Ax−)1/x·nH2O (251-LRH) and Ln2(OH)4(Ax−)2/x·nH2O (241-LRH) phases. The luminescent properties of the LRHs themselves, the oxide, oxysulfate, and oxysulfide phosphors derived from the LRHs via controlled calcination, and the highly oriented transparent phosphor films of enhanced luminescence and/or novel emission features are summarized.

Crystal Structure of NaLuW2O8·2H2O and Down/Upconversion Luminescence of the Derived NaLu(WO4)2:Yb/Ln Phosphors (Ln = Ho, Er, Tm)

Hydrothermally reacting Lu(NO)3 and Na2WO4·2H2O at 200 °C and pH = 8 produced the new compound NaLuW2O8·2H2O, which was analyzed via the Rietveld technique to crystallize in the orthorhombic system (space group: Cmmm) with cell parameters a = 21.655(1), b = 5.1352(3), and c = 3.6320(2) Å and cell volume V = 403.87(4) Å3. The crystal structure presents -(NaO6)-(NaO6)- and -(LuO4(H2O)2WO5)-(LuO4(H2O)2WO5)- alternating layers linked together by the O2- ion common to NaO6 octahedron and WO5 triangle bipyramid. Tetragonal structured and phase-pure Na(Lu0.87Ln0.03Yb0.1)(WO4)2 phosphors (Ln = Ho, Er, and Tm) were directly produced by calcining their NaLuW2O8·2H2O analogous precursors at 600 °C for 2 h, followed by a detailed study of their downconversion/upconversion (DC/UC) photoluminescence. It was shown that the UC luminescence is dominated by a red band at ∼650 nm for Ho3+ (5F5 → 5I8 transition), green bands at ∼500-575 nm for Er3+ (2H11/2/4S3/2 → 4I15/2 transitions) and a blue band at ∼476 nm for Tm3+ (1G4 → 3H6 transition), all via a three-photon process. DC luminescence of the phosphors is characterized by a ∼545 nm green emission for Ho3+ (5F4/5S2 → 5I8 transition, λex = 453 nm), ∼500-575 nm green emissions for Er3+ (2H11/2/4S3/2 → 4I15/2 transitions, λex = 380 nm), and a ∼455 nm blue emission for Tm3+ (1D2 → 3F4 transition, λex = 360 nm), with CIE chromaticity coordinates of around (0.27, 0.71), (0.26, 0.72), and (0.15, 0.04), respectively.

NaLaW2O7(OH)2(H2O): Crystal Structure and RE3+ Luminescence in the Pristine and Annealed Double Tungstates (RE = Eu, Tb, Sm, and Dy)

Hydrothermal reaction of La(NO3)3 and Na2WO4·2H2O at 100 °C and pH 8 resulted in the formation of new compound NaLaW2O7(OH)2(H2O), as confirmed by the X-ray diffraction results, chemical composition, Fourier transform infrared, thermogravimetric/differential thermal analysis, and transmission electron microscopy analyses. The crystal structure was determined in the triclinic system (space group P1̅), with lattice constants a = 5.8671(2) Å, b = 8.2440(2) Å, and c = 9.0108(3) Å, axis angles α = 93.121(2)°, β = 75.280(2)°, and γ = 94.379(2)°, and cell volume V = 420.03(2) Å3. The structure contains two-dimensional layers of -(W1O6)-(W1O6)-(W2O6)-(W2O6)-(W1O6)-(W1O6)- and -LaO9-LaO9- chains alternating in the a-b plane, which are linked together through NaO6 octahedral trigonal prisms by edges to form a three-dimensional net. Dehydration of the compound proceeds up to a low temperature of ∼350 °C and results in the formation of technologically important NaLa(WO4)2 double tungstate, which is thus a unique precursor for the latter. Na(La,RE)W2O7(OH)2(H2O) and Na(La,RE)(WO4)2 solid solutions separately doped with the practically important activators for which RE = Eu, Tb, Sm, and Dy were also successfully synthesized and investigated for their structural features and photoluminescence properties, including excitation, emission, quantum yield, emission color, and fluorescence decay kinetics. The compounds were shown to exhibit dominantly strong red (∼616 nm for Eu3+; λex = 395 or 464 nm), green (∼545 nm for Tb3+; λex = 278 or 258 nm), deep red (∼645 nm for Sm3+; λex = 251 nm), and yellow (∼573 nm for Dy3+; λex = 254 nm) emission upon being irradiated with the peak wavelengths of their strongest excitation bands.

Selective Crystallization of Four Tungstates (La2W3O12, La2W2O9, La14W8O45, and La6W2O15) via Hydrothermal Reaction and Comparative Study of Eu3+ Luminescence

Hydrothermal reaction at 200 °C was systematically undertaken in wide ranges of solution pH (4-13) and W/La molar ratio ( R = 0.5-2), without using any organic additive, to investigate the effect of hydrothermal parameter on product property and the underlying mechanism. Combined analysis by X-ray diffraction (XRD), inductively coupled plasma (ICP) spectroscopy, elemental mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that either a decreasing pH or increasing R value yielded a product richer in W and, conversely, richer in La. The results were interpreted from the solution chemistry of La3+ and tungstate ions. As an outcome of our 40 well-designed experiments, four La tungstates-La2W3O12, La2W2O9, La14W8O45, and La6W2O15-were successfully obtained in a phase-pure form by calcining their hydrothermal precursors. Phase and morphology evolution, structure features, and properties of Eu3+ emission were, for the first time, comparatively investigated for the four compounds. Spectral analysis found that the 5 at.u202f% Eu3+-doped La2W3O12 phosphor exhibits the highest quantum efficiency (∼47%), more red component, and the shortest fluorescence lifetime of luminescence (∼0.72 ms).

Multi-color emission in monodispersed spheres of tetragonal yttrium phosphate: microwave-assisted fast synthesis, formation mechanism, temperature-dependent luminescence, and application in anti-fake labeling

Well-dispersed uniform spheres (∼0.7–1.5 μm in diameter) of YPO4 and (Y,Ln)PO4 (Ln = Eu, Tb, Dy, Ho, Tm, Ce, and Eu/Tb) tetragonal phosphate hydrates have been directly synthesized via microwave-assisted processing. The polycrystalline spheres, which are made of confined 10–17 nm nanocrystals, are direct solid solutions rather than a mechanical mixture of individual particles. Their growth follows an aggregation process, maintaining the composition of the material. The phosphate hydrates converted into anhydrous tetragonal materials via calcination, and the spherical shape and excellent dispersion of the original particles were well retained up to 1000 °C. Upon UV excitation, the YPO4:Ln3+ samples exhibit characteristic emissions of Ln3+ (Ln = Eu, Tb, Dy, Ho, Tm, and Ce) and yield red, green, near white, green, blue, and blue-green emissions, respectively. The color-tunable emissions (from green to red) can be varied by increasing the Eu content, and the Tb3+ → Eu3+ energy transfer process was observed for the tetragonal phosphate in the Y/Tb/Eu system. The efficiency of the Tb3+ → Eu3+ energy transfer was discovered to be ∼28% for (Y0.88Tb0.02Eu0.10)PO4. The thermal stability of (Y0.88Tb0.02Eu0.10)PO4 is lower than that of (Y0.98Tb0.02)PO4 and (Y0.95Eu0.95)PO4. Thus, the 5D4 → 7F5 transition of Tb3+ at 546 nm splits into → 7F5 (at ∼538–546 nm) and → 7F5 (at ∼557 nm) transitions during the heating process. This phenomenon has been ascribed to the variation of the Tb3+ crystal environment. The energy level gradually shifts to a higher one at an elevated temperature, inducing more and more excited electrons transferring from the higher energy level to the lower one via nonradiative relaxation. Therefore, an enhanced emission intensity at ∼557 nm ( → 7F5 transition) and a weakened one at ∼538–546 nm ( → 7F5 transition) were observed. (Y0.95Eu0.05)PO4/PVA transparent films were successfully fabricated by solution casting, and they exhibit both outstanding flexibility and high transmittances of ≥83% in the 400–850 nm region. The hybrid films can be coated on paper money for anti-fake labeling applications.

Efficient and controllable photoluminescence in novel solid solution Ca1−xSrxHf4(PO4)6:Eu2+ phosphors with high thermal stability for white light emitting diodes

To explore a new family of phosphate phosphors for application in ultraviolet-based light emitting diodes (LEDs), novel promising blue emission solid solution phosphors, Ca1−xSrxHf4(PO4)6:Eu2+, with efficient and controllable photoluminescence were synthesized and characterized in the full range of 0 ≤ x ≤ 1. To deeply understand the relationship between the structure and properties of the phosphors, the phase purities, crystal structures and linear structural evolutions of Ca1−xSrxHf4(PO4)6:Eu2+ solid solutions were investigated in detail by FT-IR spectroscopy and Rietveld structure refinement of their powder XRD patterns and verified by high transmission electron microscopy (HRTEM). The linear structural evolutions induced controllable blue emission with high quantum efficiency and excellent thermal stability in the Ca1−xSrxHf4(PO4)6:Eu2+ solid-solution phosphors. Moreover, the photoluminescence properties in Ca1−xSrxHf4(PO4)6:Eu2+ can be optimized by adjusting the components. The mechanism of the controllable luminescence properties was also investigated in detail. Excellent thermal stability was obtained in Ca1−xSrxHf4(PO4)6:Eu2+, and the thermal quenching mechanism was investigated. Finally, white LEDs were fabricated and their electroluminescence properties were investigated. This simple and effective method to optimize the photoluminescence properties of phosphors can be used for designing other novel solid-solution phosphors.

Combining complexing agent and solvothermal reaction for the morphology controlled synthesis of (Y,Eu)PO4 crystals with size-dependent photoluminescence

Hexagonally structured YPO4 (h-YPO4) micro/nanocrystals with the multiform morphologies of quasi-equiaxed microprisms, quasi-ellipsoidal nanoparticles, and sphere-like nanoparticles, have been successfully synthesized via a facile solvothermal reaction in the co-presence of appropriate amounts of ethylene glycol (EG) and ethylene diamine tetraacetic acid (EDTA) or citric acid (CA). A series of precisely controlled experiments indicated that EG can significantly amplify the abilities of EDTA and CA in modifying the growth regimes of YPO4. Detailed characterizations of the products were implemented by the combined techniques of XRD, FE-SEM, TEM, LPSA (laser particle size analysis), FTIR, and optical spectroscopy, and the mechanisms of the morphology/phase evolution of the YPO4 crystals were investigated via altering the EDTA/Y and CA/Y molar ratios, and the EG content. The particle size-dependent luminescence properties of h-(Y0.95Eu0.05)PO4, including the luminescence intensity, fluorescence lifetime, asymmetry factor of luminescence ((5D0 → 7F2)/(5D0 → 7F1) intensity ratio), and CIE chromaticity coordinates, were also investigated in detail.