•NH2-Tb-MOF is facilely synthesized under microwave irradiation condition.•Structure characterization demonstrates the amino groups in the micropores.•The activity of NH2-Tb-MOF as solid base catalyst is evaluated.•The NH2-Tb-MOF shows a selective catalysis towards small sized reactants.•The stable structure affords the heterogeneous catalyst recyclable and reusable.Lanthanide metal-organic frameworks (Ln-MOFs) are featured by their tolerance to water and dense structure. In this work, an amine-functionalized Ln-MOF was facilely synthesized by coordination of terbium with 2-aminoterephthalic acid under the condition of microwave irradiation. The crystal structure was characterized by single crystal X-ray diffraction, FT-IR, Raman, TG-DTA and XPS analysis. The basic catalytic activity of the NH2-Tb-MOF was evaluated for Knoevenagel condensation and Henry reactions. Apart from the high activity and 100% selectivity to the condensation product, the NH2-Tb-MOF catalyst could be easily recycled and reused owing to the high stability of the MOF framework formed by coordination of Tb3+ with carboxylic groups. Remarkably, the NH2-Tb-MOF exhibited size-selective catalysis to substrates. For the small-sized reactants, it displayed comparable activity to the homogeneous catalyst of aniline owing to the high dispersion of NH2− active sites and the low diffusion limits. However, in the same reaction system, extremely poor activity in Knoevenagel condensation and Henry reaction for the bulky substrate 4-(tert-butyl) benzaldehyde was observed due to the both effects of substitute and inhibition of diffusion into the micropores. Crystal structure analysis provided a mechanistic evidence that the heterogeneous base catalysis arose from the amino groups densely distributed inside the micropores.Download high-res image (251KB)Download full-size image

•A dual-emitting Eu/Zr-MOF is synthesized.•Small molecules-dependent luminescence enhancing and quenching is observed.•A drastic enhancement of fluorescence at 465 nm is induced by formaldehyde.•Ratiometric fluorescent sensing of formaldehyde is performed.•The binding interaction of the free amino groups in the micropores to guest is demonstrated.A dual-emitting luminescent lanthanide/transition heterometal-organic frameworks of Eu/Zr-MOF was synthesized by incorporation of Eu3+ ions into NH2-UiO-66 under microwave irradiation condition. The multiband fluorescence derived from the characteristic emission of Eu3+ and linker-to-cluster (Eu- oxo or Zr- oxo) charge transfer (LCCT) transition was fabricated. By combination of the luminescent property with the intrinsic porosity and open sites of amine group to bind target analytes, the Eu/Zr-MOF exhibited small molecules-dependent luminescence enhancement and quench effects. Notably, a drastic enhancement of fluorescent at 465 nm induced by formaldehyde was observed. Thus, a ratiometric fluorescent sensing for formaldehyde was performed based on the intensity ratio of two emission bands at 465 and 615 nm for Eu/Zr-MOF. Under the excitation of 365 nm, the increase in intensity ratio of the two emission bands was nearly linearly proportional to the amount of formaldehyde. By this Eu/Zr-MOF sensor, the detection limit of formaldehyde was 0.2 mg/L. This sensing mechanism was ascribed to the binding interaction of free amino groups in Eu/Zr-MOF with the guest. An added electron transfer from amino group containing lone pair electrons to the positively charged formaldehyde leads to a drastic enhancement of luminescence at about 465 nm, while the characteristic emission of Eu3+ at 615 nm enhances slightly. These studies demonstrate that the strategy of multiband emissive heterometal-MOFs can be served as a facile method to fabricate sensitive and specific fluorescent probes of polluting organic small molecules.Download high-res image (152KB)Download full-size image

A series of single-phase broadband white-light-emitting Sr5(PO4)3F:Eu2+,Mn2+ phosphors were prepared by a solid state reaction. The luminescence property, and the crystal and electronic structures of the fluorophosphates were studied by photoluminescence analysis, XRD Rietveld refinement and density functional theory calculation (DFT), respectively. Under near ultraviolet excitation in the 250 to 430 nm wavelength range, the phosphors exhibit two emission bands centered at 440 and 556 nm, caused by the Eu2+ and Mn2+ ions. By altering the relative ratios of Eu2+ and Mn2+ in the compounds, the emission color could be modulated from blue to white. The efficient energy transfer from the Eu2+ to Mn2+ ions could be ascribed to the well crystallized host lattice and the facile substitution of Eu2+ and Mn2+ for Sr2+ sites due to similar ionic radii. A series of fluxes were investigated to improve the photoluminescence intensity. When KCl was used as flux in the synthesis, the photoluminescence intensity of Sr5(PO4)3F:Eu2+,Mn2+ was enhanced by 85% compared with no fluxes added. These results demonstrate that the single-phase Sr5(PO4)3F:Eu2+,Mn2+ with enhanced luminescence efficiency could be promising as a near UV-convertible direct white-light-emitting phosphor for WLED applications.

A series of La3BW1–xMoxO9:Eu3+ (x = 0–0.4) polycrystalline powders were prepared by using solid-state reactions. The phase structure, UV–vis absorption spectra, and photoluminescence properties were studied as a function of the Mo/W ratio. When Mo6+ ions are incorporated into the lattice, the characteristic sharp lines in the excitation spectra of Eu3+ monitored at 617 nm are prominently enhanced, which join the ligand-to-metal charge transfer (LMCT) band of La3BW1–xMoxO9:Eu3+ into a broad band ranging from 250 to 450 nm centered at 375 nm. The intensity of the broad excitation band reaches a maximum when the content of Mo6+ ions increases to x = 0.3. On the other hand, the LMCT band around 306 nm decreases and shifts toward the longer wavelength. These features are advantageous to near-UV or blue light GaN-based LED applications. Orbital population analysis by density functional theory calculation (DFT) reveals that the near-UV excitation of La3BW1–xMoxO9:Eu3+ red phosphor is due to the electronic transition from the O 2p orbital to the W 5d and Mo 4d orbitals, respectively. With the introduction of Mo6+ into the lattice, the band gap of La3BW1–xMoxO9 becomes narrower than that of the pure phase La3BWO9.

The effect of alkali-metal ions on the local structure and luminescence properties for alkali-metal europium double tungstate compounds AEu(WO4)2 (A = Li, Na, K) has been investigated by a dual-space structural technique, atomic pair distribution function (PDF) analysis, and the Rietveld method of powder X-ray diffraction. The compounds AEu(WO4)2 (A = Li, Na) crystallize in the isostructure with the tetragonal space group I41/a (No. 88) and show the same luminescence properties in spite of the different doped alkali metals. However, KEu(WO4)2 crystallizes in monoclinic symmetry with the space group C2/c (No. 15). Compared with the two other counterparts, KEu(WO4)2 exhibits a more effective charge-transfer excitation, a larger Stokes shift, and a broader 612 nm emission band. This phenomenon is ascribed to the lower crystal symmetry in KEu(WO4)2, which influences bond distances and the coordination number of Eu3+. Two complementary methods, the Rietveld method and PDF analysis, reveal that both LiEu(WO4)2 and NaEu(WO4)2 afford the same local surroundings of Eu3+. The local structure determined by the Rietveld and PDF methods well account for the observed luminescent properties.

The compound Y2WO6 is prepared by solid state reaction at 750 °C using sodium chloride as mineralizer. Its structure is solved by ab-initio methods from X-ray powder diffraction data. This low temperature phase of yttrium tungstate crystallizes in tetragonal space group P4/nmm (No. 129), Z=2, a=5.2596(2) Å, c=8.4158(4) Å. The tungsten atoms in the structure adopt an unusual [WO6] distorted cubes coordination, connecting [YO6] distorted cubes with oxygen vacancies at the O2 layers while other yttrium ions Y2 form [YO8] cube coordination. Y3+ ions occupy two crystallographic sites of 2c (C4v symmetry) and 2a (D2d symmetry) in the Y2WO6 host lattice. With Eu3+ ions doped, the high resolution emission spectrum of Y2WO6:Eu3+ suggests that Eu3+ partly substituted for Y3+ in these two sites. The result of the Rietveld structure refinement shows that the Eu3+ dopants preferentially enter the 2a site. The uniform cube coordination environment of Eu3+ ions with the identical eight Eu–O bond lengths is proposed to be responsible for the intense excitation of long wavelength ultraviolet at 466–535 nm.Graphical abstractThe excitation and emission spectra of Y2WO6:Eu3+ indicates the effect of long wavelength-excited property, which is ascribed to the cube coordination environment of Eu3+ ions with the uniform Eu–O bond length.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Y2WO6 is prepared using sodium chloride as mineralizer. ► The structure of the compound Y2WO6 is solved by ab-initio methods. ► The site occupation of Eu3+ is determined. ► The cube coordination of Eu3+ ions is responsible for the long wavelength excitation.

White xerogel powder of yttrium tungstate-chloride was synthesized, and its photoluminescence properties were investigated. Under the excitation of 254 nm, the xerogel phosphor exhibits emission ranging from 300 to 650 nm. This luminescent spectrum is identified as two emission bands of 300–400 and 400–650 nm due to different emission mechanism. While the emission band of 300–400 nm is ascribed to the charge transfer (CT) from O to metal W, the emission of 400–650 nm is attributed to electron–hole (e−–h+) carrier emission related to oxygen vacancies. By calcining the sample in reducing atmosphere, the number of oxygen vacancies acting as luminescence centers is increased. As a result, the emission intensity of 400–650 nm is significantly enhanced. Based on electron paramagnetic resonance and spectral analysis, the mechanism of peroxy-radical hole traps (PRHT) is proposed for the luminescence of 400–650 nm.

•A series of single phased warm-white-emissive and color-tunable phosphors were prepared.•The crystal structure and photoluminescence property of the phosphors were systematically investigated.•The energy transfer process from Tb3+ to Eu3+ in Sr7Zr(PO4)6 was verified by fluorescence spectra and decay time.•Excellent thermal quenching property predicts the promising application in near-UV LEDs.A series of Sr7Zr(PO4)6:Tb3+, Eu3+ phosphors of eulytite-type structure have been synthesized via a high temperature solid-state reaction. The crystal structure and luminescence property of the samples was systematically investigated by XRD Rietveld refinement, excitation and photoluminescence, respectively. For Tb3+ and Eu3+ ions single doped phosphor, the characteristic emission from f-f transitions of Tb3+ at 544 nm and Eu3+ at 613 nm was observed, while for Tb3+ and Eu3+ co-doped phosphor Sr7Zr(PO4)6:Tb3+, xEu3+, various emission color from green to red can be achieved by trimming the relative ratio of Eu3+ to Tb3+ in Sr7Zr(PO4)6 host. Moreover, a warm white light emission of single phased with CIE coordinates at (0.359, 0.327) and correlated color temperature (CCT) at 4264 K was realized under 374 nm excitation. The energy transfer process from Tb3+ to Eu3+ in Sr7Zr(PO4)6 was verified by fluorescence spectra and decay time, where a resonant type via a dipole-dipole mechanism was demonstrated. In addition, the Sr7Zr(PO4)6:Tb3+, Eu3+ phosphors exhibited an excellent thermal quenching luminescence property owing to the structural stability. These results indicate that Sr7Zr(PO4)6:Tb3+, Eu3+ could be anticipated as color-tunable and direct white-light-emitting phosphor for near-UV-pumped w-LEDs application.

A series of single-phase broadband white-light-emitting Sr5(PO4)3F:Eu2+,Mn2+ phosphors were prepared by a solid state reaction. The luminescence property, and the crystal and electronic structures of the fluorophosphates were studied by photoluminescence analysis, XRD Rietveld refinement and density functional theory calculation (DFT), respectively. Under near ultraviolet excitation in the 250 to 430 nm wavelength range, the phosphors exhibit two emission bands centered at 440 and 556 nm, caused by the Eu2+ and Mn2+ ions. By altering the relative ratios of Eu2+ and Mn2+ in the compounds, the emission color could be modulated from blue to white. The efficient energy transfer from the Eu2+ to Mn2+ ions could be ascribed to the well crystallized host lattice and the facile substitution of Eu2+ and Mn2+ for Sr2+ sites due to similar ionic radii. A series of fluxes were investigated to improve the photoluminescence intensity. When KCl was used as flux in the synthesis, the photoluminescence intensity of Sr5(PO4)3F:Eu2+,Mn2+ was enhanced by 85% compared with no fluxes added. These results demonstrate that the single-phase Sr5(PO4)3F:Eu2+,Mn2+ with enhanced luminescence efficiency could be promising as a near UV-convertible direct white-light-emitting phosphor for WLED applications.