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Tài liệu Eu doped mgal2o4 2016

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J. Am. Ceram. Soc., 99 [8] 2561–2564 (2016) DOI: 10.1111/jace.14383 © 2016 The American Ceramic Society Journal Rapid Communication Europium(III)-Doped MgAl2O4 Spinel Nanophosphor Prepared by CO2 Laser Co-Vaporization Christoph Wenisch, Heinz-Dieter Kurland,† Janet Grabow, and Frank A. M€ uller Otto Schott Institute of Materials Research (OSIM), Friedrich Schiller University Jena, Jena 07743, Germany radiation, the raw powder heats up, vaporizes, and forms a plasma. The vaporization proceeds in a flowing process gas at atmospheric pressure. In the process gas, plasma and vapor rapidly cool down and ultrafine particles arise by gasphase condensation. Recently, we used the laser co-vaporization (CoLAVA) to prepare europium-doped SrAl2O4 NPs from a homogeneous powder mixture of SrO, Al2O3, and Eu2O3.19 However, the as-prepared amorphous NPs were only weakly photoluminescent. Only after annealing under reducing conditions, a crystalline photoluminescent powder was obtained. However, as a negative side effect the primary NPs fused into hard aggregates. In contrast, MgAl2O4 spinel with its very stable cubic structure should be suitable to form crystalline NPs even at the high quenching rates19–21 of the CoLAVA process. Beketov et al. prepared Eu3+-doped MgAl2O4 NPs from a powder mixture of MgO, Al2O3, and Eu2O3 using another implementation of the laser vaporization technique with an ytterbium fiber laser.21 However, photoluminescence was also achieved only after subsequent annealing which resulted in the undesired fusion of the as-prepared well-dispersed NPs. Photoluminescent nanoparticles (NPs) are of specific interest for biomedical applications, bioimaging, and cell tracking. Here, we report on the synthesis of europium(III)-doped MgAl2O4 spinel NPs by the CO2 laser co-vaporization of a homogeneous raw powder mixture consisting of micrometersized MgAl2O4 and Eu2O3 (2 and 4 mol%, respectively). The resulting NPs are spherically shaped, show a narrow size distribution (mean diameter: ~30 nm), and are well dispersed. The as-prepared NPs are highly crystalline and consist of MgAl2O4 with small amounts of the secondary phases MgO (~10 mass %) and Eu2O3 (<0.5 mass%). The photoluminescence spectra of the doped spinel nanopowders show an intense red emission (kem = 615 nm) resulting from the 5D0?7F2 transition with a maximum intensity at an excitation wavelength of 470 nm. Keywords: nanoparticles; photoluminescence; spinels; europium; phosphors I. D Introduction to their electro-optical properties, spinel structures are of particular interest as host material in bright luminescent phosphors for displays and lighting devices. Magnesium aluminate (MgAl2O4) spinel with a wide band gap of 7.8 eV (wavelength: 159 nm) provides excellent visible transparency, thus preventing absorption of visible emissions from an activator.1 Consequently, rare-earth ions like Ce3+,2 Eu2+,3 Dy3+,4 Er3+,5 and Yb3+5 have been investigated as potential dopants. Doped with lanthanide ions, e.g., Eu3+, MgAl2O4 spinel provides high quantum efficiencies.6 Typically, only a small amount (~2 mol%) of the dopant is used to prevent concentration quenching.6 By using nanoparticles (NPs), the application spectrum of MgAl2O4:Eu3+ could be expanded to e.g., markers in biomedical imaging and diagnostics.7 For this purpose, the nanophosphors have to be nontoxic, well dispersed, crystalline, and homogeneously doped.5,8,9 Different approaches, including solid-state reactions,10 sol–gel processing,6,11 combustion methods,12 and hydrothermal syntheses,13,14 were used to prepare MgAl2O4: Eu3+ NPs. However, to the best of our knowledge, the synthesis of MgAl2O4:Eu3+ NPs meeting the requirements of biomedical applications has not been reported so far. The CO2 laser vaporization technique has proven its versatility to prepare a broad range of functional, single or multiphase ceramic NPs with excellent properties.15–18 Briefly, a CO2 laser beam is focused onto the surface of a coarsegrained raw powder. By absorbing the high-intensity laser UE II. Experimental Procedure MgAl2O4 powder (Alfa Aesar, Karlsruhe, Germany; magnesium aluminum oxide, product no. 22950, purity 99%, grain size <44 lm) as host material was homogeneously mixed with 2 and 4 mol% of Eu2O3 powder as dopant (Alfa Aesar, Karlsruhe, Germany, REactonÒ, europium(III) oxide, product no. 11300, purity 99.9%), respectively. Starting from pure MgAl2O4 powder as well as from the powder mixtures, the nanopowder samples MA0, MA2, and MA4, respectively, were prepared using the CoLAVA method (continuous laser radiation, focus intensity: 175 kW/cm2, process gas: air, total flow rate: 14.5 m3/h). Under these conditions, the production rate of nanopowder was ~25 g/h. Transmission electron microscopy (TEM; JEM 3010, JEOL, Tokyo, Japan, accelerating voltage: 300 kV) was used in order to evaluate morphology and diameter distributions of the NPs. Diameters of 1000 NPs per sample were measured from TEM micrographs in order to compile the diameters’ percental density distributions on number and length basis (q0 and q1, respectively).22 Logarithmic normal distributions were fitted to each measured distribution in order to obtain the geometrical mean diameters lg(q0) and lg(q1). The characteristic diameters d10, d50, and d90 were evaluated from the number based cumulative distributions (Q0) fitted with a sigmoid function. The specific surface areas SBET of the nanopowders (dried and degassed for 2 h at 200°C) were measured using the Brunauer–Emmett–Teller (BET) method (Gemini 2370 surface area analyzer, Micromeritics Co., Aachen, Germany). Crystallographic properties of the nanopowders were analyzed using X-ray diffraction (XRD; D5000, Siemens, Karlsruhe, Germany; accelerating voltage: 40 kV, beam current: H. Wu—contributing editor Manuscript No. 38300. Received March 10, 2016; approved June 10, 2016. † Author to whom correspondence should be addressed. e-mail: [email protected] 2561 2562 Rapid Communications of the American Ceramic Society Vol. 99, No. 8  step size: D2h = 0.02°). 40 mA, CuKa wavelength: 1.5418 A, The mean crystallite sizes dhkl of the NPs were calculated from the (311) reflection of MgAl2O4 with the Scherrer formula (shape factor: 0.9).23 The residual Eu2O3 content in the nanopowders MA2 and MA4 was estimated from the areas under the characteristic Eu2O3 reflections at 2h angles of 28.5°, 32.9°, and 47.3°. Photoluminescence excitation (PLE) and emission (PL) spectra were recorded (excitation wavelength kex: 350– 545 nm, emission wavelength kem: 550–800 nm, step size: 5 nm) using a 75 W Xe arc lamp (Tunable PowerArc Illuminator, OBB Corp., Edison, NJ). III. Results and Discussion TEM micrographs (Fig. 1) of the nanopowders show almost spherical NPs that are only softly agglomerated. The lattice planes visible at higher magnification indicate that the NPs are predominantly polycrystalline. XRD measurements (Fig. 2) confirm cubic MgAl2O4 spinel as main phase with ~10 mass% of cubic MgO as secondary phase in all nanopowders. This is due to local inhomogeneities in the CoLAVA condensation zone. Here, excess MgO separately condenses (boiling points: MgO 3600°C, Al2O3 2980°C) and crystallizes as periclase.24 With decreasing temperature, a defect spinel forms with excess Al3+ occupying Mg2+ sites.25,26 Samples MA2 and MA4 contain less than 0.5 mass % (0.2 mol%) of residual cubic Eu2O3 indicating that most of the Eu3+ ions are incorporated into the spinel lattice. A more precise estimation of the Eu2O3 content was not possible because the intensities of the underlying Eu2O3 reflections are at the resolution limit of the diffractometer. Mean particle diameters lg(q0), widths d90–d10 of the corresponding diameter distributions, mean crystallite sizes dhkl, and specific surface areas SBET of the nanopowders are Fig. 2. XRD diagrams of nanopowders MA0 (a), MA2 (b), and MA4 (c). The reflections are indexed according to the ICDD PDF-2 (2002): cubic MgAl2O4 (▲), cubic MgO ( ), and cubic Eu2O3 ( ). summarized in Table I. Crystallite sizes less than half the NPs’ mean diameters confirm that the NPs are predominantly polycrystalline. With increasing Eu3+ concentration, the NPs’ mean diameters and crystallite sizes increase. This is due to the higher ionic radii of Eu3+ (95 pm) compared with Al3+ (54 pm) and Mg2+ (65 pm).14 Additionally, in order to keep charge balance, the partial substitution of Mg2+ introduces vacancies on cationic lattice sites. This results in structural disorder and consequently, in increased mean crystallite and particle sizes and decreased specific surface areas.6,26 PLE and PL spectra (Fig. 3) of the undoped sample MA0 show only the weak luminescence of the spinel host, while the spectra of MA2 and MA4 show the characteristic intense Eu3+ emissions. The green emission of Eu2+ was not observed. The PL spectra (kex = 470 nm) of the doped nanopowders (Fig. 3, right) show the bright red luminescence of the Eu3+ ions. Besides this strongest emission at 615 nm from the Fig. 1. TEM (top) and high-resolution TEM (bottom) micrographs of the nanoparticles of samples MA0 (left), MA2 (middle), and MA4 (right). August 2016 Table I. 2563 Rapid Communications of the American Ceramic Society Mean Diameters lg(q0) and lg(q1), Widths d90–d10 of the Diameter Distributions, Specific Surface Areas SBET, and Mean Crystallite Sizes d(311) of the Nanopowders MA0, MA2, and MA4 Sample lg (q0) (nm) lg (q1) (nm) d90–d10 (nm) SBET (m2/g) d(311) (nm) MA0 MA2 MA4 28.2  0.3 31.7  0.5 33.0  0.7 32.5  0.5 46.7  0.6 38.0  0.9 29.3  0.4 34.5  0.4 35.7  0.6 52.2  0.1 47.4  0.1 44.4  0.1 12.0  0.4 13.3  0.3 14.9  0.3 (CoLAVA) method. Without any additional thermal treatment, polycrystalline and strongly photoluminescent NPs were obtained. The most intense emission of Eu3+ at 615 nm arising from the 5D0?7F2 transition is reached at an excitation wavelength of 470 nm. The NPs prepared from the starting powder with 4 mol% Eu2O3 yielded the brightest luminescence. References 1 Fig. 3. Photoluminescence excitation spectra at kem = 615 nm (left) and photoluminescence emission spectra at kex = 470 nm (right) of the nanopowders MA0 ( ), MA2 ( ), and MA4 (—). 5 D0?7F2 transition, the emissions of the 5D0?7FJ transitions (J = 0, 1, 3, and 4 at kem = 580, 593, 657, and 697 nm, respectively) are also visible.10 The emission intensity of the magnetic dipole transition 5D0?7F2 exceeds that of the electric dipole transition 5D0?7F1 by far. This indicates that Eu3+ substitutes ions at lattice sites without inversion symmetry, i.e., Mg2+.10,11,14 Despite differing valences, this substitution is preferred because the ionic radius of Eu3+ is closer to that of Mg2+ than to that of Al3+.14 With increasing Eu3+ content, the emission intensity of the 5D0?7F2 transition increases while there is almost no intensity gain of the other emissions. Here, the increasing Eu3+ content in the spinel host causes concentration quenching due to cross relaxation and fast energy migration of the activator reducing the emission intensity.10 Several studies found the optimum Eu3+ content to be between 1 and 2 mol%.10,11 The excitation peaks in the PLE spectra of the doped nanopowders (Fig. 3, left) measured at kem = 615 nm are attributed to the 7F0?5D4 (kex = 365 nm), 7F0?5L7 (kex = 385 nm), 7F0?5L6 (kex = 400 nm), 7F0?5D3 (kex = 420 nm), 7 F0?5D2 (kex = 470 nm), and 7F0?5D1 (kex = 540 nm) transitions.6 The most intense peaks are at kex = 400, 470, and 540 nm. The doped samples yield comparable emission intensities except for the 7F0?5D2 transition. Its emission intensity significantly increases from sample MA2 to MA4, i.e., with increasing proportion of Eu2O3 in the raw powder mixture. Furthermore, according to literature, the 7F0?5L6 transition (kex = 400 nm) of MgAl2O4:Eu3+ should show the maximum emission intensity.6,10,11 Here, however, the 7 F0?5D2 (kex = 470 nm) transition yields the highest intensity just as in the PLE spectrum of pure Eu2O3 particles.9,27,28 These findings indicate (i) the presence of free Eu2O3 in the doped nanopowders concordant with the XRD measurements (Fig. 2), and (ii) that further increasing the Eu2O3 content of the raw powder mixture would merely result in an increasing amount of free Eu2O3 in the nanopowder without improvement of the fluorescence properties of the doped NPs. IV. Conclusions Spherical, narrowly size-distributed, and well-dispersed Eu3+doped MgAl2O4 NPs with different europium contents were successfully prepared using the CO2 laser co-vaporization M. Hachemaoui, F. Semari, R. Khenata, A. Bouhemadou, and M. Rabah, “Structural, Elastic and Electronic Properties of XAl2O4 (X=Mg, Zn) Compounds,” J. Sci. 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