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. Res., 0 [1] 112–6 (2010).
2
D. Jia and W. Yen, “Enhanced V3+k Center Afterglow in MgAl2O4 by
Doping With Ce3+,” J. Lumin., 101 [1] 115–21 (2003).
3
F. C. Palilla, A. K. Levine, and M. R. Tomkus, “Fluorescent Properties of
Alkaline Earth Aluminates of the Type MAl2O4 Activated by Divalent Europium,” J. Electrochem. Soc., 115 [6] 642–4 (1968).
4
A. S. Maia, R. Stefani, C. A. Kodaira, M. C. Felinto, E. E. Teotonio, and
H. F. Brito, “Luminescent Nanoparticles of MgAl2O4:Eu, Dy Prepared by
Citrate Sol-Gel Method,” Opt. Mater., 31, 440–4 (2014).
5
V. Singh, V. K. Rai, S. Watanabe, T. G. Rao, L. Badie, et al., “Synthesis,
Characterization, Optical Absorption, Luminescence and Defect Centres in
3+
Er
and Yb3+ Co-Doped MgAl2O4 Phosphors,” Appl. Phys. B, 108 [2] 437–
46 (2012).
6
S. Saha, S. Das, U. K. Ghorai, N. Mazumder, B. K. Gupta, and K.
K. Chattopadhyay, “Charge Compensation Assisted Enhanced Photoluminescence Derived From Li-Codoped MgAl2O4:Eu3+ Nanophosphors for
Solid State Lighting Applications,” Dalton Trans., 42 [36] 12965–74
(2013).
7
Y. Liu, D. Tu, H. Zhu, R. Li, W. Luo, and X. Chen, “A Strategy to
Achieve Efficient Dual-Mode Luminescence of Eu3+ in Lanthanides Doped
Multifunctional NaGdF4 Nanocrystals,” Adv. Mater., 22 [30] 3266–71 (2010).
8
Y.-S. Lin, C.-P. Tsai, H.-Y. Huang, C.-T. Kuo, Y. Hung, et al., “WellOrdered Mesoporous Silica Nanoparticles as Cell Markers,” Chem. Mater., 17
[18] 4570–3 (2005).
9
J. Feng, G. Shan, A. Maquieira, M. E. Koivunen, B. Guo, et al., “Functionalized Europium Oxide Nanoparticles Used as a Fluorescent Label in an
Immunoassay for Atrazine,” J. Anal. Chem., 75 [19] 5282–6 (2003).
10
I. Omkaram, B. V. Rao, and S. Buddhudu, “Photoluminescence Properties
of Eu3+:MgAl2O4 Powder Phosphor,” J. Alloys Compd, 474 [1] 565–8 (2009).
11
R. Wiglusz, T. Grzyb, A. Lukowiak, P. Głuchowski, S. Lis, and W. Strek,
“Comparative Studies on Structural and Luminescent Properties of Eu3+:
MgAl2O4 and Eu3+/Na+:MgAl2O4 Nanopowders and Nanoceramics,” Opt.
Mater., 35 [2] 130–5 (2012).
12
V. Singh, M. Haque, and D.-K. Kim, “Investigation of a New Red-Emitting, Eu3+-Activated MgAl2O4 Phosphor,” Bull. Korean Chem. Soc., 28 [12]
2477–80 (2007).
13
X. Y. Chen, C. Ma, and S. P. Bao, “MgAl2O4:Eu3+ Nanoplates and
Nanoparticles as Red-Emitting Phosphors: Shape-Controlled Synthesis and
Photoluminescent Properties,” Solid State Sci., 12 [5] 857–63 (2010).
14
X. Y. Chen, C. Ma, Z. J. Zhang, and X. X. Li, “Structure and Photoluminescence Study of Porous Red-Emitting MgAl2O4:Eu3+ Phosphor,” Microporous Mesoporous Mater., 123 [1] 202–8 (2009).
15
C. St€
otzel, H.-D. Kurland, J. Grabow, S. Dutz, E. M€
uller, et al., “Control
of the Crystal Phase Composition of FexOy Nanopowders Prepared by CO2
Laser Vaporization,” Cryst. Growth Des., 13 [11] 4868–76 (2013).
16
C. St€
otzel, H.-D. Kurland, J. Grabow, and F. A. M€
uller, “Gas Phase
Condensation of Superparamagnetic Iron Oxide-Silica Nanoparticles - Control
of the Intraparticle Phase Distribution,” Nanoscale, 7 [17] 7734–44 (2015).
17
J. F. Bartolome, A. Smirnov, H.-D. Kurland, J. Grabow, and F. A.
M€
uller, “New ZrO2/Al2O3 Nanocomposites Fabricated From Hybrid
Nanoparticles Prepared by CO2 Laser Co-Vaporisation,” Sci. Rep., 6, 20589,
11pp (2016).
18
F. Wesarg, F. Schlott, J. Grabow, H.-D. Kurland, N. Heßler, et al., “In
Situ Synthesis of Photocatalytically Active Hybrids Consisting of Bacterial
Nanocellulose and Anatase Nanoparticles,” Langmuir, 28 [37] 13518–25
(2012).
19
C. Zollfrank, S. Gruber, M. Batentschuk, A. Osvet, F. G€
otz-Neunhoeffer,
et al., “Synthesis of Eu-Doped SrAl2O4 Nanophosphors by CO2 Laser Vaporization,” Acta Mater., 61 [19] 7133–41 (2013).
20
H.-D. Kurland, J. Grabow, and F. A. M€
uller, “Preparation of Ceramic
Nanospheres by CO2 Laser Vaporization (LAVA),” J. Eur. Ceram. Soc., 31
[14] 2559–68 (2011).
2564
Rapid Communications of the American Ceramic Society
21
I. Beketov, A. Medvedev, O. Samatov, A. Spirina, and K. Shabanova,
“Synthesis and Luminescent Properties of MgAl2O4:Eu Nanopowders,” J.
Alloys Compd, 586, S472–5 (2014).
22
H. D. Kurland, C. St€
otzel, J. Grabow, I. Zink, E. M€
uller, et al., “Preparation of Spherical Titania Nanoparticles by CO2 Laser Evaporation and Process-Integrated Particle Coating,” J. Am. Ceram. Soc., 93 [5] 1282–9 (2010).
23
P. Scherrer, “Bestimmung der Inneren Struktur und der Gr€
oße von Kolloidteilchen Mittels R€
ontgenstrahlen,” Nachr. Ges. Wiss. G€
ottingen, 1918, 98–
100 (1918).
24
B. Hallstedt, “Thermodynamic Assessment of the System MgO-Al2O3,” J.
Am. Ceram. Soc., 75 [6] 1497–507 (1992).
Vol. 99, No. 8
25
L. Cain, G. Pogatshnik, and Y. Chen, “Optical Transitions in NeutronIrradiated MgAl2O4 Spinel Crystals,” Phys. Rev. B, 37 [5] 2645–52 (1988).
26
R. D. Shannon, “Revised Effective Ionic Radii and Systematic Studies of
Interatomic Distances in Halides and Chalcogenides,” Acta Crystallogr., Sect.
A, 32 [5] 751–67 (1976).
27
L. Chen, J. Zhang, X. Zhang, F. Liu, and X. Wang, “Optical Properties
of Trivalent Europium Doped ZnO:Zn Phosphor Under Indirect Excitation of
Near-UV Light,” Opt. Express, 16 [16] 11795–801 (2008).
28
S. Lima, F. Sigoli, M. Davolos, and M. Jafelicci, “Europium(III)-Containing Zinc Oxide From Pechini Method,” J. Alloys Compd, 344 [1] 280–4
(2002).
h