Synthesis and Fluorescence Properties of Eu2+-Doped KMgF3 Nanoparticles1

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CHEM.

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RES. CHINESE U. 2006, 22( 3 ) , 274-277

Synthesis and Fluorescence Properties of Eu2+-DopedKMgF, Nanoparticles * YAN Jing-hui' , ZHANG Ming'" , LIAN Hong-zhou2, LIU Jie' , LI Zhong-tian' , CAO Jie' and SHI Chun-shan"

*

1. College .f Materials and Chemical Engineering, Changchun University of Science and Technology, Changchun 130022 , P. R. China ; 2. Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China Received Aug. 22, 2005 The phase diagram of a cetyltrimethyl ammonium bromide ( CTAB)/n-butanoVn-octane/K"O, -Mg ( NO3 ) system was drawn. Nanoparticles of EuZ' -doped KMgF, were prepared from the quaternary microemulsions of cetyltrimethy1 ammonium bromide( CTAB) , n-butanol, n-octane and water. The X-ray diffraction( XRD) patterns were indexed to a pure KMgF, cubic phase. The environmental scanning electron microscopic( ESEM) images show the presence of spherical EuZ+ -doped KMgF, nanoparticles with a diameter of ca. 20 nm. The emission of KMgF,: Eu2+ nanoparticles peaks at 360 nm. The excitation band was observed at 250 nm with a blue shift of ca. 70 nm compared with that of KMgF, : Eu" single crystal. The preparation method of nano-KMgF, : Eu2'/PMMA composite films was inquired into. Keywords Fluoride ; Microemulsion phase diagram ; Nanostructure ; X-ray diffraction ; Luminescence Article ID loO5-904O(2006) 43-274-04

Introduction Nanomaterials usually show some novel optical, electronic, magnetic and chemical properties significantly different from those of the bulk materials because of their extremely small size, large specific surface areas and peculiar quantum size. The potential of their applications in various fields of science and technology is tremendous. Therefore, nanomaterials have been attracting much attention in recent years. There are several methods for nanopartices preparation, which are gas-phase synthesis, solid-phase synthesis and liquidphase synthesis. Liquid-phase synthesis includes many different kinds of techniques, such as coprecipitation , hydrothermal synthesis, sol-gel processing, solvothermal synthesis, microemulsion techniques, etc. . A microemulsion is generally defined as a thermodynamically stable, optically isotropic dispersion of two immiscible liquids ( usually water and hydrocarbon ) , one or both of which are dispersed as microdomains, which are 10-50 nm in dimension"]. Microemulsions

have been widely utilized for the synthesis' of nanoparticles , such as Cu, Ho, Oizl , cobalt ferriteL3], polyaniliner4], erbium-doped BaF;'], cerium-doped BaFF] , CdE( E = S , Se, Te) semicond~ctor"~ , etc. , because of easy operation and uniformity of particle products. KMgF, is a typical complex fluoride with a cubic perovskite structure. In this structure, F - ions are situated at the face-center positions of a cube, 12-fold K' ions are situated at the corners, whereas 8-fold Mg2' ions are situated at the body center"]. The well-established interest in the fluorescence properties of E u 2 + doped KMgF, has been enhanced recently by its various technological applications, such as lamp phosphors and tunable solid state l a ~ e r s [ ~ - ' ~ The ~ . line emission of Eu2' in KMgF, single crystal peaks at 360 nm and the band excitation peaks at 320 nm"'] . In this study, KMgF, : EuZ+ nanoparticles were prepared, and their fluorescence properties were studied at an excitation wavelength of 40 (266 nm) with a pump source YAG: Nd. KMgF,: Eu" nanoparti-

* Supported by the National Natural Science Foundation of China( No. 90201032) and the Science and Technology Department of Jilin Province( No. 20050507). * * To whom correspondence should be addressed. E-mail: cshi@ ciac. jl. cn

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cles that can be embedded into composite films acting as the activator has potential applications in the field of film laser materials. Hua Rui-nian et al. [ I 3 ] prepared Eu2+-doped KMgF, nanocrystalline via a water/cetyltrimethyl ammonium bromide( CTAB ) /2-octanol microemulsion and discussed its spectral properties. To further study the influences of a microemulsion system on composite fluoride preparation and on KMgF, doped rare earth ion fluorescent properties, a phase diagram of a new microemulsion system was drawn, and KMgF, : ELI,+ nanoparticles were prepared under the super conditions determined by the phase diagram. With KMgF,: Eu2+ as an active component and PMMA as the host, the preparation of nano-KMgF, : Eu2'/PMMA composite films was studied.

were dispersed in absolute ethanol by ultrasonication in a KQ-250B ultrasonic bath, and allowed to dry on a silicon slice. The shape and size of particles were examined with a Philips XI30 environmental scanning electron microscope ( ESEM ). The emission and excitation spectra were measured at room temperature by using a Hitachi F-4500 fluorescence spectrometer with a xenon lamp, operated at 700 V , and a scanning speed of 2400 nm/min. The preparation of nano-KMgF, : Eu2+ /PMMA composite films was finished as follows. An organic solution was produced by dissolving 0.5 g of PMMA completely in 10 mL of methylbenzene. To this 0.025 g of KMgF, : EuZ+ was added with supersonic stirring till the organic solution became transparent, and then the organic solution was dried into films.

Experimental

Results and Discussion

Eu ( NO, ), was prepared in our laboratory by dissolving 99.99% purity Eu, 0, into an HNO, solution. Mg( NO,), * 6H,O, KNO, , NH,F, cetyltrimethyl ammonium bromide( CTAB) , n-butanol, all were of analytical reagent grade, were supplied by Beijing Chemical Inc. n-Octane( analytical reagent grade ) was purchased from Shanghai Chemical Inc.. Two organic solutions were prepared separately with 3.8 g of CTAB , 3. 1 g of n-butanol, and 10.6 g of n-octane , respectively. An aqueous solution( 3 mL) of 1.0 x mol of Mg ( NO,),, 1.0 x lo-, mol of KNO, , and 2 x 10 -'mol of Eu ( NO, ), was added dropwise into one of the above-prepared organic solutions under magnetic stirring until clear microemulsion I was obtained. Another aqueous solution ( 3 mL) of 3.6 x lo-, mol of NH,F was added dropwise into another above-prepared organic solution, and clear microemulsion Iz was obtained. Microemulsions I and II were mixed quickly, and stirred vigorously at room temperature for 15 min. The reaction products were centrifuged at 6000 r/min for 20 min, and then washed five times with a 1 : 1 ( volume ratio) mixture of methanol and dichloromethane to remove the organic phase and the surfactant. The final products were dried under an infrared lamp, lightly crushed with an agate mortar, and calcined at 300 9= for 1 h under the protection of N, to remove residual water and organic impurities. The phase purity of the final products was characterized by X-ray diffraction ( XRD ) measurement performed on a Rigaku D/max II B X-ray diffractometer under monochromatic Cu Ka radiation ( A = 0.15418 nm) . The step-scan covered an angular range from 20" to 80" with a step size of 0.02". The powder samples

Fig. 1 shows the phase diagram of a cetyltrimethyl ammonium bromide ( CTAB ) /n-butanown-octane/ KN0,-Mg( NO,), system, in which two phase regions are relatively larger, while the double continuous phase region and the O/W region are relatively smaller. According to the relationship between the conductivity and the variation in water content, in the W/O region of the phase diagram, conditions corresponding to point T in Fig. 1 were selected as the super preparation conditions of KMgF,: Eu2+ nan~particles"~~.

o.fio.8 n-Octane

(

0.4/

0. 6 0.8

OIW

/-

T

\ ,*'

.rwo , phase

W I O_--- ____ -__.---_---_.--CYFB

KNOs-Mg(NOs)z 0.2 I 0 . 4 0.6 solution Bicontinuous

0.8 CTAB+n -butanol

Fig. 1 Phase diagram of a CTAB/n-butanoVn-octane/ KN0,-Mg( NO, ) I system.

The formation of KMgF, can be verified by the positions and intensities of the peaks of XRD patterns of the final products according to the standard JCPDS card No. 18-1033.It can also be seen from the XRD patterns of KMgF, : Eu nanoparticles. Although 2% ( molar fraction) Eu was doped into KMgF,, the crystal structure of KMgF, : Eu nanoparticles is still cubic lattice, which is the same as that of KMgF, without doping. The average size of the nanoparticles was calculated according to the Debye-Scherrer equation ( d = 0.89A/

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Bcose, where! A is the wavelength of Cu Ka radiation, B is the calibrated half-width of the peak in radian, and 8 is the diffraction angle of a crystal face). The crystalline sizes of three main faces of the KMgF,: Eu nanoparticlesare 15.2 n m ( l l O ) , 15.4 n m ( l l l ) , and 14.7 nm( 200) , respectively. Fig. 2( A) and ( B ) are the ESEM micrographs of the KMgF, : Eu nanoparticles, which reveal that the KMgF,: Eu nanoparticles are spherical and uniform. individual particles of the sample are slightly aggregative, which is inevitable because of the essential properties of nuorides nanopartic~es"~'. The average value of the diameter of the KMgF,: Eu nanoparticles is 20 nm, which is slightly larger than the calculated values by the Debye-Scherrer equation.

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the substitution of K + cations by Eu" cations in the host lattice, which act partly as luminescent centers. The emission of the KMgF, : Eu2+ nanoparticles at

360 nm that can be attributed to the 6P,,2-+sS7/2transition of Eu2+ is the same as that of the KMgF,: EuZt single c r y ~ t a l [ ~ ~ "On * ~the ~ ~ basis . of an earlier study[161 , it can be assumed that the emission of the KMgF,: Eu2+single crystal at ca. 420 nm arising from trace oxygen and color centers does not appear in the emission spectrum of the KMgF, : Eu" nanoparticles. In Fig. 4 , spectrum lines ( A ) and ( B ) , both of which were monitored at 360 nm, are the band excitation peaks of the KMgF, : Eu'+ nanoparticles and the KMgF, : Eu2+ single crystal"']

, respectively.

Fig. 4 ( A) is in the range of 230-320 nm. The 4f65d level splits into the 4f 6 5 d ( e B ) level and the 4f 65d ( t t g ) level because of the crystal field effect. The excitation band of the KMgF, : Eu2+ nanoparticles peaks at 250 nm, which has blue shifted about 70["] or 80 nm[ 1'1 compared with that of the KMgF,: Eu2+ single crystal. There may be two reasons for this phenomenon. The first can be ascribed to small size effect and the quantum size effect is caused by the formation of small nanoparticles. An appreciable proportion of atoms in the particles are on or near the surface, so the sur-

j

1400

r

1000

I

2oo

--. 800 x d

(A)

n

c

'g c e

200

Fig. 2 ESEM micrographs of KMgF,:Eu' + nanoparticles in merent resolutions.

Fig. 3 is the emission spectrum of the KMgF,: Eu2+nanoparticles at room temperature. A characteristic line emission at 360 nm excited at 250 nm is due to 400 2 1000

1c

180 1400

-

1 200

-

220

260 A/nm

300

340

(B)

2 800 c .600 400 200 j

1000

d

1

c

n,

180

- 200 250

I

350

450 A/nm

1

I

550

650

Fig. 3 Emission spectun of KMgF,:Eu" nanoparticles excited at 250 nm and room temperature.

Fig.4

220

260 A/nm Comparison between the

300

340

excitation spectrum of KMgF,:Eu*' nanoparticks(A) and KMgF,: Ed' single crystal( B) monitored at 360 NU. ( 9 ) is from ref. [ 121.

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face electronic states exert a strong influence on the energy gap. In particular, they can localize and trap charge carriers, which lead to the shift of the excitation band"']. Another possible reason may be that the content of oxygen in the complex fluoride nanoparticles synthesized by means of micmemulsions is much lower than that in the single crystals prepared by the vertical Bridgeman method"*] . Because the excitation band blue shifts to 250 nm , a pump source YAG: Nd with 40 (266 nm ) is more suitable for studying the KMgF, : Eu2' nanoparticles system. The excitation spectrum and the emission spectrum of nano-KMgF, : Eu2'/PMMA composite films were determined. Compared with those of the KMgF, : Eu2' nanoparticles, the spectral peak of the excitation band of nano-KMgF, : Eu2'/PMMA composite films red shifts, while the emission peak shifts to a shorter wavelength. It is likely related to the properties of host materials or to the existence of hybridism; this is still under study.

Conclusion The phase diagram of. a cetyltrimethyl ammonium bromide ( CTAB ) In-butanoVn-octane/KNO, Mg( NO,), system was drawn. In this research, KMgF, : Eu2' nanoparticles were prepared by the microemulsion method. The particle size ( about 15 nrn in diameter) calculated by the Debye-Schemer equation is slightly smaller than that ( about 20 nm in diameter) observed by ESEM. The emission band of KMgF, : Eu2' nanoparticles peaks at 360 nm, and the excitation band peaks at 250 nm that has blue shifted about 80 nm compared with that of the single crystal of KMgF, :

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Eu2' . The preparation method of KMgF, : Eu2'/PMMA composite nanofilms was investigated.

References Singhal M. , Chhabra V. , Kang P. , et al. , Mater. Res. E d l . , 1997, 32, 239 Francesca P. , Concetta B. , Paola F. , et al. , Colloid S w f a e A , 1999,160,281 Pillai V. , Shah D. D. , J. Magn. Mater. ,1996, 163, 243 Kim B. J. , Oh S. G. , Han M. G . , et al. , Synthetic Met. , 2001, 122, 297 Lian H. Z . , Liu J., Ye 2. R. , et al. , Chem. Phys. Lett. , 2004, 386, 291 Hua R. N. , Zang C. Y. , Shao C. , et al. , Nanotechnology, 2003, 1 4 , 588 Murray C. 8. , Noms D. J. , Bawendi M. G. , J. Am. Chem. Soc. , 1993, 115, 8706 Abraham M. M. , Finch C. B. , Kolopus J. L. , et al. , Phys. Rev. B , 1971, 3 , 2855 Sadar D. K. , Sibley W. A. , Alcala R . , J. ,Cumin. , 1982, 27, 401 Joubert M. F . , Boulon G . , Gaurne F. , Chem. Phys. Lett. , 1981, 80,367 Ellens A , , Meijennk A . , Blasse G . , J. b i n . , 1994, 59, 293 Jia Z. H. , Yu L. , Shi C. S. , Spctroehimica Acta A , 2003, 59, 2943 Zhu G. X. , Yan J. H. , Mo F. S . , et al. , Chem. J. Chinese Universities, u)o6, 27( 3 ) , 401 Yan J. H. , Zhang M. , Lian H. Z. , et al. , Chem. J. Chinese Universities, 2005, 2 6 ( 6 ) , 1006 Bender C. M. , Burlitch J. M. , Barber D. , ef al. , Chem. Mater. , 2000, 12, 1969 Su H. Q. , Jia Z. H. , Shi C. S. , Chem. Mater. , 2002, 14, 310 Ball P. , Gamin L. , Nature, 1992, 355, 761 Zhu G. X. , Lian H. 2. , Mo F. S. , et al. , Chem. J. Chinese Universities, 2006, 2 7 ( 2 ) : 215