Preparation of KMgF3 and Eu-doped KMgF3 nanocrystals in water-in-oil microemulsions

Materials Research Bulletin 42 (2007) 249–255 www.elsevier.com/locate/matresbu

Preparation of KMgF3 and Eu-doped KMgF3 nanocrystals in water-in-oil microemulsions Ruinian Hua a,*, Zhihong Jia c, Chunshan Shi b a

b

College of Life Science, Dalian Nationalities University, Dalian 116600, PR China Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China c Department of Geosciences, Material Sciences Center, Philipps University, Hans-Meerwein-Strasse, D-35032 Marburg, Germany Received 18 October 2005; received in revised form 1 May 2006; accepted 5 June 2006 Available online 12 July 2006

Abstract In this study, KMgF3:Eu2+ luminescent nanocrystals (NCs) were prepared in water/cetyltrimethylammonium bromide (CTAB)/ 2-octanol microemulsions. The KMgF3:Eu2+ NCs were characterized by transmission electron microscopy (TEM), X-ray diffractometer (XRD), fluorescence spectrum, infrared spectroscopy (IR) and elementary analysis. The results showed that the size of the KMgF3:Eu2+ NCs was hardly affected by water content and surfactant (CTAB) concentration. The emission spectrum showed that the position of the 362 nm peak is due to the K+ sites substituted Eu2+. Two emission peaks located at 589 and 612 nm can be attributed to Eu3+, which exist at two different types of Eu3+ centers: one is Eu3+ at a K+ site, the other is clustering of Eu3+ ions in the interstices of KMgF3 host lattice. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Fluoride; A. Nanostructures; B. Chemical synthesis; D. Luminescence

1. Introduction Nanometer-sized particles have attracted great interest in recent years because of their unique properties in physics and chemistry as well as their potential application in industry [1,2]. Many luminescent particles of different chemical compositions, shapes and size distributions have been prepared by many different kinds of methods such as gas evaporation [3], laser vaporization [4], ionized beam deposition [5], sol–gel process [6,7], hydrothermal [8,9], solvothermal process [10–12], etc. In comparison, the synthesis of luminescent particles in reverse microemulsion is more attractive than that of many other methods, in terms of being able to have homogeneous morphology and being nanosized. Since Boutonnet et al. have reported the synthesis of the monodispersed metallic particles of Pt, Pd, Rh and Ir using microemulsions [13], reverse microemulsions have been successfully utilized for the synthesis of nanoparticles such as CeF3 [14], BaF2:Nd [15], BaF2:Ce [16], BaF2:Er [17] nanoparticles, BaF2 nanowires [18] and the complex fluorides LiBaF3 NCs [19]. To our knowledge, there is no report of preparation of KMgF3 and KMgF3:Eu2+ NCs using reverse microemulsions. As KMgF3 is a good laser host material, and Eu-doped could make this material a good * Corresponding author. Tel.: +86 411 81382259; fax: +86 411 87537259. E-mail address: [email protected] (R. Hua). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.06.004

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candidate for tunable laser [20]. However, formation of defect-free crystals can be time-consuming, expensive and require specialized equipment, or may even be impossible for many materials. Beecroft and Ober have used a composite consisting of light-amplifying nanoparticles placed in a glass or polymer matrix to circumvent these problems [21]. In this study, luminescent nanomaterials of europium-doped KMgF3 were prepared in a reverse microemulsion. With the KMgF3 and Eu-doped KMgF3 NCs obtained, particle size, shape, crystal structures, chemical composition and fluorescent properties were measured by transmission electron microscopy (TEM), X-ray diffractometer (XRD), elementary analysis, infrared spectroscopy (IR) and fluorescence spectrum. 2. Experimental 2.1. Chemical reagents All the chemicals used in this study were analytically pure grade without further purification. The initiative materials such as Mg(NO3)26H2O, KNO3 and NH4F were used as the Mg source, K source and F source, respectively. The cetyltrimethylammonium bromide was chosen as the surfactant, and 2-octanol was used as solvents. 2.2. Synthesis Two types of reverse microemulsions, denoted as microemulsion I and II, were prepared separately. Both microemulsion I and II contain two common components, that is 10 g of cetyltrimethylammonium bromide dissolved in 40 g of 2-octanol (weight ratio is 1:4). This mixture was stirred magnetically for 1 h, and then salt aqueous solution I [0.0025 mol (0.641 g) Mg(NO3)26H2O and 0.003 mol (0.3033 g) KNO3 were dissolved in 4.6 mL H2O] (for synthesis of KMgF3:Eu2+, the salts were Mg(NO3)26H2O, KNO3 and Eu(NO3)3, the molar ratios of Eu:Mg are 0.02:1, 0.04:1, 0.06:1, 0.08:1 and 0.10:1, respectively.) and salt aqueous solution II [0.01 mol (0.3704 g) NH4F is dissolved in 4.6 mL H2O] were poured slowly in microemulsion I and II, respectively. The water content in the microemulsions is 8.6% (w/ w). These two microemulsions were stirred separately for 2 h, and then they were combined and stirred at 20 8C for 5– 15 min before collection of the product began. Finally, the emulsion mixture was centrifuged at 6000 rpm for 30 min, which caused sedimentation of the nanoparticles and allowed removal of the mother liquor. The particles were then washed with ethanol (5  8 mL) and centrifuged (20 min, 6000 rpm), dried under a heat lamp for 40 min, lightly crushed in an agate mortar and pestle. 2.3. Characterization All above-prepared products were characterized by XRD, using a Japan Rigaku D/max-IlB diffractometer with Cu Ka1 radiation (l = 0.1541 nm). The XRD data for index and cell-parameter calculations were collected by a scanning mode with a step of 0.028 in the 2u range from 108 to 1008 and a scanning rate of 4.08 min1 with silicon used as an internal standard. The particles size and shape was examined by a JEOL-3010FX transmission electron microscope, equipped with a 2k*2k CCD camera and operated at 300 kV. IR spectra were obtained with a Magna 560 spectrometer in the range 400–4000 cm1. The samples were pressed KBr pellets for the spectral measurements. Elementary analysis was conducted on a POEMS-II inductively coupled plasma-atomic emission spectrometry. Luminescence spectra under steady-state excitation were measured using a Hitachi F-4500 fluorescence spectrometer. 3. Results and discussion 3.1. Elementary analysis of the products KMgF3 and Eu-doped KMgF3 luminescent NCs were prepared in water/cetyltrimethylammonium bromide/2octanol microemulsions. Elementary analysis showed that mole ratios of K:Mg are ca. 1:1 in KMgF3 and KMgF3:Eu2+ NCs. In the synthesis of KMgF3 and KMgF3:Eu2+ NCs, trace excess K ions can cause Mg ions to react completely. Because KF is soluble in the water, therefore, analysis results of products can keep the molar ratios of K:Mg as 1:1. The molar ratios of Eu:Mg in the KMgF3:Eu nanoparticles are Eu:Mg = 0.0133, 0.0255, 0.0422, 0.0533 and 0.0652, respectively. These values are less than that of the stoichiometric concentration (Eu:Mg = 0.02, 0.04, 0.06, 0.08 and

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Fig. 1. Powder XRD patterns of KMgF3 from microemulsion reactions: (a) KMgF3 (2-octanol:CTAB = 4/1, water content is 10.6%, 15 min reaction); (b) KMgF3 (2-octanol:CTAB = 4/1, water content is 8.60%, 15 min reaction); (c) KMgF3 (2-octanol:CTAB = 4/1, water content is 6.60%, 15 min reaction); (d) KMgF3 (2-octanol:CTAB = 6/1, water content is 10.6%, 15 min reaction); (e) KMgF3 (2-octanol:CTAB = 6/1, water content is 8.60%, 15 min reaction); (f) KMgF3:6.52 mol Eu% (2-octanol:CTAB = 4/1, water content is 8.60%, 5 min reaction).

0.10). Since it is solution reaction, the solid products usually contain different molar ratios compared with the starting reaction solution, so it is not surprising that Eu ions content is less than expected. 3.2. Structural characterization of the products KMgF3 and KMgF3:Eu2+ NCs were examined by XRD and the patterns are shown in Fig. 1. KMgF3 ˚ ) and KMgF3:Eu2+ (a = 4.0706  0.0067 A ˚ ) NCs are found that the positions and the (a = 4.0112  0.0083 A intensities of the peaks are consistent with those for the KMgF3 single crystal, which appeared in JCPDS card 18-1003. This means that the KMgF3 and KMgF3:Eu2+ NCs are cubic lattice. The average sizes of particles were calculated by the Debye–Scherrer equation from the half-width of the peak (1 1 0) of XRD. The equation for calculations is d = 0.89 l/B cos u, where l is the wavelength of Cu Ka1 radiation (here l = 0.1541 nm), B the calibrated half-width of the peak in radian and u is the diffraction angle of the (1 1 0) peak in degree. The calculational values of KMgF3 and KMgF3:Eu2+ NCs were estimated to be ca. 16 nm. With increasing or decreasing of water content, surfactant Table 1 Prepare of KMgF3 and KMgF3:Eu2+ NCs in microemulsion of water/cetyltrimethylammonium bromide (CTAB)/2-octanol systems Starting materials d

a:b:c:d mole ratio

2-Octanol/ CTAB (w/w)

Water (wt.%)

Reaction time (min)

Phase in products

Diameter (nm) TEM

Scherrer

4/1 4/1 4/1 4/1 4/1 4/1 6/1 6/1 6/1 6/1 4/1

10.6 10.6 8.60 8.60 6.60 6.60 10.6 10.6 8.60 8.60 8.60

15 5 15 5 15 5 15 5 15 5 5

KMgF3 KMgF3 KMgF3 KMgF3 KMgF3 KMgF3 KMgF3 KMgF3 KMgF3 KMgF3 KMgF3:1.33 mol Eu% KMgF3:2.55 mol Eu% KMgF3:4.22 mol Eu% KMgF3:5.33 mol Eu% KMgF3:6.52 mol Eu%

15 15 15 15 16 16 15 16 16 15 15

16.5 16.5 16.5 16.5 16.0 16.0 16.5 16.5 16.5 16.0 15.8

16

15.8

15

15.8

15

15.5

16

15.6

a

b

c

Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2 Mg(NO3)2

KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3

NH4F NH4F NH4F NH4F NH4F NH4F NH4F NH4F NH4F NH4F NH4F

Eu(NO3)3

1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4 1:1.2:4:0.02

Mg(NO3)2

KNO3

NH4F

Eu(NO3)3

1:1.2:4:0.04

4/1

8.60

5

Mg(NO3)2

KNO3

NH4F

Eu(NO3)3

1:1.2:4:0.06

4/1

8.60

5

Mg(NO3)2

KNO3

NH4F

Eu(NO3)3

1:1.2:4:0.08

4/1

8.60

5

Mg(NO3)2

KNO3

NH4F

Eu(NO3)3

1: 1.2:4:0.10

4/1

8.60

5

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concentration and reaction times, the sizes of the particles are hardly changing according to the Debye–Scherrer equation from the half-width of the XRD peak (1 1 0). The calculational values and TEM observational values of KMgF3 and KMgF3:Eu2+ NCs are listed in Table 1. A water-in-oil (W/O) microemulsion is a transparent and isotropic liquid medium with nanosized water pools dispersed in a continuous phase by surfactant molecules at the water/oil interface. These water pools offer ideal microreactors for the formation of KMgF3 and KMgF3:Eu2+ NCs under experimental conditions. These results also indicate that water molecules in the water core of the microemulsions are relatively immobilized and bound to the hydrophilic portion via hydrogen bond. 3.3. Transmission electron microscopy of the products TEM was useful for qualitative assessment of the reaction products despite the inevitable aggregation of particles that occurred during sample preparation. Fig. 2 shows that the products of KMgF3 and KMgF3:Eu NCs. There was no significant difference in shape and size between KMgF3 and different concentration Eu-doped KMgF3 NCs. Both KMgF3 and KMgF3:Eu NCs are cubic particles, and the particles sizes of KMgF3 and KMgF3:Eu2+ NCs are all about 15–16 nm, in good agreement with calculational values from the XRD peak (1 1 0).

Fig. 2. TEM images of microemulsion reaction products: (a) KMgF3 (2-octanol:CTAB = 4:1, water content is 10.6% and 15 min reaction); (b) KMgF3 (2-octanol:CTAB = 4:1, water content is 8.6% and 15 min reaction); (c) KMgF3 (2-octanol: CTAB = 6:1, water content is 10.6% and 15 min reaction ); (d) KMgF3:6.52 mol Eu% (2-octanol:CTAB = 4:1, water content is 8.6% and 5 min reaction).

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Fig. 3. The emission of KMgF3:Eu2+ powder excited at 254 nm.

3.4. Infrared spectroscopy of the products IR spectra were used to determine whether surfactant and other anions were removed from the products after washing. Being used nitrate salts in the synthesis, the asymmetric stretching vibration for NO3 was clearly visible in the IR spectrum at 1440 cm1. This results shows that there are trace NO3 were not removed from the products after washing with ethanol. Water, or possibly hydroxyl groups, was also found to be present in powders as shown by absorptions at 3436 and 1635 cm1. This result is consistent with the report previously [15]. 3.5. Optical properties of the KMgF3:Eu nanoparticles Fig. 3 was the emission spectrum of the KMgF3:Eu2+ powder. As can be seen, the overall appearance of the emission spectrum consisted of one sharp peak emission located at 362 nm, which arisen from f ! f transition of Eu2+ in the host lattice which substitute the K+ ion. The intensity of the 362 nm emission peak enhance when the Eu content increase. In the region range from 570 to 650 nm, there are two emission peaks located at 589 and 612 nm (in the inset of Fig. 3), they can be attributed to the Eu3+. It is known that there are two types of Eu3+ in KMgF3:Eu, one is Eu3+ at K+ site, the other is clustering of Eu3+ ions in the interstice of the KMgF3 lattice host [22]. It need to noted that from the emission spectrum, no any emission of Eu substitution of the Mg ion can be seen. Based on previous report of Eu2+ in KMgF3:Eu2+ single crystal, the emission about 420 nm risen from the trace oxygen and color center not appear in our powder [23]. The Eu2+ ion substitution of K+ ion in the KMgF3:Eu2+ powder have been studied previously, based on the crystal field calculation of the Eu2+ in cubic symmetry lattice [24], it Hamiltonian function can be written as following: rffiffiffiffiffirffiffiffiffiffi rffiffiffiffiffi rffiffiffiffiffiffiffiffi rffiffiffi rffiffiffiffiffi   6 7 4 5 6 14 1 6 7 6 6 6  ðu þ u4 Þ ; H ¼ 16B4 u þ ðu þ u4 Þ þ 320B6 u þ 11 12 0 25 4 429 8 0 16 4 Based on the above function, the matrix element of Eu2+ 6p state can be written as: rffiffiffiffiffi rffiffiffiffiffiffiffiffi   6 14 n a n b b a ð6G 1 ; Jmb G j jJ aj G i Þr lm ; h f Jl G i jH cr j f Jm G j i ¼ dðG i ; G j 16B4 ð4G 1 ; Jm G j jJl G i Þbl;m þ 320B6 11 429 the crystal field parameters can be get from above equation, the values are B4 = 46 and B6 = 25. So the Eu2+ ion in the KMgF3 host should substitute the K+ ions in the dodecacoordinate site but not in the Mg2+ site. The results have been proved by the emission spectrum mentioned above. The excitation spectrum of the KMgF3:Eu2+ powder is shown

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Fig. 4. The excitation of KMgF3:Eu powder monitored at 362 nm.

as Fig. 4 monitored at 362 nm emission. From the excitation spectrum, it can be seen that there is one broad peak maximum at about 254 nm. The excitation band due to the 4f7–4f6 5d transition of Eu2+ in KMgF3 is normally peaked at 254 nm not only for the KMgF3:Eu2+ nanoparticles but also for the KMgF3:Eu2+ crystals [25,26]. In the f–d transitions of rare earth ions, the wavefunction is confined within the nearest- and next-nearest-neighbor ions. Therefore, it is not possible to observe a change in spectral feature by reducing the size of host material to nanoscale. 4. Conclusion In summary, KMgF3 and europium-doped KMgF3 NCs have been successfully prepared for the first time from a water/cetyltrimethylammonium bromide/2-octanol microemulsion process. The final products are cubic lattice NCs with 16 nm in size and the sizes of the particles are hardly changing with increasing or decreasing of water content, surfactant concentration and reaction time, which indicate that water molecules in the water core of the microemulsions are relatively immobilized and bound to the hydrophilic portion via hydrogen bond. The emission spectrum consisted of one sharp peak emission located at 362 nm (f ! f transition of Eu2+) and two small emission peaks located at 589 and 612 nm (attributed to the 5D0–7F1 and 5D0–7F2 emission of Eu3+), respectively. The excitation spectrum of the KMgF3:Eu2+ NCs monitored at 362 nm shows one broad peak maximum at about 254 nm. The IR spectrum results show traces of NO3 and water, or possibly hydroxyl groups, which were not removed from the products after washing with ethanol. The preparation of KMgF3 and KMgF3:Eu2+ NCs via microemulsion process has almost no oxygen containing impurities in the product. It had good reproducibility and simple maneuverability. The possibility of synthesizing other rare earth ions-doped fluoride nanostructures with a similar method are underway. Acknowledgments This work was supported by the Scientific Research Startup Foundation of Introducing the Person with Ability into Dalian Nationalities University (20056110) and the National Natural Science Foundation of China (90201032). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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