Tuning of Crystal Phase and Luminescence Properties of Eu3+ Doped

J. Phys. Chem. C 2008, 112, 3223-3231

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Tuning of Crystal Phase and Luminescence Properties of Eu3+ Doped Sodium Yttrium Fluoride Nanocrystals Pushpal Ghosh and Amitava Patra* Department of Materials Science and Centre for AdVanced Materials, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: October 11, 2007; In Final Form: December 3, 2007

We demonstrated an emulsion based wet chemical method for preparing Eu3+ doped cubic NaYF4 and hexagonal Na(Y1.5Na0.5)F6 nanocrystals. Here, we report the control of the crystal phase as a function of the Y3+/F- ratio and pH of the solution. It was found that cubic NaYF4 and hexagonal Na(Y1.5Na0.5)F6 nanocrystals were prepared at 1:4 and 1:8 Y3+/F- ratios, respectively. The hexagonal Na(Y1.5Na0.5)F6 and cubic NaYF4 phases appeared in the pH range of 6-7 and 2-3, respectively, indicating that the pH of the solution also plays an important role in tuning the crystal structure. The mechanism related to crystal phase control is proposed and discussed. It was also found that the emission intensity of the peak at 614 nm (5D0 - 7F2) for Eu3+ ions and decay time are sensitive to the crystal structure, which is again controlled by the Y3+/F- ratio and pH of the solution. Again, it is seen that the interlayered water molecules strongly influence the photoluminescence of Eu3+ ions up to 300 °C, which was confirmed by XRD and TGA analysis.

Introduction The optical properties of doped nanocrystals continue to command research attention in terms of both their fundamental and their technological importance.1-3 Over the past few years, much attention has been paid to rare-earth doped sodium yttrium fluoride nanocrystals, and several groups have already demonstrated the efficient up-conversion luminescence properties of rare-earth doped cubic NaYF4 and hexagonal Na(Y1.5Na0.5)F6 nanoparticles.4-10 Precise control of the size and shape allows manipulation of the properties of the nanocrystals as desired. In addition, phase control is crucially important, as the properties of the materials are determined first by their phase. From a fundamental point of view, the physical understanding of the luminescence properties of rare-earth ions in nanocrystals and the way the properties change with crystal phase and local structure is very important. It is well-established that in the luminescence of rare-earth ions, the highest phonon frequencies of the host lattice are responsible for nonradiative relaxations. To overcome the phonon decay problem, it is necessary to choose a lattice that has a lower phonon energy. The fluoride matrix seems to be an ideal medium for the preparation of highly luminescent materials because it has a low phonon energy as compared to oxide hosts.11 As we move toward nanotechnology, it is worthwhile to investigate the efficiency of rare-earth ions in nanocrystals because nanoscopic interactions play key roles in controlling the excitation dynamics. Liu et al.12 have demonstrated the higher efficiency of the hexagonal phase than the cubic phase in transparent glass ceramics due to the multisite character of the hexagonal phase. At ambient temperature and pressure, NaYF4 exists in two polymorphs: the hexagonal structure and the cubic structure, depending on the synthesis conditions. The crystal structure of sodium yttrium fluoride has been the subject of debate for a long time. Zachariasen13 proposed a trigonal structure for NaLaF4 crystals. However, * Corresponding author. E-mail: [email protected]; tel.: (91)-33-24734971; fax: (91)-33-2473-2805.

Sobolev et al.14 proposed a gagarinite structure of NaYF4 crystals. The crystal structure of NaYF4 has been elucidated by Burns.15 In the hexagonal form of NaYF4, space group P6, the Y3+ ions are distributed over two crystallographic sites, namely, 1a and 1f, both sites having C3h point symmetry. Recently, Kra¨mer et al.10 found the gagarinite structure for a slightly nonstoichiometric mixed Na3-xLn2xF6 (x ) 0.45). Ammonium fluorolanthanates are another group of interesting materials. The structure of NH4Ln2F7 can be visualized as layers of LnF6 octahedra (built up by mutual sharing of corners and edges) with interweaving layers of NH4 groups.16 Recently, Matsumoto et al.17 showed the photoluminescence (PL) of various rare-earth ions in the interlayers of Ti layered oxide, and they found a significant increase in emission intensity due to interlayered water molecules. In the present study, we prepared cubic and hexagonal sodium yttrium fluoride with a small amount of NH4Y2F7 (which is a layered structure itself). The identification of a layered structure of NH4Y2F7 and its role in the luminescence properties of Eu3+ ions are discussed in detail. Normally, a hexagonal to cubic or cubic to hexagonal phase transformation of sodium yttrium fluoride is performed by heat treatment. In the present study, we tuned the crystal phase by varying the Y3+/F- ratio and the pH of the solution. In particular, to the best of our knowledge, there has been no such detailed study on the modification of a crystal phase by varying the Y3+/F- ratio and the pH of the solution. Of particular interest is how the physical properties of rare-earth ions vary with the modification of the crystal phase, which is again tuned by changing the Y3+/F- ratio and the pH of the solution with the hope that such knowledge will enable us to construct efficient nanomaterials. Here, we address the role of the Y3+/F- ratio and the pH of the solution on the modification of the crystal phase and the luminescence properties of Eu3+ doped sodium yttrium fluoride nanocrystals derived from emulsion techniques.

10.1021/jp7099114 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

3224 J. Phys. Chem. C, Vol. 112, No. 9, 2008

Ghosh and Patra

Experimental Procedures Preparation of Hexagonal Na(Y1.5Na0.5)F6 and Cubic NaYF4 Nanocrystals. A simple emulsion based wet chemical method was employed to prepare Eu3+ doped cubic NaYF4 and hexagonal Na(Y1.5Na0.5)F6 nanocrystals. Wang and Li5 prepared Na(Y1.5Na0.5)F6 nanocrystals using a wet chemical method. Mai et al. 4b used a co-thermolysis method, whereas other researchers have used thermal decomposition,4c a liquid-solid two phase approach,9 a coprecipitation technique,4a a complex assisted hydrothermal method,18 or a solid-state synthesis route for preparing these nanocrystals. In the present study, we used a water-in-oil (w/o)-type emulsion for preparing these nanocrystals. Here, the nanoreactor was prepared through a -w/o-type emulsion with cyclohexane and sorbitan monooleate (Span 80, Fluka) as the organic liquid phase (oil phase) and a nonionic surfactant with a low hydrophilic-lyophilic balance number of 4.3, respectively. The support solvent containing 5 vol % Span 80 in cyclohexane was used for emulsification. Preparation of Cubic NaYF4:Eu Nanoparticles (Y3+/F1:4 and pH 6-7). A clear solution of 0.425 M Y(NO3)3‚6H2O (Indian Rare Earth Ltd.) ,0.425 M NaCl (Merck), and a required amount of europium nitrate (for 1 mol % Eu3+, Aldrich) were dissolved in 12.5 mL of water under vigorous stirring for 10 min at room temperature. In this preparation technique, the Y3+/ F- ratio was maintained at 1:4. This mixed solution was added to the previously explained microemulsion and stirred for 5 min followed by dropwise addition of 12.5 mL of a 1.7 M solution of NH4F (Merck). The total solution mixture was stirred for 30 min. The pH of this mixture was around 6-7. Finally, a white precipitate was obtained. Particles were collected by centrifugation (6000 rpm), then the particles were washed twice with acetone and methanol and dried at 80 °C for 12 h in a vacuum oven, and the crystalline particles were collected. The samples were heated at 120, 200, 250, 300, 400, 500, and 600 °C for 1 min with a rate of heating of 12 °C/min, which is similar to TG-DTA measurement conditions. Preparation of Hexagonal Na(Y1.5Na0.5)F6:Eu Nanocrystals (Y3+/F- 1:8 and pH 6-7). The synthesis procedure was same as stated previously. However, we used 3.4 M NH4F to maintain a Y3+/F- ratio of 1:8. All other concentrations of the constituents were similar to that of the cubic nanocrystal preparation. The final pH of the solution was 6-7. Finally, the particles were washed, centrifuged, heated, and collected like the previous method. Preparation of Cubic NaYF4:Eu Nanoparticles (Y3+/F1:8 and pH 2-3). The synthesis procedure was same as hexagonal Na(Y1.5Na0.5)F6:Eu nanocrystals, only the pH of solution was changed. To decrease the pH value to 2-3, 3.0 mL of CF3COOH (Merck) was added, and the solution was stirred. Finally, the particles were washed, centrifuged, heated, and collected like the previous method. Transmission electron microscopy (TEM, JEOL Model 200) was used to study the morphology and particle size of the resulting powders. The crystalline phases of heated powders were identified by X-ray diffraction (XRD) using a Siefert XRD 3000 P. The crystallite sizes of the nanocrystals were calculated following Scherrer’s equation

D ) Kλ/β cos θ

(1)

where K ) 0.9, D represents the crystallite size (Å), λ is the wavelength of Cu KR radiation, and β is the corrected halfwidth of the diffraction peak. The volume fractions of the cubic (Xc) and hexagonal (Xh) phases were estimated from the integrated peak intensity of the (100)h, (110)h, (101)h, (201)h,

Figure 1. X-ray powder diffraction patterns of different temperatures of heated 1.0 mol % Eu-doped NaYF4 nanocrystals prepared at Y3+/ F- 1:4 and pH 6-7.

and (211)h planes of the hexagonal phase and the (111)c plane of the cubic phase using the following equation:

Xc ) Ic(111)/[Ih(100) + Ih(110) + Ic(111) + Ih(101) + Ih(201) + Ih(211)] (2) The excitation, emission spectra, and decay times were recorded in a Fluro MaX-P (Horiba Jobin Yvon) spectrometer, using a solid sample holder at room temperature. Thermogravimetric and differential thermal analysis (TG and DTA) curves were obtained from SDT (Quantachrome) with a heating rate of 12 °C/min using N2 gas. Infrared spectra of powders (FTIR) were recorded in the range of 400-4000 cm-1 on a Fourier transformation spectrometer (Nicolet Magna IR 750 Series 2). A small amount of sample was mixed with KBr and then pressed to make a thin pellet for FTIR studies. Results and Discussion Structural Investigations. Figure 1 depicts the XRD pattern of different temperatures of heated 1 mol % Eu3+ doped sodium yttrium fluoride nanoparticles prepared at a 1:4 ratio of Y3+/ F-. The peaks at 27.93° (111), 32.35° (200), and 46.436° (220) match with the cubic NaYF4 phase (JCPDS card no. 6-342, Fm3m, Z ) 2). The peaks marked with asterisks in the XRD patterns are well-indexed with a small amount of cubic NH4Y2F7 (JCPDS card no-. 43-0847). A small amount of NH4Y2F7 was observed along with the NaYF4, which is shown by asterisked peaks in Figure 1a-c. Here, both NH4Y2F7 and NaYF4 phases are present. Figure 1d depicts the XRD pattern of the 600 °C heated sample. Here, all the peaks are well-indexed with cubic NaYF4. Considering the (111) plane of cubic NaYF4, the crystallite sizes are calculated using Scherrer’s equation as seen in Table 1. Figure 2 depicts the XRD pattern of different temperatures of heated 1 mol % Eu3+ doped sodium yttrium fluoride nanoparticles prepared at a 1:8 ratio of Y3+/F-. The peaks at 17.009° (100), 29.787° (110), 30.624° (101), 34.712° (200), 39.454° (111), 43.262° (201), 46.474° (210), and 53.439 o (301) (JCPDS card no.16-334) clearly suggest the formation of a Na(Y1.5Na0.5)F6 hexagonal phase. The peaks marked with the asterisks such as 13.06° (200), 14.91° (210), 19.77° (300), 25.93°, 27.355° (410), 32.646° (422), 40.798° (616), 49.141° (640), and 51.908° (642) suggest the formation of NH4Y2F7 (JCPDS card no. 43-0847). The peak at 27.33° (asterisk marked)

Eu3+ Doped Sodium Yttrium Fluoride Nanocrystals

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3225

Figure 2. X-ray powder diffraction patterns of different temperatures of heated 1.0 mol % Eu-doped Na(Y1.5Na Y3+/F- 1:8 and pH 6-7.

0.5)F6

nanocrystals prepared at

TABLE 1: Phase Composition, Crystallite Sizes, and Cell Parameters of NaYF4 and Na(Y1.5Na0.5)F6 NaYF4 Nanocrystals condition

temp (°C)

crystal phase

crystallite size (nm)

cell param (Å)

cell vol (Å3)

Y3+/F- 1:4 and pH 6-7

80 120 200 250 300 400 600 80 80 120 200 250 300 400 600

cubic cubic cubic cubic cubic cubic cubic cubic hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal

21.5 21.0 21.0 21.0 20.3 21.0 24.3 11.0 18.2 22.5 21.7 22.5 22.0 22.0 26.4

a ) 5.52594 a ) 5.52362 a ) 5.52302 a ) 5.52270 a ) 5.52237 a ) 5.51487 a ) 5.51577 a ) 5.5357 a ) 5.97547, c ) 3.53785 a ) 5.97708, c ) 3.53068 a ) 5.97617, c ) 3.52975 a ) 5.97608, c ) 3.52619 a ) 5.97643, c ) 3.52458 a ) 5.97857, c ) 3.51735 a ) 5.97193, c ) 3.51885

168.740 168.536 168.473 168.443 168.405 167.908 167.810 169.66 109.399 109.237 109.174 109.061 109.024 108.878 108.683

Y3+/F- 1:8 and pH 2-3 Y3+/F- 1:8 and pH 6-7

is the 100 intensity peak of NH4Y2F7. Similar patterns were observed for 200 and 300 °C heated samples as shown in Figure 2b,c. This indicates that both Na(Y1.5Na0.5)F6 (major) and NH4Y2F7 (minor) are present up to 300 °C. In our system, the major and minor phases are Na(Y1.5Na0.5)F6 and NH4Y2F7, respectively. The calculated lattice volumes are 109.399 and 2402.290 Å3 for Na(Y1.5Na0.5)F6 and NH4Y2F7, respectively. The lattice volume of Na(Y1.5Na0.5)F6 is much less than that of NH4Y2F7 (see Table 1). According to Rajeshwar and Secco,16 the ammonium fluorolanthate structure may be visualized as layers of LnF6 octahedra (built up by mutual sharing of corner and edges) with interlaying layers of NH4 groups. Huang et al.19 also discussed the layered structure of NH4LnF4 (Ln ) Nd, Sm, Eu, Gd, and Tb), which is potentially stacked along the (100) direction linked by NH4+ ions. Meyer and Plitzko20 also showed a layered structure of NH4DyF4. According to Burns,15 there are three types of cation sites: a one-fold site occupied by Nd3+, a one-fold site occupied randomly by 1/2 Na+ and 1/2 Nd3+, and a two-fold site occupied randomly by Na+; vacancies of the structure of hexagonal NaNdF4 and the fluoride coordination about the first two sites is nine-fold. The

structure of NaYF4 is similar to NaNdF4. Therefore, we believe that the fluoride ion of Na(Y1.5Na0.5)F6 is connected to the NH4+ ion of NH4Y2F7 and forms a weak N-H f F bond along the c-axis and forms a layered structure in the present case. Figure 3 shows a schematic diagram of a layered structure between Na(Y1.5Na0.5)F6 and NH4Y2F7 with the decomposition process. This layered structure is further confirmed by TG-DTA, TEM, and IR analyses. The peaks due to the NH4Y2F7 phase are eliminated (Figure 2d) at 400 °C. It is interesting to note that the 100 intensity peak of NH4Y2F7 at 27.33° is shifted to 27.909° on increasing the temperature from 80 to 400 °C, respectively. Actually, this peak at 27.909° (111) is the 100 intensity peak of orthorhombic YF3 (JCPDS card no. 32-1431, SG-Pnma). A similar result was obtained by Huang et al.19 during the preparation of the 1-D nanostructure LnF3 from NH4LnF4. Rajeshwar and Secco16 have also observed similar results during the preparation and decomposition of various ammonium flurolanthates to flurolanthate. They suggested that the ionic radius parameter p (i.e., rNH4+/rM3+) plays an important role in the production of NH4LnF4 or (NH4)3Ln2F9. It is reported that the product will be NH4Y2F7 instead of NH4YF4 when


Ghosh and Patra

Figure 3. Schematic diagram of the layer structure between Na(Y1.5Na0.5)F6 and NH4Y2F7 with decomposition process.

Figure 4. Variation of 1/β vs heating temperature for hexagonal and cubic 1.0 mol % Eu-doped NaYF4 and Na(Y1.5Na0.5)F6 nanocrystals prepared at pH 6-7.

Figure 5. Normalized X-ray powder diffraction patterns of a particular peak at different temperatures of 1.0 mol % Eu-doped Na(Y1.5Na0.5)F6 nanocrystals prepared at Y3+/F- 1:8 and pH 6-7.

rNH4+/rM3+ is 1.607, which is valid in the present study. NH4Y2F7 will finally decompose to YF3 at 400 °C. The decomposition reaction can be written as

400 °C is used to break the N-H-F bond (i.e., to destroy the layer structure by liberating NH3 gas). Therefore, the Ostwald ripening process is hindered at this temperature range, and there is no change in particle size. It could be the reason that the peak width of the XRD peaks does not change with changing the temperature of heating. Structural changes for longer heat treatments of the samples are discussed in the Supporting Information (see S1). Figure 5 shows the shifting of normalized XRD peaks with increasing the temperature of heating. The peak obtained at 27.33° for the 80 °C dried samples is due to the formation of NH4Y2F7. This peak is shifted from 27.33 to 27.908° with increasing the temperature from 80 to 400 °C, respectively. The peak at 27.908° is due to the formation of YF3 at 400 °C, and the peak at 28.024° confirms the formation of cubic NaYF4 at 600 °C where the annealing time is 1 min. With increasing the annealing time from 1 min to 1 h at 600 °C, the peak is shifted from 28.024 to 28.209°, which is the 100 intensity peak of cubic NaYF4. The shifting of this peak from 80 to 400 °C reveals the

NH4Y2F7 f 2YF3 + NH3(g) + HF(g)

(3)

Figure 2e depicts the XRD pattern of the 600 °C heated sample. Here, all the peaks are well-indexed with hexagonal Na(Y1.5Na0.5)F6 except for two peaks at 28.024° (111) and 58.89° (311). These peaks are indexed for cubic NaYF4 (JCPDS card no. 6-342, Fm3m, Z ) 2). Finally, we observed a 91.1% hexagonal phase with an 8.9% cubic phase at the 600 °C heated sample. We know that the corrected half-width of the diffraction peak (β) is related to the particle size (eq 1). This indicates that the particle size decreases with increasing the β value. Figure 4 shows the plot of 1/β versus the heating temperature where the 1/β value is more or less the same up to 400 °C for both cubic and hexagonal phases, but there is a change at 600 °C. It is clear from Figure 4 that the heating temperature from 80 to



Figure 6. Variation of cell parameter (c value) with temperature of 1.0 mol % Eu doped Na(Y1.5Na0.5)F6 nanocrystals prepared at Y3+/F1:8 and pH 6-7.

decomposition of NH4Y2F7 to YF3. However, a mixture of the Na(Y1.5Na0.5)F6 (major) and NaYF4 (minor) phases is obtained at 600 °C (1 h of heating). Hund21 showed that the solid solution of YF3 in cubic NaYF4 occurs by substituting Y3+ for Na+ ions and by filling some of the vacancies with F- ions to produce electroneutrality. Thoma et al.22 presented the phase equilibrium diagram of NaF‚YF3 and obtained the first eutectic temperature as 638 °C at which temperature a solid mixture is obtained. In the present study, we prepared samples at 600 °C, which is below the eutectic temperature. Therefore, we believe that YF3 (produced at 400 °C) will help to produce an 8.9% NaYF4 phase along with the Na(Y1.5Na0.5)F6 phase in a solid solution at 600 °C. Figure 6 depicts the variation of cell parameter (only c) with temperature. It is seen that the c value decreases (0.6%) with increasing the temperature from 80 to 400 °C, which supports the formation of a layered structure of Na(Y1.5Na0.5)F6. The layered structure potentially stacks along the direction of (100) linked by the NH4+ ion along the c-axis. Therefore, the 0.6% decrease in the c value can be explained by the liberation of bound water and the decomposition of NH4Y2F7 to YF3, which will finally destroy the layered structure. Again, a slight increase of the c value is observed with increasing the temperature from 400 to 600 °C. In the case of cubic NaYF4, the a value decreases (0.2%) with increasing the temperature from 80 to 400 °C and then further slightly increases with increasing the temperature up to 600 °C. Mathews et al.23 also observed similar results at higher temperatures. Now, we examined the effect of fluoride concentration on crystal phase. Figure 7 shows the XRD pattern of 80 °C dried fluoride nanocrystals with a varying Y3+/F- molar ratio. Here, we compare the crystal structure of 600 °C heated samples prepared by varying the Y3+/F- molar ratios of 1:4 and 1:8. In both the cases, the pH was maintained at 6-7. It is clearly seen from Figure 5 that the cubic and hexagonal phases are obtained in Y3+/F- ratios of 1:4 and 1:8, respectively. We also observed a change in lattice volume during the phase transformation of cubic (167.810 Å3) to hexagonal (108.683 Å3) phases as indicated in Table 1. The cubic structure of NaYF4 has been compared with a CaF2 structure,21 where the Na+ and Ln3+ ions randomly replace the Ca2+ ionic sites and each cation is coordinated by F- ions. Again, it is well-established10,11 that the hexagonal phase is a more ordered and thermodynamically stable phase and that the transition from a cubic to a hexagonal phase [Fm3m f P63/m] is a disorder-to-order character with respect to cations. This phase transition behavior can be controlled by changing the environment or changing the energy barrier. It is reported that the phase transition from cubic

Figure 7. X-ray powder diffraction patterns of 600 °C heated 1.0 mol % Eu-doped NaYF4 and Na(Y1.5Na0.5)F6 nanocrystals prepared at pH 6-7 and varying Y3+/F- ratios of 1:4 and 1:8, respectively.

Figure 8. X-ray powder diffraction patterns of 80 °C dried 1.0 mol % Eu doped NaYF4 and Na(Y1.5Na0.5)F6 nanocrystals prepared at different pH values.

R-NaYF4 to β-NaYF4 is mainly due to the modification of the environment of Y3+ occupation sites, including coordination number.23,24 Thoma et al.22 reported that the kinetic energy influences the phase change from cubic to hexagonal NaYF4. Recently, Mai et al.4b also reported that the energy barrier is the main reason for β-NaReF4 formation. Analysis suggests that Y3+ or other cationic sites are conveniently coordinated by Fions with increasing the fluoride concentration (1:8), which will decrease the energy barrier, and as a result, an ordered hexagonal structure appears. However, a less ordered cubic structure is obtained at Y3+/F- 1:4. Therefore, the fluoride concentration plays an important role in tuning the crystal structure, which is an important observation in this study. Now, we examined the effect of pH on the crystal phase. Figure 8 shows the XRD pattern of sodium yttrium fluoride nanocrystals prepared at different pH values. Here, the Y3+/Fratio is 1:8 in both the cases. We obtained the hexagonal Na(Y1.5Na0.5)F6 phase in the pH range of 6-7. The peaks marked with asterisks indicate the formation of NH4Y2F7, which was explained earlier. All other peaks are well-indexed with the hexagonal peak. However, a cubic nanocrystal is obtained (along with the formation of NH4Y2F7 marked by asterisks) at lower pH values of 2-3 (by the addition of CF3COOH during preparation). This reveals that pH plays an important role in


Ghosh and Patra

Figure 10. TEM micrograph (a), HRTEM image (b), and SAED pattern (c) of 80 °C dried Eu doped Na(Y1.5Na0.5)F6 nanocrystals.

Figure 9. TG and DTA curves of 1.0 mol % Eu-doped NaYF4 and Na(Y1.5Na0.5)F6 nanocrystals.

controlling the crystal phase. A similar cubic crystal phase was observed using another acid (i.e., HNO3 instead of CF3COOH) to lower the pH value. The H+ ion was produced by the addition of acids, either CF3COOH or HNO3, which will capture the excess F- ion and form HF. As a result, the Y3+ or other cationic sites are not conveniently coordinated by F-, which is an important factor in forming a hexagonal structure. So, the disordered cubic structure was formed at low pH values instead of forming an ordered thermodynamically stable hexagonal structure. Figure 9 shows the TGA curves of 1.0 mol % Eu3+ doped hexagonal and cubic nanocrystals prepared at pH 6-7, respectively. The weight loss in the temperature range between room temperature and 80 °C is due to the removal of relatively free water. The 6% weight loss within the temperature range of 100250 °C for the hexagonal phase is due to the liberation of interlayered water or bound water. However, an 11% weight loss is observed within the temperature range of 170-250 °C for the cubic phase, which is also due to the release of interlayer water. It is already seen from the XRD study that the layered structure of sodium yttrium fluoride decomposed above 300 °C. Recently, Matsumoto et al.17 showed the loss of intercalated water in a titanate layered oxide structure in the range of 100400 °C. Huang et al.19 showed the decomposition of NH4LnF4 (Ln ) Nd, Sm, Eu, Gd, or Tb) to LnF3 within the temperature range of 250-280 °C by TG-DTA analysis. The decomposition of NH4Y2F7 to YF3 began at 250 and 280 °C in the DTA curves and was completed at around 360 °C, which is comparable to Liang’s data.25 Analysis suggests that the sharp weight loss (below 250 °C) is not due to the formation or liberation of NH4Y2F7: it is solely due to bound or interlayered water. It is seen from the TGA pattern of pure surfactant (Span-80) that the main weight loss begins after 280 °C and continues up to 450 °C. Therefore, analysis suggests that the weight loss in the range of 100-250 °C is definitely due to bound water or interlayered water, which is formed during the formation of a layered structure of sodium yttrium fluoride. The morphology and microstructural detail can be obtained from the TEM study. Figure 10a,b shows the low magnification TEM and HRTEM images of 80 °C dried Eu- doped Na(Y1.5Na0.5)F6 nanocrystals. The interplanar distance is 2.96 Å, which corresponds to the (110) plane of the hexagonal Na(Y1.5Na0.5)F6 nanocrystals. Figure 10c shows the selected area electron diffraction (SAED) pattern, and this diffraction pattern also confirms the presence of the (110) plane. Figure 11a,b represents the low magnification TEM and HRTEM images of a 400 °C heated Na(Y1.5Na0.5)F6 sample. It is clearly seen from Figure



Figure 13. TEM micrograph (a), HRTEM image (b), and SAED pattern (c) of 80 °C dried Eu doped NaYF6 nanocrystals.

11a,b that hollow interior sphere-type structures are formed. This may be due to diffusion out of the solute and leaving a hollow structure that is clearly seen from the HRTEM images (Figure 11b). Recently, Liang et al.25 reported the formation of nanocages from NH4Ln2F7. This layered structure of NH4Y2F7 would collapse due to decomposition of interweaving layers of NH4 groups above 300 °C. Therefore, we believe that this hollow structure is being formed at 400 °C. The interplanar distance is 2.96 Å, which corresponds to the (110) plane of the hexagonal Na(Y1.5Na0.5)F6 nanocrystals. The SAED pattern study (Figure 11c) also confirms this (110) lattice plane. The HRTEM observations confirmed that the formation of the nanocages may have a close relationship with the inherent layered structure of NH4Y2F7. Figure 12a shows the low magnification TEM image of Na(Y1.5Na0.5)F6 prepared at 600 °C. However, we did not observe any hollow structure in the HRTEM image of the 600 °C heated samples (Figure 12b). Figure 12c shows the SAED pattern, and this diffraction pattern also confirms the presence of the (101) plane. This indicates that coalescence occurs with increasing the heat treatment. Figure 13a,b shows the low magnification TEM and HRTEM images of 80 °C dried Eu- doped NaYF4 cubic nanocrystals. The interplanar distance is 3.23 Å, which corresponds to the (111) plane of the cubic NaYF4 nanocrystals. Figure 13c shows the SAED pattern, and this diffraction pattern also confirms the presence of the (111) plane. Luminescence Properties. Figure 14 depicts the excitation and photoluminescence spectra of different temperature heated 1 mol % Eu3+ doped Na(Y1.5Na0.5)F6 nanoparticles. The sharp


Figure 14. Excitation and photoluminescence spectra of 1 mol % Eudoped Na(Y1.5Na0.5)F6 nanoparticles (pH 6-7 and Y3+/F- 1:8) prepared at different temperatures (λex ) 394 nm).

Figure 15. Normalized PL decays of different temperatures of heated 1 mol % Eu3+ doped Na(Y1.5Na0.5)F6 nanoparticles, monitored at the 5 D0 f 7F2 transition (617 nm).

lines in the excitation spectra correspond to direct excitation of Eu3+ from the ground state to higher levels of the 4f manifold, and their assignments are marked in Figure 14.1c The prominent emission bands at 616 and 594 nm are due to 5D0 f 7F2 and 5D - 7F transitions, respectively, after being excited at 394 0 1 nm. In europium, the 5D0 - 7F1 (594 nm) transition is mainly magnetically allowed (a magnetic-dipole transition), while 5D0 - 7F2 (614 nm) is a hypersensitive forced electric-dipole transition that is allowed only at low symmetries with no inversion center. It is interesting to note that the overall photoluminescence intensity decreases with increasing the temperature up to 300 °C, and then the PL intensity further increases with increasing the temperature. Figure 15 shows the normalized PL decays of different temperatures of heated 1 mol % Eu3+ doped Na(Y1.5Na0.5)F6 nanoparticles, monitored at the 5D f 7F transition (617 nm). The decay times are 7.76, 7.14, 0 2 6.49, 5.93, and 6.38 ms for 80, 200, 250, 300, and 600 °C heated 1 mol % Eu3+ doped Na(Y1.5Na0.5)F6 nanoparticles, respectively. This result also agrees with the emission spectra. It is already seen from XRD and TG-DTA analyses that the bound water or interlayered water exits below 300 °C. The maximum PL intensity was observed for the 80 °C dried sample. However, the PL intensity decreases at the 200 °C heated sample because physisorbed as well as some bound water is removed as is evidenced from the TG analysis.The PL intensity decreases


Figure 16. PL spectra of 200 °C heated 1 mol % Eu-doped Na(Y1.5Na0.5)F6 nanoparticles prepared at different relative humidities (λex ) 394 nm).

again at 250 °C, where a 6% water loss is observed. The drastic decrease in PL intensity at 300 °C from the 80 °C dried sample is due to the destruction of the layered structure and decomposition of NH4Y2F7 to YF3, as is evident from TG and XRD analyses. In the present study, we have seen that the PL intensity decreases with increasing the temperature up to 300 °C, which is a new and interesting result in sodium yttrium fluoride nanocrystals. Recently, Matsumoto et al.17 observed similar results in a titanate layered oxide structure intercalated with Eu3+ in TiO2 thin films. They showed a higher emission intensity for the Eu3+ ion at room temperature than the 400 °C heated sample. They also observed a higher emission intensity at 100% RH (relative humidity) than at 5% RH. They suggested that coordinating water molecules to the Eu3+ cation play an important role in the luminescence properties of the Eu3+ intercalated layered oxide. In the present study, we believe that bound water or interlayered water plays a significant role in the luminescence properties of Eu3+ ions. Previously, we explained the presence of three cationic sites inside the hexagonal Na(Y1.5Na0.5)F6 nanoparticle. Recently, spectroscopic analysis of NaEuF419 and NaYF4:Pr24 indicated that the sodium ion sites are accommodated by a small portion of rare-earth ions. In the present study, Eu3+ ions are present in those cationic sites and are coordinated by interlayered water molecules that are released due to heat treatment (Figure 3). According to Matsumoto et al.,17 the interlayered water molecules will be fixed via hydrogen bonding, as in ice, leading to a decrease in radiationless quenching. This kind of ice-like behavior of the water molecule due to strong hydrogen bonding was observed in confined water molecules as in montmorillonite or other layered or porous compounds.26-29 The crystal field strength applied on the Eu3+ ion is stronger when it is coordinated with water molecules and gives a higher emission intensity. As we increase the temperature, water molecules are released, and the crystal field strength becomes weaker and gives a lower emission intensity of Eu3+ ions. After 300 °C, when the layer structure is destroyed, we observed an increase in the emission intensity of the Eu3+ ion for the 400 and 600 °C heated samples. The PL intensity again increases with increasing the temperature from 400 to 600 °C and is due to the removal of hydroxyl and other organic groups that are responsible for nonradiative relaxation. Similar results were obtained in the case of cubic NaYF4 crystals (see S2). To understand the effect of interlayered water on PL intensity, we studied the humidity effects on


Figure 17. FTIR spectra of 80, 300, and 400 °C heated 1 mol % Eudoped Na(Y1.5Na0.5)F6 nanoparticles prepared at pH 6-7 and Y3+/F1:8.

Figure 18. Excitation and PL spectra of 600 °C heated 1 mol % Eudoped NaYF4 and Na(Y1.5Na0.5)F6 nanocrystals prepared at pH 6-7 (λex ) 394 nm).

photoluminescence properties. Figure 16 clearly shows that the emissions intensity increases with increasing the RH. The maximum PL intensity was observed at 96% RH for 15 h, which reveals that interlayered or bound water has a profound effect on the emission intensity of the Eu3+ ion. Again, the existence and loss of water are confirmed by FTIR study. Figure 17 shows the FTIR spectra of the 80, 300, and 400 °C heated samples. A broad band in the region of 3000-3600 cm-1 is due to water O-H stretching vibrations that prove the existence of water in the 80 °C dried sample. The two strong peaks located at 1405 and 1364 cm-1 correspond to the NH4+ ion of NH4Y2F7, which indicates the formation of NH4Y2F7 along with sodium yttrium fluoride.16,19 Heating at 300 °C, the bands due to O-H stretching, and the NH4+ ion decreases, indicating the destruction of the layered structure due to the decomposition of NH4Y2F7. At 400 °C, the peaks due to O-H stretching and NH4+ nearly vanish, indicating that the layered structure is totally destroyed. This result supports the formation of layered structures below 300 °C. Again, we studied the effect of the Eu3+ concentration on luminescence properties given in the Supporting Information (S3). The effect of crystal phase upon the PL property of fluoride nanocrystals can be elucidated from Figure 18. Figure 18 depicts the photoluminescence spectra of 600 °C heated 1 mol % Eu3+ doped hexagonal Na(Y1.5Na0.5)F6 and cubic NaYF4 samples

Ghosh and Patra

Figure 19. Excitation and photoluminescence spectra of 80 °C dried 1 mol % Eu doped NaYF4 and Na(Y1.5Na0.5)F6 nanoparticles prepared at different pH values (λex ) 394 nm).

prepared at different Y3+/F- ratios and pH 6-7. It was already seen from the XRD study that the hexagonal phase and cubic phase appeared at 1:8 and 1:4 ratios, respectively. The PL intensity at the hexagonal phase is a 3.55 times stronger emission than the cubic phase sample. The decay times are 6.38 and 5.36 ms for 1:8 (hexagonal) and 1:4 (cubic) ratios, respectively. This result agrees with the emission spectra. Several researchers4a,b,10 reported that the PL intensity is higher in the hexagonal phase than in the cubic phase. We have already reported that the PL properties of rare-earth ions are crystal phase sensitive.30 The presence of three cationic sites inside the hexagonal form is the main reason for the higher efficiency of luminescence than the cubic phase.24 From the analysis, it is clear that the PL property can be tuned by judiciously controlling the fluoride concentration and crystal phase. Figure 19depicts the photoluminescence spectra of 80 °C dried 1 mol % Eu3+ doped hexagonal Na(Y1.5Na0.5)F6 and cubic NaYF4 (at Y3+/F- 1:8) at pH 6-7 and 2-3, respectively. Here, also the PL intensity is 3.2 times higher in the hexagonal phase than the intensity in the cubic phase sample. The decay times are 7.76 and 5.92 ms for pH 6-7 (hexagonal) and pH 2-3 (cubic), respectively. This result agrees with the emission spectra. The higher efficiency of the luminescence in the hexagonal form is due to the multisite character of the crystal lattice. This reveals that the efficiency of the luminescence property can be tuned by changing the crystal phase and that the crystal phase can be tuned by controlling the fluoride concentration and the pH value. Conclusion Chemical synthesis through an emulsion method is a promising route for the preparation of Eu doped sodium yttrium fluoride nanocrystals. Cubic NaYF4 and hexagonal Na(Y1.5Na0.5)F6 nanocrystals were prepared at Y3+/F- ratios of 1:4 and 1:8, respectively. Again, it was found that the pH of the solution played an important role in tuning the crystal structure. The hexagonal Na(Y1.5Na0.5)F6 and cubic NaYF4 phases appeared in the pH range of 6-7 and 2-3, respectively. The mechanism related to crystal phase control was proposed and discussed. Analysis suggests that the formation of interlayered water is due to the layered structure of NH4LnF4, which is potentially stacked along the direction of (100) linked by NH4+ ions. The presence of interlayered water molecules was found to be inevitable for emissions with high intensity. It was also found that PL properties of Eu3+ ions are sensitive to the crystal structure, which is again controlled by the Y3+/F- ratio and pH of the solution. The emission intensity of the peak at 614 nm

Eu3+ Doped Sodium Yttrium Fluoride Nanocrystals (5D0 f 7F2) for Eu3+ ions for hexagonal nanocrystals is greater than the cubic sample. Acknowledgment. The Department of Science and Technology (NSTI) and “Ramanujan Fellowship” are acknowledged for financial support. Supporting Information Available: Structural changes for longer treatments (S1), effect of cubic phase on luminescence (S2), and effect of Eu3+ ion concentration (S3) on luminescence. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sivakumar, S.; Veggel, V. M. J. C. F.; May, S. P. J. Am. Chem. Soc. 2007, 129, 620. (b) Bovero, E.; Veggel, V. M. J. C. F. J. Phys. Chem. C 2007, 111, 4529. (c) Meyssamy, H.; Riwotzki, K.; Kornowski, A.; Naused, S.; Haase, M. AdV. Mater. 1999, 11, 840. (2) (a) Chen, W.; Joly, G. A.; Zhang, Z. J. Phys. ReV. B: Condens. Matter Mater. Phys. 2001, 64, 41202. (b) Chen, W.; Malm, O. J.; Zwiller, V.; Huang, Y.; Liu, S.; Wallenberg, R.; Bovin, O. J.; Samuelson, L. Phys. ReV. B: Condens. Matter Mater. Phys. 2000, 61, 11021. (c) Smith, A. B.; Zhang, Z. J.; Joly, A.; Liu, J. Phys. ReV. B: Condens. Matter Mater. Phys. 2000, 62, 2021. (3) (a) Song, H.; Yu, L.; Lu, S.; Wang, T.; Liu, Z.; Yang, L. Appl. Phys. Lett. 2004, 85, 470. (b) Yu, L.; Song, H.; Lu, S.; Liu, Z.; Yang, L.; Kong, X. J. Phys. Chem. B 2004, 108, 16697. (c) De La Rosa-Cruz, E.; Diaz-Torres, L. A.; Rodriguez-Rojas, R. A.; Meneses-Nava, M. A.; BarbosaGarcia, O.; Salas, P. Appl. Phys. Lett. 2003, 83, 4903. (4) (a) Yi, G.; Lu, H.; Zhao, S.; Ge, Y.; Yang, W.; Chen, D.; Guo, H. L. Nano Lett. 2004, 4, 2191. (b) Mai, X. H.; Zhang, W. Y.; Si, R.; Yan, G. Z.; Sun, D. L.; You, P.; Li Yan, H. C. J. Am. Chem. Soc. 2006, 128, 6426. (c) Boyer, C. J.; Vetrone, F.; Cuccia, A. L.; Capobianco, A. J. J. Am. Chem. Soc. 2006, 128, 7444. (5) (a) Wang, L.; Li, Y. Nano Lett. 2006, 6, 1645. (b) Wang, L.; Li, Y. Chem. Commun. 2006, 2558, 2557. (6) Zeng, H. J.; Su, J.; Li, H. Z.; Yan, X. R.; Li, D. Y. AdV. Mater. 2005, 17, 2119. (7) Scha¨fer, H.; Ptacek, P.; Ko¨mpe, K.; Haase, M. Chem. Mater. 2007, 19, 1396.

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Tuning of Crystal Phase and Luminescence Properties of Eu3+ Doped

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