NaYF4:Eu2+ Microcrystals: Synthesis and Intense Blue Luminescence

Jun 18, 2008 - ... Graduate School of Chinese Academy of Sciences, Fuzhou 350002, ... NaYF4:Eu2+ microcrystals with an intense blue luminescence were ...
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CRYSTAL GROWTH & DESIGN

NaYF4:Eu2+ Microcrystals: Synthesis and Intense Blue Luminescence Yiguo Su, Liping Li, and Guangshe Li* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Graduate School of Chinese Academy of Sciences, Fuzhou 350002, P. R. China

2008 VOL. 8, NO. 8 2678–2683

ReceiVed June 25, 2007; ReVised Manuscript ReceiVed March 24, 2008

ABSTRACT: In this work, NaYF4:Eu2+ microcrystals with an intense blue luminescence were successfully fabricated via a solutionbased route. During the sample preparation, oleic acid and cetyltrimethylammonuim bromide were used as the surfactants to tune the morphology, while citric acid was taken as a ligand to stabilize the β-phase NaYF4 and to completely reduce Eu3+ to Eu2+ for activation of blue luminescence. As confirmed by transmission electron microscopy and scanning electron microscopy measurements, all as-prepared samples crystallized in hexagonal rods or tubes, depending on the types of surfactants used. The rod samples had a diameter of about 1.7 µm and a length in the range of 2.6-3 µm, while the tube samples showed an external diameter in the range of 0.8-1 µm and a length in the range of 2.5-3.5 µm with an internal diameter of about 0.5 µm. Both rods and tubes were capped with surfactants, which enabled them to stably disperse in glycol to form transparent solutions. This work seems to be the first example of the successful preparation of stable dispersion of micron luminescence solids. These transparent solutions showed an intense blue luminescent emission of Eu2+ with a quantum yield of about 14%. Finally, the mechanism for the fabrication of the rods and tubes as well as the Eu3+ reduction was discussed.

1. Introduction As a class of building blocks, well-defined micro- and nanostructured materials are expected to play key roles in many aspects of applications due to their unique electronic, optical, and magnetic properties. It is necessary to develop methods for organizing them into larger entities and to explore the potential applications of these entities.1–3 Many efforts have been made in designing and organizing organic or inorganic phosphor materials in microscale,4–6 among which lanthanide ions doped luminescence materials are of the utmost prospective interest.7–10 In particular, europium-doped luminescent microcrystals have garnered extensive research attention because of the unique feature of changeable valence states (Eu3+ vs Eu2+) that may result in tunable emission and long-lasting luminescence decay times in comparison with other luminescent materials such as CdS/CdSe and ZnS:Mn.11,12 Eu3+ is chemically stable in a variety of host materials and generally emits a characteristic red emission; as a result, the preparation of Eu3+-doped phosphor can always get facile success by sol-gel methods, solid-state reaction, or liquid phase techniques. In contrast, Eu2+ featured by a blue emission is chemically metastable in an oxidizing atmosphere and can be very difficult to prepare or stabilize in host materials because of its easy oxidization to Eu3+ via Eu2+ f Eu3+ + e-. In the view of the promising applications of Eu2+-doped phosphors for plasma display phosphors and light emitting diodes,13,14 it appears very important to find proper host materials and to explore new methodologies for the assembly of Eu2+-based luminescent materials. Until now, most literature reports of luminescent host materials have concentrated on oxygen-based systems such as metal oxides and inorganic salts, whereas the conventional oxygen-based systems often have large phonon energy. Comparatively, rare-earth metal fluorides are advantageous as the host matrix for their lower phonon energy, which would * To whom correspondence should be addressed. Tel: +86-591-83792846. Fax: +86-591-83714946. E-mail: [email protected].

significantly reduce the quenching possibility of the excited states of the rare ions. Moreover, the bonds between rare-earth and fluorine ions are primarily ionic, which can lead to a wide bandgap and transparency in visible and ultraviolet range. These two factors are propitious to the usability of fluorides based host materials in optical applications under vacuum ultraviolet (VUV) excitation. β-Phase (hexagonal-phase) NaYF4 is a prototype fluoride that shows merits of high infrared-to-visible up-conversionefficiencyviaEr3+/Yb3+ orTm3+/Yb3+ codoping,15,16 while the down-conversion phosphors can also be achieved in it when doped with special ions such as Eu2+ and Tb3+. Nevertheless, the current methodologies for Eu2+-doped blue phosphors are still very complicated and difficult. Hightemperature disposal technology was always taken as the traditional strategies for the formation of crystalline Eu2+-doped luminescent materials, in which high temperature always results in a wide particle size distribution and great extravagance of energy sources.17–20 Consequently, it remains a vital challenge to prepare well-defined, monodispersed, single-crystalline, sizeand shape-controlled NaYF4:Eu2+ with a blue emission. Herein, we designed a facile solution route to synthesize β-phase NaYF4:Eu2+ microcrystals. The formation conditions were optimized primarily using oleic acid and cetyltrimethylammonuim bromide as the surfactants for morphological control and using citric acid as the ligand to stabilize Eu2+. The asprepared microcrystals were stably dispersed in solvents and showed an intense blue emission. The methodology reported in this work for blue luminescence fluoride phosphors may open the door for future applications in solid and fluid environments.

2. Experimental Procedures 2.1. Preparation of NaYF4: Eu2+ hexagonal microrods. Analytical grade chemicals of NaF, Y2O3, Eu2O3, citric acid, and NaOH were purchased from Shanghai Chemical Industrial Co. and used as the starting materials without further purification. A total of 1.07 g of Y2O3 (99.99%) and 0.09 g of Eu2O3 (99.99%) were sufficiently mixed and dissolved in diluted nitrate acid on heating with stirring. The mixed nitrate solution thus formed was allowed to cool down to room

10.1021/cg070574g CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

NaYF4:Eu2+ Microcrystals

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Figure 1. XRD patterns of (A) sample I, (B) sample II, and (C) sample III. Vertical bars below the patterns are the standard diffraction data (JCPDS card No. 16-334; 77-2042) for β- and R-NaYF4, respectively.

temperature. Then, 2.10 g of citric acid and 0.06 g of cetyltrimethylammonuim bromide (CTAB) were added into the mixed nitrate solution. After mixing with 2.52 g of NaF (99.9%) until a white homogeneous dispersion appeared, a well-controlled amount of NaOH (99.9%) solution was added with magnetic stirring to reach pH ) 8, and the total solution volume was carefully adjusted to 60 mL. The mother solution containing the white suspension was sealed in 25 mL Teflon-lined stainless steel autoclaves which were allowed to react at 200 °C for 16 h. The as-prepared white suspensions were washed with ethanol and deionized water several times and then dried at 80 °C for 3 h. The obtained product was named as sample I. 2.2. Preparation of NaYF4: Eu2+ microtubes. The procedure of preparing NaYF4:5%Eu2+ microtubes was similar to that for microrods with the exception that CTAB was replaced by 20 mL of oleic acid. The final products were named as sample II. Using a similar procedure but with no citric acid addition, sample III was obtained. 2.3. Preparation of Transparent Solutions of NaYF4:Eu2+ Microcrystals. Transparent solutions containing 0.2 wt% microcrystals in glycol were obtained by treating the desiccated particle powders in glycol in an ultrasonic bath for 10 min. Transparent solutions containing tubes or rods with the same concentration displayed similar optical properties and quantum efficiencies. 2.4. Sample Characterization. The phase structure and purity of the as-prepared samples were characterized by X-ray power diffraction (XRD) on Rigaku DMAX2500 X-ray diffractometer using a copper target (λ ) 0.154 nm) for 2θ ranging from 5 to 85°. The particle sizes and morphologies of the samples were determined using transmission electron microscopy (TEM) on a JEM-2010 apparatus with an acceleration voltage of 200 kV and a field emission scanning electron microscopy (SEM) on a JSM6700F apparatus working at 10 kV. Infrared spectra of the samples were measured on a Perkin-Elmer IR spectrometer using a KBr pellet technique. 2.5. Photoluminesence Measurements. The photoluminescence spectra of all samples were measured on Cray Eclipse fluorescence spectrophotometer with a Xe lamp excited at 355 nm and recorded at a scan rate of 120 nm/min.

3. Results and Discussion We first optimized the synthetic conditions of pure β-NaYF4 with and without addition of citric acid. The phase purity and crystallinity of the as-prepared samples were monitored using XRD. Figure 1 shows XRD patterns of the as-prepared samples. When citric acid was involved in the reaction systems, as indicated in Figure 1, both samples I and II showed diffraction peaks that match well the standard data of β-phase NaYF4 (Joint Committee for Power Diffractions Standards, JCPDS card No. 16-334). No traces of impurity peaks are observed, which indicates that both samples crystallized in a single phase of β-NaYF4. Comparatively, sample III prepared in the citric acid

Figure 2. (A) SEM, (B) TEM, (C) SAED, and (D) EDS data of the sample I; (E) SEM, (F) TEM, (G) SAED, and (H) EDS data of the sample II.

free condition exhibited several additional weak peaks at 28.1°, 32.6°, and 46.8°. XRD data analysis indicated that these additional peaks were associated with R-NaYF4 (JCPDS card No. 77-2042). These observations clearly demonstrate that citric acid plays an important role in the formation of pure β-NaYF4. From Figure 1, it is also seen that the diffraction peaks (011) and (110) for samples I and II showed inverse intensities, which are likely associated with the distinct morphologies. The full widths at half-maximum (fwhm) for diffraction peaks of both samples are relatively small, which indicates a large size and high crystallinity. These assumptions are confirmed by scanning electron microscope (SEM) and transmission electron microscope (TEM). Morphology and particle size of the as-prepared samples were examined using SEM and TEM. As shown in Figure 2A,B, sample I fabricated using CTAB as the surfactant consisted of

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Figure 3. FTIR spectra of (A) sample I and (B) sample II.

primary hexagonal rod-like microcrystals which coexisted with a small quantity of hexagonal tube-like ones. The microrods exhibited a diameter of about 1.7 µm and a length in the range of 2.6-3 µm. The aspect ratio is intermediately between 1.6 and 1.8. All these microcrystals displayed a relatively narrow dimension distribution as is indicated by SEM observations in larger scale regions (S1). A selected-area electron diffraction (SAED) pattern in Figure 2C for a single microrod clearly shows the characteristic spots of single-crystal NaYF4:Eu as is confirmed by HRTEM images (S2). While for sample II that was prepared using oleic acid as the surfactant, microcrystals were achieved to show a tube shape with an external diameter in the range of 0.8-1 µm, an internal diameter of about 0.5 µm, and a length in the range of 2.5-3.5 µm (Figure 2E,F). The spots in the SAED pattern (Figure 2G) indicate that the tubes were single crystals. It should be mentioned that the SEM and TEM images were obtained from the randomly selected areas of the samples and were thus representative of the overall sizes and shapes of the as-prepared β-phase NaYF4:Eu microcrystals. Moreover, the EDS data (Figure 2D,H) confirmed that the main elemental components are Na, Y, and F. The signals of minor dopant Eu ions and those of the Cu from the TEM grid were also detected. The Eu concentration in the rods and tubes were determined to be 9.61 and 9.47%, respectively. In addition, the weak signals in the EDS data were originated from the surface overlayers like oleic acid or CTAB. Surface layers of the as-prepared samples were examined by FTIR spectra. As indicated in Figure 3A, the rod samples showed a set of complicated absorption bands: two strong bands at 3343 and 1630 cm-1 are attributed to the stretching vibration of O-H, and the weak bands observed at 1159 and 1064 cm-1 may be assigned to the vibration of C-N bonds in surfactant CTAB when comparing to the IR spectrum of the product prepared using citric acid only (S3). For the tube samples, no characteristic vibrations for the citric acid were observed (S3), which suggests almost the absence of the citric acid residues after the formation reactions. Nevertheless, as indicated in Figure 3B, in addition to the vibrations of O-H bonds at 3443 and 1630 cm-1, several new bands appeared at 2927, 2855, 1466, and 1713 cm-1, respectively. The former three bands are assigned to the asymmetric (vas) and symmetric (vs) stretching vibrations and the deformation vibrations of methylene (CH2) in the long alkyl chain of the oleic acid molecules,21 while the latter one is attributed to the CdO stretching vibration. These features indicate that the surfaces of the as-prepared NaYF4:Eu microcrystals were capped by CTAB and oleic acid. These organic capping agents are fundamentally important, which may produce distinct morphologies and, more specifically, give rise

Figure 4. Schematic diagrams proposed for the formation of (A) sample I and (B) sample II.

to a long-term colloidal stability of NaYF4:Eu microcrystals necessary for intense luminescence, just like what is observed in poly(acrylic acid) capped CdS in aqueous solution.22 In the following, we will address these issues. With regard to the formation mechanism for the distinct morphologies, surfactants oleic acid and CTAB can play the primary roles in assembling nano- or microscale building blocks into the various crystalline shapes. The formation processes of hexagonal rod- and tube-like NaYF4:Eu microcrystals are illustrated in Figure 4. In a typical procedure for the synthesis of rod-like NaYF4 micocrystals, CTAB was first dissolved in water to form micelles. During the subsequent process, yttrium ions are likely chemically bonded with citric acid to form two types of complexes via van der Waals forces.23 Then, these

NaYF4:Eu2+ Microcrystals

complexes would attach on branches of the micelles and react with Na+ and F- to form NaYF4 seeds. On the basis of the SEM observations, we supposed that two types of micelles may exist as illustrated in Figure 4A. As the reaction goes, the surface of the seeds would be capped with the organic surfactant (Figure 3S), which likely reduces the activity of the nanocrystals to promote the ordered crystal growth.24 Therefore, the majority of NaYF4 seeds may grow into plates via van der Waals attractions or polar interparticle interactions.25 Meanwhile, a small quantity of the seeds grew into annuluses. Finally, the plates and annuluses grew into hexagonal-shaped rods and tubes, most likely following an oriented aggregation process as what occurred for gold nanorods formed in the aqueous solution at the presence of CTAB.26 In order to verify the importance of CTAB on the morphology control, double amounts of CTAB were added to this reaction system. Irregularly shaped NaYF4 crystals were obtained (S4), probably because excessive surfactants may destroy the balance of the forces of interparticle attraction and van der Waals interaction between NaYF4 seeds and micelles,27 and therefore change the forms of the micelles for the final formation of NaYF4 microcrystals. Sun and co-workers have proved that the hexagonal flake shape of NaYF4 microcrystals is obtained using trisodium citrate without any surfactants.28 Having these in mind, it can be concluded that CTAB was responsible for the formation of rod-like NaYF4 microcrystals. Oleic acid is another surfactant in common use to control the morphology and dimension of the final particles,29,30 As for the tubes prepared using surfactant oleic acid, the formation mechanism could be somewhat different from that of rod-like NaYF4 microcrystals, since the synthesis process is mainly based on a phase-transfer and separation mechanism,31 which involves the precipitation of the yttirium salts and Na+, F- ions in the presence of excess oleic acid and citric acid. During the reaction process, as illustrated in Figure 4B, Y3+ first reacted with citric acid to form two type of complexes.23 Then oleic acid was added with vigorous stirring and forms a two-phase water/oil system. Since oleic acid is largely in stoichiometric excess in the system, it is likely that the above-mentioned complexes would react with the excess oleic acid to form a more complicated complex. During this procedure, the concentration of Y3+ ions got a balance in the two-phase system. Under hydrothermal conditions, oleic acid capped complex had a great possibility to react with Na+ and Y3+ ions to form nuclei. Obviously, the organic adsorption on NaYF4 crystal surfaces may change the activity of the crystal planes32 and therefore become the primary cause to the morphological control, though the shape-control procedures may also occur through noncovalent interactions that include hydrogen bonding, hydrophilic, hydrophobic, van der Waals forces, and electrostatic networks.33 As indicated by the above SEM and TEM observations, all as-prepared samples have an excellent crystalline nature. Though at micron scale, the particles of the as-prepared samples were still able to be stably dispersed in glycol to form transparent solutions. All dispersions showed an intense blue luminescence when excited at the wavelength of 355 nm (Figure 5, right). Figure 5 shows the excitation and emission luminescent spectra of the colloidal solutions that contain 0.2 wt% of the samples. Two excitation bands were detected at 260 and 355 nm, which could be associated with Eu2+, because the host NaYF4 hardly shows any absorption in the wavelength range between 260 and 355 nm. We first address the origin of the excitation band at 260 nm. It is known that ionic radius of fluoride ions is 1.33 Å, which is close to that of 1.40 Å for O2- ions. Consequently, it

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Figure 5. Left: Excitation and emission luminescent spectra of colloidal solutions containing 0.2 wt% sample I (A) and sample II (B). Right: eye-visible luminescence photos of the corresponding colloidal solutions with and without excitation at 355 nm. The dot lines represent the data fit using Gaussian functions.

can be impossible to avoid the presence of traces of oxygen species in lattice during the crystal-grown process in solutionbased systems. On the basis of the assignments for the excitations in KMgF3:Eu and LiBaF3:Eu,34,35 the excitation band at 260 nm is ascribed to the transitions between O2- and Eu2+. On the other hand, the excitation band at 355 nm is characteristic of the transition from ground state (4f) to the 5d levels of Eu2+. Figure 5 also shows a broad emission for the as-prepared samples. To understand the nature of this broad emission, we did a comparative study on the emission features of Eu2+ in several compounds. It is known that the emission of Eu2+ due to 4f7 f 4f65d1 transition is very sensitive to the host materials, which usually gives several bands in the broad violet-red region. For example, the emission band is located in the red region of 616-670 nm for Sr2Si5N8:Eu2+ phosphors obtained by solidstate reactions in a protective atmosphere of nitrogen.36 Ba2B5O9Cl: Eu2+ films prepared using a rapid thermal annealing method in air show a blue luminescence in the range of 360-550 nm even though Eu3+ emission can still be observed.37 For the present NaYF4:Eu2+ microcrystals, a very broad emission was observed in the violet-green range. Careful data analysis indicates that the broad emission band consisted of three sub-bands peaking at 410, 430, and 453 nm. The existence of multibands might be due to the split 5d levels of Eu2+ ions that are locating in hexagonal NaYF4 matrix. It is noted that there are no emissions at 593 and 616 nm from Eu3+ under 355 nm excitation, and that the shape and position of the emission band did not show any apparent changes when the excitation wavelength varies from 260 to 355 nm. These observations strongly suggested that the majority of Eu ions in the present samples are divalent. Alternatively, when Eu ions were in mixed valence of Eu2+ and Eu3+ in crystals, the energy transfer from the excited Eu2+ ions to the neighboring Eu3+ ions through a nonradiative relaxation should be easily detected by the red shift of emission band with increasing the excitation wavelength.38 Strikingly, both samples I and II exhibited almost the same excitation and emission spectra (Figure 5), which indicates a weak impact of the micron scale morphology on the luminescence. Nevertheless,

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the morphology control over the luminescent materials reported in this work is fundamentally important and may have many implications in determining the maintenance of the phosphors in the field emission display39 and in improving the screen brightness as well.40 Sample characterizations as stated above have shown that the as-prepared NaYF4 microcrystals exhibited a high sample uniformity, which may have some effects on the quantum efficiency. Here, the quantum efficiency of the samples was determined by the following equation according to Silva:41

Qx⁄Qr)[Ar(λr)/Ax(λx)][I(λr)/I(λx)][n2x/n2r ][Dx ⁄ Dr]

(1)

where subscript r stands for the reference and x is for the samples; A is the absorbance at the excitation wavelength, I is the intensity of the excitation light at the same wavelength, n is the refractive index (n ) 1.4318, in C2H6O2), and D is the measured integrated luminescence intensity. Rhodamine B with a concentration of 1.58 × 10-7 M in ethanol (n ) 1.36048, 20 °C, Qabs ) 0.65) was used as the standard to determine the quantum efficiency of NaYF4:Eu2+ microcrystals. The quantum efficiencies of rod- and tube-like NaYF4:Eu2+ microcrystals dispersed in glycol were determined to be almost the same at about 14%, which is lower than those of 80% for Sr2Si5N8: Eu2+,38 97% for spherical CaMgSi2O6:Eu2+,42 and 70% for BaMgAl11O17:Eu2+.43,44 It has to be mentioned that for the present work, the luminescence measurements were performed for the samples dispersed in glycol solutions, while the luminescence data reported in literature were achieved for the Eu2+ containing solids with irregular shape, wide particle size distribution, and serious aggregation. The reduction in quantum efficiency for the present samples should be closely related to the existence of organic molecules adsorbed on the sample surfaces since, as in many wet chemical routes, it is inevitable to introduce OH ions which are easily coordinated to the rare earth ions at the surface of the microcrystals. These OH ions can act as the quenchers to the luminescence of rare earth ions through multiphonon relaxation.45,46 Yan and co-workers47 reported that the photoluminescence quantum efficiency of YBO3:Eu prepared by hydrothermal method is lower than that of the samples prepared by solid-state reactions. Sabbatini et al.48 demonstrated that the replacement of the OH group by the low-frequency OD group diminishes the vibronic deactivation pathway and thus enhances the luminescent efficiency. On the other hand, organic molecules with high-energy C-C and C-H vibrational oscillators can also act as the luminescence quenchers for nearby lanthanide ions.49,50 In spite of the relatively low quantum efficiency, the methodology reported in this work also show merits of morphological control and most importantly, the environmentally friendly valence reduction of Eu3+ to Eu2+ at low temperatures. Comparatively, when using the traditional methods, Eu2+ has to be achieved at high temperature under reductive gases but also facing great difficulties in morphological control. It is kinetically unusual that Eu ions were reduced from +3 to +2 even in solution at a temperature below 200 °C. Previous studies have concluded that the three-dimensional rigid network of BO4 tetrahedron is necessary to stabilize the divalent rare earth ions in an oxidizing atmosphere.51,52 However, the reduction of Eu3+ in the network of BO4 tetrahedron often needs high-temperature disposal. Because of the urgent demand for achieving blue luminescence, several groups have tried to stabilize Eu2+ via wet chemical routes,37,53 but most of these literature reports show mixed red and blue emissions from Eu3+ and Eu2+ ions.53 Compared to these previous literature works,

the methodology reported in this work is superior as Eu3+ was completely reduced to Eu2+ with the addition of citric acid. It appears that citric acid can be crucial for the reduction reaction of Eu3+ as in the preparation of Ag and Au nanoparticles,54–57 which however conflicts with the analysis of redox potential of europium ions.23 As a consequence, the reduction mechanism of Eu3+ to Eu2+ reported in this work is still unclear, which deserves further investigations.

4. Summary 2+

Eu doped NaYF4 microcrystals with high crystallinity and intense blue emission were synthesized by a facile solution route at low temperatures. During the sample preparation, oleic acid and CTAB molecules were used as the surfactants to tailor NaYF4 into microrods and microtubes, and citric acid was applied as a ligand to stabilize the β-phase NaYF4 microcrystals and furthermore to reduce Eu3+ into Eu2+. Both rod and tube samples capped with the surfactants were stably dispersed in glycol and formed a transparent solution. Such transparent solution showed an intense blue luminescence with a quantum yield of about 14%. The results reported in this work represented an important step toward the development of Eu2+-based fluoride phosphors. Moreover, the successful preparation of NaYF4:Eu2+ microcrystals would open a realm of novel ways for the preparation of Eu2+-doped luminescent materials. Acknowledgment. This work was financially supported by NSFC under the contract (Nos. 20671092, 20773132, 20771101), 973 program (No. 2007CB613301), Fund of Fujian Key Laboratory of Nanomaterials (No. 2006L2005), and Directional program (KJCXZ-YW-M05) of Chinese Academy of Sciences. Supporting Information Available: TEM and HRTEM images of sample I at low and high magnification; IR spectra of (a) sample I, (b) sample II, and (c) the sample prepared using citric acid only; TEM image of the sample prepared using excessive CTAB as surfactant. This material is available free of charge via the Internet at http:// pubs.acs.org.

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