Morphology-Controlled Synthesis, Physical Characterization, and

Dec 30, 2008 - Three-dimensional pomponlike europium-doped sodium gadolinium tungstate NaGdWO4(OH)x:Eu3+ microarchitectures that exhibit efficient whi...
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J. Phys. Chem. C 2009, 113, 1074–1082

Morphology-Controlled Synthesis, Physical Characterization, and Photoluminescence of Novel Self-Assembled Pomponlike White Light Phosphor: Eu3+-Doped Sodium Gadolinium Tungstate Fang Lei and Bing Yan* Department of Chemistry, Tongji UniVersity, Siping Road 1239, Shanghai 200092, People’s Republic of China ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: NoVember 26, 2008

Three-dimensional pomponlike europium-doped sodium gadolinium tungstate NaGdWO4(OH)x:Eu3+ microarchitectures that exhibit efficient white-light photoluminescence properties have been successfully synthesized via a facile hydrothermal process in the presence of the surfactants cetyl trimethyl ammonium bromide, poly(vinyl pyrrolidone), and poly(ethylene glycol)-block-poly(propylene glycol)-block-ploy(ethylene glycol). The white light sodium gadolinium tungstate phosphor contains three emission bands: the blue-green band at 468 nm is ascribed to the ligand to metal charge transfer transition from OfW; the orange band at 590 nm and red band at 610 nm are attributed to the 5D0f7F1 and 5D0f7F2 transitions of Eu3+. The luminescence color can be tuned from blue to white to red by adjusting the doping concentration of Eu3+. Both scanning electron and transmission electron microscopy indicate that the obtained microspheres have a uniform particle size distribution. The three-dimensional sodium gadolium tungstate pompon-shaped structures were constructed layer-by-layer from a large number of two-dimensional nanoflakes with a mean diameter of ∼100 nm. The whole time-dependent process is interpreted as an example of self-assembly process. The emission spectra are dominated by a 5D0f7F2 transition of Eu3+. The optimum concentration for the white light was to keep the ratio of Eu3+ and Gd3+ at about 0.02. Introduction Large-scale self-assembled structures with highly specific morphologies and novel properties are of great interest in the area of materials synthesis and device fabrication.1-4 Many ordinary compositions exhibit attractive properties owing to their unique microstructures.5 Therefore, artificial building of microspheres with three-dimensional (3D) architectures is an attractive pursuit with many challenges. The alignment of nanostructure building blocks (nanoparticles, nanoflakes, and nanoribbons/nanoplates) into 3D ordered superstructures by bottom-up approaches has been an exciting field in recent years.6 Many methods have been employed in synthesizing inorganic materials, such as microemulsion, hydrothermal (solventhermal), sol-gel, molten salt, microwave, sonochemical route, and so on. Among the many synthetic routes of inorganic materials, hydrothermal synthesis route is one of the most prevalent methods for controlled synthesis with various morphologies. Self-assembly is an efficient and often preferred process to build micro- and nanoparticles into ordered 3D macroscopic structure. Up to now, different driving mechanisms of the self-assembly processes have been proposed: such as, surface tension, capillary effect, oriented attachment, electric and magnetic forces, hydrophobic interaction, van der Waals interaction, aromatic interaction, hydrogen-bonding, electrostatic interaction, etc.7-17 Versatile surfactant-assisted hydrothermal methods can lead to various morphologies of inorganic crystals. Previous works have demonstrated that surfactants have great influence on controlling the morphology of inorganic materials.18-20 Tungstate compounds are a large class of inorganic functional materials that exhibit interesting physical properties and have * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. Fax: +86-21-65982287. Phone: +86-21-65984663.

technological applications in the field of optical material, photocatalysis, quantum electronics, and so on.21-24 Some tungstate hosts as one kind of self-activating materials can emit blue light itself under ultraviolet or X-ray excitation.25,26 Rareearth metal ions (RE3+) doped tungstates are widely known as multifunctional material having unique physical and chemical properties. These compounds are extensively used as laser, scintillator and luminescent materials.27,28 Brito et al. gave the luminescence investigation of Eu3+ in the RE2(WO4)3 (RE ) La, Gd) matrix using the Pechini method.29 Up to now, tungstate systems have been developed and investigated by Yu 19,30,31 and Hu32 in morphology-controlled synthesis. All these investigations greatly enriched the theory of crystallization for self-assembly microarchitectures. However, despite these improvements in crystal growth of nanocrystals with predictable size, shape, and crystal structures are still hard challenges, due to the complexity of crystal growth mechanism and composition of materials. More studies are still needed to clearly clarify these issues. Herein, in this paper we report a facile surfactant-assisted hydrothermal approach under mild conditions for the preparation of nearly monodisperse 3D white-light sodium gadolinium tungstate (NaGdWO4(OH)x:Eu3+) micropompons self-assembled by multilayer nanoflakes with controllable morphology and size, and they have tunable color diversity properties which is due to the different doping concentration of Eu3+. When the phosphor was doped with a certain amount of Eu3+, it can be used as potential white-light phosphor for the combination of blue-green light related to the charge transfer (CT) from OfW of tungstate and red light of characteristic transition of Eu3+. The influence of reaction temperature and the concentration of surfactant on the formation of sodium gadolinium tungstate have been systematically examined in such a reaction system.

10.1021/jp8082634 CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

Eu3+-Doped Gadolinium Tungstate 2. Experimental Section 2.1. Chemicals. Europium oxide (Eu2O3 (99.99%)), gadolinium oxide (Gd2O3 (99.99%)), nitric acid (HNO3), sodium tungstate (Na2WO4 · H2O (AR)), and surfactants (cetyltrimethyl ammonium bromide (CTAB), poly(vinyl pyrrolidone) (PVP, K30), poly(ethylene glycol)-block-poly(propyleneglycol)-blockploy(ethylene glycol) (pluronic P123 (EO20PO70EO20, Aldrich)) were used as the raw materials without further purification. 2.2. Synthesis Procedures. The given amounts of rare-earth nitrates (Gd3+ and Eu3+) were prepared by dissolving corresponding Eu2O3-Gd2O3 ((2-x) mmol Gd3+ and x mmol Eu3+ (x ) 0, 0.02, 0.04, 0.06, and 0.08), the molar ratio of Eu3+/ Gd3+ vary from 0.01∼0.05) in HNO3 and excess HNO3 was removed by evaporation. Solution A was prepared by dissolving the obtained rare earth nitrate (containing Gd3+ and Eu3+) in 5 mL of deionized water; solution B was prepared by dissolving Na2WO4 · 2H2O (the molar ratio of RE/W is 2: 1) and ∼0. 0005 mol CTAB in a 5 mL of heated deionized water (∼75 °C) with initial pH 8.0 and stirred for ∼10 min at room temperature. Then solution A was added into solution B, and the mixture was vigorously stirred for about 30 min to ensure that all reagents were dispersed homogeneously. The mixture was transferred into a 25-mL Teflon-lined stainless steel autoclave and filled with deionized water up to a 70% filling capacity of the total volume. The autoclave was sealed and maintained at 170 °C for 48-72 h and then cooled to room temperature naturally. After the above hydrothermal treatment, the product was centrifuged and washed with deionized water for several times. And then the precipitate was dried at 70 °C for 24 h and collected for characterization. In addition, we get part of the final products for calcinations at 630 °C for about 2 h to characterize their structure. For comparison, another experiment was repeated with the existence of surfactants CTAB, PVP, and P123 on the same hydrothermal reaction. 2.3. Characterization. The samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), Fourier tranform (FT-IR), UV-vis spectroscopy, and photoluminescence (PL). XRD analyses were carried out on a Bruker D8-Advance diffractometer with graphite-monochromatized Cu KR radiation (40 KV/60 mA, graphite monochromator). Thermogravimetric analysis was performed on a STA-409PC/4/H LUXX TG-DSC instrument at a heating rate of 10 K/min to a maximum temperature of 1273 K. The morphology was characterized with a Philip XL30 environmental scanning electron microscope (ESEM). TEM images and EDS were recorded on a JEOL200CX microscope with an accelerating voltage of 200 kV. PL spectra were obtained using a fluorescence spectrophotometer (RF5301) with a xenon lamp as the excitation source. FT-IR data were collected on Perkin-Elmer 2000 FT-IR spectrophotometer in the range of 4000-400 cm-1 using KBr pellets. UV-vis diffuse reflectance spectra were recorded on a Shimadzu UV-3101 spectrophotometer equipped with an integrating sphere, using BaSO4 as a reference. 3. Results and Discussion 3.1. Crystal Structure of the Pomponlike Sodium Gadolinium Tungstate NaGdWO4(OH)x:Eu3+. The phase and purity of the as-synthesized samples were determined by XRD patterns. It can be seen from parts a-g of Figure 1 the XRD patterns of the products obtained by hydrothermal method at 170 °C for different reaction times of (a) 0 h, (b) 3 h, (c) 12 h, (d) 24 h, (e) 48 h, (f) 72 h, and (g) 120 h, respectively. Figure 1a is the

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Figure 1. XRD patterns of the product synthesized by hydrothermal method at 170 °C in the presence of CTAB for different reaction times: (a) 0 h, (b) 3 h, (c) 12 h, (d) 24 h, (e) 48 h, (f) 72 h, and (g) 120 h

Figure 2. (a) TG-DSC of the product obtained by hydrothermal method at 170 °C for 72 h in the presence of CTAB. (b) XRD pattern of the same hydrothermal product with heat treatment at 630 °C for 2 h.

XRD pattern of the precipitation precursor; there is no obvious diffraction peaks, so the product exists as amorphous state. The products obtained for 3 h also exhibit amorphous XRD pattern (Figure 1b). When the reaction time increased at 12 h, we can observe obvious diffraction peaks, which corresponding the crystallization process. With increasing hydrothermal reaction time, most of these diffraction peaks intensity increase significantly. Usually, the longer reaction time is good for the better crystallization. When the reaction time keeps at 72 h, there is not much difference from the products obtained at 120 h. Its structure can not be clearly figured out now because it can not be matched well with the database of XRD. Several main peaks match with the compound NaGd(WO4)2 (NGW). The crystal of the hydrothermal product may contain hydroxy group. It will be verified in the following thermogravimetric differential scanning calorimetry (TG-DSC) and FT-IR discussion. NGW crystal belongs to the scheelite family of crystals.33 The lattice constants were reported as: a ) 5.243 Å and c ) 11.368 Å. The space group is I41/a.34,35 To investigate the thermal stability and phase transformation temperature of the hydrothermal products, we employed TGDSC measurements. Figure 2a shows the TG-DSC curve of the pompon-ike sodium gadolinium tungstate phosphor obtained by hydrothermal method at 170 °C in the presence of CTAB for 72 h. From the TG curve, the weight loss stage (∼2%) of the hydrothermal products with a weak endothermal peak at

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Figure 3. (a) Low- and (b) high-magnification TEM images of the Eu3+-doped gadolinium tungstate micropompons obtained in the presence of CTAB with hydrothermal process in 170 °C for 72 h. (c) One nanoflake of the micropompons. (d) EDS spectrum of the nanoflake in part c.

∼630 °C on the DSC curve corresponds to the evaporation of coordinated water in crystal lattice. Figure 2b is the XRD pattern of the hydrothermal product sodium gadolinium tungstate calcined at ∼630 °C for 2 h; it can be indexed to the NaGd(WO4)2 phase.35 The NGW crystal is in the tetragonal system, space group I41/a.34,36 After calcination at 600 °C, we can obtain a pure tetragonal phase of NGW. That is, during the heat treatment, some gas got off and caused the weigh loss at ∼630 °C, so the hydrothermal products may contain coordinated water or rudimental surfactants. And the product with heat treatment is NGW also verifies that the hydrothermal product contains Na element. Further evidence will be given in the following FT-IR analysis. 3.2. Morphogenesis of Gadolinium Tungstate in the Presence of CTAB, PVP, and P123 as Surfactants. The morphology of the hydrothermal products was further examined with TEM. Parts a and b of Figure 3 show the typical TEM imagines of the sodium gadolinium tungstate phosphors obtained by hydrothermal process in the presence of CTAB at 170 °C for 72 h; there are several pomponlike microspheres in Figure 3a. These microspheres are seen to be nearly monodisperse with average diameter of ∼1 µm. Figure 3b is the high-magnification view of the pompon-like microspheres, we can observe the microsphere was self-assembled by numerous nanoflakes. The width of the nanoflake is ∼ 100 nm (Figure 3c). The chemical composition of the nanoflake was determined by using EDS.

The EDS analysis (Figure 3d) shows that the nanoflake crystal contains Gd, W, and O elements with a molar ratio of Gd:W:O of 1:1:3.5. Nowadays surfactant-assisted synthesis attracted great attention in morphology controlled inorganic synthesis field.37-40 In most of these reactions, surfactants act as templates in structure directing process. To data, the morphology of micro- and nanocrystals has mostly been controlled in the presence of stabilizing reagents,41,42 such as polymers or surfactants, or strong chelating ligands. Li et al. have reported the influence of PVP in morphology control of Bi2WO6; they found the selective adsorption of PVP on various crystallographic planes of Bi2WO6 nanoplates was of great importance at the initial stage.43 CTAB, PVP, and P123 as the self-organization directing surfactant were found to play a crucial role in achieving the micro- and nanocrystals with controlled morphology. In our work, we investigated the behavior of surfactants CTAB, PVP, and P123 in hydrothermal reaction. The morphologies of the pompon-shaped sodium gadolium tungstate particles obtained by the hydrothermal process in the presence of CTAB, PVP, and P123 with a certain concentration at 170 °C for 72 h are shown in parts a-c of Figure 4, respectively. The three surfactants all direct the products with microspherical morphology self-assembled by a great many nanoflakes. The average diameter of the products obtained in the presence of CTAB, PVP, and P123 is ∼2 µm, ∼4 µm and 2 µm, respectively. The

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Figure 4. SEM images of gadolinium tungstate microspherical particles obtained by hydrothermal method in the presence of (a) CTAB, (b) PVP, and (c) P123.

Figure 5. SEM images of gadolinium tungstate microrods obtained by hydrothermal process without the influence of surfactant. (a) Low-magnification view of high-yield products. (b) High-magnification view of individual microrods.

larger diameter of the products obtained in the presence of PVP may be due to the larger steric hindrance of PVP. The diameter of the products observed in SEM image is a little larger than the particles in TEM images because the samples need prolonged ultrasonic treatment in solution before having TEM measurement. To further investigate the influence of surfactants, we also studied the same experiments without the influence of surfactant as a comparison. Parts a and b of Figure 5are the SEM imagines of the products obtained in surfactant-free hydrothermal process. The overview of the samples is shown in Figure 5a. The flowerlike structures are distributed randomly and are made up of several micrometer-sized square columns. Close observation (Figure 5b) reveals that the one flowerlike structure is composed of numerous square columns that extend outward from the center of the microstructure. The lengths of the flowerlike microstructure are ∼15 µm. The branching microrods have a uniform length of about 10 µm, and the diameter of one cylinder section is ∼2 µm. In comparison with Figure 4, the size of the microstructure is much larger than the microspheres obtained

in presence of surfactant. The morphology changed dramatically with the surfactant-assisted hydrothermal process. This results imply that surfactant play an important role as a template in the formation of the microspheres. In addition to the variation of different surfactants, the concentration of surfactant also plays a key role in the formation of the self-assembly process. A series of experiments were performed in order to further investigate the formation process in the presence of CTAB, PVP, and P123 with different concentrations. The SEM images elucidate the influence of surfactant concentration on the size and morphology variation. From parts a-e of Figure 6, we can observe that with the increasing of the corresponding surfactant concentration the nanoflakes self-assembled into more compacted spheres, the micropompons seem more uniform, and they tend to assemble in an orderly way layer by layer of nanoflakes. The selfassembled spherical or flowerlike morphologies varied from loosening state to tightening state. That is, when the surfactant concentration reached a given degree, the morphology of the

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Figure 6. SEM images of gadolinium tungstate micropompons obtained by hydrothermal method in the presence of various surfactant with different concentration of (a) 0.01 mol/L CTAB, (b) 0.025 mol/L CTAB, (c) 0.076 g PVP, (d) 0.173 g PVP, (e) 0.010 g P123, and (e) 0.600 g P123

SCHEME 1: Schematic Illustration of the Products Obtained by the Hydrothermal Process (a) without the Attending of CTAB, (b) 0.2 mol CTAB, and (c) 0.5 mol CTAB

products exhibit more ordered pompons. Moreover, different surfactants show diversity behavior in the formation of the products. Parts a, c, and e of Figure 6 show the morphology of products obtained in the presence of CTAB, PVP, and P123 with relatively low concentration, respectively. Figure 6a exhibits a loosening sphere composed of numerous nanoflakes and nanoflakes. Figure 6b shows two different morphologies; one is a flowerlike cluster morphology self-assembled by many rods and the other is microspheres assembled by numerous nanoflakes. Figure 6c shows the echinus-like structure radiated from the center of this microstructure in low concentration of P123; divergent spherical-like gadolinium tungstate can be

observed. To the best of our knowledge, such gadolinium tungstate structures have not been reported hitherto, and it is a novel morphology for gadolinium and some other rare earth tungstates in the same reaction. We can easily conclude that the concentration of surfactant plays an important role in the formation of hydrothermal product NaGdWO4(OH)x:Eu3+ crystal with unique morphologies. On the basis of the above analysis, a possible mechanism of the surfactant concentration influence on morphology in the hydrothermal process is schematically shown in Scheme 1 (taking the surfactant CTAB as an example). The square column clusterlike products were obtained by the hydrothermal process

Eu3+-Doped Gadolinium Tungstate

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Figure 7. SEM images of time-dependent products synthesized by hydrothermal process at 170 °C for (a) 3 h, (b) 12 h, and (c) 72 h.

without the influence of surfactant (Scheme 1a). When a small amount of CTAB were added in the solution, the directed aggregation and self-assembly took place; the products presented radiated flowerlike microrod clusters (Scheme 1b). And the size of the final products decreased obviously. As the concentration of CTAB increased to a certain degree, the nanoflakes selfassembled into compacted pompons layer by layer (Scheme 1c). 3.3. Growth Mechanism. The crystal growth is controlled by the extrinsic and intrinsic factors, including the degree of supersaturation, diffusion of the reaction, surface energy, crystal structure, and solution parameters.44,45 Several crystal growth mechanisms in solution system are the well-known oriented attachment, Ostwald ripening process, and Kirkendall effect.46-49 In the formation of sodium gadolinium tungstate spheres, we believe the self-assembly process play a key role in this process. By controlling the solution reaction conditions, we synthesized morphology-controlled micropompons NaGdWO4(OH)x: Eu3+. The formation mechanism was discussed on the basis of time-dependent experiments. To understand the growth process of the sodium gadolinium tungstate phosphors, we investigated the products obtained by hydrothermal process at 170 °C for different reaction time in the presence of CTAB (Figure 7). Figure 7a is the SEM image of the products obtained by hydrothermal method at 170 °C for 3 h, which exhibits amorphous state layer by layer, the XRD pattern is well agreed with the result. With prolonging the reaction time for 12 h, small nucleus came into being (Figure 7b). The process from the amorphous state to the formation the nucleus can be regarded as nucleation process. The pomponlike final products (Figure 7c) obtained for 72 h are much larger than the products obtained for 12 h, and this process corresponding to the crystal growth process. The pomponlike spheres are self-assembled by great many nanoflakes. On the basis of the time-dependent SEM images, it can be concluded that the formation of such intricate micropompons is achieved via assembly process. That is, the original precursor exists as amorphous, with the prolonging of

Figure 8. FT-IR spectrum of (a) the products synthesized by hydrothermal method without the influence of CTAB, (b) the products synthesized by hydrothermal method in the presence of CATB, (c) the hydrothermal product with CTAB calcined at 630 °C for 2 h, and (d) surfactant CTAB

the reaction time for nearly 3 h, they begin to form nucleus by the habitude of themselves. 3.4. FT-IR Spectra of the Products. Figure 8 shows the FT-IR spectra of (a) the products synthesized by hydrothermal method without the influence of CTAB at 170 °C for 72 h, (b) the same products synthesized by hydrothermal method in the presence of CTAB at 170 °C for 72 h, (c) the product of part b with heat treatment at 630 °C for 2 h, and (d) surfactant CTAB. There are broad bands at 3445 and 1642 cm-1 in spectra a-d, corresponding to the surface-absorbed water and hydroxyl groups, respectively. The weak band at 3551 cm-1 attributed to the O-H vibration of the crystal structure50 (parts a and b of Figure 6). The result was also verified by the TG-DSC curve we just discussed. So in combination with the XRD, TG-DSC,

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Figure 9. UV-vis diffuse reflectance of the products synthesized by hydrothermal method with different ratio between Eu3+ and Gd3+ at 170 °C for 72 h. (a) 0, (b) 0.01 (c) 0.02, (d) 0.03, (e) 0.04.

and EDS analyses, we can have a primary conclusion that the composition of the hydrothermal product is NaGdWO4(OH)x: Eu3+. The IR absorption bands at 2362 cm-1 in the gadolinium tungstate can be attributed to the characteristic frequencies of residual materials (CO2). Compared with parts a and b of Figure 8, the only difference is the band at 1387 cm-1 in spectrum a, corresponding to the NO3- characterization vibration. There is no NO3- absorption band of the final products obtained in the influence of CTAB in the hydrothermal process because the CTAB chains absorbed on the surface of precipitation precursors and formed the directional arrangement, which prevent the approaching of NO3- in solution. Figure 8c is the spectrum of the product with heat treatment; the band at 3551 cm-1 disappeared, which indicates that when heated at 630 °C, the O-H group in the crystal structure disappeared. Figure 7d is the FT-IR spectrum of CTAB, the IR bands at 3015, 2918, and 2847 cm-1 (ν-CH, ν-CH3, and ν-CH2) as well as 1474 cm-1 (δCH) the bending of C-H vibration, the band at 724 cm-1 is the characteristic vibration of -CH2 long-chain. In comparison with parts b, c, and d of Figure 8, we can’t find the characteristic absorption bands of CTAB on the IR spectra of hydrothermal products for the rudimental surfactant must be removed by washing process. In this hydrothermal process, CTAB just be used as a structure directing reagent. Parts a-c of Figure 8 contain the bands at the 950-650-cm-1 region corresponding to the symmetric and asymmetric stretching vibrations of the terminal (short) W-O bonds.51,52 The bands at the 830-530cm-1 interval presents antisymmetric stretching and at the 470-400-cm-1 region that displays bands due to the bending modes of the O-W bonds.53 In general, the optical absorption energy of the host lattice might be obtained by measuring the diffused reflection spectrum. Figure 9 shows the UV-visible diffuse reflectance of the products synthesized by hydrothermal method with different ratio of Eu3+ and Gd3+ at 170 °C for 72 h at (a) 0, (b) 0.01 (c) 0.02, (d) 0.03, (e) 0.04. The optical absorption spectra of parts a-e were similar. There is a broad absorption band range from 220 to 350 nm, peaking at about 247.6 nm in parts a-e of Figure 9, ascribed to the charge transfer transition of OfW and OfEu3+, which does not have considerable changes when doped with the different concentration of Eu3+. The similar CT transition was also observed in the perovskite phosphor La0.90Eu0.05Nb2O754 and Eu3+-doped sheelite-type phosphor CaMoO4.55 In these absorption spectra, the downward bands

Lei and Yan

Figure 10. Excitation and emission spectra of the hydrothermal product synthesized in the presence of CTAB at 170 °C for 72 h.

Figure 11. (I) Emission spectra of gadolinium tungstate in the hydrothermal process at 170 °C for 72 h with different ratios of Eu3+ and Gd3+ (a) Eu3+/Gd3+ ) 0, (b) Eu3+/Gd3+ ) 0.01, (c) Eu3+/Gd3+ ) 0.02, (d) Eu3+/Gd3+ ) 0.03, and (e) Eu3+/Gd3+ ) 0.04. (II) The photo of these five phosphors with different ratios of Eu3+ and Gd3+ from 0-0.04 (from left to right).

are the characteristic emission of Eu3+. With the increasing of doping concentration of Eu3+, the intensity of f-f transition of Eu3+ at 613 nm increased. 3.5. PL Properties. Room temperature PL spectra of the synthesized gadolinium tungstate were investigated. Figure 10 shows the excitation and emission spectra of NaGdWO4(OH)x: Eu3+ synthesized by hydrothermal method in the presence of CTAB at 170 °C for 72 h. The excitation spectrum under the 613 nm monitoring wavelength shows a broadband along with sharp lines of Eu3+ at ∼359 nm, ∼379 nm, ∼392 nm, and ∼410 nm, which correspond to the transitions of 7F0f5D4,7F0f5L7, 7 F0f5L6 and 7F0f5D2, respectively. The high-intensity broad-

Eu3+-Doped Gadolinium Tungstate

Figure 12. Temperature-dependent luminescence spectra of the gadolinium tungstate synthesized by hydrothermal process at different temperature for 72 h: (a) 120 °C, (b) 170 °C, and (c) 220 °C.

band at about 270 nm in the short wavelength ranging from 200 to 300 nm is attributed to the charge transfer transitions of OfW ligand to metal charge transfer (LMCT).25 The weak excitation band at around 323 nm is related to the 8Sf6P transition of Gd3+, which is possibly to be detected due to the Gd3+f Eu3+ energy transfer.56 The f-f transitions of Gd3+ at 275 and 313 nm are corresponding to 8Sf6I and 8Sf6P, respectively. The 8Sf6I transition at about 275 nm maybe overlapped with the broad LMCT band peaking at 270 nm. The tungstate group partially transferred its energy to Eu3+ and Gd3+ ions.29 The emission spectrum of gadolinium tungstate recorded in the range of 350-700 nm under 270 nm excitation. It includes a broadband and two narrow 5D0f7FJ (J ) 1, 2) emission bands appearing at 468, 590, and 611 nm, respectively. The broadband ranged from 380 to 580 nm, peaking at 468 nm in the blue region, which assigned to the OfW LMCT states. Eu3+ is a good probe for the chemical environment of the rare-earth ions because of the 5D0f7F2 transition (allowed by electric dipole) is very sensitive to the surroundings, while the 5D0f7F1 transition (allowed by magnetic dipole) is insensitive to the environment. In a site with inversion symmetry, the 5D0f7F1 transition is dominating, while in a site without inversion symmetry, the 5D0f7F2 transition is dominating. The fact that the dominant emission is from the parity forbidden electric dipole transition rather than from the magnetic dipole transition indicates that Eu3+ is located at the site with no inversion symmetry.54,57,58 The doping concentration of Eu3+ plays a key role in adjusting the color of NaGdWO4(OH)x:Eu3+ phosphor. The sodium gadolinium tungstate host without doping with Eu3+ can emit blue light itself; with the increasing of doping concentration of Eu3+, the photoluminescence can be tuned from blue to white to red (Figure 11). Figure 11 shows the emission spectra of gadolinium tungstate in the hydrothermal process at 170 °C for 72 h with different ratio of Eu3+ and Gd3+ (a) Eu3+/Gd3+ ) 0, (b) Eu3+/Gd3+ ) 0.01, (c) Eu3+/Gd3+ ) 0.02, (d) Eu3+/Gd3+ ) 0.03, and (e) Eu3+/Gd3+ ) 0.04. The emission band of parts b-e of Figure 11 consists of two parts: one is the broad intense band peaking at 468 nm; the other is the characteristic transition of Eu3+ at 588, 592, and 613 nm, respectively. Without doping with Eu3+, there is the only one broad emission band at 468 nm, ascribed to charge transfer of OfW (Figure 11a). With

J. Phys. Chem. C, Vol. 113, No. 3, 2009 1081 the increasing of doping concentration of Eu3+, the intensity of the broadband at 468 nm decreased, the characteristic emission band at 613 nm increased obviously, the ratio of red and orange increased, too. The bands at 588 and 592 nm do not change with the doping concentration. The light color under 270-nm excitation is composed of the blue-green, orange, and red bands; when the ratio of Eu3+ and Gd3+ was kept at 0.02, we can obtain bright white light. The hydrothermal temperature is as well the key factor in the formation of the final products, it also has influence on the luminescent intensity. Figure 12 shows the emission spectra of gadolinium tungstate synthesized by hydrothermal process at different temperature (a) 120 °C, (b) 170 °C, and (c) 220 °C for 72 h. There is no obvious blue or red shift in these three spectra, with the increasing of hydrothermal temperature, the luminescent intensity increased, too, especially the ratio of red and orange. Relative high hydrothermal temperature is good to the crystal growth. 4. Conclusion In summary, the self-organization of NaGdWO4(OH)x:Eu3+ microspheres composed of multilayered nanoflakes were successfully synthesized by means of a hydrothermal method using CTAB, PVP, and P123 as the surfactants. The formation and growth mechanism of NaGdWO4(OH)x:Eu3+ particles were elucidated by self-assembly process. Surfactant-assisted hydrothermal approach has been developed for the controllable synthesis of unique micropompon sodium gadolinium tungstate. WhitelightphosphorswithnominalcompositionofNaGdWO4(OH)x: Eu3+ suitable for UV excitation were prepared. The phosphor have tunable color from blue-green to red by adjusting the Eu3+ concentration. When the ratio of Eu3+ and Gd3+ at about 0.02, the phosphor exhibits bright white light, which is due to the combination of blue-green, orange, and red light from charge transfer transition of the OfW and the transition between 5 D0f7F1 and 5D0f7F1 of Eu3+. We also found the surfactants of CTAB, PVP, and P123 could affect the morphology. Meanwhile, the concentration of the surfactant also plays an important role on the shape and size of the hydrothermal products. The present results, demonstrate that the crystallization procedure by a surfactant-assisted hydrothermal reaction provide a facile way for shape control on a number of inorganic functional materials. Their intriguing self-assemble capability enables them to serve as novel nanobuiding blocks for new nanodevice applications. Furthermore, this method also presents a way for the controlled synthesis of multicomponent metal oxides. Acknowledgment. The authors gratefully acknowledge the financial support from the National Science Foundation of China (No. 20671072) References and Notes (1) Antonietti, M. O.; G., A. Chem.-Eur. J. 2004, 10, 28. (2) Bu, W. B.; Xu, Y. P.; Zhang, N.; Chen, H. R.; Hua, Z. L.; Shi, J. L. Langmuir 2007, 23, 9002. (3) Wu, H.; Thalladi, V. R.; Whitesides, S.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 14495. (4) Zhang, N.; Bu, W. B.; Xu, Y. P.; Jiang, D. Y.; Shi, J. L. J. Phys. Chem. C 2007, 111, 5014. (5) Li, H.; Bian, Z.; Zhu, J.; Zhang, D.; Li, G.; Huo, Y.; Li, H.; Lu, Y. J. Am. Chem. Soc. 2007, 129, 8406. (6) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (7) Peng, Y.; Xu, A. W.; Deng, B.; Antonietti, M.; Colfen, H. J. Phys. Chem. B 2006, 110, 2988. (8) Xu, A. W.; Antonietti, M.; Colfen, H.; Fang, Y. P. AdV. Funct. Mater. 2006, 16, 903.

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