Morphology-Controlled Synthesis, Physical Characterization, and Photoluminescence ofNovel Self-Assembled Pomponlike White Light Phosphor: Eu3+-Doped Sodium GadoliniumTungstate

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+ mi-croarchitectures that exhibit efficient white-light photoluminescence properties have been successfullysynthesized via a facile hydrothermal process in the presence of the surfactants cetyl trimethyl ammoniumbromide, poly(vinyl pyrrolidone), and poly(ethylene glycol)-block-poly(propylene glycol)-block-ploy(ethyleneglycol). The white light sodium gadolinium tungstate phosphor contains three emission bands: the blue-greenband at 468 nm is ascribed to the ligand to metal charge transfer transition from OfW; the orange band at590 nm and red band at 610 nm are attributed to the 5D0f7F1 and 5D0f7F2 transitions of Eu3+. Theluminescence 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 auniform particle size distribution. The three-dimensional sodium gadolium tungstate pompon-shaped structureswere 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. Theemission spectra are dominated by a 5D0f7F2 transition of Eu3+. The optimum concentration for the whitelight was to keep the ratio of Eu3+ and Gd3+ at about 0.02.

Introduction

Large-scale self-assembled structures with highly specificmorphologies and novel properties are of great interest in thearea of materials synthesis and device fabrication.1-4 Manyordinary compositions exhibit attractive properties owing to theirunique microstructures.5 Therefore, artificial building of micro-spheres with three-dimensional (3D) architectures is an attractivepursuit with many challenges.

The alignment of nanostructure building blocks (nanoparticles,nanoflakes, and nanoribbons/nanoplates) into 3D ordered su-perstructures by bottom-up approaches has been an exciting fieldin recent years.6 Many methods have been employed insynthesizing inorganic materials, such as microemulsion, hy-drothermal (solventhermal), sol-gel, molten salt, microwave,sonochemical route, and so on. Among the many synthetic routesof inorganic materials, hydrothermal synthesis route is one ofthe most prevalent methods for controlled synthesis with variousmorphologies. Self-assembly is an efficient and often preferredprocess to build micro- and nanoparticles into ordered 3Dmacroscopic structure. Up to now, different driving mechanismsof the self-assembly processes have been proposed: such as,surface tension, capillary effect, oriented attachment, electricand magnetic forces, hydrophobic interaction, van der Waalsinteraction, aromatic interaction, hydrogen-bonding, electrostaticinteraction, etc.7-17 Versatile surfactant-assisted hydrothermalmethods can lead to various morphologies of inorganic crystals.Previous works have demonstrated that surfactants have greatinfluenceoncontrollingthemorphologyof inorganicmaterials.18-20

Tungstate compounds are a large class of inorganic functionalmaterials that exhibit interesting physical properties and have

technological applications in the field of optical material,photocatalysis, quantum electronics, and so on.21-24 Sometungstate hosts as one kind of self-activating materials can emitblue light itself under ultraviolet or X-ray excitation.25,26 Rare-earth metal ions (RE3+) doped tungstates are widely known asmultifunctional material having unique physical and chemicalproperties. These compounds are extensively used as laser,scintillator and luminescent materials.27,28 Brito et al. gave theluminescence investigation of Eu3+ in the RE2(WO4)3 (RE )La, Gd) matrix using the Pechini method.29 Up to now, tungstatesystems have been developed and investigated by Yu 19,30,31 andHu32 in morphology-controlled synthesis. All these investigationsgreatly enriched the theory of crystallization for self-assemblymicroarchitectures. However, despite these improvements incrystal growth of nanocrystals with predictable size, shape, andcrystal structures are still hard challenges, due to the complexityof 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-assistedhydrothermal approach under mild conditions for the preparationof nearly monodisperse 3D white-light sodium gadoliniumtungstate (NaGdWO4(OH)x:Eu3+) micropompons self-assembledby multilayer nanoflakes with controllable morphology and size,and they have tunable color diversity properties which is dueto the different doping concentration of Eu3+. When thephosphor was doped with a certain amount of Eu3+, it can beused as potential white-light phosphor for the combination ofblue-green light related to the charge transfer (CT) from OfWof tungstate and red light of characteristic transition of Eu3+.The influence of reaction temperature and the concentration ofsurfactant on the formation of sodium gadolinium tungstate havebeen systematically examined in such a reaction system.

* Towhomcorrespondenceshouldbeaddressed.E-mail:byan@tongji.edu.cn.Fax: +86-21-65982287. Phone: +86-21-65984663.

J. Phys. Chem. C 2009, 113, 1074–10821074

10.1021/jp8082634 CCC: $40.75 2009 American Chemical SocietyPublished on Web 12/30/2008

2. Experimental Section

2.1. Chemicals. Europium oxide (Eu2O3 (99.99%)), gado-linium oxide (Gd2O3 (99.99%)), nitric acid (HNO3), sodiumtungstate (Na2WO4 ·H2O (AR)), and surfactants (cetyltrimethylammonium bromide (CTAB), poly(vinyl pyrrolidone) (PVP,K30), poly(ethylene glycol)-block-poly(propyleneglycol)-block-ploy(ethylene glycol) (pluronic P123 (EO20PO70EO20, Aldrich))were used as the raw materials without further purification.

2.2. Synthesis Procedures. The given amounts of rare-earthnitrates (Gd3+ and Eu3+) were prepared by dissolving corre-sponding 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 wasremoved by evaporation. Solution A was prepared by dissolvingthe obtained rare earth nitrate (containing Gd3+ and Eu3+) in 5mL of deionized water; solution B was prepared by dissolvingNa2WO4 ·2H2O (the molar ratio of RE/W is 2: 1) and ∼0. 0005mol CTAB in a 5 mL of heated deionized water (∼75 °C) withinitial pH 8.0 and stirred for ∼10 min at room temperature.Then solution A was added into solution B, and the mixturewas vigorously stirred for about 30 min to ensure that allreagents were dispersed homogeneously. The mixture wastransferred into a 25-mL Teflon-lined stainless steel autoclaveand filled with deionized water up to a 70% filling capacity ofthe total volume. The autoclave was sealed and maintained at170 °C for 48-72 h and then cooled to room temperaturenaturally. After the above hydrothermal treatment, the productwas centrifuged and washed with deionized water for severaltimes. And then the precipitate was dried at 70 °C for 24 h andcollected for characterization. In addition, we get part of thefinal products for calcinations at 630 °C for about 2 h tocharacterize their structure. For comparison, another experimentwas repeated with the existence of surfactants CTAB, PVP, andP123 on the same hydrothermal reaction.

2.3. Characterization. The samples were examined by X-raydiffraction (XRD), scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), energy-dispersive X-rayspectroscopy (EDS), Fourier tranform (FT-IR), UV-vis spec-troscopy, and photoluminescence (PL). XRD analyses werecarried out on a Bruker D8-Advance diffractometer withgraphite-monochromatized Cu KR radiation (40 KV/60 mA,graphite monochromator). Thermogravimetric analysis wasperformed on a STA-409PC/4/H LUXX TG-DSC instrumentat a heating rate of 10 K/min to a maximum temperature of1273 K. The morphology was characterized with a Philip XL-30 environmental scanning electron microscope (ESEM). TEMimages and EDS were recorded on a JEOL200CX microscopewith an accelerating voltage of 200 kV. PL spectra wereobtained using a fluorescence spectrophotometer (RF5301) witha xenon lamp as the excitation source. FT-IR data were collectedon Perkin-Elmer 2000 FT-IR spectrophotometer in the rangeof 4000-400 cm-1 using KBr pellets. UV-vis diffuse reflec-tance spectra were recorded on a Shimadzu UV-3101 spectro-photometer equipped with an integrating sphere, using BaSO4

as a reference.

3. Results and Discussion

3.1. Crystal Structure of the Pomponlike Sodium Gado-linium Tungstate NaGdWO4(OH)x:Eu3+. The phase and purityof the as-synthesized samples were determined by XRD patterns.It can be seen from parts a-g of Figure 1 the XRD patterns ofthe products obtained by hydrothermal method at 170 °C fordifferent 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

XRD pattern of the precipitation precursor; there is no obviousdiffraction peaks, so the product exists as amorphous state. Theproducts obtained for 3 h also exhibit amorphous XRD pattern(Figure 1b). When the reaction time increased at 12 h, we canobserve obvious diffraction peaks, which corresponding thecrystallization process. With increasing hydrothermal reactiontime, most of these diffraction peaks intensity increase signifi-cantly. Usually, the longer reaction time is good for the bettercrystallization. When the reaction time keeps at 72 h, there isnot much difference from the products obtained at 120 h. Itsstructure can not be clearly figured out now because it can notbe matched well with the database of XRD. Several main peaksmatch with the compound NaGd(WO4)2 (NGW). The crystalof the hydrothermal product may contain hydroxy group. It willbe verified in the following thermogravimetric differentialscanning calorimetry (TG-DSC) and FT-IR discussion. NGWcrystal belongs to the scheelite family of crystals.33 The latticeconstants 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 transformationtemperature of the hydrothermal products, we employed TG-DSC measurements. Figure 2a shows the TG-DSC curve of thepompon-ike sodium gadolinium tungstate phosphor obtained byhydrothermal method at 170 °C in the presence of CTAB for72 h. From the TG curve, the weight loss stage (∼2%) of thehydrothermal products with a weak endothermal peak at

Figure 1. XRD patterns of the product synthesized by hydrothermalmethod 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 methodat 170 °C for 72 h in the presence of CTAB. (b) XRD pattern of thesame hydrothermal product with heat treatment at 630 °C for 2 h.

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∼630 °C on the DSC curve corresponds to the evaporation ofcoordinated water in crystal lattice. Figure 2b is the XRD patternof the hydrothermal product sodium gadolinium tungstatecalcined at ∼630 °C for 2 h; it can be indexed to theNaGd(WO4)2 phase.35 The NGW crystal is in the tetragonalsystem, space group I41/a.34,36 After calcination at 600 °C, wecan obtain a pure tetragonal phase of NGW. That is, during theheat treatment, some gas got off and caused the weigh loss at∼630 °C, so the hydrothermal products may contain coordinatedwater or rudimental surfactants. And the product with heattreatment is NGW also verifies that the hydrothermal productcontains Na element. Further evidence will be given in thefollowing FT-IR analysis.

3.2. Morphogenesis of Gadolinium Tungstate in the Pres-ence of CTAB, PVP, and P123 as Surfactants. The morphol-ogy of the hydrothermal products was further examined withTEM. Parts a and b of Figure 3 show the typical TEM imaginesof the sodium gadolinium tungstate phosphors obtained byhydrothermal process in the presence of CTAB at 170 °C for72 h; there are several pomponlike microspheres in Figure 3a.These microspheres are seen to be nearly monodisperse withaverage diameter of ∼1 µm. Figure 3b is the high-magnificationview of the pompon-like microspheres, we can observe themicrosphere was self-assembled by numerous nanoflakes. Thewidth of the nanoflake is ∼ 100 nm (Figure 3c). The chemicalcomposition of the nanoflake was determined by using EDS.

The EDS analysis (Figure 3d) shows that the nanoflake crystalcontains Gd, W, and O elements with a molar ratio of Gd:W:Oof 1:1:3.5.

Nowadays surfactant-assisted synthesis attracted great atten-tion in morphology controlled inorganic synthesis field.37-40 Inmost of these reactions, surfactants act as templates in structuredirecting process. To data, the morphology of micro- andnanocrystals has mostly been controlled in the presence ofstabilizing reagents,41,42 such as polymers or surfactants, orstrong chelating ligands. Li et al. have reported the influenceof PVP in morphology control of Bi2WO6; they found theselective adsorption of PVP on various crystallographic planesof Bi2WO6 nanoplates was of great importance at the initialstage.43 CTAB, PVP, and P123 as the self-organization directingsurfactant were found to play a crucial role in achieving themicro- and nanocrystals with controlled morphology. In ourwork, we investigated the behavior of surfactants CTAB, PVP,and P123 in hydrothermal reaction. The morphologies of thepompon-shaped sodium gadolium tungstate particles obtainedby the hydrothermal process in the presence of CTAB, PVP,and P123 with a certain concentration at 170 °C for 72 h areshown in parts a-c of Figure 4, respectively. The threesurfactants all direct the products with microspherical morphol-ogy self-assembled by a great many nanoflakes. The averagediameter of the products obtained in the presence of CTAB,PVP, and P123 is ∼2 µm, ∼4 µm and 2 µm, respectively. The

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

1076 J. Phys. Chem. C, Vol. 113, No. 3, 2009 Lei and Yan

larger diameter of the products obtained in the presence of PVPmay be due to the larger steric hindrance of PVP. The diameterof the products observed in SEM image is a little larger thanthe particles in TEM images because the samples need prolongedultrasonic treatment in solution before having TEM measurement.

To further investigate the influence of surfactants, we alsostudied the same experiments without the influence of surfactantas a comparison. Parts a and b of Figure 5are the SEM imaginesof the products obtained in surfactant-free hydrothermal process.The overview of the samples is shown in Figure 5a. Theflowerlike structures are distributed randomly and are made upof several micrometer-sized square columns. Close observation(Figure 5b) reveals that the one flowerlike structure is composedof numerous square columns that extend outward from the centerof the microstructure. The lengths of the flowerlike microstruc-ture are ∼15 µm. The branching microrods have a uniformlength of about 10 µm, and the diameter of one cylinder sectionis ∼2 µm. In comparison with Figure 4, the size of themicrostructure is much larger than the microspheres obtained

in presence of surfactant. The morphology changed dramaticallywith the surfactant-assisted hydrothermal process. This resultsimply that surfactant play an important role as a template inthe formation of the microspheres.

In addition to the variation of different surfactants, theconcentration of surfactant also plays a key role in the formationof the self-assembly process. A series of experiments wereperformed in order to further investigate the formation processin the presence of CTAB, PVP, and P123 with differentconcentrations. The SEM images elucidate the influence ofsurfactant concentration on the size and morphology variation.From parts a-e of Figure 6, we can observe that with theincreasing of the corresponding surfactant concentration thenanoflakes self-assembled into more compacted spheres, themicropompons seem more uniform, and they tend to assemblein an orderly way layer by layer of nanoflakes. The self-assembled spherical or flowerlike morphologies varied fromloosening state to tightening state. That is, when the surfactantconcentration reached a given degree, the morphology of the

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-magnificationview of high-yield products. (b) High-magnification view of individual microrods.

Eu3+-Doped Gadolinium Tungstate J. Phys. Chem. C, Vol. 113, No. 3, 2009 1077

products exhibit more ordered pompons. Moreover, differentsurfactants show diversity behavior in the formation of theproducts. Parts a, c, and e of Figure 6 show the morphology ofproducts obtained in the presence of CTAB, PVP, and P123with relatively low concentration, respectively. Figure 6aexhibits a loosening sphere composed of numerous nanoflakesand nanoflakes. Figure 6b shows two different morphologies;one is a flowerlike cluster morphology self-assembled by manyrods and the other is microspheres assembled by numerousnanoflakes. Figure 6c shows the echinus-like structure radiatedfrom the center of this microstructure in low concentration ofP123; divergent spherical-like gadolinium tungstate can be

observed. To the best of our knowledge, such gadoliniumtungstate structures have not been reported hitherto, and it is anovel morphology for gadolinium and some other rare earthtungstates in the same reaction. We can easily conclude thatthe concentration of surfactant plays an important role in theformation of hydrothermal product NaGdWO4(OH)x:Eu3+ crystalwith unique morphologies.

On the basis of the above analysis, a possible mechanism ofthe surfactant concentration influence on morphology in thehydrothermal process is schematically shown in Scheme 1(taking the surfactant CTAB as an example). The square columnclusterlike products were obtained by the hydrothermal process

Figure 6. SEM images of gadolinium tungstate micropompons obtained by hydrothermal method in the presence of various surfactant with differentconcentration 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 ofCTAB, (b) 0.2 mol CTAB, and (c) 0.5 mol CTAB

1078 J. Phys. Chem. C, Vol. 113, No. 3, 2009 Lei and Yan

without the influence of surfactant (Scheme 1a). When a smallamount of CTAB were added in the solution, the directedaggregation and self-assembly took place; the products presentedradiated flowerlike microrod clusters (Scheme 1b). And the sizeof the final products decreased obviously. As the concentrationof CTAB increased to a certain degree, the nanoflakes self-assembled into compacted pompons layer by layer (Scheme 1c).

3.3. Growth Mechanism. The crystal growth is controlledby the extrinsic and intrinsic factors, including the degree ofsupersaturation, diffusion of the reaction, surface energy, crystalstructure, and solution parameters.44,45 Several crystal growthmechanisms in solution system are the well-known orientedattachment, Ostwald ripening process, and Kirkendall effect.46-49

In the formation of sodium gadolinium tungstate spheres, webelieve the self-assembly process play a key role in this process.

By controlling the solution reaction conditions, we synthe-sized morphology-controlled micropompons NaGdWO4(OH)x:Eu3+. The formation mechanism was discussed on the basis oftime-dependent experiments. To understand the growth processof the sodium gadolinium tungstate phosphors, we investigatedthe products obtained by hydrothermal process at 170 °C fordifferent reaction time in the presence of CTAB (Figure 7).Figure 7a is the SEM image of the products obtained byhydrothermal method at 170 °C for 3 h, which exhibitsamorphous state layer by layer, the XRD pattern is well agreedwith the result. With prolonging the reaction time for 12 h, smallnucleus came into being (Figure 7b). The process from theamorphous state to the formation the nucleus can be regardedas nucleation process. The pomponlike final products (Figure7c) obtained for 72 h are much larger than the products obtainedfor 12 h, and this process corresponding to the crystal growthprocess. The pomponlike spheres are self-assembled by greatmany nanoflakes. On the basis of the time-dependent SEMimages, it can be concluded that the formation of such intricatemicropompons is achieved via assembly process. That is, theoriginal precursor exists as amorphous, with the prolonging of

the reaction time for nearly 3 h, they begin to form nucleus bythe habitude of themselves.

3.4. FT-IR Spectra of the Products. Figure 8 shows theFT-IR spectra of (a) the products synthesized by hydrothermalmethod without the influence of CTAB at 170 °C for 72 h, (b)the same products synthesized by hydrothermal method in thepresence of CTAB at 170 °C for 72 h, (c) the product of partb with heat treatment at 630 °C for 2 h, and (d) surfactantCTAB. There are broad bands at 3445 and 1642 cm-1 in spectraa-d, corresponding to the surface-absorbed water and hydroxylgroups, respectively. The weak band at 3551 cm-1 attributedto the O-H vibration of the crystal structure50 (parts a and b ofFigure 6). The result was also verified by the TG-DSC curvewe just discussed. So in combination with the XRD, TG-DSC,

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.

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

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and EDS analyses, we can have a primary conclusion that thecomposition of the hydrothermal product is NaGdWO4(OH)x:Eu3+. The IR absorption bands at 2362 cm-1 in the gadoliniumtungstate can be attributed to the characteristic frequencies ofresidual materials (CO2). Compared with parts a and b of Figure8, the only difference is the band at 1387 cm-1 in spectrum a,corresponding to the NO3

- characterization vibration. There isno NO3

- absorption band of the final products obtained in theinfluence of CTAB in the hydrothermal process because theCTAB chains absorbed on the surface of precipitation precursorsand formed the directional arrangement, which prevent theapproaching of NO3

- in solution. Figure 8c is the spectrum ofthe product with heat treatment; the band at 3551 cm-1

disappeared, which indicates that when heated at 630 °C, theO-H group in the crystal structure disappeared. Figure 7d isthe FT-IR spectrum of CTAB, the IR bands at 3015, 2918, and2847 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 isthe characteristic vibration of -CH2 long-chain. In comparisonwith parts b, c, and d of Figure 8, we can’t find the characteristicabsorption bands of CTAB on the IR spectra of hydrothermalproducts for the rudimental surfactant must be removed bywashing process. In this hydrothermal process, CTAB just beused as a structure directing reagent. Parts a-c of Figure 8contain the bands at the 950-650-cm-1 region correspondingto the symmetric and asymmetric stretching vibrations of theterminal (short) W-O bonds.51,52 The bands at the 830-530-cm-1 interval presents antisymmetric stretching and at the470-400-cm-1 region that displays bands due to the bendingmodes of the O-W bonds.53

In general, the optical absorption energy of the host latticemight be obtained by measuring the diffused reflection spectrum.Figure 9 shows the UV-visible diffuse reflectance of theproducts synthesized by hydrothermal method with differentratio 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 partsa-e were similar. There is a broad absorption band range from220 to 350 nm, peaking at about 247.6 nm in parts a-e of Figure9, ascribed to the charge transfer transition of OfW andOfEu3+, which does not have considerable changes whendoped with the different concentration of Eu3+. The similar CTtransition was also observed in the perovskite phosphorLa0.90Eu0.05Nb2O7

54 and Eu3+-doped sheelite-type phosphorCaMoO4.55 In these absorption spectra, the downward bands

are the characteristic emission of Eu3+. With the increasing ofdoping concentration of Eu3+, the intensity of f-f transition ofEu3+ at 613 nm increased.

3.5. PL Properties. Room temperature PL spectra of thesynthesized gadolinium tungstate were investigated. Figure 10shows the excitation and emission spectra of NaGdWO4(OH)x:Eu3+ synthesized by hydrothermal method in the presence ofCTAB at 170 °C for 72 h. The excitation spectrum under the613 nm monitoring wavelength shows a broadband along withsharp lines of Eu3+ at ∼359 nm, ∼379 nm, ∼392 nm, and ∼410nm, which correspond to the transitions of 7F0f5D4,7F0f5L7,7F0f5L6 and 7F0f5D2, respectively. The high-intensity broad-

Figure 9. UV-vis diffuse reflectance of the products synthesized byhydrothermal method with different ratio between Eu3+ and Gd3+ at170 °C for 72 h. (a) 0, (b) 0.01 (c) 0.02, (d) 0.03, (e) 0.04.

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

Figure 11. (I) Emission spectra of gadolinium tungstate in thehydrothermal 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 photoof these five phosphors with different ratios of Eu3+ and Gd3+ from0-0.04 (from left to right).

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band at about 270 nm in the short wavelength ranging from200 to 300 nm is attributed to the charge transfer transitions ofOfW ligand to metal charge transfer (LMCT).25 The weakexcitation band at around 323 nm is related to the 8Sf6Ptransition of Gd3+, which is possibly to be detected due to theGd3+f Eu3+ energy transfer.56 The f-f transitions of Gd3+ at275 and 313 nm are corresponding to 8Sf6I and 8Sf6P,respectively. The 8Sf6I transition at about 275 nm maybeoverlapped with the broad LMCT band peaking at 270 nm. Thetungstate group partially transferred its energy to Eu3+ and Gd3+

ions.29

The emission spectrum of gadolinium tungstate recorded inthe range of 350-700 nm under 270 nm excitation. It includesa broadband and two narrow 5D0f7FJ (J ) 1, 2) emission bandsappearing at 468, 590, and 611 nm, respectively. The broadbandranged from 380 to 580 nm, peaking at 468 nm in the blueregion, which assigned to the OfW LMCT states. Eu3+ is agood probe for the chemical environment of the rare-earth ionsbecause 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 theenvironment. In a site with inversion symmetry, the 5D0f7F1

transition is dominating, while in a site without inversionsymmetry, the 5D0f7F2 transition is dominating. The fact thatthe dominant emission is from the parity forbidden electricdipole transition rather than from the magnetic dipole transitionindicates that Eu3+ is located at the site with no inversionsymmetry.54,57,58

The doping concentration of Eu3+ plays a key role in adjustingthe color of NaGdWO4(OH)x:Eu3+ phosphor. The sodiumgadolinium tungstate host without doping with Eu3+ can emitblue light itself; with the increasing of doping concentration ofEu3+, the photoluminescence can be tuned from blue to whiteto red (Figure 11). Figure 11 shows the emission spectra ofgadolinium tungstate in the hydrothermal process at 170 °C for72 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 partsb-e of Figure 11 consists of two parts: one is the broad intenseband peaking at 468 nm; the other is the characteristic transitionof Eu3+ at 588, 592, and 613 nm, respectively. Without dopingwith Eu3+, there is the only one broad emission band at 468nm, ascribed to charge transfer of OfW (Figure 11a). With

the increasing of doping concentration of Eu3+, the intensity ofthe broadband at 468 nm decreased, the characteristic emissionband at 613 nm increased obviously, the ratio of red and orangeincreased, too. The bands at 588 and 592 nm do not changewith the doping concentration. The light color under 270-nmexcitation is composed of the blue-green, orange, and red bands;when the ratio of Eu3+ and Gd3+ was kept at 0.02, we can obtainbright white light.

The hydrothermal temperature is as well the key factor inthe formation of the final products, it also has influence on theluminescent intensity. Figure 12 shows the emission spectra ofgadolinium tungstate synthesized by hydrothermal process atdifferent temperature (a) 120 °C, (b) 170 °C, and (c) 220 °Cfor 72 h. There is no obvious blue or red shift in these threespectra, with the increasing of hydrothermal temperature, theluminescent intensity increased, too, especially the ratio of redand orange. Relative high hydrothermal temperature is good tothe crystal growth.

4. Conclusion

In summary, the self-organization of NaGdWO4(OH)x:Eu3+

microspheres composed of multilayered nanoflakes were suc-cessfully synthesized by means of a hydrothermal method usingCTAB, PVP, and P123 as the surfactants. The formation andgrowth mechanism of NaGdWO4(OH)x:Eu3+ particles wereelucidated by self-assembly process. Surfactant-assisted hydro-thermal approach has been developed for the controllablesynthesis of unique micropompon sodium gadolinium tungstate.WhitelightphosphorswithnominalcompositionofNaGdWO4(OH)x:Eu3+ suitable for UV excitation were prepared. The phosphorhave 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 thecombination of blue-green, orange, and red light from chargetransfer transition of the OfW and the transition between5D0f7F1 and 5D0f7F1 of Eu3+. We also found the surfactantsof CTAB, PVP, and P123 could affect the morphology.Meanwhile, the concentration of the surfactant also plays animportant role on the shape and size of the hydrothermalproducts. The present results, demonstrate that the crystallizationprocedure by a surfactant-assisted hydrothermal reaction providea facile way for shape control on a number of inorganicfunctional materials. Their intriguing self-assemble capabilityenables them to serve as novel nanobuiding blocks for newnanodevice applications. Furthermore, this method also presentsa way for the controlled synthesis of multicomponent metaloxides.

Acknowledgment. The authors gratefully acknowledge thefinancial support from the National Science Foundation of China(No. 20671072)

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