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Morphology-controlled synthesis of NaGd(WO4)2:Eu3+ microcrystals by hydrothermal process
Superlattices and Microstructures 80 (2015) 188–195
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Morphology-controlled synthesis of NaGd(WO4)2:Eu3+ microcrystals by hydrothermal process Xiaomin Yang a,⇑, Hongxia Tang b, Zhenfu Guo a a b
Department of Chemistry, College of Science, Hebei North University, Zhangjiakou 075000, China The First Hospital of Endocrinology, Zhangjiakou 075000, China
a r t i c l e
i n f o
Article history: Received 8 January 2015 Accepted 10 January 2015 Available online 17 January 2015 Keywords: NaGd(WO4)2:Eu3+ Hydrothermal synthesis Microcrystal Morphology
a b s t r a c t Spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals were synthesized by hydrothermal process with the addition of PVP and Na3Cit. The XRD and FTIR results show that synthesized samples have the pure tetragonal phase. The addition of PVP in the synthesis induces the formation of spindle morphology, but the addition of Na3Cit induces the obtaining of hierarchical petaloid structure. The hydrothermal time has obvious influence on the morphology and size of the NaGd(WO4)2:Eu3+ samples. The excitation and emission spectra of spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals show characteristic bands corresponding to the WO24 groups and Eu3+ ions. Although there is no obvious red or blue shift in the emission spectra of spindle and petaloid NaGd(WO4)2:Eu3+ samples, the emission intensity of spindle NaGd(WO4)2:Eu3+ sample is higher than that of the petaloid NaGd(WO4)2:Eu3+ sample. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction It is known that the chemical and physical properties of nano/micro inorganic powders have close relation with their structure, crystal phase, shape, size and dimensionality [1]. Consequently, controlled synthesis of inorganic powders with a certain morphology and well-defined size became ⇑ Corresponding author. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.spmi.2015.01.008 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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one of important research topics in the past few decades. A range of synthesis routes have been applied to obtain these inorganic powders, such as hydrothermal/solvothermal reaction, sol–gel synthesis, microemulsion method and co-precipitation. Among them, hydrothermal/solvothermal is one of effective routes to synthesize inorganic nano/micro powders with controllable morphology and size. This method exploits the solubility of almost all inorganic substances in water at elevated temperatures and pressures and subsequent crystallization of the dissolved material from the fluid [2]. The tungstate and double tungstate families have been long known as functional materials and have been used in some fields because of their luminescence properties corresponding to the electron transitions within the 4f shell, such as CaWO4 [3–5], PbWO4 [6], CdWO4 [7], BaWO4 [8,9], KY(WO4)2: Yb3+ [10], NaLaWO4 [11], K1 xNaxGd(WO4)2 [12] and NaGd(WO4)2 [13]. In recent years, one of important applications of tungstates and double tungstates is to be as host materials for rare earth (RE) ions. RE ions doped materials may be the promising candidates as light emitting phosphors for the applications of light-emitting dioxides (LEDs). The luminescent materials doped with RE ions are of particular interest as light emitting devices allowing three-dimensional displays, biological detection, lasers, and many others, due to their unique spectral property to transform ultraviolet or infrared light into visible and stable fluorescence [14]. Among these tungstates, sodium based double tungstates have less destructive phase transition than potassium based double tungstates and do not exhibit a polymorphic phase transformations below their melting points, such as NaGd(WO4)2 [15]. Since the microstructure of the host material is one of key factors to influence the luminescence of the doped RE ions, the controllable synthesis of host materials with a certain morphology and size may be important for powder phosphors. Herein, we report on the controllable synthesis of NaGd(WO4)2:2 mol% Eu3+ phosphors by a hydrothermal process with the addition of different surfactants. We found that the addition of polyvinylpyrrolidone (PVP) induces the formation of spindle powders, but the addition of sodium citrate (Na3Cit) induces the formation of hierarchical petaloid microstructure. And the morphology further influences the luminescent properties of the samples.
2. Materials and methods NaGd(WO4)2:2 mol% Eu3+ phosphors were synthesized by a hydrothermal process. Europium oxide (Eu2O3), gadolinium oxide (Gd2O3), sodium tungstate (Na2WO4H2O) were chosen as raw materials. PVP and Na3Cit were used as surfactant. Nitric acid (HNO3) was used in the obtaining of Eu(NO3)3 and Gd(NO3)3. All chemicals were of analytical grade and used directly without further purification. Deionized water was used as solvent. For the obtaining of Eu(NO3)3 and Gd(NO3)3, Eu2O3 and Gd2O3 (the mole ratio of Eu3+/Gd3+ is 0.02) were dissolved into dilute HNO3 solution and the excess HNO3 was removed by evaporation. In a typical synthesis, 0.4 g of PVP (0.6 g of Na3Cit) was dissolved into 30 mL of deionized water firstly under continuous stirring for 15 min. Secondly, 10 mL of solution containing 2 mmol of Eu(NO3)3 and Gd(NO3)3 (the mole ratio of Eu3+/Gd3+ is 0.02) was added into the above solution. Then the mixed solution was stirred for 15 min. Thirdly, 10 mL of Na2WO4 solution containing 4 mmol of Na2WO4 was added into the mixture. Then the pH value of the mixture was adjusted to 8–9 by the addition of NH3H2O. After continuous stirring for about 20 min, the mixture was transferred to an autoclave, sealed, and heated at 180 °C for 20 h. As the system cooled to room temperature naturally, the production was separated by centrifugation, washed with deionized water, and dried at 80 °C for 24 h in air. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with graphite-monochromatized Cu Ka radiation (k = 0.15405 nm) in the 2h range from 20° to 60°. Fourier transform infrared spectroscopy (FTIR) spectra were obtained by using a Perkin–Elmer Rx1 instrument in the range of 400–4000 cm 1. Scanning electron microscope (SEM) images were obtained on a JSM-6480A scanning electron microscope. Transmission electron microscopy (TEM) images were performed on an FEI Tecnai G2 S-Twin instrument. Excitation and emission spectra were obtained by an F-7000 spectrophotometer equipped with a Xe lamp as the excitation source (Hitachi, Japan). All measurements were performed on at room temperature.
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3. Results and discussion The XRD patterns of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit are shown in Fig. 1. All the diffraction peaks of indexing to the pure tetragonal phase of NaGd(WO4)2 are shown, and they are well according with JCPDS Card No. 25-0829. There are no other peaks from impurities can be detected in XRD patterns. These results indicate that crystallographic NaGd(WO4)2: Eu3+ samples can be obtained under current synthesis conditions, and the doped Eu3+ ions have no influence on the structure of NaGd(WO4)2. The NaGd(WO4)2 material has the similar structure with CaWO4, which has the cell parameters a = b = 5.243 Å, c = 11.368 Å, and z = 4. Generally, the Eu3+ ions will prefer to occupy either the eight-coordinated Na+ site or the eight-coordinated Gd3+ site, because of the similar effective radius of Eu3+ (1.066 Å) with those of Na+ (1.180 Å) and Gd3+ (1.193 Å) ions. However, on the basis of the valence state analysis, Gd3+ sites are much more probable to be occupied [16]. Fig. 2 shows the FTIR spectra of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit. The bands below 1000 cm 1 are characteristic vibrations of W–O bands [17]. The weak absorption band at 728 cm 1 comes from the stretching vibrations of the W–O bond, which corresponds to the reduction state of tungsten (W5+). The strong absorption band at 806 cm 1 originates from the O–W–O stretches of WO4 tetrahedron. The absorption bands at 1628 and 3436 cm 1 can be ascribed to the O–H bending and stretching vibrations of residual water in the samples. The absorption band at 1386 cm 1 (dCH) is induced by the residual organic materials. Conclusively, the FTIR spectra further confirm the successful synthesis of NaGd(WO4)2:Eu3+ samples. The SEM images of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit are shown in Fig. 3. It is obvious that the different surfactants can influence the morphology and dimension of the samples greatly. The samples are spindles when there is the addition of PVP in the synthesis, as shown in Fig. 3a. The length and the width of the spindles are about 5 and 1.5 lm, respectively. In addition, the spindles have smooth surfaces, uniform dimensions, and good dispersion. However, the NaGd(WO4)2:Eu3+ sample has a hierarchical petaloid microstructure when Na3Cit is added in the synthesis, as shown in Fig. 3b. And the hierarchical petaloid microstructure is constructed by thin nanoplate subunits. To study the influence of hydrothermal time on morphology and size of the spindle NaGd(WO4)2: Eu3+ samples, the TEM images in different reaction stages are obtained, as shown in Fig. 4. The TEM images exhibit the morphology evolution with the increase of hydrothermal time. The product consists of irregular nanoparticles with aggregation when the hydrothermal time is 2 h, as shown in
Fig. 1. XRD patterns of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
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Fig. 2. FTIR spectra of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
Fig. 3. The SEM images of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
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Fig. 4. The TEM images of obtained NaGd(WO4)2:Eu3+ samples in different hydrothermal stages with the addition of PVP in the synthesis a: 2 h; b: 5 h; c: 10 h; d: 20 h.
Fig. 4a. When the hydrothermal time increases to 5 h, the spindles with length about 1 lm and width about 300 nm are obtained, as shown in Fig. 4b. Also, lots of irregular nanoparticles still exist. As the hydrothermal time prolongs to 10 h, spindles with increased length (1.5 lm) and width (500 nm) are obtained (Fig. 4c). And hardly any nanoparticles can be seen. At a hydrothermal time of 20 h, the spindles grow larger in all directions and the morphology uniformity is improved highly, as shown in Fig. 4d. Based on these results, one can conclude that the spindle NaGd(WO4)2:Eu3+ samples grow up continuously with the hydrothermal time. The amorphous particles disappear slowly and the spindle NaGd(WO4)2:Eu3+ samples with smooth surface are obtained finally. Generally, the key role in the formation of spindle NaGd(WO4)2:Eu3+ samples is the Ostwald ripening. In the hydrothermal process, NaGd(WO4)2:Eu3+ nucleates from the solution, and forms lots of nanoparticles. Some of these nanoparticles are small, and some of them are large. The larger nanoparticles will grow up continuously at the cost of the small ones. In addition, PVP will absorb selectively on the planes with highest surface energy to lower their surface energies, which induces a lower growth in this direction. Finally, spindle NaGd(WO4)2:Eu3+ samples are obtained. Fig. 5 gives the SEM images in different hydrothermal stages when there is the addition of Na3Cit in the synthesis of NaGd(WO4)2:Eu3+ samples. When the hydrothermal time is 2 h, only irregular nanoparticles are obtained, as shown in Fig. 5a. After increasing the hydrothermal time to 5 h, numerous nanoparticles and some nanoplates are found, as shown in Fig. 5b. Further increasing the hydrothermal time to 10 h, numerous microcrystals with petaloid structure are formed (Fig. 5c). In addition, it
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Fig. 5. The SEM images of obtained NaGd(WO4)2:Eu3+ samples in different hydrothermal stages with the addition of Na3Cit in the synthesis a: 2 h; b: 5 h; c: 10 h; d: 20 h.
should be noted that there still lots of nanoparticles and nanoplates. After 20 h of hydrothermal treatment, the nanoparticles disappear completely and uniform petaloid microcrystals are obtained finally. In the synthesis of NaGd(WO4)2:Eu3+ sample with the hierarchical petaloid microstructure, Na3Cit plays an important role. When Eu(NO3)3 and Gd(NO3)3 was added into the solution containing Na3Cit. Gd3+–Cit3 (Eu3+–Cit3 ) compounds are formed immediately. In the hydrothermal process, the energy will induce the fracture of Gd3+–Cit3 bonds and the gradual combinations with WO24 , which induces the formation of NaGd(WO4)2:Eu3+ nuclei. However, these nuclei are metastable and prefer to aggregate to minimize the interface energy. In addition, the Na3Cit can be used as crystal growth modifier. These result in the highly anisotropic growth of the NaGd(WO4)2:Eu3+ particles. As a result, petaloid microcrystals assembled by 2D nanoplates can be synthesized. Such an aggregation process and anisotropic growth mechanism can be observed in the formation of CaCO3 mesocrystals [18]. Fig. 6 gives the excitation spectra of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit. Clearly, excitation spectra of NaGd(WO4)2:Eu3+ samples with different morphologies all consist of a broad band and four narrow bands. The broad bands peaking at 266 nm originate from the charge transfer transitions of O2–W6+ within the WO24 groups (CTB), which indicates the effective energy transfer between the WO24 group and Eu3+ ions in NaGd(WO4)2:Eu3+ samples. The narrow bands peaking at 361 nm, 381 nm, 393 nm, and 414 nm correspond to the 7F0 ? 5D4, 7F0 ? 5L7, 7F0 ? 5D6, and 7F0 ? 5D3 transitions of Eu3+ ions, respectively. It should be noted that there are shoulders in
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Fig. 6. The excitation spectra of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
the range of 275–350 nm in the excitation spectra. These shoulders should originate from the overlaps of the 8S ? 6I and 8S ? 6P transitions of Gd3+ ions [19]. Fig. 7 shows the emission spectra of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit. Under the excitation of 266 nm, spindle and petaloid NaGd(WO4)2:Eu3+ samples show bands peaking at 594 nm, 618 nm, 654 nm, and 704 nm, which correspond to the 5D0 ? 7Fj (j = 1, 2, 3, 4) transitions of Eu3+ ions, respectively [20]. Eu3+ ion is a good probe for the chemical environment of the RE ions because that the 5D0 ? 7F2 transition allowed by electric dipole is sensitive to the surroundings, while 5D0 ? 7F1 transition allowed by magnetic dipole is sensitive to the environment. The 5D0 ? 7F1 transition will be dominate if Eu3+ ion in a site with inversion symmetry. On the contrary, The 5D0 ? 7F2 transition will be dominate if Eu3+ ion in a site without inversion symmetry. The fact that the dominant emission from the 5D0 ? 7F2 transition rather than the 5D0 ? 7F2 transition, indicates Eu3+ ions are located at the site without inversion symmetry in NaGd(WO4)2 hosts. The microstructure of the sample has influence on the intensity. Although there is no obvious red or blue
Fig. 7. The emission spectra of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
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shift in the emission spectra of spindle and petaloid NaGd(WO4)2:Eu3+ samples, the emission intensity of spindle NaGd(WO4)2:Eu3+ sample is higher than that of the petaloid NaGd(WO4)2:Eu3+ sample. 4. Conclusion We synthesized spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals successfully by a hydrothermal process with the addition of PVP and Na3Cit. The XRD and FTIR results show that synthesized samples have the pure tetragonal phase. The hydrothermal time and surfactants play the key role in the formation of spindle and hierarchical petaloid structures. The addition of PVP in the synthesis induces the formation of spindle morphology, but the addition of Na3Cit induces the obtaining of hierarchical petaloid structure. The excitation and emission spectra of spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals show characteristic bands corresponding to the WO24 groups and Eu3+ ions. Although there is no obvious red or blue shift in the emission spectra of spindle and petaloid NaGd(WO4)2:Eu3+ samples, the emission intensity of spindle NaGd(WO4)2:Eu3+ sample is higher than that of the petaloid NaGd(WO4)2:Eu3+ sample. Acknowledgements This work is supported financially by Zhangjiakou Science and Technology Bureau Projects (No. 11110024D and No. 1321022B), and the Youth Fund of Natural Science of Hebei North University (No. Q201101). References [1] A.M. Kaczmarek, R.V. Deun, Rare earth tungstate and molybdate compounds – from 0D to 3D architectures, Chem. Soc. Rev. 42 (2013) 8835–8848. [2] C. Burda, X. Chen, R. Narayanan, M.A. EI-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102. [3] Y. Yang, X. Wang, B. Liu, Synthesis of CaWO4 and CaWO4:Eu microspheres by precipitation, Nano (2014) 1450008. [4] X. Yang, Y. Wang, N. Wang, S. Wang, G. Gao, Effects of co-doped Li+ ions on luminescence of CaWO4:Sm3+ nanoparticles, J. Mater. Sci.: Mater. Electron. 25 (2014) 3996–4000. [5] M.S. Silva, L.M. Jesus, L.B. Barbosa, D.R. Ardila, J.P. Andreeta, R.S. Silva, Crucibleless crystal growth and radioluminescence study of calcium tungstate single crystal fiber, Opt. Mater. 37 (2014) 51–54. [6] O. Chukova, S. Nedilko, V. Scherbatskyi, Effect of annealing on luminescence properties of the undoped and rare earth doped lead tungstate crystals, Opt. Mater. 34 (2012) 2071–2075. [7] X.-C. Chu, Y.-N. Xu, L.-T. Li, Hydrothermal synthesis and effects of pH values on properties of CdWO4 powders, J. Mater. Sci.: Mater. Electron. 25 (2014) 2601–2604. [8] L. Zhang, J.-S. Dai, L. Lian, Y. Liu, Dumbbell-like BaWO4 microstructures: surfactant-free hydrothermal synthesis, growth mechanism and photoluminescence property, Superlattices Microstruct. 54 (2013) 87–95. [9] A. Phuruangrat, T. Thongtem, S. Thongtem, Precipitate synthesis of BaMoO4 and BaWO4 nanoparticles at room temperature and their photoluminescence properties, Superlattices Microstruct. 52 (2012) 78–83. [10] H.-G. Liu, W.-C. Zheng, Crystal-field study for Yb3+ doped KY(WO4)2 crystal – revisited based on superposition model, Opt. Mater. 36 (2013) 575–580. [11] X. Liu, W. Hou, X. Yang, J. Liang, Morphology controllable synthesis of NaLa(WO4)2: the morphology dependent photoluminescent properties and single-phased white light emission of NaLa(WO4)2:Eu3+/Tb3+/Tm3+, CrystEngComm 16 (2014) 1268–1276. [12] A. Durairajan, D. Thangaraju, S. Moorthy Babu, Synthesis, structure and vibrational studies on mixed alkali metal gadolinium double tungstate, K1 xNaxGd(WO4)2, Opt. Mater. 35 (2013) 735–739. [13] S. Perets, R.Z. Shneck, R. Gajic, A. Golubovic, Z. Burshtein, Vibrational spectra of sodium gadolinium tungstate NaGd(WO4)2 single crystals: observation of spatial dispersion, Vib. Spectrosc. 49 (2009) 110–117. [14] Z. Piskula, J. Czajka, K. Staninski, S. Lis, Luminescence properties of calcium tungstate activated by lanthanide(III) ions, J. Rare Earth 32 (2014) 221–225. [15] A. Durairajan, D. Thangaraju, D. Balaji, S. Moorthy Babu, Sol–gel synthesis and characterizations of crystalline NaGd(WO4)2 powder for anisotropic transparent ceramic laser application, Opt. Mater. 35 (2013) 740–743. [16] Y. Liu, G. Liu, X. Dong, J. Wang, W. Yu, Luminescence, energy-transfer and tunable color properties of single-component Tb3+ and/or Sm3+ doped NaGd(WO4)2 phosphors with UV excitation for use as WLEDs, RSC Adv. 4 (2014) 58708–58716. [17] L.J. Burcham, I.E. Wachs, Vibrational analysis of the two non-equivalent, tetrahedral tungstate (WO4) units in Ce2(WO4)3 and La2(WO4)3, Spectrochim. Acta A 54 (1998) 1355–1368. [18] A.-W. Xu, M. Antonietti, H. Colfen, Y.-P. Fang, Uniform hexagonal plates of vaterite CaCO3 mesocrystals formed by biomimetic mineralization, Adv. Funct. Mater. 16 (2006) 903–908. [19] L. Zhou, W.C.H. Choy, J. Shi, M. Gong, H. Liang, T.I. Yuk, Synthesis, vacuum ultraviolet and near ultraviolet-excited luminescent properties of GdCaAl3O7:RE3+ (RE = Eu, Tb), J. Solid State Chem. 178 (2005) 3004–3009. [20] Y. Yang, Synthesis and luminescent properties of LaPO4:Eu3+ microspheres, Mater. Sci. Eng. B 178 (2013) 807–810.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Morphology-controlled synthesis of NaGd(WO4)2:Eu3+ microcrystals by hydrothermal process Xiaomin Yang a,⇑, Hongxia Tang b, Zhenfu Guo a a b
Department of Chemistry, College of Science, Hebei North University, Zhangjiakou 075000, China The First Hospital of Endocrinology, Zhangjiakou 075000, China
a r t i c l e
i n f o
Article history: Received 8 January 2015 Accepted 10 January 2015 Available online 17 January 2015 Keywords: NaGd(WO4)2:Eu3+ Hydrothermal synthesis Microcrystal Morphology
a b s t r a c t Spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals were synthesized by hydrothermal process with the addition of PVP and Na3Cit. The XRD and FTIR results show that synthesized samples have the pure tetragonal phase. The addition of PVP in the synthesis induces the formation of spindle morphology, but the addition of Na3Cit induces the obtaining of hierarchical petaloid structure. The hydrothermal time has obvious influence on the morphology and size of the NaGd(WO4)2:Eu3+ samples. The excitation and emission spectra of spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals show characteristic bands corresponding to the WO24 groups and Eu3+ ions. Although there is no obvious red or blue shift in the emission spectra of spindle and petaloid NaGd(WO4)2:Eu3+ samples, the emission intensity of spindle NaGd(WO4)2:Eu3+ sample is higher than that of the petaloid NaGd(WO4)2:Eu3+ sample. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction It is known that the chemical and physical properties of nano/micro inorganic powders have close relation with their structure, crystal phase, shape, size and dimensionality [1]. Consequently, controlled synthesis of inorganic powders with a certain morphology and well-defined size became ⇑ Corresponding author. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.spmi.2015.01.008 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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one of important research topics in the past few decades. A range of synthesis routes have been applied to obtain these inorganic powders, such as hydrothermal/solvothermal reaction, sol–gel synthesis, microemulsion method and co-precipitation. Among them, hydrothermal/solvothermal is one of effective routes to synthesize inorganic nano/micro powders with controllable morphology and size. This method exploits the solubility of almost all inorganic substances in water at elevated temperatures and pressures and subsequent crystallization of the dissolved material from the fluid [2]. The tungstate and double tungstate families have been long known as functional materials and have been used in some fields because of their luminescence properties corresponding to the electron transitions within the 4f shell, such as CaWO4 [3–5], PbWO4 [6], CdWO4 [7], BaWO4 [8,9], KY(WO4)2: Yb3+ [10], NaLaWO4 [11], K1 xNaxGd(WO4)2 [12] and NaGd(WO4)2 [13]. In recent years, one of important applications of tungstates and double tungstates is to be as host materials for rare earth (RE) ions. RE ions doped materials may be the promising candidates as light emitting phosphors for the applications of light-emitting dioxides (LEDs). The luminescent materials doped with RE ions are of particular interest as light emitting devices allowing three-dimensional displays, biological detection, lasers, and many others, due to their unique spectral property to transform ultraviolet or infrared light into visible and stable fluorescence [14]. Among these tungstates, sodium based double tungstates have less destructive phase transition than potassium based double tungstates and do not exhibit a polymorphic phase transformations below their melting points, such as NaGd(WO4)2 [15]. Since the microstructure of the host material is one of key factors to influence the luminescence of the doped RE ions, the controllable synthesis of host materials with a certain morphology and size may be important for powder phosphors. Herein, we report on the controllable synthesis of NaGd(WO4)2:2 mol% Eu3+ phosphors by a hydrothermal process with the addition of different surfactants. We found that the addition of polyvinylpyrrolidone (PVP) induces the formation of spindle powders, but the addition of sodium citrate (Na3Cit) induces the formation of hierarchical petaloid microstructure. And the morphology further influences the luminescent properties of the samples.
2. Materials and methods NaGd(WO4)2:2 mol% Eu3+ phosphors were synthesized by a hydrothermal process. Europium oxide (Eu2O3), gadolinium oxide (Gd2O3), sodium tungstate (Na2WO4H2O) were chosen as raw materials. PVP and Na3Cit were used as surfactant. Nitric acid (HNO3) was used in the obtaining of Eu(NO3)3 and Gd(NO3)3. All chemicals were of analytical grade and used directly without further purification. Deionized water was used as solvent. For the obtaining of Eu(NO3)3 and Gd(NO3)3, Eu2O3 and Gd2O3 (the mole ratio of Eu3+/Gd3+ is 0.02) were dissolved into dilute HNO3 solution and the excess HNO3 was removed by evaporation. In a typical synthesis, 0.4 g of PVP (0.6 g of Na3Cit) was dissolved into 30 mL of deionized water firstly under continuous stirring for 15 min. Secondly, 10 mL of solution containing 2 mmol of Eu(NO3)3 and Gd(NO3)3 (the mole ratio of Eu3+/Gd3+ is 0.02) was added into the above solution. Then the mixed solution was stirred for 15 min. Thirdly, 10 mL of Na2WO4 solution containing 4 mmol of Na2WO4 was added into the mixture. Then the pH value of the mixture was adjusted to 8–9 by the addition of NH3H2O. After continuous stirring for about 20 min, the mixture was transferred to an autoclave, sealed, and heated at 180 °C for 20 h. As the system cooled to room temperature naturally, the production was separated by centrifugation, washed with deionized water, and dried at 80 °C for 24 h in air. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with graphite-monochromatized Cu Ka radiation (k = 0.15405 nm) in the 2h range from 20° to 60°. Fourier transform infrared spectroscopy (FTIR) spectra were obtained by using a Perkin–Elmer Rx1 instrument in the range of 400–4000 cm 1. Scanning electron microscope (SEM) images were obtained on a JSM-6480A scanning electron microscope. Transmission electron microscopy (TEM) images were performed on an FEI Tecnai G2 S-Twin instrument. Excitation and emission spectra were obtained by an F-7000 spectrophotometer equipped with a Xe lamp as the excitation source (Hitachi, Japan). All measurements were performed on at room temperature.
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3. Results and discussion The XRD patterns of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit are shown in Fig. 1. All the diffraction peaks of indexing to the pure tetragonal phase of NaGd(WO4)2 are shown, and they are well according with JCPDS Card No. 25-0829. There are no other peaks from impurities can be detected in XRD patterns. These results indicate that crystallographic NaGd(WO4)2: Eu3+ samples can be obtained under current synthesis conditions, and the doped Eu3+ ions have no influence on the structure of NaGd(WO4)2. The NaGd(WO4)2 material has the similar structure with CaWO4, which has the cell parameters a = b = 5.243 Å, c = 11.368 Å, and z = 4. Generally, the Eu3+ ions will prefer to occupy either the eight-coordinated Na+ site or the eight-coordinated Gd3+ site, because of the similar effective radius of Eu3+ (1.066 Å) with those of Na+ (1.180 Å) and Gd3+ (1.193 Å) ions. However, on the basis of the valence state analysis, Gd3+ sites are much more probable to be occupied [16]. Fig. 2 shows the FTIR spectra of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit. The bands below 1000 cm 1 are characteristic vibrations of W–O bands [17]. The weak absorption band at 728 cm 1 comes from the stretching vibrations of the W–O bond, which corresponds to the reduction state of tungsten (W5+). The strong absorption band at 806 cm 1 originates from the O–W–O stretches of WO4 tetrahedron. The absorption bands at 1628 and 3436 cm 1 can be ascribed to the O–H bending and stretching vibrations of residual water in the samples. The absorption band at 1386 cm 1 (dCH) is induced by the residual organic materials. Conclusively, the FTIR spectra further confirm the successful synthesis of NaGd(WO4)2:Eu3+ samples. The SEM images of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit are shown in Fig. 3. It is obvious that the different surfactants can influence the morphology and dimension of the samples greatly. The samples are spindles when there is the addition of PVP in the synthesis, as shown in Fig. 3a. The length and the width of the spindles are about 5 and 1.5 lm, respectively. In addition, the spindles have smooth surfaces, uniform dimensions, and good dispersion. However, the NaGd(WO4)2:Eu3+ sample has a hierarchical petaloid microstructure when Na3Cit is added in the synthesis, as shown in Fig. 3b. And the hierarchical petaloid microstructure is constructed by thin nanoplate subunits. To study the influence of hydrothermal time on morphology and size of the spindle NaGd(WO4)2: Eu3+ samples, the TEM images in different reaction stages are obtained, as shown in Fig. 4. The TEM images exhibit the morphology evolution with the increase of hydrothermal time. The product consists of irregular nanoparticles with aggregation when the hydrothermal time is 2 h, as shown in
Fig. 1. XRD patterns of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
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Fig. 2. FTIR spectra of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
Fig. 3. The SEM images of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
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Fig. 4. The TEM images of obtained NaGd(WO4)2:Eu3+ samples in different hydrothermal stages with the addition of PVP in the synthesis a: 2 h; b: 5 h; c: 10 h; d: 20 h.
Fig. 4a. When the hydrothermal time increases to 5 h, the spindles with length about 1 lm and width about 300 nm are obtained, as shown in Fig. 4b. Also, lots of irregular nanoparticles still exist. As the hydrothermal time prolongs to 10 h, spindles with increased length (1.5 lm) and width (500 nm) are obtained (Fig. 4c). And hardly any nanoparticles can be seen. At a hydrothermal time of 20 h, the spindles grow larger in all directions and the morphology uniformity is improved highly, as shown in Fig. 4d. Based on these results, one can conclude that the spindle NaGd(WO4)2:Eu3+ samples grow up continuously with the hydrothermal time. The amorphous particles disappear slowly and the spindle NaGd(WO4)2:Eu3+ samples with smooth surface are obtained finally. Generally, the key role in the formation of spindle NaGd(WO4)2:Eu3+ samples is the Ostwald ripening. In the hydrothermal process, NaGd(WO4)2:Eu3+ nucleates from the solution, and forms lots of nanoparticles. Some of these nanoparticles are small, and some of them are large. The larger nanoparticles will grow up continuously at the cost of the small ones. In addition, PVP will absorb selectively on the planes with highest surface energy to lower their surface energies, which induces a lower growth in this direction. Finally, spindle NaGd(WO4)2:Eu3+ samples are obtained. Fig. 5 gives the SEM images in different hydrothermal stages when there is the addition of Na3Cit in the synthesis of NaGd(WO4)2:Eu3+ samples. When the hydrothermal time is 2 h, only irregular nanoparticles are obtained, as shown in Fig. 5a. After increasing the hydrothermal time to 5 h, numerous nanoparticles and some nanoplates are found, as shown in Fig. 5b. Further increasing the hydrothermal time to 10 h, numerous microcrystals with petaloid structure are formed (Fig. 5c). In addition, it
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Fig. 5. The SEM images of obtained NaGd(WO4)2:Eu3+ samples in different hydrothermal stages with the addition of Na3Cit in the synthesis a: 2 h; b: 5 h; c: 10 h; d: 20 h.
should be noted that there still lots of nanoparticles and nanoplates. After 20 h of hydrothermal treatment, the nanoparticles disappear completely and uniform petaloid microcrystals are obtained finally. In the synthesis of NaGd(WO4)2:Eu3+ sample with the hierarchical petaloid microstructure, Na3Cit plays an important role. When Eu(NO3)3 and Gd(NO3)3 was added into the solution containing Na3Cit. Gd3+–Cit3 (Eu3+–Cit3 ) compounds are formed immediately. In the hydrothermal process, the energy will induce the fracture of Gd3+–Cit3 bonds and the gradual combinations with WO24 , which induces the formation of NaGd(WO4)2:Eu3+ nuclei. However, these nuclei are metastable and prefer to aggregate to minimize the interface energy. In addition, the Na3Cit can be used as crystal growth modifier. These result in the highly anisotropic growth of the NaGd(WO4)2:Eu3+ particles. As a result, petaloid microcrystals assembled by 2D nanoplates can be synthesized. Such an aggregation process and anisotropic growth mechanism can be observed in the formation of CaCO3 mesocrystals [18]. Fig. 6 gives the excitation spectra of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit. Clearly, excitation spectra of NaGd(WO4)2:Eu3+ samples with different morphologies all consist of a broad band and four narrow bands. The broad bands peaking at 266 nm originate from the charge transfer transitions of O2–W6+ within the WO24 groups (CTB), which indicates the effective energy transfer between the WO24 group and Eu3+ ions in NaGd(WO4)2:Eu3+ samples. The narrow bands peaking at 361 nm, 381 nm, 393 nm, and 414 nm correspond to the 7F0 ? 5D4, 7F0 ? 5L7, 7F0 ? 5D6, and 7F0 ? 5D3 transitions of Eu3+ ions, respectively. It should be noted that there are shoulders in
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Fig. 6. The excitation spectra of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
the range of 275–350 nm in the excitation spectra. These shoulders should originate from the overlaps of the 8S ? 6I and 8S ? 6P transitions of Gd3+ ions [19]. Fig. 7 shows the emission spectra of obtained NaGd(WO4)2:Eu3+ samples with addition of PVP and Na3Cit. Under the excitation of 266 nm, spindle and petaloid NaGd(WO4)2:Eu3+ samples show bands peaking at 594 nm, 618 nm, 654 nm, and 704 nm, which correspond to the 5D0 ? 7Fj (j = 1, 2, 3, 4) transitions of Eu3+ ions, respectively [20]. Eu3+ ion is a good probe for the chemical environment of the RE ions because that the 5D0 ? 7F2 transition allowed by electric dipole is sensitive to the surroundings, while 5D0 ? 7F1 transition allowed by magnetic dipole is sensitive to the environment. The 5D0 ? 7F1 transition will be dominate if Eu3+ ion in a site with inversion symmetry. On the contrary, The 5D0 ? 7F2 transition will be dominate if Eu3+ ion in a site without inversion symmetry. The fact that the dominant emission from the 5D0 ? 7F2 transition rather than the 5D0 ? 7F2 transition, indicates Eu3+ ions are located at the site without inversion symmetry in NaGd(WO4)2 hosts. The microstructure of the sample has influence on the intensity. Although there is no obvious red or blue
Fig. 7. The emission spectra of obtained NaGd(WO4)2:Eu3+ samples with the addition of PVP and Na3Cit in the synthesis.
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shift in the emission spectra of spindle and petaloid NaGd(WO4)2:Eu3+ samples, the emission intensity of spindle NaGd(WO4)2:Eu3+ sample is higher than that of the petaloid NaGd(WO4)2:Eu3+ sample. 4. Conclusion We synthesized spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals successfully by a hydrothermal process with the addition of PVP and Na3Cit. The XRD and FTIR results show that synthesized samples have the pure tetragonal phase. The hydrothermal time and surfactants play the key role in the formation of spindle and hierarchical petaloid structures. The addition of PVP in the synthesis induces the formation of spindle morphology, but the addition of Na3Cit induces the obtaining of hierarchical petaloid structure. The excitation and emission spectra of spindle and hierarchical petaloid NaGd(WO4)2:Eu3+ microcrystals show characteristic bands corresponding to the WO24 groups and Eu3+ ions. 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