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Novel ionic liquid-assisted hydrothermal method for the assembly of luminescent lanthanide fluorides with controllable morphologies
Journal of Molecular Liquids 212 (2015) 799–803
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Novel ionic liquid-assisted hydrothermal method for the assembly of luminescent lanthanide fluorides with controllable morphologies Aiqi Wang b, Zhi Zeng b,⁎, Jinwei Gao d, Qianming Wang a,b,c,⁎⁎ a
Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, 510006, China d Institute for Advanced Materials, Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China b c
a r t i c l e
i n f o
Article history: Received 8 May 2015 Received in revised form 1 September 2015 Accepted 16 October 2015 Available online xxxx Keywords: Nanoparticle Luminescence Ionic liquid Lanthanide
a b s t r a c t An amphiphilic ionic liquid (diallyldimethylammonium tetrafluoroborate) assisted hydrothermal method has been applied for the synthesis of lanthanide fluoride nanoparticles. It has been carried out under mild conditions and large scale amounts of samples can be facilely achieved by this efficient way. Luminescence features of the phosphors were studied by steady state fluorescence and the samples could present striking 5D0 → 7F1 transition with orange-red color, suggesting that Eu3+ ions are located in the crystal lattice with inversion symmetry. The particle size distribution was investigated by scanning electron microscope and the results showed that different microstructures such as nano-rods and nanoplates were obtained. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) have been paid much attention for the synthesis of inorganic nano-materials owing to the outstanding properties like good thermal stability, negligible vapor pressure and extremely high ionic conductivity. They can not only play as solvents, but also use as template and ligand reagent [1]. Excellent capability for the dissolution and stabilization of metal cations were shown by these amphiphilic compounds, which will benefit the homogeneous synthesis of nano-sized inorganic materials [2]. Due to intrinsic structure and excellent luminescent performances, rare earth materials have been extensively studied [3–6]. As a wellknown luminescent host, lanthanide fluorides (LnF3) have great advantageous features in comparison with the conventional oxygen-based system such as low vibration energies, good optical transparency over a wide wavelength range and minimal quenching of the excited state of doped ions [7]. Up to now, most researchers have tried NaF, NH4F, NH4BF4, NaBF4 or KBF4 as fluoride sources [8]. Though these inorganic reagents are easily accessible, ILs still demonstrated the promising property of multi-functional positions in the reaction [9–13]. Recently, a few teams have reported the fabrication of lanthanide solid state materials in
the presence of polymerizable ionic liquid under irradiations [9,14]. This important organic salt (here is diallyldimethylammonium tetrafluoroborate ([DADMA]BF4)) with environmentally benign features could be utilized as the structure directing reagent. It displays great potentials in optical application and phosphor industry. BF− 4 acted as a good replacing source of F− upon decomposition. In addition, DADMA+ moiety is commercially available and easily polymerizable cation [14]. In brief, this specific ionic liquid molecule is a preferable fluoride donor and efficient synthesis of lanthanide fluorides could be possibly realized by this way. The main advantage of choosing this ammonium cation type ionic liquid in the inorganic synthesis is its effectiveness for the dissolution and combination of metal salts, which permits it with the behavior of acting as surfactants. In addition, its melting point and viscosity were not very high. Therefore, in our work, [DADMA]BF4 was incorporated to assemble LnF3:Eu3+(Ln = Y, Gd, La) nanoparticles. Different controllable morphologies were achieved and the photoluminescence properties of the nanocrystals were also discussed in detail.
2. Experimental 2.1. Preparation
⁎ Corresponding author. ⁎⁎ Correspondence to: Q. Wang, Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China. E-mail address: [email protected] (Q. Wang).
http://dx.doi.org/10.1016/j.molliq.2015.10.033 0167-7322/© 2015 Elsevier B.V. All rights reserved.
Y2O3, La2O3, Eu2O3, Gd2O3, NaF and NaBF4 were purchased from Sigma-Aldrich Company. Diallyldimethylammonium chloride (60% aqueous) was provided by Aladdin. All the other reagents were purchased from Guangzhou Chemical Reagent Factory and used without
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further purification. Lanthanide nitrates were obtained by dissolving their oxides in concentrated nitric acid. Ionic liquid and phosphor synthesis: diallyldimethylammonium chloride (5 g, 0.031 mol) in dichloromethane (10 ml) was added together with NaBF4 (3.4 g, 0.031 mol) and stirred under room temperature for 72 h. The white precipitate was removed by filtration. The clear solution was washed with small quantities of water to remove remaining chloride. The solvent was removed in vacuo and the clear ionic liquid ([DADMA]BF4) was dried at 80 °C under dynamic vacuum for 24 h. Following procedures were the assembly of phosphors, 5 ml of 0.1 mol/l Ln(NO3)3 and 2.5 ml of 0.01 mol/l Eu(NO3)3 were mixed. Subsequently, the [DADMA]BF4 (0.1 M)solution was added dropwise to the previous solution under magnetic stirring. Until a homogeneous solution was formed, the above mixed solution was transferred to Teflon-toclave and maintained at 160 °C for 10 h. After naturally cooling down to room temperature, the product was collected by centrifugation and dried in vacuum at 70 °C. Similar synthesis procedures were carried out to prepare YF3 nanocrystals through employing other fluoride sources (NaF and NaBF4). The determination of Eu(III) was briefly described as follows: 0.5 g sample, 3 ml HF, 7 ml HNO3, 3 ml HCl and 2 ml H2O2 were transferred into the tetrafluorometoxil reaction vessels and microwave assisted digestion was performed. After cooling, the resultant sample was diluted with ultrapure water to final volume of 50 ml. 115In (for Eu) was used as internal standard elements. Corresponding ICP-MS analysis results were added in Table 1. 2.2. Characterization The X-ray powder diffraction was investigated on Bruker D8 diffractometer with Cu Kα radiation (k = 0.1541 nm) in the range of 2θ = 10–90. Scanning electron microscope (SEM) was measured with JSM6360LV. Luminescence spectra were measured on an Edinburgh FLS920 spectrometer. The crystal phase identification of the obtained samples was examined using a MDI Jade 5.0 system. Inductively coupled plasma mass spectrometry (abbreviated as ICP-MS) was carried out on an Agilent7500a (Agilent, USA). The samples were digested in a Mars 5 microwave reaction system (CEM Corporation, USA). Overall quantum yields were detected at room temperature based on an integrating sphere (Edinburgh FLS920 spectrometer) method and the sample was placed inside the integrating sphere. The excitation light was entered into the sphere by the optical fiber. When specific excitation wavelength (394 nm) was selected, the quantum yield values were calculated automatically. 3. Results and discussion X-ray diffraction analysis is an effective way for the identification of the composition of the phosphors. It has been considered as the reliable method to determine the crystalline structures of different compounds. In order to study the feasibility of the ionic liquid as the precursor, we assembled a group of YF3:Eu3 + phosphors by using various fluoride sources such as NaF, NaBF4 and [DADMA]BF4. The collected results supported that the materials crystallized well (Fig. 1). Peak positions
Table 1 Elemental measurements of LaF3:Eu3+ (sample 1), GdF3:Eu3+(sample 2) and YF3:Eu3+ (sample 3) by ICP-MS after microwave digestion (n = 6). Sample
Certified concentration (mg/g)
Added concentration (mg/g)
Experimentally measured concentration by ICP-MS (mg/g)
Relative standard deviation (%)
1 2 3
37.30 34.22 49.45
10 10 10
47.44 44.38 59.26
2.2 1.9 2.0
Fig. 1. XRD patterns of orthorhombic phase YF3:Eu3+(F = NaF,NaBF4,[DADMA]BF4). The reference pattern for orthorhombic YF3 from JCPDS file is also included.
and intensities of the three samples were analogous and consistent with the pure orthorhombic phases as indicated in the JCPDS card (No. 70-1935). It demonstrated that [DADMA]BF4 could replace conventional fluoride source like NaF in the fabrication of europium doped phosphors. For the sake of confirming the reliability of ionic liquid, we carried out photoluminescence measurements concerning the three optical materials (Fig. 2). The typical intense transitions observed in the emission spectra were derived from the 5D0 level to the sublevels of 7F1, 7 F2, 7F3 and 7F4. There were no apparent changes among the three samples, showing that the ionic liquid would act as an alternative precursor to prepare the desired solid state materials. The variation of lanthanide emission curves and shapes was also studied in a series of inorganic hosts including LaF3, GdF3 and YF3. Fig. 3 gave the excitation (monitored at 593 nm) and emission spectra (excited at 394 nm) of LaF3 assembled under hydrothermal conditions. The excitation band consists of a group of sharp peaks covering from 300 nm to 500 nm due to the direct transitions from ground state into higher excited states from the europium f-electrons. The characteristic peaks in terms of f-f transitions in Eu3+ were attributed to 7F0 → 5H6 (317 nm), 7F0 → 5D4 (361 nm), 7 F0 → 5G2(383 nm), 7F0 → 5L6 (394 nm), 7F0 → 5D3 (414 nm), 7 F0 → 5D2 (464 nm) and 7F0 → 5D1 (525 nm). Under excitation at 394 nm, it can be observed that the derived narrow peaks were corresponding to 5D0 → 7F1 (593 nm), 5D0 → 7F2 (615 nm), 5D0 → 7F3 (650 nm) and 5D0 → 7F4 (702 nm) transitions. It has been accepted that 5D0 → 7F2 transition (615 nm) is a forced electric dipole transition which would be prevalent due to small deviation from inversion symmetry. 5D0 → 7F1 transition (593 nm) is the magnetic dipole transition which is insensitive to environmental differences. In this research, it is clear that europium ion occupies in the crystal lattice a site with inversion symmetry [15]. The results indicated that 5D0 → 7FJ emissions were very favorable to investigate the transition probabilities of sharp spectral features of lanthanide elements. In these lanthanide fluorides, 5D0 → 7F1 emission peak was dominating and transition to level with even J values (J = 2) was forbidden. When Gd or Y takes place of La in the matrices, the majority bands were also identified (Figs. 4 and 5). It demonstrated that the 4f orbitals of Eu3+ are hardly affected by the crystal field and host lattices because of the shielding effect of the 5s25p6 electrons. It is known that the size, shape and positioning of the microstructures were very attractive for potential applications in optical fields. We have carried out SEM measurements to have a better understanding of the morphological results (Figs. 6–8). The images showed a very high yield of the regular shaped nanostructures. YF3 and GdF3 exhibited one
A. Wang et al. / Journal of Molecular Liquids 212 (2015) 799–803
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Fig. 2. Excitation and emission spectra of the Eu3+-doped YF3 prepared with NaF, NaBF4 and [DADMA]BF4 as fluoride sources.
Fig. 3. Excitation and emission spectra of the Eu3+-doped LaF3.
dimensional nanorods with sizes of 100 nm (Figs. 6 and 7). A coin like structure with a diameter of 80–100 nm was identified in LaF3 (Fig. 8). In this work, ionic liquid has been attached on the surface of the crystal seeds based on the coulomb interaction. The electrostatic forces between electrically charged ions (such as lanthanide cations, BF− 4 and
DADMA+) might contribute to the spontaneous aggregation of smaller granules into well-defined nanostructures. With the aim of clarifying the formation strategy, LaF3:Eu3 + has been prepared based on the same hydrothermal conditions (by using NaF and NaBF4 as the reagents) in the absence of [DADMA]BF4 (Figs. 9 and 10). The images
Fig. 4. Excitation and emission spectra of the Eu3+-doped GdF3.
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Fig. 5. Excitation and emission spectra of the Eu3+-doped YF3.
clearly showed the morphology of material structures that was highly affected by changing the ionic liquid stabilizer. When NaF and NaBF4 were used, irregular clusters were predominant and disordered aggregates were observed. In this sense, it has been found that the selective adsorption on the crystal faces through ionic liquid would be very
important in the post-growth treatment due to the surface energy control on the LnF3 nanoparticles [16]. In addition, the nucleation and growth of the nanostructures during hydrothermal synthesis has been realized through dissolution–reprecipitation. The solubility that was improved by the ionic liquid would be helpful for the direction growth.
Fig. 6. SEM image of Eu3+-doped YF3 prepared by [DADMA]BF4 as fluoride source.
Fig. 8. SEM image of Eu3+-doped LaF3 prepared by [DADMA]BF4 as fluoride source.
Fig. 7. SEM image of Eu3+-doped GdF3 prepared by [DADMA]BF4 as fluoride source.
Fig. 9. SEM image of Eu3+-doped LaF3 prepared by NaF as fluoride source.
A. Wang et al. / Journal of Molecular Liquids 212 (2015) 799–803
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Acknowledgments Q. M. appreciates National Natural Science Foundation of China (No. 21371063) excellent university young scholar fund of Guangdong province (Yq2013053), Science and Technology Project in Guangzhou (2014J4100054) and Guangdong Science and Technology Plan (2013B010403025). J. W. thanks the supports from the Guangdong Province Fund (2014B09091505).
References
Fig. 10. SEM image of Eu3+-doped LaF3 prepared by NaBF4 as fluoride source.
The dissolution has been enhanced by the hydrothermal conditions, while the diffusivity of the dissolved species was increased by the reduced viscosity of water. Furthermore, the shape of the prepared samples changed from nanorods to a nano-disk like structure, which presented that the ionic radius of different lanthanide elements had a significant effect on the morphology of the fluorides. The oriented attachment results might be useful in the controllable synthesis in the direction of crystal growth. Moreover, we have performed the quantum yield measurements for the three samples (LaF3:Eu3+, GdF3:Eu3+ and YF3:Eu3+). The results showed that the efficiency of particle-shape material (LaF3:Eu3+, 10.3%) was relatively higher than the other two materials (GdF3:Eu3 +, 7.9% and YF3:Eu3 +, 5.7%). It is estimated that the regular disc-like structure may have stiff lattices and could effectively suppress the non-radiative transitions. 4. Conclusion Luminescent LnF3:Eu3+ nano-sized phosphors were synthesized via the novel ionic liquid tunable-based approach. In contrast to normal coprecipitation and sintering treatment, the reactive lanthanide salts can be miscible together due to the incorporation of ionic liquid. Photoluminescence studies showed the derived materials that exhibited red emissions even under visible light excitations which will be very favorable for optical applications. It has been proved that the ionic liquid acted as the role of capping reagents and induced the assembly and evolution of nano-particles. Interestingly, we also identified different morphologies of lanthanide fluorides and nano-rods and nanoplates were achieved. The research may open a new start for the fabrication of lanthanide phosphors and establish structure–property relationship.
[1] C. Lorbeer, F. Behrends, J. Cybinska, H. Eckert, A.-V. Mudring, Charge compensation in RE3+ (RE ¼ Eu, Gd) and M+ (M ¼ Li, Na, K) co-doped alkaline earth nanofluorides obtained by microwave reaction with reactive ionic liquids leading to improved optical properties, J. Mater. Chem. C 2 (2014) 9439–9450. [2] A.M. Fernandes, R. Gracia, G.P. Leal, M. Paulis, D. Mecerreyes, Simple route to prepare stable liquid marbles using poly(ionic liquid)s, Polymer 55 (2014) 3397–3403. [3] X.C. Wu, Y.R. Tao, C.Y. Song, C.J. Mao, L. Dong, J.J. Zhu, Morphological control and luminescent properties of YVO4: Eu nanocrystals, J. Phys. Chem. B 110 (2006) 15791–15796. [4] J.W. Stouwdam, F.C.J.M. Veggel, Near-infrared emission of redispersible Er3+, Nd3+, and Ho3+ doped LaF3 nanoparticles, Nano Lett. 2 (2002) 733–737. [5] Q.L. Luo, S.D. Shen, G.Z. Lu, X.Z. Xiao, D.S. Mao, Y.Q. Wang, Synthesis of cubic ordered mesoporous YPO4: Ln3+ and their photoluminescence properties, J. Mater. Chem. 19 (2009) 8079–8085. [6] A.K. Gulnar, V. Sudarsan, R.K. Vatsa, R.C. Hubli, U.K. Gautam, A. Vinu, A.K. Tyagi, CePO4: Ln (Ln = Tb3+ and Dy3+) nanoleaves incorporated in silica sols, Cryst. Growth Des. 9 (2009) 2451–2456. [7] C.X. Li, J. Lin, Rare earth fluoride nano-/microcrystals: synthesis, surface modification and application, J. Mater. Chem. 20 (2010) 6831–6847. [8] X.B. Li, S.L. Gai, C.X. Li, D. Wang, N. Niu, F. He, P.P. Yang, Monodisperselanthanide fluoride nanocrystals: synthesis and luminescent properties, Inorg. Chem. 51 (2012) 3963–3971. [9] N.O. Nunez, M. Ocana, An ionic liquid based synthesis method for uniform luminescent lanthanide fluoride nanoparticles, Nanotechnology 18 (2007) 455606 (7 pp.). [10] C. Zhang, J. Chen, Y.C. Zhou, D.Q. Li, Ionic liquid-based “all-in-one” synthesis and photoluminescence properties of lanthanide fluorides, J. Phys. Chem. C 112 (2008) 10083–10088. [11] H.X. Zhong, J.M. Hong, X.F. Cao, X.T. Chen, Z.L. Xue, Ionic-liquid-assisted synthesis of YF3 with different crystalline phases and morphologies, Mater. Res. Bull. 44 (2009) 623–628. [12] C.X. Li, P.A. Ma, P.P. Yang, Z.H. Xu, G.G. Li, D.M. Yang, C. Peng, J. Lin, fine structural and morphological control of rare earth fluorides REF3 (RE = La–Lu, Y) nano/ microcrystals:microwave-assisted ionic liquid synthesis, magnetic and luminescent properties, CrystEngComm 13 (2011) 1003–1013. [13] T. Zhang, H. Guo, Y.M. Qiao, Facile synthesis, structural and optical characterization of LnF3: RE nanocrystals by ionic liquid-based hydrothermal process, J. Lumin. 129 (2009) 861–866. [14] P.S. Campbell, C. Lorbeer, J. Cybinska, A.-V. Mudring, One-pot synthesis of luminescent polymer-nanoparticle composites from task-specific ionic liquids, Adv. Funct. Mater. 23 (2013) 2924–2931. [15] G.H. Li, Y.W. Lai, W.W. Bao, L.L. Li, M.M. Li, S.C. Gan, T. Long, L.C. Zou, Facile synthesis and luminescence properties of highly uniform YF3:Ln3+ (Ln = Eu, Tb, Ce, Dy) nanocrystals in ionic liquids, Powder Technol. 214 (2011) 211–217. [16] Y.Y. Li, S.Q. Xu, A new sonication-assisted ionic liquid-based route to microcrystals of lanthanide fluorides and their photoluminescent properties, J. Alloys Compd. 601 (2014) 195–200.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Novel ionic liquid-assisted hydrothermal method for the assembly of luminescent lanthanide fluorides with controllable morphologies Aiqi Wang b, Zhi Zeng b,⁎, Jinwei Gao d, Qianming Wang a,b,c,⁎⁎ a
Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, 510006, China d Institute for Advanced Materials, Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China b c
a r t i c l e
i n f o
Article history: Received 8 May 2015 Received in revised form 1 September 2015 Accepted 16 October 2015 Available online xxxx Keywords: Nanoparticle Luminescence Ionic liquid Lanthanide
a b s t r a c t An amphiphilic ionic liquid (diallyldimethylammonium tetrafluoroborate) assisted hydrothermal method has been applied for the synthesis of lanthanide fluoride nanoparticles. It has been carried out under mild conditions and large scale amounts of samples can be facilely achieved by this efficient way. Luminescence features of the phosphors were studied by steady state fluorescence and the samples could present striking 5D0 → 7F1 transition with orange-red color, suggesting that Eu3+ ions are located in the crystal lattice with inversion symmetry. The particle size distribution was investigated by scanning electron microscope and the results showed that different microstructures such as nano-rods and nanoplates were obtained. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) have been paid much attention for the synthesis of inorganic nano-materials owing to the outstanding properties like good thermal stability, negligible vapor pressure and extremely high ionic conductivity. They can not only play as solvents, but also use as template and ligand reagent [1]. Excellent capability for the dissolution and stabilization of metal cations were shown by these amphiphilic compounds, which will benefit the homogeneous synthesis of nano-sized inorganic materials [2]. Due to intrinsic structure and excellent luminescent performances, rare earth materials have been extensively studied [3–6]. As a wellknown luminescent host, lanthanide fluorides (LnF3) have great advantageous features in comparison with the conventional oxygen-based system such as low vibration energies, good optical transparency over a wide wavelength range and minimal quenching of the excited state of doped ions [7]. Up to now, most researchers have tried NaF, NH4F, NH4BF4, NaBF4 or KBF4 as fluoride sources [8]. Though these inorganic reagents are easily accessible, ILs still demonstrated the promising property of multi-functional positions in the reaction [9–13]. Recently, a few teams have reported the fabrication of lanthanide solid state materials in
the presence of polymerizable ionic liquid under irradiations [9,14]. This important organic salt (here is diallyldimethylammonium tetrafluoroborate ([DADMA]BF4)) with environmentally benign features could be utilized as the structure directing reagent. It displays great potentials in optical application and phosphor industry. BF− 4 acted as a good replacing source of F− upon decomposition. In addition, DADMA+ moiety is commercially available and easily polymerizable cation [14]. In brief, this specific ionic liquid molecule is a preferable fluoride donor and efficient synthesis of lanthanide fluorides could be possibly realized by this way. The main advantage of choosing this ammonium cation type ionic liquid in the inorganic synthesis is its effectiveness for the dissolution and combination of metal salts, which permits it with the behavior of acting as surfactants. In addition, its melting point and viscosity were not very high. Therefore, in our work, [DADMA]BF4 was incorporated to assemble LnF3:Eu3+(Ln = Y, Gd, La) nanoparticles. Different controllable morphologies were achieved and the photoluminescence properties of the nanocrystals were also discussed in detail.
2. Experimental 2.1. Preparation
⁎ Corresponding author. ⁎⁎ Correspondence to: Q. Wang, Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China. E-mail address: [email protected] (Q. Wang).
http://dx.doi.org/10.1016/j.molliq.2015.10.033 0167-7322/© 2015 Elsevier B.V. All rights reserved.
Y2O3, La2O3, Eu2O3, Gd2O3, NaF and NaBF4 were purchased from Sigma-Aldrich Company. Diallyldimethylammonium chloride (60% aqueous) was provided by Aladdin. All the other reagents were purchased from Guangzhou Chemical Reagent Factory and used without
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further purification. Lanthanide nitrates were obtained by dissolving their oxides in concentrated nitric acid. Ionic liquid and phosphor synthesis: diallyldimethylammonium chloride (5 g, 0.031 mol) in dichloromethane (10 ml) was added together with NaBF4 (3.4 g, 0.031 mol) and stirred under room temperature for 72 h. The white precipitate was removed by filtration. The clear solution was washed with small quantities of water to remove remaining chloride. The solvent was removed in vacuo and the clear ionic liquid ([DADMA]BF4) was dried at 80 °C under dynamic vacuum for 24 h. Following procedures were the assembly of phosphors, 5 ml of 0.1 mol/l Ln(NO3)3 and 2.5 ml of 0.01 mol/l Eu(NO3)3 were mixed. Subsequently, the [DADMA]BF4 (0.1 M)solution was added dropwise to the previous solution under magnetic stirring. Until a homogeneous solution was formed, the above mixed solution was transferred to Teflon-toclave and maintained at 160 °C for 10 h. After naturally cooling down to room temperature, the product was collected by centrifugation and dried in vacuum at 70 °C. Similar synthesis procedures were carried out to prepare YF3 nanocrystals through employing other fluoride sources (NaF and NaBF4). The determination of Eu(III) was briefly described as follows: 0.5 g sample, 3 ml HF, 7 ml HNO3, 3 ml HCl and 2 ml H2O2 were transferred into the tetrafluorometoxil reaction vessels and microwave assisted digestion was performed. After cooling, the resultant sample was diluted with ultrapure water to final volume of 50 ml. 115In (for Eu) was used as internal standard elements. Corresponding ICP-MS analysis results were added in Table 1. 2.2. Characterization The X-ray powder diffraction was investigated on Bruker D8 diffractometer with Cu Kα radiation (k = 0.1541 nm) in the range of 2θ = 10–90. Scanning electron microscope (SEM) was measured with JSM6360LV. Luminescence spectra were measured on an Edinburgh FLS920 spectrometer. The crystal phase identification of the obtained samples was examined using a MDI Jade 5.0 system. Inductively coupled plasma mass spectrometry (abbreviated as ICP-MS) was carried out on an Agilent7500a (Agilent, USA). The samples were digested in a Mars 5 microwave reaction system (CEM Corporation, USA). Overall quantum yields were detected at room temperature based on an integrating sphere (Edinburgh FLS920 spectrometer) method and the sample was placed inside the integrating sphere. The excitation light was entered into the sphere by the optical fiber. When specific excitation wavelength (394 nm) was selected, the quantum yield values were calculated automatically. 3. Results and discussion X-ray diffraction analysis is an effective way for the identification of the composition of the phosphors. It has been considered as the reliable method to determine the crystalline structures of different compounds. In order to study the feasibility of the ionic liquid as the precursor, we assembled a group of YF3:Eu3 + phosphors by using various fluoride sources such as NaF, NaBF4 and [DADMA]BF4. The collected results supported that the materials crystallized well (Fig. 1). Peak positions
Table 1 Elemental measurements of LaF3:Eu3+ (sample 1), GdF3:Eu3+(sample 2) and YF3:Eu3+ (sample 3) by ICP-MS after microwave digestion (n = 6). Sample
Certified concentration (mg/g)
Added concentration (mg/g)
Experimentally measured concentration by ICP-MS (mg/g)
Relative standard deviation (%)
1 2 3
37.30 34.22 49.45
10 10 10
47.44 44.38 59.26
2.2 1.9 2.0
Fig. 1. XRD patterns of orthorhombic phase YF3:Eu3+(F = NaF,NaBF4,[DADMA]BF4). The reference pattern for orthorhombic YF3 from JCPDS file is also included.
and intensities of the three samples were analogous and consistent with the pure orthorhombic phases as indicated in the JCPDS card (No. 70-1935). It demonstrated that [DADMA]BF4 could replace conventional fluoride source like NaF in the fabrication of europium doped phosphors. For the sake of confirming the reliability of ionic liquid, we carried out photoluminescence measurements concerning the three optical materials (Fig. 2). The typical intense transitions observed in the emission spectra were derived from the 5D0 level to the sublevels of 7F1, 7 F2, 7F3 and 7F4. There were no apparent changes among the three samples, showing that the ionic liquid would act as an alternative precursor to prepare the desired solid state materials. The variation of lanthanide emission curves and shapes was also studied in a series of inorganic hosts including LaF3, GdF3 and YF3. Fig. 3 gave the excitation (monitored at 593 nm) and emission spectra (excited at 394 nm) of LaF3 assembled under hydrothermal conditions. The excitation band consists of a group of sharp peaks covering from 300 nm to 500 nm due to the direct transitions from ground state into higher excited states from the europium f-electrons. The characteristic peaks in terms of f-f transitions in Eu3+ were attributed to 7F0 → 5H6 (317 nm), 7F0 → 5D4 (361 nm), 7 F0 → 5G2(383 nm), 7F0 → 5L6 (394 nm), 7F0 → 5D3 (414 nm), 7 F0 → 5D2 (464 nm) and 7F0 → 5D1 (525 nm). Under excitation at 394 nm, it can be observed that the derived narrow peaks were corresponding to 5D0 → 7F1 (593 nm), 5D0 → 7F2 (615 nm), 5D0 → 7F3 (650 nm) and 5D0 → 7F4 (702 nm) transitions. It has been accepted that 5D0 → 7F2 transition (615 nm) is a forced electric dipole transition which would be prevalent due to small deviation from inversion symmetry. 5D0 → 7F1 transition (593 nm) is the magnetic dipole transition which is insensitive to environmental differences. In this research, it is clear that europium ion occupies in the crystal lattice a site with inversion symmetry [15]. The results indicated that 5D0 → 7FJ emissions were very favorable to investigate the transition probabilities of sharp spectral features of lanthanide elements. In these lanthanide fluorides, 5D0 → 7F1 emission peak was dominating and transition to level with even J values (J = 2) was forbidden. When Gd or Y takes place of La in the matrices, the majority bands were also identified (Figs. 4 and 5). It demonstrated that the 4f orbitals of Eu3+ are hardly affected by the crystal field and host lattices because of the shielding effect of the 5s25p6 electrons. It is known that the size, shape and positioning of the microstructures were very attractive for potential applications in optical fields. We have carried out SEM measurements to have a better understanding of the morphological results (Figs. 6–8). The images showed a very high yield of the regular shaped nanostructures. YF3 and GdF3 exhibited one
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Fig. 2. Excitation and emission spectra of the Eu3+-doped YF3 prepared with NaF, NaBF4 and [DADMA]BF4 as fluoride sources.
Fig. 3. Excitation and emission spectra of the Eu3+-doped LaF3.
dimensional nanorods with sizes of 100 nm (Figs. 6 and 7). A coin like structure with a diameter of 80–100 nm was identified in LaF3 (Fig. 8). In this work, ionic liquid has been attached on the surface of the crystal seeds based on the coulomb interaction. The electrostatic forces between electrically charged ions (such as lanthanide cations, BF− 4 and
DADMA+) might contribute to the spontaneous aggregation of smaller granules into well-defined nanostructures. With the aim of clarifying the formation strategy, LaF3:Eu3 + has been prepared based on the same hydrothermal conditions (by using NaF and NaBF4 as the reagents) in the absence of [DADMA]BF4 (Figs. 9 and 10). The images
Fig. 4. Excitation and emission spectra of the Eu3+-doped GdF3.
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A. Wang et al. / Journal of Molecular Liquids 212 (2015) 799–803
Fig. 5. Excitation and emission spectra of the Eu3+-doped YF3.
clearly showed the morphology of material structures that was highly affected by changing the ionic liquid stabilizer. When NaF and NaBF4 were used, irregular clusters were predominant and disordered aggregates were observed. In this sense, it has been found that the selective adsorption on the crystal faces through ionic liquid would be very
important in the post-growth treatment due to the surface energy control on the LnF3 nanoparticles [16]. In addition, the nucleation and growth of the nanostructures during hydrothermal synthesis has been realized through dissolution–reprecipitation. The solubility that was improved by the ionic liquid would be helpful for the direction growth.
Fig. 6. SEM image of Eu3+-doped YF3 prepared by [DADMA]BF4 as fluoride source.
Fig. 8. SEM image of Eu3+-doped LaF3 prepared by [DADMA]BF4 as fluoride source.
Fig. 7. SEM image of Eu3+-doped GdF3 prepared by [DADMA]BF4 as fluoride source.
Fig. 9. SEM image of Eu3+-doped LaF3 prepared by NaF as fluoride source.
A. Wang et al. / Journal of Molecular Liquids 212 (2015) 799–803
803
Acknowledgments Q. M. appreciates National Natural Science Foundation of China (No. 21371063) excellent university young scholar fund of Guangdong province (Yq2013053), Science and Technology Project in Guangzhou (2014J4100054) and Guangdong Science and Technology Plan (2013B010403025). J. W. thanks the supports from the Guangdong Province Fund (2014B09091505).
References
Fig. 10. SEM image of Eu3+-doped LaF3 prepared by NaBF4 as fluoride source.
The dissolution has been enhanced by the hydrothermal conditions, while the diffusivity of the dissolved species was increased by the reduced viscosity of water. Furthermore, the shape of the prepared samples changed from nanorods to a nano-disk like structure, which presented that the ionic radius of different lanthanide elements had a significant effect on the morphology of the fluorides. The oriented attachment results might be useful in the controllable synthesis in the direction of crystal growth. Moreover, we have performed the quantum yield measurements for the three samples (LaF3:Eu3+, GdF3:Eu3+ and YF3:Eu3+). The results showed that the efficiency of particle-shape material (LaF3:Eu3+, 10.3%) was relatively higher than the other two materials (GdF3:Eu3 +, 7.9% and YF3:Eu3 +, 5.7%). It is estimated that the regular disc-like structure may have stiff lattices and could effectively suppress the non-radiative transitions. 4. Conclusion Luminescent LnF3:Eu3+ nano-sized phosphors were synthesized via the novel ionic liquid tunable-based approach. In contrast to normal coprecipitation and sintering treatment, the reactive lanthanide salts can be miscible together due to the incorporation of ionic liquid. Photoluminescence studies showed the derived materials that exhibited red emissions even under visible light excitations which will be very favorable for optical applications. It has been proved that the ionic liquid acted as the role of capping reagents and induced the assembly and evolution of nano-particles. Interestingly, we also identified different morphologies of lanthanide fluorides and nano-rods and nanoplates were achieved. The research may open a new start for the fabrication of lanthanide phosphors and establish structure–property relationship.
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