Er3+) nano- and microstructures

Optical Materials 34 (2012) 1007–1012

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Solvothermal synthesis and luminescence properties of NaYF4:Ln3+ (Eu3+, Tb3+, Yb3+/Er3+) nano- and microstructures Xuesong Qu a,b, Guohui Pan c, Hyun Kyoung Yang a, Yeqing Chen a, Jong Won Chung a, Byung Kee Moon a, Byung Chun Choi a, Jung Hyun Jeong a,⇑ a b c

Department of Physics, Pukyong National University, Korea Busan 608-737, South Korea Department of Physics, Changchun Normal University, Changchun 130032, China Institut de Recherche Interdisciplinaire (IRI, USR-3078), Parc de la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France

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Article history: Received 29 September 2011 Received in revised form 12 December 2011 Accepted 13 December 2011 Available online 20 January 2012 Keywords: Luminescence NaYF4:Ln3+ Solvothermal synthesis

a b s t r a c t NaYF4 and NaYF4 (Eu3+, Tb3+, Yb3+/Er3+) nano- and microstructures were successfully synthesized through a facile and effective polyol-mediated route with ethylene glycol (EG) as solvent. The factors including the molar ratio of F/Y3+, reaction temperature and reaction time were well studied in the influences on size, morphology and phase transition of samples. The experimental results indicate that the F/Y3+ ratio and reaction temperature play key roles in crystal phase and morphology of the product. Higher molar ratio of F/Y3+ and higher temperature as well as longer reaction time facilitate a cubic structure-to-b hexagonal structure phase transition, which behaves as a dissolution–recrystallization transformation process. Photoluminescence measurements demonstrated that the as-prepared b-NaYF4 is one good up- and downconversion luminescent host material. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction In the past several decades, rare-earth ion-doped fluorides based on ALnF4 (A = alkaline metal, Ln = RE element) system with unique luminescent, ferromagnetic, insulating/magnetic, and piezoelectric properties have drawn particular attention because of their wide applications as solid electrolytes, solid-state lasers, and upconversion (UC) or downconversion (DC) hosts [1,2]. Among them, NaYF4, as one of the most efficient DC and UC host lattices, has attracted most of the attention in the field of materials science. It is well-established that hexagonal NaYF4 (b-NaYF4) is the most efficient UC host material known to date and has great potential applications in display, integrated optical system, solid state laser and bio-labels [3–5]. Dramatic research efforts have been devoted to its synthesis, characterization, and optical properties using various wet chemical routes such as solid-state treatment [6], modified co-precipitation [7,8], hydrothermal method [9–18], liquid–solid-solution (LSS) procedure [19,20], solvothermal methods [3], co-thermolysis method [21,22], and a liquid–solid two phase approach [23], and so forth. Up to the present, one of the particular interests is still lying on the exploration of new approaches for fabrication with controlled size/shape/phase and investigation of the underlying nucleation and growth mechanism [22]. Among various solution-phase routes, the organometallic and

⇑ Corresponding author. Tel.: +82 51 629 5564; fax: +82 51 629 5549. E-mail address: [email protected] (J.H. Jeong). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.12.010

its alternatives are relatively successful synthetic methods. Through the co-thermolysis of Na(CF3COO) and Y(CF3COO)3 in hot surfactant solutions, monodisperse a-NaYF4 and b-NaYF4 nanocrystals have been synthesized by Mai et al. at the same time, the growth mechanism and a-to-b phase transition were deeply discussed and a delayed nucleation was considered as a unique pathway for size monodispersity [22]. By liquid–solid two phase approach Chen’s group [23] got the oil-dispersible NaYF4 nanoplates. Through hydrothermal process, both a and b phase NaYF4 with different morphology such as nanocrystals [5,9,10], nanowires [11], microprisms [12–15], microrods [13,14,16], octadecahedron [14] and microtube [10] have been reported. The polyol process was initially used for the preparation of elemental metals and was then extended to synthesize nanoscale oxide, sulfide, and phosphate materials [24,25]. In such a system, a high-boiling alcohol such as glycerol or glycol, is always used and the surface of the nuclei is surrounded by polyol medium immediately after formation which limits the growth of particles and stabilizes them against agglomeration. For the fluoride materials, Wang et al. [26] prepared CeF3:Tb3+ nanoparticles with diameter of 7 nm by the polyol method and demonstrated that as-formed product could be well-dispersed in ethanol due to the polyol capping. a-NaYF4 nanoparticles and nanocubes were also reported by Wei et al. [27] and Qin et al. [28] respectively. Very recently, our group reported hexagonal NaCeF4 nanorods with different aspect ratios via polyol route and studied the shape-dependent PL properties [29]. In this work, we performed a systematic synthesis of a-NaYF4 nanocubes and b-NaYF4 microstructure using polyol

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2.2. Measurements and characterization X-ray diffraction (XRD) patterns of the samples were obtained on a Philips XPert/MPD diffraction system with Cu Ka radiation (k = 1.54056 Å). Field emission scanning electron micrograph (FESEM) images were taken on a Hitachi S-4200 electron microscope. Fluorescence and excitation downconversion spectra were recorded on a QuantaMaster™ 30 fluorometer (Photon Technology International (PTI)) at room temperature, equipped with a 60 W Xe-arc lamp and the high sensitivity multi-mode PTI photomultiplier tube (PMT) detector Model 914. The emission spectra were measured at a fixed band-pass of 0.25 nm, an excitation split of 0.3 nm and an emission split of 0.2 nm. In the upconversion experiments, a 978 nm continuous diode laser was employed to pump the samples; emission was then focused into a 75-cm monochromator and detected by a thermoelectrically cooled photomultiplier tube. The signal was finally fed to a digital oscilloscope. 3. Results and discussion 3.1. Effect of F/Y3+ ratio In this paper, NH4F was used as the F source for the preparation of NaYF4 samples. Different F/Y3+ ratios of 4:1, 5:1, 6:1, 7:1, 8:1, 12:1, 16:1, and 24:1, were selected to study the production of NaYF4. Fig. 1 depicts XRD patterns of as-prepared NaYF4 samples and the standard data for a-NaYF4 (JCPDS 77-2042) and b-NaYF4 (JCPDS 16-0334). It can be seen that the ratio of F/Y3+ could affect the crystal structure of the product greatly in our experimental conditions. The present series of samples tend to exhibit cubic phase with lower F/Y3+ ratio and hexagonal phase with higher F/Y3+ ratio. In the case of sample which is synthesized with F/ Y3+ ratio of 4:1, namely chemically stoichiometric, the sample exhibits the peak of pure crystalline cubic NaYF4 and no second

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All chemicals were of analytical grade and were used as received without further purification. In a typical procedure for the preparation of NaYF4 sample, 2 mmol Y(NO3)3 .6H2O and 2 mmol NaNO3 was dissolved in 20 mL of ethylene glycol (EG) solution. Afterward, 20 mL EG solution of 24 mmol NH4F was added into the above solution. After stirring for 30 min, the mixed complex precursor solution was transferred into a Teflon bottle of 70 mL held in a stainless steel autoclave, sealed, and maintained at 180 °C for 48 h. As the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence, and then dried in air at 70 °C for 12 h. In our experiments, different samples have been synthesized to study the relationship between reaction conditions and product morphology and phase. It should be pointed out that when the effect of one reaction condition was studied, the other reaction conditions were kept the same as those for the typical synthesis.

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method. The effects of the synthetic conditions including content of NH4F, reaction temperature and reaction time on the morphology and structure of the final products were studied in detail. At the same time, the formation process of microstructure b-NaYF4 resulting from a-to-b phase transition was also checked on the basis of temporal experiments. Furthermore, the DC and UC luminescence properties of Eu3+, Tb3+, and Yb3+/Er3+ have been studied in as-obtained b-NaYF4 samples.

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2 Theta (Degree) Fig. 1. XRD patterns of as-obtained samples synthesized with different F/Y3+ ratios ((a) 4:1, (b) 5:1, (c) 6:1, (d) 7:1, (e) 8:1, (f) 12:1, (g) 16:1, and (h) 24:1 mmol) and standard data of a-NaYF4 (JCPDS 77–2042) and b-NaYF4 (JCPDS 16–0334).

phase isdetected (Fig. 1a). Increasing the F/Y3+ ratio to 7:1, the XRD patterns has no obvious changes. When the ratio of F/Y3+ approached to 8:1, hexagonal NaYF4 started to form and a combination of cubic and hexagonal phase was present. Pure hexagonal NaYF4 was obtained until increasing the ratio of F/Y3+ to 12:1, the corresponding XRD pattern is shown in Fig. 1f. Upon further increasing the F/Y3+ ratio to 16:1 and 24:1, pure hexagonal phase NaYF4 were still prepared. It is worth pointing out that there is a difference from each other in the relative intensities based on (1 1 0), (1 0 0), (1 0 1), and (2 0 1) peaks for three b-NaYF4 samples (Fig. 1f–h), indicating the possibility of different preferential orientation growth under different F/Y3+ ratio conditions. Here it is also noted that the a-to-b phase transition process occurred during the present solvothermal treatment due to the sample without solvothermal process is the cubic phase. Base on the above XRD results, we can consider that the high molar ratio of F/Y3+ would benefit to the a-to-b phase transition and formation of hexagonal phase products under the present synthetic conditions. Moreover, in our experiment, the lowest temperature for formation of pure b-NaYF4 under different F/Y3+ ratios has been determined. The value of 220 °C, 200 °C, 180 °C and 150 °C is corresponding to the F/ Y3+ ratios of 8:1, 10:1, 12:1 and 16:1, respectively. This result further confirmed the conclusion that the F/Y3+ ratio plays an important role in the crystal phase of the final products and the presence of the excess of F ions favors the growth of the b-NaYF4 crystals. It is reported that that F ions were not only a reactant but also a mineralizer and lower the crystallization temperature remarkably in the other synthesis of the NaLnF4 crystals [30,31]. Fig. 2 and Fig. S1 present FE-SEM images of as-prepared NaYF4 samples synthesized with different F/Y3+ ratios. As can be seen from Fig. 2A and B and Fig. S1A and B, all a-NaYF4 samples consist of well-dispersed nanocubes with uniform size. The edge lengths of the nanocubes, corresponding to F/Y3+ ratios of 4:1, 5:1, 6:1 and 7:1, are about 30, 35, 55, and 65 nm, respectively. That is, the edge lengths of nanocubes increased with increasing ratio of F/Y3+. For the sample prepared at an 8:1 ratio of F/Y3+, besides nanocubes there existed a small amount of sub-microrods with length about 0.8 lm, as shown in Fig. 2C. When the F/Y3+ ratio is 12:1, the sample is of only irregular microrods about 2–3 lm in length without any nanocubes (see Fig. 2D). With increasing the ratio to 16:1 and 24:1, irregular microplate-like morphology with diameter of 1 lm was observed. Considering the evolutions shown in XRD and FESEM, we could index these microstructures to b-NaYF4. Note that the morphology of microstructure b-NaYF4 changed from microrod-like to microplate-like upon increasing the F/Y3+ ratios from 12:1 to 16:1. This result demonstrated that the F has a great

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Fig. 2. FE-SEM images of samples synthesized with different F/Y3+ ratios. (A) 4:1; (B) 7:1; (C) 8:1; (D) 12:1; (E) 16:1 (F): 24:1. (Scale bars are all 1 lm for inset).

influence on the morphology of b-NaYF4. Here we consider that it may be related to the selection adsorption of F to different faces. Lots of experimental results have demonstrated that selective adhesion of capping ligands onto specific crystal planes could control its growth rate and be of critical in the epitaxial growth of nanocrystals [29]. For a hexagonal crystal structure, the surfaces are typically {0 0 0 1} for top/bottom planes and a family of six  0g side planes (ð1 0 1  0Þ, ð1  0 1 0Þ, energetically equivalent f1 0 1  1 0), (0 1 1  0), (1 1  0 0), and (1  1 0 0)) on the basis of the known (0 1 [12]. According to the experimental results, the effect of F on the change of b-NaYF4 morphology can be explained as follows. Due to the coordination effect between F and Y3+, F inevitably capped on crystal surfaces of NaYF4 unit cell but the capping effect of F on the {0 0 0 1} crystal planes was greater than that on the  0} planes in our experimental conditions. When increasing {1 0 1 F/Y3+ ratio, the surface energy of the {0 0 0 1} crystal plane was decreased dramatically and hence the growth rate along {0 0 0 1} direction was markedly prohibited because of the stronger capping effect of F to such facets. In this case, the growth is driven along  0), (1  0 1 0), (0 1  1 0), (0 1 1  0), six symmetric directions: ((1 0 1  0 0), and (1  1 0 0)), which directly resulted in the formation of (1 1 b-NaYF4 microplates.

3.2. Effect of reaction temperature In this work, the effect of the reaction temperature on the crystal phase and morphology of the final products was also examined. The experimental result shows that the lower temperature favors the formation of the small nanoparticles of cubic phase, whereas higher temperature benefits the formation of hexagonal big microrods and microplates. Fig. 3A presents the XRD patterns of samples prepared at different temperature (180 °C, 200 °C, 220 °C) in the case of F/Y3+ ratio of 8:1. It can be seen that the fraction of hexagonal phase in the mixture increased remarkably with increasing the reaction temperature from 180 to 200 °C. When the temperature increased to 220 °C, pure hexagonal phase microrods were obtained. (Fig. 3B) Therefore, it can be concluded that high temperature could provide enough energy that is required the transformation from a to b phase to overcome the any barrier. 3.3. Effect of reaction time As is well known, the structure of NaYF4 exhibits two polymorphic forms, that is, cubic (a-) and hexagonal (b-). The cubic phase is a high-temperature metastable phase, and the hexagonal

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phase is a thermodynamically stable phase. However, a-NaYF4 tends to form first and then transforms to b-NaYF4 in most cases for solution-phase synthesis of NaYF4 [9]. In this work, we select F/Y3+ ratio of 12:1 as an example to investigate the phase transformation process. Fig. 4 shows the XRD pattern (A) as a function of the reaction time. The different XRD patterns at various reaction time indicated that the as-prepared b-NaYF4 underwent a phase transition from a-to-b process with time prolongation. The sample obtained at t = 2 h exhibits pure a phase (Fig. 4A(a)). When the time extends to 10 h (Fig. 4A(b)), a small amount of diffraction peaks characteristic of the b phase were detected. Further extending the

reaction time, the faction of b phase increased gradually on the basis of the XRD patterns. It took 40 h to transfer from the a phase to the b phase completely. At the same time, FE-SEM images collected from the corresponding reaction time were presented in Fig. 4B–F. At t = 2 h, SEM image reveals small cubic nanocubes with the edge length of about 80 nm. The morphology is the same as the sample prepared with lower F/Y3+ ratios (4:1–7:1). When the reaction is prolonged to 10 h, a small number of irregular sub-microstructures appeared. Base on the XRD results, it should correspond to the hexagonal phase NaYF4. With increasing the reaction time to 18 h, it can be seen that an amount of microrods with 1 lm in length were

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formed. Further increasing the reaction time to 28 h, the microrods become larger in both diameter and length and the microrods morphology is predominant. As the reaction time as long as 40 h, the a small nanocubes disappeared completely and only the b phase microrods with 2–3 lm in length could be detected from the SEM images. According to the above results, we can conclude that the a phase nanocubes are unstable upon increasing the reaction time in the present synthetic conditions and an enhanced Ostwald-ripening process [13,14,21,22,32,33], that is, dissolution and recrystallization for nanocubes takes place, and therefore more stable b-NaYF4 start to form. b-NaYF4 was obtained from a-NaYF4 monomers by both enhanced and restricted Ostwald-ripening process were reported in co-thermolysis of trifluoroacetate precursors system [21,22].

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Due to the lower phonon energies b-NaYF4 is regarded as an efficient host lattice for DC and UC process. In this work, we select Eu3+, Tb3+, and Yb3+/Er3+ as the doping ions to investigate the photoluminescence (PL) properties of the as-prepared b-NaYF4. Fig. 5 shows the room-temperature PL excitation (left) and emission (right) spectra of 2 mol% Eu3+ doped b-NaYF4. In the excitation spectrum, it consists of several sharp lines which are identical to the characteristic excitation lines of Eu3+ within 4f [6] configuration from 200 to 450 nm. In general, most of the excitation lines can be clearly assigned (319 nm, 7F0–5H6; 363 nm, 7F0–5D4; 382 nm, 7F0–5G2; 396 nm, 7F0–5L6; 418 nm, 7F0–5D3) [34,35]. The result is in agreement with the excitation spectra reported for Eu3+ doped other fluoride materials [36]. Upon excitation into the strongest 7F0–5L6 transition at 396 nm, it can be seen that the emission spectrum is composed of sharp lines associated with the Eu3+ transition from the excited 5D0–2 level to 7FJ (J = 1, 2, 3, 4) and the strongest emission line is at 617 nm. Generally the emission from higher levels 5D1–3 excited states of Eu3+ would be quenched by multiphonon relaxation if the phonon energy of host lattice is high enough [37]. Due to the lower phonon energy in b-NaYF4, the emission lines from 5D1 and 5 D2 states could therefore be observed in as-prepared b-NaYF4:Eu3+ (2 mol%) sample. Fig. 6 shows the excitation (left) and emission (right) spectra of b-NaYF4:Tb3+ (10 mol%) sample. In the excitation spectrum, a band in the range from 200 to 240 nm and series of characteristic f–f transition lines between 240 and 400 nm could be observed by monitoring the green emission of Tb3+ at 544 nm. The former are due to the spin-allowed 4f–5d transitions of the Tb3+ ions and the latter are attributed to the 4f–4f transitions in Tb3+ 4f8 configuration. The main excitation lines can be assigned, that is, 256 nm

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(7F6–5K9), 274 nm (7F6–5I7), 286 nm (7F6–5F5), 304 nm (7F6–5H6), 319 nm (7F6–5D0), 342 nm (7F6–5G2), 354 nm (7F6–5D2), 370 nm (7F6–5G6), and 379 nm (7F6–5D3), respectively [38]. Upon excitation into the 7F6–5D3 transition at 379 nm, the sample exhibits the green emission locating at 488, 544, 584, and 619 nm, respectively. These green-light emitting lines should be ascribe to transitions from the excited 5D4 level to the 7FJ (J = 3–6) levels of Tb3+ ions. Fig. 7 shows the UC spectrum for Yb3+ (10 mol%) and Er3+ (2 mol%) doped b-NaYF4 sample upon 978 nm excitation. In the spectrum the dominant green emissions in the range of 500– 580 nm correspond to the 2H11/2, 4S3/2–4I15/2 transitions and some red lines between 640 and 690 correspond to the 4F9/2–4I15/2 transitions. The emission spectrum is similar to those of Yb3+ and Er3+ codoped b-NaYF4 system reported previously [22]. 4. Conclusions

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In summary, we have demonstrated a polyol-mediated method for fabricating NaYF4 and NaYF4 (Eu3+, Tb3+, Yb3+/Er3+) nano/microstructure with EG as solvent. The influences of various factors including the molar ratio of F/Y3+, reaction temperature and reaction time on the phase structure and morphology have been demonstrated in detail. The experimental results indicate that the ratio of F/Y3+ and reaction temperature are crucial in controlling product morphology and crystal phase. The higher F/Y3+ ratio favors the growth of the bigger hexagonal NaYF4 microcrystals and lower the forming temperature accordingly. Moreover, the temporal experiments show that the dissolution and recrystallization is

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responsible to the a-to-b phase transition and determine the morphology and phase. Based on the analysis of the experimental results, it was found that the a-to-b phase transition was favored by the high molar ratio of F/Y3+ and high temperature as well as long reaction time. PL measurements demonstrated that as-prepared b-NaYF4 microcrystals are good UC and DC luminescent host materials. Acknowledgement This work was supported by the Pukyong National University Research Fund in 2010 (PK-2011-31). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.optmat.2011.12.010. References [1] R.T. Wegh, H. Donker, K.D. Oskam, A. Meijerink, Science 283 (1999) 663–666. [2] J.F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K.W. Kramer, C. Reinhard, H.U. Gudel, Opt. Mater. 27 (2005) 1111–1130. [3] J.L. Sommerdijk, J. Lumin. 6 (1973) 61–67. [4] H. Insley, G.M. Hebert, Inorg. Chem. 5 (1966) 1222–1229. [5] L.Y. Wang, R.X. Yan, Z.Y. Hao, L. Wang, J.H. Zeng, J. Bao, X. Wang, Q. Peng, Y.D. Li, Angew. Chem. Int. Ed. 44 (2005) 6054–6057. [6] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185– 1189. [7] J.S. Lee, Y.J. Kim, Opt. Mater. 33 (2011) 1111–1115. [8] G.S. Yi, H.C. Lu, S.Y. Zhao, G. Yue, W.J. Yang, D.P. Chen, L.H. Guo, Nano Lett. 4 (2004) 2191–2196. [9] J. Zhao, Y. Sun, X. Kong, L. Tian, Y. Wang, L. Tu, J. Zhao, H. Zhang, J. Phys. Chem. B 112 (2008) 15666–15672. [10] J. Zhuang, L. Liang, H.H.Y. Sung, X. Yang, M. Wu, I.D. Williams, S. Feng, Q. Su, Inorg. Chem. 46 (2007) 5404–5410.

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