Controlled synthesis and formation mechanism of sodium yttrium fluoride nanotube arrays

JOURNAL OF RARE EARTHS, Vol. 30, No. 12, Dec. 2012, P. 1260

Controlled synthesis and formation mechanism of sodium yttrium fluoride nanotube arrays TIAN Li (田 俐)1,2, TAN Li (谭 丽)1, SUN Qiliang (孙起亮) 1, XIANG Shaobin (向绍斌)1, XIAO Qiuguo (肖秋国)1, TANG Jianting (汤建庭)1, ZHU Guangshan (朱广山)2 (1. School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China; 2. State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China) Received 21 August 2012; revised 11 October 2012

Abstract: Cubic and hexagonal sodium yttrium fluoride were successfully synthesized from yttrium nitrate, sodium fluoride and polyethanediol in propanetriol solvent under a facile hydrothermal route. By regulating the molar ratio of yttrium and fluoride, hydrothermal temperature and reaction time, the phase and shape of sodium yttrium fluoride were commendably controlled. The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDS) techniques. It was revealed that the hollow-structured Na(Y1.5Na0.5)F6 nanotubes self-assembled and arrayed orientedly to be bamboo raft-shaped. The formation of hexagonal Na(Y1.5Na0.5)F6 nanotube arrays was attributed to solid-liquid-solid process and Oswald ripening. This study provided a simple method to prepare hexagonal bamboo raft-shaped Na(Y1.5Na0.5)F6 on a large scale, which broadened their practical applications. Keywords: hydrothermal synthesis; fluorides; self-reassembly; nanomaterials; rare earths

The fabrication of inorganic functional materials with shape and phase control has received plenty of interest due to their special properties and potential applications in photoelectric device, catalysis, gas sensor, luminescent crystal, and so on. It is true that the properties of functional materials can be heavily influenced by their chemical components and phase structures, but also by their crystalline, and by the particle size and shape on the part of micro-crystalline powders. To date, a great deal of endeavor has been made on renewal of preparative technique in order to tune the microstructure and modulate the performances of functional materials[1–5]. Sodium yttrium fluoride (NaYF4) is one of the most valid up-conversion phosphor hosts[6–17]. Increasing attention has been paid to the preparation of sodium yttrium fluoride crystals in the last ten years, in particular for hexagonal β-form denoted as Na(Y1.5Na0.5)F6, which has been detected to be much moral efficient up-conversion phosphor host than its cubic α-polymorph denoted as Na0.41Y0.59F2.18[6]. For instance, α-NaYF4 were synthesized by co-precipitation at room temperature[7], and high quality β-NaYF4 nanomaterials were grown via thermal decomposition by several research groups[8]. But the latter requires high temperature and the byproducts are toxic. It is indicated that α-NaYF4 is the kinetic product and precipitates first in these synthetic routes and stable β-NaYF4 is thermodynamical product with progressive phase transformation under further heat treat-

ment[9–11]. As a result, the complete precipitation of β-NaYF4 from initially as-formed α-nanocrystals requires quite forcing conditions. In this study, β-NaYF4 arrays self-assembled from nanotubes were prepared under facile hydrothermal conditions with the addition of polyethanediol and propanetriol, which provided a simple method to obtain hexagonal bamboo raftshaped Na(Y1.5Na0.5)F6 on a large scale. And the formation of hexagonal Na(Y1.5Na0.5)F6 nanotube arrays was investigated detailedly for broadening their practical applications.

1 Experimental 1.1 Preparation of Na(Y1.5Na0.5)F6 nanotube arrays Sodium fluoride (NaF), yttrium oxide (Y2O3, 99.99%), polyethanediol (PEG) and propanetriol were of analytical grade and used as purchased from Shanghai Chemical Reagent Company. All materials were used without further purification. Yttrium oxide was dissolved in nitric acid by heating to obtain the stock solution of yttrium nitride. In a typical synthesis, 2 ml of yttrium nitride (0.20 mol/L) and 0.05 g PEG were introduced into 4 ml of propanetriol, followed by adding 4 ml of NaF solution (0.6 mol/L), then stirred vigorously for 0.5 h and transferred into a 20 ml Teflon-lined stainless autoclave. The resulting outcome was

Foundation item: Project supported by National Natural Science Foundation of China (51202066), China Postdoctoral Science Foundation Project (20100480947, 201104510), Scientific Research Found of Hunan Provincial Education Department (12A047), State Key Laboratory Program of Inorganic Synthesis and Preparative Chemistry of China (2013-26) and Doctoral Start-up Research Fund of Hunan University of Science and Technology (E51079) Corresponding author: TIAN Li (E-mail: [email protected]; Tel.: +86-731-58290147) DOI: 10.1016/S1002-0721(12)60217-8

TIAN Li et al., Controlled synthesis and formation mechanism of sodium yttrium fluoride nanotube arrays

cooled down naturally at room temperature after hydrothermal reaction at 220 ºC for 10 h. Precipitates were collected by centrifugation, and rinsed by distilled water and anhydrous ethanol in turn several times. After drying at 60 ºC in an oven, the final products were gained and dried in a desiccator for further characterizations. 1.2 Characterization The crystallinity and crystal structure of the products were examined by an X-ray diffractometer (XRD, Rigaku D/max 2200 VPC) with Cu Kα radiation (λ=0.15418 nm) and the scanning rate of 10o/min. The morphology of the products was characterized by thermal field environment scanning electron microscopy (SEM, JEOL JSM-6330F, 40 kV) with gold coating. The composition of the products was analyzed by an energy dispersive X-ray spectrometer (EDS, Oxford ISIS-300, 40 kV).

2 Results and discussion The phase of the synthesized Na(Y1.5Na0.5)F6 nanotube arrays was identified by XRD characterization. The Na(Y1.5Na0.5)F6 nanotube arrays were prepared with the Y:F ratio of 1:6 at 220 ºC for 24 h. As shown in Fig. 1, every diffraction peak is in well accord with the standard diffraction patterns of hexagonal sodium yttrium fluoride (JCPDS 160334), the characteristic peaks at 2θ of 17.20°, 30.06°, 30.78°, 34.83°, 39.67°, 43.49°, 46.61°, 52.04°, 53.28°, 53.75°, 55.19°, 61.12°, 62.35°, 63.73° and 65.24° are corresponding to (100), (110), (101), (200), (111), (201), (210), (002), (300), (211), (102), (112), (220), (202) and (310) crystallographic nucleation planes of hexagonal Na(Y1.5 Na0.5)F6 phase, respectively. There is no characteristic diffraction peaks detected for other impurities, showing the high purity of the as-prepared products. In addition, the peaks of (100) and (110) planes at 2θ=17.20° and 2θ=30.06° indicate fine preferential growth directions of Na(Y1.5Na0.5)F6 nanotube arrays. The EDS measurement data show that the as-prepared products are

Fig. 1 XRD patterns (a) Standard Na(Y1.5Na0.5)F6 (JCPDS No.16-0334); (b) As-prepared hexagonal sodium yttrium fluoride nanotube arrays prepared with the Y:F ratio of 1:6 at 220 ºC for 24 h

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composed of sodium, yttrium and fluorine elements with the Na:Y:F ratio of 1:1:4, which is in good agreement with the XRD measurement result. Fig. 2 shows phase compositions of the samples prepared with the Y:F molar ratios of 1:4, 1:5 and 1:6. Cubic sodium yttrium fluoride, namely α-NaYF4, was obtained when the molar ratio of Y(NO3)3 and NaF is 1:4. As shown in Fig. 2(1), the diffraction peaks are in good agreement with the values in the standard card (JCPDS 39-0724), with the space group of Pm3m (225) and the cell parameter of 0.5503 nm. With the change of the Y:F molar ratio to 1:5, cubic sodium yttrium fluoride, denoted as (Na0.41Y0.59F2.18), and hexagonal sodium yttrium fluoride, denoted as Na(Y1.5Na0.5)F6, could been concomitantly produced for 24 and 48 h reactions (Fig. 2 and 4). If the Y:F molar ratio is modulated to 1:6, diffraction peaks of cubic sodium yttrium fluoride (Na0.41Y0.59 F2.18) were not discerned in the sample obtained for 18 h, as shown in Fig. 2(4). And the product is phase-pure hexagonal sodium yttrium fluoride. SEM images of the products prepared with different Y:F molar ratios at 220 ºC for 24 h are shown in Fig. 3. It could be seen that cubic sodium yttrium fluoride (Na0.41Y0.59F2.18) was produced when the Y:F molar ratio was 1:4, shown in Fig. 3(a). With the change of Y:F molar ratio to 1:5, bamboo raft-shaped hexagonal Na(Y1.5Na0.5)F6 could be observed accompanying with abundant cubic Na0.41Y0.59F2.18 nanoparticles (Fig. 3(b)), which is coincident with the XRD analytical results (Fig. 2(b)). When the Y:F molar ratio was adjusted to 1:6, uniform hexagonal bamboo raft-shaped Na(Y1.5Na0.5)F6 arrays was found, as shown in the low-magnification SEM image in Fig. 3(c1). Seen from the high-magnification SEM images of the bamboo raft-shaped Na(Y1.5Na0.5)F6 arrays in Fig. 3(c2–c4), the as-prepared Na(Y1.5Na0.5)F6 are composed of numerous nanotubes with similar micrometer-sized dimensions (Fig. 3(c2)) and hollow interiors (Fig. 3(c3) and Fig. (c4)). It is indicated that hexagonal sodium yttrium fluoride arrays consist of hollow-structured nanotubes self-assembled side by side and arrayed orientedly to take on bamboo raft morphology. And the even internal aperture of

Fig. 2 XRD patterns of samples prepared at 220 ºC (1) Y3+:F=1:4 for 24 h; (2, 3) Y3+:F=1:5 for 24 and 48 h, respectively; (4) Y3+:F=1:6 for 18 h (the dots denote the diffraction peaks of α-NaYF4)

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Fig. 3 SEM images of the samples prepared at 220 ºC for 24 h (a) Y3+:F=1:4; (b) Y3+:F=1:5; (c) Y3+:F=1:6

Na(Y1.5Na0.5)F6 nanotubes is about 300–400 nm and wall thickness is 50–100 nm. It is believed that the novel oriented arrays and hollow structures of Na(Y1.5Na0.5)F6 functional materials would optimize the photoelectric performances and have potential applications in photoelectric crystals. Further research is underway. In order to understand the formation mechanism of bamboo raft-shaped hexagonal Na(Y1.5Na0.5)F6, further timedependence and temperature-dependence experiments were carried out. Keeping the Y:F molar ratio of 1:6, the products were yielded at 180 and 220 ºC for different reaction times. Fig. 4(1–3) shows the progress of the reaction at 140 ºC after 24, 48 and 72 h respectively, indicating the commensal outcomes of cubic sodium yttrium fluoride and hexagonal sodium yttrium fluoride with β-NaYF4 formation increasing over time. After the reaction at 180 ºC for 72 h, phase-pure hexagonal sodium yttrium fluoride obtained (Fig. 4(3)). The XRD patterns of the products with the reaction at 220 ºC for 5, 10, 18 and 48 h respectively are shown in Fig. 4(4–7). With the reaction time increasing from 5 to 48 h, cubic

Na0.41Y0.59F2.18 translated into hexagonal Na(Y1.5Na0.5)F6 gradually. After the reaction at 220 ºC for 18 h, cubic Na0.41Y0.59F2.18 disappeared completely and hexagonal Na(Y1.5Na0.5)F6 is found to be the exclusive product, shown as Fig. 4(6). As anticipated, the reaction rate is accelerated so that the hexagonal Na(Y1.5Na0.5)F6 product is phase pure after 18 h. Fig. 5 presents typical SEM images of the products yielded at 180 and 220 ºC for different reaction times. It could be seen that solid irregular rod-like Na(Y1.5Na0.5)F6 was obtained at 180 ºC after 72 h, with scattered nanoparticles falling out on its surface (Fig. 5(a)). After the reaction at 220 ºC for 5 h, uniform cubic sodium yttrium fluoride nanoparticles were produced, as shown in Fig. 5(b). With the reaction time increasing to 10 h, cubic sodium yttrium fluoride nanoparticles and hexagonal sodium yttrium fluoride arrays coexisted (Fig. 5(c)), matching with the XRD data (Fig. 4(5)). When the reaction time increases to 18 h, uniform hexagonal sodium yttrium fluoride arrays grow on a large scale, shown as in Fig. 5(d). Based on the above analysis, it is suggested that hexagonal sodium yttrium fluoride arrays form in a three-step process. (1) At the primary reaction stage or low temperature, cubic sodium yttrium fluoride (Na0.41Y0.59F2.18) nanoparticles were yielded. It has been well recognized that the kinetic product, i.e. cubic Na0.41Y0.59F2.18, is formed as the intermediate phase at earlier solution growth stage. That is the growth of cubic Na0.41Y0.59F2.18 is related to a liquid to solid (LS) mechanism. (2) With the temperature increased or the reaction time prolonged, the kinetic intermediate Na0.41Y0.59F2.18 nanoparti-

Fig. 4 XRD patterns of the samples (1–3) 180 ºC for 24, 48 and 72 h; (4–7) 220 ºC for 5, 10, 18 and 48 h

Fig. 5 SEM images of the samples prepared under different conditions (a) 180 ºC, 72 h; (b) 220 ºC, 5 h; (c) 220 ºC, 10 h; (d) 220 ºC, 18 h

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Fig. 6 Schematic illustration showing the growth behavior and hollowing process of the hexagonal bamboo raft-shaped Na(Y1.5Na0.5)F6 arrays, prepared at 220 ºC for 24 h

cles slowly transforms to the thermodynamically stable bamboo raft-shaped Na(Y1.5Na0.5)F6 polymorph through redissolution and re-growth process. That is the growth of hexagonal Na(Y1.5Na0.5)F6 related to a solid (re-dissolution of cubic polymorph) to liquid (mass transfer) to solid (renucleation and re-growth for hexagonal polymorph) mechanism, namely SLS process. Generally, owing to a slow conversion from solid cubic Na0.41Y0.59F2.18 to solid hexagonal Na(Y1.5Na0.5)F6 on kinetic opinion, the as-grown cubic Na0.41Y0.59F2.18 could be preserved for a much longer time and even phase-pure cubic Na0.41Y0.59F2.18 can be readily gained at a much lower reaction temperature or shorter reaction time. (3) The bamboo raft-shaped Na(Y1.5Na0.5)F6 arrays hollowed gradually due to Oswald ripening[16]. And finally, hexagonal bamboo raft-shaped Na(Y1.5Na0.5)F6 arrays consisting of hollow-structured nanotubes formed with the increased reaction temperature and the prolonged reaction time. The formation of the hexagonal bamboo raft-shaped Na(Y1.5Na0.5)F6 arrays and the hollowing process are schematically illustrated in Fig. 6.

3 Conclusions Cubic and hexagonal sodium yttrium fluoride was synthesized through a controllable hydrothermal routine in propanetriol solvent with yttrium nitrate and sodium fluoride as raw materials. By regulating the molar ratio of yttrium and fluoride, hydrothermal temperature and reaction time, the phase and shape of sodium yttrium fluoride was commendably controlled. At the primary reaction stage or low temperature or with high molar ratio of yttrium and fluoride, cubic intermediate sodium yttrium fluoride nanoparticles were yielded, and then transformed to the thermodynamically stable hexagonal Na(Y1.5Na0.5)F6 arrays through a re-dissolution and re-growth process. The formation of hexagonal Na(Y1.5Na0.5)F6 arrays composed of self-assembled nano-

tubes was attributed to solid-liquid-solid process and Oswald ripening. This study provided a simple method to prepare bamboo raft-shaped Na(Y1.5Na0.5)F6 on a large scale, which broadened their practical applications. Acknowledgement: L. Tian is grateful to Prof. M. M. Wu at Sun Yat-sen University and Prof. Y. N. Liu at Central South University in China for helpful direction.

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