Control of the morphology and composition of yttrium fluoride via a salt-assisted hydrothermal method

Abstract

Yttrium fluoride such as Na(Y_1.5Na_0.5)F₆ and α-NaYF₄; YF₃ has been successfully synthesized via a salt-assisted hydrothermal method in the presence of sodium nitrate. The morphology and chemical composition of the as-obtained products can be controlled by monitoring the reaction parameters inclusive of the time-zero concentration of salt solution, reaction temperature and time. The as-prepared products cover Na(Y_1.5Na_0.5)F₆, α-NaYF₄ and YF₃, while the corresponding morphology changes from spherical aggregation to octahedron, which were characterized by XRD, SEM, and TEM. Moreover, LnF₃ (Ln = La, Nd, Eu, Gd) can also be synthesized using the same method. Further studies reveal that the morphology and composition of the as-synthesized lanthanide fluoride can be determined mainly by the ionic radius of rare earth and the concentration of NaNO₃. It is emphasized that the sodium nitrate plays an important role in such control of the differentiated products. Apart from these findings, the photoluminescent properties of Eu³⁺:YF₃ octahedral crystals are further investigated.

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Introduction

It is of importance to control the inorganic crystals of uniform dimension and shape so as to obtain their differentiated properties.¹ To date, many chemical methods have been developed for the synthesis of varied inorganic crystals in well-defined shapes, including rods, whiskers and microboxes.^2–4 The commonly used methods take advantage of monitoring the nucleation and growth processes, for example, manipulating the controllable growth parameters using different molecular precursors and capping agents at reaction temperature for certain growth time. In general, organic additives functioning as the “shape modifiers” are favoring the control of the morphology and size of the crystals because they either promote or inhibit crystal growth through modifying the crystal facets dynamically.⁵ Besides, a cost-effective and environmentally-friendly synthetic method becomes more popular for obtaining such nano-crystals. Nowadays, one of the challenges facing the chemists and scientists is to develop a facile approach for the creation of controllable morphology.

Although organic surfactants or polymers appear to be effective in tailoring the crystal growth and the morphology of products through controlling the reaction rate and selectively altering the growth kinetics of different crystal facets, some organic additives are required for different purposes, which pose potential dangers of changing the properties of products. Therefore, it is indispensable to choose suitable and novel additives in the synthetic process. In the past several years, some attractive attempts have been made in the synthesis of nanomaterials through a salt-mediated process.⁶ The addition of salt avoids the occurrence of impurities and effectively controls the morphology of the final product. On the other hand, when a metal ionic compound (such as precursor) is dissolved in the aqueous solution, the metal ions are usually covalently bonded to other ions or water molecules because of their particular value of charge and some characteristic “combining power” according to Alfred Werner's proposal. The specific number of molecules or ions combined with a metal ion is now referred to as the coordination numbers of the element (typically four or six). The water molecules covalently bonded to the central metal ion are called hydrate ligands. It is worth mentioning that NO₃⁻ ions can replace water molecules and combine with the rare earth metal ion to form a new ligand in high NO₃⁻(∼8 M) ionic solution, which would further affect the chemical reactions.⁷ In the salt-mediated process, the effects of inorganic salts on the inorganic materials include: first, the inorganic ions (in the salt-assisted synthesis) are expected to have a more pronounced influence on the nucleation process of nanocrystallites than that of organic surfactants or polymers because of their smaller size, which can increase the ability to form complexes with reactive species and decrease the reaction rate of reaction ions in the solution; second, the addition of inorganic ions might also affect the growth stages through selectively altering the growth kinetics of different crystal facets of the final products, similar to the role of surfactants in controlling the formation of nanomaterials.⁸ Herein, sodium nitrate is chosen to synthesize rare earth fluorides and is proven to be useful for controlling the composition and morphology.

YF₃, a well-known functional rare earth fluoride, has been extensively studied and employed in various fields including lamps and display devices,⁹ phosphors,¹⁰ and scintillators.¹¹ Stimulated by both the promising applications and the interesting properties, much attention has been directed to the controlled synthesis of YF₃ nanostructures. Over the past few years, considerable progress has been made in the synthesis of YF₃ nanostructures of different shapes and in the investigation of their size/shape-dependent properties.¹² YF₃ crystals with different morphologies according to different procedures and synthetic techniques are obtained, such as truncated octahedral-, quadrilateral-, hexagonal-, spherical-, octahedral-, and bundle-like nanocrystals.^10,13 Of these methods, the hydrothermal method is by far the most reported for its merits. However, in previous reports about the nanosized YF₃ prepared by the hydrothermal method, some special organic surfactants or templates were used.^{12, 14} So, it is necessary to develop a facile, economical and effective method for preparing YF₃ with a controllable composition and morphology. On the other hand, though various morphologies of YF₃ have been obtained, the investigation of the relationship between different morphologies is still crucial for realizing the morphology-controlled synthesis.

In this paper, a salt-assisted hydrothermal method has been developed to control-synthesize yttrium fluorides, such as Na(Y_1.5Na_0.5)F₆ shuttles, α-NaYF₄ nanospheres, YF₃ particles with a spindle, octahedral and truncated octahedral shape. It is found that the composition and morphology of the obtained product can be controlled using inorganic species like sodium nitrate. Furthermore, other light rare earth fluorides (LaF₃, PrF₃, EuF₃, GdF₃) have also been prepared by this method.

Experimental

Synthesis of YF₃ nanooctahedron

All reagents (analytical grade) were purchased from Shanghai Chemical Reagent Company and used without further purification. In a typical procedure, yttrium nitrate hexahydrate (Y(NO₃)₃·6H₂O) (0.107 g, 0.28 mmol) was first dissolved into 28 ml NaNO₃ (7 M) solution, and then 0.035 g (0.84 mmol) NaF was added and stirred for 10 min. The clear and colorless solution was transferred into a 35 ml Teflon-lined autoclave, and then the autoclave was filled up to 80% of its total volume. Afterwards the autoclave was heated up to 140 °C and held for 20 h, then allowed to cool down to room temperature naturally. The obtained samples were collected after being centrifugally separated at 3500 rpm for 20 min, washed with deionized water and dried at 60 °C in air. The sample Eu³⁺ : YF₃ (molar ratio Eu³⁺/Y³⁺ = 1 : 9) was prepared using similar procedures. The detailed reaction parameters and the corresponding results are summarized in Table 1.

Table 1 All reaction parameters and the corresponding results^a

Sample	[NaNO3]/M	Time/h	Temperature/°C	Compounds	Morphology of product
a All samples were obtained with NaF and Ln(NO3)3·6H2O (Ln = Y, La, Nd, Eu, Gd) as precursors.
S1	0	20	25	YF3	nanobones
S2	0.1	20	25	YF3	nanospindles
S3	1	20	25	Na(Y1.5Na0.5)F6	nanoshuttles
S4	7	20	25	α-NaYF4	nanospheres
S5	7	20	60	α-NaYF4	nanospheres
S6	7	20	100	YF3	truncated and regular octahedra
S7	7	20	140	YF3	monodisperse octahedra
S8	7	1	140	α-NaYF4	nanospheres
S9	0	20	140	YF3	nanobones
S10	0.1	20	140	YF3	nanoplates
S11	3	20	140	YF3	nanoparticles
S12	7	20	140	LaF3	nanoplates
S13	7	20	140	NdF3	nanoplates
S14	7	20	140	EuF3	nanoplates
S15	7	20	140	GdF3	olive-like nanoparticles

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Structural characterization

The phase of as-prepared products was carried out using powder X-ray diffraction (XRD, Shimadzu XRD-6000) with Cu Kα radiation (λ = 1.5406 Å) 2θ from 10 to 80° at a scanning rate of 6° min⁻¹. The X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. The morphologies and structure of the samples were characterized using high-resolution transmission electron microscopy (HRTEM; JEOL, 2100F, with an accelerating voltage of 200 kV) and field-emission scanning electron microscopy (FESEM; JEOL, JSM-6700F with an accelerating voltage of 5 kV), respectively. The obtained solid samples were suspended in absolute ethanol by an ultrasonic treatment for 5 min. Then a drop of the well-dispersed suspension was trickled on a piece of carbon-coated copper grid, and the copper grid was air-dried in ambient conditions. Photoluminescence (PL) was investigated on Perkin Elmer LS50B photoluminescence spectrophotometer by dispersing samples in alcohol.

Results and discussion

LnF₃ (Ln = Y, La, Nd, Eu, Gd) was synthesized in a salt-assisted solution, as summarized in Table 1. A typical XRD pattern of the as-obtained products (sample 7) is shown in Fig. 1a. It can be seen that the pattern fits well with the orthorhombic type YF₃ [space group: pnma (62)] with lattice constants a = 6.353, b = 6.850 and c = 4.393 Å (JCPDS 74–0911). No peaks of any other phases or impurities can be detected, indicating its high purity. The chemical composition of the as-prepared product was further confirmed by EDX. It can be seen from Fig. 1b that only yttrium and fluorine are detected in the spectrum, and the atomic ratio of F to Y is 73.34 : 26.66, which is very close to the stoichiometric YF₃.

Fig. 1 (a) XRD pattern and (b) EDX spectrum of the YF₃ particles obtained at 140 °C for 20 h.

The as-obtained product was further characterized by field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). A typical SEM image of sample 6 is shown in Fig. 2a. The monodisperse YF₃ particles can be prepared on a large scale, and the as-synthesized particles possess a regular and well-defined octahedral shape. It is detected that YF₃ octahedra have triangular surfaces and sharp edges, and the edge length ranges from 3 to 3.5 µm (Fig. 2b). The bright field TEM image of an individual YF₃ particle also reveals that the as-obtained YF₃ is in rhombic shape (Fig. 2c). The diffraction spots are indexed to the (020) and (211) planes of orthorhombic YF₃, respectively, indicating that the YF₃ octahedron is a well-developed single crystal (inset of Fig. 2c). As illustrated in Fig. 2d, the 0.33 nm interplanar lattice shown in HRTEM image (recorded on the tip of the rhombus) corresponds to the (111) planes of YF₃, indicating that the well-defined octahedral YF₃ crystal is enclosed by eight {111} planes.

Fig. 2 Images of YF₃ particles prepared at 140 °C for 20 h: (a) low-magnification SEM image; (b) enlarge SEM image; (c) TEM image of an individual YF₃ octahedron, the inset gives the corresponding SAED; (d) HRTEM image of the YF₃ octahedron.

In the salt-assisted synthetic process, the concentration of NaNO₃, the reaction temperature and time have a great effect on the chemical composition and morphology of the final product. A series of condition-dependent experiments have been carried out to understand the characteristic effects of inorganic salt on the nucleation and crystal growth processes, and further to uncover the underlying mechanism for the controllable composition and shape.

The effect of NaNO₃ concentration

The influence of NaNO₃ concentration on the chemical composition and morphology of the final products was investigated. When the reaction was carried out at room temperature, a large quantity of uniform and dumbbell-like YF₃ (sample 1) structures, with 160 nm in diameter and 3.5 µm in length, could be obtained in the absence of NaNO₃ salt. The obtained nanobones consist of many nanowires with a small size (Fig. 3a). However, when 0.1 M NaNO₃ was added into the reaction system, spindle-like YF₃ (sample 2) particles were obtained with length ranging from 150 nm to 3.5 µm. They are still composed of bundles of small nanowires (Fig. 3b). Further increasing the concentration of NaNO₃ to 1 M, nearly monodiseperse Na(Y_1.5Na_0.5)F₆ (sample 3) nanoshuttles were obtained in large quantity (see the ESI, Fig. S1).† These shuttles of 1.2–2 µm in length actually comprised many little nanoparticles (Fig. 3c). When the concentration of NaNO₃ was increased to 7 M, monodisperse α-NaYF₄ (sample 4) nanospheres (Fig. 3d) formed (see the ESI, Fig. S2).† These nanospheres of 100–500 nm in diameter also aggregated from many primary nanoparticles.

Fig. 3 SEM images of the products prepared at room temperature for 20 h with different concentrations of NaNO₃: (a) YF₃, 0 M; (b) YF₃, 0.1 M; (c) Na(Y_1.5Na_0.5)F₆, 1 M; (d) α-NaYF₄, 7 M.

By increasing the reaction temperature up to 140 °C, YF₃ could be obtained even in the higher concentration of NaNO₃. However, the concentration of NaNO₃ still affected the morphology of the obtained products. When the reaction was carried out without NaNO₃, bone-like YF₃ (sample 9) nanostructures were obtained (Fig. 4a). In comparison with the sample prepared at room temperature, these uniform bone-like YF₃ particles are assembled by the original nanoparticles of about 400–600 nm in diameter and 700–1500 nm in length, respectively. By increasing NaNO₃ concentration to 0.1 M, the YF₃ nanoplates of 40–500 nm in size (sample 10) formed and aggregated loosely (Fig. 4b). When the concentration of NaNO₃ increased up to 3 M, irregular YF₃ particles (sample 11) conglomerated into larger particles in an octahedral shape (Fig. 4c). When a higher concentration of NaNO₃ (7 M) was used, monodisperse YF₃ octahedra were prepared on a large scale (Fig. 2a). To further confirm the uniqueness in changing the morphology and composition of YF₃, other salts such as KNO₃ and NaNO₂ at an equal molar concentration were used. It was found that no YF₃ or other pure compounds could be obtained, suggesting that these salts could not play the same role as did NaNO₃ in the synthesis process. The reasonable explanation for the uniqueness of NaNO₃ include: first, NaNO₃ can replace water molecules and attach the Y³⁺ ion to form a ligand; second, it can affect the reaction rate according to Le Chatelier's principle; third, the properties of NaNO₃ (such as the ionic radius, molecular weight, solubility, etc) have some influence on the synthesis.

Fig. 4 SEM images of YF₃ obtained at 140 °C for 20 h with different concentrations of NaNO₃: (a) 0 M; (b) 0.1 M; (c) 3 M.

In traditional crystal growth, both nucleation and growth get involved in the preparation of nanostructured materials. To meet the requirements of homogeneous nucleation, the reaction should be carried out at high temperature to surmount the energy barrier, and the source materials should reach high supersaturation to promote nucleation. After the formation of nuclei, the constituents will be incorporated to the existing nuclei with less energy than that for nucleation, and such post-growth would be largely controlled by the kinetic parameters adopted in the reaction system and responsible for the morphology variation.^15,16 Based on the above results, an acceptable explanation can be presented in the salt-assisted process. In a low salt concentration, the improvement of the reaction temperature may diminish the separation between the two stages. At room temperature, the rare earth ions are presumably more favorable to nucleation and oriented growth, resulting in formation of nanowires. On the contrary, when the reaction temperature increases, particle nucleation is fast followed by the aggregation of small particles caused by nanoparticles with reduced surface energy. This result proves that temperature is crucial to the shape of products in a low salt concentration. The higher concentration of salt would increase the ionic intensity and decrease the diffusion of ions or growing units, which would further reduce the net reaction rate according to Le Chatelier's principle and suppress the nucleation formation of YF₃ nanocrystal at room temperature. Thus YF₃ can be obtained with less NaNO₃ at lower reaction temperature or with more NaNO₃ at high reaction temperature (140 °C) because it is easy to reach a degree of supersaturation for the nucleation of YF₃. However, the thermodynamic stable hexagonal Na(Y_1.5Na_0.5)F₆ shuttle and cubic α-NaYF₄ nanospheres are apt for the formation at high NaNO₃ concentration and lower reaction temperature because they reach easily their supersaturation degree in advance in the micro-environment.^17–19 The formation of shuttles and nanospheres can be well understood by the reported two-stage growth model in which nanosized Na(Y_1.5Na_0.5)F₆ and α-NaYF₄ are nucleated first in supersaturated solution and then the initially formed original particles aggregate into large secondary particles.¹⁹ The above experiments clearly demonstrate the presence of NaNO₃ is beneficial for the control of both the shape and chemical composition of the products.

The effect of the reaction temperature

Under a high concentration of NaNO₃, the reaction temperature is also crucial to the composition and morphology of the final products. Monodisperse α-NaYF₄ (sample 5) nanospheres were obtained if the reaction temperature was lower than 60 °C (Fig. 5a). Their size is also similar to that of sample 4. However, YF₃ (sample 6) particles in truncated and regular octahedral shape, with an average edge size of about 700 nm in length, were obtained when the reaction was carried out at 100 °C under similar conditions (Fig. 5b). It can be observed that the truncated octahedra also have a smooth surface and sharp edges (inset in Fig. 5b). Regular and well-defined YF₃ octahedra could be obtained if the reaction temperature was further increased to 140 °C (Fig. 2a). The size of the obtained YF₃ is increased to 3–3.5 µm. On the basis of the above experiment, it is reasonable to conclude that the elevated reaction temperature accelerates the motion of ions and facilitates the nucleation of YF₃ between the two constituent ions in solution. These experiments further demonstrate the high motion/diffusion of ions is in favor of the formation of YF₃ and the NaNO₃ can effectively regulate its morphology.

Fig. 5 SEM images of the products obtained with 7 M NaNO₃ for 20 h at different reaction temperatures: (a) α-NaYF₄, 60 °C; (b) YF₃, 100 °C.

The effect of reaction time

To further investigate the role of NaNO₃ in the composition and shape of the as-prepared products, samples taken from different time intervals at a high NaNO₃ concentration (7 M) were characterized by XRD and TEM. The XRD patterns reveal that only α-NaYF₄ (sample 8) is obtained after 1 h. With increasing reaction time, YF₃ appears gradually and then α-NaYF₄ disappears. α-NaYF₄ completely transforms into YF₃ as the reaction time is up to 20 h (Fig. 6a). This result implies that a longer reaction time would be favorable for the formation of the YF₃ product at high NaNO₃ concentration. Similarly, the corresponding SEM images are consistent with the observation. Monodisperse α-NaYF₄ nanospheres are obtained when the reaction is 1 h (Fig. 6b). Then, truncated or regular octahedral YF₃ particles appear gradually with the reaction time increasing to 15 h and the size of YF₃ particles grows from 1 to 2 µm (Fig. 6c,d). As the reaction proceeds to 20 h, octahedral YF₃ particles are obtained with all nanospheres vanishing and the size of octahedral YF₃ particles in the range of 3–3.5 µm (Fig. 6e). It is evident to conclude that the YF₃ octahedra formed through the dissolution-crystallization process at the cost of α-NaYF₄ nanospheres and the transformation of chemical composition was from the metastable α-NaYF₄ phase to the stable YF₃ phase with extending the reaction time.

Fig. 6 (a) XRD patterns of the products synthesized at 140 °C with 7 M NaNO₃ for different reaction times; (b–e) SEM images of the as-prepared product for (b) 1 h; (c) 5 h; (d) 15 h; (e) 20 h, respectively. Other reaction parameters were unchanged.

The formation mechanism of octahedron

For the growth of inorganic crystals, the organic additives including surfactants or polymers play an important role in the controlled synthesis of the final products because of their coordination and selective absorption effects on different crystal facets. Furthermore, most of the reactions occur in an aqueous solution, thus the ion concentrations and the effects from salts also exert a great influence on the crystal growth. In our experiment, when Y(NO₃)₃ and NaF are dissolved in the aqueous of NaNO₃, the Y³⁺ and F⁻ ions would be surrounded by the opposite charge NO₃⁻ and Na⁺ ions owing to electrostatic interactions. This result would decrease the collision probability and the net reaction rate between the Y³⁺ and F⁻ ions, and further reduce the nucleation of YF₃ crystallites. However, increasing the reaction temperature and time can overcome these shortcomings, and promote the nucleation of YF₃. On the other hand, the added NaNO₃ also has similar influence on the shape-controlled crystal growth to the organic additives selectively modifying the different facets of inorganic crystal. Furthermore, from the crystal structure of YF₃ (Fig. 7), the zirconium type of structure of YF₃ consists of YF₈ dodecahedron, and the YF₈ units are stacked alternately along its axes. The interplanar lattice of (100), (010) and (001) is 6.353, 6.858 and 4.393 Å, respectively, which is much larger than that of (111) faces (3.1959 Å), leaving much space along the axes for other ions attacking. This structure results in a relatively low growth rate in the [111] direction because the active points of (111) faces are greatly restricted by NaNO₃, and further leads to the formation of YF₃ octahedron. As illustrated in Scheme 1, the formation of the YF₃ octahedron can be rationally expressed as a kinetically controlled dissolution-crystallization mechanism through the reaction temperature and time.²⁰ In conclusion, the main effect of NaNO₃ on the growth of YF₃ nanocrystals comprises two aspects, one is to limit the rate of the chemical reaction through interaction between the yttrium and fluorine ions; the other is to exert dynamic effect on the growth of different crystal facets.

Fig. 7 Crystal structure of YF₃ octahedral nanocrystals viewed (a) along the [111] direction and (b) along the [010] direction.

Scheme 1 Schematic illustration of the formation of YF₃ octahedron.

Optical properties of Eu³⁺ : YF₃ crystals

Fig. 8 shows the room temperature emission spectra of Eu³⁺ : YF₃ nanocrystals excited at 224 nm short-wavelength. All emission peaks at λ = 579 (⁵D₀-⁷F₀), 592 (⁵D₀-⁷F₁), 612 (⁵D₀-⁷F₂), 657 nm (⁵D₀-⁷F₃) have been assigned in Fig. 8, and there is no notable shift in the positions of emission peaks compared to other Eu³⁺-doped system,²¹ because the 4f energy levels of Eu³⁺ are hardly affected by the crystal field.¹² The electric dipole transition ⁵D₀-⁷F₂ is the primary group in the emission spectrum of the sample. The ⁵D₀-⁷F₁ and ⁵D₁-⁷F₁ transitions are magnetic dipole allowed and their intensities are almost independent of the local environment around Eu³⁺ ions.²² The ⁵D₀-⁷F₂ transition is allowed due to an admixture of opposite parity 4fⁿ⁻¹5d states by odd parity crystal-field component.^21,23 Therefore, its intensity is sensitive to the local structure around the Eu³⁺ cation. The ⁵D₀-⁷F₃ transition exhibits a mixture of magnetic dipole and electric dipole character.²¹ Reduction of particle size and the high concentration of Eu³⁺ cause the degree of lattice distortion and corresponding lower symmetry of the crystal field around the Eu³⁺ ions, results in the blue shift of excitation, the primary emission of ⁵D₀-⁷F₂ transition and no splitting of ⁵D₀-⁷F_j (J = 1, 2, 3) transition.^12,24

Fig. 8 Room temperature (a) excitation (λ_em = 614 nm) and (b) emission (λ_ex = 224 nm) spectra of a Eu³⁺ : YF₃ sample.

The preparation of light lanthanide fluoride LnF₃ (Ln = La, Nd, Eu, Gd)

It is well known that the series of rare earth element is frequently divided into two groups based on the atomic weight and chemical properties. The “light” rare earths consist of elements with atomic number 57 to 63, while the “heavy” rare earths consist of elements 64–71 as well as scandium and yttrium because of similar chemical behavior. Although the crystal phase of LnF₃ (Ln = La–Lu) belongs to the hexagonal or orthorhombic system, it is interesting to find that only light rare earth fluoride, such as LnF₃ (Ln = La, Nd, Eu, Gd) (sample 12–15), can be hydrothermally synthesized at 7 M NaNO₃ under similar conditions (see ESI, Fig. S3†). The TEM of LaF₃, NdF₃, EuF₃ and GdF₃ are shown in Fig. 9, nanoplates of LaF₃, NdF₃ and EuF₃ can be obtained (Fig. 9a-c). While the as-prepared GdF₃ (Fig. 9d) has an olive-like structure with about 250–600 nm in length and 300 nm in diameter, assembled by many nanoplates, which is much different from the YF₃ octahedron prepared in similar conditions. Owing to the lanthanide contraction, there is a gradual decrease in the radius of rare earth with the increase of its atomic number. So LnF₃ (Ln = La, Nd, Eu, Gd) can also be prepared at high NaNO₃ concentration because the larger radius of light rare earth element increases the probability of ionic collision between Ln³⁺ and F⁻ and the nucleation of LnF₃. These results imply that the shape and composition of LnF₃ are principally dependent on the radius of rare earth and the concentration of NaNO₃.

Fig. 9 TEM images of LnF₃ obtained at 140 °C with 7 M NaNO₃ for 20 h: (a) LaF₃; (b) NdF₃; (c) EuF₃; (d) GdF₃.

Conclusion

In summary, different kinds of yttrium fluoride (Na(Y_1.5Na_0.5)F₆; α-NaYF₄; YF₃) have been successfully fabricated by using a one-step and facile hydrothermal procedure. The results indicate that a different chemical composition and morphology of yttrium fluoride can be controlled effectively by adjusting the reaction condition. A possible growth mechanism of the YF₃ octahedron is proposed. This method can be extended to synthesizing light rare earth fluoride, and their chemical composition and morphology can also be controlled effectively. Further studies reveal that the morphology and composition of the as-synthesized LnF₃ are determined mainly by the ionic radius of rare earth and the concentration of NaNO₃. This work may present a way for the shape-controlled synthesis of other inorganic materials. The simple composition and morphology controllable synthesis of yttrium fluoride holds promise for application in photoluminescence devices.

Acknowledgements

The work described here was supported by the National Science Foundation of China (No: 20671061), the Programme for New Century Excellent Talents of Education Ministry of China, and National Basic Research Programme of China (2009CB930400 and 2007CB209705).

References

1. (a) H. Li, R. Liu, R. X. Zhao, Y. F. Zheng, W. X. Chen and Z. D. Xu, Cryst. Growth Des., 2006, 6, 2795; (b) C. X. Li, X. M. Liu, P. P. Yang, C. M. Zhang, H. Z. Lian and J. Lin, J. Phys. Chem. C, 2008, 112, 2904; (c) Z. L. Wang, Z. W. Quan, P. Y. Jia, C. K. Lin, Y. Luo, Y. Chen, J. Fang, W. Zhou, C. J. O'Connor and J. Lin, Chem. Mater., 2006, 18, 2030.
2. J. Q. Hu, Q. Chen, Z. X. Xie, G. B. Han, R. H. Wang, B. Ren, Y. Zhang, Z. L. Yang and Z. Q. Tian, Adv. Funct. Mater., 2004, 14, 183.
3. J. Q. Hu, Q. Li, N. B. Wong, C. S. Lee and S. T. Lee, Chem. Mater., 2002, 14, 1216.
4. F. H. Zhao, W. J. Lin, M. M. Wu, N. S. Xu, X. F. Yang, Z. R. Tian and Q. Su, Inorg. Chem., 2006, 45, 3256.
5. (a) X. Wang, J. Zhuang, Q. Peng and Y. D. Li, Adv. Mater., 2006, 18, 2031; (b) C. X. Li, J. Yang, P. P. Yang, X. M. Zhang, H. Z. Lian and J. Lin, Cryst. Growth Des., 2008, 8, 923.
6. (a) S. K. Im, Y. T. Lee, B. Wiley and Y. Xia, Angew. Chem., Int. Ed., 2005, 44, 2154; (b) L. Yan, R. B. Yu, J. Chen and X. R. Xin, Cryst. Growth Des., 2008, 8, 1474; (c) Q. Wu, F. Zhang, P. Xiao, H. S. Tao, X. Z. Wang, Z. Hu and Y. N. Lü, J. Phys. Chem. C, 2008, 112, 17076; (d) T. Xia, Q. Li, X. D. Liu, J. Meng and X. Q. Cao, J. Phys. Chem. B, 2006, 110, 2006; (e) C. X. Li, Z. W. Quan, P. P. Yang, J. Yang, H. Z. Lian and J. Lin, J. Mater. Chem., 2008, 18, 1353.
7. Q. Su, Rare Earth of Chemistry, Science and Technology Press of Chinese Henan Province, 1993, p 108.
8. (a) T. Herricks, J. Chen and Y. Xia, Nano Lett., 2004, 4, 2367; (b) K. E. Korte, S. E. Skrabalak and Y. Xia, J. Mater. Chem., 2008, 18, 437; (c) M. H. Kim, B. Lim, E. P. Lee and Y. Xia, J. Mater. Chem., 2008, 18, 4069; (d) S. H. Yu and H. Cölfen, J. Mater. Chem., 2004, 14, 2124; (e) S. H. Yu, H. Cölfen and M. Antonietti, J. Phys. Chem. B, 2003, 107, 7396; (f) S. H. Yu, Top. Curr. Chem., 2007, 271, 79.
9. L. Beauzamy, B. Moine and P. Gredin, J. Lumin., 2007, 127, 568.
10. R. X. Yan and Y. D. Li, Adv. Funct. Mater., 2005, 15, 763.
11. M. Nikl, A. Yoshikawa, A. Vedda and T. Fukuda, J. Cryst. Growth, 2006, 292, 416.
12. (a) M. F. Zhang, H. Fan, B. J. Xi, X. Y. Wang, C. Dong and Y. T. Qian, J. Phys. Chem. C, 2007, 111, 6652; (b) F. Tao, Z. J. Wang, L. Z. Yao, W. L. Cai and X. G. Li, J. Phys. Chem. C, 2007, 111, 3241.
13. (a) J. L. Lemyre and A. M. Ritcey, Chem. Mater., 2005, 17, 3040; (b) Y. Cheng, Y. S. Wang, Y. H. Zheng and Y. Qin, J. Phys. Chem. B, 2005, 109, 11548; (c) X. Wang and Y. D. Li, Angew. Chem., Int. Ed., 2003, 42, 3497; (d) X. Wang, J. Zhuang, Q. Peng and Y. D. Li, Nature, 2005, 437, 121; (e) Y. W. Zhang, X. Sun, R. Si, L. P. You and C. H. Yan, J. Am. Chem. Soc., 2005, 127, 3260; (f) M. Wang, Q. L. Huang, J. M. Hong and X. T. Chen, Mater. Lett., 2007, 61, 1960; (g) M. Wang, Q. L. Huang, H. X. Zhong, X. T. Chen, Z. L. Xue and X. Z You, Cryst. Growth Des., 2007, 7, 2106.
14. (a) C. X. Li, J. Yang, P. P. Yang, H. Z. Lian and J. Lin, Chem. Mater., 2008, 20, 4317; (b) F. Tao, Z. J. Wang, L. Z. Yao, W. L. Cai and X. G. Li, Cryst. Growth Des., 2007, 7, 854.
15. X. Ji, X. Song, J. Li, Y. Bai, W. Yang and X. Peng, J. Am. Chem. Soc., 2007, 129, 13939.
16. Z. P. Peng, Y. S. Jiang, Y. H. Song, C. Wang and H. J. Zhang, Chem. Mater., 2008, 20, 3153.
17. L. Y. Wang and Y. D. Li, Nano Lett., 2006, 6, 1645.
18. C. X. Li, J. Yang, Z. W. Quan, P. P. Yang and J. Lin, Chem. Mater., 2007, 19, 4933.
19. (a) J. P. Ge, Y. X. Hu, M. Baisini, W. P. Beyermann and Y. D. Yin, Angew. Chem., Int. Ed., 2007, 46, 4342; (b) S. Libert, V. Gorshkov, D. Goia, E. Matijević and V. Privman, Langmuir, 2003, 19, 10679; (c) H. Deng, X. L. Li, Q. Peng, X. Wang, J. P. Chen and Y. D. Li, Angew. Chem., Int. Ed., 2005, 44, 2782; (d) X. Liang, X. Wang, Y. Zhuang, B. Xu, S. M. Kuang and Y. D. Li, J. Am. Chem. Soc., 2008, 130, 2736.
20. C. X. Li, Z. W. Quan, J. Yang, P. P. Yang and J. Lin, Inorg. Chem., 2007, 46, 6329.
21. G. S. Ofelt, J. Chem. Phys., 1962, 37, 511.
22. S. Ray, P. Pramanik, A. Singha and A. Roy, J. Appl. Phys., 2005, 97, 094312.
23. B. R. Judd, Phys. Rev., 1962, 127, 750.
24. N. Zhang, W. B. Bu, Y. P. Xu, D. Y. Jiang and J. L. Shi, J. Phys. Chem. C, 2007, 111, 5014.

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns. See DOI: 10.1039/b911401g

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