Shape-Controllable Synthesis and Upconversion Properties of

J. Phys. Chem. C 2008, 112, 13395–13404

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Shape-Controllable Synthesis and Upconversion Properties of Lutetium Fluoride (Doped with Yb3+/Er3+) Microcrystals by Hydrothermal Process Chunxia Li, Zewei Quan, Piaoping Yang, Shanshan Huang, Hongzhou Lian, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: April 2, 2008; ReVised Manuscript ReceiVed: May 12, 2008

Lutetium fluorides with different compositions, crystal phases, and morphologies, such as β-NaLuF4 hexagonal microprisms, microdisks, mirotubes, R-NaLuF4 submicrospheres, LuF3 octahedra, and NH4Lu2F7 icosahedra, prolate ellipsoids and spherical particles have been successfully synthesized via a facile hydrothermal route. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, and photoluminescence spectra were used to characterize the samples. The intrinsic structural feature of lutetium fluorides, the solution pH values, F- sources, and organic additives (Cit3- and EDTA) account for the ultimate shape evolutions of the final products. The possible formation mechanisms for products with various architectures have been presented. Additionally, we investigated the upconversion luminescence properties of β-NaLuF4: 20% Yb3+/2% Er3+ with different morphologies. This is a facile and general strategy to modulate the crystal phases, morphologies, and sizes of lutetium fluorides, which might be applicable for other lanthanide fluorides. 1. Introduction In recent years, the precisely architectural manipulation of inorganic functional materials with well-defined morphologies and accurately tunable sizes remains the research focus in science research not only for fundamental scientific interest but also for their various applications in fields such as biological labeling and imaging, catalysis, drug delivery, and sensing.1 So many efforts have been made to explore excellent synthetic approaches to the fabrication of a variety of inorganic crystals with different levels to enhance their performance in currently existing applications. But due to the complexity of crystal structures and compositions of the materials, it is still a challenging and urgent task for us to manipulate and control the morphologies of various nano- and micromaterials, consequently achieving a better understanding of the observed complex phenomena of crystal growth and revealing the underlying fundamental theories and principles. Furthermore, from the perspective of application, nano- and micromaterials are not only synthesized in large quantities with desired composition, reproducible size, shape, and structure but also prepared and assembled using green, environmentally responsible methodologies. So the development of a mild and more controlled method for creating such novel architectures will be of general interest. Recently, environmentally friendly synthetic methodologies, which include molten-salt synthesis, hydrothermal processing, and template synthesis, have gradually been implemented as viable techniques in the synthesis of a range of nano- and mcirostructures.2 Especially the hydrothermal method as a typical solution-based approach has been proven an effective and convenient process in preparing various inorganic materials with diverse controllable morphologies and architectures in terms of cost and potential for large-scale production.3 More importantly, this protocol frequently uses water as the reaction medium. Our groups have successfully * To whom correspondence should be addressed. E-mail: [email protected].

synthesized NaREF4 microcrystals with diverse morphologies and excellent photoluminescence properties via this facile hydrothermal route.4 Rare-earth (RE) fluorides such as NaREF4 and REF3, possessing a high refractive index and low phonon energy,5 have become a research focus in the material field owing to their unique applications in optical telecommunication, lasers, biochemical probes, and medical diagnostics based on the electronic, optical, and chemical characteristics arising from the 4f electrons.6–9 Among them, NaREF4 is known to exist in two modifications, namely, cubic (R-) and hexagonal (β-) phases. Currently, most of the reported research strategies are based on the cothermolysis of rare-earth trifluoroacetates in high-temperature organic solutions10 or a solvothermal route.11 However, from a green chemistry standpoint, all of these nonaqueous synthetic schemes are far from ideal. Water may eventually become a plausible medium for the growth of high-quality nanoand microcrystals with various compositions. This attractive future will likely come with systematic and quantitative studies of some carefully chosen aqueous model systems.12 Furthermore, these reported routes usually only gave rise to the nanoscale products with one crystal structure. Little has been done on the manufacture of rare-earth fluoride microcrystals with uniform morphology and size.13 Microscale inorganic materials with novel and well-defined morphologies are of special significance because their structure characteristics endow them with potential technological applications in microelectronic devices.14 In this article, via a facile and effective hydrothermal route in pure water medium, we will first report on the controlled synthesis of lutetium fluorides with different crystal phases and morphologies including β-NaLuF4 hexagonal microdisks, microprisms, microtubes, and microrods and R-NaLuF4 submicrospheres, LuF3 octahedra, and NH4Lu2F7 icosahedra, prolate ellipsoids, and spherical particles. As the focus of this article, we will discuss how a precise control over the crystal phases and morphologies of the products can be achieved by simple tuning

10.1021/jp802826k CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008

13396 J. Phys. Chem. C, Vol. 112, No. 35, 2008

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TABLE 1: Parameters of the Representative Experiments and the Morphologies and Sizes of the Corresponding Productsa sample F- source pH organic additive crystal phase P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 a

NH4F NaBF4 NH4F

3 7 10 1 3 7 3 7 10 7

3-

Cit

β-NaLuF4

Cit3EDTA

LuF3 R-NaLuF4 β-NaLuF4 NH4Lu2F7

no

β-NaLuF4 NH4Lu2F7

morphology

thickness or length (µm) diameter (µm)

hexagonal microprisms with concave centers hexagonal mciroprisms hexagonal microdisks octahedra submicrospheres hexagonal microtubes icosahedra prolate ellipsoids hexagonal microrods spherical particles

6.6 3.5 0.45 0.26 14.5 2 5.8

4.6 5.5 7.2 0.32 3.8 1 4.5 0.15

All samples were obtained via hydrothermal treatment at 180 °C for 24 h.

several critical parameters including F- sources, pH value in the initial solution, and choice of organic additives. The morphological evolvement and the growth mechanism for the synthesized lutetium fluorides under different conditions have been studied in detail. Moreover, we also investigate the upconversion (UC) photoluminescence properties of β-NaLuF4: Yb3+/Er3+ with different morphologies. 2. Experimental Section Preparation. The rare-earth oxides RE2O3 (RE ) Yb, Lu, Er) (99.999%) were purchased from Science and Technology Parent Co. of Changchun Institute of Applied Chemistry, and other chemicals were purchased from Beijing Chemical Co. All chemicals are analytical grade reagents and used directly without further purification. Rare-earth chloride stock solutions of 0.2 M were prepared by dissolving the corresponding metal oxide in hydrochloric acid at elevated temperatures. In a typical procedure, 10 mL of LuCl3 (0.2 M) was added into 20 mL of aqueous solution containing 2 mmol of trisodium citrate (labeled as Cit3-), and then a white precipitate formed. After vigorous stirring for 30 min, 25 mmol of NH4F was added into the above solution. The as-obtained mixing solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed and maintained at 180 °C for 24 h, and then air-cooled to room temperature naturally. The resulting precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence, and then dried in air at 80 °C for 12 h. The as-prepared product was denoted as P1. Other samples (P2-P10) and 20% Yb3+/2% Er3+ (molar ratio) codoped samples were prepared by similar procedures except for different F- sources, pH values, and coordination agent ethylenediaminetetraacetic acid disodium salt (EDTA). The pH of the mixture was adjusted to a specific value by adding NaOH solution (3 M), ammonia solution (25%), or HCl (1 M) solution. The final products together with the corresponding detailed experimental conditions are listed in Table 1. All samples were obtained by hydrothermal treatment at 180 °C for 24 h. Characterization. X-ray power diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 15°/min in the 2θ range from 10 to 70°, with graphite monochromatized Cu KR radiation (λ ) 0.15405 nm). Scanning electron microscipy (SEM) micrographs were obtained using a field emission (FE)-SEM (XL30, Philips). Lowto high-resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. The UC emission spectra were obtained using 980 nm laser from an optical parametric oscillator (Continuum Surelite) as the excitation source and detected by

Figure 1. XRD patterns of the as-prepared β-NaLuF4 products using NH4F as F- source and Cit3- as organic additive at different pH values of (a) 3, (b) 7, and (c) 10 and the standard data of β-NaLuF4 (JCPDS 27-0726).

R955 (Hamamatsu) from 500 to 700 nm. All the measurements were performed at room temperature. 3. Results and Discussion Table 1 summarizes the experimental conditions and the corresponding crystal phases, morphologies, and sizes of the products. From Table 1 it is found that the F- sources, pH values of the initial reaction solutions, and organic additives have important effects on the crystal phases, morphologies, and dimensions of the final products. Even if the same F- source is used, the different pH values of the initial reaction solution show a large impact on the crystal growth of lutetium fluorides, which will be discussed in detail in the following paragraphs. 3.1. Effects of F- Sources and pH Values (Cit3- as Organic Additive). The previous studies demonstrate that use of different F- sources and pH values in the initial solution influences significantly the morphology of rare-earth fluorides.4,15 Here, we select NH4F and NaBF4 as F- sources to investigate crystal structures and shape evolutions of lutetium fluorides. (A) NH4F as F- Source. When Cit3- is used as organic additive and NH4F serves as F- source, the composition and phase purity of the products prepared under various pH conditions were first examined by XRD, as shown in Figure 1. The sharp diffraction peaks indicate the good crystallization of the products. All the peaks of the three samples can be indexed as a pure hexagonal (β-) phase of NaLuF4 (JCPDS 27-0726). From Figure 1, it can be seen that the XRD peaks of the three samples show slight shift with respect to those of NaLuF4 in the JCPDS card. This may be caused by the minor alteration of the cell parameters of NaLuF4 crystals grown under different conditions or the instrument errors. It is noteworthy that the relative intensities of XRD patterns based on (110), (100), (101), and (201) peaks for three samples show a large difference from

Properties of Lutetium Fluoride Microcrystals

J. Phys. Chem. C, Vol. 112, No. 35, 2008 13397

Figure 2. Influence of the pH value on the shapes of β-NaLuF4 microcrystals with NH4F as F- source and Cit3- as organic additive. (A, B) Prismatic structures with scrappy ends obtained at pH ) 3; (C, D) uniform hexagonal microprisms obtained at pH ) 7; (E, F) hexagonal microdisks obtained at pH ) 10.

each other, indicating the possibility of different preferential orientation growth under different pH conditions. In this situation, the morphologies of the as-prepared products are illustrated in Figure 2. At pH ) 3, the regular hexagonal microprismatic shaped β-NaLuF4 crystals (P1) with average size of 6.6 µm in length and 4.6 µm in diameter are observed from Figure 2A. Interestingly, the higher magnification SEM image demonstrates the presence of scrappy ends and concave centers on the top/bottom surfaces, as shown in Figure 2B. Furthermore, the edges of six prismatic surfaces are not uneven in length. When the pH value of the initial solution is not adjusted (pH ) 7), the general image of β-NaLuF4 crystals (P2) produced is shown in Figure 2C. It clearly indicates that products are composed of large-scale, regular, and monodisperse hexagonal microprisms with uniform size of 3.5 µm in thickness and 5.5 µm in diameter. More careful examination of the magnified SEM image (Figure 2D) shows clearly that the top/bottom of the prisms have a lot of clean strains. When the experiment is performed at pH ) 10 adjusted with ammonia solution (25%) and other experimental conditions remain unchanged, the morphology of the products (P3) is the hexagonal microdisks with smooth surfaces, remarkable uniformity, and monodispersity (Figure 2E). Figure 2F shows some microdisks perpendicular to the substrate. Analysis of a number of the microdisks shows that these microcrystals have an average size of 7.2 µm in diameter and 0.45 µm in thickness. The microstructure of product is further characterized by TEM. Figure 3A is a

representative TEM image of two hexagonal microdisks parallel to the substrate. Very regular hexagonal cross sections can be clearly observed from this figure. From corresponding highresolution transmission electron microscopy (HRTEM) image (Figure 3B, taken with the electron beam perpendicular to the edge of the microdisk), the lattice fringes of the sample can be seen clearly. The interplanar distance is ∼0.51 nm, which corresponds to the d-spacing value of the (101j0) planes of β-NaLuF4. From the above analysis, it can be concluded that, with the increase of pH value, the dimension in thickness of particles reduces while that in diameter increases systematically (Table 1). The reason for that can be ascribed to the different interactions of Cit3- with β-NaLuF4 specific crystal planes under different pH. Simultaneously, it is noteworthy that, for β-NaLuF4 crystals with hexagonal shapes, the surfaces are typically {0001} top/bottom planes and six energetically equivalent {101j0} family of prismatic side planes on the basis of the known or similar models.16 The velocity of different crystallographic planes descends in the order, ν(0001) > ν(101j0) > ν(0001j).17 In fact, the differences of pH values show a significantly effect on the existent form and complexing ability of Cit3- to Lu3+, consequently affecting the selective adsorption on the different facets of growing NaLuF4 crystallites, giving rise to the difference of the growth rates between different crystallographic directions. At pH ) 3, Cit3- would partly combine H+ in the solution and exists as HxCitx-3, directly decreasing its complexing ability with


Li et al.

Figure 3. TEM (A) and HRTEM (B) images of β-NaLuF4 hexagonal microdisks (P3).

metal ions to a great degree. In such an environment, Cit3adsorbs selectively the (101j0) family surfaces of β-NaLuF4 crystallites. So the growth along the [0001] direction has a faster rate than that along other directions and is much more favorable. Thus, prisms with longer length are easily formed. The presence of uneven length of side edges implies that the lower solution pH does not facilitate the equilibrium and equivalent growth of different side surfaces. Furthermore, during the growth process, when the vicinal monomers of the top/bottom surfaces are consumed, the concentration of monomers near the hexagonal prisms decreased and the monomers at a distance would move to supplement. Thus, the growth rate of prismatic planes is a little faster than top/bottom planes, leading to the formation of more depth concave center.18 Under pH ) 7-10, citrate exists as Cit3- and the predominant species in solution remains a 1:1 stronger complex from the coordination reaction between Lu3+ and Cit3-. At pH ) 7, Cit3- can absorb onto the {101j0} and {0001} facets simultaneously, leading to formation of shorter hexagonal microprisms. However, at pH ) 10, Cit3- interacts more strongly with {101j0} facets than the {0001} facets, resulting in growth predominantly along [101j0] directions to form the hexagonal microdisks. In summary, using NH4F as F- source, with increasing pH value, the growth rate along the [0001] direction is prohibited and the growth along [101j0] directions is promoted. As a result, the longitudinal size decreases while latitudinal size boots up gradually. In order to substantiate the important influence of Cit3- on the β-NaLuF4 shape evolution in the current system, a controlled experiment was carried out in the absence of Cit3- and the other parameters remained unchanged. The XRD pattern (Figure 4A) shows that a new crystalline phase appears, which is basically identified as cubic NH4Lu2F7 (JCPDS 43-0846) except for one weak impurity peak (denoted as *, which can be assigned to the unreacted NH4F, JCPDS 35-0758). The sample is composed of spherical particles with a mean diameter of 150 nm, as shown in Figure 4B. This demonstrates that Cit3- plays an important role in the phase and morphology for the formation of β-NaLuF4. The roles of Cit3- may lie in three aspects: providing a Na+ source, acting as coordination agent slowing down the nucleation and subsequent crystal growth of NaLuF4 particles, and serving as a shape modifier affecting its adsorption onto different crystal facets. (B) NaBF4 as F- Source. If NaBF4 substitutes for NH4F as the F- source and the other conditions remain the same, the situations become quite different. Figure 5 shows the XRD patterns of as-prepared products at different pH values (1, 3, 7). Unexpectedly, the product (P4) obtained at pH ) 1 is LuF3 with an orthorhombic crystal phase (space group Pnma) (Figure

Figure 4. XRD pattern (A) and SEM images (B) of the as-prepared sample (P10) in the absence of Cit3- (NH4F as F- source).

Figure 5. XRD patterns of the as-prepared different products using NaBF4 as F- source and Cit3- as organic additive at different pH values of (a) LuF3, pH ) 1; (b) R-NaLuF4, pH ) 3; and (c) β-NaLuF4, pH ) 7 and the standard data of LuF3 (JCPDS 32-0612).

5a), which is in good agreement with the standard literature data (JCPDS 32-0612). If the pH in the initial solution is not adjusted (pH ) 3), the as-prepared product (P5) is R-NaLuF4, as illustrated in Figure 5b. This is quite different from the



Figure 6. Influence of the pH value on the shapes of products with NaBF4 as F- source and Cit3- as organic additive. SEM (A) image of LuF3 octahedra obtained at pH ) 1; SEM (B), TEM (C), SAED (D), and HRETM (E) images of R-NaLuF4 submicrospheres obtained at pH ) 3; SEM (F, G), TEM (H), and HRTEM (inset in H) images of β-NaLuF4 microtubes obtained at pH ) 7.

β-NaLuF4 products harvested using NH4F as F- source. However, when the pH value is increased up to 7 with 3 M NaOH, the crystal structure of the sample (P6) is β-NaLuF4 (Figure 5c). Because the other parameters are basically identical to those using NH4F as the F- source, the difference of the crystal structures of the products might have a close relationship with NaBF4. Furthermore, NaBF4 not only alters the crystal phases of the products but also influences greatly their morphologies. As shown in Figure 6A, LuF3 crystals (P4) have octahedral shape, whereas the size distribution is not very uniform. Careful examination indicates that their surfaces are extremely smooth without obvious defects and small particles attached on them. The product P5 is composed of spheres with mean diameters of 320 nm (Figure 6B). A typical TEM image of the sample is presented in Figure 6C, which clearly reveals that the spheres are hierarchical structures and constructed from the smaller particles. The SAED pattern shown in Figure 6D of the spherical structure is composed of cubic (R)-NaLuF4 with strong ring patterns indexed to the (111), (200), (220), and (311) planes, demonstrating the polycrystalline nature of the spheres. The HRTEM image of a single particle (Figure 6E) confirms the distance of 0.32 nm between the adjacent lattice planes, ascribed to that of (111) crystal planes. A typical SEM image of P6 is shown in Figure 6F, indicating that the integral morphology of products is microtubes with average diameter

of 3.8 µm and length up to 14.5 µm. A magnified SEM image (Figure 6G) indicates that the tubes have open ends. Furthermore, the outer surfaces of the tube form a hexagonal prism, while the inner surfaces of the tubes are not very regular and have the broken ends. Figure 6H displays the TEM image of a single microtube, but it is a pity that the entire tube is too large and thick-walled to show its inner structure clearly. The inset of Figure 6H is a corresponding HRTEM image recorded from the tip of an individual tube, verifying that the lattice fringe separation of 0.37 nm. This plane is coincident well with the distances between (0001) crystal planes, confirming that the microtubes show a preferred growth along the c-axis, namely, the [0001] direction. Because the other parameters are basically identical to those using NH4F as F- sources, the formation of different crystal structures and morphologies may be dependent on pH values in the initial solution and the special properties of NaBF4. At first, in aqueous solution, NaBF4 is slowly hydrolyzed to produce BO33- and F- anions, as shown in eq 2, which has been proven by other groups.19 Especially at an acid environment (pH ) 1), from the view of the reaction equilibrium, this situation is not favorable for the release of F-. Furthermore, the composition analysis of evaporating the clear solution after centrifugation demonstrates the formation of H3BO3 and Na2B2O4 (eq 3).20 Finally, Lu3+ released from the complexes reacts with F-


Li et al.

produced during the slow hydrolysis of NaBF4 to form LuF3 nuclei, as presented in eq 4. In this case, the particle growth of the precipitated LuF3 solid is very slow due to the very low Fconcentration. Furthermore, the lower pH value is favorable to the segregate of LuF3 along with Na+ in the solution, instead of forming NaLuF4. The possible reaction processes for the formation of LuF3 can be summarized as follows:

Lu3++Cit3- f Lu3+-Cit3-(complex)

(1)

-

BF4- + 3H2O T 3HF + F +H3BO3 +

(2) +

2H3BO3+Na f Na2B2O4+2H2O + 2H -

Lu -Cit +3F f LuF3+Cit 3+

3-

3-

(3) (4)

As for the formation and evolution process of R-NaLuF4 spheres, it may be interpreted in terms of the aggregation mechanism of crystal growth proposed by Matijevic´ et al.,21 which can be divided into three steps: the initial nucleating process, the continued growth of the nuclei, and the subsequent “separation” process. At the very beginning, the precursor is turbid, which demonstrates the formation of Lu3+-Cit3complex, resulting in the slow release speed of Lu3+ in the solution. When the nucleation rate is relatively slow, Cit3- ions are adsorbed onto the crystal surfaces of the formed R-NaLuF4 nuclei tightly in all directions, which thus prevents their anisotropic growth. Then, large amounts of the newly formed NaLuF4 nuclei grow by a diffusive mechanism into the primary particle units, which will aggregate together to form much larger particles in the process dominated by irreversible capture of the single particles. In the last step, these larger particles adopt a 3D spherical structure with larger sizes in order to keep their surface energy low and segregate from each other to form their final morphology.22 However, in the present case, the final product has not experienced the transformation from R- to β-phase. It is highly possible that, in the presence of the NaBF4, the aggregation of R-NaLuF4 nuclei does not facilitate the occurrence of phase transformation. This further confirms that the inherent crystal structure of seeds plays an important role in the formation of nano- and microstructures. The inherent isotropic unit cell structure of cubic R-NaLuF4 seeds induces the isotropic growth for all different facets, and therefore, spherical particles are observed to minimize the surface energy of crystal facets. The formation of β-NaLuF4 microtubes experiences dissolution of solid crystals of R-NaLuF4, the mass transfer in liquid solution, and the renucleation-reconstruction process occurring at the circumferential edge of each β-NaLuF4 seed. Subsequent fast growth on the edges leads to a depletion of substances in the centers and eventually produces tubular structure of with well-defined cross sections.23 The above results powerfully demonstrate that the use of different F- sources plays a crucial role in determining the crystal compositions and morphologies of the products. 3.2. Effects of Organic Additives (NH4F as F- Source). In a solution-phase synthesis, various organic additives are used to direct rationally the anisotropic growth of the crystals, for instance, micellar assemblies (e.g., cetyltrimethylammonium bromide),24 coordinating ligands,25 biological reagents,26 etc. When the F- source is fixed, two types of organic additives, EDTA and Cit3-, are used to perform the contrastive experiments with an aim to investigate their effects on compositions and morphologies of the products through different coordination modes and specific molecular complementarity with lutetium fluorides. When Cit3- is used as organic additive, the as-obtained products (P1, P2, P3) are β-NaLuF4 crystal phase with

hexagonal microprism and microdisk shapes, respectively, as stated above. However, when other experimental conditions are identical and EDTA acts as organic additive, the crystal structures and shapes of the products are quite different from the former. Figure S1(Supporting Information, SI) shows the XRD patterns of the as-prepared products in the presence of EDTA at different pH values. At pH ) 3 or 7, the crystal phases of the products (P7 and P8) are also basically identified as cubic NH4Lu2F7 (JCPDS 43-0846) expect for two weak impurities peaks (denoted as *), as shown in Figure S1a,b (SI). But with the increase of pH to 10 adjusted with NaOH, the sample P9 consists of pure hexagonal β-NaLuF4 (JCPDS 27-0726) (Figure S1c, SI). It is noteworthy that when the pH value of the initial solution is 10 adjusted with NH3 · H2O, the solution after hydrothermal treatment is still clear and transparent and no product could be obtained. The morphologies and microstructural details can be observed from the SEM, TEM, and HRTEM techniques. Figure 7A shows the SEM images of P7, indicating the exclusive formation of NH4Lu2F7 icosahedra. From the enlarged SEM image (inset in Figure 7A), one can clearly resolve the exact shape. Each icosahedron is composed of 2 top/bottom hexagonal faces and 18 rhombic faces. Figure 7B shows a TEM image of P7 with different orientations. The corresponding HRTEM image reveals the presence of clear crystal lattices from various directions with the calculated value of ∼0.32 nm, consistent with the crystal plane of (410) of NH4Lu2F7, as shown in Figure 7C. Upon a further increase of the pH value up to 7, the product P8 is prolate ellipsoids with average length of 2 µm and diameter of 1 µm. When pH of the solution is elevated to 10, the microrod-shaped β-NaLuF4 crystals (P9) are viewed from Figure 7D. The mean length and diameter of rods are estimated to be 5.8 and 4.5 µm, respectively. Moreover, there is a small quantity of microrods interconnecting of the centers to form a flowerlike structure. The magnified SEM image reveals that the microrods have solid interiors and very regular hexagonal cross sections (inset in Figure 7D). Interestingly, the top/bottom surfaces are protruding and exhibit a 6-fold axis. On the basis of the above analysis, it is reasonable to conclude that use of different organic additives is also an important factor in the controllable synthesis of lutetium fluorides. NH4+ and EDTA are found to responsible for the phase formation of NH4Lu2F7.27 For samples P3 and P9, they exhibit the same crystal phase (β-NaLuF4). Moreover, they are prepared under the identical conditions of the same F- source, pH value, reaction temperature, and time except for different organic additives (Table 1); however, their morphologies and sizes have obvious differences, which are related to the differences of the chelating constants of Cit3- and EDTA with Ln3+ and the adsorption ability of the different crystal facets of NaLuF4 particles. The chelating constant is larger for EDTA (lg β )18-19) than for Cit3- (lg β ) 8-9),28 leading to the different nucleation rates of NaLnF4. The smaller the chelating constant, the faster the nucleation rate. On the other hand, the coordination modes between EDTA and Cit3- with Ln3+ are clearly different,29 so the selective adsorption binding on the specific crystal facets of NaLuF4 particles is different, resulting in the different growth rates of different facets, consequently forming the different morphologies and sizes. In addition, the difference of XRD patterns (Figures 1c and 7c) further confirms the different preferential orientation growths of the crystals under these two conditions. 3.3. Effects of Reaction Time. Because the crystal structure of NaREF4 exhibits cubic (R-) and hexagonal (β-) polymorphic forms, the R f β phase transformation process can easily occur



Figure 7. Morphologies of the different products with NH4F as F- source and EDTA as organic additive. SEM (A), TEM (B), and HRTEM (C) images of NH4Lu2F7 icosahedra obtained at pH ) 3; SEM (D) images of NH4Lu2F7 prolate ellipsoids obtained at pH ) 7; SEM (E) images of β-NaLuF4 microrods obtained at pH ) 10.

Figure 8. Morphological evolution process of sample P2: XRD patterns (A) and the corresponding SEM images of the samples at 180 °C for different periods of reaction time of 1 (B), 2 (C), and 3 h (D).

at the designed reaction conditions.30 For β-NaLuF4 products (P2 example), the integration of XRD patterns and the corresponding TEM and SEM images of the different intermediate samples at different reaction stages at 180 °C clearly reveal the growth process of a hexagonal microprism structure, as shown in Figure 8. At a short reaction time of 1 h, the sample obtained is pure cubic (R-) NaLuF4 (part a in Figure 8A), which consists of spherical-like nanoparticles with a mean diameter of 40 nm (Figure 8B). But in the present environment, the R-phase of NaLuF4 is unstable and is susceptible to slow phase transformation. A dissolution-renucleation process for nanospheres takes

place and the more stable crystalline phase, namely, β-NaLuF4, emerges with the reaction proceeding for 2 h. Part b in Figure 8A shows the mixture of predominantly the R-phase and minor β-phase. This intermediate product consists of hexagonal microdisks of 2.5 µm in diameter and some irregular larger particles (Figure 8C). Furthermore, a large amount of nanoparticles are attached on the surfaces of microdisks (inset in Figure 8C). At a reaction time of 3 h, the R-phase disappears completely and only the β-phase exists (part c in Figure 8A). The corresponding shape is fairly uniform hexagonal microdisks with an average thickness and diameter of 0.4 and 3 µm, respectively,


Li et al.

SCHEME 1: Schematic Illustration for the Possible Formation Processes of Lutetium Fluorides with Different Crystal Phases and Morphologies under Various Experimental Conditions

as shown in Figure 8D. These phenomena indicate that R f β phase transition directly results in the dramatic change in morphology of NaLuF4 crystals along with reaction time. This further substantiates that R f β phase transition process of NaREF4 is an endothermic process and β-phase is more thermodynamically stable than R-phase.31 This experiment has proved from another point of view that the inherent cell structure of seeds strongly affects further crystal growth.32 The R-NaLuF4 seeds have isotropic unit cell structures, which generally induce the isotropic growth of particles, and therefore spherical particles are observed. In contrast, β-NaLuF4 seeds have anisotropic unit cell structures, which can induce anisotropic growth along crystallographically reactive directions, resulting in the formation of hexagonal-shaped β-NaLuF4 structures. On the basis of the above analysis, it can be found that, besides inherent unit cell structures of nucleated seeds, Fsources, organic additives, pH values in the initial solution, and reaction times are four important factors for the phase and morphology evolution of the different products. Scheme 1 summarizes the possible shape formation processes of different lutetium fluorides under various experimental conditions. 3.4. Upconversion Luminescence Properties of the Products. The UC luminescence of rare-earth ions has been investigated extensively because of their potential application in various fields, i.e., lighting or display, IR detection, and medical imaging.33 Here, we select Yb3+/Er3+ as the codoped ions to investigate the UC luminescent properties of the β-NaLuF4 products P1 (hexagonal microprisms), P6 (hexagonal microtubes), and P9 (hexagonal microrods). It is noted that

doping with 20% Yb3+/2% Er3+ (molar ratio) alters neither the crystal structure nor the morphology of the host materials. Under 980-nm laser excitation, the room-temperature UC emission spectra of P1, P6, and P9 are presented in Figure 9, respectively. It can be seen clearly that, in all three samples, the emission spectra are similar in shape; namely, they exhibit three obvious emission bands centered at 521, 539, and 654 nm that are assigned to 2H11/2 f 4I15/2, 4S3/2 f 4I15/2, and 4F9/2 f 4I15/2 transitions of Er3+, respectively, with green emission at 539 nm as the most prominent group.8–10a The upconversion mechanisms in Yb3+, Er3+-codoped materials are well investigated;34 i.e.,

Figure 9. UC emission spectra of 20% Yb3+/2% Er3+ codoped β-NaLuF4 products P1 (black line), P6 (red line), and P9 (blue line). All the samples are measured under the same conditions with 980-nm laser as excitation source.

Properties of Lutetium Fluoride Microcrystals an initial energy transfer from an Yb3+ ion in the 2F5/2 state to an Er3+ ion populates the 4I11/2 level of Er3+. A second 980-nm photon transferred by the excited Yb3+ ions can then populate a higher 4F7/2 energetic state of the Er3+ ions, whose energy lies in the visible region. The Er3+ ion can then relax nonradiatively by a fast multiphoton decay process to the 2H11/2 and 4S3/2 levels and the dominant green 2H11/2 f 4I15/2 and 4S3/2 f 4I15/2 emissions occur. Alternatively, the electron can further relax and populate the 4F9/2 level resulting in the occurrence of red 4F9/2 f 4I15/2 emission. The above energy-transfer and upconversion emission process is schematically shown in Figure S2 (SI). Note that although the three samples (P1, P6, P9) with different morphologies show much different upconversion emission intensity as reported previously,35 here we would not like to discuss it in more detail. A comparison of emission intensity does not make much sense in our current experiments based on the following situations. In order to draw the conclusion on optical properties of an individual luminescence center from such a comparison, one has to assume three preconditions. First, the absorption coefficients for three samples must be verified. However, the morphologies and sizes of three samples are different, which affect the scattering and absorption of incident light, so it is difficult for us to confirm the absorption coefficients of the samples. Second, the actual doping concentrations of Yb3+ and Er3+ ions should be equal. Because the samples are synthesized by a hydrothermal method, it is hard to ensure that all final samples have the same doping concentrations as they did initially in the experiment. Third, the number of the luminescence centers under the excitation area should be the same. However, it is very difficult to satisfy this precondition experimentally in this case since the sizes of the microcrystals are different. Moreover, the samples are large enough to ignore the surface-state influence on the optical properties of the dominant luminescence centers inside. As a result, the different emission intensities among P1, P6, and P9 samples might be caused by quite a few uncertain factors and will not be discussed here in more detail. 4. Conclusions Though the precise manipulation of a series of experimental conditions in pure water medium, such as the usage of different F- sources, choice of organic additives, and the adjustment of pH value in the initial reaction solution, we obtain β-NaLuF4 hexagonal microdisks, hexagonal microprisms, microtubes, R-NaLuF4 spheres, LuF3 octahedra, and NH4Lu2F7 icosahedra, prolate ellipsoids, and spherical particles. Simplicity, low costs, ease of scale-up, diverse morphologies, and relatively greenness (aqueous solution) constitute the key traits of this method. The possible formation mechanisms for products with various crystal phases and architectures have been presented in detail. Additionally, Yb3+ and Er3+-codoped β-NaLuF4 samples (P1, P6, P9) show a strong green (visible, 539 nm) emission from Er3+ (4S3/2 f 4I15/2) under 980-nm laser excitation. This is a facile and general strategy to modulate the crystal phases, morphologies, and sizes of lutetium fluorides, which is helpful for the crystal design and morphology-controlled synthesis of other rareearth fluoride compounds. Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua” of Chinese Academy of Science, the MOST of China (2003CB314707, 2007CB935502), and the National Natural Science Foundation of China (NSFC 50572103, 20431030 and 50702057).

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