Two-Dimensional Î²-NaLuF4 Hexagonal Microplates - Crystal Growth

CRYSTAL GROWTH & DESIGN

Two-Dimensional β-NaLuF4 Hexagonal Microplates Chunxia Li, Jun Yang, Piaoping Yang, Xiaoming Zhang, Hongzhou Lian, and Jun Lin* State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China

2008 VOL. 8, NO. 3 923–929

ReceiVed August 9, 2007; ReVised Manuscript ReceiVed October 22, 2007

ABSTRACT: A simple hydrothermal method has been developed to synthesize monodisperse β-NaLuF4 microplates in a large scale. The microcrystals have a perfect hexagonal shape with a diameter of about 5.2 µm and a thickness of 300 nm. Trisodium citrate (Cit3-), which is introduced into the reaction mixture and acts as the chelating agent and shape modifier, plays a key role in fine-tuning the microstructures. The dominant adsorption of Cit3- onto the {0001} facets lowers the surface energy of these facets. Consequently, the typical growth in the [0001] direction is prohibited, and the growth of nuclei is driven along the six symmetric directions ([101j0], ([11j00], and ([011j0], which directly results in the formation of β-NaLuF4 microplates. The microstructure and growth mechanism of β-NaLuF4 microplates are discussed in detail. Additionally, we investigated the photoluminescence properties of β-NaLuF4:Ln3+ (Ln ) Eu, Tb, and Yb/Er). This powerfully demonstrates that β-NaLuF4 is an excellent host lattice for downconversion and up-conversion luminescence of various optically active lanthanide ions.

1. Introduction In recent years, the shape control of anisotropic nano- and microcrystals has received considerable attention because the morphology, dimensionality, and size of materials are wellknown to have great effects on their physical, chemical, magnetic, and catalytic properties, as well as on their application in optoelectronic devices.1,2 Among a variety of morphologies, one-dimensional (1D) nano- and microstructures, including rods, wires, tubes, prisms, and belts, are extensively studied because of their potential applications in a wide range of fields.3–7 However, in comparison with 1D nano- and microstructures, 2D sheet, platelet, or disk-shaped morphologies, which have potential applications in information storage, whisper gallery mode lasers, transducer, light emitter, catalyst, and sensor,8–10 have drawn little attention. Currently, much effort is focused on the realization of 2D nano- and microstructures by controlling the sizes and shapes of inorganic solids. In particular, nanoplates and nanoprisms are studied widely because they are better than spherical nanocrystals as building blocks for constructing nanodevices with crystal orientation controlled by a bottom-up method, owing to their anisotropic structures.11 During the crystal growth process, the phase control is crucially important, because the properties of the materials are determined first by their phase. Furthermore, the phases of the crystalline seeds can induce different growth kinetics (isotropically or anisotropically) of the crystals because of their characteristic unit cell structures, which subsequently determine the product shapes.12 Hence, the control of the key factors affecting the crystalline phases of seeds and the subsequent growth is of particular interest. Another crucial factor influencing the crystal growth is the organic additives utilized, which can adsorb selectively onto particular crystallographic facets and change their growth kinetics and surface energies, ultimately leading to the anisotropic growth of low-symmetry nano- and microstructures, such as 1D rods and 2D disks and platelets.13 The concept of selective adsorption is to use an organic molecule to inhibit the growth of a particular crystallographic direction.14 * Author to whom any correspondence should be addressed. E-mail: jlin@ ciac.jl.cn.

As an important kind of rare earth fluoride materials, NaREF4 (RE ) rare earth) has received more and more attention because of its potential applications in a number of fields, such as solidstate lasers,15 3D flat-panel displays,16 biological detection,17,18 low-intensity IR imaging,19 and so forth. Thus, in the past decades, much effort has been devoted to the shape-controlled synthesis of NaREF4 micro- and nanocrystals. Nevertheless, to the best of our knowledge, most of the reported research work was predominantly focused on the fabrication of NaREF4 nanocrystals with different morphologies.20–24 Little has been done on the manufacture of NaREF4 microcrystals with a uniform morphology.25,26 Inorganic microcrystals with novel morphologies are of special significance in understanding the growth behavior and potential technological applications in microelectronic devices.27 To date, there is no report concerning the well-defined and uniform NaLuF4microcrystals with a 2D platelike morphology via a direct facile synthesis route without template or the aid of other techniques. Accordingly, in this paper, we report for the first time the hydrothermal synthesis of β-NaLuF4 hexagonal microplates in a large scale, assisted by trisodium citrate (Cit3-) as a shape modifier. The results demonstrate that the anisotropic unit cell structure of the nucleated β-NaLuF4 seeds is an important factor for the shape control. On the other hand, the Cit3- anions play a critical role in the formation of the final morphology. The morphological evolution and the growth mechanism of the synthesized β-NaLuF4 microcrystals have been studied in detail. In addition, the down-conversion (DC) emission for 5 mol % Eu3+ and 5 mol % Tb3+ doped β-NaLuF4 and up-conversion (UC) emission for 20 mol % Yb3+/2 mol % Er3+ co-doped β-NaLuF4 were investigated. These results demonstrate that β-NaLuF4 is an excellent host lattice for the luminescence of various optically active lanthanide ions.

2. Experimental Section Preparation. The rare earth oxides RE2O3 (RE ) Lu3+, Eu3+, Yb3+, and Er3+) (99.999%) and Tb4O7 (99.999%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry, and other chemicals were purchased from Beijing Chemical Company. All chemicals were analytical grade reagents and used as purchased without further purification. LuCl3 stock solution

10.1021/cg7007528 CCC: $40.75  2008 American Chemical Society Published on Web 01/16/2008

924 Crystal Growth & Design, Vol. 8, No. 3, 2008

Figure 1. (a) XRD patterns of the as-prepared NaLuF4 sample. Standard data of hexagonal (b) β-NaLuF4 (JCPDS 27-0726) and (c) β-NaYF4 (JCPDS 16-0334), used for reference. (0.2 M) was prepared by dissolving Lu2O3 in hydrochloric acid at a high temperature. In a typical procedure, 10 mL of LuCl3(0.2 M) was added into a 20 mL aqueous solution containing 2 mmol of Cit3- to form the metal-Cit3- complex. After vigorous stirring for 30 min, 30 mL of an aqueous solution containing 25 mmol of NaF was introduced into the above solution. After additional agitation for 15 min, the asobtained mixed solution was transferred into a Teflon bottle held in a stainless-steel autoclave, which was sealed and maintained at 180 °C for 24 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 dried in air at 80 °C for 12 h. Additionally, some organic additives, such as ethylenediamine tetraacetic acid disodium salt (EDTA), ammonium oxalate (AO), and polyvinylpyrrolidone (PVP), and different hydrothermal treatment times (1 or 2 h) were selected to investigate the dependence of the morphological and structural properties of the samples on these factors. A similar process was employed to prepare 5 mol % Eu3+ (and 5 mol % Tb3+) doped β-NaLuF4 and 20 mol % Yb3+/2 mol % Er3+ codoped β-NaLuF4 samples, by using Eu2O3, Tb4O7, Yb2O3, and Er2O3 together with Lu2O3 as the starting materials. Characterizations. Powder X-ray 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 80°, with graphite monochromatized Cu KR radiation (λ ) 0.15405 nm). Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) spectra were obtained by using a field emission scanning electron microscope (XL30, Philips). Low- and high-resolution transmission electron microscopy (TEM) was performed by using an FEI Tecnai G2 S-Twin instrument with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. The samples for TEM observations were prepared by dispersing some products in ethanol. This procedure was followed by ultrasonic vibration for 20 min and deposition of a drop of the dispersion onto a carbon-coated copper grid. The excess liquid was wicked away with a filter paper, and the grid was dried at 70 °C. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The UC emission spectra were obtained by using a 980 nm laser from an optical parametric oscillator (OPO, Continuum Surelite, USA) as the excitation source and detected by an R955 instrument (Hamamatsu) from 400 to 700 nm. The PL lifetimes of the samples were measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) by using a tunable laser (pulse width ) 4 ns) as the excitation source (Continuum Sunlite OPO). All the measurements were performed at room temperature.

3. Results and Discussion Structure and Morphology. The composition and phase purity of the products were first examined by XRD, as shown in Figure 1. The diffraction peaks from 10 to 70° can be indexed to those of the hexagonal β-NaLuF4 with calculated lattice constants of a ) 5.890 Å and c ) 3.469 Å, which are very close to the data reported in literature (JCPDS 27-0726, a ) 5.901 Å and c ) 3.453 Å) (Figure 1b). Because β-NaLuF4 and

Li et al.

β-NaYF4 are isostructural, they have the same space group (P63/ m) and crystalline diffraction planes.20b According to the standard card of β-NaYF4 (JCPDS 16-0334, Figure 1c), the three diffraction peaks between 70 and 80° can be indexed as the (311), (212), and (302) planes. Stronger diffraction peaks appear at 17.3°, 30.1°, 31.2°, 44.0°, and 52.9°. They can be attributed to the (100), (110), (101), (201), and (002) planes of β-NaLuF4, respectively. No impurity peaks are observed, indicating the high purity of the final product. The as-prepared β-NaLuF4 sample is investigated by SEM and TEM. Figure 2 shows the SEM images of the product. The low-magnification SEM image reveals that the product consists of hexagonal microplates with a uniform size over the entire surface of the substrate, which indicates the high yield of the product obtained by this simple hydrothermal method. Although a small quatity of microplates are stacked together (Figure 2A), most of them lie parallel to the substrate, and some of them stand vertically, as shown in parts b and c of Figure 2, respectively. Further close investigation under higher magnification also reveals that almost all microplates have a perfectly hexagonal shape and smooth surfaces (Figure 2B,C). The analysis of a number of the microprisms shows that these microplates have an average diameter of 5.2 µm and a thickness of 300 nm . Figure 2D shows the EDS pattern for the β-NaLuF4 microplates, which reveals the presence of Na, Lu, and F. The Au peak in the figure comes from the coating for the measurement. TEM images can provide further insight into the micrometer-scale details of the hexagonal plated shape. Figure 3A is a TEM image of the product, which clearly shows the regular hexagonal cross section. In the corresponding HRTEM image, taken with the electron beam perpendicular to the edge of the microplate, the interplanar distances between adjacent lattice fringes are determined as ∼0.52 and 0.32 nm (Figure 3B). These distances can be well indexed as the d-spacing values of the (101j0) and (112j0) planes of the β-NaLuF4 crystals, respectively.20b,28 The corresponding fast Fourier transform (FFT) (inset in Figure 3B) shows the characteristic hexagonal pattern. Growth Mechanism. A series of controlled experiments demonstrate that the shape evolution of β-NaLuF4 microcrystals is influenced by external factors and the intrinsic crystallographic structure of β-NaLuF4 crystals simultaneously. Intrinsic Crystalline Phase of the Nuclei. One of the critical factors responsible for the shape determination of the microcrysals is the crystallographic phase of the initial seeds during the nucleation process. The seeds can have a variety of crystallographic phases, but the stable phase is highly dependent on the environment, especially on temperature.3 The crystal structure of NaLuF4 exhibits two polymorphic forms, namely, cubic (R-) and hexagonal (β-) phases, depending on the synthesis conditions and methods. To shed light on the evolution of the NaLuF4 microcrystals’ shape, the reactions were quenched at different time intervals. Figure 4 shows the XRD patterns and SEM images of the products prepared at different reaction times at the same hydrothermal temperature (180 °C). The samples display distinctively different XRD patterns associated with the distinct morphological variation at different reaction times. At t ) 1 h, the obtained sample shows a unique XRD pattern due to the pure NaLuF4 R-phase.20b The corresponding typical SEM image reveals that R-NaYF4 products are composed of spherical particles with a mean diameter of 80 nm, as shown in Figure 4a. At t ) 2 h, the R-phase transfers completely to the β-phase. The corresponding products are uniformly hexagonal microplates, similar to the sample obtained at t ) 24 h, except that

Two-Dimensional β-NaLuF4 Hexagonal Microplates

Crystal Growth & Design, Vol. 8, No. 3, 2008 925

Figure 2. SEM images and EDS spectrum of β-NaLuF4 microplates: (A) dense microplates with bulk quantity, (B) microplates parallel to the substrate, (C) microplates standing vertically, and (D) EDS of β-NaLuF4 sample.

Figure 3. (A) TEM and (B) HRTEM images of a β-NaLuF4 hexagonal microplate lying flat on the substrate. The FFT (inset in (B)) is characterized by an hexagonal symmetry.

Figure 4. XRD patterns and corresponding SEM images for NaLuF4 samples at 180 °C as a function of the reaction time: (a) 1 h and (b) 2 h.

the mean diameter of the products is 2.5 µm, as shown in Figure 4b. On the basis of the above analysis, it can be concluded that

the conversion from the R- to the β-phase directly results in the dramatic morphology changes of the products. From another point of view, this experiment has proved that the inherent crystal structure of the seeds plays an important role in the formation of nano- and microstructures. 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 structures, as reported previously for the formation of hexagonal nanoplates of β-NaYF4:Yb3+/Er3+.29 In addition, because the surfaces of spherical R-NaLuF4 particles contain high-index crystallographic planes with a high surface energy, facets tend to form on the particles’ surface to increase the portion of the low-index planes to minimize the total surface energy. This leads to the appearance of the β-NaLuF4 hexagonal


Li et al.

Figure 5. SEM images of β-NaLuF4 particles formed in the absence of Cit3-.

shape with stable {101j0} and {0001} facets. It is noticeable that, in our experiment, it took only 2 h to transfer completely from the R-phase to the β-phase. Thus, it is reasonable that the β-phase of NaLuF4 is thermodynamically more stable than its R-phase under the present synthesis conditions, which also indicates that our method is facile and effective for obtaining the desirable crystalline pure products of β-NaLuF4. Thoma and co-workers30 have determined the crystallographic structures of these phases. In the case of R-NaREF4, Na+ and RE3+ cations are randomly distributed in the cationic sublattice, whereas for β-NaREF4, there are three types of cation sites: a one-fold site occupied by RE3+ (1a), a one-fold site occupied randomly by 1 /2Na+ and 1/2RE3+ (1f), and a two-fold site occupied randomly by Na+ (2h). Thus, for NaLuF4, the transformation from the R-phase to the β-phase is of a disorder-to-order character with respect to cations.20b Role of Cit3-. To substantiate the important influence of Cit3on the β-NaLuF4 shape evolution in our current synthesis, a controlled experiment was carried out in the absence of Cit3anions, the other parameters remaining the same. When the reaction was carried out without the aid of Cit3-, only irregularly shaped microplates as long as 42 µm were obtained instead of uniformly hexagonal microplates, as shown in Figure 5A. The high-magnification image (inset in Figure 5B) shows that the surface of these particles is a little coarse. In addition, other additives, such as PVP, AO, and EDTA, were also tested in this work. It is noted that the use of these organic additives does not alter the crystal phase of the products. The as-formed products with AO as the organic additive are mainly composed of hollow microtubes with a length of 32 µm. Furthermore, several particles, marked with white arrows in Figure 6A, can further demonstrate the formation of hollow tubular structures. Obviously, AO can also suppress the growth along the [0001] orientation, but not much more than can Cit3-. In the presence of PVP, the product has a microrod morphology with a length of 37 µm (Figure 6B). Additionally, when EDTA is employed as an organic additive, the β-NaLuF4 crystals, shown in Figure 6C, are uniformly microrod-shaped and have a mean length of 12.5 µm and a mean diameter of 3.9 µm. Moreover, there is a small quantity of microrods interconnecting at the center to form flowerlike structures. The enlarged SEM image clearly reveals that the microrods have solid interiors and very regular hexagonal cross sections (Figure 6D). EDTA and Cit3have similar properties because they are regarded as excellent chelating agents that slow down the nucleation rate and prevent further aggregation of the particles.31 However, they have a remarkably different impact on the morphologies of the final products, which is related to the differences of the chelating constant with Ln3+ and the adsorption ability of the different crystal facets of NaLuF4 particles. The chelating constant is

Figure 6. SEM images of β-NaLuF4 particles formed in the presence of (a) AO, (b) PVP, and (c and d) EDTA.

Scheme 1. Possible Formation Mechanism for β-NaLuF4 Hexagonal Microplates

larger for EDTA (lg β ) 18–19) than for Cit3- (lg β ) 8–9),32 leading to the different nucleation rates of NaLuF4. 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,33 so the selective adsorption binding on the specific crystal facets of NaLuF4 particles is different, resulting in the different morphologies and sizes of the products. The XRD pattern of β-NaLuF4 particles formed in the presence of EDTA is shown in Figure S1 (Supporting Information). It differs greatly from the pattern of β-NaLuF4 particles formed in the presence of Cit3-, which further confirms the different preferential orientation growths of the crystals under these two conditions. On the basis of the above analysis, we can confirm the unique role of Cit3- in the ultimate shape determination of the products. This demonstrates that the growth of oriented β-NaLuF4 microplates is quite unusual and is directly related to the function of the organic molecules (Cit3-). Laudise et al.34 claimed that the growth of crystals is related to the relative growth rate of different crystal facets, and the difference in the growth rates of various crystal facets results in a different shape of the crystallite. Previous studies on hexagonal plates have shown that the side facets of a hexagonal plate are bound by the {101j0}



Figure 7. Room temperature (a) excitation (λem ) 614 nm) and (b) emission (λex ) 397 nm) spectra of the β-NaLuF4:5% Eu3+ sample.

Figure 8. Room temperature (a) excitation (λem ) 542 nm) and (b) emission (λex ) 356 nm) spectra of the β-NaLuF4:5% Tb3+ sample.

family planes, and the top/bottom facets are enclosed by the {0001} planes.8,20b It is reasonable to assume a similar situation in our case. The Cit3- anions introduced into the reaction system inhibit remarkably the longitudinal growth of rodlike structures but promote the growth of plated structures. Concretely speaking, without Cit3-, the crystals of β-NaLuF4 grow predominantly along the [0001] direction. However, once Cit3- is introduced into the reaction system, it can selectively adsorb on different crystallographic facets, resulting in the modification of the surface free energy of the individual crystallographic facets and leading to the formation of anisotropic microplates. On the basis of our observations, Cit3- may specifically bind to the {0001} facets. The dominant adsorption of Cit3- on the {0001} facets lowers the surface energy of these facets and drives the growth of the nuclei along six symmetric directions: ([101j0], ([11j00], and ([011j0]. This ultimately gives rise to formation of the 2D anisotropic plated microstructures. This phenomenon can be explained by the fact that the growth along the [001] direction was restricted by adsorbing additive Cit3- ions35–37 on the (001) surfaces of ZnO, resulting in the formation of thin ZnO nanoplates. In summary, there are two kinds of key factors that determine the final shapes of β-NaLuF4 microcrystals. The first one is the crystallographic phase of the nucleated seeds because of its characteristic unit cell structure. The Rfβ phase conversion leads directly to the morphology variation. The reason is that the anisotropic structures of the thermodynamically stable β-NaLuF4 seeds induce the anisotropic growth along their crystallographically reactive directions; thus, anisotropic shapes of microcrystals are expected. The other factor is the Cit3anions, used as the chelating agents and shape modifiers, which play an important role in inducing the formation of the {101j0} planes to facilitate the growth of β-NaLuF4 microplates and are essential to the shape evolution. Scheme 1 shows the possible formation mechanism for the β-NaLuF4 hexagonal microplates. Photoluminescence Properties. It is well-known that the hexagonal β-NaYF4 is a much better host lattice for the luminescence of various optically active lanthanide ions.20–24 Here, we also investigate the PL properties of the as-formed β-NaLuF4:Ln3+ (Ln3+ ) Eu3+, Tb3+, and Yb3+/Er3+) hexagonal microplates, in an effort to reveal whether β-NaLuF4 is an efficient host lattice or not. It is noted that the doping with rare earth elements alters neither the crystal structure nor the morphology of the host material. All of the doping ratios of Ln3+ are molar in our experiments. (i) β-NaLuF4:5% Eu3+. The excitation and emission spectra for 5% Eu3+ doped β-NaLuF4 are shown in panels a and b of

Figure 7, respectively. The excitation spectrum (Figure 7a) consists of the characteristic excitation lines of Eu3+ within its 4f6 configuration from 200 to 550 nm, which is similar to the absorption profile for Eu3+ in a β-NaYF4 host.38 In general, most of the excitation lines can be clearly assigned (320 nm, 7 F0f5H6; 365 nm, 7F0f5D4; 384 nm, 7F0f5G2; 397 nm, 7 F0f5L6, strongest; 418 nm, 7F0f5D3), except for the weak ones at 255, 271, 289, and 301 nm (which have a little contribution to the excitation of Eu3+ and are of minor significance).36 Unlike in the excitation spectra for Eu3+ in oxide systems, in which a charge-transfer band (CTB) of Eu3+-O2- is frequently observed between 200 and 300 nm, the CTB of Eu3+F- (generally located below 200 nm) is not present in this region.39 Krupa and Queffelec40 have reported that the CTB from F- to Eu3+ is about 8.15 eV (∼152 nm) in LiYF4:Eu3+ because of the high electronegativity of pure fluoride systems. Excitation into the strongest 7F0f5L6 transition of Eu3+ at 397 nm yields the emission spectrum of the sample, as shown in Figure 7b. It consists of the emission lines associated with the Eu3+ transitions from the excited 5D0,1,2 levels to the 7FJ level, that is, 5D2f7F0 (464 nm), 5D2f7F2 (489 nm), 5D2f7F3 (511 nm), 5D1f7F1 (534 nm), 5D1f7F2 (555 nm), 5D0f7F1 (590 nm), 5D0f7F2 (614 nm), 5D0f7F3 (648 nm), and 5D0f7F4 (693 nm).41 There is no notable shift in the positions of the emission peaks compared to other Eu3+ doped systems because the 4f energy levels of Eu3+ are hardly affected by the crystal field because of the shielding effect of 5s25p6 electrons. In addition, from the emission spectrum of β-NaLuF4:5% Eu3+, it can be seen clearly that the 5D0f7F1 and 5D0f7F2 emissions have comparable intensity, indicating that the Eu3+ ions occupy the 1a sites, 1f sites (C3h symmetry with an inversion center), and 2h sites (Cs symmetry without an inversion center) simultaneously in β-NaLuF4 host lattices.30 Figure S2 (Supporting Information) shows the luminescence decay curve of Eu3+ in β-NaLuF4:5% Eu3+. The decay curve can be fitted well with a single exponential function as I(t) ) I0 exp(-t/τ) (I0 is the initial emission intensity at t ) 0 and τ is the 1/e lifetime of the emission center), and the lifetime for 5D0 (detected at 614 nm) of Eu3+ is determined to be 5.32 ms. (ii) β-NaLuF4:5% Tb3+. The β-NaLuF4:5% Tb3+ sample emits a bright-green light under UV excitation. Panels a and b of Figure 8 show the excitation and emission spectra of the sample, respectively. The excitation spectrum (Figure 8a) is composed of the characteristic f-f transition lines within the Tb3+ 4f8 configuration. Basically, the main excitation lines can be assigned as the transitions from the 7F6 ground state to the different excited states of Tb3+, that is, 288 nm (5I6), 306 nm (5H6), 321 nm (5D0), 344 nm (5G2), 356 nm (5D2), 372 nm (5G6),


Li et al.

between Cit3- and the different crystal surfaces play important roles in the formation of the final products. These results not only improve the knowledge of lanthanide and fluoride chemistry but also provide fundamental insights into the crystal growth and formation mechanism of microscale materials. Furthermore, this simple method might be useful for the synthesis of monodisperse hexagonal plates for many other complex rare earth fluorides with hexagonal symmetry structures. The investigation on the Eu3+ and Tb3+ doped β-NaLuF4 DC spectra and Yb3+/Er3+ co-doped β-NaLuF4 UC spectrum indicates that β-NaLuF4 is also an excellent host lattice for the luminescence of various optically active lanthanide ions.

Figure 9. UC emission spectra of 20% Yb3+/2% Er3+ co-doped β-NaLuF4 sample under 980 nm NIR excitation.

and 382 nm (5D3).42 Upon excitation into the 7F6f5D2 transition at 356 nm, the obtained emission spectrum exhibits four obvious lines centered at 488, 542, 584, and 619 nm, originating from the transitions from the 5D4 excited state to the 7FJ (J ) 6,5,4,3) ground states of the Tb3+ ions, respectively (Figure 8b), the 5 D4f7F5 transition at 542 nm (green) being the most intense group. No emission spectral region from the high-energy 5D3 is observed. This is typical for luminescent materials with a high concentration of Tb3+ ions, because cross-relaxation produces an increase in the population of the 5D4 states at the expense of the 5D3 state.43 The lifetime for 5D4 (detected at 542 nm for the 5D4f7F5 transition) of Tb3+ was determined to be 3.96 ms in the β-NaLuF4:5% Tb3+ sample (Supporting Information, Figure S3). (iii) β-NaLuF4:Yb3+/Er3+. Figure 9 shows the UC luminescence spectrum of the 20% Yb3+/2% Er3+ ion-pair co-doped β-NaLuF4 microcrystals under a 980 nm NIR laser excitation, which is similar to that for Yb3+/Er3+ activated β-NaYF4 nanocrystals and bulks reported previously.44 The emission bands can be assigned to the 2H9/2f4I15/2 (409 nm), 2H11/2f4I15/2 (521 nm), 4S3/2f4I15/2 (542 nm), and 4F9/2f4I15/2 (654 nm) transitions of Er3+. In the case of Er3+, the most important excitation path is 4I15/2f2I11/2f4F7/2, which requires two energytransfer processes from Yb3+. The subsequent multiphonon relaxation populates the emitting 2H11/2 and 4S3/2 states, and the dominant green 2H11/2f4I15/2 and 4S3/2f4I15/2 emissions occur. Alternatively, the electrons can further relax and populate the 2 H9/2 and 4F9/2 levels, resulting in the occurrence of the blue 2 H9/2f4I15/2 and red 4F9/2f4I15/2 emissions. The mechanism responsible for the UC fluorescence of β-NaLuF4:Yb3+/Er3+ is shown in Figure S4 (Supporting Information). To summarize, the luminescence properties of Ln3+ in β-NaLuF4 host lattices are basically identical to those in β-NaYF4 host lattices. The DC and UC luminescence results powerfully demonstrate that the as-prepared β-NaLuF4 hexagonal microplate crystals are also excellent host lattices for the luminescence of Eu3+ and Tb3+ ions and Yb3+/Er3+ ion pairs. The present results suggest that this simple method might be useful for the synthesis of monodisperse hexagonal plates for many other complex rare earth fluorides with hexagonal symmetry structures.

4. Conclusions In conclusion, β-NaLuF4 microplates with a perfect hexagonal shape have been hydrothermally formed on a large scale. The formation mechanism of microplated structures has been investigated in detail. It is believed that the inherent growth habit of the β-NaLuF4 crystals as well as the specific interaction

Acknowledgment. This project is financially supported by the foundation of Bairen Jihua of the Chinese Academy of Science, the MOST of China (2003CB314707, 2007CB935502), and the National Natural Science Foundation of China (NSFC 50572103, 20431030, and 50702057). Supporting Information Available: XRD pattern of β-NaLuF4 sample prepared using EDTA as chelating agent (Figure S1), decay curve for the luminescence (5D0f7F2) of Eu3+ in β-NaLuF4:5% Eu3+ sample (Figure S2), decay curve for the 5D4f7F5emission of Tb3+ in β-NaLuF4:5% Tb3+ (Figure S3), scheme showing the energy level and UC luminescence process for Yb3+/Er3+ in β-NaLuF4:Yb3+/Er3+ microcrystals (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Im, S. H.; Lee, Y. T.; Wiely, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2154. (2) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.W.; Alivisatos, A. P. Nature 2004, 430, 190. (3) Xia, Y. N.; Yang, Y. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F. K.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (4) Xie, B. Q.; Qian, Y. T.; Zhang, S. Y.; Fu, S. Q.; Yu, W. C. Eur. J. Inorg. Chem. 2006, 2454. (5) Zhang, X. J.; Zhang, X. H.; Shi, W. S.; Meng, X. M.; Lee, C. S.; Lee, S. T. Angew. Chem., Int. Ed. 2007, 46, 1525. (6) Zhang, J.; Liu, Z. G.; Lin, J.; Fang, J. Y. Cryst. Growth Des. 2005, 5, 1527. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) (a) Xu, C. X.; Sun, X. W.; Dong, M. B.; Yu, M. B. Appl. Phys. Lett. 2004, 85, 3878. (b) Kim, C.; Kim, Y. J.; Jang, E. S.; Yi, G. C.; Kim, H. H. Appl. Phys. Lett. 2006, 88, 093104. (9) Zhang, Y. W.; Sun, X.; Si, R.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 3260. (10) Xu, C. X.; Sun, X. W.; Dong, Z. L.; Cui, Y. P.; Wang, B. P. Cryst. Growth Des. 2007, 7, 541. (11) Zhang, H. T.; Wu, G.; Chen, X. H. Langmuir 2005, 21, 4281. (12) Jun, Y.-W.; Choi, J.-S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (13) Sigman, M. B.; Ghezelbash, J. A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050. (14) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (15) Ghosh, S.; Rosenbaum, T. F.; Aeppli, G.; Coppersmith, S. N. Nature 2003, 425, 48. (16) Downing, E.; Hesselimk, L.; Ralston, J.; Macfarlane, R. Science 1996, 273, 1185. (17) Yi, G. H.; Lu, H. C.; Zhao, S. Y.; Yue, G.; Yang, W. J.; Chen, D. P.; Guo, L. H. Nano Lett. 2004, 4, 2191. (18) Wang, L. Y.; Li, Y. D. Chem. Commun. 2006, 24, 2557. (19) Shalav, A.; Richards, B. S.; Trupke, T; Krämer, K. W.; Güdel, H. U. Appl. Phys. Lett. 2005, 86, 013505. (20) (a) Heer, S.; Kömpe, K.; Güdel, H.-U.; Haase, M. AdV. Mater. 2004, 16, 2102. (b) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (21) (a) Wang, L. Y.; Yan, R. X.; Hao, Z. Y.; Wang, L.; Zeng, J. H.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 6054. (b) Wang, L. Y.; Li, Y. D. Chem. Mater. 2007, 19, 727. (c) Zeng, J. H.; Su, J. S.; Li, Z. H.; Yan, R. X; Li, Y. D. AdV. Mater. 2005, 17, 2119. (d) Zeng, J. H.; Li, Z. H.; Su, J.; Wang, L. Y.; Yan,


(22) (23) (24) (25) (26) (27) (28) (29) (30) (31)

(32) (33) (34)

R. X.; Li, Y. D. Nanotechnology 2006, 17, 3549. (e) Wang, X.; Zhuang, J.; Peng, J.; Li, Y. D. Inorg. Chem. 2006, 45, 6661. Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. Yi, G. S.; Chow, G. M. Chem. Mater. 2007, 19, 341. Schäfer, H.; Ptacek, P.; Kömpe, K.; Haase, M. Chem. Mater. 2007, 19, 1396. Wang, Z. J.; Tao, F.; Yao, L. Z.; Cai, W. L.; Li, X. G. J. Cryst. Growth 2006, 290, 296. Liang, L. F.; Xu, H. F.; Su, Q.; Konishi, H.; Jiang, Y. B.; Wu, M. M.; Wang, Y. F.; Xia, D. Y. Inorg. Chem. 2004, 43, 1594. Wang, Q. Q.; Xu, G.; Han, G. R. Cryst. Growth Des. 2006, 6, 1776. Wang, L. Y.; Li, Y. D. Nano Lett. 2006, 6, 1645. Wei, Y.; Lu, F. Q.; Zhang, X. R.; Chen, D. P. Chem. Mater. 2006, 18, 5733. Thoma, R. E.; Insley, H.; Hbert, G. M. Inorg. Chem. 1966, 5, 1222. (a) Luo, F.; Jia, C. J.; Song, W.; You, L. P.; Yan, C. H. Cryst. Growth Des. 2005, 5, 137. (b) Tao, F.; Wang, Z. J.; Yao, L. Z.; Cai, W. L.; Li, X. G. Cryst. Growth Des. 2007, 7, 854. (c) Huignard, A.; Buissette, V.; Laurent, G.; Gacoin, T.; Boilot, J. P. Chem. Mater. 2002, 14, 2264. Moeller, T.; Martin, D. F.; Thompson, L. C.; Ferrús, R.; Feistel, G. R.; Randall, W. J. Chem. ReV. 1965, 65, 1. Sun, Y. J.; Chen, Y.; Tian, L. J.; Yu, Y.; Kong, X. G.; Zhao, J. W.; Zhang, H. Nanotechnology 2007, 18, 275609. (a) Laudise, R. A.; Ballman, A. A. J. Phys. Chem. 1960, 64, 688. (b)


(35) (36) (37) (38) (39) (40) (41) (42) (43) (44)

Laudise, R. A.; Kolb, E. D.; Caporaso, A. J. J. Am. Ceram. Soc. 1964, 47, 9. Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. Liang, J. B.; Liu, J. W.; Xie, Q.; Bai, S.; Yu, W. C.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 9436. Tian, A.; Voigt, K.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. R.; Konishi, H.; Xu, H. F. Nat. Mater. 2003, 2, 821. Li, C. X.; Quan, Z. W.; Yang, J.; Yang, P. P.; Lin, J. Inorg. Chem. 2007, 46, 6329. Deshazer, L. G.; Dieke, G. H. J. Chem. Phys. 1963, 38, 2190. Krupa, J. C.; Queffelec, M. J. J. Alloys Compd. 1997, 250, 287. Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783. Thomas, K. S.; Singh, S.; Dieke, G. H. J. Chem. Phys. 1963, 38, 2180. Robbins, D. J.; Cockayne, B.; Chang, N. C.; Clasper, J. L. Solid State Commun. 1976, 20, 673. (a) Lu, H. C.; Yi, G. S.; Zhao, S. Y.; Chen, D. P.; Guo, L. H.; Cheng, J. J. Mater. Chem. 2004, 14, 1336. (b) Krämer, K. W.; Biner, D.; Frei, G.; Güdel, H. U.; Hehlen, M. P.; Lüthi, S. R. Chem. Mater. 2004, 16, 1244. (c) Suyver, J. F.; Grimm, J.; van Veen, M. K.; Biner, D.; Krämer, K. W.; Güdel, H. U J. Lumin. 2006, 117, 1. (d) Diamente, P. R.; Raudsepp, M.; van Veggel, F. C. J. M. AdV. Funct. Mater. 2007, 17, 363.

CG7007528

Two-Dimensional Î²-NaLuF4 Hexagonal Microplates - Crystal Growth

Recommend Documents