Ba2GdF7 Nanocrystals: Solution-Based

Article pubs.acs.org/crystal

Ba2GdF7 Nanocrystals: Solution-Based Synthesis, Growth Mechanism, and Luminescence Properties Qi Zhao,†,‡ Baiqi Shao,†,‡ Wei Lü,† Yongchao Jia,†,‡ Wenzhen Lv,†,‡ Mengmeng Jiao,†,‡ and Hongpeng You*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Pseudo-octahedrons of Ba2GdF7 were successfully synthesized through a solution-based method in a hydrothermal environment for the first time. The preferential adsorption of ethylene glycol (EG) molecules on the {111} planes contributed to the formation of the octahedral shape. Hydrazine was employed as the alkaline source to adjust the adsorption affinity of EG and the hydrolysis rate of NaBF4 (fluorine supply), thereby controlling the morphology and phase structure. A series of contrast experiments were conducted to investigate the growth mechanism of the Ba2GdF7 nanocrystals. As a proof-of-concept experiment, Eu3+, Tb3+, Dy3+, and Yb3+/Er3+ ions were doped to demonstrate the potential of the Ba2GdF7 crystals as host material for phosphors.

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is more, Gd3+ ions can act as an intermediate part transferring energy to activator Ln3+ ions to enhance the luminescent efficiency.27 Therefore, it is meaningful to develop more effective and facile methods for preparing gadolinium-based manifold fluorides nanocrystals. Compared with the high temperature solid-state method, the solution-based method enjoys many advantages such as inexpensive facilities, time- and energy-saving procedure, tunable reaction parameters, and size- and morphologycontrollable products. Accordingly, the solution-based route was widely used in synthesizing high-quality nano- or microcrystals.28−30 However, the crystal growth process is very sensitive to reaction parameters because of the complex chemical environment and intricate growth mechanism in solution. In the phase- and shape-controllable synthesis, it is important to make clear the effect of reaction conditions on the target product. As far as we known, the crystal growth behavior of complex fluorides in solution was rarely investigated. In the present work, we proposed a solution-based strategy to synthesize Ba2GdF7 nanocrystals via a hydrothermal route. The ethylene glycol, hydrazine, and Gd/Ba ratio played critical roles in the phase controlling and morphological evolution. According to a series of contrast experiments, the formation mechanism of the Ba2GdF7 nanocrystals was discussed in detail. Multicolor emission was realized by doping activator ions (Eu3+, Tb3+, and Dy3+) in the Ba2GdF7 matrix. Besides, the

INTRODUCTION Historically, inorganic fluorides have been extensively studied due to their roles in metallurgy, isotope separation, catalysis, optics, etc.1 In particular, the MF2−REF3 (M = alkaline-earth element, RE = rare earth element) system has been given considerable attention because of the application in solid-state lasers, scintillator ceramics, thin optical films, high resolution color displays, and so on.2 Modern interest in the nano- and submicro-scale alkaline-earth lanthanide manifold fluorides has grown dramatically in recent years. This interest arises from their potential applications in biolabeling, bioimaging, and drug delivery as luminescent material due to the advantages of high chemical and thermodynamic stability, sharp emission bandwidths, and large Stokes (or anti-Stokes) shifts.3−6 To date, most of the research was focused on the “BaLnF5” because the high-quality nanocrystal can be prepared with many synthesis methods, such as the thermal decomposition,7,8 coprecipitation,9 and hydro(solvo)thermal route.10,11 As another important MF2−REF3 solid solution, M2REF7 has been rarely investigated. In the earlier literature, the M2REF7 crystals were usually synthesized by annealing and quenching methods12−14 or precipitated from the glass matrix.15,16 Recently, well-defined nanocrystals of Sr2YF7 and Ba2REF7 (RE = La, Sm, Ho, Er, Yb, and Y) were obtained via a hightemperature solution method and liquid−solid−solution (LSS) process, which exhibited satisfactory luminescence properties.17−21 However, to the best of our knowledge, there was no report on the synthesis of nanoscale M2GdF7 crystal. As is known, Gd3+-based compounds are good candidates as multifunctional agents for multimodal bioimaging considering that Gd3+ is an ideal paramagnetic relaxation agent.22−26 What © 2014 American Chemical Society

Received: December 30, 2013 Revised: February 18, 2014 Published: March 3, 2014 1819

dx.doi.org/10.1021/cg401933m | Cryst. Growth Des. 2014, 14, 1819−1826

Crystal Growth & Design

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reaction condition described in the Experimental Section. To shed more light on the structural characteristics, the mole ratio of Gd/Ba was determined to be 2:1.09 by ICP spectrometries. Figure 1 shows the X-ray diffraction (XRD) pattern of the

upconversion luminescent properties were investigated by codoping Yb3+/Er3+ ions.

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EXPERIMENTAL SECTION

Reagents. The rare-earth oxides RE2O3 (RE = Gd, Eu, Tb, Dy, Yb, and Er) (99.99%) were purchased from Shanghai Yuelong NonFerrous Metals Limited. The other analytical chemicals (ethylene glycol, barium chloride, sodium fluoborate, sodium hydroxide, ammonia−water, and hydrazine) were purchased from Beijing Chemical Co. and used as received without further purification. Rare-earth chloride stock solutions were prepared by dissolving the corresponding metal oxide in hydrochloric acid under heating with agitation. The excess hydrochloric acid was evaporated until the pH value was 4. Preparation. In a typical procedure, 1 mL of GdCl3 (1 mol·L−1) and 1 mL of BaCl2 (1 mol·L−1) aqueous solution were added to a mixture solution of ethylene glycol (20 mL) and water (10 mL). After vigorous stirring for 10 min, 7 mL of aqueous solution containing 1.5 mmol (0.15 g) of NaBF4 was introduced into the solution. Subsequently, l mL of hydrazine was added. When the amount of ethylene glycol, hydrazine, and BaCl2 was varied, the addition of water was changed accordingly to ensure 40 mL of the whole volume of the solution. After additional agitation for 15 min, the feedstock was transferred to a 50 mL Teflon-lined stainless autoclave and heated at 160 °C for 6 h. When the autoclave was cooled to room temperature naturally, the precursors were separated by centrifugation, washed with ethanol and deionized water several times, and dried at 60 °C in air. A similar process was employed for preparing lanthanide-doped samples. Stoichiometric amounts of LnCl3 (Ln = Eu, Tb, Dy, Yb, and Er) were added at an initial stage with GdCl3, and other processes were the same afterward. The detailed addition amount in the contrast experiments is listed in Table S1, Supporting Information. Characterization. The samples were characterized by powder Xray diffraction (XRD) performed on a D8 Focus diffractometer (Bruker). Fourier transform infrared spectroscopy (FT-IR) spectra were measured by a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. The size and morphology of the samples were inspected using a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were obtained by a JEOL-2010 transmission electron microscope at an accelerating voltage of 200 kV. Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source at room temperature. The upconversion emission spectra were obtained using a 980 nm laser from an OPO (optical parametric oscillator, Continuum Sunlite, USA) as the excitation source and detected by a R955 (HAMAMATSU) from 400 to 730 nm. All the measurements were performed at room temperature. Elemental analyses of Gd and Ba in the solid samples were carried out on inductively coupled plasma-optical emission spectroscopy (ICP-OES) (iCAP 6300, Thermo Scientific, USA). Gd(III) and Ba(II) were analyzed at wavelengths of 335.0 and 455.4 nm, and the quantitation limits were 0.01 and 0.001 μg/mL, respectively. The solid samples were treated as follows: (1) a 0.05 g powder sample was mixed with 2 mL of HClO4. (2) This mixture was heated at 180 °C until white smoke disappeared. (3) The residue was dissolved by 2 mL of HNO3. (4) The solution was made up to 25 mL with deionized water and stored at 4 °C until analysis.

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Figure 1. XRD pattern of the typical sample.

typical sample. On the basis of the JCPDS reference database, all the diffraction peaks were coincident with the cubic Ba2LaF7 (JCPDS No. 48-0099) except slight shifting toward the higher 2θ side. The lanthanide contraction, which means that the Gd3+ ion has a smaller radius than the La3+ ion, accounts for the shift of the diffractogram.31 Therefore, the as-synthesized sample can be assumed as cubic Ba2GdF7 (a = b = c = 5.926 Å, calculated by the Jade 5.0 software). The morphologies of the products were observed employing SEM and TEM techniques. As is presented in Figure 2a−d, the Ba2GdF7 crystals exhibit pseudo-

Figure 2. (a, b) SEM, (c, d) TEM, and (e) HRTEM images of the typical sample.

octahedron in shape, some of which tend to be spherical. The mean diameter is in the range of 60−90 nm. The HRTEM image (Figure 2e) of an octahedral particle shows clear lattice fringe with interplanar spacing of 0.342 nm that corresponds to the (111) plane of Ba2GdF7. Effect of the Ethylene Glycol. Ethylene glycol (EG) with two hydroxyl (−OH) groups is an important additive in the synthesis of nanomaterials, since it can combine with various metal cations and adsorb on the specific binding sites of nuclei surface, thus modulating the crystal growth kinetically.32−34 In our case, when the EG/H2O ratio was 5/35 in the initial solution, the product exhibited a special starlike appearance with six arms which stretched in six different directions in a perpendicular manner (Figure 3a). The length of each arm is

RESULTS AND DISCUSSION

Phase Identification and Morphology. The energy dispersive X-ray spectroscopy (EDS) measurement confirmed the presence of the Ba, Gd, and F (Figure S1, Supporting Information) in the typical sample which was yielded under the 1820



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are assigned to stretching vibrations, bending vibrations, and rocking mode of the −CH2− groups of EG. The absorption bands at 1087 and 1048 cm−1 come from the C−O and C−C stretching vibrations of EG.35 By contrast, the characteristic peaks corresponding to EG disappear for the sample prepared in the water. According to the experimental results, the EG served two functions in the hydrothermal process: (1) to promote the formation of the cubic Ba2GdF7 phase; (2) to favor the preferential crystal growth. In the EG-riched solution, the Gd3+ and Ba2+ ions were surrounded and protected by EG molecules through forming complexes, which would decompose gradually during the hydrothermal process.34,36 The slow release of metal ions induce the simultaneous precipitation of the Gd3+ and Ba2+ by F− ions, leading to the formation of Ba2GdF7 crystals. Furthermore, it is supposed that the affinity of ethylene glycol to Gd3+ is stronger than that to Ba2+.37 Under the circumstance that the ethylene glycol is deficient, there would remain free Ba2+ in the solution, resulting in the formation of BaF2. As is known, the −OH groups of the EG molecular can adsorb certain facets of crystal nuclei and inhibit the growth of these crystal plane. Thus, the crystal growth would be favored in the direction where crystallization hindrance is weakest. Empirically, the surfactant molecules prefer to adsorb on the lattice plane where the atomic planar density (APD) is higher. The Ba2GdF7 was determined to be face-centered cubic structure, just the same as Ba2LaF7, where the APD of {111}, {100}, and {110} is 2.3/a2, 2/a2, and 1.4/a2, respectively. Evidently, the EG would adsorb on the {111} planes preferentially because of the higher APD. The growth rate along the ⟨100⟩ was faster than that along the ⟨111⟩ directions, favoring the formation of six-armed starlike structures. Higher EG concentration might lead to more efficient adsorption of EG molecules on the {111} facets and the growth of {100} planes was further enhanced to some extent. In general, facets perpendicular to the fast directions of growth have smaller surface areas, and slower growing facets therefore determine the crystal morphology. Consequently, the fast growing {100} facets shrunk and the Ba2GdF7 crystals were bound by eight {111} surfaces which exhibited the octahedral shape eventually. This is confirmed by the HETEM image (Figure 2e) which clearly shows the (111) plane of the octahedron surface. As for the spherical nanoparticles, it is because that EG concentration was high enough to adsorb on the crystal faces in all directions to afford isotropic growth. In addition, the intermediate shapepseudo-octahedron and quasi-spherecan be obtained when the growth rate along the ⟨100⟩ and ⟨111⟩ was modulated appropriately. Effect of the Hydrazine. Hydrazine (HZ) is a well-known reducing agent in chemical synthesis, but recently the hydrazine has found application in fabricating nanomaterials and tailoring their morphology.38−40 In our case, when HZ was absent, elliptic nanostructures of orthorhombic GdF3 (Figure S3, Supporting Information) were obtained where the Ba2+ acted as an inorganic structure-directing agent, as we have reported previously.41 In the presence of 0.3 mL of HZ, cubic Ba2GdF7 emerged which took the form of irregular blocks with a diameter of 420−550 nm (Figures 5a and 6a). Some of the blocks appeared to be octahedral shape (marked by arrow). With addition of HZ 0.5 and 0.8 mL, the product was quasispheres as well as a few quasi-octahedrons (marked by arrow) which had a diameter of 70−150 nm and 50−100 nm, respectively (Figure 5b,c). But the as-obtained sub-micro-

Figure 3. SEM images of samples prepared at different EG/H2O ratios: (a) 5/35, (b, c) 10/30, (d) 15/25, (e) 25/15. (f) Schematic illustration of the morphological evolution.

about 600−800 nm, measured from the center of the structure. If 10 mL of EG was introduced in the reaction system, the arms of the starlike structure were shortened to 300−400 nm (Figure 3b,c). Unfortunately, the XRD patterns show mixed phases of cubic Ba2GdF7 and BaF2 (marked by orange dots) for the samples prepared at a EG/H2O ratio of 5/35 and 10/30 (Figure 4a,b). With further increasing the EG addition to 15

Figure 4. XRD patterns of samples prepared at different EG/H2O ratios: (a) 5/35, (b) 10/30, (c) 15/25, (d) 25/15.

mL, pure cubic Ba2GdF7 was obtained (Figure 4c). The SEM image demonstrates that the sample is composed of octahedrons with 100−200 nm in edge length (Figure 3d). Careful observation shows that the six arms of the above starlike structure further shrunk to form the octagonal shape. High content of EG up to 25 mL produced Ba 2 GdF7 nanospheres in diameter of 50−70 nm (Figures 3e and 4d). A schematic illustration of the morphological evolution is shown in Figure 3f. Figure S2, Supporting Information shows the FT-IR spectra of samples prepared with and without EG. The band centering at 3397 cm−1 can be attributed to the stretching vibrations of O−H groups of absorbed water or ethylene glycol. The bending mode of the water molecule is observed at 1635 cm−1. The peaks located at about 2955, 1400−1500, and 865 cm−1 1821



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biphase of Ba2GdF7 and BaF2 (Figure 6e,f). The more HZ that was added, the stronger the diffraction peaks of BaF2 became. It is evident that the HZ takes responsibility for the variety of crystal morphology. More HZ favors the growth of the octagonal crystals. As is discussed above, the formation of octagonal shape was attributed to the EG’s selective adsorption on the special crystal facets. Additionally, our group’s previous work has proved that the electrostatic attraction between −OH and metal ions can be weakened by the “OH− ions atmosphere”. In the case of Ba2GdF7 crystal, more HZ addition implied a higher OH− ions concentration in reaction system. As a result, the adsorption affinity of the −OH groups became weak, and the EG molecules preferentially adsorbed on the {111} planes with higher APD. What is more, the decreased adsorption of EG on metal ions caused the easy release of Ba2+ ions because of the weaker interaction between hydroxyl groups and Ba2+ compared to Gd3+. This is the reason why BaF2 was formed when HZ addition increased to 2−3 mL. With less HZ, by contrast, the electrostatic attraction between hydroxyl groups and metal ions was relatively strong to adsorb the crystal homogeneously in different directions, which favored the isotropic growth and eventually induced the formation of the spherical shape. It is also observed that the size of the products decreased as the HZ content increased. As is known, slow nucleation provides low concentration of seeds and results in large particles, whereas fast nucleation produces high particle concentration and yields small particles. In our work, the HZ increased the pH value of the solution. Higher pH value could accelerate the hydrolysis of NaBF4 which acted as precipitant and increase the concentration of F− in the reaction system. Besides, hydrazine is also a Lewis base, and it could directly attack the boron atom in the BF4− ainion, which would promote the release of the F− anion. Therefore, an elevated HZ amount gave rise to a higher concentration of F− and resulted in fast crystal nucleation and thereby produced crystals with smaller size. Moreover, it has been reported that a high concentration of F− is favorable to the coprecipitation of Ba2+ and Gd3+.41 This is confirmed by the fact that only orthorhombic GdF3 was yielded without adding HZ, indicating that the alkaline atmosphere was essential to the formation of ternary fluoride. However, if aqueous ammonia (25 wt %) and sodium hydroxide (1 mol·L−1 aqueous solution) were employed as the alkaline source, impurity was detected by the XRD analysis, namely, cubic Na5Gd9F32 and hexagonal NaGdF4 (Figure 7). Considering that aqueous ammonia and sodium hydroxide have stronger alkalinity than hydrazine, the concentration of Na+ was increased too much because of the hydrolysis of NaBF4. Hence, the Na-containing multiple fluorides were produced at this circumstance. Their morphologies are shown in Figure S4, Supporting Information. On the basis of the analysis above, the hydrazine played two significant roles in the formation of cubic Ba2GdF7: (1) to adjust the adsorption affinity of EG molecular with metal ions, inducing the morphological evolution; (2) to regulate the hydrolysis of NaBF4, affecting the size and phase structure. Effect of the Barium Amount. With respect to the preparation of complex fluorides, the ratio of starting materials has an important effect on the phase structure of the final product.42 The structural and morphological evolution of the Ba2GdF7 was investigated by varying the content of barium chloride, while keeping the gadolinium chloride constant (1 mmol). Uniform nanorods of hexagonal NaGdF4 with the

Figure 5. SEM images of samples obtained with different hydrazine addition: (a) 0.3 mL, (b) 0.4 mL, (c) 0.8 mL, (d) 1.5 mL, (e) 2.0 mL, (f) 3.0 mmol.

Figure 6. XRD patterns of samples obtained with different hydrazine addition: (a) 0.3 mL, (b) 0.4 mL, (c) 0.8 mL, (d) 1.5 mL, (e) 2.0 mL, (f) 3.0 mmol.

crystals had broad size distribution and rough surface. The XRD patterns shown in Figure 6 confirmed their pure cubic Ba2GdF7 nature (Figure 6b,c). Following the increase of HZ content to 1.5 mL in the initial solution, the Ba2GdF7 crystals tended to be pseudo-octahedrons with several nanospheres remaining (Figures 5d and 6d). The size distribution became narrower, and the mean diameter was 75−110 nm. Upon further increasing the HZ addition to 2 mL, the octahedrons were clearly observed with rounded edges with the length of 65−120 nm (Figure 5e). Product prepared by introducing 3 mL of HZ evolved to octahedrons with sharpened edges having 60−100 nm in length (Figure 5f). However, the XRD patterns show that high content of HZ (2−3 mL) would produce a 1822



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Figure 7. XRD patterns of samples prepared with (a) aqueous ammonia and (b) sodium hydroxide as the alkaline source.

length and diameter of 0.6−1 μm and 60−100 nm were yielded when no BaCl2 was used (Figures 8a and 9a). The XRD

Figure 9. XRD patterns of samples obtained with different amounts of BaCl2: (a) 0 mmol, (b) 0.1 mmol, (c) 0.3 mmol, (d) 0.5 mmol, (e) 0.8 mmol, (f) 1.5 mmol.

system containing 0.8 mmol of BaCl2 (Figures 8e and 9e). A high barium amount of up to 1.5 mmol would lead to the formation of BaF2 (Figure 9f), but the morphology did not show significant change (Figure 8f). The observation on the phase and morphology variation by the barium amount revealed that the barium chloride not only enabled the formation of cubic Ba2GdF7 but also promoted the morphological evolution. When the BaCl2 content was low, the formation of NaGdF4, rather than the Ba2GdF7, was favorable because of the deficiency of Ba2+ ions. What is more, the crystal growth was limited by the low concentration of Ba2+ ions so that the inhibitor effect of EG was weakened. This is why the irregular nanoparticles were obtained instead of octahedrons. Actually, the 1.5 mmol of BaCl2 was still insufficient stoichiometrically for the Ba2GdF7 crystals. But, as was discussed above, only the free Gd3+ and Ba2+ ions released from the metal−EG complexes were available in the precipitation of Ba2GdF7. Taking the stability difference of the Gd- and Ba-based complexes into consideration, the exact concentration of Gd3+ and Ba2+ ions was difficult to control. When BaCl2 addition increased to a certain value, the free Ba2+ ions was superfluous in the reaction solution, resulting in the formation of BaF2. Formation Mechanisms. To shed more light on the growth mechanism of the Ba2GdF7 crystal, a series of timedependent experiments were conducted. Figure 10 shows the TEM and SEM images and XRD patterns of the intermediate products for different hydrothermal times. Before hydrothermal treatment, the initial solution was clear and transparent, indicating that fluoride precipitation has not been formed. Amorphous colloids were yielded in a short reaction time of 0.5 h, which had very low crystallinity (Figure S5, Supporting Information). When the hydrothermal time was increased to 1 h, quasi-spheres with a mean diameter of 50 nm came into being, some of which were nearly pseudo-octahedron in shape (Figure 10a). The XRD pattern shows that the product has crystallized into cubic Ba2GdF7 (Figure 10d). After reacting for 3 h, the Ba2GdF7 crystals tended to pseudo-octahedrons with rounded edges with a mean length of 75 nm. By further

Figure 8. SEM images of samples obtained with different amounts of BaCl2: (a) 0 mmol, (b) 0.1 mmol, (c) 0.3 mmol, (d) 0.5 mmol, (e) 0.8 mmol, (f) 1.5 mmol.

patterns (Figure 9b) show that the cubic Ba2GdF7 came into being (marked by green dots) when 0.1 mmol of BaCl2 was added. The SEM images (Figure 8b) demonstrated that the products still exhibited nanorods but with a lower aspect ratios (length = 400−650 nm; diameter = 60−120 nm). When the BaCl2 increased to 0.3 mmol, pure cubic Ba2GdF7 crystals were obtained (Figure 9c), which consisted of semiformed microtubes and irregular nanoparticles with a mean diameter of 75 nm (Figure 8c). The tubular structure disappeared, and agglomerated nanoparticles of Ba2GdF7 emerged with a diameter of 50−100 nm if 0.5 mmol of BaCl2 was introduced into the initial solution (Figures 8d and 9d). It is interesting that quasi-octahedrons of Ba2GdF7 were produced which had rounded edges with a length of 400−650 nm in the reaction 1823



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Luminescent Properties. It is well-known that alkalineearth lanthanide manifold fluoride is an efficient host lattice for the luminescence of various optically active ions. The luminescence properties of the Ba 2 GdF 7 crystal were investigated by singly doping Eu3+, Tb3+, and Dy3+ as activator. The PL excitation spectra for Ln3+-doped Ba2GdF7 octahedron were obtained by monitoring the characteristic emissions of the Eu3+, Tb3+, and Dy3+ ions at 590, 545, and 477 nm, respectively. As is presented in Figure 11, the peaks located at 274, 298, 306,

Figure 10. SEM images of samples synthesized with different reaction times: (a) 1 h, (b) 3 h, (c) 24 h, and (d) their corresponding XRD patterns.

prolonging the reaction time to 24 h, the octahedrons grew up to 80−140 nm in length and the edges were sharpened. As is shown in the XRD patterns (Figure 10d), the diffraction intensity become stronger as the reaction time extended, indicating that crystallinity increased as the hydrothermal treatment proceeded. According to the experimental results, the growth process of the octahedral Ba2GdF7 crystals can be illustrated as follows: in the first stages, the Ba2GdF7 crystal nuclei were formed by the F− ions reacting with the Gd3+ and Ba2+ ions released from the metal−EG complex. The hydroxyl groups of EG preferentially adsorbed on specific planes of the initial nuclei surfaces, regulating the growth direction and controlling the surface morphology. The Ba2GdF7 crystals underwent morphological evolution from sphere to octagonal shape because of the enhanced growth rate along the ⟨100⟩ versus ⟨111⟩. Afterward, the Ostwald-ripening was mainly going on to make larger crystal. As is discussed above, various shapes could be formed depending on the EG content, HZ addition, and BaCl2 amount. The effect of reaction parameters and crystal growth process are summarized in Scheme 1.

Figure 11. (a−c) Excitation (left) and emission (right) spectra of Ln3+-doped (Ln = Eu, Tb, Dy) samples.

and 311 nm originate from the transitions of 8S7/2 → 6I7/2, 8S7/2 → 6P3/2, 8S7/2 → 6P5/2, and 8S7/2 → 6P7/2 of the Gd3+ transitions, respectively. The weaker lines in the long wavelength regions of 315−400 nm are assigned to the f−f intraconfiguration transitions of the Ln3+ ions, and their assignments are marked in the figure. Under excitation of the Gd3+ ions at 274 nm, intense and characteristic emission patterns of the Ln3+ are detected in the visible region. As is presented in Figure 11a, the most intense band at 590 nm corresponds to the 5D0 → 7F1 transitions of the Eu3+ ions and the peaks locating at 470, 490, 512, 527, 538, 556, and 615 nm come from the 5D2 → 7FJ (J = 1, 2, 3), 5D1 → 7FJ (J = 0, 1, 2), and 5D0 → 7F2 transitions. Figure 11b shows that the emission spectrum of the Tb3+-doped Ba2GdF7 consists of a group of lines peaking at 382, 416, 437, 471, 490, 545, 585, and 623 nm, which are explicitly assigned to 5D3 → 7F J (J = 6, 5, 4, 2) and 5 D4 → 7F J (J = 6, 5, 4, 3) transitions of the Tb3+ ions. In Figure 11c, the emission spectrum of Dy3+-doped sample is dominated by strong lines centered at 477 and 570 nm due to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions, respectively. No emission from the Gd3+ ions is observed in all the three emission spectra, indicating that an efficient energy transfer occurs from the Gd3+ to Ln3+ ions. Generally, the Er3+ ions are a popularly used emitter to obtain upconversion (UC) luminescence, and Yb3+ ions are usually codoped as a sensitizer to enhance the emission efficiency.43,44 Herein, we prepared the Yb3+/Er3+ codoped

Scheme 1. Schematic Illustration of Crystal Growth Process and Effect of Reaction Parameters on Morphological Evolution

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(EG) amount. The EG molecular preferentially adsorbed on the {111} facets because of the higher atomic planar density and favored the formation of the octagonal shape. Hydrazine played important roles as alkaline source. It can control the shape, size, and phase of the products by adjusting the adsorption affinity of EG molecular with metal ions and regulating the hydrolysis rate of NaBF4. The amount of BaCl2 also had significant effect on the growth process of the Ba2GdF7 crystal dynamically. Desirable multicolor outputs were generated under excitation of 274 nm by choosing Eu3+, Tb3+, and Dy3+ ions as emitter where the Gd3+ ions served as energy intermediate. Furthermore, the as-synthesized Ba2GdF7 crystals were proven to be excellent host materials for upconversion luminescence by codoping Yb3+ and Er3+ ions.

Ba2GdF7 crystals and investigated their UC emission properties. Under 980 nm NIR excitation, the spectra of Ba2GdF7:20% Yb3+, 2% Er3+ crystals exhibit two emission bands attributed to the 4f inter shell electronic transitions of the Er3+ ions. As is shown in Figure 12, the green emission corresponding to the

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ASSOCIATED CONTENT

S Supporting Information * 3+

Figure 12. Upconversion emission spectra of Ba2GdF7:Yb /Er crystals under 980 nm excitation.

Figure S1: EDS spectrum of a typical sample. Figure S2: FT-IR spectra of samples prepared with and without EG. Figure S3: SEM images and XRD pattern of sample prepared without hydrazine. Figure S4: SEM images of samples prepared with aqueous ammonia and sodium hydroxide as the alkaline source. Figure S5: SEM image and XRD pattern of sample after reacting for 0.5 h. Table S1: Detailed addition amount in the contrast experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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transitions of 2H11/2 → 4I15/2 (521 nm) and 4S3/2 → 4I15/2 (540 nm) predominates over the red emission ascribed to the transition of 4F9/2 → 4I15/2 (655 nm). To study the UC mechanism, excitation power density dependence of the UC luminescence was measured. For the unsaturated upconversion process, the relationship between the upconvertion emission intensity (Iup) and the pump laser intensity (Ip) is held by the formula: Iup ∝ Ipn, where n is the number of pump photons absorbed per upconverted photon emitted.45 A plot of ln Iup versus ln Ip yields a straight line with slope n. As is shown in Figure 13, the slopes (n values) were calculated to be 1.96, 1.93,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-431-85262798. Fax: +86-431-85698041. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 21271167 and 11304309) and the Fund for Creative Research Groups (Grant No. 21221061), and the National Basic Research Program of China (973 Program, grant no 2014CB6438003).

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Figure 13. Pump power dependence of UC emission intensities of Ba2GdF7:Yb3+/Er3+ crystals.

and 1.77 for 4S3/2 → 4I15/2, 4F9/2 → 4I15/2, and 2H11/2 → 4I15/2 transition of the Er3+ ions, respectively, indicating that the green and red emissions were both two-photon processes. Accordingly, the as-synthesized Ba2GdF7 crystals can be used as host material for upconversion luminescence.

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CONCLUSIONS In summary, cubic Ba2GdF7 crystals have been prepared through a simple and facile hydrothermal method. The morphology evolved from starlike architecture to pseudooctahedrons and quasi-spheres by varying the ethylene glycol 1825



Article

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Ba2GdF7 Nanocrystals: Solution-Based

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