Monodispersed β-NaYF4 Mesocrystals: In Situ Ion Exchange and

Article pubs.acs.org/crystal

Monodispersed β‑NaYF4 Mesocrystals: In Situ Ion Exchange and Multicolor Up- and Down-Conversions Jianle Zhuang,†,‡ Xianfeng Yang,† Junxiang Fu,† Chaolun Liang,† Mingmei Wu,*,† Jing Wang,*,† and Qiang Su† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry/State Key Laboratory of Optoelectronic Materials and Technologies/Key Laboratory of Environment and Energy Chemistry of Guangdong Higher Education Institutes, School of Chemistry and Chemical Engineering, Instrumental Analysis and Research Center, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, P. R. China. ‡ Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: Monodispersed β-NaYF4 spindlelike mesocrystals have been successfully synthesized directly from Y(OH)xF3−x as a precursor via a facile route. Detailed structural analyses demonstrated that the crystal framework structures of Y(OH)xF3−x and β-NaYF4 were identical. A possible mechanism based on in situ ion-exchange transformation has been proposed. The monodispersed β-NaYF4 can be obtained through carefully controlled synthesis of the precursor. The photoluminescence properties of the β-NaYF4 doped with various rare earth ions (such as Yb/Er, Yb/Tm, Pr, Nd, Sm, Eu, Tb, Dy, and Ho) were investigated and multicolor emission was achieved.

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INTRODUCTION Up-conversion (UC) phosphors have attracted increasing attention due to their potential applications in solid-state lasers, flat-panel displays, solar cells, biolabels, and so on.1 It is well-known that NaYF4 is an excellent host material for UC emission.2−4 NaYF4 has two crystal forms: cubic (denoted as αNaYF4) and hexagonal (denoted as β-NaYF4). β-NaYF4 is more efficient than α-NaYF4 in UC emission as reported.4,5 However, α-NaYF4 as a preferentially kinetic product would always form at an initial stage in solution-based reactions.5,6 The direct growth of highly efficient β-NaYF4 from a rationally selected precursor is of great interest. β-NaYF4 microdisks of single crystals with tunable size and thickness were directly grown from yttrium oxide in our group.7 Since the properties of materials, especially as the host-sensitive rare-earth-doped phosphors can be tuned by adjusting their morphologies, it is reasonable that size- and shape-controlled NaYF4 materials with novel morphologies may exhibit extensive applications due to their unique luminescence properties.7−9 In the past several years, nano/microsized single- or poly crystals of NaYF4 with various morphologies, such as nonorods,10,11 nanoparticles,6,12,13 branched nanocrystals,14 nanostructure arrays,8,15 microdisks,7,16 microrods,17 microtubes,5,18 and nanoassemblies19a have been synthesized via different methods. However, the rational growth of monodispersed β-NaYF4 mesocrystals has not been achieved up to now. As mentioned in literature, © XXXX American Chemical Society

morphology can play a significant factor in affecting properties of single- and even mesocrystals. For example, it was reported that the luminescent properties of YF3:Ce3+ mesocrystals can be tuned with morphologies changes,19b and bismuth oxybromide with different morphologies exhibited different optical properties and photocatalytic activities.19c Thus, the successful synthesis of β-NaYF4 mesocrystals will be of great importance for studying NaYF4. Recently, a synthetic route based on ion-exchange reactions has been regarded as an efficient way to obtain nanomaterials, especially those nano- or microstructures which are difficult to be achieved through general methods.20−22 This route is very attractive because it can “duplicate” the morphologies from precursors to final products. For example, CdSe nanocrystals and CdTe tetrapods can be converted into Ag2Se nanocrystals and Ag2Te tetrapods, respectively.20 Columnar and tubular ZnS have been transferred to Ag2S, Cu2S, Bi2S3, or Sb2S3 with preservation of initial morphologies.21 Although such an ionexchange route has been adopted to synthesize abundant materials, there are few successful applications in the synthesis of rare earth fluorides. Very recently, rare earth fluoride nanorods, nanowires, and nanotubes have been prepared by in Received: November 29, 2012 Revised: April 23, 2013

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situ ion-exchange transformation from rare earth hydroxides.11,23,24 Therefore, it is highly desired in nanochemistry and crystal engineering to obtain rare earth fluorides with novel nano- and mesostructures by an in situ ion exchange route from suitable precursors. In this work, we report the synthesis of monodispersed spindlelike β-NaYF4 mesocrystals via in situ ion-exchange transformation from nanorod bundles of Y(OH) x F 3−x precursors. The possible mechanism for the formation of such a novel structure of β-NaYF4 and the luminescence properties of various rare earth ions doped β-NaYF4 were proposed. Other inorganic materials with mesocrystal structures were fabricated and characterized abundantly in the literature.25 However, to the best of our knowledge, such a novel kind of NaYF4 mesocrystal has not been reported before. Herein, the topotactic conversion from Y(OH)xF3−x to NaYF4 mesocrystals could open up a new strategy for synthesizing new rare earth mesocrystals, which may be used in biolabeling and detecting technology.1g,19b Multimodal bioimaging combining more than one imaging modality, such as optical, nuclear, ultrasound, and magnetic resonance imaging (MRI), is a new frontier in biology and medicine.26 Among all the medical imaging techniques, optical imaging is the only technique that can provide cellular- or molecular-level information with almost single molecule sensitivity. Meanwhile, it is inexpensive, robust, and portable and has the highest sensitivity and spatial resolution for in vitro imaging.27 However, most research has been focused on the near infrared (NIR)-to-vis UC properties of rare earth ionscodoped NaYF4.4,13,18,28 Recently, Li and coauthors demonstrated that dual-mode [down-conversion (DC) and UC] luminescent nanomaterials may potentially serve as luminescent probes with multiplex capability in highly sensitive bioassays.29 Unfortunately, Ln3+ (Ln = Eu, Tb, Ce−Tb)-doped NaYF4 only shows green and red DC emissions with a much lower signalto-noise ratio. In order to advance their direct applications in highly sensitive and multifunctional bioassays, color-point tuning, including multicolor and high color purity, is an important challenge for dual-mode (DC and UC) luminescent nanomaterials. Here, we reported the DC and UC properties of Ln3+-doped NaYF4. It is of great interest that multicolors, including blue, green, orange, red, and even white, can be tuned by simply doping or codoping with different Ln3+ ions. More importantly, white, including bluish white, yellowish white, and pure white, can be obtained by singly doping with Dy3+, Ho3+, and Pr3+ ions, respectively.

Figure 1. XRD patterns of the (a) precursor and (b) final product. Vertical bars are the standard diffraction data of Y(OH)1.57F1.43 and βNaYF4, respectively (JCPDS cards no. 80-2008 and 16-0334).

alternately for several times and then dried at 60 °C overnight (Figure 1b). Characterization. Products were characterized by powder X-ray diffraction [Rigaku D/MAX 2200 VPC using Cu Kα radiation (λ = 0.1541 nm)] at 40 kV and 30 mA with a scanning rate of 10°/min. SEM images were taken on a Philips FEI Quanta 400 instrument. TEM measurements and selected area electron diffraction (SAED) analyses were performed on a JEOL 2010 high-resolution transmission electron microscope equipped with an Oxford Instrument EDS system. The downconversion luminescence spectra were measured on a Fluorolog-3 spectrofluorometer (Jobin Yvon Inc./Specx) equipped with a 450 W Xe lamp, double excitation monochromators, and single emission monochromator. Upconversion emission spectra were detected by the same instrument equipped with an external 980 nm diode laser and a R928P photomultiplier tube; the pump power of the laser was fixed at 450 mW/mm2.

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RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the (a) precursor and (b) final product. The final product was obtained from the asprepared precursor (Figure 1a) after a postsolvothermal treatment as described in Experimental Section. Although both of the XRD patterns seem to be similar, difference can be found between Figure 1 (panels a and b), especially those diffraction peaks indicated by the asterisk (Figure 1b). The XRD pattern of the precursor can be indexed to hexagonal Y(OH)xF3−x [the JCPDS number cannot be found in the database, and the pattern is close to JCPDS no. 80-2008, Y(OH)1.57F1.43]. The EDX spectrum indicates that the precursor is composed of Y, O, and F (Figure SI-1a of the Supporting Information) but absent of Na. After the solvothermal treatment, all the diffraction peaks of the resulting sample can be indexed to β-NaYF4 (JCPDS no. 16-0334), indicating a successful conversion from Y(OH)xF3−x. EDX analysis suggests the presence of Na in the final product (Figure SI-1b of the Supporting Information). Figure 2 (panels a1 and a2) show the typical SEM images of the precursor. It can be seen that the sample is composed of monodispersed spindlelike nanorod bundles with diameters of

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EXPERIMENTAL SECTION Synthesis of Y(OH)xF3−x Precursors. In a typical procedure, NaF (5 mL, 0.3 M) aqueous solution was added to Y(NO3)3 (5 mL, 0.2 M), under vigorous stirring. The pH value of the mixture solution was adjusted by adding a NaOH aqueous solution carefully. The mixture was further stirred for 10 min and then transferred to a 30 mL Teflon-lined stainless autoclave and heated at 100 °C for 5 h. The white precipitate was centrifuged and washed with deionized water several times and then dried at 60 °C overnight (Figure 1a). Synthesis of β-NaYF4 Bundles. Y(OH)xF3−x (0.07 g), NaF (0.063 g), and NH4HF2 (0.171 g) were mixed in 10 mL ethanol in a 23 mL Teflon-lined stainless autoclave. After stirring vigorously for 30 min, the autoclave was sealed and heated at 180 °C for 24 h. The obtained products were then centrifuged and washed with deionized water and ethanol B

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Figure 2. SEM images of the Y(OH)xF3−x precursors (a1−a4) and β-NaYF4 mesocrystals (b1−b4) with different magnifications.

about 1.5 μm and lengths of about 2.7 μm. Each bundle is an assembly of aligned nanorods with diameters of about 20 nm [Figure 2 (panels a3 and a4) and Figure SI-2 of the Supporting Information]. After solvothermal treatment, the obtained βNaYF4 crystals preserved the profile and size of the precursor ones, as shown in Figure 2 (panels b1 and b2) and Figure SI-3 of the Supporting Information. Further observation reveals that the microspindles consist of hexagonal particles [Figure 2 (panels b3 and b4)], which are different from the nanorod bundles of the precursor. Such monondispersed β-NaYF4 spindles with mesotructure are quite novel and have never been reported before. To further study the detailed structure relationship between Y(OH)xF3−x and β-NaYF4, two related crystals were cut into slices to be clearly observed with TEM measurement. Figure 3 (panels a and b) show the TEM images of the cross sections of a Y(OH)xF3−x crystal and a NaYF4 crystal, respectively, along their long axes and corresponding SAED patterns (insets). It can be observed that the SAED patterns are very similar to each other. Each pattern can be identified as a single crystalline

diffraction pattern with a zone axis of [010], indicating that either a spindlelike Y(OH)xF3−x particle or a β-NaYF4 one was comprised of smaller primary crystals with almost the same orientations. The c axes of all the building blocks were grown along the longitudinal direction of the spindle particle. HRTEM images and related FFT patterns further attest to the direction of each smaller crystal of Y(OH)xF3−x and β-NaYF4 [Figure 3 (panels c and d), respectively]. The interplanar spacing of 0.51 nm was indexed to be {100} lattice fringes for both Y(OH)xF3−x and β-NaYF4 [Figure 3 (panels c and d)]. These results suggest that the {100} planes of a Y(OH)xF3−x precursor crystal are converted into the {100} planes of βNaYF4 one and the long axes of either Y(OH)xF3−x or β-NaYF4 microspindles is the c axis. This fact can also be supported by the structural analysis of the cross section perpendicular to the c axis of NaYF4 (Figure SI-4 of the Supporting Information). The SAED pattern with circle spots indicated a mesostructure feature of these β-NaYF4 spindles (Figure SI-4b of the Supporting Information). It has been well-documented in literature to synthesize products by solid precursors. However, there are limited reports about the synthesis of β-NaYF 4 by such a method.11,23,24 We reported the synthesis of β-NaYF4 directly from solid Y2O3 by a topotactic growth mechanism.7 The solvothermal treatment of Y(OH)xF3−x here was similar to that for Y2O3;7 however, there may be a different mechanism for the transformation of Y(OH)xF3−x to β-NaYF4. The XRD patterns indicate that the structures of Y(OH)xF3−x and β-NaYF4 are very similar, both of which are hexagonal phases. The Y(OH)xF3−x crystals were transferred to β-NaYF4 crystals, keeping their profiles almost unchanged. Furthermore, the long axes of the Y(OH)xF3−x crystals are in agreement with those of β-NaYF4 ones. Figure 4 reveals the schematic representation of crystallographic structures of Y(OH)xF3−x and β-NaYF4 viewed along their c axes, respectively. It can be identified that the framework structures of these two compounds are identical. The OH− ions in Y(OH)xF3−x can easily be substituted by F− ions. The Na+ ions can easily be trapped into the tunnel structure to form a stable β-NaYF4 phase.11,23 From the analysis above, we conclude that the β-NaYF4 crystals were synthesized by in situ transformation of Y(OH)xF3−x. The Y(OH)xF3−x microspindles were formed by the self-assemble of nanorods stacking parallel along their c axes in hydrothermal reaction. After heat treatment in ethanol with NaF and NH4HF2, Y(OH)xF3−x nanorods transformed to β-NaYF4 gradually and some particles merged together. However, the spindlelike

Figure 3. (a and b) TEM images of slices of a Y(OH)xF3−x crystal and a β-NaYF4 crystal taken along the [010] zone axes with corresponding SAED patterns (insets). (c and d) HRTEM images recorded from the square area marked in (a and b), respectively, with the corresponding FFT patterns (insets). C

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Figure 4. Schematic representations of the crystallographic structures of (a) Y(OH)xF3−x and (b) β-NaYF4, viewed along their c axes.

morphology was maintained during the transformation. This in situ transformation is unique and can be extended to synthesize the complex structure of β-NaYF4. As the β-NaYF4 product was synthesized via in situ transformation of the precursor, the morphologies and phases of the precursor play a key role in the formation of β-NaYF4. To obtain the resulting monodispersed mesocrystals, precursor particles with monodispersed morphologies have to first be prepared. It is found that different pH values lead to a yield of different phases during the growth of the precursor. Monodispersed bundles of the precursor described here were fabricated with a pH value of 6. When the pH value was decreased to 4, only YF3 particles can be obtained, which were not an ideal precursor for the synthesis of β-NaYF4. When the pH value was increased to 7, monodispersed nanorod bundles still remained but were slimmer in shape (Figure SI-5 of the Supporting Information). A higher pH value can break the uniform nanorod bundles. That is, irregular bundles were synthesized with their pH at 8 (Figure SI-6a of the Supporting Information). For this pH value, in addition, the XRD pattern of the products can undoubtedly be ascribed to the known compound Y(OH)1.57F1.43 (Figure SI-6b of the Supporting Information). Increasing the molar ratio of F and Y to 3:1 with a pH value of 6, YF3 existed in the products and no nanorod bundles can be obtained. From the above results, we find that the monodispersed precursor nanorod bundles only appeared with careful control of the pH value and mole ratio of the reactants. Furthermore, larger spindlelike particles assembled by larger nanorod bundles were formed at a higher reaction temperature (especially at 180 and 220 °C), keeping other conditions unchanged (Figure SI-7 of the Supporting Information). Other rare earth ions can easily be codoped into the precursor and finally existed in β-NaYF4. Here, precursor codoped with 20% Yb3+ and 2% Er3+ ions was prepared and then transformed to β-NaYF4:Yb3+/Er3+. It is shown that Y(OH)xF3−x:Yb3+/Er3+ does not exhibit any UC luminescence upon excitation at 980 nm (Figure 5, red curve). However, the final product displays strong UC emission with 980 nm excitation (Figure 5). This fact can further demonstrate that βNaYF4, well-known as one of the most efficient host materials for UC emission so far, can be rationally grown from Y(OH)xF3−x. The UC emission peaks of β-NaYF4:Yb3+/Er3+ can be attributed to 2H9/2 → 4I15/2, 2H11/2, 4S3/2 → 4I15/2, and 4 F9/2 → 4I15/2 transitions of Er3+, respectively, as shown in Figure 5, which have been well-described in other reports.3,4,12a Another pair of common codoped ions (Yb3+ and Tm3+) can also be doped into the product and blue up-conversion

Figure 5. Up-conversion emission spectra of the Y(OH)xF3−x:Yb3+/ Er3+ precursor and synthesized β-NaYF4:Yb3+/Er3+ mesocrystals with photo of green emission (inset). The pump power of the 980 nm diode laser was fixed at 450 mW/mm2.

emission can be detected (Figure SI-8 of the Supporting Information). Color point tuning, including multicolor and high color purity, is an important challenge for dual-mode (DC and UC) luminescent nanomaterials. In order to advance their direct applications in multiplexed and highly sensitive bioassays, we demonstrate here that multicolors can be obtained and tuned by simply doping with specified Ln3+ ions. Figure 6 shows the down-conversion (DC) emission spectra of the synthesized βNaYF4:Ln3+ (Ln = Pr, Nd, Sm, Eu, Tb, Dy, and Ho, 10%) mesocrystals. For β-NaYF4:Eu3+ mesocrystals, when excited into the transition of 7F0 → 5L6 at 395 nm, there are many sharp emission peaks, due to the transitions from the excited state 5DJ=2,1,0 to the ground state 7FJ=0,1,2,3,4.7 The hypersensitive forced electric-dipole 5D0 → 7F2 transition is predominant at 617 nm, indicating that the doped Eu3+ ions are located mainly in ancentrosymmetric sites.7 For β-NaYF4:Sm3+ mesocrystals, three main peaks at 560, 594, and 644 nm are corresponding to 4 G5/2 → 6HJ (J = 5/2, 7/2, and 9/2) transitions, respectively. When excited into the 6H5/2 → 4K11/2 transition at 402 nm, the prominent emission at about 594 nm is assigned to the 4G5/2 → 6 H7/2 transition.30 For β-NaYF4:Tb3+ mesocrystals, under excitation into the 7F6 → 5G6 transition at 370 nm, there are four obvious sharp lines centered at 488, 544, 584, and 619 nm, originating from the transitions from the 5D4 excited state to 7FJ (J = 6, 5, 4, and 3) ground states of the Tb3+ ion. The most prominent emission at 544 nm is derived from 5D4 → 7F5 transition.31 For β-NaYF4:Nd3+ mesocrystals, under excitation into the 4I9/2 → 4DJ (J = 1/2, 3/2, and 5/2) transition at 355 nm, there are many sharp emission peaks, due to the transitions from the excited states, 4D3/2, 4GJ=5/2,7/2, and 2H11/2, to the ground state, 4IJ=9/2,11/2,13/2,15/2. The predominating emissions at 416 and 449 nm are due to the 4 D 3/2 → 4 I J=13/2,15/2 transitions.32 The CIE color coordinates of all the above emissions are summarized in Figure 7. It is of great interest that multicolors, including red, orange, green, and blue emitting light, can be obtained by simply doping with Eu3+, Sm3+, Tb3+, and Nd3+, respectively. More significant is the fact that whiteemitting light, including bluish white, yellowish white and pure D

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Figure 6. Photoluminescence emission spectra of β-NaYF4:Ln3+ (Ln = Pr, Nd, Sm, Eu, Tb, Dy, and Ho) mesocrystals.

crystals may potentially advance their applications in multiplexed and highly sensitive bioassays.

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CONCLUSIONS In summary, we have presented a simple and special route to synthesize monodispersed β-NaYF4 spindlelike mesocrystals via in situ transformation of Y(OH)xF3−x precursor crystals with spindlelike nanorod bundles. The relationship between Y(OH)xF3−x and β-NaYF4 was investigated in detail and the in situ ion-exchange transformation mechanism was proposed. Such monodispersed β-NaYF4 can only be obtained through careful control synthesis of the precursor. These complex structures of β-NaYF4 may have wide potential applications in up/down-conversion science and technology. The two steps synthesis should become a new route for the rational growth of β-NaYF4 with exceptional complex nanostructures. Such an ion exchange growth strategy to design highly efficient photoluminescence materials with complex but ordered nanostructure can also be applied to other functional nanostructured materials.

Figure 7. CIE 1931 chromaticity diagram of β-NaYF4:Ln3+ (Ln = Pr, Nd, Sm, Eu, Tb, Dy, and Ho) mesocrystals. (★): the ideal white point of (0.33, 0.33).

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white, can also be tuned by simply doping with Dy3+, Ho3+, or Pr3+ ions. For β-NaYF4:Dy3+ mesocrystals, under excitation into the 6H15/2 → 4I11/2 transition at 366 nm, there are two obvious emissions centered at 482 and 572 nm, originating from the transitions from the 4F9/2 excited state to the 6HJ (J = 15/2, 13/ 2) ground states of the Dy3+ ion.33 For β-NaYF4:Ho3+ mesocrystals, under excitation into 5I8 → 5F1+5G6 transitions at 448 nm, there are two obvious emissions centered at 540 and 645 nm, originating from the transitions from the excited states 5 S2 and 5F5 to the ground state 5I8 of the Ho3+ ion.34 For βNaYF4:Pr3+ mesocrystals, under excitation into the 3H4 → 3P2 transition at 444 nm, there are many emission peaks corresponding to the transitions from the 3P0 excited state to the ground states 3HJ (J = 4, 5, and 6) and 3FJ (J = 2, 3, and 4) of the Pr3+ ion.35 Additionally, it is worth noting that among all the trivalent rare earth ions, Sm3+- and Eu3+-activated β-NaYF4 mesocrystals exhibit high color purity up to 91% and 99%, respectively. These results indicate that β-NaYF4:Ln3+ meso-

ASSOCIATED CONTENT

S Supporting Information *

EDX spectra, more TEM images, SAED patterns, and SEM images of the precursor and the product, as well as upconversion emission spectrum of the synthesized β-NaYF4:Yb3+/Tm3+ mesocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.W.: e-mail, [email protected]; tel, +86 20 8411 1823; fax, +86 20 8411 1038. J.W.: e-mail, [email protected]. cn. Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (NSF) of China and the Government of Guangdong Province for NSF and industrial applications of rare earth nanomaterials (Grants 20571087, U0734002, 21271190, and S2012020011113).

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dx.doi.org/10.1021/cg301751c | Cryst. Growth Des. XXXX, XXX, XXX−XXX