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Mild, Quasireverse Emulsion Route to Submicrometer Lithium Niobate Hollow Spheres Chao Luo and Dongfeng Xue* State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian 116012, China ReceiVed July 26, 2006. In Final Form: September 18, 2006 A water/ethylene glycol (H2O/EG) system has been designed to synthesize lithium niobate (LiNbO3) powders by a mild, one-step quasireverse emulsion method. A morphology transformation from initial nuclei to flowerlike structures and then to hollow spheres is confirmed by the time-dependent experiment. The as-obtained LiNbO3 hollow spheres are formed via Ostwald ripening under solvothermal conditions, and their absorption edge in UV/vis diffuse reflectance spectra can be effectively tuned by the current morphology control strategies. This facile, efficient, and economic work provides a new route to simply and mildly synthesize hollow LiNbO3 particles and is a good initiation in the morphology control study of LiNbO3 powders.
Introduction LiNbO3 has been used for all-optical wavelength conversion and ultrafast optical signal processing due to its outstanding physical properties, such as rapid nonlinear optical response, low switching power, broad conversion bandwidth, and high Curie temperature.1 Recently, with the development of functional devices, much interest has been stimulated in ultrafine LiNbO3 powders.2-5 LiNbO3 powders have attracted a great deal of attention due to their potential applications, such as sensor arrays, piezoelectric antenna arrays, optoelectronic devices, and electronic devices for nano- and microelectromechanical systems.4 Furthermore, this material has also been proven to exhibit high photocatalytic activity without a trace of photocatalytic deactivation.5 It is well-accepted that there is a close relationship between the morphology and properties of inorganic materials; that is, the morphology determines properties because the crystal shape dictates the interfacial atomic arrangement of the material.6 In previous investigations, a wide range of methods have been developed to synthesize LiNbO3 powders, such as template route,4 sol-gel,7 molten salt synthesis,8 thermal decomposition of suitable precursors,9 hydrothermal route,10 combustion method,11 and wave radiation.12 Although these methods are available to * Corresponding author. E-mail:
[email protected]. (1) (a) Wang, X. L.; Wang, K. M.; Chen, F.; Fu, G.; Li, S. L.; Liu, H.; Gao, L.; Shen, D. Y.; Ma, H. J.; Nie, R. Appl. Phys. Lett. 2005, 86, 041103. (b) Xue, D.; Wu, S.; Zhu, Y.; Terabe, K.; Kitamura, K.; Wang, J. Chem. Phys. Lett. 2003, 377, 475. (2) Zhang, X.; Xue, D.; Liu, M.; Ratajczak, H.; Xu, D. J. Mol. Struct. 2005, 754, 25. (3) Popa, M.; Kakihana, M. Catal. Today 2003, 78, 519. (4) Zhao, L. L.; Steinhart, M.; Yosef, M.; Lee, S. K.; Schlecht, S. Sens. Actuators, B 2005, 109, 86. (5) Li, Z. S.; Yu, T.; Zou, Z. G.; Ye, J. H. Appl. Phys. Lett. 2006, 88, 071917. (6) (a) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. AdV. Mater. 2004, 16, 831. (b) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. J. Am. Chem. Soc. 2003, 125, 16196. (c) Yan, C.; Xue, D. J. Phys. Chem. B 2006, 110, 7102. (d) Xu, J.; Xue, D. J. Phys. Chem. B 2006, 110, 7750. (7) (a) Zeng, H. C.; Tung, S. K. Chem. Mater. 1996, 8, 2667. (b) Pitcher, M. W.; He, Y. N.; Bianconi, P. A. Mater. Chem. Phys. 2005, 90, 57. (8) Afanasiev, P. Mater. Lett. 1998, 34, 253. (9) Dey, D.; Kakihana, M. J. Ceram. Soc. Jpn. 2004, 112, 368. (10) (a) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. Angew. Chem., Int. Ed. 2004, 43, 2270. (b) Liu, M.; Xue, D. Mater. Lett. 2005, 59, 2908. (11) Liu, M.; Xue, D.; Luo, C. J. Am. Ceram. Soc. 2006, 89, 1551. (12) Brooks, D. J.; Brydson, R.; Douthwaite, R. E. AdV. Mater. 2005, 17, 2474.
synthesize LiNbO3 powders, almost no work is reported about the morphology control of LiNbO3 products because of the severe synthesis conditions at high temperatures. Therefore, it is a big challenge for us to realize the morphology control of LiNbO3 products by available synthesis strategies. In recent years, micro- and nanospheres with a hollow interior have aroused intense interest, mainly owing to their promising devices, drug delivery, active-material encapsulation, ionic intercalation, robust catalysts/carriers, and size-selective reactions.13 Concerning the fabrication of hollow materials, traditional ways focus on two main categories of methods: (i) templatedirected synthesis14-16 and (ii) emulsion synthesis.17 The basis of the template-directed synthesis is the adsorption of nanoparticles on the template, such as SiO214,15 or polystyrene.16 However, the subsequent removal of the template by calcinations or dissolution with solvents usually makes the process complicated. In the emulsion synthesis, the solution is emulsified, and the adsorption or reaction takes place on the surface of micelles to form the hollow spheres. It can also be regarded as a soft template method, and after reaction, the template can be removed directly from the formed hollow spheres. In this work, we report a facile and controllable one-step method to produce uniform LiNbO3 hollow spheres under quasimicroemulsion conditions. To our knowledge, LiNbO3 hollow spheres have not yet been reported. Even though the alcohols and water are infinitely miscible, the solution is still far from homogeneous.18 Therefore, we can design a mixed-solvent system with the microheterogeneity to synthesize LiNbO3 products. Herein, we select the system of H2O/EG as an example to synthesize LiNbO3 hollow spheres by precise control of the volume ratio of H2O/EG, reaction time, (13) (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Li, X. X.; Xiong, Y. J.; Li, Z. Q.; Xie, Y. Inorg. Chem. 2006, 45, 3493. (14) Dhas, N. A.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 2368. (15) Arnal, P. M.; Weidenthaler, C.; Schuth, F. Chem. Mater. 2006, 18, 2733. (16) (a) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. AdV. Mater. 2006, 18, 801. (b) Wang, P.; Chen, D.; Tang, F. Q. Langmuir 2006, 22, 4832. (17) (a) Pileni, M. P. Nat. Mater. 2003, 2, 145. (b) Sun, Q. Y.; Kooyman, P. J.; Grossmann, J. G.; Bomans, P. H. H.; Frederik, P. M.; Magusin, P. C. M. M.; Beelen, T. P. M.; van Sanetn, R. A.; Sommerdijk, N. A. J. M. AdV. Mater. 2003, 15, 1097. (18) (a) Roney, A. B.; Space, B. J. Phys. Chem. B 2004, 108, 7389. (b) Yang, H. G.; Zeng, H. C. Angew. Chem. Int. Ed. 2004, 43, 5206.
10.1021/la062193v CCC: $33.50 © 2006 American Chemical Society Published on Web 10/28/2006
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Table 1. Experimental Conditions for Typical LiNbO3 Samples and Their Morphologies samplea
solution
1
EG (20 mL)/H2O (4 mL) 1.5 mmol Nb2O5 / 3.0 mmol LiOH same as sample 1 same as sample 1 sample 1 + 0.5 mmol SDS sample 1 + 0.5 mmol CTAB
2 3 4 5 a
time (days)
morphology
5
hollow spheres
4 3 3 3
flowerlike structures nanoparticles hollow spheres hollow spheres
All samples were prepared by solvothermal method at 220 °C.
and surfactant. On the basis of the excellent properties of LiNbO3, this kind of novel, hollow spheres can lead to modification of the surface properties and even provide new functions in the applications of LiNbO3 powders when used as a microdevice or photocatalyst.19 Experimental Section The starting materials, lithium hydroxide (LiOH‚H2O, >90%), niobium oxide (Nb2O5, 99.99%), hydrofluoric acid (HF, 40%), aqueous solution of ammonia (25-28%), ethylene glycol (EG, >99.9%), and cetyltrimethylammonium bromide (CTAB, >99.0%), were of analytical grade, and sodium dodecyl sulfate (SDS, 85%) was chemically pure. All the reagents were purchased and used without further purification. Deionized water used in all syntheses was purified to a resistivity of 18 MΩ cm. In a typical experiment, after dissolving Nb2O5 (0.40 g) in HF (10 mL) solution, aqueous solution of ammonia was added until all precipitate appeared. The fresh niobium acid was then obtained after filtering and washing several times with water, and subsequently, LiOH (0.13 g) and the obtained fresh niobium acid were added to the EG (20 mL) using different volumes of H2O. After being stirred vigorously, the white mixture was transferred into a Teflon-lined stainless steel autoclave. The autoclave was heated to 200-220 °C and maintained for 3-5 days and then cooled to room temperature naturally. The white powders were collected and washed several times with distilled water and absolute ethanol to remove impurities. The final LiNbO3 samples were dried at 60 °C (more than 5 h) for further characterizations. In some experiments, CTAB or SDS was also added to the solution to adjust the morphology of the LiNbO3 product. The experimental conditions for typical LiNbO3 samples and their final morphologies are listed in Table 1. The phase composition and crystallinity of the synthesized LiNbO3 powders were characterized using X-ray powder diffraction (XRD, D/Max 2400, Rigaku Corp., Tokyo, Japan; equipped with graphite monochromatized CuKR radiation) in the 2θ angle ranging from 10 to 80°. FT-IR spectra were recorded on a Fourier transform infrared spectrometer (FT-IR, KBr disk method; NEXUS) at wavenumbers 400-4000 cm-1. The microstructure of LiNbO3 samples was studied by a scanning electron microscope (SEM, JSM-5600LV, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, Philips, Tecnai G2 20, operated at 200 kV). UV/vis diffuse reflectance spectra were obtained using a UV-vis-NIR spectrophotometer (JASCO, V-550).
Results and Discussion EG has been proven to be a versatile solvent and reactant for controlling the crystallization of nanoparticles.20 The diffusion of ions in EG at the intermediate temperature typically employed (100-250 °C) is considerably rapid, which leads to the acceleration in the solubilization of starting materials and the subsequent crystal growth. Figure 1a shows the XRD pattern of (19) Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540. (20) (a) Zheng, Y. H.; Cheng, Y.; Wang, Y. S.; Zhou, L. H.; Bao, F.; Jia, C. J. Phys. Chem. B 2006, 110, 8284. (b) Wang, S. M.; Guo, F.; Zhang, Y. Q.; Zheng, W. W.; Zhang, Y. G.; Qian, Y. T. Cryst. Growth Des. 2004, 4, 413.
Figure 1. XRD patterns of the reactants (1.5 mmol Nb2O5 + 3.0 mmol LiOH) reacted at (a) 200 °C for 5 days in 20 mL of EG, (b) 220 °C for 5 days in the system of 20 mL of EG and 7 mL of H2O, (c) 220 °C for 5 days in the system of 20 mL of EG and 4 mL of H2O, and (d) 220 °C for 5 days in 20 mL of EG. The inset is the corresponding XRD pattern of the sample synthesized in pure H2O at 250 °C for 5 days.
LiNbO3 samples prepared at 200 °C for 5 days with pure EG as the solvent. We can see that LiNbO3 has a hexagonal structure (JCPDS card file no. 20-0631). By replacing EG with H2O, the XRD pattern shows that the phase of LiNbO3 does not appear, even at 250 °C (inset of Figure 1). When the mixture of EG and H2O is used as the solvent, the peak intensity of LiNbO3 increases significantly when the content of H2O is reduced from 7 to 0 mL (Figures 1b-d). Although the concentration of reactants is changed, as shown in Figure 1, XRD patterns (Figure S1 of the Supporting Information) clearly verify that the dilution of reactants did not affect the crystallinity of the as-obtained samples without changing the content of H2O. Therefore, EG as a solvent positively affects the formation and crystallinity of LiNbO3 powders. In this work, it is confirmed that the volume ratio of H2O/EG strongly affects the morphology of the as-obtained LiNbO3 samples. It is verified that the proper volume ratio (H2O/EG ) 1/5) is the key factor to form monodisperse hollow spheres, which cannot be generated by increasing or decreasing this value (as shown in the Supporting Information, Figures S2 and S3). Figure 2a shows a lower-magnification SEM image of sample 1 prepared at the proper volume ratio of H2O/EG, from which monodisperse submicrometer spheres can be clearly seen. TEM images of sample 1 in Figures 2b, c clearly show the bright center, obviously confirming that the interior of sample 1 is hollow. It seems that EG has a property similar to that of 2-propanol, which has been reported to possess a self-hydrophobic performance to form a quasireverse emulsion, even though it is miscible with H2O.18 It was proved that even though 2-propanol and water are infinitely miscible, the solution is still far from a homogeneous system. On the other hand, this case leads to a water-in-propanol system. Therefore, in this experiment, H2O molecules in aqueous phase interact preferentially among themselves through strong hydrogen-bonding interactions, whereas EG molecules have only a small disrupting effect on the hydrogen bonds, which results in microheterogeneities of the H2O/EG system. It is concluded that 1/5 is the suitable ratio of H2O/EG to form the “quasireverse emulsion”, in which EG presents a weaker hydrophobic effect than the oil hydrocarbons used in standard reverse-emulsion methods.16 The generation of hollow spheres could be attributed to the formation of a “quasireverse emulsion”. To obtain a better understanding of the formation and evolution of LiNbO3 hollow spheres, a time-dependent experiment was
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Figure 4. XRD patterns of the as-obtained LiNbO3 samples: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5.
Figure 2. SEM (a) and TEM (b, c) images of the as-obtained LiNbO3 sample 1 (5 days without surfactants).
Figure 3. SEM and TEM of samples: (a) sample 3 (3 days without surfactants) and (b) sample 2 (4 days without surfactants). The inset of Figure 3b is the corresponding TEM of sample 2.
carried out. The shape evolution at the middle reaction time of spherical growth is shown in Figures 3a, b. Nanoparticles (less than 100 nm in diameter) with a large scale are gradually formed after 3 days (Figure 3a). It is interesting that monodisperse, flowerlike structures with a diameter of ∼0.8-1.0 µm can be obtained when increasing the reaction time up to 4 days (Figure
3b). The TEM image of sample 2 shown in the inset of Figure 3b indicates that the particle does not have the characteristic of a hollow interior. In this time-dependent experiment, the morphology transformation from nanoparticle to flowerlike structure and then to hollow sphere is confirmed when the reaction time increases gradually. Figure 4a-c shows XRD patterns corresponding to LiNbO3 samples 1, 2, and 3. The phase of these samples is well-crystallized with the hexagonal structure (JCPDS card file no. 20-0631), and the peak intensity of LiNbO3 becomes stronger and sharper when the reaction time increases. Significantly, a longer reaction time leads to the precipitation of larger agglomerates. It can be concluded that the morphology and size of the LiNbO3 samples are sensitive to the reaction time. The effect of different surfactants on the morphology of the as-obtained LiNbO3 samples was also investigated. LiNbO3 could not be synthesized when excess surfactant was added. It is clear that the surfactant is necessary to act as a capping reagent binding to the surface of crystal, which directly affects the faced growth and crystallinity of the crystallites. The absorbance of surfactant molecules onto specific facets of the niobium acid decreases their surface energy and, finally, their activity. An appropriate amount (0.5 mmol) of surfactant (SDS or CTAB) was selected as a morphology inducer. It was found that the formation time of hollow spheres was shortened to 3 days in the case of when a surfactant was added. Figure 5a displays a SEM image of sample 4. It is observable that these obtained spheres have a less compact surface. A typical TEM image of sample 4 was shown in Figure 5b, which shows that the center portion of the structure is lighter than the edge, confirming the hollow interior of these unique spheres. When CTAB was introduced, uniform and monodisperse spheres (Figure 5c) were also formed; however, the shell wall became more compact than sample 4. The TEM image of sample 5 (Figure 5d) also clearly shows the white contrast at the center of the sphere with the black thick shell, suggesting the hollow center inside the microspheres. Moreover, the broken sphere (indicated by a white arrow) in the higher magnification (Figure S6 of the Supporting Information) further confirms our conclusion. From samples 1, 4, and 5, it can be concluded that three kinds of hollow spheres were formed at different conditions, and the structure of the shell wall was sensitive to the surfactant (the higher-magnification SEM images of these samples are shown in Figures S4 to S6 of the Supporting Information). XRD patterns of samples 4 and 5 in Figure 4d, e show the same phase as sample 1, which means that samples 4 and 5 are well-crystallized LiNbO3 crystals. IR spectra (Figure S7) of these hollow spheres further confirm XRD analysis of LiNbO3 samples. The current experiment indicates that the
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Figure 6. UV/vis diffuse reflectance spectra of the as-obtained LiNbO3 samples: (a) sample 3, (b) sample 1, and (c) sample 2. Figure 5. SEM (a, c) and TEM (b, d) images of the as-obtained LiNbO3 samples: (a, b) sample 4 (3 days with SDS) and (c, d) sample 5 (3 days with CTAB).
surfactant plays a great role as the morphology inducer, since it shortens the formation time of the hollow sphere and affects the morphology of the shell wall. A mechanism of Ostwald ripening could be proposed for the formation of LiNbO3 hollow spheres. A schematic representation is shown in Figure S8 of the Supporting Information. At the beginning of the solvothermal reaction, LiNbO3 units are produced in the “quasireverse emulsion” that consisted of EG and H2O. With an increasing concentration of LiNbO3 units in the solution, the aggregation of these units then occurs to form small LiNbO3 nuclei, as shown in Figure 3a. The supersaturation of the system significantly decreases due to the reactant exhaustion after the primary nucleation stage, and nucleation is thus restrained. Being small could make the surface of the nanoparticles unstable due to the high surface energy and the large surface curvature.21 Therefore, an oriented outgrowth step of LiNbO3 nuclei occurs subsequently to form the flowerlike particles (Figure 3b) for eliminating the higher surface-energy faces. A high density of defects at the center of the flowerlike particle results in a high local dissolving/reaction rate.22 With increasing reaction time, LiNbO3 crystallites located in the inner cores begin to dissolve, while new depositions grow on the edge, which leads to the formation of hollow spheres. In particular, the crystallinity of LiNbO3 products is, indeed, increased with increasing reaction time, indicating that Ostwald ripening, in which large crystallites grow at the expense of the smaller ones,23 is an underlying mechanism in this hollowing process. The optical properties of the as-obtained LiNbO3 samples shown in Figure 6 are determined by UV/vis spectroscopy in an absorbance mode. It has been reported that the absorption position of the obtained sample is affected by its size effect of the particles.24 From Figure 6a, it is observed that sample 3 with the nano size exhibits an absorption edge at ∼320 nm. Subsequently, the absorption edge has a red shift to ∼340 nm when the reaction time is increased up to 4 days (Figure 6c), with the evolution of (21) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (22) (a) Li, F.; Ding, Y.; Gao, P. X.; Xin, X. Q.; Wang, Z. L. Angew. Chem., Int. Ed. 2004, 43, 5238. (b) Miao, J. J.; Fu, R. L.; Zhu, J. M.; Xu, K.; Zhu, J. J.; Chen, H. Y. Chem. Commun. 2006, 28, 3013. (23) Burleson, D. J.; Penn, R. L. Langmuir 2006, 22, 402. (24) Liu, Q. S.; Lu, W. G.; Ma, A. H.; Tang, J. K.; Lin, J.; Fang, J. Y. J. Am. Chem. Soc. 2005, 127, 5276.
the initial smaller particles into well-developed submicrometer flowerlike structures. As the aging process continues, the evacuation process takes place, leading to the hollow interior and additionally a blue shift of the absorption edge (Figure 6b), similar to the previous report.25 Therefore, our observation on the tunable optical absorption of LiNbO3 particles may be rationalized by considering that the changing of the absorption edge can be achieved by our current morphology control. Different UV absorption edge positions of LiNbO3 crystals obviously affect their optical performances, which is of particular interest for tuning operating frequency bands26 or photocatalytic properties27 in practical applications.
Conclusions In the present work, submicrometer LiNbO3 hollow spheres have been successfully formed via the Ostwald ripening mechanism. Our current work here provides a useful way to improve various properties of LiNbO3 products by tuning their morphologies on the basis of our designed one-step, mild, quasireverse emulsion route. The proper volume ratio (water/ EG ) 1/5) plays a key role in the formation of the hollow spheres in our “quasireverse emulsion” system. The time-dependent experiment verifies a morphology transformation process from initial nuclei to flowerlike structures and then to hollow spheres. The formation time of hollow spheres can be clearly shortened in the case of adding surfactant. The UV absorption edge position of the as-obtained LiNbO3 samples can be modified by controlling their shape; therefore, their optical properties can also be tuned in this way. The advantages of this route for the preparation of inorganic materials lie in its simplicity, economics, controllability, large scale, and mild reaction conditions. This mild solvothermal method can be extended to the synthesis of other kinds of nanoor microstructures with unique morphologies. Future work may be carried out to investigate the detailed mechanism and to synthesize other materials through a similar method. Acknowledgment. Financial support from the Program for New Century Excellent Talents in University (Grant No. NCET05-0278); the National Natural Science Foundation of China (Grant No. 20471012); a Foundation for the Author of National (25) Lu, C. H.; Qi, L. M.; Yang, J. H.; Wang, X. Y.; Zhang, D. Y.; Xie, J. L.; Ma, J. M. AdV. Mater. 2005, 17, 2562. (26) Bhatt, R.; Kar, S.; Bartwal, K. S.; Wadhawan, V. K. Solid State Commun. 2003, 127, 457. (27) Yin, J.; Zou, Z. G.; Ye, J. H. J. Phys. Chem. B 2003, 107, 4936.
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Excellent Doctoral Dissertation of China (Grant No. 200322); the Research Fund for the Doctoral Program of Higher Education (Grant No. 20040141004); and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged.
Luo and Xue
Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. LA062193V
