ARTICLE pubs.acs.org/crystal
Hierarchical Laminar Superstructures of Rhombic Priceite (Ca4B10O19 3 7H2O): Facile Hydrothermal Synthesis, Shape Evolution, Optical, and Thermal Decomposition Properties Wancheng Zhu,*,† Xiaoli Wang,† Xiao Zhang,† Heng Zhang,† and Qiang Zhang*,‡ † ‡
Department of Chemical Engineering, Qufu Normal University, Shandong 273165, China Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: The assembly of nanosheets/nanoplates into three-dimensional hierarchical superstructures brings unexpected properties and unique applications. However, to organize borate nanostructures into ordered superstructures is still a great challenge. Herein, uniform hierarchical laminar priceite (Ca4B10O19 3 7H2O) superstructures were synthesized via a facile hydrothermal route, in the absence of any templates, surfactants, or capping reagents. The as-prepared hierarchical laminar Ca4B10O19 3 7H2O superstructures containing rhombic nanosheets were formed by the dissolutionrecrystallization controlled growth and subsequent hydrothermal self-assembly and could also be further disassembled into the rhombic nanosheets by hydrothermal treatment over time. The superstructures exhibited transmittance vibrational bands concentrating on the wavenumber range 1750450 cm1, lost weight continuously within the temperature range 125586 °C and kept constant weight thereafter, and demonstrated weak absorptions within the wavelength range 220245 nm and 580610 nm. The as-synthesized uniform hierarchical laminar Ca4B10O19 3 7H2O superstructures and nanosheets were expected as a sort of potential building blocks for advanced functional materials, as well as a flame retardant in polymer-based composites with a quasi transparent characteristic within the UVvis region.
1. INTRODUCTION The assembly of nanoplates, nanotubes, nanowires, and nanospheres into two-dimensional (2D) and three-dimensional (3D) hierarchical superstructures with unexpected properties and unique applications has received considerable attention recently.17 The as-obtained superstructures were widely explored for their potential applications in nanocomposites, energy conversion and storage, catalysis, optoelectronics, and drug delivery systems.13,6 Among them, nanoplates, or nanosheets, such as graphene and graphene oxide,8,9 layered double hydroxides,6,9 and natural clay (e.g., vermiculite, kaolinite, montmorillonite, smectite)1,2,10 were one of the most basic building blocks for advanced functional materials. Developing novel nanosheet-like superstructures is an emerging scientific issue in crystal engineering. Recently, borate nanostructures (e.g., Ca2B2O5 nanogrooves,11 Ca4B10O19 3 7H2O oval-like microspheres,12 calcium-containing rare earth oxyborates,13 Mg3B2O6 nanotubes,14 Mg2B2O5 nanorods,15,16 nanowires,17,18 nanowhiskers,19,20 MgBO2(OH) nanowhiskers,15,21 lithium borate (Li3B5O8(OH)2),22 aluminum borate (Al18B4O33) nanowires,2326 zinc borates (Zn4B2O7 3 nH2O), nano/microstructures,27,28 and barium borate (Ba3B6O9(OH)6) nanorods29 have been paid much attention for their excellent mechanical properties, lightweight, good chemical inertness, high stability under high temperature, and low thermal expansion coefficients.11 Among them, calcium borates (xCaO 3 yB2O3 3 nH2O) have great potential in applications for glass fibers, reinforcements in electronic ceramics, r 2011 American Chemical Society
flame retardants, dielectric devices, antiwear additives, ultraviolet light sources, lubricating oil additives, agriculture, and luminescent phosphors.11,15,18,3034 Many borate minerals, such as borax (Na2B4O5(OH)4 3 8(H2O)), colemanite (CaB3O4(OH)3 3 (H2O)), priceite (Ca4B10O19 3 7H2O), and ulexite (NaCaB5O6(OH)6 3 5(H2O)) were found in evaporite contexts. In those natural borate minerals, the borate (BO3) units may be polymerized and result in B2O5, B3O6 anions, as well as more complex structures which include halogen or hydroxide anions. Thus, the as-obtained natural borates were always a mixture of various kinds of borates, whose structure and composition were hard to control. To get high purity bulk calcium borate materials, various strategies, such as solid state reactions between boric acid and calcium oxide, and hydrothermal and microemulsion methods, have been developed. For instance, Liu reported uniform priceite microspheres with oval-like morphology by a hydrothermal method, in the presence of the surfactant polyethylene glycol (PEG-300).12 The hydrothermal treatment technique was also employed to recover boron as recyclable precipitate Ca2B2O5 3 H2O from aqueous solutions.35 Very recently, Li and coauthors found a facile catalystfree hydrothermal method to synthesize Ca2B2O5 3 H2O nanobelts at low temperature.11 However, the assembly and organization of those monomorph borate nanostructures into complex hierarchical
Received: January 25, 2011 Revised: April 15, 2011 Published: May 05, 2011 2935
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Figure 1. XRD pattern (a), SEM image (b and c), size distribution (b1), TEM image (d), and SAED pattern (d1) of the hierarchical rhombic priceite Ca4B10O19 3 7H2O formed at 180 °C for 12.0 h. Molar ratio: CaCl2/H3BO3/NaOH = 1:6:2. Red /, Ca4B10O19 3 7H2O (PDF No. 09-0147); (, Ca3B4(OH)18 (PDF No. 46-1466); r, unknown phase.
superstructures in the absence of any templates, surfactants, or capping reagents still remained a great challenge to date. In this contribution, we synthesized for the first time, to the best of our knowledge, the uniform hierarchical laminar rhombic priceite (Ca4B10O19 3 7H2O) superstructures containing rhombic nanosheets via a facile hydrothermal route, without the aid of any templates, surfactants, or capping reagents. The structure, morphology, and even assembly behavior of the Ca4B10O19 3 7H2O superstructures could be well tuned by facile hydrothermal treatment at various temperatures for various growth times, based on which a possible dissolutionrecrystallization controlled growth and subsequent hydrothermal self-assembly formation mechanism was proposed. Moreover, the optical and thermal decomposition properties of the as-obtained Ca4B10O19 3 7H2O superstructures were also explored, to further reveal the extraordinary performance of the unique Ca4B10O19 3 7H2O superstructures.
2. EXPERIMENTAL SECTION All of the reagents were analytical grade and used directly without further purification. Priceite Ca4B10O19 3 7H2O hierarchical superstructures were synthesized by a facile hydrothermal route. In a typical procedure, 4.0 mL of NaOH (4.0 mol L1) solution was mixed with 2.929 g of H3BO3, 4.0 mL of CaCl2 (2.0 mol L1) solution, and 18.4 mL of deionized (DI) water under constant magnetic stirring at room temperature, keeping the molar ratio of CaCl2/H3BO3/NaOH 1:6:2. The resultant slurry was transferred into a Teflon-lined stainless steel autoclave with a capacity of 44.0 mL. The autoclave was sealed, heated to 180 °C (heating rate: 5 °C min1), and kept in an isothermal state for
12.0 h. After the hydrothermal synthesis, the autoclave was cooled down to room temperature naturally. The as-obtained precipitate was washed with DI water three times, then filtered, and dried at 60 °C for 24.0 h for further characterization. The structure of the sample was identified by an X-ray powder diffractometer (XRD, D8-Advance, Bruker, Germany) using Cu KR radiation (λ = 1.54178 Å) and a fixed power source (40.0 kV, 40.0 mA). The morphology and microstructure of the sample were examined by field emission scanning electron microscopy (SEM, JSM 7401F, JEOL, Japan, at 3.0 kV) and high resolution transmission electron microscopy (TEM, JEM2010, JEOL, Japan, at 120.0 kV). The selected area electron diffraction (SAED) was also performed by JEM-2010 TEM. The size distribution of the as-synthesized hierarchical superstructures was estimated by direct measuring ca. 100 particles from the typical SEM images. The chemical bonds in the molecules of the hydrothermal product were determined by the Fourier transform infrared spectrum (FT-IR, Nexus 470, Nicolet, USA) within the wavenumber range 4000400 cm1. The optical properties were examined with a UVvis spectrophotometer (UV-3600 230VCE, Shimadzu, Japan), with absolute ethanol or DI water as the dispersive medium. The thermal decomposition behavior of the sample was detected with a thermogravimetric analyzer (Netzch Sta 409 PC/PG, Germany) from 30 to 900 °C, and the experiments were carried out in dynamic air with a heating rate of 10 °C min1.
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology Characterization. Figure 1 shows the composition, morphology, and size distribution of the hydrothermally synthesized product (180 °C, 12.0 h). The main 2936
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Figure 2. XRD patterns (a) and SEM images (bf) of the hydrothermal products obtained at different temperatures: molar ratio, CaCl2/H3BO3/ NaOH = 1:6:2; time (h), 12.0; temperature (°C), (a1,b) 60, (a2,c) 90, (a3,d) 120, (a4,e) 150, (a5,f) 210; b, CaB2O4 3 H2O (PDF No. 16-0366); r, Ca2B6O11 3 5H2O (PDF No. 33-0267); /, Ca4B10O19 3 7H2O (PDF No. 09-0147).
phase was readily indexed into the priceite Ca4B10O19 3 7H2O (PDF No. 09-0147).36 Several low intensity diffraction peaks could be assigned to those of Ca3B4(OH)18 (PDF No. 46-1466), coexisting with some very low intensity diffraction peaks of unknown phase. The overall and dominant chemical reaction involved in the room temperature coprecipitation and hydrothermal process was written as follows: 4CaCl2 þ 10H3 BO3 þ 8NaOH f Ca4 B10 O19 3 7H2 O þ 8NaCl þ 12H2 O
ð1Þ
The as-prepared particles exhibited uniform hierarchical laminar morphology, containing rhombic nanosheet-like subunits, as shown in the SEM images (Figure 1b and c). Around 2050 nanosheet-like subunits were packed into a laminar superstructure particle, similar to that of exfoliated vermiculite.37 The nanosheet-like subunits were interlinked with each other, leading to multilayered structural booklike superstructures. This was quite different from the hydrothermally synthesized oval-like Ca4B10O19 3 7H2O microspheres assisted by PEG-300.12 The statistic data showed that ca. 89.5% of the hierarchical
superstructures had a ratio of long diagonal to short one within the range 1.42.2, revealing a narrow size distribution of the asprepared hierarchical rhombic superstructures (Figure 1b1). The TEM image recorded from an individual sheet derived from the superstructure via the ultrasonic treatment in process of TEM sample preparation (Figure 1d) reconfirmed that the individual subunit demonstrated a quasi rhombic nanosheet-like shape. The corresponding SAED pattern (Figure 1d1) indicated the subunit of single crystalline-like high crystallinity, in accordance with the XRD results. 3.2. Shape Evolution and Possible Formation Mechanism. The composition and morphology evolution of the hydrothermal products obtained at different temperatures for 12.0 h are shown in Figure 2. The compound nanoparticles (NPs) containing boron and calcium were formed at a low temperature (6090 °C), with irregular shape and poor crystallinity (Figure 2a1-a2 and b-c). With the temperature going up to 120 °C, the precipitate turned into Ca4B10O19 3 7H2O, with a relatively regular rhombic nanosheet-like morphology (Figure 2a3 and d), coexisting with sporadic fragments. With the temperature further increasing to 150180 °C, the crystallinity of Ca4B10O19 3 7H2O became higher (Figure 2a4, Figure 1a). Meanwhile, the morphology of the rhombic nanosheet-like 2937
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Figure 3. XRD patterns (a) and SEM images (bd) of the hydrothermal products: molar ratio, CaCl2/H3BO3/NaOH = 1:6:2; temperature (°C), 180. Time (h): (a1 and b) 3.0; (a2 and c) 6.0; (a3 and d) 18.0.
Ca4B10O19 3 7H2O tended to be uniform (Figure 2e), and it further assembled into hierarchical laminar rhombic superstructures (Figure 1b and c). However, when the temperature reached 210 °C, the products changed into the mixture containing CaB2O4 3 H2O (PDF No. 16-0366) and Ca2B6O11 3 5H2O (PDF No. 330267), with a nanobelt-like morphology (Figure 2a5 and f). Thus, the hydrothermal temperature determined the phase formation and also the morphology evolution of the priceite Ca4B10O19 3 7H2O, which was fixed as 180 °C hereafter, to obtain the uniform hierarchical laminar rhombic Ca4B10O19 3 7H2O superstructures. Figure 3 demonstrates the composition and morphology evolution of the hydrothermal products obtained at 180 °C for various hydrothermal times. Apparently, all products consisted of the Ca4B10O19 3 7H2O phase. However, with the time increasing from 3.0 to 6.0 to 18.0 h, the crystallinity of the product tended to be higher (Figure 3a1-a3). When hydrothermally treated for 3.0 h, the asobtained products exhibited dominant individual rhombic nanosheets, coexisting with some very tiny NPs (Figure 3b). When treated for 6.0 h, some thin rhombic nanosheet-assembled superstructures have emerged, coexisting with less tiny NPs (Figure 3c). With the time increasing to 12.0 h, the products demonstrated uniform hierarchical laminar rhombic morphology with high crystallinity, as previously shown in Figure 1. However, the hierarchical laminar rhombic superstructures did not further evolve into those containing significant thicker sheets or entirely solid superstructures when hydrothermally treated for longer time, such as 18.0 h. On the contrary, some thinner and even individual rhombic nanosheets appeared (Figure 3a3 and d) again. In other words, the hierarchical laminar rhombic superstructures were disassembled into the discrete
Figure 4. Possible formation mechanism of the hierarchical rhombic priceite Ca4B10O19 3 7H2O.
rhombic nanosheets by hydrothermal treatment over time, indicating relatively weak force existed among the assembled superstructures. Consequently, the hydrothermal time played a key role in the assembly as well as the disassembly of the rhombic superstructures. Thus, as shown in Figures 2 and 3, the hydrothermal temperature and time had great influences on the composition, morphology, as well as the assembly behavior of the hydrothermal products. The possible formation mechanism of the hierarchical laminar rhombic priceite Ca4B10O19 3 7H2O superstructures was assumed, as shown in Figure 4. The irregular amorphous compounds containing boron and calcium were formed first at a low temperature hydrothermal treatment (lower than 90 °C), which evolved into the priceite with dominant regular rhombic nanosheet-like morphology at an appropriate higher temperature (120150 °C). With the temperature increasing to 180 °C, rhombic nanosheets were assembled into uniform hierarchical laminar rhombic priceite superstructures. The conversion from irregular amorphous compounds to rhombic nanosheets was believed to be controlled by the dissolution recrystallization process, taking the composition and morphology 2938
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Figure 5. FT-IR (a) and UVvis (b and c) spectra of the rhombic priceite Ca4B10O19 3 7H2O hierarchical superstructures (b) and nanosheets (c) dispersed in absolute ethanol (b1 and c1) or DI water (b2 and c2).
change before and after the moderate hydrothermal treatment (120150 °C) into consideration. Since no additional templates, surfactants, or capping reagents were employed during the dissolutionrecrystallization process, the formation of the individual rhombic nanosheets at the early stage of the hydrothermal treatment should be attributed to the specific anisotropic crystal structure of the rhombic priceite Ca4B10O19 3 7H2O phase, similar to the catalyst-free hydrothermal growth of Ca2B2O5 3 H2O nanobelts11 and high anisotropic crystal structure induced hydrothermal formation of the MgBO2(OH) nanowhiskers.21 However, the self-assembly of the previously formed individual rhombic nanosheets into the uniform laminar rhombic superstructures was apparently different from poly(styrene-alt-maleic acid) assisted CaCO3 unusual superstructures,38 ethylenediaminetetraacetic acid assisted SrCO3 submicrometer spheres,39 SrSO4 dumbbell-like architectures,40 Ni2þ assisted Co11(HPO3)8(OH)6 superstructures,41 block copolymer P123 assisted nanorods-based urchinlike and nanosheets-based flowerlike cobalt basic salt nanostructures,42 and polymeric poly(sodium 4-styrenesulfonate) assisted Bi2WO6 flowers.43 In contrast, the present laminar rhombic Ca4B10O19 3 7H2O superstructures seemed to have experienced a similar self-assembly process with those complex superstructures obtained in the absence of any surfactants, such as hierarchical Cu2PO4OH,44 cantaloupe-like AlOOH,45 hierarchical dendrites of NdOHCO3,46 and porous NiO nano-/microspherical superstructures.47 As a matter of fact, in addition to the effects of the hydrothermal temperature and time, influences of the molar ratio of the reactants CaCl2/H3BO3/NaOH, such as 2:3:4, 2:6:15, and 1:9:12, on the hydrothermal products were also explored in the extensive experiments, which however resulted in a Ca2B2O5 3 H2O phase with beltlike morphology at 120210 °C for 12.0 h. Thus, an appropriate amount of OH anions (originated from different
molar ratios of the reactants) within the solution phase was also necessary for the formation of a rhombic Ca4B10O19 3 7H2O phase. This was analogous to the formation of the hierarchical Cu2PO4OH44 to some extent. In addition, as an effective approach for the hierarchical superstructures, self-assembly has been widely investigated. Self-assembly of the nanocrystals could be driven by the van der Waals forces and hydrogen bonding among certain organic molecules adsorbed on the surfaces of particles.48 In the present case, the formerly confirmed weak force among the laminar Ca4B10O19 3 7H2O superstructures comprising nanosheets over long time hydrothermal treatment (Figure 3a3 and d) was considered as the van der Waals forces, rather than the stronger hydrogen bonding. On the other hand, from a thermodynamic perspective, the surface energy of an individual rhombic Ca4B10O19 3 7H2O nanosheet is much higher than that containing two main exposed planes.49 As a result, individual Ca4B10O19 3 7H2O nanosheets tended to aggregate each other in a direction perpendicular to the surface planes to decrease the surface energy by reducing exposed areas, leading to the final laminar rhombic Ca4B10O19 3 7 H2O superstructures. Nevertheless, the relatively weak van der Waals forces among the rhombic nanosheets was not strong enough to bond them together and result in further coalescence or oriented attachment growth of the laminar rhombic Ca4B10O19 3 7H2O superstructures into completely solid hierarchical superstructures, which gave rise to the disassembly of the laminar rhombic superstructures into discrete rhombic nanosheets when hydrothermally treated overtime. 3.3. FT-IR and UVVis Spectra. Figure 5a shows the FT-IR spectrum of the hierarchical laminar rhombic priceite Ca4B10O19 3 7H2O superstructures. The vibrational bands concentrated in the wavenumber range 1750450 cm1. According to the FT-IR spectroscopic study results of hydrated borates,50 the bands at 2939
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further dehydration from Ca4B10O19 3 7H2O: Ca4 B10 O19 3 7H2 O f Ca4 B10 O19 3 5H2 O þ 2H2 O
ð3Þ
With temperature going up to 430 °C, the weight loss became 7.9%, compared to the theoretical value (7.8%) for the further dehydration of Ca4B10O19 3 7H2O: Ca4 B10 O19 3 7H2 O f Ca4 B10 O19 3 4H2 O þ 3H2 O
ð4Þ
When the temperature reached 586 °C, the weight loss reached 18.4%, due to the ultimate dehydration of Ca4B10O19 3 7H2O: Figure 6. TG-DSC curves of the hierarchical rhombic priceite Ca4B10O19 3 7H2O superstructures: molar ratio, CaCl2/H3BO3/NaOH = 1:6:2; temperature (°C), 180; time (h), 12.0.
3560, 3461, and 3101 cm1 were ascribed to the OH stretching; the characteristic bands at 2578 and 2539 cm1 were attributed to the OH stretching because of hydrogen bonding; the band at 1658 cm1 was due to the HOH bending of lattice water; the bands at 1396 and 1307 cm1 were ascribed to the asymmetric stretching of B(3)O; the band at 1060 cm1 corresponded with the asymmetric stretching of B(4)O; characteristic bands at 902 and 786 cm1 were attributed to the symmetric stretching of B(3)O and B(4)O, respectively; the band at 715 cm1 was ascribed to the out-of-plane bending of B(3)O; the band at 511 cm1 was due to the bending of B(3)O and B(4)O; and the band at 472 cm1 was in accordance with the bending of B(4)O. These results were in well agreement with those assignments in the literature.20,5153 The UVvis spectra of the hierarchical laminar rhombic Ca4B10O19 3 7H2O superstructures and also nanosheets dispersed in ethanol as well as DI water are also recorded, as shown in Figure 5b and c. The laminar Ca4B10O19 3 7H2O superstructures demonstrated weak absorptions within the wavelength ranges 220245 nm (UV region) and 580610 nm (visible region) when dispersed in ethanol and exhibited weak absorptions within the wavelength ranges 250300 nm (UV region) and 520580 nm (visible region). In contrast, the Ca4B10O19 3 7H2O nanosheets revealed weak absorption within the wavelength ranges 280320 nm (UV region), 550610 nm, and 750790 nm (visible region), whenever dispersed in ethanol or DI water. The weak absorption indicated the quasi transparent characteristic29 of the hierarchical laminar rhombic Ca4B10O19 3 7H2O superstructures as well as nanosheets within the UVvis region. 3.4. Thermal Decomposition Property. Thermogravimetric analysisdifferential scanning calorimetry (TG-DSC) curves of the laminar rhombic Ca4B10O19 3 7H2O superstructures are shown in Figure 6. The sample weight decreased at a relatively slow rate below 294 °C, a faster rate within the range 294433 °C, and a fastest rate between 433 and 586 °C, and it was kept almost constant thereafter. The weight loss within 125294 °C was 2.8%, very similar to the theoretical weight loss (2.6%) for the dehydration of one molar of crystal water from Ca4B10O19 3 7H2O: Ca4 B10 O19 3 7H2 O f Ca4 B10 O19 3 6H2 O þ H2 O ð2Þ There was a 5.1% weight loss when the temperature increased to 362 °C, quite analogous to the theoretical value (5.2%) for the
Ca4 B10 O19 3 7H2 O f Ca4 B10 O19 þ 7H2 O
ð5Þ
With the weight of the sample decreasing, the DSC curve showed a significant endothermic peak within the temperature range 300520 °C, corresponding with the remarkable dehydration of Ca4B10O19 3 6H2O. Notably, there emerged a sharp exothermic peak at 660820 °C during the weight constant stage. This was probably due to the recrystallization of the Ca4B10O19 phase, akin to the recrystallization phenomena existing for some other borates.54 Taking the continuous weight loss and also remarkable endothermic effect of the samples within the temperature range 300520 °C as well as the quasi transparent characteristic into consideration, the asprepared uniform hierarchical laminar rhombic Ca4B10O19 3 7H2O superstructures and even nanosheets might be expected as a promising candidate for the building blocks of advanced functional materials, and also a flame retardant in high molecular or polymerbased composites.34
4. CONCLUSION Uniform hierarchical laminar rhombic priceite Ca4B10O19 3 7 H2O superstructures were synthesized by a facile hydrothermal route at 180 °C for 12.0 h, by using CaCl2, H3BO3, and NaOH as the raw materials and keeping the molar ratio Ca/B/Na as 1:6:2. The formation of the hierarchical laminar Ca4B10O19 3 7H2O superstructures experienced the dissolutionrecrystallization controlled growth at the early stage of the hydrothermal treatment and subsequent hydrothermal self-assembly, which could also be further disassembled into the rhombic nanosheets by hydrothermal treatment over time. The hierarchical laminar rhombic Ca4B10O19 3 7H2O superstructures showed transmittance vibrational bands concentrating on the wavenumber range 1750450 cm1 (FT-IR spectrum), lost weight continuously within the temperature range 125586 °C, and kept constant weight thereafter (TG-DSC curves), and they demonstrated weak absorptions within the wavelength ranges 220245 nm and 580610 nm (UVvis spectrum). The as-synthesized uniform hierarchical laminar rhombic Ca4B10O19 3 7H2O superstructures as well as the nanosheets were expected as potential candidates for the advanced functional materials building blocks and also flame retardants in high molecular or polymer-based composites, with a quasi transparent characteristic within the UVvis region. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ86-537-4456301. Fax: þ86-537-4456305. E-mail:
[email protected] (W.Z.); zhang-qiang@mails. tsinghua.edu.cn (Q.Z.). 2940
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’ ACKNOWLEDGMENT This work was supported by the State Key Laboratory of Chemical Engineering, China (No. SKL-ChE-09A02), a Project of the Shandong Province Higher Educational Science and Technology Program, China (J10LB15), the Excellent MiddleAged and Young Scientist Award Foundation of Shandong Province, China (BS2010CL024), and the Youth Foundation of Qufu Normal University, China (XJ200926). ’ REFERENCES (1) Lin, L.; Liu, M. J.; Chen, L.; Chen, P. P.; Ma, J.; Han, D.; Jiang, L. Adv. Mater. 2010, 22, 4826–4830. (2) Zhang, Q.; Zhao, M. Q.; Liu, Y.; Cao, A. Y.; Qian, W. Z.; Lu, Y. F.; Wei, F. Adv. Mater. 2009, 21, 2876–2880. (3) Song, J. M.; Mao, C. J.; Niu, H. L.; Shen, Y. H.; Zhang, S. Y. CrystEngComm 2010, 12, 3875–3881. (4) Ling, Y. C.; Zhou, L. A.; Tan, L.; Wang, Y. H.; Yu, C. Z. CrystEngComm 2010, 12, 3019–3026. (5) Cai, W. Q.; Yu, J. G.; Gu, S. H.; Jaroniec, M. Cryst. Growth Des. 2010, 10, 3977–3982. (6) Zhao, M. Q.; Zhang, Q.; Jia, X. L.; Huang, J. Q.; Zhang, Y. H.; Wei, F. Adv. Funct. Mater. 2010, 20, 677–685. (7) Sun, X. J.; Wang, J. W.; Xing, Y.; Zhao, Y.; Liu, X. C.; Liu, B.; Hou, S. Y. CrystEngComm 2011, 13, 367–370. (8) Tang, Z. H.; Shen, S. L.; Zhuang, J.; Wang, X. Angew. Chem., Int. Ed. 2010, 49, 4603–4607. (9) Li, H. J.; Zhu, G.; Liu, Z. H.; Yang, Z. P.; Wang, Z. L. Carbon 2010, 48, 4391–4396. (10) Prikryl, R.; Weishauptova, Z. Appl. Clay Sci. 2010, 47, 163–170. (11) Bao, L. H.; Xu, Z. H.; Li, R.; Li, X. D. Nano Lett. 2010, 10, 255–262. (12) Liu, Z. H.; Xue, L. Mater. Lett. 2008, 62, 2692–2695. (13) Liu, N.; Zhao, D.; Yu, L. X.; Zheng, K. Z.; Qin, W. P. Colloids Surf., A 2010, 363, 124–129. (14) Ma, R. Z.; Bando, Y.; Golberg, D.; Sato, T. Angew. Chem., Int. Ed. 2003, 42, 1836–1838. (15) Zhu, W. C.; Li, G. D.; Zhang, Q.; Xiang, L.; Zhu, S. L. Powder Technol. 2010, 203, 265–271. (16) Xu, B.; Li, T.; Zhang, Y.; Zhang, Z.; Liu, X.; Zhao, J. Cryst. Growth Des. 2008, 8, 1218–1222. (17) Tao, X. Y.; Li, X. D. Nano Lett. 2008, 8, 505–510. (18) Zeng, Y.; Yang, H. B.; Fu, W. Y.; Qiao, L.; Chang, L. X.; Chen, J. J.; Zhu, H. Y.; Li, M. H.; Zou, G. T. Mater. Res. Bull. 2008, 43, 2239–2247. (19) Zhu, W. C.; Zhang, Q.; Xiang, L.; Wei, F.; Sun, X. T.; Piao, X. L.; Zhu, S. L. Cryst. Growth Des. 2008, 8, 2938–2945. (20) Zhu, W. C.; Zhang, Q.; Xiang, L.; Zhu, S. L. CrystEngComm 2011, 13, 1654–1663. (21) Zhu, W. C.; Zhu, S. L.; Xiang, L. CrystEngComm 2009, 11, 1910–1919. (22) Li, P.; Liu, Z. H. J. Chem. Eng. Data 2010, 55, 2682–2686. (23) Tao, X. Y.; Wang, X. N.; Li, X. D. Nano Lett. 2007, 7, 3172–3176. (24) Wang, J.; Ning, G. L.; Lin, Y. Mater. Lett. 2008, 62, 2447–2449. (25) Zhou, J.; Su, D. G.; Luo, J. M.; Zhong, M. F. Mater. Res. Bull. 2009, 44, 224–226. (26) Song, H. S.; Luo, J. J.; Zhou, M. D.; Elssfah, E.; Zhang, J.; Lin, J.; Liu, S. J.; Huang, Y.; Ding, X. X.; Gao, J. M.; Tang, C. C. Cryst. Growth Des. 2007, 7, 576–579. (27) Shi, X. X.; Yuan, L. J.; Sun, X. Z.; Chang, C. X.; Sun, J. T. J. Phys. Chem. C 2008, 112, 3558–3567. (28) Gao, Y. H.; Liu, Z. H. J. Chem. Eng. Data 2009, 54, 2789–2790. (29) Li, R.; Tao, X. Y.; Li, X. D. J. Mater. Chem. 2009, 19, 983–987. (30) Jing, F. L.; Fu, P. Z.; Wu, Y. C.; Zu, Y. L.; Wang, X. Opt. Mater. 2008, 30, 1867–1872.
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