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Synthesis of Inorganic-Organic Layered Compounds Using Immiscible Liquid-Liquid Systems under the Distribution Law Sara Inoue and Shinobu Fujihara* Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received July 19, 2010. Revised Manuscript Received September 11, 2010 A method for synthesizing inorganic-organic layered compounds is proposed using a biphasic liquid-liquid system in one pot. Layered basic zinc benzoate (LBZB) compounds were chosen, and their formation was investigated starting from a xylene-water system. In a typical synthesis, a xylene phase dissolving benzoic acid was allowed to stand in contact with an equal amount of an aqueous phase dissolving zinc nitrate hexahydrate and urea. A role of urea is to supply OH- gradually by hydrolysis at an elevated temperature. The biphasically separated solutions were maintained at 80 °C, and then LBZB was obtained in the aqueous phase. Two kinds of layered structures with a basal spacing of 27.14 and 14.77 A˚ were formed by changing a C6H5COOH/Zn molar ratio. Chemical compositions of the 27.14 and the 14.77 A˚ layered phases were estimated to be Zn(OH)1.74(C6H5COO)0.26 3 0.29H2O and Zn(OH)1.12(C6H5COO)0.88 3 0.21H2O, respectively. The 27.14 A˚ phase could also be deposited as a film on substrates by heterogeneous nucleation. The film consisted of standing platelike particles and exhibited a two-dimensional structure, which could be converted to ZnO by heating. The relationship between the initial solution compositions and the final solid products was systematically examined on the basis of distribution law for benzoic acid in the xylene-water system.
Introduction Layered metal hydroxide compounds with a brucite-like structure can be classified into two types. One is layered double hydroxides (LDHs) with a general formula of [M2þ1-xM0 3þx(OH)2]An-x/n 3 mH2O, where M2þ and M0 3þ are different kinds of divalent and trivalent metal ions, respectively, in the octahedral site of brucitetype hydroxide layers and An- is interlayered anions to keep a whole charge balance.1 The other is layered basic metal salts (LBMSs) with a general formula of [M2þ(OH)2-x]An-x/n 3 mH2O, which contains only one kind of metal ion. Because of their lamellar structure, the layered compounds have attracted much attention as materials for anion exchange,2 intercalation,3 catalysis,4 twodimensionally confined reaction space,5 and exfoliation to nanosheets.6 In addition, unlike LDHs basically containing at least two different metal ions, LBMSs are decomposed into single metal oxides such as ZnO,7 CuO,8 NiO,9,10 and Co3O411,12 with specific electrical and optical properties. We have focused our attention on layered zinc hydroxide compounds, Zn(OH)2-xAn-x/n 3 mH2O (An- = Cl-, Br-, I-, NO3-, CO32-, CH3COO-, etc.), that can be converted into ZnO by lowtemperature heat treatments.13,14 ZnO is an n-type semiconductor *Corresponding author. E-mail:
[email protected].
(1) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (2) Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5201. (3) Carlino, S. Solid State Ionics 1997, 98, 73. (4) Choudary, B. M.; Kavita, B.; Chowdari, N. S.; Sreedhar, B.; Kantam, M. L. Catal. Lett. 2002, 78, 373. (5) Rey, S.; Merida-Robles, J.; Han, K. S.; Guerlou-Demourgues, L.; Delmas, C.; Duguet, E. Polym. Int. 1999, 48, 277. (6) Chen, W.; Feng, L.; Qu, B. Chem. Mater. 2004, 16, 368. (7) Moriya, M.; Yoshikawa, K.; Sakamoto, W.; Yogo, T. Inorg. Chem. 2009, 48, 8544. (8) Liang, Z. H.; Zhu, Y. J. Chem. Lett. 2005, 34, 214. (9) Zhu, J.; Gui, Z. Mater. Chem. Phys. 2009, 118, 243. (10) Nishizawa, H.; Kishikawa, T.; Minami, H. J. Solid State Chem. 1999, 146, 39. (11) Xu, R.; Zeng, H. C. Chem. Mater. 2003, 15, 2040. (12) Xu, Z. P.; Zeng, H. C. Chem. Mater. 2000, 12, 3259. (13) Kakiuchi, K.; Hosono, E.; Kimura, T.; Imai, H.; Fujihara, S. J. Sol-Gel Sci. Technol. 2006, 39, 63. (14) Hosono, E.; Fujihara, S.; Kimura, T.; Imai, H. J. Colloid Interface Sci. 2004, 272, 391.
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with a wide, direct band gap of 3.35 eV, a large exciton binding energy of 60 meV, and high electronic mobility. Because of these unique electrical and optical properties, ZnO is utilized in various applications such as ultraviolet-light emitting devices,15 field emitters,16 transparent conductors,17 gas sensors,18 photoanodes,19 and so on. The physical properties of ZnO are strongly dependent on its structures, including the morphology, aspect ratio, size, orientation, and density of crystals.20,21 However, wurtzite-type ZnO tends to grow along the c-axis to form one-dimensional (1D) morphology such as rods, wires,22 tubes,23 whiskers,24 and belts25 because of its crystal structure where charged oxygen and zinc metal layers are stacked alternately along the c-axis.26 Thus, the layered zinc hydroxide compounds with the lamellar structure are promising as a precursor for two-dimensionally (2D) structured ZnO with large specific areas. Additionally, the 2D sheetlike morphology of the layered compounds is varied depending on kinds of the interlamellar species intercalated into the brucite layers.27 In the previous works, the layered zinc hydroxide compounds were prepared by the precipitation,28 the anion exchange,29 the (15) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (16) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. (17) Minami, T.; Ida, S.; Miyata, T.; Minamino, Y. Thin Solid Films 2003, 445, 268. (18) Rao, G. S. T.; Rao, D. T. Sens. Actuators, B 1999, 55, 166. (19) Saito, M.; Fujihara, S. Energy Environ. Sci. 2008, 1, 280. (20) Zhao, Q.; Zhang, H. Z.; Zhu, Y. W.; Feng, S. Q.; Sun, X. C.; Xu, J.; Yu, D. P. Appl. Phys. Lett. 2005, 86, 203115. (21) Zhang, J.; Sun, L.; Yin, J.; Su, H.; Liao, C.; Yan, C. Chem. Mater. 2002, 14, 4172. (22) Vayssieres, L. Adv. Mater. 2003, 15, 464. (23) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (24) Satoh, M.; Tanaka, N.; Ueda, Y.; Ohshio, S.; Saitoh, H. Jpn. J. Appl. Phys. 1999, 38, L586. (25) Wen, X.; Fang, Y.; Pang, Q.; Yang, C.; Wang, J.; Ge, W.; Wong, K. S.; Yang, S. J. Phys. Chem. B 2005, 109, 15303. (26) Laudise, R. A.; Ballman, A. A. J. Phys. Chem. 1960, 64, 688. (27) Ogata, S.; Tagaya, H.; Karasu, M.; Kadokawa, J. J. Mater. Chem. 2000, 10, 321. (28) Rocca, E.; Caillet, C.; Mesbah, A.; Francois, M.; Steinmetz, J. Chem. Mater. 2006, 18, 6186.
Published on Web 09/28/2010
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reconstruction of metal oxides or hydroxides,27,30,31 the chemie douce method,32 the chemical bath deposition (CBD),13,14 or the cathodic deposition.33 All of these methods are carried out using the single liquid-phase medium, which has restricted chemical compositions of the formable layered compounds so far. That is, hydrophobic species having a potential ability of intercalation are difficult to be dissolved except in nonpolar, hydrophobic solvents, while building blocks of zinc hydroxide layers (Zn2þ and OH-) are soluble only in polar, hydrophilic solvents. Therefore, the single-phase synthesis methods generally require at least two steps to obtain inorganic-organic hybrid compounds. Even so, they suffer from lower yields of desired products. Here we propose the utilization of immiscible liquid-liquid biphasic systems to synthesize inorganic-organic layered compounds having new compositions and morphologies. Our synthesis method is based on the principle of distribution equilibrium of solutes between two liquid phases and is potentially applicable to a variety of inorganic-organic hybrid materials. In this first report, we have examined a system where hydrophobic organic acids are dissolved in an organic phase and zinc salts and OHsources are in an aqueous phase. According to our preliminary experiments, we failed in the synthesis of layered compounds when OH- was present in the aqueous phase at the beginning of the procedures. We then conceived a gradual release of OH- by thermal decomposition of urea. As a result, organic acid could be continuously delivered from the organic phase to the aqueous phase to form solids by getting supersaturated. We recognize that the idea of using immiscible liquid-liquid interfaces is known for solvent extraction,34 assembly of monolayers of nanoparticles,35 and limited reaction fields distinct from the bulk phase.36,37 Our approach is new in that one liquid phase works as a supplier of components that are hard to be dissolved in the other phase. A target compound was set to layered basic zinc benzoate (LBZB) in the present work. By using continuous transport of benzoic acid from xylene to water, LBZB with a unique morphology was obtained as powders or deposited on glass substrates as 2D structured uniform films in one pot and one process.
Experimental Section Synthesis. Zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O, 99.0% purity, Wako Pure Chemicals Co., Ltd., Japan) and urea ((NH2)2CO, 99.0%, Wako) were dissolved in deionized water with the assistance of ultrasonication at room temperature. Separately, benzoic acid (C6H5COOH, 99.5%, Wako) was dissolved in xylene as well. A concentration of Zn2þ in the aqueous solution was fixed at 0.1 mol/dm3, while those of urea in the aqueous solution and benzoic acid in the organic solution were varied between 0.1-1.0 and 0-1.0 mol/dm3, respectively. 7 mL of the xylene solution was injected gently into the equal volume of the aqueous solution using a syringe, keeping disturbance minimal. The resultant solutions were biphasically separated and were maintained at 80 °C for 24 h in a dry block bath without stirring. A solid phase was then formed in the aqueous phase. In some cases, (29) Arizaga, G. G. C.; Gardolinski, J. E. F. C.; Schreiner, W. H.; Wypych, F. J. Colloid Interface Sci. 2009, 330, 352. (30) Morioka, H.; Tagaya, H.; Karasu, M.; Kadokawa, J.; Chiba, K. Inorg. Chem. 1999, 38, 4211. (31) Miao, J.; Xue, M.; Itoh, H.; Feng, Q. J. Mater. Chem. 2006, 16, 474. (32) Poul, L.; Jouini, N.; Fievet, F. Chem. Mater. 2000, 12, 3123. (33) Sofos, M.; Goldberger, J.; Stone, D. A.; Allen, J. E.; Ma, Q.; Herman, D. J.; Tsai, W.; Lauhon, L. J.; Stupp, S. I. Nature Mater. 2009, 8, 68. (34) Saito, K.; Masuda, Y.; Sekido, E. Anal. Chim. Acta 1983, 151, 447. (35) Rao, C. N. R.; Kulkarni, G. U.; Rhomas, P. J.; Agrawal, V. V.; Saravanan, P. J. Phys. Chem. B 2003, 107, 7391. (36) Song, X.; Sun, S.; Zhang, W.; Yu, H.; Fan, W. J. Phys. Chem. B 2004, 108, 5200. (37) Mlondo, S. N.; Andrews, E. M.; Thomas, P. J.; O’Brien, P. Chem. Commun. 2008, 2768.
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Figure 1. Appearance of the biphasic xylene-water systems with
[B]:[Z]:[U] = 1:1:10, 2:1:10, and 5:1:10 after the reaction at 80 °C for 24 h. Sample A, B, and D were formed in the aqueous phase of the system of [B]:[Z]:[U] = 1:1:10, 2:1:10, and 5:1:10, respectively. Sample C was formed in the xylene phase of only the system of [B]:[Z]:[U] = 5:1:10.
however, solids were also formed in the organic phase, depending on the initial concentrations of the solutes. For the isolation of solid samples in the aqueous phase, the xylene and the aqueous solutions were sequentially removed by using a syringe. The final samples were obtained after being dried under the ambient condition. Attempts were also made to deposit films of layered compounds on substrates through heterogeneous nucleation and subsequent crystal growth. Quartz glass slides 1 mm in thickness were used as substrates for the deposition. The substrate was put into bottles filled with the biphasically separated solutions to contact only with the aqueous phase. After the deposition, the film samples obtained were dried under the ambient condition. Characterization. The crystal structure of the samples was identified by X-ray diffraction (XRD) analysis with a Bruker D8 ADVANCE diffractometer using Cu KR radiation (λ = 1.5405 A˚). To examine the layered structure of the samples, the θ-2θ scan was started from a low diffraction angle of 2θ = 2°. The organic species present in the samples were examined by Fourier transform infrared (FT-IR) spectroscopy with a Varian FTS60A/896 spectrometer using a KBr method. The sample morphology was observed by field emission scanning electron microscopy (FE-SEM) with a Hitachi S-4700 and an FEI Sirion microscope. The microstructure was observed by transmission electron microscopy (TEM) with a Philips TECNAI F20 microscope. The thermal decomposition behavior of the samples was examined by thermogravimetry-differential thermal analysis (TG-DTA) with a Mac Science 2020S analyzer using a heating rate of 2 °C/ min in flowing air. The specific surface area of the samples was determined by the Brunauer-Emmett-Teller (BET) method based on the nitrogen adsorption isotherm (77 K) with a Shimadzu Trister 3000 micrometric analyzer. For FT-IR, TG-DTA, and BET measurements, the film samples were removed from the substrate by scratching and treated as powdery samples.
Results and Discussion Hereafter, the biphasic xylene-water systems are distinguished with an initial concentration ratio of the solutes as “[B]:[Z]:[U]” ([B] for benzoic acid in xylene and [Z] for Zn2þ and [U] for urea in water). Figure 1 shows the appearance of glass containers after the reaction at 80 °C for 24 h with the different initial concentration ratios. It is seen that white precipitates are obtained in the aqueous phase at the lower concentrations of benzoic acid in the xylene phase. Sample A ([B]:[Z]:[U] = 1:1:10) was precipitated preferentially on the internal surface of the reaction container in the aqueous phase by the heterogeneous nucleation. In contrast, sample B ([B]:[Z]:[U] = 2:1:10) was precipitated as a gel-like solid in the aqueous phase. At the higher concentration of benzoic acid in the xylene phase ([B]:[Z]:[U] = 5:1:10), sample C was floating on the xylene-water interface while sample D was formed in the aqueous phase. Figure 2 shows XRD patterns of samples A to D. Diffraction peaks for samples C and D can be completely indexed as zinc DOI: 10.1021/la1028542
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Figure 3. FT-IR spectra of samples A and B. A spectrum of benzoic acid is also shown for comparison.
Figure 2. XRD patterns of samples A to D. Peaks appearing exclusively in the sample A and B are labeled with b and 0, respectively. A pattern of zinc benzoate is also shown as a reference.
benzoate (Zn(C6H5COO)2; ICDD 30-1998), which is not our target material. On the other hand, samples A and B show the patterns typical of the layered structure with strong peaks appearing in the lower 2θ range. Both of the two samples exhibit equally spaced peaks with d-values of 27.14 (2θ = 3.25°), 13.57 (6.51°), 9.05 (9.76°), 6.79 (13.03°), and 5.43 A˚ (16.31°) for sample A and 14.77 (2θ = 5.98°), 7.38 (11.98°), and 4.92 A˚ (18.01°) for sample B, which can be indexed as (00l) planes. For sample A, another peak also appears clearly at 2.73 A˚ (2θ = 32.78°), which is in good agreement with the (100) plane commonly found in any other layered zinc hydroxide compounds.32 The (100) peak shows an asymmetric broadening because the diffraction peak from the two-dimensional crystal generally becomes asymmetric with a long tail on the high angle side. A close look at the pattern of sample B reveals that it has mixed layered structures with two basal spacing of 14.77 and 13.19 A˚ (2θ = 6.70°), which is also found in LBZB reported by Miao et al.31 However, a peak at 27.96 A˚ (2θ = 3.16°), which is expected from a sum of the interlayer distance of 14.77 and 13.19 A˚, is not observed for sample B. This is also the case with the report by Miao et al.31 Instead, sample B exhibits two minor peaks at 25.93 A˚ (2θ = 3.41°) and 22.65 A˚ (3.90°), indicating that sample B possibly contains a trace of other phases different from the main phase with the two basal spacing of 14.77 and 13.19 A˚. The XRD data indicate that two kinds of layerstructured phases with the basal spacing of 27.14 and 14.77 A˚ are formed by changing the [B]:[Z] ratio between 1:1 and 2:1. The presence of the benzoate anion (C6H5COO-) in samples A and B was examined by FT-IR spectroscopy. FT-IR spectra of samples A and B as well as benzoic acid (as a reference) are compared in Figure 3. At a higher wavenumber range for samples A and B, intense and broad absorption bands are observed between 3700 and 3100 cm-1, resulting from the stretching vibration mode of the hydroxyl group (-O-H) of crystallization water and zinc hydroxide layer (Zn(OH)x). The intensity of these absorption bands significantly decreased after the conversion of samples A and B to ZnO by a heat treatment. Additionally, a sharp absorption peak at 3613 cm-1 for sample B is assigned to a free hydroxyl group derived from the adsorbed water on the surface. The free hydroxyl group is also detected in sample A as a weak signal at 3606 cm-1. For benzoic acid, broad and structured 15940 DOI: 10.1021/la1028542
absorptions detected between 3070 and 2560 cm-1 can be assigned to the stretching vibration mode of -O-H in the carboxyl group. At a lower wavenumber range, the aromatic ring of benzoic acid is evidenced by four bands at 1598, 1501 (CdC stretching mode), 710, and 683 cm-1 (-C-H antiplane bending mode) for all the samples. Although an intense absorption band is observed at 1687 cm-1 due to the CdO stretching vibration mode of aromatic carboxylic acid for benzoic acid, this band is completely absent for the samples A and B. Instead, benzoate anion displays two absorption bands at 1541 and 1394 cm-1 arising from COOasymmetric and symmetric stretching mode, respectively, in the spectra of samples A and B. In addition, weak bands resulting from the -O-H bending mode of water are detected at 1653 cm-1 for samples A and B. The FT-IR analysis then shows that samples A and B obtained in the aqueous phase of the biphasic xylenewater system include the benzoate anions and the water molecules. The thermal decomposition behavior of samples A and B was examined by TG-DTA analysis conducted in flowing air. As shown in Figure 4, a first gradual weight loss is observed at temperatures up to 130 °C for both the samples due to the release of water intercalated into or adsorbed onto the brucite-type hydroxide layers. A second weight loss occurs around 203 and 192 °C for sample A and B, respectively, accompanied by endothermic peaks. This is attributed to the dehydration of the zinc hydroxide layers. It is known that zinc hydroxide undergoes a decomposition reaction to form ZnO in the temperature range 70-140 °C.38 In the case of LBZB, however, this reaction temperature shifts to a higher temperature range of 160-220 °C.31 Third and fourth weight losses with exothermic peaks appear around 350 and 399 °C (sample A) and 338 and 392 °C (sample B). These behaviors can be assigned to the decomposition and oxidation of the benzoate group to CO2 and H2O in air because the two-step thermal decomposition is known to take place for benzoic acid in the temperature range 300-600 °C.31 Chemical compositions of samples A and B were determined from the above experimental data and the following consideration. Urea in the aqueous solution is hydrolyzed through heating to form ammonia and carbon dioxide, which react with water to produce OH- and CO32-, respectively.13 ðNH2 Þ2 CO þ H2 O f 2NH3 þ CO2
ð1Þ
NH3 þ H2 O f NH4 þ þ OH -
ð2Þ
CO2 þ H2 O f CO3 2 - þ 2Hþ
ð3Þ
(38) Kasai, A.; Fujihara, S. Inorg. Chem. 2006, 45, 415.
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Figure 5. Crystal structures of (a, c) sample A and (b) sample B.
Figure 4. TG-DTA curves for (a) sample A and (b) sample B.
Under the distribution law, a small amount of benzoic acid is transferred to the aqueous phase, and it is partially dissociated as follows: C6 H5 COOH T C6 H5 COO - þ Hþ
ð4Þ
As a result, four kinds of anionic species (NO3-, OH-, CO32-, and C6H5COO-) from starting Zn(NO3)2 3 6H2O, (NH2)2CO, and C6H5COOH are present in the aqueous phase. In forming the layered zinc hydroxides, the carbonate ion (CO32-) and the nitrate ion (NO3-) also have the potential to be incorporated into the brucite-type layers. However, at least the FT-IR and TGDTA data excluded the possible existence of CO32- and NO3- in our layered compounds. Based on this interpretation, approximate chemical compositions of the sample A and B were determined to be Zn(OH)1.74(C6H5COO)0.26 3 0.29H2O and Zn(OH)1.12(C6H5COO)0.88 3 0.21H2O, respectively, from the TG curves in Figure 4. A yield was calculated based on the Zn content in the sample and in the initial solution. The Zn content in the sample was determined from the weight of ZnO obtained by heating the sample. Yields of our compounds through the liquid-liquid synthesis were found to be relatively high: 88.8 and 89.2% for sample A and B, respectively. Referring to the literature, crystal structures of samples A and B, namely LBZB with the basal spacing of 27.14 and 14.77 A˚, respectively, have also been considered in terms of the interlayer structure and are depicted in Figure 5. The structure of sample A is similar to that of zinc hydroxide nitrate (Zn5(OH)8(NO3)2 3 2H2O),39 chloride (Zn5(OH)8Cl2 3 2H2O),40 and carboxylates (Zn5(OH)8(RCOO)2 3 2H2O; R = CH3 or C6H5).27,32 The large (39) St€ahlin, W.; Oswald, H. R. Acta Crystallogr. B 1970, 26, 860. (40) Allmann, R. Z. Kristallogr. 1968, 126, 417.
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basal spacing (27.14 A˚), the low C6H5COO/Zn ratio in the chemical composition (
