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Mesostructural Transformation of Vanadium Oxide-Hexadecyltrimethylammonium Composite by Low-Temperature Calcination Yasuhiro Yagi,† Haoshen Zhou,‡ Masaru Miyayama,*,† Tetsuichi Kudo,† and Itaru Honma‡ Institute of Industrial Science, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan, and Energy Fundamentals Division, Electrotechnical Laboratory, Umezono, Tsukuba, Ibaraki 305-8568, Japan Received October 6, 2000. In Final Form: December 26, 2000 Mesostructural transformation and deformation of vanadate-hexadecyltrimethylammonium composite powders by thermal treatment were investigated. This composite, prepared from vanadate and surfactant solutions, had a lamellae-like structure and displayed mesophase transformations to two-dimensional monoclinic (p2) and hexagonal (p6m) structures during calcination at 160 °C. The formation of the p2 mesophase was found for the first time in the self-organization processing system. The origin of the phase transformation was confirmed by thermal analysis to result from removal of the organic component during the heat treatment. It is speculated that those results are due to the strong interaction between the inorganic framework and the organic template.
The discovery of the mesoporous silica and aluminosilicate by Beck et al.1 has led to the creation of new pathways for the synthesis of mesostructured materials, and numerous subsequent papers have further exhibited reaction mechanisms and engineering applications.2 The usual processing methods for mesoporous materials, typically used for silica-based surfactant systems, have been reported using the following route: the silicasurfactant composite is first prepared by mixing a silica source (e.g., tetraethyl orthosilicate) with a template source of an organic surfactant (e.g., hexadecyltrimethylammonium bromide, C16TMABr) in a solvent, and then the organic component is removed from the composite by calcination or solvent extraction.3 The mesostructure of the final porous product is formed in the self-organizing reaction in solution, and structural deformation does not occur during the surfactant removal process. The factors determining the shape of the mesopore include the ratio of the inorganic source to the surfactant,4 temperature,5 pH,6 and reaction time.7 All factors relate to the reaction conditions in the solution. The factors which determine the size of the mesopore are alkyl chain length and additives,1 and these are related to the template material. * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81-3-3402-6231 (ex 2427). Facsimile: +81-3-5474-2137. † The University of Tokyo. ‡ Electrotechnical Laboratory. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2075. (3) Hitz, S.; Prins, R. J. Catal. 1997, 168, 194. (4) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Chem. Mater. 1994, 6, 2317. (5) Chen, C. Y.; Li, H. Y.; Davis, M. E. Microporous Mater. 1993, 2, 17. (6) Monier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (7) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176.
Although mesoporous silica materials are known, mesoporous vanadium(V) oxides have not yet been synthesized using the same methods as published for the silicasurfactant systems. Luca et al. reported that the calcination of a vanadate-C16TMA composite causes mesophase deformation.8 Zhou et al. reported the phase transformation of the mesostructure in a vanadate-C16TMA thin film during heat treatment at 160 °C.9 These papers lead to the conclusion that the typical selforganization process used for preparing mesoporous silica cannot be successfully applied to a vanadate-surfactant system. The details of the phase transformation and deformation are thus far unclear. Owing to many potential applications of vanadium(V) oxides as electrode materials, a successful synthesis of a mesoporous vanadium oxide could lead to a major breakthrough in the field of energy materials applications such as lithium ion batteries, electrochemical capacitors, and fuel cell reformers.10-12 The deformation of the mesostructure has also been reported to occur during the calcination of germanium sulfide mesostructured materials,13 and it therefore should be recognized as a common problem for the preparation of mesoporous inorganic materials, with the exception of mesoporous silica. It appears that there is a difference between the formation of silica-surfactant and the other inorganic-surfactant mesostructured materials. However, to our knowledge there have been no reports which explain this difference or clarify the mechanisms involved in the mesostructural transformation and deformation of the vanadate-surfactant composite. In this paper, we describe the mesostructural transformation of a vanadate-C16TMA composite powder. The composite powder was systematically calcined at various temperatures and (8) Luca, V.; MacLachlan, D. J.; Hook, J. M.; Withers, R. Chem. Mater. 1995, 7, 2220. (9) Zhou, H.; Honma, I. Mater. Res. Soc. Symp. Proc. 1999, 549, 261. (10) Spahr, M. E.; Stoschitzki-Bitterli, P.; Nesper, R.; Haas, O.; Nova´k, P. J. Electrochem. Soc. 1999, 146, 2780. (11) Nishizawa, M.; Mukai, K.; Kuwabata, S.; Martin, C. R.; Yoneyama, H. J. Electrochem. Soc. 1997, 144, 1923. (12) Che, G.; Jirage, K. B.; Fisher, E. R.; Martin, C. R.; Yoneyama, H. J. Electrochem. Soc. 1997, 144, 4296. (13) MacLachlan, M. J.; Coombs, N.; Bedard, R. L.; White, S.; Thompson, L. K.; Ozin, G. A. J. Am. Chem. Soc. 1999, 121, 12005.
10.1021/la001417l CCC: $20.00 © 2001 American Chemical Society Published on Web 02/06/2001
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Figure 1. Powder XRD pattern of vanadate-C16TMA composite before calcination.
for varying lengths of time to observe the sequence of the mesophase transformation. The purpose of this work is to determine and describe the difference between silicasurfactant and vanadate-surfactant systems from the discussion of the experimental results. The synthetic method for vanadate-surfactant composite powders has already been reported by Luca et al.8 We prepared the composite powder in the same manner using hexadecyltrimethylammonium chloride (C16TMACl) as the surfactant source and ammonium vanadate(V) (NH4VO3) as the vanadate source. All starting materials were purchased from Wako Pure Chemical Industries, Ltd. The surfactant solution was prepared by dissolving 3.84 g of C16TMACl in 50 mL of water. The vanadate solution was prepared separately by dissolving 1.17 g of NH4VO3 in 50 mL of aqueous sodium hydroxide solution (1.0 mol/L). After the vanadate solution was stirred for 2 h, hydrochloric acid (1.2 mol/L) was added dropwise until the pH was decreased to 6.5. The surfactant solution was then added to the resulting vanadate solution, quickly giving a yellow precipitate of the vanadate-C16TMA composite. The composite was filtered, washed with ethanol and water, and then dried at 65 °C for 10 h. Mesophase transformation and deformation of the vanadate-C16TMA composite were investigated by calcination of the powder in a furnace, and the results are discussed as a function of both calcination temperature and time. The purified composite powder (330 mg in a sample container) was calcined by either (1) increasing the temperature to a target temperature (150-400 °C) and holding the temperature constant for 2 h or (2) increasing the temperature to 160 °C and holding the temperature constant for 1-64 h. In both cases, the heating rate was 1 °C/min. After heating, the powder was furnace-cooled for 2 h. The sample was characterized before and after heat treatment by the assignment of lowangle powder X-ray diffraction (XRD) peaks. The XRD patterns of the samples were taken on a Rigaku RINT 2400 diffractometer using Cu KR radiation. To investigate the origin of phase transformation during the calcination at 160 °C, thermogravimetry/differential thermal analysis (TG-DTA) was performed for the sample before calcination using a Sinku-Riko TGD 7000 differential thermogravimetric analyzer under an air atmosphere (flow rate, 2.00 mL/s). Figure 1 shows the XRD pattern of the composite before calcination. From the three indicative peaks (3.65, 7.31, and 11.0°), this XRD pattern looks similar to that of a lamellar mesostructured material. Indeed, Luca et al. synthesized the same composite previously and asserted that it consists of a lamellar mesostructure.8 However, we determined that the mesostructure of the composite is not lamellar but is in fact lamellae-like, for the following two reasons: (1) peaks other than the three above are
Figure 2. Powder XRD patterns of the samples obtained by calcining the vanadate-C16TMA composite at various temperatures for 2 h.
Figure 3. Powder XRD patterns of the samples obtained by calcining the vanadate-C16TMA composite at 160 °C for various times.
clearly observed and (2) the d-value of the main peak (2.42 nm) is much smaller than that of an authentic lamellar mesostructure (3.6 nm for silica with the C16TMA template).1,4 The XRD patterns of samples after calcination at various temperatures are shown in Figure 2. Characteristic peaks for a mesostructure were observed in samples calcined at temperatures below 250 °C; however, the mesostructure was deformed at higher temperatures. To observe in detail the mesostructure and its transformation sequence at lower temperatures, we carried out a calcination experiment at 160 °C for a varying number of hours. The results are shown in Figure 3. Samples calcined for 1-4 h consist of two-dimensional monoclinic p2 structures because all of the peaks can be assigned to the p2 structure reported for the water-C16TMABr system by Auvray et al.14 Indexes for the peaks of the sample calcined for 1 h are included in Figure 3. On the other hand, the peaks in the XRD patterns of samples calcined for more than 8 h are indicative of the hexagonal p6m structure, as is often observed in the silica-surfactant system. Accordingly, the sequence of structural transformation in the vanadateC16TMA composite was found to be from a lamellae-like (for the as-prepared composite) structure to monoclinic p2 and hexagonal p6m mesostructures. This monoclinic (14) Auvray, X.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A. J. Phys. Chem. 1989, 93, 7458.
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Figure 4. Result of TG-DTA of the vanadate-C16TMA composite powder before calcination. TG and DTA curves are represented by a solid line (left axis) and a solid line with open circles (right axis), respectively, as a function of time. TG-DTA was carried out by controlling the temperature as shown in this figure, in which the temperature curve is represented by a dotted line (left axis outside the plotted area) as a function of time.
phase, usually referred as “oblique”,15 was discovered for the first time in the inorganic-organic composite system. With the progress of calcination, a change in the parameters of the p2 lattice was observed as follows: the length of the a axis remains unchanged (approximately 4.4 nm), whereas the length of the b axis increases (from 3.6 to 4.3 nm), and the angle (a b, B b) approaches 120° from 105°. As for the p6m mesostructure, the lattice parameter decreases with increasing heating time. Because the change in the mesostructure of the vanadate-C16TMA composite (p2 to p6m) corresponds to the concentration of C16TMABr against water,14 the calcination of this material at 160 °C is believed to result in the baking out of the organic template component, leading to a decrease in its concentration against the vanadate framework. Therefore, we carried out a thermal analysis in order to determine the reaction of the surfactant during the heat treatment. The result of TG-DTA for the sample before calcination is shown in Figure 4. Two kinds of weight loss are observed: one is due to an endothermic physical desorption of adsorbed water in the temperature range of 25-130 °C and the other is due to a gradual exothermic reaction at temperatures from 130 to 160 °C and during further heating at 160 °C. On the basis of the weight loss at 130-160 °C, it was confirmed that the organic component was baked out of the matrix and that the mesostructural transformation of the vanadate-C16TMA composite is a direct result of the decrease of the surfactant concentration. From the above experimental results, the difference between vanadate-surfactant and silica-surfactant composites becomes clearer. Heating causes the vanadatesurfactant composite to transform its mesostructure corresponding to the surfactant concentration, but the same treatment for a silica-surfactant system does not give the same result. In addition, the formation of the p2 (15) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Liq. Cryst. 1994, 17, 157.
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phase is peculiar to the mesostructure in the vanadateC16TMA composite, because there is no region of the p2 structure in the silica-C16TMA mesophase diagram.4 We believe that the difference between the vanadate-surfactant and silica-surfactant composites is strongly related to the behavior of the cationic surfactant in polar solvents. Auvray et al. examined the phase transformation sequences of C16TMABr in water, formamide, and glycerol and found that the formation of the p2 mesostructure is observed only in water.14 The change in the mesostructure of the vanadate-C16TMA composite as a function of the surfactant concentration is the same as that observed for C16TMABr-water, and that of the silica-C16TMA composite (hexagonal, p6m T cubic, Ia3d T lamellar) is the same as that for C16TMABr-formamide and C16TMABrglycerol. Although the interaction between the surfactant and polar solvents remains unclear, we speculate that the origin of the difference between the vanadate-C16TMA and silica-C16TMA composites relates to the ionic character of the inorganic framework, and our speculation is described below. The vanadate framework and water both have a strong ionic character, whereas the silica framework and the above polar organic solvents are weaker by comparison. Because of a strong ionic character, the vanadate framework is attracted to the ionic headgroup of the surfactant, and therefore a strong interfacial tension applies to the region between the framework and the surfactant. This results in the mesophase transformation and deformation during the calcination. In contrast, the weak ionic character of the silica framework does not cause enough strong interfacial tension to result in the same phase transformation. In the silica-surfactant system, the framework is maintained even after the complete removal of the surfactant and results in the formation of the mesopore. In summary, mesostructural transformation and deformation of the vanadate-hexadecyltrimethylammonium composite powder were observed at various calcination temperatures and times. We investigated the transformation sequence and found for the first time the formation of the two-dimensional monoclinic phase in the self-organization processing system. The transformation and deformation during heating are caused by the baking out of the surfactant, leading to the decrease in its concentration. This concept was confirmed by thermal analysis. The change in the mesostructure of the composite is discussed as a function of the surfactant concentration with relation to that of hexadecyltrimethylammonium bromide in water. The driving force for these phenomena is speculated to be an attractive interaction between the vanadate framework and the surfactant template, because of the strong ionic character of the vanadate framework and its interaction with its surroundings. This speculation is based on the comparison in behavior between inorganic frameworks and polar solvents when those compose mesostructures with the cationic surfactant. This paper demonstrates one viewpoint which clarifies the difference between vanadate-surfactant and silica-surfactant systems. LA001417L
