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Highly Crystalline Spindle-Shaped Mesoporous Anatase Titania Particles: Solution-Phase Synthesis, Characterization, and Photocatalytic Properties Xinling Liu,†,‡ Yanfeng Gao,*,† Chuanxiang Cao,† Hongjie Luo,† and Wenzhong Wang† †
Shanghai Institute of Ceramics, Chinese Academy of Sciences,1295 Dingxi Road, Shanghai 200050, China, and ‡ Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China Received March 7, 2010. Revised Manuscript Received April 14, 2010
Spindlelike mesoporous anatase titania particles were directly synthesized at a low temperature (95 °C) by using an aqueous peroxotitanium solution with polyacrylamide (PAM). The mesoporous titania had a BET-specific surface area of 89.6 m2 g-1 and showed high crystallinity, thermal stability, and good photocatalytic activity in the degradation of rhodamine B. PAM, as an additive, was confirmed to be crucial in the evolution of the specific structure, morphology, and crystalline phase, and a possible formation mechanism was suggested.
Titania (TiO2) has been widely studied because of its promising applications in pigments, photocatalysis, water splitting, and lithium rechargeable batteries.1 Recently, considerable attention has been paid to mesoporous TiO2, which can be used in chromatography, separations, solar cells, catalyst supporters, and sensors.2 In particular, anatase TiO2 with a large surface area in the form of mesoporous anatase particles should exhibit better photocatalytic activity. Although many methods have been developed for preparing mesoporous TiO2, most of these processes can produce only amorphous or weakly crystalline powders and further calcination or hydrothermal treatment is generally needed to crystallize the material.3 Until now, some success has been found with the low-temperature synthesis of mesoporous anatase titania, but only low crystallinity or mixed phases can be obtained.4 It remains a challenge to obtain mesoporous, highlycrystalline TiO2 with controllable crystalline phases by a solutionbased, simple, cheap, environmentally friendly process at a low temperature (below 100 °C). Organic matrices have been shown to be vital for the formation of inorganic biominerals in living organisms. For example, the *To whom all correspondence should be addressed. E-mail: yfgao@mail. sic.ac.cn. (1) (a) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891–2959. (b) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (c) Wagemaker, M.; Kentgens, A. P. M.; Mulder, F. M. Nature 2002, 418, 397–399. (d) Hashimoto, K.; Irie, H.; Fujishima, A. Jpn. J. Appl. Phys. 2005, 44, 8269–8285. (e) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243–2245. (2) (a) Lee, K.; Park, S. W.; Ko, M. J.; Kim, K.; Park, N. G. Nat. Mater. 2009, 8, 665–671. (b) Jing, W. H.; Huang, W.; Xing, W. H.; Wang, Y.; Jin, W. Q.; Fan, Y. Q. ACS Appl. Mater. Interfaces 2009, 1, 1607–1612. (c) Yang, W. G.; Wan, F. R.; Chen, Q. W.; Li, J. J.; Xu, D. S. J. Mater. Chem. 2010, 20, 2870–2876. (d) Wang, D. H.; Ma, Z.; Dai, S.; Liu, J.; Nie, Z. M.; Engelhard, M. H.; Huo, Q. S.; Wang, C. M.; Kou, R. J. Phys. Chem. C 2008, 112, 13499–13509. (e) Martinez-Ferrero, E.; Franc, G.; Mazeres, S.; Turrin, U. O.; Boissiere, U.; Caminade, A. M.; Majoral, J. P.; Sanchez, C. Chem.;Eur. J. 2008, 14, 7658–7669. (3) (a) Soler-Illia, G.; Louis, A.; Sanchez, C. Chem. Mater. 2002, 14, 750–759. (b) Peng, T. Y.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005, 109, 4947– 4952. (c) Kim, D. S.; Kwak, S. Y. Appl. Catal., A 2007, 323, 110–118. (d) Zhang, X.; Li, X. G.; Wu, J. S.; Yang, R. C.; Tian, L. M.; Zhang, Z. H. J. Sol-Gel Sci. Technol. 2009, 51, 1–3. (4) (a) Goutailler, G.; Guillard, C.; Daniele, S.; Hubert-Pfalzgraf, L. G. J. Mater. Chem. 2003, 13, 342–346. (b) Bosc, F.; Ayral, A.; Albouy, P. A.; Guizard, C. Chem. Mater. 2003, 15, 2463–2468. (c) Liu, Y.; Li, J.; Wang, M. J.; Li, Z. Y.; Liu, H. T.; He, P.; Yang, X. R.; Li, J. H. Cryst. Growth Des. 2005, 5, 1643–1649. (d) Shibata, H.; Ogura, T.; Mukai, T.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2005, 127, 16396–16397. (e) Shibata, H.; Mihara, H.; Mlikai, T.; Ogura, T.; Kohno, H.; Ohkubo, T.; Sakait, H.; Abe, M. Chem. Mater. 2006, 18, 2256–2260. (f) Hao, H. Y.; Zhang, J. L. Mater. Lett. 2009, 63, 106–108.
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self-assembled matrix of two types of organic molecules (a polycationic component and a polyanionic regulator) by electrostatic interactions can function as templates for the diverse biosilica patterns that exist in diatoms and sponges.5a In addition to acting as structure scaffolds, organic molecules also serve as polymorph modifiers by modulating the balance of thermodynamics and kinetics. For instance, vaterite (the most unstable phase of CaCO3) can be stabilized by proteins.5b Inspired by biomineralization, many soluble polymers mimicking the organic matrix in an organism have been employed to control the morphology, phase, and texture of TiO2.5c-f In this work, a soluble polyacrylamide (PAM) polymer was employed to modify the formation of TiO2 at 95 °C in an aqueous peroxotitanium solution. Highly crystalline, spindle-shaped mesoporous anatase TiO2 powders were directly obtained. These powders showed good thermal stability and photocatalytic activity in the degradation of rhodamine B (RhB). The PAM molecules played an important role in the structure and stabilization of the anatase phase. Figure 1a shows the XRD pattern of the product (defined as sample A) that was synthesized in the presence of PAM. The pattern is consistent with the anatase phase (JCPDS card no. 211272), and the sharp peaks are indicative of high crystallinity. However, rutile TiO2 (defined as sample R) was obtained in the absence of PAM (JCPDS card no. 21-1276, Figure 1b), in accord with a previous report.6a The difference in the crystalline phase implies that PAM may stabilize the anatase phase, and the possible mechanism will be discussed later in this letter. The TEM graphs of the products are shown in Figure 2. Anatase TiO2 (sample A) obtained in the presence of PAM exhibits spindlelike morphology and a wormlike mesoporous structure (Figure 2a); the particle’s mean center diameter is (5) (a) Poulsen, N.; Sumper, M.; Kroger, N. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 12075–12080. (b) Wang, X. Q.; Kong, R.; Pan, X. X.; Xu, H.; Xia, D. H.; Shan, H. H.; Lu, J. R. J. Phys. Chem. B 2009, 113, 8975–8982. (c) Dickerson, M. B.; Sandhage, K. H.; Naik, R. R. Chem. Rev. 2008, 108, 4935–4978. (d) Kroger, N.; Sandhage, K. H. MRS Bull. 2010, 35, 122–126. (e) Yuan, J. J.; Jin, R. H. Langmuir 2010, 26, 4212–4218. (f) Fang, Y. N.; Wu, Q. Z.; Dickerson, M. B.; Cai, Y.; Shian, S.; Berrigan, J. D.; Poulsen, N.; Kroger, N.; Sandhage, K. H. Chem. Mater. 2009, 21, 5704–5710. (6) (a) Gao, Y. F.; Nagai, M.; Seo, W. S.; Koumoto, K. Langmuir 2007, 23, 4712–4714. (b) Gao, Y. F.; Masuda, Y.; Peng, Z. F.; Yonezawa, T.; Koumoto, K. J. Mater. Chem. 2003, 13, 608–613. (c) Gao, Y.; Luo, H.; Mizusugi, S.; Nagai, M. Cryst. Growth Des. 2008, 8, 1804–1807.
Published on Web 05/03/2010
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Figure 1. XRD patterns of TiO2 synthesized (a) with PAM (sample A) and (b) without PAM (sample B).
Figure 3. (a) FTIR spectra of (1) anatase TiO2 synthesized with PAM (sample A) and (2) anatase TiO2 after heat treatment (samples A-H). (b) TG-DTA curves of anatase TiO2 synthesized with PAM (sample A).
Figure 2. TEM photographs (left), SAED patterns (inset), and corresponding HRTEM photographs (right) of (a) anatase TiO2 synthesized with PAM (sample A), (b) rutile TiO2 synthesized without PAM (sample R), and (c) anatase TiO2 after heat treatment (sample A-H).
about 50 nm, and the length is about 100 nm. Again, the high crystallinity is apparent from the SAED pattern and the HRTEM photograph. Interestingly, the crystalline rutile TiO2 (sample R) that was obtained without PAM is spindlelike but solid (Figure 2b). To remove the polymer and test the thermal stability, sample A was subjected to heat treatment at around 500 °C for 10 min. After the thermal treatment, the polymer of PAM was removed because no obvious characteristic peaks of PAM (e.g., (7) (a) Tang, Q. W.; Lin, J. M.; Wu, Z. B.; Wu, J. H.; Huang, M. L.; Yang, Y. Y. Eur. Polym. J. 2007, 43, 2214–2220. (b) Wang, X. J.; Hu, D. D.; Yang, J. X. Chem. Mater. 2007, 19, 2610–2621.
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3500-3300 cm-1 (νN-H), 1688 cm-1 (νCdO), 1406 cm-1 (δCH2))7 were present in the FTIR spectra of the obtained sample (defined as sample A-H) (Figure 3a). However, as shown in the TEM graphs in Figure 2c, the spindlelike shape and mesoporous structure remain, suggesting that the powders showed a relatively high thermal stability. The mesoprous TiO2 powders after heat treatment (sample A-H) are single-crystalline as judged by the SAED pattern and HRTEM photograph. Comparatively, the wall of mesoporous TiO2 in most previous reports is polycrystalline. It is worth noticing that all of the samples are spindlelike and crystalline as shown in Figure 2. This finding indicates that highly crystalline TiO2 can be acquired even in an aqueous solution at low temperature. The current process is superior to the traditional methods in which a long-time thermal treatment at a high temperature is usually required to remove organic templates and crystallize the materials. Obviously, the crystalline precursor is beneficial to retaining the mesoporous structure because it can suffer less structural collapse during thermal treatment. To research the mesoporous structure and pore-size distribution further, the nitrogen adsorption-desorption isotherms were measured (Figure 4). The isotherms were attributed to type IV, suggesting the presence of mesopores. The BET specific surface areas were estimated to be 89.6 and 64.7 m2 g-1 for samples A and A-H, respectively. In spite of a lower surface area for sample A-H, the BJH pore-size distribution curve (Figure 4b, inset) with a sharp peak at 4.5 nm demonstrates that the pore size ranges mainly from 2 to 10 nm, similar to the pore-size distribution of sample A (Figure 4a, inset). Together with the TEM analysis, the BET data further confirms that the powders are quite thermally stable. The photocatalytic activity of samples A and A-H was evaluated by the photocatalytic degradation of rhodamine B (RhB). Figure 4c shows the time-dependent changes in the Langmuir 2010, 26(11), 7671–7674
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Figure 4. Nitrogen adsorption-desorption isotherm and corresponding BJH pore-size distribution curve (inset) of (a) anatase TiO2 synthesized with PAM (sample A) and (b) anatase TiO2 after heat treatment (sample A-H). (c) Photocatalytic degradation of RhB by anatase TiO2 synthesized with PAM (sample A) (O), anatase TiO2 after heat treatment (sample A-H) (2), and no TiO2 (9). (d) Concentration variation of titanium in the solution without (O) and with (9) PAM. The inset shows the concentration variation during the first 10 h.
concentration of rhodamine B (RhB). In the absence of TiO2, the degradation of RhB proceeded slowly, and only about 25% of RhB was degraded after irradiation for 80 min. In the presence of TiO2 powders, however, at least 80% of the RhB was degraded after 30 min. The total degradation of RhB was complete only after 70 min for sample A and after 50 min for sample A-H. Those data gave evidence of good photocatalytic activity in the photodegradation of RhB because of the high surface area, crystalline framework, and accessible diffusion pathways. PAM is believed to impose control over the formation of TiO2. TG-DTA curves (Figure 3b) illustrated that the mass ratio of PAM in sample A was about 8% (estimated from the mass loss that occurred at above 150 °C) and the PAM molecules attached to TiO2 may play an important role in the evolution of crystalline phases. In an aqueous solution, the phase stability of TiO2 is pHdependent. The adsorbed species and surface charges can vary with the pH value, thus changing the surface energy that consists of both surface electrostatic energy and interfacial energy. It was thermodynamically and experimentally determined that the more stable phase under acidic conditions is rutile and, in contrast, anatase is the more stable phase under alkaline conditions.8a Indeed, rutile was obtained without PAM at pH 1. In the presence of PAM, however, anatase was obtained. Because the phase transformation involves both thermodynamic and kinetic factors, it appears that PAM may affect the kinetic phase transformation. The way in which the TiO6 octahedrons connect (linear for rutile (8) (a) Finnegan, M. P.; Zhang, H. Z.; Banfield, J. F. J. Phys. Chem. C 2007, 111, 1962–1968. (b) Wang, Y. W.; Huang, Y.; Ho, W. K.; Zhang, L. Z.; Zou, Z. G.; Lee, S. C. J. Hazard. Mater. 2009, 169, 77–87.
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and spiral for anatase) determines the phase of TiO2. The PAM molecules contain amino and carbonyl functional groups, which can interact with metal ions by chemical or physical adsorption. After PAM is adsorbed onto the TiO6 octahedrons, the long chains of PAM can create a sterically crowded environment. As a result, the spiral connections of anatase are favored, and the linear connections of rutile are obstructed.8b Hence, the transformation into rutile is suppressed, and the anatase phase is stabilized by PAM. The existence of pores may be ascribed to the synergistic effect of the mutual interactions between TiO2 and PAM, and several phenomena should be mentioned. First, the precipitate began to occur after 1.5 h at 95 °C in the solution without PAM, whereas it occurred after 4.5 h with PAM. Second, the concentration of titanium decreased faster in the solution without PAM than with PAM (Figure 4d). Third, the final concentration of titanium in the solution without PAM was 0.45 μg/mL, less than the value of 4.84 μg/mL with PAM. According to the formation mechanism suggested by Beck et al.9a for mesoporous silicates, the polymerization of inorganics and the formation of organic templates are mutually affected. When the titanium-oxygen clusters attach to PAM chains, the clusters will promote the unfolding of PAM molecules. Simultaneously, the PAM chains will hinder the massive aggregation of nonporous TiO2, resulting in a delay in (9) (a) 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, 10834–10843. (b) Sanchez, C.; Soler-Illia, G.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061–3083. (c) Chen, W.; Geng, Y.; Sun, X. D.; Cai, Q.; Li, H. D.; Weng, D. Microporous Mesoporous Mater. 2008, 111, 219–227.
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Letter Scheme 1. Schematic Illustration of the Formation Mechanism of Mesoporous TiO2
precipitation, more residual titanium ions in the solution, and looser anatase spindlelike particles compared to solid rutile paricles.9b Because of the mutual interactions among TiO6 clusters and PAM, the rates of crystallization, framework formation, and PAM assembly are balanced,4d and mesoporous anatase TiO2 is finally generated. However, the rearrangement and mobility of the PAM chains may be weakened because of the adhesion to titanium-oxygen clusters, and the pores are kept in an irregular and disordered intermediate state.9c Consequently, the wormlike disordered structure is produced. In Figure 4d, there exists a sharp decrease (about 80%) in the concentration of titanium during the first 10 h in the solution with PAM, indicating that the precipitation process proceeded mostly in that stage. The precipitate collected at 13.5 h was characterized. It could be seen that only aggregates with a few spindles on the edge were observed from the TEM graph (Figure S1 in the Supporting Information), and weak crystallinity was indicative in the XRD pattern (not shown). Those results showed that
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spindlelike particles were not formed in the first stage. A previous report showed that the process is kinetically controlled (i.e., amorphous TiO2 forms first and crystalline TiO2 follows).6 Therefore, it is considered that there exists a dissolution-crystallization process during which the amorphous phase transforms into the anatase phase and the spindlelike TiO2 particles begin to form. On basis of the discussions above, a possible formation mechanism is shown in Scheme 1. An amorphous TiO2-PAM hybrid is formed by the interactions among the PAM molecules and the titanium-oxygen clusters. Then, amorphous TiO2 transforms into anatase TiO2 through a dissolution-crystallization process. Meanwhile, anatase TiO2 and PAM attach to each other. Finally, they coassemble into spindlelike mesoporous TiO2. In summary, we have described a novel, low-temperature, onestep process for preparing highly crystalline mesoporous anatase TiO2. The spindlelike TiO2 exhibited good photocatalytic activity for the degradation of RhB. Also, the formation pathway was discussed, and it was suggested that the product was obtained through TiO2-PAM coassembly, which was accompanied by an amorphous-to-crystalline transformation. Acknowledgment. This work is partially supported by the Century Program of the Chinese Academy of Sciences. We thank Dr. W. Z. Yin, Dr. H. Y. Xing, Dr. L. J. Wang, and Dr. J. K. Chen for the characterization of photocatalytic properties and helpful discussions. Supporting Information Available: Experimental section and a TEM photograph. This material is available free of charge via the Internet at http://pubs.acs.org.
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