Materials Science and Engineering A 458 (2007) 44–47
Preparation of TiO2/PS complex nanoparticles Bitao Su ∗ , Zhanying Ma, Shixiong Min, Shixiong She, Zhiyuan Wang Key Labor of Gansu Polymer Materials, Department of Chemistry, Northwest Normal University, Lanzhou 730070, China Received 12 May 2006; received in revised form 8 December 2006; accepted 11 December 2006
Abstract TiO2 /polystyrene (PS) complex nanoparticles were successfully prepared through a novel two-step preparation method (TSPM) with styrene monomer and TiCl4 and characterized by XRD, TEM, TG-DTA, elemental analysis and IR techniques. Herein, TiCl4 was used as both the catalyst for polymerizing styrene and Ti source. The IR spectrum of TiO2 /PS showed the growths of the group C O on the chain of PS and Ti–O–C bond between TiO2 and PS. From its TEM, particles were observed to be nanosized and the average size was about 7 nm. The nanosize and the polar groups are advantageous for good absorptive property and strong interaction of PS and TiO2 and bringing their functions in the single material. Catalytic experiments indicated that the complex nanomaterial could fully decolorize dye MB in 10 min under the sunlight. © 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 ; PS; Complex nanoparticles; Preparation; Characterization
1. Introduction Semiconductor photocatalysis had received considerable attention as an alternative remediation technology of hazardous wastes, contaminated groundwater, and air contaminants. In recent years, considerable efforts had been put into TiO2 due to its unique properties [1] such as inexpensive, safe biologically and chemically inert, and stability with respect to photocorrosion. Although titanium dioxide is the most popular photocatalytic material [2], it is active only in the ultraviolet (UV) region because of its wide band gap. Therefore, many researchers [3–7] have attempted to modify the electronic properties of TiO2 in order to improve its catalytic activity under natural light. Although the dye-sensitized TiO2 showed excellent power conversion performances, their catalytic applications were still limited due to stability problems such as dissolution and the photocatalytic degradation of the dye. The capability of some polymers to act as sensitizer of TiO2 , for the visible light, had been demonstrated for PPVs and PTS [8–11]. The photo-induced charge transfer from the polymers to TiO2 had been found to be possible when the thickness of the polymer was below the exciton diffusion length (approx. 20 nm) [11]. However, the major problem, which had to be solved in order to improve the per-
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formances of polymer/TiO2 materials, was the poor interaction and absorptive properties of the polymer to nanosized TiO2 . In this paper, we reported a new and simple preparation method successfully used to prepare TiO2 /PS complex nanoparticles with an average size of 7 nm from styrene monomer and TiCl4 using this method. Using this method, TiO2 and PS were closely linked by some chemical force. The sample was characterized by TEM, XRD, TG-DTA, elemental analysis, IR and catalytic experiments. 2. Experimental 2.1. Reagent Titanium chloride, monomer styrene, and methylene blue (MB) were reagent grade; anhydrous ethanol and deionized water were used through out this study. 2.2. Preparation of the TiO2 /PS complex nanoparticles We developed a simple and effective two-step preparation method (TSPM), ionic polymerization and heat conversion, as described below. (1) Ionic polymerization step: under room temperature and vigorous stirring conditions, a suitable amount of TiCl4 , was slowly added drop wise to 20 ml styrene. Styrene was catalytically polymerized by Ti4+ via an ionic polymerization
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route to produce polystyrene (PS) and during the polymerizing process, catalyst Ti4+ ions were spontaneously mixed into PS base to form the orange precursor Ti4+ /PS. (2) Heat conversion step: this precursor Ti4+ /PS was treated at 300 ◦ C for a proper time under air atmosphere and the black product, TiO2 /PS, was obtained. The appropriate treatment could bring about the conversion of Ti4+ to TiO2 and a new structure of PS and some strong interactions between TiO2 and PS, proved by the XRD and TG-DTA and IR results of the sample. 2.3. Apparatus TEM observations were made using a JEM-100SX Electron Microscope. The sample was dispersed in alcohol and a drop of the dispersion was cast onto a carbon film supported a copper grid. XRD pattern was obtained by a Riggaku D/max-3C Xray diffraction meter. TG-DTA measurement (DT-40) were also made. IR spectra were recorded on Nicolet NEXUS 670 FT-IR spectrophotometer. Elemental analysis of TiO2 /PS was performed using different methods. C and H contents were analyzed on an elemental analyzer (PE-2400). Ti was determined by burning the sample at an enough high temperature in order to bring about the complete transformation of PS to CO2 and H2 O, and thus the residue was TiO2 . The O was calculated from C and H and Ti contents. 3. Results and discussion TEM image of the sample is showed in Fig. 1. It was observed that the average size was about 7 nm without the separation of organic and inorganic phases. The poor dispersion was possibly related to high surface energy of nanoparticles with such a
Fig. 1. TEM image of TiO2 /PS complex particles.
Fig. 2. XRD pattern of TiO2 /PS.
small size and structural property of PS. A large interface on a nanometer scale of donor and acceptor materials was advantageous for closely linking TiO2 and PS together and bringing their functions in the single material [11]. At the same time, nanoparticles with such small size could exhibit much larger efficient areas and more efficient active center, in favor of preadsorption of organic pollutants [12–14], which was important for the photodegradation when it was used as a catalyst. Shown in Fig. 2 is the XRD pattern of the sample. PS of the complex was amorphous and a characteristic signal of TiO2 appeared in the pattern. The appearance of TiO2 phase indicated the TiO2 could be formed by the above-mentioned method. In addition, the characteristic reflection peaks were broadened and considerably changed, indicating that the crystal lattice of TiO2 was locally distorted because of the strong interaction of TiO2 and PS [7], and also suggesting a small crystalline domain size [15,16] in agreement with that from TEM. TG-DTA property of the precursor Ti4+ /PS was investigated and curves are shown in Fig. 3. From them, two endothermic peaks and three weight loss regions were observed. Below 230 ◦ C, the weight loss could be assigned to the release of HCl, inferred by the released gas smell when Ti4+ /PS was heated from the room temperature to 250 ◦ C in an electric oven, the desorption of adsorbed water and the removal of H2 O from Ti–OH. And the significant ones were endothermic and respectively occurred at about 394 and 438 ◦ C, assigned to the decomposition of main chain and side group of PS. In the complex, the decomposition temperatures of PS were lower than pure PS’ at about 405 and 551 ◦ C. Lowing the temperatures could be attributed to the
Fig. 3. TG-DTA curves of the Ti4+ /PS.
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Fig. 5. The activity comparison of: (a) TiO2 /PS complex and (b) P25 under the sunlight.
Fig. 4. IR spectra of the precursor Ti4+ /PS (a) and TiO2 /PS nanocomplex (b).
catalysis of Ti4+ . Ti4+ played a role of catalyst by forming the coordination with π electron of phenyl ring and/or polar group C O on the chain of PS and weakening the bonds of PS. Residue after 450 ◦ C was pure TiO2 and about 47%. So the heat treatment of Ti4+ /PS at 300 ◦ C does not lead to the decomposition of PS while it can bring about the conversion of Ti4+ to TiO2 . The elemental analytic results showed that in the complex, the ratio of C to H was higher than 1:1, theoretical value in PS, and the ratio of O and Ti about 3:1, higher than 2:1 in TiO2 . It implied that in the complex, the unsaturated degree of the PS was higher and there was some polar group with O on the chain of the PS, supported by its IR spectrum. TiO2 was about 50 wt.%, consistent with TG-DTA’ result. The IR spectra of Ti4+ /PS (a) and TiO2 /PS (b) are shown in Fig. 4. In Fig. 4a, the broad band at about 3185.19 cm−1 was contributed to –C–H and C–H of PS and O–H of the adsorbed H2 O. The existence of H2 O is beneficial to the conversion of Ti4+ to TiO2 . The peaks near 1600.60, 1491.17 and 1449.45 cm−1 were attributed to the characteristic skeleton vibration peak of phenyl ring [17]. The peaks at about 748 and 697 cm−1 were related to single-substrate of phenyl ring. So the styrene was catalytically polymerized by Ti4+ via an ionic polymerization route. In addition, the strong band about 500 cm−1 was characteristic one of TiO2 . The TiO2 /PS’ spectrum (curve b) showed the growth of C O at 1700.61 cm−1 and C–O–Ti at 1265.78 cm−1 [18]. The existence of C O demonstrated the oxidation of some –CH2 – to –CO– on the chain of PS during the heat treatment. The oxidation resulted in the increase of unsaturated degree of PS. Characteristic peaks of phenyl ring were broadened and became one and red-shifted. These were possibly related to the increase of the conjugated chain length and a series of quantum size effects [19] and an efficient coordination bond between phenyl ring and Ti4+ and C–O–Ti bond [20]. The existence of polar groups such as C–O–Ti and C O groups led to asymmetric electron structure, which is important for enhancing the activities of catalysts.
Therefore, this present method was successful for preparing the complex nanomaterials with some strong interactions and absorptive properties of the polymer to TiO2 and improving the performance of polymer/TiO2 material. The method was simple to prepare nanosized TiO2 /PS complex material and efficient to control the growth of their particles. The catalytic property of the sample was evaluated by measuring the decolorizing efficiency of MB solution and compared with P25, a commercial TiO2 under the sunlight. The catalytic reaction was performed in a flask under the sunlight (1–15 September 2005). Thirty milligrams of powder was added in 30 ml MB solution of 10 mg/l, the system was stirred for some time under the sunlight, and then the solution, from which the powder was removed, was analyzed to determine the absorption value A. The decolorization efficiency is calculated based on the equation: D (%) = (A0 − At )/A0 × 100%, in which, D (%) means the decolorization efficiency of dye solution, A0 and At the absorption values of dye solution at initial time t = 0 and reaction time t, respectively. From Fig. 5, it could be found that the decolorization efficiency of MB on TiO2 /PS particles reached 93% in less than 10 min while on P25, the efficiency was about 25% in 20 min and did not changed with the further increase of the time, which was only attributed to the absorption of MB molecules on the surface of P25 particles. The catalytic mechanism will be further investigated. 4. Conclusion In summary, we firstly designed a novel method TSPM and successfully prepared a new class of TiO2 /PS complex with an average size of 7 nm through using this method. The nanosize of the sample and electron structures of PS and Ti4+ brought about the strong absorptive property and interaction between TiO2 and PS phases, bringing their functions in the single material and improving the performances of the material. The catalytic experiment demonstrated these complex nanoparticles showed extremely high catalytic activity for the decolorization of MB under the mild condition, indicating that the effective complex of
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PS and TiO2 remarkably improved the performance of TiO2 . So this complex material was a promising one for effective application in environment treatment under the sunlight without using any special UV light source. Acknowledgement This work was financially supported by the Natural Science Foundation of Gansu province (No. 3ZS041-A25-034). References [1] M.R. Hoffmann, S.T. Martin, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] S. Monredon, A. Cellot, F. Ribot, J. Mater. Chem. 12 (2002) 2396. [3] H. Chen, X.L. Jin, B.T. Su, Y.J. Fang, K. Zhu, R.Q. Yang, Ind. J. Chem. B 39A (2000) 685. [4] Y.P. Wang, J.G. Yu, X.J. Zhao, Zh.M. Shu, China Environ. Sci. 18 (1998) 244. [5] Zh.C. Wang, X.J. Li, P. Wang, China Environ. Sci. 23 (2003) 535. [6] R. Asahi, T. Morikawa, T. Ohwaki, A. Aoki, Y. Taga, Science 293 (2001) 269.
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[7] T. Ohno, T. Mitsui, M. Matsumura, Chem. Lett. 32 (2003) 364. [8] R.A.J. Janssen, P.A. Van Hal, M.M. Wienk, J. Phys. Chem. B. 103 (1999) 4352. [9] T.J. Savenije, J.M. Warman, A. Goossens, Chem. Phys. Lett. 287 (1990) 148. [10] J.S. Salafsky, W.H. Lubberhuizen, R.E.I. Schropp, Chem. Phys. Lett. 290 (1998) 297. [11] T.J. Savenije, M.J.W. Vermeulen, M.P. De Haas, J.M. Warman, Sol. Energy Mater. Sol. Cells 61 (2000) 9. [12] Y.H. Zhang, G.X. Xiong, W.Sh. Yang, X.Z. Fu, Acta Phys.—Chim. Sin. 17 (2001) 273. [13] H.M. Sung-Suh, J.R. Choi, H.J. Hah, S.M. Koo, Y.C. Bae, J. Photochem. Photobiol. A 163 (2004) 37. [14] Y.M. Wang, S.W. Liu, M.K. L¨u, S.F. Wang, F.G.X.Z. Gai, X.P. Cui, J. Pan, J. Mol. Catal. A 215 (2004) 137. [15] J.R. Zhang, L. Gao, Chem. Lett. 32 (2003) 458. [16] Y.B. Xie, Ch.W. Yuan, Appl. Catal. B 46 (2003) 251. [17] Ch.Y. Duan, J.F. Zhou, Zh.Sh. Wu, H.X. Dang, Acta Phys.—Chim. Sin. 19 (2003) 1049. [18] Q.Y. Gao, Y.J. Zhang, X.D. Yu, Acta Polym. Sin. 3 (2001) 329. [19] L.D. Zhang, J.M. Mou, Nanomaterials Science, Liaoning Science & Technology Press, Shenyang, 1994, p. 171. [20] S.H. Jang, M.G. Han, Synth. Met. 110 (2000) 17.