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Ultrasonic Irradiation: A Novel Approach To Prepare Conductive Polyaniline/Nanocrystalline Titanium Oxide Composites Hesheng Xia and Qi Wang* The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, China Received October 11, 2001. Revised Manuscript Received February 6, 2002
A novel approach, i.e., ultrasonic irradiation, was used to prepare polyaniline/nanocrystalline TiO2 composite particles. Polymerization of aniline proceeded under ultrasonic irradiation in the presence of nanocrystalline TiO2. The aggregation of nano TiO2 can be reduced under ultrasonic irradiation, and the nanoparticles can be redispersed in the aqueous solution. The polyaniline deposits on the surface of the nanoparticle, which leads to a coreshell structure. The resulting polyaniline/nano TiO2 composite particles are spherical, and the sizes vary with the content of TiO2. The polyaniline/nano TiO2 composite particles prepared by the conventional stirring method have a “raspberry” aggregate structure, which is different from that obtained through ultrasonic irradiation. The presence of nanocrystalline TiO2 strengthens the UV absorption of polyaniline and leads to a blue shift of the π-polaron absorption of polyaniline. Ultrasound can enhance the doping level. When polyaniline deposits on the surface of nano TiO2, the crystalline behavior of polyaniline is hampered and the degree of crystallinity decreases. With increased TiO2 content, the H-bonding interaction is strengthened and the characteristic peaks of benzoid and quinoid are shifted. X-ray photoelectron spectroscopy (XPS) shows that the ratio of the number of Ti and N atoms (Ti/N) on the surface is lower than that in the bulk. This is strong evidence for a polyanilineencapsulated nano TiO2 structure. The conductivity of the composites obtained through ultrasonic irradiation decreases with increasing TiO2 content. Ultrasonic irradiation contributes to the increase in the conductivity compared with conventional stirring. When the content of polyaniline decreases to ∼10%, the conductivity of composite still remains at 10-1 S·cm-1. Ultrasonic irradiation provides us a new way to prepare 0-3-dimensional conducting polymer/nanocrystalline particle composites.
Introduction Conducting polymer/inorganic nanoparticle composites with different combinations of the two components have attracted more and more attention, since they have interesting physical properties and many potential applications.1 However, conducting polymers are not molten in nature and generally are insoluble in common solvents, and the nanoparticles are easily aggregated due to their high surface energy, so it is difficult to prepare conducting polymer/inorganic nanoparticle composites by conventional blending or mixing in solution or melt form. Recently, Armes et al. have developed a brilliant sol technique.2,3 In the presence of silica sol with a particle size of ∼20 nm, colloidal dispersions of conducting polymers/inorganic nanoparticles were prepared. Those nanocomposite particles have potential applications as marker particles for immunodiagnostic assays. A large number of sols were used in this method, * To whom correspondence should be addressed: e-mail qiwang@ sun450.scu.edu.cn; fax +86-28-5402465; tel +86-28-5405133. (1) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (2) Flitton, R.; Johal, J.; Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1995, 173, 135. (3) Butterworth, M. D.; Corradl, R.; Johal, J.; Lascelles, S. F.; Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1995, 174, 510.
and the primary objective was to keep the conducting polymer in a good dispersion. However, to expand the technological application of those nanocomposites, it is very important to have successful combination of different structures of the core materials, especially the inorganic oxide nanocrystalline structure. Clearly, the sol approach encounters some difficulties in preparing such nanocomposites, because inorganic oxide nanocrystals are usually obtained by calcinations at a high temperature, and the polymer will decompose at such a high temperature. Polyaniline is one of the most important conducting polymers because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and its excellent environment stability.4,5 Nanocrystalline TiO2 has unique physical and chemical properties and it can be used in advanced coating,6 cosmetic,7 sensor,8 solar cell,9 and photocatalyst10 ap(4) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477. (5) Sailor, M. J.; Ginsburg, E. J.; Gorman, C. B.; Kumar, A.; Grubbs, R. H.; Lewis, N. S. Science 1990, 249, 1146. (6) Wang, X.; Zu, Y.; Li, X. Adv. Chem. Technol. 2000, 1, 67. (7) Zu, Y.; Lei, Y.; Yu, X. New Mater. Chem. Technol. 1998, 6, 26. (8) Zhou, W.; Sun, C. W.; Yang, Z. Z. Acta Inorg. Mater. 1998, 13, 275.
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plications, etc. Generally most applications are related to the crystalline structure of TiO2, and amorphous TiO2 has limited applications. For example, the metastable anatase TiO2 obtained at ∼400-600 °C is good for photocatalytic activity, and the stable rutile TiO2 obtained at ∼600-800 °C is the best choice for cosmetics. Polyaniline/nanocrystalline TiO2 composites combine the merits of polyaniline and nanocrystalline TiO2 particles and have potential applications in conductive coating, charge storage, electrochromic activities, photovoltaic properties, electrocatalytic applications, and absorption materials of solar cell. As mentioned above, it is very difficult to prepare such a 0-3-dimensional nanocomposite with a core-shell structure by conventional mixing, blending, and sol technique. Yoneyama got a 0-2-dimensional nanocomposite film by electrochemical deposition of commercially available TiO2 particles (∼22 nm) into polyaniline films and succeeded in writing with UV light on polyaniline film.11 Somani prepared the highly piezoresistive conducting polyaniline-titanium oxide composites (PANI/TiO2) by an in situ deposition technique by placing the fine-grade powder of anatase TiO2 with a relatively larger particle size (∼100 nm).12 However, the preparation of 0-3dimensional polyaniline/nanocrystalline TiO2 composites particles with a core-shell structure still remains a puzzle and requires new technologies and new methods. Ultrasonic irradiation, as a new technology, has been widely used in chemical synthesis. When an ultrasonic wave passes through a liquid medium, a large number of microbubbles form, grow, and collapse in a very short time of about a few microseconds, an effect that is called ultrasonic cavitation. Sonochemical theory calculations and the corresponding experiments suggested that ultrasonic cavitation can generate local temperatures as high as 5000 K and local pressures as high as 500 atm, with heating and cooling rates greater than 109 K/s, a very rigorous environment.13 Therefore, ultrasound has been extensively applied in dispersion, emulsifying, crushing, and activation of particles. Previously we reported that the aggregates of nano silica could be broken apart and be redispersed in the aqueous medium, and thus the long-term stable polymer/nano silica composite latex was prepared through ultrasonic irradiation.14,15 In the present study, the ultrasonic irradiation technique was employed to prepare 0-3dimensional polyaniline/nanocrystalline TiO2 shell-core composite particles. By taking advantage of the multiple effects of ultrasound, one can break down the aggregates of nanocrystalline TiO2 particles. The polymerization of aniline proceeded while the nanoparticles were redispersed by ultrasound, and the synthesized polyaniline deposited on the TiO2 particle, which formed polyaniline-coated nanocrystalline composite particles.
Experimental Methods
(9) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (10) Machida, M.; Norimoto, K.; Watanabe, T.; Hashimoto, K.; Fujishima, A. J. Mater. Sci. 1999, 34, 2569. (11) Yoneyama, H. Adv. Mater. 1993, 5, 394. (12) Somani, P. R.; Marimuthu, R.; Mulik, U. P.; Sainkar, S. R.; Amalnerkar, D. P. Synth. Met. 1999, 106, 45. (13) Suslick, K. S. Science 1990, 3, 1439. (14) Wang, Q.; Xia, H. S.; Zhang, C. H. J. Appl. Polym. Sci. 2001, 80, 1478. (15) Xia, H. S.; Zhang, C. H.; Wang, Q. J. Appl. Polym. Sci. 2001, 80, 1130.
Materials. Aniline (ANI) (AR, Beijing Chemical Reagent Co., China) was distilled twice under reduced pressure and stored below 4 °C under nitrogen atmosphere. Nanocrystalline TiO2 (anatase, 27 nm, Zhoushan Nanomaterials Ltd. Co., Zhejiang, China), sodium lauryl sulfate (SLS) (CP, Shanghai Xiangde Chemical Factory, China), sodium dodecyl benzenesulfonate (SDBS) (CP, Shanghai Xiangde Chemical Factory, China), cetyl trimethylammonium bromide (CTAB) (AR, Beijing Chemical Reagent Co., China), sodium bis(2-ethylhexyl)sulfosuccinate (AOT) (Acros Organics, New Jersey), poly(ethylene glycol) mono-4-octylphenyl ether (OP-10) (CP, Beijing Chemical Reagent Co., China), 38% hydrochloric acid, and ammonium peroxydisulfate (NH4)2S2O8 (APS) were used as received. Apparatus. The reaction apparatus was described in the previous paper.14 The ultrasonic irradiation instrument is VC1500 (Sonic & Material Co.). It has the following characteristics: standard titanium horn with a diameter of 22 mm, adjustable power out, replaceable flat stainless steel tip, digital thermometer to determine temperature, and gas flow meter to measure gas flow rate. The glass reactor was self-designed and made in house. Preparation of Polyaniline/Nanocrystalline TiO2 Particle Composites through Ultrasonic Irradiation. The polyaniline/nanocrystalline TiO2 particle composites were prepared according to the following steps. First, 0.5-3.0 g of nanocrystalline TiO2 particles, 1 mL of aniline, 2.5 g of (NH4)2S2O8, 100 mL of 2.4 M concentrated HCl aqueous solution, 4 g of SLS, and 90 mL of deionized and distilled water were introduced into the reaction vessel. Second, the mixture was deoxygenated by bubbling with oxygen-free nitrogen for 2 min in the reaction vessel, and water was circulated to maintain a certain temperature. The APS/ANI molar ratio was 1:1 in all experiments in this study. Third, ultrasonic irradiation was carried out with the probe of the ultrasonic horn immersed directly into the mixture emulsion system. A thermistor probe was immersed into the solution to measure the temperature variation during polymerization. After the reaction started, the reaction temperature increased from 14 to 28 °C in 3 min. After 1 h of irradiation, the reaction was stopped. Fourth, the polymerized colloid dispersion was precipitated with 100 mL of ethanol and left standing for 48 h without stirring, then the precipitated mixture was filtrated. Finally, the obtained green polyaniline/nano TiO2 composite powder was washed with a large amount of deionized water, then 50 mL of ethanol and 30 mL of ether, and after that was dried for 48 h under vacuum at 30 °C. The drying temperature was stringently controlled. Preparation of Polyaniline/Nanocrystalline TiO2 Particle Composites through Conventional Stirring. As a comparative control experiment, we prepared polyaniline/ nanocrystalline TiO2 particle composites through conventional stirring by using the same formulation and process as those in the ultrasonic irradiation. Characterization. The stability was evaluated as follows: 30 mL of colloid dispersion was placed in a cylindrical tube with a volume scale, and the sediment volume with the time was observed. The particle morphology of all the samples was observed by TEM on a Hitachi H-600 instrument. The sample for observation was prepared as the following procedure: The 15 mL colloid dispersion was centrifuged with a Beckman J2HS centrifuge (Beckman Instruments Inc.) at 16 000 rpm for 1 h. The supernatant liquid was removed, the green sediment was redispersed in 50 mL of water and the centrifugationredispersion cycles was repeated three times, the final sediment was redispersed in 30 mL of water through ultrasonic irradiation, and then the dispersion was dropped on a copper grid to observe the morphology of polyaniline/TiO2 composite nanoparticle. The UV-vis absorption spectra were recorded on a Shimadzu UV-240 spectrophotometer with 2 nm spectral resolution at room temperature in the range of 190-800 nm. The samples for UV-vis absorption spectra analysis were prepared by the following procedure: the prepared polyaniline
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Figure 1. TEM photographs of (a) original TiO2 under conventional stirring, (b) polyaniline obtained through ultrasonic irradiation, and composite particles obtained through ultrasonic irradiation: (c) polyaniline/TiO2, 0.5 g of TiO2 charged; (d) polyaniline/TiO2, 1 g of TiO2 charged; (e) polyaniline/TiO2, 2 g of TiO2 charged; (f) polyaniline/TiO2, 3 g of TiO2 charged. colloid dispersions were diluted 250 times with water and then were used to determine the spectra. Fourier transform infrared (FTIR) spectra of the polymers in KBr pellets were recorded on a Nicolet 560 FTIR spectrometer. The spectra were collected from 4000 to 400 cm-1, with a 4 cm-1 resolution over 20 scans. X-ray diffraction patterns (XRD) for PANI powder samples
were taken on a Philip-X’Pert X-ray diffractometer with a Cu KR X-ray source. X-ray photoelectron spectroscopy (XPS) was performed on a XSAM 800 X-photoelectron spectrometer with a Mg KR X-ray source. In the data analysis, the binding energy (BE) of the core level C1s was set at 284.8 to compensate for surface-charging effects. The surface elemental stoichiometries
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were determined from the ratios of peak areas corrected with the empirical sensitivity factors. The electrical conductivity measurements were made by the conventional four-point method on pressed pellets of composite particles prepared at ambient temperature (15 °C).
Results and Discussion Polymerization. In the absence of nano TiO2, the polymerization of aniline was conducted and the color of the system changed from light gray to dark green. However, in the presence of nano TiO2, color of the system changed from light blue to deep blue to green. The different color changes for the two systems suggest that the addition of nanocrystalline TiO2 affects the light absorption behavior. Effect of Experimental Parameters on Stability. The effect of experimental parameters such as the type and concentration of surfactant, as well as the power of ultrasound on the stability of the dispersion, was examined. When cationic surfactant CTAB, long-chain anionic surfactant AOT, and nonionic surfactant OP10 were used, the colloid dispersions were poorly stable and would sediment after 1 h. However, the stability of the dispersion with anionic surfactant SLS or SDBS was much better, and no sediments were observed after 72 h. With increasing concentrations of surfactant, the stability is increased. The stability of the dispersion prepared at 600 W power is better than those prepared at 450 and 300 W with conventional stirring. Size and Morphology of Polyaniline/Nano TiO2 Composite Particles. As shown in Figure 1a, the original commercially available nanocrystalline TiO2 particles are aggregated in aqueous solution with an irregular shape and the size is in the submicrometer range. This should be attributed to the high surface energy of nanoparticles. In the absence of nano TiO2, the size of polyaniline prepared through ultrasonic irradiation is ∼60∠120 nm as shown in Figure 1b. The TEM photographs of polyaniline/nanocrystalline TiO2 composite particles in the colloid dispersion with 0.5, 1, 2, and 3 g of TiO2 particles initially charged are shown in Figure 1 panels c-f, respectively. The composite particles are spherical and the sizes are ∼30-70 ∼3050, ∼40-50, and ∼100-200 nm, respectively. The average particle size is decreased after addition of the nano TiO2 particles, and the composite particles become more uniform with the increase in TiO2 particles. However, too many TiO2 particles will lead to a larger particle size. It can be thought that there are two kinds of particles in the composite colloid dispersions. One is free polyaniline particles, with a relatively larger size close to that of the prepared polyaniline particle (∼60120 nm) in the absence of nano TiO2. The other is polyaniline-encapsulated nano TiO2 composite particles, with a smaller particle size close to that of the original nano TiO2 (27 nm). The composition of the two kinds of particles depends on the content of nano TiO2 charged. With increasing nano TiO2 content, the contact area between polyaniline and nano TiO2 increased, the absorbed polyaniline increased, and the content of free polyaniline decreased, which leads to a more uniform composite particle structure. However, upon further increasing the content of nano TiO2, aggregation of particles occurs and the size of the composite particles increases.
Figure 2. TEM photograph of polyaniline/nano TiO2 composite particles prepared through conventional stirring (stirring rate 400 rpm).
We also compared the effect of ultrasound and conventional stirring on the composite particle morphology. The TEM photograph of polyaniline/nanocrystalline TiO2 composite particles prepared through conventional stirring with 2 g of TiO2 particles charged is shown in Figure 2. Different from those obtained through ultrasonic irradiation, the composite particles are irregular “raspberry” aggregates and the size is in the submicrometer range, which is somewhat similar to the results of Armes and co-workers.2 This is because the aggregates of nanocrystalline TiO2 are difficult to break down under conventional stirring. Ultrasound can break the aggregates of nanocrystalline TiO2 and contributes to producing spherical nanoparticles due to the immense shock wave and microstream produced by ultrasonic cavitation. UV-Vis Absorption. UV-vis absorption spectra of diluted colloid dispersions obtained by ultrasonic irradiation at different contents of TiO2 charged are shown in Figure 3. Clearly, the prepared polyaniline/ nanocrystalline TiO2 composites not only can strongly absorb the UV light but also can absorb the visible and near-IR light. The characteristic peaks of polyaniline appear at ∼325-360, ∼400-430, and ∼780-826 nm, which were attributed to π-π*, polaron-π*, and π-polaron transitions, respectively.16,17 The results suggest that the prepared PANI are in the doped state. It is interesting to note that, with increased content of nanocrystalline TiO2, the UV absorption at ∼240-340 nm is strengthened and the absorption at ∼800 nm due to the π-polaron transition is shifted from 802 to 762 nm, as shown in Table 1. This result indicates there is strong interaction between polyaniline and nanocrystalline TiO2. (16) Lu, F. L.; Wudll, F.; Nowak, M.; Heeger, A. J. J. Am. Chem. Soc. 1986, 108, 8311. (17) Stafstrom, S.; Breadas, J. L.; Epstein, A. J.; Woo, H.-S.; Tenner, D. B.; Huang, W.-S.; Macdiarmid, A. G. Phys. Rev. Lett. 1987, 59, 1464.
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Figure 3. UV-vis absorption spectra of diluted colloid dispersions obtained by ultrasonic irradiation: (a) TiO2; (b) polyaniline; (c) polyaniline/TiO2, 0.5 g of TiO2 charged; (d) polyaniline/TiO2, 1 g of TiO2 charged; (e) polyaniline/TiO2, 2 g of TiO2 charged; (f) polyaniline/TiO2, 2.5 g of TiO2 charged; (g) polyaniline/TiO2, 3 g of TiO2 charged; (h) polyaniline/TiO2, 4 g of TiO2 charged.
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Figure 4. UV-vis absorption spectra of diluted colloid dispersions of polyaniline particles obtained through (a) ultrasonic irradiation and (b) conventional stirring.
Table 1. Shift of Wavelength of Absorption Peak Due to π-Polaron with the Amount of TiO2 Charged amount of TiO2 charged
shift of wavelengths of absorption peak due to π-polaron
0 0.5 1.0 2 2.5 3 4
802 800 798 795 780 762 762
Also, we compared the effect of ultrasound and conventional stirring on the UV-vis absorption spectra of diluted colloid dispersions. Figure 4 shows the UVvis absorption spectra of diluted colloid dispersions of polyaniline particles obtained through ultrasonic irradiation and conventional stirring. It can be seen that two curves are intercrossed at ∼720 nm and the π-polaron absorption of polyaniline particles obtained through ultrasonic irradiation is stronger. This result suggests that ultrasound improves the doping degree of polyaniline. It can be reasonably concluded that ultrasonic cavitation promotes the diffusion of HCl molecules into the polyaniline chain. Figure 5 shows UV-vis absorption spectra of diluted colloid dispersions of polyaniline/nanocrystalline TiO2 particles obtained through ultrasonic irradiation and conventional stirring. It can be noted that the UV-vis absorption spectra of polyaniline/nano TiO2 particles obtained through conventional stirring are similar to that of the pure polyaniline system, and the UV absorption at ∼240340 nm is lower than that obtained through ultrasonic irradiation. These results indicate that ultrasound improves the dispersion of nanocrystalline TiO2 in aqueous solution, which contributes to exploit the full potential of nanocrystalline TiO2. FTIR. Figure 6 shows the FTIR spectra of the samples obtained through ultrasonic irradiation at different polyaniline concentrations. The characteristic peaks of TiO2 at ∼1630.2 and ∼523 cm-1 and polyaniline at ∼1560, ∼1467, ∼1293, ∼1103, and ∼876 cm-1 appear in the FTIR spectra of polyaniline/nanocrystal-
Figure 5. UV-vis absorption spectra of diluted colloid dispersions of polyaniline/nano TiO2 particles obtained through (a) ultrasonic irradiation and (b) conventional stirring.
line TiO2 particle composites. From Figure 6, it can be noted that the hydrogen-bond absorption at ∼3230 cm-1 is strengthened with increased content of nano TiO2. In addition, the incorporation of nano TiO2 leads to the shift of some FTIR bands of polyaniline. With the increased content of nano TiO2, the absorption peaks of CdC of benzoid ring at 1457 cm-1 and quinoid ring at 1103 cm-1 are both shifted to higher wavenumbers except for the 34.6 wt % PANI sample as shown in Table 2. The results also suggest that there is strong interaction between the polyaniline and nanocrystalline TiO2. XRD. Figure 7 shows the X-ray diffraction patterns of polyaniline (a), polyaniline/nanocrystalline TiO2 particle composites with different polyaniline contents obtained through ultrasonic irradiation (b-d), and anatase nano TiO2 (e). The Bragg diffraction peaks of 2θ ∼ 25.1° (d ∼ 3.54 Å), 2θ ∼ 20.1° (d ∼ 4.41 Å), and 2θ ∼ 16.5° (d ∼ 5.38 Å) can be found in the polyaniline, as shown in Figure 7a. Also it can be noted that when the content of polyaniline in the polyaniline/nanocrystalline TiO2 particle composites is ∼51.9% (Figure 7b), the peak of diffraction of polyaniline at 2θ ∼ 20.1° (d ∼ 4.41 Å) becomes very weak and disappears at higher content of nano TiO2 (Figure 7c,d). The result suggests that the addition of nanocrystalline TiO2 hampers the crystallization of the polyaniline molecular chain. This is because when the deposited polyaniline is absorbed on
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Figure 6. FTIR spectra of the samples obtained through ultrasonic irradiation: (a) polyaniline; (b) polyaniline/nano TiO2 composites, 51.9% (wt) polyaniline; (c) polyaniline/nano TiO2 composites, 34.6% (wt) polyaniline; (d) polyaniline/nano TiO2 composites, 21.6% (wt) polyaniline; (e) polyaniline/nano TiO2 composites, 11.6% (wt) polyaniline.
Figure 7. X-ray diffraction patterns: (a) polyaniline; (b) polyaniline/nano TiO2 composites, 51.9% (wt) polyaniline; (c) polyaniline/ nano TiO2 composites, 34.6% (wt) polyaniline; (d) polyaniline/nano TiO2 composites, 21.6% (wt) polyaniline; (e) anatase TiO2. Table 2. Shift of FTIR Band of Polyaniline
PANI PANI/TiO2 (51.9% PANI) PANI/TiO2 (34.6% PANI) PANI/TiO2 (21.6% PANI) PANI/TiO2 (11.5% PANI)
CdC benzoid ring (cm-1)
CdC quinoid ring (cm-1)
1457.8 1467.6 1482.4 1468.0 1485.1
1103.6 1106.9 1096.0 1122.0 1126.1
the surface of the nano TiO2 particle, the molecular chain of absorbed polyaniline is tethered, and the degree of crystallinity decreases. Figure 8 shows X-ray diffraction patterns of polyaniline obtained through ultrasonic irradiation and conventional stirring. For ultrasonic irradiation, the peak of 2θ ∼ 25.1° (110 face) is stronger than that of 2θ ∼ 20.1° (100 face), which is similar to that of highly doped emeradine salt.18 However, for conventional stir-
ring, the peak of 2θ ∼ 25.5° (110 face) is weaker than that of 2θ ∼ 20.1° (100 face), which is similar to that of less doped emeradine salt. The results suggest that ultrasound improves the doping degree of polyaniline, which is in agreement with the analysis of UV-vis spectra. XPS. Figure 9 shows survey X-ray photoelectron spectra of the samples obtained through ultrasonic irradiation at different polyaniline conetents. The peak of binding energy of every element was slightly shifted due to the change of environment. The ratio of the atom number of Ti and N (Ti/N) in the surface for polyaniline/ nanocrystalline TiO2 particle composites was determined from the ratio of peak areas corrected with the (18) Pouget, J. P.; Jozefowicz, M. E.; Epstein, A. J.; Tang, X.; Macdiarmid, A. G. Macromolecules 1991, 24, 779.
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Figure 8. X-ray diffraction patterns of polyaniline obtained through (a) ultrasonic irradiation or (b) conventional stirring.
Figure 9. Survey X-ray photoelectron spectra: (a) polyaniline obtained through ultrasonic irradiation; (b) polyaniline/nanocrystalline TiO2 composites [51.9% (wt) polyaniline] obtained through ultrasonic irradiation; (c) polyaniline/nanocrystalline TiO2 composites [21.6% (wt) polyaniline] obtained through ultrasonic irradiation; (d) polyaniline/nanocrystalline TiO2 composites [24.8% (wt) polyaniline] obtained through conventional stirring.
empirical sensitivity factors. The Ti/N ratio in the surface for polyaniline/nanocrystalline TiO2 composite particles with ∼51.9 wt % polyaniline content (sample b) is ∼0.5, which is lower than that in the bulk (∼1.08). And the Ti/N ratio in the surface for polyaniline/ nanocrystalline TiO2 composite particles with ∼21.6 wt % polyaniline content (sample c) is ∼2.93, lower than that in the bulk (∼4.22). The results provide supporting evidence for the polyaniline-encapsulated nanocrystalline TiO2 structure. Also, we compared the XPS spectra for the samples obtained through ultrasonic irradiation and conventional stirring. For conventional stirring, the ratio of the atom number of Ti/N in the surface for polyaniline/nanocrystalline TiO2 particle composites at the same condition is ∼3.03, which is higher than that for ultrasonic irradiation (∼2.93). This indicates a lower ratio of encapsulation for conventional stirring. This can be attributed to the fact that under conventional stirring the aggregates are difficult to break apart, and the surface areas of the nanoparticles coated by the shell of polyaniline are relatively lower.
Figure 10. Variation of conductivity of polyaniline/nanocrystalline TiO2 composites at ambient temperature (15 °C) with the content of polyaniline. Table 3. Electrical Conductivity of Samples conductivity (S·cm-1) PANIa 1.4 PANIb 0.6 PANI/TiO2a (21.6%) 0.72 b PANI/TiO2 (25.4%) 0.4 a Prepared by ultrasonic irradiation. b Prepared by conventional stirring.
Conductivity. Figure 10 shows variation of conductivities of polyaniline/nanocrystalline TiO2 composites obtained through ultrasonic irradiation at ambient temperature (15 °C) with the content of polyaniline. The conductivities of nanocomposites are in the range of ∼10-2-10 S·cm-1 and decrease with increasing content of TiO2. When the content of polyaniline decreases to ∼10%, the conductivity of composite still remains at ∼10-1 S·cm-1. The electrical conductivities of samples obtained through ultrasonic irradiation and conventional stirring are shown in Table 3. Clearly, ultrasonic irradiation contributes to the increase in conductivity compared with conventional stirring. This is because ultrasonic irradiation improves the dispersion of nanocrystalline TiO2 in the composites and the doping degree of polyaniline. So the conducting polymer/inorganic particle composites with low percolation thresholds can be prepared through ultrasonic irradiation.
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Figure 11. Formation of core-shell polyaniline/nanocrystalline TiO2 composite particles: (a) TEM photograph of aggregates of nanocrystalline TiO2 in aqueous solution; (b) TEM photograph of polyaniline/nanocrystalline TiO2 composites particles obtained through ultrasonic irradiation.
Formation of Core-Shell Polyaniline/Nanocrystalline TiO2 Composite Particles. On the basis of the above analysis, a mechanism of formation of core-shell polyaniline/nanocrystalline TiO2 composite particles was proposed, as shown in Figure 11. Under ultrasonic irradiation, the aggregates of nanocrystalline TiO2 are broken down and nanoparticles are redispersed in the aqueous solution at the nano scale, while at the same time, aniline monomer is polymerized in the presence of (NH4)2S2O8 and the synthesized polyaniline is absorbed on the surface of nanocrystalline TiO2 particles, which forms the shell of conductive polyaniline. The surfactant molecules are absorbed on the surface of the composite particles and have a stabilizing effect. Conclusion A novel approach, i.e., ultrasonic irradiation, was successfully employed to prepare polyaniline/nanocrystalline TiO2 particle composites. The difficulty in preparation of 0-3-dimensional conducting polymer/nanocrystalline particle composites at the nano scale was overcome through ultrasonic irradiation. Polymerization of aniline proceeded under ultrasonic irradiation in the presence of nanocrystalline TiO2 particles. The aggregation of nano TiO2 in the aqueous solution can be broken down under ultrasonic irradiation, and the formed
polyaniline deposits on the surface of the nanoparticle, which leads to a core-shell structure. The resulted polyaniline/nano TiO2 composite particles are spherical, and the sizes vary with the content of TiO2. The introduction of nanocrystalline TiO2 leads to a blue shift of the π-polaron absorption. Ultrasound can enhance the doping level. The incorporation of nano TiO2 hampers the crystalline behavior of polyaniline and reduces its crystallinity. The results of FTIR, UV-vis, and XPS analyses show that the interaction between polyaniline and nanocrystalline is strong. XPS shows that the ratio of the number of Ti and N atoms (Ti/N) in the surface is lower than that in the bulk. This is strong evidence for polyaniline-encapsulated nano TiO2 structure. Ultrasonic irradiation contributes to the increase in the conductivity compared with conventional stirring. Therefore, ultrasonic irradiation provides us a new approach to prepare 0-3-dimensional conductive polymer/nanocrystalline particle composites. Acknowledgment. This work is supported by National Science Foundation of China (29974020, 200340061) and Ministry of Education of China. We thank Mr. Yongming Liao at the Physical Department of Sichuan University for his help to our work. CM0109591
