Preparation and characterization of poly(3-methylthiophene)/CeY zeolite composites

Abstract

Poly(3-methylthiophene)/CeY zeolite nanocomposites (P3MT/CeY) were prepared by chemical oxidative polymerization using anhydrous FeCl₃ as the oxidizing agent. The physical and chemical properties of the prepared samples were measured using various characterization techniques, such as conductivity measurements, Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and carbon–sulfur analysis. The prepared P3MT/CeY composites retained the crystalline structure of Y zeolite, which also enhanced thermostability. A 2000-fold increase in conductivity relative to a blank prepared using P3MT was observed for the composite prepared using a 3 : 1 g mL⁻¹ ratio of CeY and P3MT.

---

1 Introduction

Conducting polymers have shown tremendous prospect for applications in the fields of electronics,¹ energy² and biology.³ Polythiophene and its derivatives are of particular interest due to their ease of synthesis and high environmental stability, and have been actively researched for more than two decades.^4,5 When the conductivity of polythiophene is higher than 10⁻⁶ S cm⁻¹, it has the potential to be used as a semi conductive polymer, and can be used in light emitting diodes, field-effect transistors, photodetectors, photovoltaic cells, optocouplers and light modulators.⁶ Guo et al.⁷ synthesized a LiV₃O₈/polythiophene composite, and found that the composite had high electronic conductivity (∼10⁻³ S cm⁻¹) and good cycling performance, which could be used as a promising cathode material for lithium ion batteries. Poly(3-hexylthiophene) with high carrier density and conductivity was achieved by electrostatic charge injection, and thus its field-effect mobility was improved dramatically compared with oligomeric semiconductors.⁸ Tong et al.⁹ reported an Au/polythiophene/Au device with a sandwich structure, which has high conductivity (∼10 S cm⁻¹) and exhibits strong photoresponses. One such derivative, poly(3-methylthiophene) (P3MT), is of importance due to its application in a variety of devices, such as sensors, super capacitors, light-emitting devices and photovoltaic cells.

Many studies^10,11 have attempted to develop hybrid P3MT-inorganic nanocomposites. The addition of inorganic materials can not only improve composite stability, but can also enhance their performance in other applications. Hybrid nanocomposites composed of P3MT and gold, platinum, palladium or copper metal nanoparticles have been shown to exhibit excellent electrocatalytic activity towards the oxidation of iodate,¹² dopamine,^13,14 uric acid,¹⁴ acetaminophen,¹⁵ nitrite,^12,16 methanol,¹⁶ ascorbic acid^13–15 and glucose.¹⁷ Bhattacharya et al.¹⁸ synthesized a P3MT/graphene composite via chemical oxidative polymerization and reported its use as a supercapacitor with a specific capacitance of 240 F g⁻¹. Lagoutte et al.¹⁹ reported that nanocomposites synthesized by the electrodeposition of P3MT onto long, thick and vertically aligned carbon nanotubes exhibited a large capacitance of 126 F g⁻¹. P3MT composites also presented excellent photoluminescence properties. When Cu was coated onto the P3MT film using a sequential electrochemical method and an anodic alumina oxide nanoporous template, the PL peak intensity for the hybrid P3MT/Cu film showed a 100-fold enhancement when compared to that of the P3MT single nanotube.²⁰ After the gold nanoparticles were fabricated on the surface of the P3MT nanotubes, the emission spectrum of P3MT dramatically changed from green to bright red, and the PL peak intensity increased by ∼200 times relative to that of P3MT nanotube.²¹ Wang et al.²² prepared stable P3MT/WO₃ nanocomposite films with high electrochemical activity and stability, whilst Lin et al.²³ prepared organic–inorganic hybrid materials based on P3MT and TiO₂. The photovoltaic cell fabricated from the P3MT/TiO₂ donor–acceptor hybrid showed good photoelectric performance. Sydorov et al.²⁴ produced ZnO and P3MT composites which exhibited photovoltaic activity due to their hybrid organic–inorganic heterostructure. Mokhtari et al.²⁵ prepared P3MT-coated polyester fabric using a chemical polymerization process, which demonstrated electrochromic and piezochromic behavior.

Y zeolite demonstrates remarkable stability and is commonly used as a commercial adsorbent and catalyst. Previous work showed that it is possible to ion-exchange the metal ion into the zeolite channel, leading to an improvement in conductivity. Additionally, it is possible to enhance the stabilization of P3MT by forming a composite with Y zeolite.²⁶ From our previous studies, CeY zeolite (CeY) demonstrates excellent adsorption of thiophene and its derivatives from crude benzene solutions when compared with ZSM-5, silica gel and γ-Al₂O₃.^27–31 The main focus of this work is to recycle the used CeY, along with the thiophene and any of its derivatives extracted from crude benzene. In this study, a series of P3MT/CeY samples were prepared by chemical oxidative polymerization using FeCl₃ as the oxidant so as to investigate their properties, which to the best of our knowledge have not yet been determined. The compositions, morphologies, structural features and thermal stabilities of the P3MT/CeY composites were analyzed using carbon–sulfur analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR spectroscopy and thermogravimetric analysis (TGA). The conductivities of the prepared samples were measured and correlated to the observed structures.

2 Experimental

2.1 Preparation of the samples

CeY with excellent adsorption capabilities towards thiophene present in crude benzene was prepared from commercial NaY zeolite by the following method. The commercial NaY zeolite, having a Si/Al ratio of 5 : 1, was ion-exchanged in Ce(NO₃)₃ solution (0.1 mol L⁻¹) at 100 °C in heated reflux apparatus, using 10 mL Ce(NO₃)₃ solution per 1 g of NaY zeolite. The sample was washed thoroughly with deionized water, oven-dried at 120 °C for 10 h, and then calcined at 700 °C for 2 h. The above steps were repeated to increase the amount of ion-exchanged Ce in the zeolite.

P3MT was prepared in a 250 mL conical flask. Firstly, 10 g of FeCl₃ was dispersed in 25 mL of CHCl₃ using 30 minutes of ultrasonication. 1.45 mL of 3-methylthiophene monomer (3MT) was added to a flask whilst maintaining the reaction temperature at 0 °C, after which the reaction mixture was magnetically stirred for 12 h. The product was then filtered and then dried in a vacuum oven at 50 °C for 24 h. The P3MT/CeY composites were synthesized using the same approach, with the CeY zeolite being added prior to addition of 3MT. Samples having different ratios of CeY zeolite mass (m_CeY, g) to 3MT volume (V_3MT, mL) were prepared as listed in Table 1, for which r in PMTYr denotes the CeY zeolite dosage per 10 mL of 3MT used during the preparation of the P3MT/CeY composites. PMTYB is the blank sample prepared via the same process except for the addition of 3MT.

Table 1 Codes for the samples prepared in different conditions

mCeY : V3MT (g mL^−1)	2.0	2.5	3.0	3.5	4.0	—
Code for the sample	PMTY20	PMTY25	PMTY30	PMTY35	PMTY40	PMTYB

---

2.2 Characterization of the samples

Crystallographic analysis of the samples was performed using a D/max-2500 model X-ray diffractometer (Rigaku, Japan) with a Cu-Kα radiation source (λ = 0.154 nm), 40 kV tube voltage and 100 mA tube current. Thermal stability measurements were carried out on a STA409C Thermogravimetric Analyser (Netzsch, Germany) in air with a flow rate of 80 mL min⁻¹ and a heating rate of 10 K min⁻¹. Surface morphology characterization was performed using a JSM-6700F Scanning Electron Microscope (JEOL Ltd., Japan) with an accelerating voltage of 10 kV.

FT-IR spectra of the samples were obtained using a Bruker VERTEX 70 model instrument (Bruker, Germany) in absorbance mode at room temperature, with a repetition of 16 scans at 4 cm⁻¹ resolution. For FT-IR analysis, 1 mg of the sample and 100 mg of KBr were finely ground together and then pressed into a translucent pellet. Fig. 1(a) shows a typical FT-IR spectrum of the P3MT/CeY composite. The peak at 1447 cm⁻¹ is attributed to symmetric C=C stretching vibrations of the 3MT ring,³² and the peak observed at 797 cm⁻¹ is the characteristic peak of C_α–C_α′ bonds between the 3MT rings in the polymer.³³ Because the C=C bonds can exist in several poly(3-methylthiophene) polymers with different C–C connection forms, if we divide the area of the peak at 797 cm⁻¹ by that of the peak at 1447 cm⁻¹ as shown in eqn (1), the R_A can be used to describe the relative content of the poly(3-methylthiophene) with C_α–C_α connection modes.

where A_Cα–Cα′ and A_ν=1442 are the area of the peak around 797 cm⁻¹ and 1447 cm⁻¹ in FT-IR spectrum of the sample, respectively.

Fig. 1 A typical FTIR spectrum of the composite (a) and the deconvolution of the bands around 653–890 cm⁻¹ (b) and 1318–1550 cm⁻¹ (c).

In order to calculate R_A, the bands around 797 cm⁻¹ and 1447 cm⁻¹ were deconvoluted as show in Fig. 1(b) and (c) respectively. The bands around 708 cm⁻¹ and 821 cm⁻¹ belong to out-of-plane C–H deformation of α and β positions of 3MT, respectively.²¹ The weak peak around 740 cm⁻¹ can be attributed to the δ(C_β–H) vibration band,³³ and the peak at 1392 cm⁻¹ corresponds to deformation of methyl groups in P3MT.^21,31

Sample conductivity was studied using a ZC-36 ohmmeter (Shanghai Qiangjia Electrc Co., Ltd., China). P3MT and the composite materials were ground and then characterized on pressed wafers. The wafer thickness and diameter were measured using a vernier caliper.

The carbon and sulfur content of the sample were measured using a HCS-140 infrared carbon–sulfur analyzer (Shanghai Dekai Instrument Co., Ltd., China). The theoretical sulfur content of the CeY/P3MT composite based on CeY zeolite (S_Theoretical) was calculated on the basis of the assumption that all the added 3MT successfully polymerized to form the P3MT present in the resulting CeY/P3MT composite, as shown in eqn (2). The conversion of 3MT (X_3MT) can be calculated using eqn (3).

where ρ_3MT is the density of 3MT (g mL⁻¹), M_sulfur and M_3MT are the relative molecular mass of sulfur and 3MT, respectively, and S_Measured is the sulfur content measured in the CeY/P3MT sample based on CeY zeolite using infrared carbon–sulfur analysis.

3 Results and discussion

3.1 Conductivity of the samples

The measured results of the conductivity of the different samples are shown in Table 2. The conductivity of PMTYB is very low, whereas that of the other P3MT/CeY samples is much higher. The ratio of CeY zeolite mass and 3MT volume obviously influences the electrical properties of the composites, with PMTY30 having the highest conductivity, with a value of 3.2 × 10⁻⁶ S cm⁻¹, which is about two thousand times higher than that of PMTYB. There are two probable reasons for the increase in conductivity of the samples. Firstly, cerium in CeY zeolite is a metal ion which itself is a good electrical conductor, leading to an increase in the conductivity for P3MT/CeY. Secondly, in the P3MT/CeY nanocomposite, the Ce–S bond could be formed by moving of the 3MT lone-pair electrons to the empty orbitals of the cerium ion; the resulting reduction in electron density around the 3MT would yield a more stable composite, which in turn could improve the conductivity.

Table 2 Conductivities of the samples

Sample	PMTY20	PMTY25	PMTY30	PMTY35	PMTY40	PMTYB
Conductivity (×10^−8 S cm^−1)	8	21	320	110	66	0.13

---

3.2 Composition and structural characteristics of the samples

In order to verify the existence of P3MT, the prepared samples were characterized using FT-IR spectroscopy, as shown in Fig. 2. It can be seen that the P3MT sample prepared without CeY presents a number of characteristic peaks. The peaks at 1636 cm⁻¹ and 1447 cm⁻¹ pertain to C–C stretching vibrations and C=C stretching vibrations of the thiophene ring,²¹ respectively. The peaks at 1383 cm⁻¹ and 1298 cm⁻¹ correspond to deformation of methyl groups in P3MT.^21,31 The bands around 708 cm⁻¹ and 821 cm⁻¹ belong to out-of-plane C–H deformation of α and β positions of 3MT, respectively.²¹ The peak at 856 cm⁻¹ corresponds to the C–S bond in the P3MT polymer.^21,31 The FT-IR spectra of the P3MT/CeY composites present very different peak results to those discussed previously. The peaks at 1066 cm⁻¹ and 460 cm⁻¹ of the blank sample (PMTYB) are assigned to Si–O stretching vibrations and bending vibrations of CeY zeolite, respectively,³⁴ whereas the peaks at 805 cm⁻¹ and 575 cm⁻¹ are indicative of Si–O–Si vibrations of CeY.³⁵ Compared with PMTYB, new peaks appear in the FT-IR spectra of P3MT/CeY at 1392 cm⁻¹, 1319 cm⁻¹, 797 cm⁻¹ and 699 cm⁻¹, which are shifted relative to the peaks in the P3MT FT-IR spectrum with values of 1383 cm⁻¹, 1298 cm⁻¹, 821 cm⁻¹ and 708 cm⁻¹. This shift is probably due to the chemical interactions between P3MT and CeY zeolite. Formation of the S–Ce bonds by Ce³⁺ in CeY and S from 3MT, leads to an increase in conjugation of the P3MT, which results in enhancement of the electron withdrawing effect of the π-bond on C_α–H and C_β–H in 3MT, which in turn leads to shifting of the peaks to the lower wavenumbers of 708 cm⁻¹ and 821 cm⁻¹, respectively. On the contrary, increased conjugation weakens the methyl-thiophene ring bonds, and the IR peaks of 1383 cm⁻¹ and 1298 cm⁻¹ move to higher wavenumbers. For the prepared P3MT/CeY samples, the extent of the peak shift did not change on varying the ratio of CeY mass and 3MT volume during synthesis of the composite, although the peak intensity was different.

Fig. 2 FT-IR spectra of P3MT and P3MT/CeY composites.

In order to explain the differences in electrical conductivity, it is necessary to understand the physical and chemical properties of the prepared samples. It can be observed from Fig. 3 that the R_A values of the P3MT/CeY samples show a linear relationship with the nature logarithm of their conductivities, which should explain the conductivity results in Table 3. It can be concluded that the PMTY30 sample shows the highest conductivity and C_α–C_α′ connection content of the prepared P3MT samples, and hence should be further analyzed in terms of crystalline structure, surface morphology and thermal stability.

Fig. 3 Effect of the relative peak area of C_α–C_α′ in P3MT on the conductivity of P3MT/CeY (a) and the fitted results (b).

Table 3 C and S content, theoretical sulfur content, and the conversion of 3MT of the prepared samples

Sample	Carbon content (wt%)	SMeasured (wt%)	STheoretical (wt%)	X3MT (%)
PMTY20	30.2	13.6	16.6	81.9
PMTY25	19.2	9.2	13.3	69.2
PMTY30	13.9	6.9	11.0	62.7
PMTY35	12.0	5.7	9.5	60.0
PMTY40	9.6	4.5	8.3	54.2

---

Fig. 4 displays the SEM and EDS images of P3MT, PMTYB and PMTY30. As shown in Fig. 4(a), P3MT has a smooth surface appearance and uniform color, while for PMTYB lots of particles could be observed (Fig. 4(b)). From Fig. 4(c), it can be seen that the surface of PMTY30 is similar to that of PMTYB, indicating that the P3MT polymer does not alter the surface morphology of CeY zeolite after formation of the composite. From the EDS results of these three samples, it can be seen that P3MT mainly contains carbon and sulfur from the 3MT monomer, and also contains iron and chlorine which are from the anhydrous FeCl₃ used as an oxidizing agent in the reaction. Moreover, PMTYB and PMTY30 consist primarily of Si, Al, O and Ce, which are all present within CeY zeolites. The presence of C and S in PMTY30 also indicates that P3MT was composited with CeY zeolite.

Fig. 4 SEM and EDS images of P3MT (a), PMTYB (b) and PMTY30 (c).

Fig. 5 shows the XRD patterns of P3MT, PMTYB and PMTY30. Obviously, the structure of P3MT is non-crystalline, which is consistent with the literature.²⁵ The positions of the peaks in both the PMTYB and PMTY30 samples are very similar to each other, indicating that P3MT/CeY composites still can retain the crystal structure of zeolite.

Fig. 5 XRD spectra of the P3MT, PMTYB and PMTY30 samples.

The C–S analysis results, theoretical sulfur content, and 3MT conversion of the samples are illustrated in Table 3. The C and S content both decrease with increasing CeY mass and 3MT volume ratio, which is understandable due to carbon and sulfur only being provided by the 3MT. The relationship between the actual measured (y%) values and corresponding theoretical (x%) values of the sulfur content based on CeY zeolite in the samples is shown in Fig. 6, which is used to describe the 3MT utilization during preparation. It can be seen that the sulfur content y (%) and x (%) exhibit a linear relationship to which an equation can be fit, having the formula y = 1.09x − 4.76 with R² = 0.9859. Theoretically, for this equation, the intercept should be zero, but instead is −4.76. This could be due to loss of 3MT during synthesis, since it is a volatile substance. This indicates that this preparation method has the advantage of less 3MT loss (4.76%).

Fig. 6 Relationship between the observed and corresponding theoretical sulfur content for the samples.

3.3 Thermal stability of the samples

The TGA/DTG results of P3MT, PMTYB and PMTY30 are shown in Fig. 7. It can be seen that there are two weight loss steps in the TGA curves of the P3MT, PMTYB and PMTY30 samples. The temperature of the first weight loss step below 200 °C can be attributed to loss of water and organic solvent adsorbed by the zeolite, and the temperature of the other step is higher than 300 °C, which could be due to combustion of the polymer. As illustrated in Table 4, the ignition temperature, maximum combustion temperature and burn-out temperature of PMTY30 are all higher than these of P3MT, and almost all of the P3MT decomposes before 400 °C, whereas only approximately 30 percent of the P3MT in PMTY30 decomposes at the same temperature. This suggests that the thermal stability of the composite could be enhanced by CeY zeolite. Moreover, the burnable content, which represents the polymer content, is 11% and 85% for PMTY30 and P3MT, respectively. This indicates that after combustion, P3MT still retains approximately 15% of its starting mass, which can probably be attributed to the presence of FeCl₃, the existence of which was evidenced by the EDS result of P3MT, as shown in Fig. 4(a).

Fig. 7 TGA (a) and DTG (b) curves of the P3MT, PMTYB and PMTY30 samples.

Table 4 Thermal analysis results of the samples

Sample	Ignition temperature (°C)	Maximum combustion temperature (°C)	Burn-out temperature (°C)	Burnable content (%)
P3MT	220	323	430	85
PMTY30	290	480	625	11

---

4 Conclusions

P3MT/CeY composites were successfully synthesized by chemical oxidative polymerization of 3MT and CeY zeolite in CHCl₃ using anhydrous FeCl₃ as the oxidant. P3MT exists in a non-crystalline structure, whereas the poly(3-methylthiophene)/CeY composites retain the crystal structure of Y zeolite. The conductivity of the P3MT/CeY composite is higher than that of PMTYB and up to 3.2 × 10⁻⁶ S cm⁻¹. In comparison to P3MT, the thermostability of the P3MT/CeY composite is greatly improved by the zeolite.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (51372161, 21406151), Research Fund for Doctoral Program of High Education (20131402110010) and Shanxi Scholarship Council of China (2012-0039).

References

1. D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A. J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne and H. Sirringhaus, Nature, 2014, 515, 384–388.
2. M. J. Sailor, E. J. Ginsburg, C. B. Gorman, A. Kumar, R. H. Grubbs and N. S. Lewis, Science, 1990, 249, 1146–1149.
3. P. Sista, K. Ghosh, J. S. Martinez and R. C. Rocha, J. Nanosci. Nanotechnol., 2014, 14, 250–272.
4. K. Jen, G. G. Miller and R. L. Elsenbaumer, J. Chem. Soc., Chem. Commun., 1986, 17, 1346–1347.
5. J. Ruiz, B. Gonzalo, J. M. Laza, L. M. Leon, M. I. Moreno, J. L. Vilas and E. Bilbao, Adv. Polym. Technol., 2014, 33, 7.
6. R. D. McCullough, in Handbook of Oligo- and Polythiophenes, ed. D. Fichou, WILEY-VCH, N Y, 1999.
7. H. Guo, L. Liu, H. Shu, X. Yang, Z. Yang, M. Zhou, J. Tan, Z. Yan, H. Hu and X. Wang, J. Power Sources, 2014, 247, 117–126.
8. M. J. Panzer and C. D. Frisbie, Adv. Funct. Mater., 2006, 16, 1051–1056.
9. L. Tong, C. Li, F. E. Chen, H. Bai, L. Zhao and G. Shi, J. Phys. Chem. C, 2009, 113, 7411–7415.
10. N. Lingapppan, J. H. Jeong, J. M. Park and K. T. Lim, J. Nanosci. Nanotechnol., 2014, 14, 5708–5712.
11. H. Dolas and A. S. Sarac, Synth. Met., 2014, 195, 44–53.
12. X. Huang, Y. Li, Y. Chen and L. Wang, Sens. Actuators, B, 2008, 134, 780–786.
13. N. F. Atta and M. F. El-Kady, Sens. Actuators, B, 2010, 145, 299–310.
14. X. Huang, Y. Li, P. Wang and L. Wang, Anal. Sci., 2008, 24, 1563–1568.
15. N. F. Atta and M. F. El-Kady, Talanta, 2009, 79, 639–647.
16. Y. Zhou, H. Xian, F. Li, S. Wu, Q. Lu, Y. Li and L. Wang, Electrochim. Acta, 2010, 55, 5905–5910.
17. C. Malitesta, M. R. Guascito, E. Mazzotta, T. Siciliano and A. Tepore, Sens. Actuators, B, 2013, 184, 70–77.
18. P. Bhattacharya and C. K. Das, J. Nanosci. Nanotechnol., 2012, 12, 7173–7180.
19. S. Lagoutte, P. Aubert, M. Pinault, F. Tran-Van, M. Mayne-L’Hermite and C. Chevrot, Electrochim. Acta, 2014, 130, 754–765.
20. D. H. Park, M.-Y. Jeong, M. Kim, E. H. Cho, D.-C. Kim, H. Song, J. Kim, T. K. Shim, D. Kim and J. Joo, J. Nanosci. Nanotechnol., 2009, 9, 3112–3118.
21. D. H. Park, Y. K. Hong, M. S. Kim, E. H. Cho, W. J. Choi, K. H. Kim, Q. H. Park, D.-C. Kim, H. Song, J. Kim and J. Joo, Synth. Met., 2010, 160, 604–608.
22. X. Wang, X. C. Song, Y. F. Zheng, R. Ma and H. Y. Yin, J. Exp. Nanosci., 2013, 8, 546–554.
23. Y.-J. Lin, L. Wang and W.-Y. Chiu, Thin Solid Films, 2006, 511–512, 199–202.
24. D. Sydorov, P. Smertenko, Y. Piryatinski, T. Yoshida and A. Pud, Electrochim. Acta, 2012, 81, 83–89.
25. J. Mokhtari and M. Nouri, Fibers Polym., 2012, 13, 139–144.
26. M. Jaymand, RSC Adv., 2014, 4, 33935–33954.
27. J. Liao, L. Bao, W. Wang, Y. Xie, J. Chang, W. Bao and L. Chang, Fuel Process. Technol., 2014, 117, 38–43.
28. J. Liao, W. Bao, Y. Chen, Y. Zhang and L. Chang, Energy Sources, Part A, 2012, 34, 618–625.
29. J. Liao, W. Wang, H. Wang, Q. Jin and L. Chang, Mod. Chem. Ind., 2009, 29, 219–221.
30. J. Liao, W. Wang, Y. Xie, Y. Zhang, L. Chang and W. Bao, Sep. Sci. Technol., 2012, 47, 1880–1885.
31. J. Liao, Y. Zhang, W. Wang, Y. Xie and L. Chang, Adsorption, 2012, 18, 181–187.
32. M. Akimoto, Y. Furukawa, H. Takeuchi, I. Harada, Y. Soma and M. Soma, Synth. Met., 1986, 15, 353–360.
33. T. Yamamoto, K. Sanechika and A. Yamamoto, Bull. Chem. Soc. Jpn., 1983, 56, 1497–1502.
34. G. Bidan, O. Jarjayes, J. M. Fruchart and E. Hannecart, Adv. Mater., 1994, 6, 152–155.
35. L. Lin, Y. Zhang, H. Zhang and F. Lu, J. Colloid Interface Sci., 2011, 360, 753–759.

---