Synthesis and characterization of self-assembled three-dimensional flower-like iron(III) oxide–indium(III) oxide binary nanocomposites

Abstract

Three-dimensional (3D) flower-like iron(III) oxide–indium(III) oxide (Fe₂O₃–In₂O₃) binary metal oxide nanocomposites were successfully fabricated by a simple and economical route based on an efficient ethylene glycol mediated process. Effects of the experimental parameters such as the ratio of Fe to In, type of acid absorber, solvent, and reaction temperature and time on the morphology of the nanocomposite were discussed in detail. The nanocomposites Fe₂O₃–In₂O₃ with flower-like morphology were readily obtained by annealing the precursor. The possible reaction mechanism leading to the precursor and the self-assembly process was also proposed. The results of the thermogravimetric analyses indicated that 3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites can be used as fillers to significantly enhance the thermal resistance of silicone rubber under nitrogen.

---

Introduction

Iron(III) oxide (Fe₂O₃), as an important transition metal oxide, has been extensively used as a semiconductor compound, magnetic material, catalyst, gas sensor, and heat-resistant additive in polymer systems.^1–8 Moreover, Fe₂O₃ is also environmentally friendly and used as a low cost adsorbent for environmental applications including decontamination of toxic metal ions, fluoride ions, and organic dyes from aqueous solutions. Compared to Fe₂O₃, indium(III) oxide (In₂O₃) is an n-type semiconductor, which exhibits good electrical conductivity and high optical transparency in the visible light region, and acts as a promising candidate in various applications, such as solar cells, liquid crystal displays, and gas sensors for detection of hazardous gases.^9–14 Transition metal oxides have promising gas sensing performance due to their catalytic properties. Some transition metal oxides are stable, have low electric resistance, and good gas sensing properties. However, others have high electric resistivity and require high operating temperatures. Some of the oxides are more sensitive to oxidizing gases; however, others are more to reducing gases. Therefore, it is a natural approach to combine metal oxides with different properties in an appropriate proportion in order to modify the gas sensor performance as desired. In recent years, significant attention has been paid to the fabrication of micro- and nano-structures of Fe₂O₃–In₂O₃ binary metal oxide due to their fascinating electrical conductivity, and their potential applications, mainly in optoelectronics, catalysis, and gas sensing.^9–12

Nanotechnology has attracted tremendous attention attributed to the unusual properties of the nanostructured materials known for their stability, green chemistry, and diverse technical applications. In particular, 3D nanostructures exhibit fascinating and extraordinary properties superior to their bulk and particle counterpart. Moreover, 3D binary metal oxides are obtaining progressive attention nowadays because they not only provide the synergistic effect of both the metal oxides, but also offer the advantage over the individual material in tuning the optical band gap and controlling the inter-particle electron transfer. In general, the mixed Fe₂O₃–In₂O₃ materials are prepared by conventional methods, including sol–gel procedure, hydrothermal method under critical conditions, co-precipitation method, microwave radiation method, solution combustion method, and high energy ball milling method;^15–22 however, these methods result in the formation of large crystal agglomerates or low dimensional (0D, 1D, or 2D) nanoparticles. Moreover, abovementioned methods require some specials instruments in order to meet the critical conditions, thus resulting in complicated procedures.

Nowadays, an efficient polyol process, through rapid and simple economic route, has been used to fabricate a series of 3D micro/nano metal oxide materials, such as Fe₂O₃, Ce₂O₃, Co₃O₄, and ZnO.^23–26 3D micro/nano structured metal oxide is composed of nano-sized hierarchically organized building blocks, the particle size is in the micrometer scale. Furthermore, with advancements in nanoscience and nanotechnology, multicomposite hybrid nanostructures have attracted significant attention because of their multicomponent structures which not only provide novel functions unavailable in single-composite materials or structures, but also achieve enhanced properties which break the natural constraints of single-phase materials.²⁷ However, till date, most of the Fe₂O₃–In₂O₃ materials obtained by conventional methods have exhibited large crystal agglomerates or low dimensional nanoparticles, and the synthesis of 3D micro/nano Fe₂O₃–In₂O₃ materials, in particular, by polyol process has rarely been investigated.

In this study, 3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites were successfully fabricated by a simple and economical route based on efficient ethylene glycol mediated process. Furthermore, effects of the reaction temperature and time, solvent, different molar ratios of Fe/In, templates, and type of acid absorber on the formation of Fe₂O₃–In₂O₃ binary metal oxide phase and its shape and size were also investigated. The possible reaction mechanism leading to the precursor and self-assembly process was also proposed. Moreover, although several studies have investigated the antioxidant functions of single metal oxide as filler in silicone rubber, this study provides a new perspective to the understanding of the 3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites which can be used as fillers to significantly enhance the thermal resistance of silicone rubber under nitrogen.^28–32

Results and discussion

A possible formation mechanism of the flower-like Fe and In alkoxides as the precursor was obtained based on the detailed time-dependent evolutions of morphology examined by scanning electron microscopy (SEM) at different time intervals starting from the precipitate that appeared in the reaction solution. Fig. 1a exhibits that at the early stage; the samples consist of nanoparticles with diameter of ∼50 nm. Fig. 1b shows the SEM image of the sample collected 30 min later, exhibiting the coexistence of nanoparticles and sheet-like micro/nanostructure, the length is ∼1 μm and width is ∼150 nm. With the progress in the reaction, the sheet-like structure changes from being single-layered to double-layered, and overlaps between the layers, and the size is ∼400 nm, as shown in Fig. 1c. Fig. 1d shows the coexistence of sheet-like and flower-like micro/nanostructures, with the size of ∼1 μm, respectively. Fig. 1e shows that at the reaction time of 3 h, the ratio of the flower-like micro/nanoparticles increases at the expense of the sheet-like micro/nanoparticles, and the size of flower-like particles is also ∼1 μm. Fig. 1f exhibits that at the completion of the reaction, the samples consist of only 3D flower-like micro/nanostructures with diameter of ∼1 μm, and the nanoparticles and sheet-like micro/nanoparticles are not observed. This result was consistent with the previous reports on a so-called two-stage growth process, involving a rapid nucleation of amorphous primary particles followed by a slow aggregation and crystallization of primary particles.^33–35

Fig. 1 SEM images of the as-obtained Fe and In precursors with Fe/In molar ratio of 4 : 1 collected at different intervals since the precipitate occurred: (a) 5 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 3 h, and (f) 4 h.

Influence of the reaction temperature and calcination temperature on the morphology and size of the nanocomposite

It was observed that the reaction temperature had significant effect on the morphology and size of the as-prepared precursor. When the reaction was performed at 200 °C, the precursor exhibited the coexistence of nanoparticles and sheet-like micro/nanostructure. Even though the reaction time was increased to 12 h, the 3D flower-like structure could not be formed, similar to that at 150 °C. However, the average diameter of nanosheets was 2.5 μm at 200 °C, which was larger than that obtained at 150 °C. Moreover, compared to the single-layered nanosheets at 150 °C, the nanosheets at 200 °C were multi-layered with thickness of about 500 nm. Both of them were similar to those intermediate procedures when the reaction at 240 °C. The transformation process for Fe₂O₃/In₂O₃ precursors occurred at higher temperature and took more time than for pure Fe₂O₃ precursor previously reported by Wan et al.³⁶ This is mainly attributed to the fact that low temperature (lower than 240 °C) does not possess sufficient energy to complete the Ostwald ripening process. Thus, steps 2 and 3 shown in Scheme 1 are suppressed. Therefore, the reaction temperature significantly influenced the morphology of the obtained structures. Notably, the average diameter of Fe₂O₃/In₂O₃ micro/nanostructure was 1 μm, which was lower than that of pure Fe₂O₃ precursor (2 μm) previously reported by Chen et al.⁴⁰ This indicated that addition of In into Fe₂O₃ system could limit the crystal growth.

Scheme 1 Schematic illustration of the proposed mechanism of formation of 3D flower-like nanostructures.

Thermal decomposition of the metal alkoxides is a simple route toward the synthesis of metal oxides. In this study, sample was calcined at 300, 450, 600, and 800 °C for 3 h in air at a ramping rate of 2 °C min⁻¹, respectively. Fig. 2 shows that the SEM images of the calcined product mainly consist of ample flower-like architecture, except at the calcination temperature of 800 °C. With the increase in the calcination temperature, the surface morphology of the product becomes relatively rough, indicating insignificant effect of the calcination temperature on the morphology of the flower-like architectures. Fig. 2e exhibits that at the calcination temperature of 800 °C, the morphology of the sample is irregular, thus exorbitant thermal treatment could destroy the flower-like architectures.

Fig. 2 Effect of different calcination temperature on the morphology of the product: (a) the as-prepared precursors, (b) 300 °C, (c) 450 °C, (d) 600 °C, and (e) 800 °C.

The distribution of Fe, In, and O on the surface of xFe₂O₃·(1 − x)In₂O₃ for x = 0.8 was characterized by the SEM integrated with X-ray diffraction (XRD) analysis (SEM-EDX) as shown in Fig. 3. The diagram of the sample is the image of the entire chemical element, and the spots for which the grayscales are different from the backgrounds in the other diagrams represent that the distribution of Fe, In, and O is homogeneous. The result of inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was 3.99, which was almost similar to the original proportion.

Fig. 3 SEM-EDX maps of the distributions of Fe, In, and O on the surface of the Fe and In oxides.

Influence of tetrabutylammonium bromide (TBAB) and solvents on the morphology and size of the nanocomposite

The results indicated that TBAB played a crucial role in the synthesis of Fe₂O₃/In₂O₃ precursors in contrast to the preparation of pure Fe₂O₃ precursor where the absence and presence of TBAB resulted in the same morphology. However, in multicomposite hybrid nanostructures, as shown in Fig. 4a, in the absence of TBAB, mainly nanoparticles are observed and flower-like structure is not obtained. This result indicates that initial nanoparticles tend to assemble into flower-like architecture only in the presence of TBAB. This could be attributed to the fact that solution of TBAB acts as a soft template to form micelles which can direct the growth of the nanostructures and control the size of the nanosheets that constitute the flower-like structures³³ (Scheme 1 step 2). According to the literature,^34,35 soft-template method, using surfactants and supramolecular aggregates, such as micelles and emulsions as templates, has been proven to be an efficient technique to direct the morphology control. Moreover, TBAB molecules also facilitate the assembly of initial nanoparticles into 3D flower-like binary metal oxide nanocomposite structure through the interaction between TBAB molecules and nanoparticles, similar to that reported in the literature.³⁶

Fig. 4 Effect of TBAB and urea on the morphology of the nanocomposite: (a) in the absence of TBAB, (b) in the presence of sodium carbonate instead of urea, and (c) in the presence of sodium acetate instead of urea.

Urea present in the reaction system acted as an acid absorber. Urea plays an efficient role by providing a steady supply of OH⁻ ions which are capable of neutralizing the central metal ion releasing H⁺ ions, thus allowing the reaction to continue (Scheme 1 step 1). Hydroxyl groups (–OH) formed by the hydrolysis of urea neutralized the HCl and allowed the coordination reaction to be completed.²¹ Herein; we utilized sodium acetate and sodium carbonate to replace urea. They offered the same acid absorber action; however, with different alkalescency. Fig. 4b and c clearly show that the morphology of the as-obtained samples is not flower-like, which was similar to those intermediate procedures when the reaction with urea added as the acid absorber. It indicated that alkalescency of the acid absorber was very critical in this system.

Moreover, solvent is an important parameter for synthesis. In this study, it was found that ethylene glycol was important for the formation of hierarchically organized flower-like Fe₂O₃/In₂O₃ micro/nanostructure. When glycerol was substituted for ethylene glycol, flower-like morphology was not obtained. No obvious flower-like product was observed when ethylene glycol was replaced with 1,2-propylene glycol. This is attributed to the reductive ability of polyols which determines the formation and growth rate of nanostructures.

Influence of different molar ratios of Fe/In on the morphology and size of the nanocomposite

Seven samples were prepared under the same reaction conditions except with different molar ratios of Fe/In. When the molar ratio of Fe/In was equal to or greater than 4, the flower-like structure is obtained as shown in Fig. 5a and b. Fig. 5c shows the coexistence of sheet-like and flower-like micro/nanostructures when the molar concentration of both the metal elements is same (i.e., molar ratio is 1). Sequentially, as the molar ratio reduces to 1/4, the morphology of the obtained sample consists of sheet-like structures and nanoparticles (Fig. 5d). Finally, only nanoparticles are observed for the molar ratio of Fe/In below 1/10 (Fig. 5e). Notably, with increasing molar ratio of Fe/In, the variation in morphology of the as-prepared samples was similar to the formation mechanism of the flower-like Fe and In alkoxides as the precursor (Scheme 1).

Fig. 5 Effect of different molar ratios of Fe/In on the morphology and size of the product: (a) 10 : 1, (b) 4 : 1, (c) 1 : 1, (d) 1 : 4, and (e) 1 : 10.

The XRD patterns of a series of prepared samples calcined in the air at 450 °C for 3 h were analyzed to study the phase structure in relation with different molar ratios of Fe/In in this system. Fig. 6a shows the selected XRD spectra corresponding to all concentration range. For xFe₂O₃·(1 − x)In₂O₃ (x = 0.95), In₂O₃ phase is not detected, indicating the complete substitution of In³⁺ ions by Fe³⁺ ions in the hematite lattice. With a decrease in the concentration of Fe³⁺ ions, for x = 0.9 and x = 0.8, (222) faces of In₂O₃ appear and the diffraction intensity of (104) faces of Fe₂O₃ decreases. In contrast, from x = 0.5 to x = 0.05, (104) faces of Fe₂O₃ disappear; however, (211) faces of In₂O₃ are observed. Moreover, the diffraction intensity of (104) faces of In₂O₃ increased. Fig. 6b shows the amplified (104) and (222) diffraction peaks of the samples revealing that (222) diffraction peaks successively shift to lower degrees with the increase in the In content in the samples. This shift could be attributed to an increase in the size of the crystallographic unit cell due to the incorporation of more In possessing larger atomic size than Fe. Moreover, the diffraction lines of Fe₂O₃ in all the Fe–O–In system samples are shifted toward higher values with respect to those of pure Fe₂O₃, indicating an increase in the cell volume of Fe₂O₃. However, the shift of the reflection toward greater angle values is the evidence of substitution of In ions by smaller Fe ions in the In₂O₃ crystal lattice. The concentration regions for Fe₂O₃/In₂O₃ micro/nanostructure together with the solubility limits of In³⁺ ions in the hematite lattice and of Fe³⁺ ions in the cubic In₂O₃ structure have been evidenced.⁴²

Fig. 6 (a) The XRD patterns of samples with various Fe/In ratios: H1 0.95, H2 0.9, H3 0.8, H4 0.5, H5 0.2, H6 0.1, H7 0.05; (b) the amplified diffraction peaks of the samples.

Nitrogen adsorption isotherms indicated that the Brunauer–Emmett–Teller (BET) specific surface area of the as-prepared xFe₂O₃·(1 − x)In₂O₃ for x = 0.8 was 29 m² g⁻¹. Fig. 7a and b show the complete N₂ adsorption–adsorption isotherms and pore size distribution of the obtained sample. Fig. 7a shows that the isotherm can be categorized as type H3, which is characteristic of slit-type pores associated with the inter-particle porosity generated in solids having plate- or fiber-like morphology. The pore size distribution curve of the sample exhibits a broad peak around 8–40 nm.

Fig. 7 (a) N₂ adsorption–adsorption isotherms and (b) pore size distribution of xFe₂O₃·(1 − x)In₂O₃ for x = 0.8.

3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites as fillers to enhance the thermal resistance of silicone rubber under nitrogen

Fig. 8 exhibits the TGA thermogram, revealing that the temperature at which 5% weight loss (Td₅) occurs is around 415 °C for silicone rubber without metal oxide or with well dispersed common Fe₂O₃ as filler. However, Td₅ is around 505 °C for silicone rubber with Fe₂O₃–In₂O₃ nanocomposites as fillers when the TGA analysis was performed under N₂. Apparently, the common Fe₂O₃ filler could not improve the thermal resistance of silicone rubber; however, the thermal resistance of silicone rubber was significantly enhanced nearly by 90 °C by 3D flower-like Fe₂O₃–In₂O₃ nanocomposites. This indicated that the 3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites could efficiently inhibit the back-biting reactions at high temperature.^37–42 Therefore, this study provides a new perspective to the understanding of an important method to enhance the thermal resistance of silicone rubber. Thus, the thermal resistance can be increased by controlling the morphology and structure of nanocomposites or by using different types of metal oxides. However, the mechanism has not been investigated sufficiently and it has not been clearly understood. Undeniably, a lot more systematic explorations are demanded in order to better understand the mechanism of this phenomenon which will be pursued in our successive studies.

Fig. 8 The TGA trace of silicone rubber without and with different fillers, the Fe₂O₃–In₂O₃ nanocomposites is xFe₂O₃·(1 − x)In₂O₃ for x = 0.8.

Conclusions

3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites were successfully fabricated by an efficient polyol process. The results indicated that it was possible to control the morphology and structure of Fe₂O₃–In₂O₃ composites by adjusting the experimental parameters such as reaction temperature and time, solvent, different molar ratios of Fe/In, templates, and acid absorber. This synthetic route allowed us to obtain the 3D flower-like composite structure and solid solutions in an extended concentration range. Furthermore, the 3D flower-like Fe₂O₃–In₂O₃ binary metal oxide nanocomposites could be efficiently used as filler to significantly enhance the thermal resistance of silicone rubber under nitrogen.

Experimental

Materials and synthesis

All the chemicals were of analytical grade and used as received without further purification. In a typical procedure involving the fabrication of Fe₂O₃–In₂O₃ binary metal oxide nanocomposite, ferric chloride (FeCl₃·6H₂O, 0.96 g, 3.80 mmol), indium nitrate (In(NO₃)₃·5H₂O, 0.37 g, 0.95 mmol), urea (2.7 g), and tetrabutylammonium bromide (TBAB, 7.2 g) were dissolved in ethylene glycol (180 mL). The solution was refluxed at 220 °C for 4 h. Subsequently, the contents were cooled down and the as-synthesized Fe and In oxides precursors were collected by four centrifugation and ethanol washing cycles. The obtained samples were dried in an oven at 70 °C for 10 h, followed by calcination at 450 °C for 3 h in air at a ramping rate of 2 °C min⁻¹. The yield of the obtained product was ca. ∼0.39 g (89.7%). (In_xFe_1−x)₂O₃ crystals with different molar ratio of Fe/In were prepared according to the typical procedure described above. The total concentration of metal ions was kept at 0.007 mol L⁻¹; however, the initial concentrations of In and Fe in solution were varied. The silicone rubber was vulcanized by tetraethoxysilane with 25% SiO₂ and 5% metal oxide as fillers.

Characterizations

The structure of the nanocomposites was characterized by the powder X-ray diffraction (XRD) measurements performed on a Shimadzu XRD-6000 X-ray diffractometer (Cu Kα radiation, λ = 15 406 Å). Morphologies of the samples were investigated using a Cambridge S-4300 scanning electron microscope (SEM). The specimens were coated with gold prior to their observation at the accelerating voltage of 15 kV. A Cambridge S-6700 SEM at the accelerating voltage of 5 kV was also employed in this study. The nitrogen adsorption–desorption isotherms at 77 K were measured using a micromeritics ASAP 2020 V3.00 H system. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was performed on Plasma1000 to determine the composition of the nanocomposite before the samples were completely solved and the ion concentrations were kept at 50 ppm. Thermogravimetric analyses (TGA) were performed on a SII EXTRA 6300 TG/DTA apparatus, using 6–10 mg of polymer, at a heating rate of 10 K min⁻¹ with nitrogen as the purge gas at 20.0 mL min⁻¹.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (no. 50803070).

Notes and references

1. Q. Guo, S. Kim, M. Kar, W. Shafarman, R. Birkmire, E. Stach, R. Agrawal and H. Hillhouse, Nano Lett., 2008, 8, 2982–2987.
2. S. Karthikeyan, C. Judia Magthalin, A. B. Mandala and G. Sekaran, RSC Adv., 2014, 4, 19183–19195.
3. M. Hu, J.-S. Jiang, F.-X. Bu, X.-L. Cheng, C.-C. Lin and Y. Zeng, RSC Adv., 2012, 2, 4782–4786.
4. G. X. Do, B. J. Paul, V. Mathew and J. Kim, J. Mater. Chem. A, 2013, 1, 7185–7190.
5. Y. Kido, K. Nakanishi and K. Kanamori, RSC Adv., 2013, 3, 3661–3666.
6. N. Basavegowda, K. Mishra and Y. R. Lee, RSC Adv., 2014, 4, 61660–61666.
7. L. Gu, R. H. Fang, M. J. Sailor and J.-H. Park, ACS Nano, 2012, 6, 4947–4954.
8. Y. Kido, K. Nakanishi, A. Miyasaka and K. Kanamori, Chem. Mater., 2012, 24, 2071–2077.
9. B. Koo, S. Chattopadhyay, T. Shibata, V. B. Prakapenka, C. S. Johnson, T. Rajh and E. V. Shevchenko, Chem. Mater., 2013, 25, 245–252.
10. L. Wortmann, S. Ilyas, D. Niznansky, M. Valldor, K. Arroub, N. Berger, K. Rahme, J. Holmes and S. Mathur, ACS Appl. Mater. Interfaces, 2014, 6, 16631–16642.
11. E. Boros, A. M. Bowen, L. Josephson, N. Vasdev and J. P. Holland, Chem. Sci., 2015, 6, 225–236.
12. Y. Jung, S. Lee, D. Ko and R. Agarwal, J. Am. Chem. Soc., 2006, 128, 14026–14027.
13. J. Luther, M. Law, M. Beard, Q. Song, M. Reese, R. Ellingson and A. Nozik, Nano Lett., 2008, 8, 3488–3492.
14. M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel, J. Am. Chem. Soc., 2008, 130, 16770–16777.
15. H. Peng, C. Xie, D. Schoen and Y. Cui, Nano Lett., 2008, 8, 1511–1516.
16. H. Tu and D. Kelley, Nano Lett., 2006, 6, 116–122.
17. K. Park, K. Jang, S. Kim, H. Kim and S. Son, J. Am. Chem. Soc., 2006, 128, 14780–14781.
18. D. T. Schoen, H. L. Peng and Y. Cui, J. Am. Chem. Soc., 2009, 131, 7973–7975.
19. Y. Yang and Y. Chen, J. Phys. Chem. B, 2006, 110, 17370–17374.
20. M. Ng, C. Boothroyd and J. Vittal, J. Am. Chem. Soc., 2006, 128, 7118–7119.
21. X. Sun, B. Yu, G. Ng, T. Nguyen and M. Meyyappan, Appl. Phys. Lett., 2006, 89, 233121.
22. B. Yu, S. Ju, X. Sun, G. Ng, T. Nguyen, M. Meyyappan and D. Janes, Appl. Phys. Lett., 2007, 91, 133119.
23. M. Ettenberg, Adv. Imaging, 2005, 20, 29–32.
24. E. Fossum, IEEE Micro, 1998, 18, 8–15.
25. S. Kim, Y. T. Lim, E. G. Soltesz, A. M. DeGrand, J. Lee, A. Nakayama, J. A. Parker, T. Mihaljevic, R. G. Laurence, D. M. Dor, L. H. Cohn, M. G. Bawendi and J. V. Frangioni, Nat. Biotechnol., 2004, 22, 93–97.
26. J. Tang, G. Konstantatos, S. Hinds, S. Myrskog, A. Pattantyus-Abraham, J. Clifford and E. Sargent, ACS Nano, 2008, 3, 331–338.
27. J. Wang, M. Gudiksen, X. Duan, Y. Cui and C. Lieber, Science, 2001, 293, 1455–1457.
28. T. Suzuki, Polymer, 1989, 30, 333–337.
29. M. Cypryk, B. Delczyk, A. Juhari and K. Koynov, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 1204–1216.
30. T. H. Thomas and T. C. Kendrick, J. Polym. Sci., Part A-2, 1970, 8, 1823–1830.
31. T. H. Thomas and T. C. Kendrick, J. Polym. Sci., Part A-2, 1969, 7, 537–549.
32. P. B. Kevin, C.-M. Kuo, L. M. Robert and C. S. John, Macromolecules, 1995, 28, 790–792.
33. J. Zhao, Y. Guo, H. Guo, Y. Chai, Y. Li, Y. Liu and C. Liu, Inorg. Chem. Commun., 2012, 18, 21–24.
34. C. Cheng, F. Xu and H. Gu, New J. Chem., 2011, 35, 1072–1079.
35. M.-K. Jia, G.-J. Su, M.-H. Zhang, B. Zhang and S.-J. Lin, Sci. China: Chem., 2010, 53, 1266–1272.
36. L.-S. Zhong, J.-S. Hu, H.-P. Liang, A.-M. Cao, W.-G. Song and L.-J. Wan, Adv. Mater., 2006, 18, 2426–2431.
37. H.-F. Fei, W. Xie, Q. Wang, X. Gao, T. Hu, Z. Zhang and Z. Xie, RSC Adv., 2014, 4, 56279–56287.
38. M. Nishizawa, K. Mukai, S. Kuwabata, C. R. Martin and H. J. Yoneyama, Electrochem. Soc., 1997, 144, 1923–1926.
39. Y. Wang, K. Takahashi, H. Shang and G. Cao, J. Phys. Chem. B, 2005, 109, 3085–3088.
40. J. Chen, L. N. Xu, W. Y. Li and X. L. Gou, Adv. Mater., 2005, 17, 582–586.
41. H. Morimoto, S. I. Tobishima and Y. J. Iizuka, Power Sources, 2005, 146, 315–318.
42. Q. R. Zhao, Y. Gao, X. Bai, C. Z. Wu and Y. Xie, Eur. J. Inorg. Chem., 2006, 8, 1643–1648.

---