Uniform copper oxide–poly(m-phenylenediamine) microflowers: synthesis and application for the adsorption of methyl orange

Abstract

A rapid, template-free and surfactant-free approach is proposed for the preparation of copper oxide–poly(m-phenylenediamine) (CuO–PmPD) microflower composites by simply mixing aqueous Cu(NO₃)₂ and m-phenylenediamine (mPD) at 10 °C. The concentrations of reactants are found to play an important role in the formation of different CuO–PmPD morphologies. The morphologies of the composites can be varied from microflowers to spheres by changing the concentrations of reactants. Moreover, the CuO–PmPD microflower composite exhibits a superior capacity for methyl orange (MO) adsorption.

---

1. Introduction

During recent years, a great deal of research interest has been focused on one-dimensional (1D) structures (rods, wires, tubes and belts), owing to their unique properties and potential to revolutionize broad areas of nanotechnology and microtechnology.^1–3 However, the assembly and integration of 1D materials into three-dimensional (3D) arrays or hierarchical structures are desirable for many applications, such as drug delivery, sensing, energy conversion and storage, light-emitting displays, catalysis and adsorption.^4–8 Various physical and chemical routes have been developed to fabricate 3D hierarchical ordered structures^9–11 for facile searching, transport, contact and detection. However, these methods usually require rigorous conditions, using either high temperatures or special techniques, which can possibly lead to high costs and further limit their potential applications. Therefore, the fabrication of desired hierarchical micro/nanostructures via a rational, simple and cost-effective technique remains a significant challenge.

Polymers based on aniline derivatives have also been extensively investigated,¹² among which poly(phenylenediamine) (PPD) is one of the most studied conducting polymers due to its high thermostability and sensitivity.^13,14 As a result, intensive efforts have been put into synthesizing PPD with various morphologies, including microparticles, nanobelts, microfibrils, etc.,^15–18 to meet the demand for the design and construction of devices.¹⁹ Min and co-workers reported a shape-controlled method for the synthesis of poly(p-phenylenediamine) (PpPD) microstructures using a UV lamp as the oxidation energy source and poly(N-vinylpyrrolidone) (PVP) as the surfactant.²⁰ Zhang et al. fabricated poly(m-phenylenediamine) (PmPD) core–shell nanoparticles by directly mixing aqueous silver nitrate and m-phenylenediamine (mPD) solutions at room temperature.²¹ So far, a variety of PPD based micro/nanostructures have also been successfully synthesized through the template method,²² electropolymerization method,²³ photopolymerization method,²⁴ and so on. However, there are few simple, economical and productive synthesis approaches. Despite the fact that Wang et al. prepared poly(o-phenylenediamine) (PoPD) microfibrils using cupric sulfate as a facile oxidant in the absence of any external surfactant,²⁵ the PPD hierarchical structures that are advantageous for adsorption have not been produced by a simple and economical method. Moreover, to the best of our knowledge, the synthesis of copper oxide–poly(m-phenylenediamine) (CuO–PmPD) flower-like composites via a liquid synthesis process has not yet been reported.

Herein, we demonstrate the preparation of uniform CuO–PmPD flower-like composites by directly mixing aqueous solutions of Cu(NO₃)₂ and mPD at 10 °C. This simple and cost-effective process does not need any catalysts or surfactants. Meanwhile, the morphology of the CuO–PmPD composites can be modulated under the control of reactant concentrations. In our research, a series of techniques were applied to characterize the crystallographic phase, morphology, microstructure and adsorption of the products. More importantly, the as-synthesized CuO–PmPD flower-like composite exhibits great promise for the removal of methyl orange (MO) from aqueous solution.

2. Experimental

2.1 Materials and apparatus

Cu(NO₃)₂ and MO were purchased from Shanghai Chemical Factory (Shanghai, China). HNO₃, NaOH, (NH₄)₂S₂O₈ and mPD were purchased from Kelong (Chengdu, China). All the chemicals were analytical reagent grade and used without further purification. The water used throughout all experiments was purified through a Millipore system.

The crystallographic phase, microstructure, thermal stability, morphology and elemental composition of sample 1 were characterized using Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet 6700 FT-IR Spectrometer), thermogravimetric analysis (TGA, STA 449 F3 Jupiter), X-ray photoelectron spectroscopy (XPS, XSAM 800 spectrometer), and scanning electron microscopy (SEM, JEOL JSM-6510LV) coupled with energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max). The Brunauer–Emmett–Teller (BET) surface areas of the samples were measured using a micromeritics analyser (ASAP, Autosorb-IQ-MP).

The concentration of leached Cu²⁺ in the supernatant was determined using atomic absorption spectroscopy (AAS, WFX-120, Rayleigh Analytical Instrument Corp.).

2.2 Preparation of CuO–PmPD composites

The CuO–PmPD flower-like composites were prepared as follows: 2 mL of aqueous mPD was directly introduced into 8 mL of aqueous Cu(NO₃)₂ at 10 °C. The concentrations for mPD and Cu(NO₃)₂ were equal at 0.1 M, 0.2 M and 0.4 M for samples 1, 2 and 3 (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4). The color of the mixture gradually changed from blue to dark. Ten hours later, a large quantity of dark precipitate was observed. The formed precipitate was washed with distilled water three times, and then dried at 50 °C under vacuum for further characterization.

2.3 Adsorption experiments

The adsorption of MO on the CuO–PmPD composite (sample 1) has been examined using a series of experiments. A known weight of CuO–PmPD (5 mg) was equilibrated with 20 mL of 65 mg L⁻¹ aqueous MO solution in a 100 mL Erlenmeyer flask at 30 °C using a shaking bath. The initial pH was adjusted with 0.1 M HNO₃ or 0.1 M NaOH. After shaking for 1 h to ensure full equilibration, the suspension was separated by centrifugation and the obtained supernatant was analysed using UV-vis absorption spectroscopy (UV, Tsushima Japan 2550 UV-vis spectrometer) at the maximum absorbance (464 nm).

The adsorption capacity was calculated from the following expressions:

where Q_e is adsorption capacity of the adsorbent at equilibrium (mg g⁻¹), C_o and C_e are the initial and equilibrium concentrations of MO (mg L⁻¹) respectively, V is the volume of solution (mL) and m is the dose of CuO–PmPD composite (g).

3. Results and discussion

3.1 Characterization of CuO–PmPD composites

Fig. 1 shows typical SEM images of the product (sample 1). The low-magnification SEM image (Fig. 1A) clearly indicates that a large number of microflowers composed of microplates are formed in this process. A close view of this sample, shown in Fig. 1B, further reveals that these microplates have diameters ranging from 3 to 4 μm, and a thickness of about 350 nm.

Fig. 1 (A) Low and (B) high magnification SEM images of the product synthesized with aqueous solutions of 0.1 M mPD and 0.1 M Cu(NO₃)₂ (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4).

EDS analysis (Fig. 2) shows the chemical composition of the product (sample 1). The peaks corresponding to C, N, O, and Cu elements are observed, indicating the possible existence of CuO and PmPD in the formed products.

Fig. 2 EDS spectrum of the product synthesized with aqueous solutions of 0.1 M mPD and 0.1 M Cu(NO₃)₂ (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4).

The FTIR spectrum of the product (sample 1) is shown in Fig. 3. The adsorption peaks around 3400 cm⁻¹ correspond to the N–H stretching mode.¹³ The peaks at 1624 and 1427 cm⁻¹ are assigned to C–N and C–C stretching vibrations in the phenazine structure, respectively.²⁶ The peak at 1343 cm⁻¹ is associated with C–N stretching in the benzenoid and quinoid imine units.²⁷ The adsorption peak at 1041 cm⁻¹ is ascribed to the aromatic C–H in plane bending mode.^28,29 The results agree well with the FTIR spectra of PPD previously reported.^30–32 Considering the Cu–O vibrations, the adsorption peaks in Fig. 3 are consistent with vibration frequencies of Cu–O in the range of 500 cm⁻¹ and above,³³ proving that CuO may have been produced.

Fig. 3 FT-IR spectrum of the product synthesized with aqueous solutions of 0.1 M mPD and 0.1 M Cu(NO₃)₂ (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4).

Fig. 4 shows the Cu_2p XPS spectra of the product (sample 1). The Cu_2p spectra show two main peaks located at about 933.4 and 953.3 eV, corresponding to Cu_2p3/2 and Cu_2p1/2, respectively, which is in agreement with the standard spectrum of CuO.³⁴ The peak at 941.6 eV is attributed to a satellite peak. All the above results further indicate the successful fabrication of CuO–PmPD flower-like composites.

Fig. 4 Cu_2p XPS spectra of the product synthesized with aqueous solutions of 0.1 M mPD and 0.1 M Cu(NO₃)₂ (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4).

Thermogravimetric analysis (Fig. 5) of the CuO–PmPD composite (sample 1) was carried out under a nitrogen atmosphere with a heating rate of 10 °C per minute. The CuO–PmPD composite underwent a three-stage degradation: (a) loss of water (at about 100 °C); (b) decomposition of NO₃¹⁻ (at about 275 °C); (c) degradation of the polyaniline backbone and benzene, and quinonoid ring opening (above 400 °C).^35,36 It can been seen that 47% of the weight still remained as the temperature increased to 800 °C, so the thermal stability of the CuO–PmPD flower-like composite is really good.

Fig. 5 TG and DTG curves of the product synthesized with aqueous solutions of 0.1 M mPD and 0.1 M Cu(NO₃)₂ (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4).

To evaluate the surface areas and porous properties of the samples, nitrogen adsorption and desorption measurements were performed. The BET surface area of sample 1 is 23.207 m² g⁻¹, which is bigger than that of sample 2 (5.583 m² g⁻¹) or sample 3 (3.773 m² g⁻¹). The larger BET surface area of sample 1 is attributed to its microflower morphology, and it is advantageous for the adsorption process.

3.2 Shape control of the CuO–PmPD composites

The remarkable changes in the self-assembly behavior of the hierarchical microstructures were investigated with different concentrations of reactants (Fig. 6). Fig. 6A shows a typical SEM image of the composite (sample 1) that was synthesized using aqueous solutions of 0.1 M mPD and 0.1 M Cu(NO₃)₂. It can be clearly seen that the product contains microflowers composed of well-defined microplates. Interestingly, when the concentrations of the aqueous solutions of mPD and Cu(NO₃)₂ both increase to 0.2 M (sample 2), the microplates are interconnected to form symmetrical hydrangea-shaped microspheres (Fig. 6B). As the reactant concentrations are further increased to 0.4 M (sample 3), microspheres are formed again. However, their surfaces are much smoother than those of sample 2 (Fig. 6C). It should be pointed out that the reactant concentrations have little effect on the size of the products (1.5–3 μm). The microplates are likely, in terms of energy reduction, to attach to each other and weld together, which could occur whenever the adjacent CuO–PmPD microplates are overlapped or in physical contact with each other, and the higher reactant concentrations can make the attachment much easier. Therefore, it is reasonable to conclude that microplate attachment and welding may contribute to the morphology evolution of CuO–PmPD from microflowers to microspheres.

Fig. 6 SEM images of CuO–PmPD composites at formed at different concentrations: (A) 0.1 M, (B) 0.2 M, and (C) 0.4 M (the molar ratio of mPD to Cu(NO₃)₂ is 1 : 4).

3.3 Adsorption of MO

To demonstrate the adsorption application of the CuO–PmPD composites, a series of experiments based on the adsorption of MO have been examined. It is exciting to find that the prepared CuO–PmPD composites exhibit a superior adsorption capacity for MO. Besides π–π stacking interactions between MO molecules and the CuO–PmPD composites, electrostatic adsorption is another important factor for the adsorption capacity of the CuO–PmPD composites towards MO.

Effect of pH. The acidity of the solution can affect the adsorption capacity of the CuO–PmPD composites for MO, because both the charges of the CuO–PmPD backbone and MO can be strongly affected by the pH value of the solution. The chemical structures of MO at different pH values are shown in Fig. 7. The acidity-dependent mass uptake of MO on the CuO–PmPD sample is illustrated in Fig. 8. It can be seen that the adsorption capacity of the CuO–PmPD sample towards MO increases with increasing pH and reaches a maximum at a pH value of 6.5. This can be attributed to the strong electrostatic adsorption between the negatively charged MO and positively charged CuO–PmPD composite in the aqueous solution. However, the adsorption capacity decreases when the pH is further increased. This is most likely due to the fact that the CuO–PmPD composite can be de-doped easily in alkali solution and consequently changed into neutral materials. The electrically neutral materials then have no electrostatic interaction with the ionic MO molecules. Therefore, the solution at pH 6.5 is selected as the medium for the adsorption process.

Fig. 7 The structures of the MO molecule at different pH values: (A) pH > 4.4 and (B) pH < 3.1.

Fig. 8 Effect of initial pH values (2.9, 3.8, 6.0, 6.5, 9.1, and 10.4) on the adsorption capacity of MO on the CuO–PmPD composite. [MO] = 65 mg L⁻¹; t = 1.0 h; T = 30 °C; each dose of CuO–PmPD is 5 mg.

Kinetics. The kinetics of adsorption of MO on the CuO–PmPD composite was investigated using real-time monitoring as shown in Fig. 9. It is clearly observed that the adsorption capacity of the CuO–PmPD composite becomes fully saturated after an hour, and then does not increase any more (Fig. 9a). Fig. 9b shows the kinetics plot for the adsorption of MO by the CuO–PmPD composite according to the pseudo-second-order equation:where Q_e and Q_t are the mass uptake of MO adsorbed on CuO–PmPD composites at equilibrium and different time intervals, respectively, and K₂ is the rate constant (g min mg⁻¹). A good linear correlation is observed in Fig. 9b (R² = 0.9993), suggesting that the reaction follows pseudo-second-order kinetics. In addition, the value of Q_e can be calculated from the slope and was found to be 264.55 mg g⁻¹ for the flower-like CuO–PmPD composite, which agrees well with the experimental value.

Fig. 9 (A) Adsorption kinetics curve and (B) pseudo-second-order kinetics plot for the adsorption of MO on the CuO–PmPD composite. [MO] = 65 mg L⁻¹; pH = 6.5; T = 30 °C; each dose of CuO–PmPD is 5 mg.

Isotherm. Fig. 10 shows the adsorption isotherm and typical Freundlich-type isotherm of MO on the CuO–PmPD composite at different concentrations of MO, ranging from 10 to 80 mg L⁻¹. Note that the adsorption amount of MO increases quickly with increasing initial concentration of MO and flattens out later (Fig. 10a), which agrees well with the nature of the Freundlich equation. It is further proved by fitting the experimental equilibrium adsorption data using the Freundlich equation:

Q_e = K_fC_e^1/n (3)

where C_e and Q_e are the equilibrium concentration (mg L⁻¹) of MO and equilibrium adsorption capacity (mg g⁻¹) of CuO–PmPD for MO, respectively. K_f and n are the Freundlich constants. The result shows that the adsorption of MO molecules on the CuO–PmPD composite follows the Freundlich isotherm well (R² = 0.9915), as shown in Fig. 10b. The Freundlich constants for the adsorbents are also calculated and are presented in Table 1.

Fig. 10 (A) Adsorption isotherm of MO on CuO–PmPD and (B) plot of lg(Q_e) vs. lg(C_e) for methyl orange adsorption onto CuO–PmPD. pH = 6.5; T = 30 °C; t = 1.0 h; each dose of CuO–PmPD is 5 mg.

Table 1 Freundlich parameters of the CuO–PmPD composite

Sample	Kf	n	R^2
CuO–PmPD	12.7912	0.9010	0.9915

---

Thermodynamics. In order to clarify the thermodynamic performance of MO on CuO–PmPD, we developed a controlled-environment system, in which the temperature is adjusted in the range of 30 to 70 °C (Fig. 11). Note that the adsorption capacity of the flower-like CuO–PmPD decreases as the temperature increases (Fig. 11a), indicating that the adsorption is an exothermic process. To confirm this conclusion, the curve of lg(Q/C_e) versus 1/T for the CuO–PmPD composite was obtained (Fig. 11b) by fitting the experimental data (R² = 0.9910). The thermodynamic parameters, such as the change in enthalpy (ΔH^θ), entropy (ΔS^θ) and Gibb’s free energy (ΔG^θ), are obtained using the following equations:

ΔG^θ = −ΔH^θ − TΔS^θ (6)

Fig. 11 (A) Effect of temperature on adsorption capacity and (B) plot of lg(Q/C_e) vs. 1/T for MO on CuO–PmPD. [MO] = 65 mg L⁻¹; pH = 6.5; t = 1.0 h; each dose of PmPD–CuO is 5 mg.

As listed in Table 2, the negative value of ΔH^θ obtained from eqn (5) indicates that the adsorption is an exothermic process. Thus, a lower temperature makes the adsorption process easier. The negative value of ΔS^θ indicates that there is a decrease in the degree of order at the solid/solution interface during the adsorption process. In addition, the negative value of ΔG^θ obtained from eqn (6) confirms the spontaneous nature of the adsorption.

Table 2 Thermodynamic parameters for MO adsorbance on the CuO–PmPD composite

Temperature (°C)	ΔG^θ (kJ mol^−1)	ΔH^θ (kJ mol^−1)	ΔS^θ (J mol^−1)
30	−26.1	−41.4	−50.5
40	−27.1	−41.4	−45.7
50	−28.0	−41.4	−41.5
60	−29.0	−41.4	−37.2
70	−30.0	−41.4	−33.2

---

Reusability of adsorbent. To investigate the reusability of the CuO–PmPD adsorbent, the CuO–PmPD composite saturated with MO was first separated by centrifugation, and then regenerated by treatment with 0.5 M NaOH at 80 °C, according to ref. 37 with a slight modification. Finally, the NaOH-treated CuO–PmPD composite was washed repeatedly with distilled water until neutral, dried, and recycled as described in the adsorption experiment. Fig. 12 shows the amount of MO adsorption of CuO–PmPD undergoing four cycles. It can be seen that the amount of adsorption of MO on the CuO–PmPD composite almost remains unchanged over all the cycles, suggesting that the as-prepared CuO–PmPD composite can be recycled and reused without the loss of active sites.

Fig. 12 Recyclability of the as-prepared CuO–PmPD adsorbent.

Comparison of maximum adsorption capacities. To evaluate the superiority of CuO–PmPD for adsorption, we performed a comparison experiment on the maximum adsorption capacities of MO between CuO–PmPD and PmPD adsorbents. The results suggested that the CuO–PmPD adsorbent shows a stronger adsorption for MO compared with the PmPD adsorbent (Fig. S1, ESI†).

3.4 Stability of adsorbents

Considering the stability of the CuO–PmPD adsorbent at different pH levels, we performed a leaching test in aqueous solution. 5 mg CuO–PmPD was dispersed in 20 mL aqueous solution with the pH ranging from 1.0 to 11.0 and agitated in a temperature-controlled shaker at 30 °C for 300 min. Fig. 13 shows the concentrations of leached Cu²⁺ at different pH levels. The concentration of Cu²⁺ is negligible at a pH of over 3.0 and increases significantly when the pH is lower than 3.0. At pH 1.0, the concentration of leached Cu²⁺ is 17.48 mg L⁻¹. These results imply that the CuO–PmPD composite could maintain good stability in weakly acidic, neutral and alkaline aqueous media.

Fig. 13 Leached Cu²⁺ content from the CuO–PmPD adsorbent at different pH levels.

4. Conclusions

In summary, we have reported the one-step synthesis of a CuO–PmPD flower-like composite through a direct redox reaction between Cu(NO₃)₂ and mPD in an aqueous medium at 10 °C. The morphology of the CuO–PmPD samples can be modulated by changing the concentrations of reactants. As a proof of concept, we demonstrated the successful use of such a CuO–PmPD composite as an adsorbent for MO. The operational parameters, such as the pH of the solution, equilibrium time, initial concentration of MO, and adsorption temperature have been studied during the experiments. The adsorption kinetics and isotherms are well-described by pseudo-second-order kinetics and the Freundlich model, respectively. The calculated thermodynamic parameters indicate that the adsorption process is feasible, spontaneous and endothermic in nature. Most importantly, the prepared flower-like composites are insoluble and exhibited a superior adsorption capacity (264.55 mg g⁻¹) for MO, suggesting a potential application for the removal of MO from wastewater.

Notes and references

1. Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim and H. Yan, Adv. Mater., 2003, 15, 353–389.
2. C. Rao, G. Gundiah, F. L. Deepak, A. Govindaraj and A. Cheetham, J. Mater. Chem., 2004, 14, 440–450.
3. Y. Li, F. Qian, J. Xiang and C. M. Lieber, Mater. Today, 2006, 9, 18–27.
4. H. J. Koo, Y. J. Kim, Y. H. Lee, W. I. Lee, K. Kim and N. G. Park, Adv. Mater., 2008, 20, 195–199.
5. K. C. Popat, M. Eltgroth, T. J. Latempa, C. A. Grimes and T. A. Desai, Biomaterials, 2007, 28, 4880–4888.
6. G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Nano Lett., 2005, 5, 191–195.
7. X. Feng, J. Zhai and L. Jiang, Angew. Chem., Int. Ed., 2005, 44, 5115–5118.
8. S.-H. Jung, E. Oh, K.-H. Lee, Y. Yang, C. G. Park, W. Park and S.-H. Jeong, Cryst. Growth Des., 2007, 8, 265–269.
9. S. Sattayasamitsathit, A. M. O’Mahony, X. Xiao, S. M. Brozik, C. M. Washburn, D. R. Wheeler, W. Gao, S. Minteer, J. Cha and D. B. Burckel, J. Mater. Chem., 2012, 22, 11950–11956.
10. L.-P. Zhu, H.-M. Xiao, W.-D. Zhang, G. Yang and S.-Y. Fu, Cryst. Growth Des., 2008, 8, 957–963.
11. L. Vayssieres and M. Graetzel, Angew. Chem., 2004, 116, 3752–3756.
12. T. Sulimenko, J. Stejskal, I. Křivka and J. Prokeš, Eur. Polym. J., 2001, 37, 219–226.
13. R. Gangopadhyay and A. De, Chem. Mater., 2000, 12, 608–622.
14. E. K. Miller, C. J. Brabec, H. Neugebauer, A. J. Heeger and N. Serdar Sariciftci, Chem. Phys. Lett., 2001, 335, 23–26.
15. X.-G. Li, X.-L. Ma, J. Sun and H. a. Mei-Rong, Langmuir, 2009, 25, 1675–1684.
16. J. J. Wang, J. Jiang, B. Hu and S. H. Yu, Adv. Funct. Mater., 2008, 18, 1105–1111.
17. L. Zhang, L. Chai, H. Wang and Z. Yang, Mater. Lett., 2010, 64, 1193–1196.
18. Y. Zhang, L. Wang, J. Tian, H. Li, Y. Luo and X. Sun, Langmuir, 2011, 27, 2170–2175.
19. Q. Deng and S. Dong, J. Electroanal. Chem., 1994, 377, 191–195.
20. Y.-L. Min, T. Wang, Y.-G. Zhang and Y.-C. Chen, J. Mater. Chem., 2011, 21, 6683–6689.
21. Y.-W. Zhang, H.-L. Li and X.-P. Sun, Chin. J. Anal. Chem., 2011, 39, 998–1002.
22. Y. Yang, Y. Chu, F. Yang and Y. Zhang, Mater. Chem. Phys., 2005, 92, 164–171.
23. Y. Du, H. Wang, A. Zhang and J. Lu, Chin. Sci. Bull., 2007, 52, 2174–2178.
24. O. Abdulrahman, Catal. Sci. Technol., 2012, 2, 711–714.
25. L. Wang, S. Guo and S. Dong, Mater. Lett., 2008, 62, 3240–3242.
26. Q. Hao, B. Sun, X. Yang, L. Lu and X. Wang, Mater. Lett., 2009, 63, 334–336.
27. A. Kitani, T. Akashi, K. Sugimoto and S. Ito, Synth. Met., 2001, 121, 1301–1302.
28. K. Mallick, M. J. Witcomb, A. Dinsmore and M. S. Scurrell, Langmuir, 2005, 21, 7964–7967.
29. A. Drelinkiewicz, M. Hasik and M. Kloc, Catal. Lett., 2000, 64, 41–47.
30. H. Jiang, X. Sun, M. Huang, Y. Wang, D. Li and S. Dong, Langmuir, 2006, 22, 3358–3361.
31. D. Ichinohe, N. Saitoh and H. Kise, Macromol. Chem. Phys., 1998, 199, 1241–1245.
32. D. Ichinohe, T. Muranaka, T. Sasaki, M. Kobayashi and H. Kise, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, 2593–2600.
33. M. Stavola, D. Krol, W. Weber, S. Sunshine, A. Jayaraman, G. Kourouklis, R. Cava and E. Rietman, Phys. Rev. B: Condens. Matter Mater. Phys., 1987, 36, 850–853.
34. C. Wagner, W. Riggs, L. Devis, J. Moulder and G. Muilenberg, Phys. Elect. Div., Eden Prairie, Minnesota, USA, 1979.
35. S. Wang, Z. Tan, Y. Li, L. Sun and T. Zhang, Thermochim. Acta, 2006, 441, 191–194.
36. C. Yang and C. Chen, Synth. Met., 2005, 153, 133–136.
37. X. Guo, G. T. Fei, H. Su and L. De Zhang, J. Mater. Chem., 2011, 21, 8618–8625.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15606d

---