Study of the dielectric properties of graphene/CuS/ZnO hybrid nanocomposites for high performance supercapacitor applications

Abstract

We report here the dielectric characteristics of a potential nanocomposite consisting of graphene doped with CuS nanoparticles and ZnO nanotubes for supercapacitance applications. The as synthesized nanoparticles were characterized using X-ray diffraction (XRD), Raman spectroscopy and High Resolution Transmission Electron Microscopy (HRTEM). By varying temperature and frequency, the dielectric characteristics of the as synthesized graphene (Gr), graphene–CuS–ZnO hybrid nanocomposite (GCZ) and graphene–ZnO hybrid nanocomposite (GZ), were investigated and compared. Complex impedance analysis showed that the capacitance characteristics of the as synthesized samples had increased with increasing frequency and temperature. The Nyquist plots of the as synthesized samples at different temperatures and frequencies confirmed that the as synthesized GCZ show better conductivity than the as synthesized GZ and Gr. The temperature and frequency dependency of the dielectric permittivity (ε′) and dielectric loss (tan δ) of all the as synthesized samples were investigated. The as synthesized GCZ shows the highest dielectric permittivity than the as synthesized GZ and Gr due to the interfacial polarisation. The order of dielectric loss of the as synthesized samples is as follows GCZ ≫ GZ > Gr due to a significant increase in the conductivity of GCZ. The results indicated that the as synthesized GCZ possesses a high surface area and high electric conductivity which provide a promising material for capacitance applications.

---

1. Introduction

Supercapacitors have generated a lot of interest among energy storage devices over the lastfew decades. This is due to their higher power capacity, high energy density and long cycle life. Supercapacitors store energy via two mechanisms: electrochemical double layer capacitance and pseudocapacitance. The main problem associated with the commercialization of perfect supercapacitors is the stabilisation of higher energy density for longer times. In light of this concern, many studies are being developed on the production of electrodes with higher specific capacitance. An excellent example of this type of promising material is graphene.

Graphene, one atom thick, flat and monolayer of sp² carbon atom, is an attracting material for making supercapacitors due to its ballistic electron transport, high surface area, good mechanical properties and excellent thermal conductivities compared with other nanoparticles.^1–3 However in respect to the self-agglomeration of graphene due to the strong van der Waal forces of attraction and pi–pi interaction, the practical applications of graphene is limited. There have been various studies conducted for the reduction of Gr self-restacking and enhancing the overall performance of Gr. Non-covalent functionalization in which the formation of hybrids by incorporating nano metal oxides with graphene is one of the promising methods usually used.^4,5 The inexpensive nature, stability, nontoxicity, tuning band gap and environmental friendly are the major criteria for selecting the filler as a candidate for hybrid formation. The versatile metal oxides such as TiO₂, ZnO and SnO₂ have been used to combine Gr for making energy harvesting, storage devices, supercapacitors and rechargeable batteries. Zhou⁶ reported the fabrication of NiO on Gr sheet as an electron storage material. In another report, Jiang⁷ studied the storage capacity of Gr doped with one dimensional silver nano wire. In his report, thermal conductivity, electrical conductivity and chemical stability of Gr were dramatically enhanced by the addition of silver nano wire. Thus the type of incorporating material and the amount of fillers may be associated with the conductivity of Gr. There are various studies have been reported using Gr–ZnO hybrid nanocomposite as energy storage material due to the low cost, environmental friendliness, non-toxicity and high specific energy density of ZnO. There are very few studies reported for the use of eco-friendly and nontoxic CuS nanomaterial to modify the properties of ZnO and graphene.^8,9 But no studies on the dielectric characteristics of Gr–CuS–ZnO hybrid nanocomposite have been reported yet. In a new development, Rao et al.¹⁰ reported the fabrication of Fe₃O₄ nano filler on single layer graphene–porous carbon–polyvinyl alcohol oxide system. The Fe₃O₄ formed the localized polarized sites which improved the dielectric properties of the hybrid nanocomposites.

In the present study, we reported, the detailed analysis of dielectric characteristics like real and imaginary parts of impedance, dielectric permittivity and dielectric loss of as synthesized Gr–CuS–ZnO hybrid nanocomposite (GCZ), graphene–ZnO hybrid nanocomposite (GZ) and pristine graphene (Gr) were analysed and compared well. The dielectric characteristics of the as synthesized samples were analysed by using precision impedance spectroscopy. In addition the effect of frequencies on the dielectric characteristics of as synthesized GCZ, GZ and Gr were also evaluated. Furthermore, the relation between temperature and dielectric characteristics of the as synthesized samples were also investigated at various temperatures.

2. Experimental section

All chemicals were analytical reagent grade and used as received without any further purification. Graphite was purchased from Sigma Aldrich, while zinc acetate, copper acetate, NaOH and thiourea were from Merck.

2.1. Preparation of graphene

Graphene Oxide (GO) was prepared by Hummers method. The synthesised GO was filtered and washed several times with 10% hydrochloric acid and dried in a vacuum oven. The dried sample was kept in a furnace at 1050 °C. Exfoliation of stacked structure obtained through the evolution of carbon dioxide gas.

2.2. Synthesis of ZnO nanotubes

Pure ZnO nanotubes was synthesised by sol gel method. Zinc acetate dihydrate was the starting material. Zinc acetate dihydrate was dissolved in ethanol solution. The saturated solution was mixed with PVP under stoichiometric concentration and stirred at 80 °C for 1 hour. The clear solution was cooled in an ice cold condition. NaOH solution was added to this drop by drop for the formation of ZnO nanotubes. The white precipitate is centrifuged and dried at 80 °C in a vacuum oven.

2.3. Synthesis of CuS spherical nanoparticles

Saturated copper acetate dihydrate solution was mixed with PVP in a stoichiometric ratio and stirred well. In this solution thiourea solution was added drop by drop and stirred well. After that NaOH solution added drop by drop till the pH of the solution maintains 9. The precipitate was kept at room temperature at 12 hours without any disturbance. The green precipitate is centrifuged and dried at 60 °C in a vacuum oven.

2.4. Synthesis of graphene–ZnO composites (GZ)

Graphene and ZnO nanotubes were dispersed in N-methyl pyrrolidone (NMP) separately under sonication. The two solutions were mixed at weight ratio of 1 : 0.33. The mixture was centrifuged and dried well.

2.5. Synthesis of graphene–CuS–ZnO composites (GCZ)

ZnO nanotubes and CuS nanoparticles were dispersed in NMP separately under sonication. The two solutions were mixed at weight ratio of 1 (CZ). The graphene was dispersed in NMP under sonication. From this solution, CZ solution was added drop by drop at a fixed weight ratio (the ratio between graphene and CuS–ZnO is 1 : 0.33). The product was centrifuged and dried well.

2.6. Characterization

Morphological studies of the synthesised samples were characterised by Transmission Electron Microscope (TEM, JEOL-2010). Raman spectra of samples were recorded by Micro Raman system using excitation laser sources – He–Ne 632.8 nm and argon 488 nm from Jobin Yvon Horibra LABRAM-HR visible (400–1100 nm). The phase purity, crystal phase characterization of the samples were analysed with wide angle X-ray diffractometer (Bruker D8 ADVANCE) using Ni-filtered Cu Kα radiation at 40 kV and 40 mA in the 2θ, ranging from 5° to 80° with a scan rate of 0.02° per second. The dielectric properties of the as synthesised samples were measured using Wayne kerr 6500B precision impedance spectroscopy.

3. Results and discussion

3.1. Characterisation of nanocomposites

The surface morphology of the as synthesized graphene (Gr), graphene–ZnO hybrid nanocomposite (GZ) and graphene–CuS–ZnO hybrid nanocomposite (GCZ) were confirmed by TEM analysis. The typical TEM images and HRTEM images of the as synthesized Gr, GZ and GCZ are displayed in Fig. 1a–f. The TEM images of GZ and GCZ show that the ZnO and CuS–ZnO hybrid nanocomposite (CZ) are well dispersed in Gr without change its exact morphology. The HRTEM image shows that the as synthesized CuS particles with average particle size 2–8 nm are uniformly deposited on the ZnO surfaces. The HRTEM results also indicate that as synthesized CZ and ZnO are also uniformly arranged on the Gr surfaces. The results confirmed that the self-agglomeration of graphene sheets is strongly controlled by hybrid formation.

Fig. 1 (a): TEM image of graphene (b): typical HRTEM image of graphene (c): TEM image of GZ (d): HRTEM image of GZ (e): TEM image of GCZ (f): HRTEM image of GCZ.

The XRD patterns in Fig. 2 further confirmed the bonding interaction between Gr and semiconductors. In GZ, the diffraction peaks at 31.610, 34.40, 36.350, 47.380, 56.530, 62.52 and 68.530 in addition to the characteristic peaks of Gr were also observed. These seven peaks are attributed to the (100), (002), (101), (102), (110), (103) and (112) crystalline plane reflection of ZnO nanotubes. In GCZ, the reflection planes at (002), (101), (102) corresponding to the hexagonal wurtzite phase of ZnO and the (006), (103), (108), (110), (116) reflections corresponding to the CuS nanoparticles were also present in addition to the characteristic peaks of Gr. The results clearly confirmed the formation of hybrid nanocomposites without any changes in the original sp² moiety of graphitic structure.

Fig. 2 XRD patterns of Gr, GZ and GCZ.

The surface chemistry of the as synthesized nanomaterials was investigated using Raman spectroscopy. The Raman spectra of as synthesized samples are shown in Fig. 3a–b. The as synthesized Gr displayed the characteristic peaks at 1350, 1582 and 2700 cm⁻¹ corresponding to the D band G band and 2D band. In GCZ and GZ, the red shift in the position and increase in the intensity ratio of G and 2D bands indicated that the electron charge transfer from semiconductor to Gr. In addition to this the intensity of D band increases due to the strong interaction between Gr and semiconductors.

Fig. 3 (a): Raman spectra of Gr, GZ and GCZ (b): enlarged band of Gr, GZ and GCZ.

3.2. Dielectric studies of nanocomposites

3.2.1. Complex impedance analysis. Complex impedance analysis was used to characterize the electrical properties of pristine Gr and hybrid nanocomposites. Complex impedance is a complex quantity showing total ability of the system to prevent the current flow at a suitable frequency. Complex impedance has two parts: real part of complex impedance (Z′) and imaginary part of complex impedance (Z′′). The addition of metal oxide or hybrid composites in the Gr layers can change the overall electrical properties of Gr. The effect of fillers, temperatures and frequency on electrical properties of nanocomposites was easily correlated by this method.
3.2.1.1. Real part of complex impedance. Real part of complex impedance analysis indicates the resistance of the as synthesized samples. The real part of impedance (Z′) plotted as functions of frequencies at increasing temperatures are shown in Fig. 4a–c. The Z′ values of all the as synthesized samples were gradually decreased with increasing frequency and attained almost constant value in the higher frequency region. The results indicate that at varying frequencies, the Z′ values of as synthesized Gr are higher than that of as synthesized GZ and GCZ. The results also indicate that the doping materials such as nano ZnO and CuS–ZnO hybrid materials are well dispersed on the Gr surfaces which in turn prevent the agglomeration of Gr. This effect may cause the defects in the as synthesized nanocomposites, which provide space charge polarization and rotation polarization.¹¹ The Z′ values of the as synthesized nanocomposites were evaluated at different temperatures and found that Z′ values decreases with increasing temperature. The reducing resistance with rising temperature of as synthesized samples indicates the increasing electron exchange interaction of thermally activated charge carriers.

Fig. 4 (a): Variation of real part of impedance with frequency of graphene. (b): Variation of real part of impedance (Z′) with frequency of GZ. (c): Variation of real part of impedance (Z′) with frequency of GCZ.

3.2.1.2. Imaginary part of complex impedance. The variations of imaginary part of complex impedance (Z′′) with varying frequencies are shown in Fig. 5a–c. The Z′′ patterns of all the samples are decreased with increasing frequency. This indicates the better capacitive nature of the as synthesized nanocomposites due to the doping effect of ZnO and CuS–ZnO hybrid materials. The hybrid composites subjected to the external electrical field, show the movement of free charges at the interfaces. The movement of free charges may cause the dipole interaction followed by space charge polarization. For pristine Gr, the space charge formation is less due to the self-aggregation properties. The as synthesized GCZ shows better space charge polarization than as synthesized GZ and Gr. This is due the effective exfoliation of Gr layers in GCZ than in GZ. It is observed that the imaginary part of impedance values decreased with rising temperatures. These results attribute the more capacitive nature of the samples with rising temperatures due to the increased space charge polarization.

Fig. 5 (a): Variation of imaginary part of impedance (Z′′) with frequency of graphene. (b): Variation of imaginary part of impedance (Z′′) with frequency of GZ. (c): Variation of imaginary part of impedance (Z′′) with frequency of GCZ.

3.2.1.3. Nyquist plot. The relationship between real part of impedance (Z′) and imaginary part of impedance (Z′′) were checked in terms of Nyquist plot. Fig. 6a–c illustrate the Nyquist plot of Gr, GZ and GCZ at different frequencies and temperatures. The results show that, there was a semicircle in the higher frequency region and a linear line in the lower frequency region. The semicircle region in the Nyquist plot represents the faradaic process of charge transfer. Interfacial charge transfer resistance (R_ct) was directly evaluated from the diameter of the semicircle. The arc diameter of Gr in Nyquist plot was evaluated and was found to be significantly decreased by the addition of nanomaterials. The result indicates that as synthesized ZnO nanotubes and CZ act as a channel for the charge transfer in Gr layers. Hence the effective diffusion of free charges occurs from the surface of nanomaterials to Gr layers, thus increasing the conductivity of the hybrid materials. However, the R_ct for all the samples are decreases with increasing temperature.¹⁰ The straight line in the low frequency region of Nyquist plot is due to the frequency dependent transport of ions and is known as Warburg impedance. In addition to this, the results again show that the vertical line of the hybrid materials were very close to 90° compared to pristine Gr. These results indicate that the exact characteristics of a pure capacitor. Furthermore, it was found that the resistance of all the samples were decreased with increasing temperature. This property is due to the presence of dipolar displacement due to the increasing temperature. These results clearly show that the capacitance characteristics of Gr are increased with the addition of nano materials and increasing temperatures.

Fig. 6 (a): Nyquist plots of graphene. (b): Nyquist plots of GZ. (c): Nyquist plots of GCZ.

The bulk resistance (R_b), bulk capacitance (C_b), relaxation time (τ) and resonance frequency (ω) were measured from the Nyquist plot. The bulk resistance values were measured from the diameter of the semicircles in the Nyquist plots. The maximum peak value of the imaginary part of impedance specifies the resonance frequency. The resonance frequency (ω) is given by the equation

ω = 1/τ

where the relaxation time (τ) is calculated using the equation τ = C_bR_b.

The values of C_b are calculated from the frequency corresponding to the peaks of the semicircle.

The parameters such as R_b, C_b, τ and ω measured from the Nyquist plot is shown in Table 1. The results show that, the bulk resistance of Gr is gradually decreased with the addition of nanomaterials. In addition to this the bulk resistance decreased with increasing temperature. At the same time the bulk capacitance of Gr is increased with the addition of nanomaterials. The bulk capacitance is dramatically increased with increasing temperatures. The results indicate that the presence of ZnO and CZ nanomaterials increased the electron transfer from nanomaterials to Gr. The results again show that the addition of ZnO and CZ nanomaterials reduces the relaxation time of Gr. This is due to the presence of excited free charge carriers in the space charge region. In addition to this the resonance frequency of Gr is shifted towards the higher frequency region by the addition of ZnO and CZ nanomaterials. The results indicate the increased the movement of charge carriers in the sample by the doping of nanomaterials. In addition to this the resonance frequency shifted towards higher frequency region as the temperature increased from room temperature to 80 °C due to the increased mobility of charge carriers in the lattice. The results again confirmed that the resonance frequency affected the conductivity of the material. As the resonance frequency increases the materials become better conductors.

Table 1 Nyquist plot parameters of all the samples

Sample	Rb (Ω)	Cb (F)	τ (s)	ω (Hz)
Gr (300 K)	9.40 × 10^3	1.88 × 10^−12	1.77 × 10^−8	1.20 × 10^7
Gr (313 K)	8.05 × 10^3	1.90 × 10^−12	1.53 × 10^−8	1.40 × 10^7
Gr (333 K)	7.19 × 10^3	1.92 × 10^−12	1.38 × 10^−8	1.50 × 10^7
Gr (353 K)	5.59 × 10^3	2.41 × 10^−12	1.35 × 10^−8	1.60 × 10^7
GZ (300 K)	4.68 × 10^3	1.90 × 10^−12	8.87 × 10^−9	2.30 × 10^7
GZ (313 K)	2.78 × 10^3	3.02 × 10^−12	8.41 × 10^−9	2.50 × 10^7
GZ (333 K)	2.09 × 10^3	3.86 × 10^−12	8.05 × 10^−9	2.60 × 10^7
GZ (353 K)	1.79 × 10^3	4.37 × 10^−12	7.81 × 10^−9	3.10 × 10^7
GCZ (300 K)	3.93 × 10^3	3.68 × 10^−12	7.45 × 10^−9	2.60 × 10^7
GCZ (313 K)	2.21 × 10^3	4.81 × 10^−12	6.69 × 10^−9	3.00 × 10^7
GCZ (333 K)	1.49 × 10^3	6.93 × 10^−12	5.75 × 10^−9	3.30 × 10^7
GCZ (353 K)	8.47 × 10^2	2.35 × 10^−11	3.70 × 10^−9	4.00 × 10^7

---

3.2.1.4. Dielectric permittivity. Fig. 7a–c shows the effect of frequency and temperature on dielectric permittivity (ε′) of Gr, GZ and GCZ. The dielectric permittivity generally related to the free dipoles oscillating in an external electrical field. The results show that the characteristic dielectric permittivity of the as synthesized samples was gradually decreased with increase in frequency is due to the polarization of the as synthesized sample lagging behind the applied electric field. The results again showed that the dielectric permittivity of as synthesized samples showed a higher value at lower frequencies. These results can be explained based on Maxwell–Wagner polarization theory.¹² According to this theory, the heterogeneous structure of hybrid nanocomposites consists of a grain boundary that separates conducting grains and resistive grains. The volume of grain boundary controls the dielectric permittivity at lower frequency. When the nanoparticles are subjected to electric field, migration of free charges occurred, which trapped the interfacial grain boundaries. Thus the dipole moments are formed at the boundaries which lead to the space charge polarisation and ion jump polarisation. However, as the frequency increases, the dipoles cannot move sufficiently which causes the deformational polarizability. Hence they may not orient themselves in the direction of the applied electric field which decreases the lowest value of dielectric permittivity.

Fig. 7 (a): Variation of dielectric permittivity (ε′) with frequency of graphene. (b): Variation of dielectric permittivity (ε′) with frequency of GZ. (c): Variation of dielectric permittivity (ε′) with frequency of GCZ.

The dielectric permittivity of as synthesized graphene is gradually increased with the presence of ZnO and CZ nanomaterials. This is related to the movement of electron charge carriers from ZnO and CZ nanomaterials to Gr. A comparative study of the dielectric permittivity of as synthesized samples with different graphene based hybrid materials taken from the reported results is shown in Table 2.^13–17 The comparative study shows that the as synthesized Gr, GZ and GCZ with very low concentration ratio at room temperature show better dielectric permittivity than the other hybrid materials with high concentration ratio. These results clearly show that the as synthesized ZnO and CZ nanomaterials can inhibit the self-aggregation of Gr and increase the transport of charge carriers from ZnO and CZ to Gr. The dielectric permittivity of the as synthesized samples was found to have increased with increasing temperatures.^18,19 This indicates that as the temperature increased, the number of thermally activated charge carriers increased. Hence the interfacial polarisation was obtained due to the increased electron exchange interactions. Thus the interfacial polarization is the main factor in increasing the dielectric permittivity of the as synthesized samples at low frequency and at higher temperatures.

Table 2 Dielectric permittivity and dielectric loss values of different graphene based hybrid nanocomposites at room temperature

Material	Concentration ratio	ε′	ε′′	Ref.
Gr–MnO2	1 : 16	300	130	13
Gr–Fe3O4	10 : 4	0.94	0.15	14
Gr–TiO2	2.4 : 4.8	5	10^3	15
Gr–Fe3O4	1 : 10	18	5.6	16
Gr–FeCo	1 : 5	16	5	17
Gr	As synthesized	1.09 × 10^6	0.10627
GZ	1 : 0.33 (as synthesized)	9.55 × 10^16	0.13364
GCZ	1 : 0.33 (as synthesized)	2.13 × 10^17	0.76153

---

3.2.1.5. Dielectric loss. The dielectric loss tangent (tan δ) shows the lagging of polarization due to the presence of applied electric field. The dielectric loss is equal to ε′′/ε′, where ε′′ is the resistive current and ε′ is the capacitive current of the samples. The dielectric loss tangent of nano materials as functions of frequency and temperatures are shown in Fig. 8a–c. It is shown that dielectric loss decreases with increase in frequency. At a lower frequency the samples show the highest dielectric loss values and are completely independent with frequency at a higher frequency region. This anomaly is explained on the basis of hopping of electron transfer and conductivity of the material.^20,21 At a low frequency region, more energy is required for the transfer of electrons between the Gr layers. Hence the dielectric loss is shown at a higher value. At a high frequency region which corresponds to low resistivity, less energy is required for the transfer of electrons between Gr layers. Moreover, the dielectric loss is increases with increasing temperature due to the transfer of thermally activated electrons. The doping of nano materials also affected the dielectric loss values.^22,23 The doped nanocomposites show the highest dielectric loss values as compared to the pristine Gr. The GCZ shows maximum dielectric loss values compared to GZ and Gr. This is due to the synergetic effect which provides the easy hopping of electrons from the CZ to Gr layers which promote the higher conductivity of the GCZ.

Fig. 8 (a): Variation of dielectric loss tangent (tan δ) with frequency of graphene. (b): Variation of dielectric loss tangent (tan δ) with frequency of GZ. (c): Variation of dielectric loss tangent (tan δ) with frequency of GCZ.

A comparative study of the dielectric loss values of as synthesized Gr, GZ and GCZ with different graphene based reported results are shown in Table 2. The results show that the as synthesized samples with very low concentration ratio exhibit better dielectric loss values than reported results. The results clearly confirmed the better interaction between as synthesized Gr and as synthesized nanomaterials and indicate that the better charge transfer interactions between them.

4. Conclusions

We have successfully synthesized graphene based novel hybrid nanocomposites through the process of incorporating with CuS and ZnO nanoparticles. The dielectric properties at various frequencies and temperatures of as synthesized GCZ was investigated and compared with as synthesized Gr and GZ. The as synthesized GCZ shows high dielectric permittivity and dielectric loss at a lower frequency and higher temperatures. The results indicated that the capacitance characteristics of GCZ are better than as synthesized GZ and Gr due to the better synergistic effect. Hence the results again confirmed that as synthesized GCZ is a promising material for capacitance applications.

Acknowledgements

Jini Varghese gratefully thanks the Department of Science and Technology, Government of India for financial support under Women Scientist-A Scheme (WOS-A scheme) (SR/WOS-A/CS-47/2012).

References

1. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
2. Q. Zeng, J. Cheng, L. Tang, X. Liu, Y. Liu, J. Li and J. Jiang, Adv. Funct. Mater., 2010, 20, 3366–3372.
3. G. Eda and M. Chhowalla, Nanoletters, 2009, 9(2), 814–818.
4. J. A. Mann and W. R. Dichtel, J. Phys. Chem. Lett., 2013, 4, 2649–2657.
5. H.-C. Cheng, R.-J. Shiue, C.-C. Tsai, W.-H. Wang and Y.-T. Chen, ACS Nano, 2011, 5(3), 2051–2059.
6. G. Zhou, D.-W. Wang, L.-C. Yin, N. Li, F. Li and H.-M. Cheng, ACS Nano, 2012, 6(4), 3214–3223.
7. J. Chen, H. Bi, S. Sun, Y. Tang, W. Zhao, T. Lin, D. Wan, F. Huang, X. Zhou, X. Xie and M. Jiang, ACS Appl. Mater. Interfaces, 2013, 5, 1408–1413.
8. Y. Zhang, J. Tian, H. Li, L. Wang, X. Qin, A. M. Asiri, A. O. Al-Youbi and X. Sun, Langmuir, 2012, 28, 12893–12900.
9. M. Basu, N. Garga and A. K. Ganguli, J. Mater. Chem. A, 2014, 2, 7517–7525.
10. B. V. Bhaskara Rao, P. Yadav, R. Aepuru, H. S. Panda, S. Ogale and S. N. Kale, Phys. Chem. Chem. Phys., 2015, 17, 18353.
11. Y. Feng, W. L. Li, J. P. Wang, J. H. Yinb and W. D. Fei, J. Mater. Chem. A, 2015, 3, 20313.
12. W. Liu, X. Yan, J. Langa and Q. Xue, J. Mater. Chem., 2012, 22, 8853–8861.
13. T. K. Gupta, B. P. Singh, V. N. Singh, S. Teotia, A. P. Singh, I. Elizabeth, S. R. Dhakate, S. K. Dhawanc and R. B. Mathura, J. Mater. Chem. A, 2014, 2, 4256.
14. C. Hu, Z. Mou, G. Lu, N. Chen, Z. Dong, M. Hua and L. Qu, Phys. Chem. Chem. Phys., 2013, 15, 13038.
15. C. Wu, X. Huang, L. Xie, X. Wu, J. Yu and P. Jiang, J. Mater. Chem., 2011, 21, 17729.
16. X. Zheng, J. Feng, Y. Zong, H. Miao, X. Hu, J. Baiab and X. Li, J. Mater. Chem. C, 2015, 3, 4452.
17. X. Li, J. Feng, Y. Du, J. Bai, H. Fan, H. Zhang, Y. Peng and L. Fashen, J. Mater. Chem. A, 2015, 3, 5535.
18. P. Thakur, A. Kool, B. Bagchi, N. A. Hoque, D. Sukhen and P. Nandy, Phys. Chem. Chem. Phys., 2015, 17, 13082.
19. R. Ma, V. Sharma, A. F. Baldwin, M. Tefferi, I. Offenbach, M. Cakmak and G. A. Sotzing, J. Mater. Chem. A, 2015, 3, 14845.
20. C. Wu, X. Huang, L. Xie, X. Wu, J. Yu and P. Jiang, J. Mater. Chem., 2011, 21, 17729–17736.
21. K. Ahmad, W. Pan and H. Wu, RSC Adv., 2015, 5, 33607–33614.
22. R. K. Jammula, S. Pittala, S. Srinath and V. V. S. S. Srikanth, Phys. Chem. Chem. Phys., 2015, 17, 17237.
23. D. Wang, T. Zhou, J.-W. Zha, J. Zhao, C.-Y. Shi and Z.-M. Dang, J. Mater. Chem. A, 2013, 1, 6162–6168.

---