Synthesis and excellent electromagnetic absorption properties of polypyrrole-reduced graphene oxide–Co3O4 nanocomposites

Synthesis and excellent electromagnetic absorption properties of polypyrrole-reduced graphene oxide–Co3O4 nanocomposites

Journal of Alloys and Compounds 573 (2013) 151–156 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepa...

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Journal of Alloys and Compounds 573 (2013) 151–156

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Synthesis and excellent electromagnetic absorption properties of polypyrrole-reduced graphene oxide–Co3O4 nanocomposites Panbo Liu, Ying Huang ⇑, Lei Wang, Wei Zhang Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an 710129, PR China

a r t i c l e

i n f o

Article history: Received 17 January 2013 Received in revised form 2 March 2013 Accepted 29 March 2013 Available online 8 April 2013 Keywords: Polypyrrole Reduced graphene oxide Nanoparticles Nanocomposites Microwave absorption property

a b s t r a c t A novel kind of polypyrrole-reduced graphene oxide–Co3O4 (PPy–RGO–Co3O4) nanocomposites was firstly synthesized by a three-step method. The results indicate that small Co3O4 nanoparticles with the sizes in the range of 10–30 nm are anchored on the surface of PPy-RGO. The results demonstrate that the maximum reflection loss of PPy–RGO–Co3O4 is 33.5 dB at 15.8 GHz with a thickness of 2.5 mm and the absorption bandwidth with the reflection loss below 10 dB is up to 11.4 GHz (from 6.6 to 18.0 GHz) with a thickness in the range of 2–4 mm, suggesting that the microwave absorption properties and the absorption bandwidth are obviously enhanced by adding Co3O4 nanoparticles. Such strong microwave absorption materials could be used as a kind of candidate for the new types of microwave absorbing materials. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Electromagnetic (EM) wave absorbing materials have attracted much attention owing to their prospective applications. Most of the microwave absorbing materials are composed of magnetic loss powders such as ferrite [1], nickel [2,3], cobalt [4] and dielectric loss materials such as carbon nanotubes [5–7] and conducting polymers [8,9]. Compared with ferrite materials, carbon-based composites own advantages, such as low density and high complex permittivity value, which may improve the microwave absorption property and electromagnetic interference shielding effect. Reduced graphene oxide (RGO) is a two-dimensional sheet of carbon material. The development of RGO has attracted much attention due to its remarkable physical and chemical properties [10]. However, the good electric conductivity makes the microwave absorption property extremely weak [11]. According to the EM energy conversion principle, a proper matching between the dielectric loss and the magnetic loss determines the reflection and attenuation characteristics of EM absorbers. Some reports demonstrate that the combination of magnetic nanoparticles with RGO would enhance its microwave absorbing property [12,13], but the relative complementarities between the dielectric loss and the magnetic loss are not so well that the microwave absorption of the composites is mainly from dielectric loss. Polypyrrole (PPy), as one of the most promising conducting polymers, has drawn more attention due to its high electrical conductivity, excellent environ⇑ Corresponding author. Tel.: +86 29 88431636. E-mail address: [email protected] (Y. Huang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.280

mental stability, relatively low density and easy preparation [14]. PPy with magnetic nanoparticles have been prepared and the composites exhibit enhanced EM absorption properties [15,16]. However, to the best of our knowledge, the EM absorption properties of the three component composites consist of RGO, PPy and magnetic nanoparticles has not been reported. Therefore, we were motivated to prepare the PPy–RGO–Co3O4 nanocomposites with the relative complementarities between the dielectric loss and the magnetic loss in order to improve the microwave absorption property of the two component composites. In this paper, we firstly synthesized a novel kind of PPy–RGO– Co3O4 nanocomposites by a three-step method. The synthesized nanocomposites were characterized by FT-IR, XRD, Raman spectra, XPS and TEM. The maximum reflection loss of the PPy–RGO–Co3O4 nanocomposites is 33.5 dB at 15.8 GHz with a thickness of 2.5 mm and the absorption bandwidth with the reflection loss below 10 dB is up to 11.4 GHz (from 6.6 to 18.0 GHz) with a thickness in the range of 2–4 mm. We believe that the synthetic strategy may afford a new vision to prepare three component composites and the nanocomposites can be a possible candidate for the new microwave absorbing materials.

2. Experimental 2.1. Synthesis of PPy–RGO–Co3O4 nanocomposites Graphene oxide (GO) was synthesized by Hummers method [17]. GO solution (1 mg/mL) was achieved by ultrasonication in water for 2 h. Firstly, a given amount of pyrrole monomer dissolved in GO solution (100 mL) with concentrated H2SO4 (2 mL) was added. The mixture was continuously sonicated for 2 h. After cooling

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Fig. 1. FT-IR spectra of GO, RGO and PPy–RGO–Co3O4.

to 5 °C, 0.95 g of (NH4)2S2O8 dissolved in 30 mL of deionic water was added. The mixture was stirred overnight and the resulting precipitates were washed with deionic water. Secondly, the precipitates with deionic water was ultrasonicated for 2 h and an aqueous solution of CoCl26H2O (1.4 g) was added. Then, an aqueous solution of NaOH (0.94 g) was added and stirred for another 2 h. The solid was obtained via centrifugal method and then dispersed into deionized water (25 mL). H2O2 (30%, 4 mL) was added to the above mixture and the mixture was sealed into a Teflon-lined autoclave, heated to 160 °C for 24 h. The solid product was collected followed by suspending in deionized water. Thirdly, hydrazine monohydrate (0.1 mL) was added to the above suspension and heated to 95 °C for 12 h. The obtained product was washed with water and dried in a vacuum oven at 80 °C. PPyRGO composites were prepared in the same way in the absence of CoCl26H2O. RGO was obtained from GO by reduction with hydrazine monohydrate via a similar process. 2.2. Characterization Fourier-transform infrared spectra (FTIR) of the samples were recorded on iS10 (USA). XRD were identified by X-ray powder diffraction with Cu Ka radiation (XRD, Philips X-ray diffractometer, PW3040). Raman scattering was performed on a Jobin–Yvon HR800 Raman spectrometer. X-ray photoelectron spectroscopy (XPS, Thermal Scientific K Alpha) was performed with a Phoibos 100 spectrometer. The morphology was observed by field emission transmission electron microscope (TEM: Tecnai F30 G2). The relative complex permittivity (e0 and e00 ) and permeability (l0 and l00 ) were carried out by a HP8753D vector network analyzer at the frequency range of 2–18 GHz. The samples were prepared by uniformly mixing 50 wt.% of the sample with 50 wt.% of paraffin wax. The mixture was then pressed into toroidal shaped mould (H: 2.0 mm, Uout: 7.0 mm, uin: 3.0 mm).

3. Results and discussion Fig. 1 shows FT-IR spectra of GO, RGO and PPy–RGO–Co3O4. For GO, the peaks at 1730, 1224 and 1065 cm1 are associated with [email protected] group, epoxy CAO and alkoxy CAO stretching vibration, respectively. For RGO, most of the peaks related with the oxygen-containing functional groups vanished, indicating that the chemical reduction of GO was complete. For PPy–RGO–Co3O4, it is clearly seen that the characteristic peaks located at 1549 and 1468 cm1 are due to the symmetric and antisymmetric stretching vibrations of PPy rings, indicating the formation of PPy [18]. The broaden intense peak at 1549 cm1 is due to the better interaction between the aromatic ring of PPy and RGO, which indicates the formation of coating PPy onto RGO [19,20]. The strong peaks near 1305 and 1043 cm1 are ascribed to CAN and CAH stretching vibrations, respectively. The peaks centered at 1194 and 914 cm1 indicates the doping state of PPy [21]. Furthermore, the strong and sharp absorption peaks at 597 and 660 cm1 can be assigned to the vibrations of the CoAO, confirming the existence of Co3O4 nanoparticles [22]. The structure of RGO, PPy-RGO and PPy–RGO–Co3O4 are investigated by XRD in Fig. 2. For RGO, the diffraction peak at 2h = 23.8°

Fig. 2. XRD patterns of RGO, PPy-RGO and PPy–RGO–Co3O4.

can be attributed to the gaphite-like structure (002) with an interlayer spacing of 0.37 nm, suggesting the reduction of GO. For PPyRGO, the broad peak shifts from 23.8° to 25.1°, corresponding to an interlayer spacing of 0.35 nm, which is associated with the closest distance of PPy [23], implying that PPy and RGO have been completely interacted. For PPy–RGO–Co3O4, the diffraction peaks appearing at 19.2°, 31.7°, 37.0°, 38.3°, 45.1°, 56.1°, 59.6° and 65.6° can be perfectly assigned to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystal planes of the cubic spinel Co3O4 (JCPDS No. 43-1003). The relatively weak intensities of those peaks demonstrate the Co3O4 nanoparticles are small. No peaks have been detected, indicating the high purity of the PPy–RGO– Co3O4 nanocomposites successfully synthesized. The Raman spectra of RGO and PPy–RGO–Co3O4 are shown in Fig. 3. RGO shows two prominent bands at 1582 cm1 and 1349 cm1 which are contributed to the D and G band, the G band corresponding to sp2-hybridized carbon and the D band originating from disordered carbon [24]. The Raman spectrum of PPy–RGO– Co3O4 shows three peaks at 1053 cm1, 970 and 921 cm1, which are associated with the CAH in-plane deformation, the quinoid polaronic and bipolaronic structure of PPy, respectively [25], indicating the successful formation of PPy. Compared with RGO, the enhanced intensity of PPy-RGOACo3O4 for the band around 1349 cm1 indicates the interaction between PPy and RGO [26]. Furthermore, the additive peaks at 191, 491 and 596 cm1 can be attributed to the F2g mode of Co3O4, the peaks at 465 and 669 cm1 can be attributed to the Eg and A1g modes of Co3O4,

Fig. 3. Raman spectra of RGO and PPy–RGO–Co3O4.

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Fig. 4. (a) C 1s XPS spectra of GO and PPy–RGO–Co3O4; (b) XPS spectrum of PPy–RGO–Co3O4; Co 2p (c) and O 1s (d) XPS spectra of PPy–RGO–Co3O4.

respectively [27,28]. These results demonstrate the formation of PPy–RGO–Co3O4. In order to check the chemical composition and structure of the composites, the samples were characterized by XPS measurements and the corresponding results are presented in Fig. 4. The C1s spectra of GO in Fig. 4a can be deconvoluted into four differ-

ent peaks. The peaks at 284.6, 286.4, 287.8 and 289.3 eV have been assigned to the CAC/C = C in the aromatic rings, CAO of epoxy and alkoxy, C = O and OAC = O groups, respectively [29]. The C 1s XPS spectrum of PPy–RGO–Co3O4 (Fig. 4a) exhibits the same functionalities, but the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of CAO (epoxy

Fig. 5. TEM images of PPy-RGO (a) and PPy–RGO–Co3O4 (b), inset in (a) is the SAED pattern, (c) HRTEM image of PPy–RGO–Co3O4, inset is the SAED pattern, (d) EDS of PPy– RGO–Co3O4.

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Fig. 6. Frequency dependence of real and imaginary parts of the complex permittivity and the complex permeability of PPy-RGO (a) and PPy–RGO–Co3O4 (c), frequency dependence of dielectric loss tangents and magnetic loss tangents of PPy-RGO (b) and PPy–RGO–Co3O4 (d), the reflection loss of PPy-RGO (e) and PPy–RGO–Co3O4 (f).

and alkoxy), decrease dramatically, indicating that the functional groups are removed [30]. Meanwhile, a new peak centered at 285.6 eV from CAN group suggesting the presence of PPy. In Fig. 4b, XPS spectra of PPy–RGO–Co3O4 shows sharp peaks at 284.8, 399.4, 531.4 and 782.6 eV correspond to the characteristic peaks of C 1s, N 1s, O 1s, and Co 2p, respectively, indicating the existence of carbon, nitrogen, oxygen, and cobalt elements in the sample. The Co 2p XPS spectra of PPy–RGO–Co3O4 (Fig. 4c) exhibits two peaks at 781.3 and 797.4 eV, which are assigned to the binding energy of Co 2p3/2 and Co 2p1/2, respectively [31–33]. Two shake-up satellite peaks with the splitting of 6 eV above the main peaks are further suggest the formation of the Co3O4 nanoparticles [34,35]. The deconvoluted O 1s spectrum in Fig. 4d displays two peaks at 530.1 and 532.0 eV, which can be assigned to the lattice oxygen in the CoAO phase and the oxygen of the hydroxide ions, respectively [36,37]. The results indicate that the successful assembly of Co3O4 nanocrystals on PPy-RGO, in accordance with the Raman results.

The morphology of PPy-RGO and PPy-RGOACo3O4 were characterized by TEM in Fig. 5. PPy-RGO in Fig. 5a exhibits a paper-like morphology and some wrinkles are uniformly distributed on the surface. Because PPy is an amorphous material, no SAED pattern is observed, the selected area electron diffraction (SAED) pattern (inset in Fig. 5a) indicates that PPy-RGO is a lack of crystalline character and the SAED spots of RGO has disappeared, which is due to the uniform coating of PPy on the surface of RGO [38], the result is consistent with the Raman analysis. From Fig. 5b, we can see1 that small Co3O4 nanoparticles (green circles) with the sizes in the range of 10–30 nm are dispersed uniformly on the surface of PPy-RGO. Fig 5c is the HRTEM image of PPy–RGO–Co3O4 (blue region in Fig. 5b). It indicates that the lattice image of Co3O4 is about 0.24 nm, corresponding to the (3 1 1) plane of Co3O4 nanoparticles, while the lattices as shown are oriented to different directions randomly. These 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.

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results indicate the polycrystalline feature of Co3O4 nanoparticles. The SAED pattern (inset in Fig. 5c) further clearly demonstrates their polycrystalline feature. Fig. 5d shows the EDS spectra of PPy–RGO– Co3O4. The results confirm the presence of Co, C, and O elements in the nanocomposites. To understand the possible microwave absorption mechanisms, the complex permittivity real part (e0 ) and imaginary parts (e00 ), the complex permeability real part (l0 ) and imaginary parts (l00 ) are show in Fig. 6. For PPy-RGO in Fig. 6a, it is observed that both e0 and e00 values are high while both l0 and l00 values are low, which is due to the high electrical conductivity of PPy coated on the surface of RGO and the weak magnetic characteristic of the composites. The dielectric loss tangent (tan de = e00 /e0 ) and magnetic loss tangent (tan dl = l00 /l0 ) of PPy-RGO are shown in Fig. 6b. It can be seen the values of tan de and tan dl vary in the ranges of 0.17– 1.88 and 0.05–1.03, respectively, over 2–18 GHz range. Due to weak magnetism, we can infer that dielectric loss is the main microwave absorbing mechanism of PPy-RGO. For PPy–RGO– Co3O4 (Fig. 6c), both e0 and e00 values decrease gradually with several small fluctuations in the frequency range of 2–18 GHz, but all of them are much low than that of PPy-RGO. While the values of l0 and l00 values vary little from 0 to 1, similar to those of PPy-RGO. From Fig. 6d, we can seen that the dielectric loss tangents (tan de) varies from 0.85 to 0.45 while the maximum values of (tan dl) is 0.58, thus the relatively complementarities between the dielectric loss and the magnetic loss may improve the microwave absorption performance of the PPy–RGO–Co3O4 nanocomposites [39]. To clarify the EM absorption properties, the reflection losses (RL) are calculated according to

  Z in  1  RL ðdBÞ ¼ 20 log  Z in þ 1 Z in ¼

qffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi lr =er tanh½jð2pfd=cÞ er lr 

ð1Þ

ð2Þ

where Zin is the input impedance of the absorber, c is the velocity of electromagnetic waves in free space, f is the frequency, and d is the layer thickness. As shown in Fig. 6e, the maximum reflection loss of PPy-RGO is only 6.1 dB at 13.8 GHz with a thickness of only 1 mm. However, the adding of Co3O4 nanoparticles significantly improved the EM absorption properties of PPy-RGO. As shown in Fig. 6f, it can been seen that the maximum RL of PPy– RGO–Co3O4 is 33.5 dB at 15.8 GHz with a thickness of 2.5 mm. Based on previous reports [40], apart from dielectric loss and magnetic loss, another important concept relating to excellent microwave absorption is strongly dependent on the efficient complementarities between the relative permittivity and permeability [41], too high permittivity of absorber is harmful to the impedance match and results in strong reflection and weak absorption [5]. The presence of Co3O4 nanoparticles can improve the impedance match characteristic, which is the reason why PPy–RGO–Co3O4 with low permittivity exhibits stronger microwave absorption than PPy-RGO. Compared with other nanocomposites [8,42–44], PPy–RGO–Co3O4 exhibit a better microwave absorption performance and the absorption bandwidth with the RL below 10 dB is up to 11.4 GHz (from 6.6 to 18.0 GHz) with a thickness in the range of 2–4 mm, which is wider than that in previously reported results [12,45]. Taken the above together, it can be inferred that the PPy–RGO–Co3O4 nanocomposites can be used as EM absorbing materials with good absorption properties. 4. Conclusions Polypyrrole-reduced graphene oxide–Co3O4 (PPy–RGO–Co3O4) nanocomposites were firstly synthesized by a three-step method.

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The Co3O4 nanoparticles with the sizes in the range of 10–30 nm are anchored onto PPy-RGO. The PPy–RGO–Co3O4 nanocomposites have a relatively complementarities between the dielectric loss and the magnetic loss and the EM absorption properties have been improved. The maximum reflection loss of the nanocomposites is 33.5 dB at 15.8 GHz with a thickness of 2.5 mm, and the absorption bandwidth with the reflection loss below 10 dB is up to 11.4 GHz (from 6.6 to 18.0 GHz) with a thickness in the range of 2–4 mm. Thus, the PPy–RGO–Co3O4PPy–RGO–Co3O4 nanocomposites could be used as a kind of candidate absorber. Acknowledgements This work was supported by the Spaceflight Foundation of the People’s Republic of China under Grant no. NBXW0001. This work was supported by the Spaceflight Innovation Foundation of China under Grant no. NBXT0002. References [1] N. Chen, K. Yang, M.Y. Gu, J. Alloys Comp. 490 (2010) 609. [2] Y.Q. Kang, M.S. Cao, J. Yuan, L. Zhang, B. Wen, X.Y. Fang, J. Alloys Comp. 495 (2010) 254. [3] S.Y. Tong, M.J. Tung, W.S. Ko, Y.T. Huang, Y.P. Wang, L.C. Wang, J.M. Wu, J. Alloys Comp. 550 (2013) 39. [4] M.S. Cao, J. Yuan, X.Y. Fang, Appl. Phys. Lett. 95 (2009) 163108. [5] R.C. Che, L.M. Peng, X.F. Duan, Q. Chen, X.L. Liang, Adv. Mater. 16 (2004) 401. [6] H. Xu, S.M. Anlage, L. Hu, G. Grunera, Appl. Phys. Lett. 90 (2007) 183119. [7] M.S. Cao, J. Yang, W.L. Song, D.Q. Zhang, B. Wen, H.B. Jin, Z.L. Hou, J. Yuan, ACS Appl. Mater. Interfaces 4 (2012) 6949. [8] W.C. Zhou, X.J. Hu, X.X. Bai, S.Y. Zhou, C.H. Sun, J. Yan, P. Chen, ACS Appl. Mater. Interfaces 3 (2011) 3839. [9] K.Y. Chen, C. Xiang, L.C. Li, H.S. Qian, Q.S. Xiao, F. Xu, J. Mater. Chem. 22 (2012) 6449. [10] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [11] X.J. Han, P. Xu, X.L. Zhang, Y.C. Du, S.R. Hu, J.Y. Wang, X.H. Wang, Appl. Phys. Lett. 98 (2011) 072906. [12] H.L. Xu, H. Bi, R.B. Yang, J. Appl. Phys. 111 (2012) 07A522. [13] E.L. Ma, J.J. Li, N.Q. Zhao, E.Z. Liu, C.N. He, C.S. Shi, Mater. Lett. 91 (2013) 209. [14] J.F. Rubinson, Y.P. Kayinamura, Chem. Soc. Rev. 38 (2009) 3339. [15] Y.B. Li, G. Chen, Q.H. Li, G.Z. Qiu, X.H. Liu, J. Alloys Comp. 509 (2011) 4104. [16] P. Xu, X.J. Han, C. Wang, H.T. Zhao, J.Y. Wang, X.H. Wang, B. Zhang, J. Phys. Chem. B 112 (2008) 2775. [17] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [18] X.T. Zhang, J. Zhang, W.H. Song, Z.F. Liu, J. Phys. Chem. B 110 (2006) 1158. [19] S. Sahoo, G. Karthikeyan, G.C. Nayak, C.K. Das, Synth. Met. 161 (2011) 1713. [20] D.C. Zhang, X. Zhang, Y. Chen, P. Yu, C.H. Wang, Y.W. Ma, J. Power Sources 196 (2011) 5990. [21] J.T. Zhang, X.S. Zhao, J. Phys. Chem. C 116 (2012) 5420. [22] Z. Dong, Y.Y. Fu, Q. Han, Y.Y. Xu, H. Zhang, J. Phys. Chem. B 111 (2007) 18475. [23] S. Bose, T. Kuila, M.E. Uddin, N.H. Kim, A.K.T. Lau, J.H. Lee, Polymer 51 (2010) 5921. [24] Y.J. Yao, Z.H. Yang, H.Q. Sun, S.B. Wang, Ind. Eng. Chem. Res. 51 (2012) 14958. [25] S. Biswas, L.T. Drzal, Chem. Mater. 22 (2010) 5667. [26] V. Chandra, K.S. Kim, Chem. Commun. 47 (2011) 3942. [27] H. Kim, D.H. Seo, S.W. Kim, J. Kim, K. Kang, Carbon 49 (2011) 326. [28] J. Wu, Y. Xue, X. Yan, W.S. Yan, Q.M. Cheng, Y. Xie, Nano Res. 5 (2012) 521. [29] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558. [30] S. Dubin, S. Gilje, K. Wang, V.C. Tung, K. Cha, A.S. Hall, J. Farrar, R. Varshneya, Y. Yang, R.B. Kaner, ACS Nano 4 (2010) 3845. [31] B. Varghese, T.C. Hoong, Z. Yanwu, M.V. Reddy, B.V.R. Chowdari, A.T.S. Wee, T.B.C. Vincent, C.T. Lim, C.H. Sow, Adv. Funct. Mater. 17 (2007) 1932. [32] S.M. Paek, E.J. Yoo, I. Honma, Nano Lett. 9 (2009) 72. [33] X.B. Fan, W.C. Peng, Y. Li, X.Y. Li, S.L. Wang, G.L. Zhang, F.G. Zhang, Adv. Mater. 20 (2008) 4490. [34] C.C. Li, X.M. Yin, T.H. Wang, H.C. Zeng, Chem. Mater. 21 (2009) 4984. [35] H. Huang, W.J. Zhu, X.Y. Tao, Y. Xia, Z.Y. Yu, J.W. Fang, Y.P. Gan, W.K. Zhang, ACS Appl. Mater. Interfaces 4 (2012) 5974. [36] Z.S. Wu, W.C. Ren, L. Wen, L.B. Gao, J.P. Zhao, Z.P. Chen, G.M. Zhou, F. Li, H.M. Cheng, ACS Nano 4 (2010) 3187. [37] C.Z. Yuan, L. Yang, L.R. Hou, J.Y. Li, Y.X. Sun, X.G. Zhang, L.F. Shen, X.J. Lu, S.L. Xiong, X.W. Lou, Adv. Funct. Mater. 22 (2012) 2560. [38] Y. Zhao, J. Liu, Y. Hu, H.H. Cheng, C.G. Hu, C.C. Jiang, L. Jiang, A.Y. Cao, L.T. Qu, Adv. Mater. http://dx.doi.org/10.1002/adma.201203578. [39] M. Zhou, X. Zhang, J.M. Wei, S.L. Zhao, L. Wang, B.X. Feng, J. Phys. Chem. C 115 (2011) 1398. [40] P. Xu, X.J. Han, C. Wang, D.H. Zhou, Z.S. Lv, A.H. Wen, X.H. Wang, B. Zhang, J. Phys. Chem. B 112 (2008) 10443.

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