Synthesis of ZnO@Co2O3-Bi2O3-MnO core-shell structured nanoparticles for varistors applications

    Synthesis of ZnO@Co2 O3 -Bi2 O3 -MnO core-shell structured nanoparticles for varistors applications Mao-Hua Wang, Fu Zhou, Bo Zhang PII: DOI: Reference:

S0032-5910(14)00319-2 doi: 10.1016/j.powtec.2014.04.013 PTEC 10174

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Powder Technology

Received date: Revised date: Accepted date:

2 January 2014 26 March 2014 4 April 2014

Please cite this article as: Mao-Hua Wang, Fu Zhou, Bo Zhang, Synthesis of ZnO@Co2 O3 -Bi2 O3 -MnO core-shell structured nanoparticles for varistors applications, Powder Technology (2014), doi: 10.1016/j.powtec.2014.04.013

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ACCEPTED MANUSCRIPT Synthesis of ZnO@Co2O3-Bi2O3-MnO core-shell structured

Fu Zhou*

Bo Zhang

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Mao-Hua Wang

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nanoparticles for varistors applications

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School of Petrochemical Engineering，Changzhou University, Changzhou 213164；PR China Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, Changzhou 213164；PR China

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Abstract: In this paper, ZnO@Co2O3-Bi2O3-MnO nanocomposites with a homogeneous

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core-shell nanostructure have been synthesized by encapsulating Co2O3-Bi2O3-MnO on the surfaces of the as-prepared ZnO nanoparticles via an in situ precipitation method. Techniques

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of XRD, TEM, SEM and EDS have been employed to characterize the as-synthesized

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nanopowders and varistor samples. The results showed that ZnO nanoparticles were fully

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covered with additional metal oxides and Bi12Mn0.5Co0.5O20 phase was observed at the as-prepared ZnO nanopowders after calcined at 700℃. The varistor ceramics sintered at

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1100℃ for 2h in air have a density of 5.56g/cm3 corresponding to 96.2% of the theoretical density, with breakdown voltage of 3285 V/cm and nonlinear coefficient of ~5.5.

Keywords: Ceramics; Nanostructures; Chemical synthesis; Electrical properities. 1. Introduction Zinc oxide (ZnO) is a wide band-gap semiconductor oxide with a large excitation binding energy (60 meV) [1], it has been one of the most important functional materials with unique properties of electric conductivity [2], piezoelectricity [3], and photocatalytic activity [4], etc. ZnO varistors are semiconducting ceramics made from ZnO and other metal oxides such as Bi2O3, Sb2O3, Co2O3, Cr2O3 and MnO, etc. Due to their highly nonlinear current–voltage * Corresponding author E-mail address: [email protected]

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ACCEPTED MANUSCRIPT characteristics and energy handling capabilities, have been widely used as surge absorbers in small current electronic circuits as well as large current transmission lines for overvoltage

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protection [5~6].

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Homogeneous powder is necessary for the manufacture of high performance ZnO varistors, since the properties of ceramics are significantly influenced by the characteristics of preliminary powders [7]. However, homogeneous composite powders can hardly be obtained

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by conventional ball milling method, so several chemical methods have been developed in the

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past including sol-gel technique, microemulsion synthesis, thermal decomposition of organic precursor, homogeneous precipitation and so on [8~9]. Particularly, the methods to prepare

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core-shell structured nanomaterials have attracted more and more attention [10]. The

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nanoparticles with core-shell structure represent a new type of constructional units consisting

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of dissimilar compositional and structural domains[11-14]. Since core-shell structured nanomaterials have high stability, homogeneous morphologies and size, and controllable

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composition and surface structures, which can be fine tuned to satisfy specific applications [15~17], such as biological labels [18], optical resonances [19], catalysis [20], magnetics [21], ceramics [22], and pigments [23]. Generally, core-shell nanoparticles are composed of two or more than two types of matters, in which one behaves as the “core” and the other acts as the “shell”. In the present work, spherical monodisperse ZnO nanoparticles as the “core” were firstly prepared, then metal oxides (Co2O3-Bi2O3-MnO) (designated as CBMO) deposited on the surface of ZnO nanoparticles. Finally, the ZnO varistors were prepared using the as-synthesized ZnO@Co2O3-Bi2O3-MnO core-shell structured nanopowders.

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ACCEPTED MANUSCRIPT 2. Experiment procedure 2.1 Preparation of monodisperse ZnO nanoparticles

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Zn(NO3)2·6H2O (AR), Bi(NO3)3·5H2O (AR), CoCl2· 6H2O (AR), MnCl2· 4H2O (AR)

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was purchased from Shanghai Chemical Company. PEG-6000 and NaOH were purchased from Beijing Chemical Company. All reagents were of analytical grade and were used without further purification.

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A typical synthetic procedure of the spherical monodisperse ZnO nanoparticles is

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briefly shown as follows[24]: 0.03mol Zn(NO3)2·6H2O was dissolved by 30mL deionized water to obtain transparent Zn2+ aqueous solution, and 20mL polyethylene glycol (PEG-6000)

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was added as dispersant. Heating in water bath kept at 95℃ for 30min under stirring mildly.

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Then 50mL 4molL-1NaOH aqueous solution was quickly poured into the mixed solution

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under vigorous stirring, which immediately produced white precipitations in the container. After stirring for 2h, the as-produced precipitates were filtered, washed with alcohol and

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deionized water for several times, and then dried at 60℃ in a vacuum drier for 6h to obtain the final white ZnO samples. 2.2 Fabrication of monodisperse ZnO@CBMO core-shell nanoparticles The ZnO mixed powders prepared in the present study contained 95%ZnO, 1.5%Co2O3, 2%Bi2O3, and 1.5%MnO (all compositions in mol %). For the solution-coating route, the starting materials for the production of ZnO varistor powder included ZnO (as-synthesized), Bi(NO3)3·5H2O (AR), CoCl2· 6H2O(AR), MnCl2· 4H2O (AR). A mixed alcohol solution containing the required amounts of metal salts was prepared. First, the as-synthesized ZnO nanoparticles were suspended in distilled water and pH value was controlled at 8.5 by 5%

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ACCEPTED MANUSCRIPT ammonia. The suspension system was stirred at 70℃ for 30 min. Then the mixed alcohol solution was droped into the ZnO suspension solution and pH value was still controlled at 8.5

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by 5% ammonia. The whole system was kept in a water bath at about 70℃ with constant

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stirring for 2h. After stirring, the as-produced precipitates were filtered, washed with alcohol and deionized water for several times, and then dried at 60℃ in a vacuum drier for 6h to obtain the final brown samples. Then the ZnO composite powders were calcined at 600℃,

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2.3 Preparation of the ZnO-based varistor

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700℃, 800℃ for 2 h respectively to obtain the ZnO@CBMO core-shell nanopowders.

The powders were pulverized into fine particles and formed into a pellet of ~1.1mm

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thickness and ~11.0mm diameter under a uniaxial pressure of 40MPa. Then green compacts

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were sintered at 1100℃ for 2h in static air with heating and cooling rate of 4℃/min, and then

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furnace-cooled. Silver paste was printed on both sides of the sintered samples, and then calcinated at 500℃ in air for 15 min in preparation for the experiment to determine its

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electrical characteristics.

2.4 Samples characterization The ZnO powders were determined by X-ray diffraction (XRD; Rigaku D/MAX-YA) using Cu Ka radiation, λ= 0.154nm, scans were performed from (2θ) 10º to 80º by rate 5º/ min. The microstructures of powders were observed using a transmission electron microscopy (TEM; JEM-2100, Japan) operated at 200kV accelerating voltage and scanning electron microscopy (SEM; JSM-6360LA, Japan). The Zeta potentials were measured by Nanometer laser particle size and zeta potential analyzer (ZEN 3600, England). Voltage-current response was measured using a DC power supply (CJ1001, China).

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ACCEPTED MANUSCRIPT The current-voltage (V-I) characteristics were determined at room temperature using a variable dc power supply. The breakdown voltage was measured at 1.0 A/cm2. The nonlinear

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exponent (α) is defined by α = (logI2-logI1)/ (logV2-logV1) [25], where V1 and V2 are the

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breakdown voltages responding to I1=0.1A/cm2 and I2=1A/cm2. 3. Results and discussion

Fig.1 (a) shows typical XRD patterns of the as-synthesized ZnO nanoparticles. All the

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intensive and sharp reflection peaks can be indexed to ZnO reported in JCPDS card

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(NO.36-1451). The FT-infrared (FTIR) spectrum of the sample as shown in Fig.1 (b), there is a strong absorption peak at 3416.4 cm−1 which is arising from the hydroxy group and two

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peaks at 1634.7 cm-1, 1384.2 cm-1 corresponding to H2O, NO3- respectively，which are crucial

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for further surface functionalization. The nearly spherical monodisperse ZnO nanoparticles

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with particle size of about 30~50nm are clearly presented in Fig.1(c). The surface state of uncoated ZnO nanoparticles is smooth. The HRTEM image as shown in Fig.1 (d), the lattice

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fringes between two adjacent planes are about 0.27nm which is equal to the lattice constant of the ZnO and it indicates that the obtained structure having a wurtzite hexagonal phase [26].

Fig.1 Characterization of as-synthesized monodisperse ZnO nanoparticles: (a) XRD patterns; (b) FTIR spectra; (c) TEM image; (d) HR-TEM image.

The morphology of ZnO nanoparticles after nano-coating is observed by TEM as shown in Fig.2. The particle size ranging from 60 to 70nm is bigger than that of bare ZnO nanoparticles as shown in Fig.1(c). There is a thin light shell on the surface of the ZnO

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ACCEPTED MANUSCRIPT nanoparticles with the thickness of just a few nanometers. From the TEM image, we can see that the thin CBMO shell of the ZnO@CBMO sample is amorphous in character, indicating

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that the ZnO particles are coated with additional homogeneous metal oxides.

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Fig.2 (a)/(b) The morphology of ZnO powder after nano-coating.

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Fig. 3 presents XRD patterns of the as-synthesized core-shell nanoparticles before and after calcinations. All of the diffraction peaks labeled with hollow triangle in Fig.3 (a) can be

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exactly indexed as the hexagonal wurtzite ZnO, which is in good agreement with the values in

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the standard card (JCPDS 36-1451). It indicates that the precursors before calcinations are

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mainly composed of ZnO. Some extra weakly peaks are observed in addition to the peaks of main ZnO matrix phase in Fig.3 (b) and become more obvious and sharp as shown in Fig.3 (c)

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and Fig.3 (d) when the annealing temperature was elevated to 700℃ and 800℃. The peaks labeled with filled triangle in Fig.3 (c) can be indexed as the compound Bi12Mn0.5Co0.5O20 according to the standard card (JCPDS 40-1044).

Fig.3 XRD patterns of the as- synthesized ZnO-based core-shell nanoparticles before (a) and after calcined at 600℃ (b), 700℃ (c) and 800℃ (d), respectively.

To study the surface properties of the bare and coated ZnO nanoparticles, the electrokinetics of the particles was measured as zeta potentials as a function of pH value and

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ACCEPTED MANUSCRIPT the results are presented in Fig.4. Above pH 3.0, the zeta potential of ZnO@CBMO core-shell nanoparticles is more negative than that of the bare ZnO, indicating that the coated specimen

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is more negatively charged than the uncoated one. The nanoparticles of ZnO@CBMO are

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negatively charged over the whole pH value range examined. For comparison, the zeta potential of CBMO is also included. It is noted that there is an identical electrokinetic behavior between the ZnO@CBMO particles and CBMO powders over the pH range studied,

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depicting the similar surface properties between the ZnO@CBMO and CBMO particles, also

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confirming a full coverage of ZnO nanoparticles with CBMO.

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Fig.4 Zeta potential at different pH values of ZnO, ZnO@CBMO and CBMO

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nanopowders.

Based on the above results, the ZnO@CBMO core-shell nanoparticles are proposed to be

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formed via the following way, as shown schematically by Fig.5. Firstly, when ZnO nanoparticles were suspended in distilled water which pH value was controlled at 8.5 by 5% ammonia, part of NH 4 was coated on the surfaces of ZnO by virtue of the electrostatic attraction of NH 4 and ZnO nanoparticles due to their high specific surface area and abundant OH− on the surfaces [27]. Secondly, Co3+, Bi3+ and Mn2+ were preferentially deposited on the surfaces of ZnO in forms of ammonia complex ([Co(NH3)6]3+) or metal hydroxide ([Bi(OH)4]-, [Mn(OH)4]-). Finally, spherical ZnO@CBMO core-shell nanoparticles were obtained by the thermal decomposition process in air.

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ACCEPTED MANUSCRIPT Fig.5 The model about how ZnO@CBMO core-shell nanostructure is formed.

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Results of EDS spot analysis of ZnO varistors are presented in Fig.6. It can be seen that

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Co, Bi and Mn peaks are observed besides Zn which is the matrix. The weight content (wt %) of each element is shown in table. The result demonstrates that the metal elements (Co, Bi, Mn) are evenly distributed in the ZnO grain. SEM micrographs of varistor disc sintered at

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1100℃ are shown in Fig.7 (a), it can be intuitively seen that the microstructures appear

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homogeneous with a finer distribution of grain size and without any pores. The density of sintered pellets was 5.56g/cm3 corresponding to 96.2% of the theoretical density (TD) (pure

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ZnO, TD= 5.78g /cm3) [28]. The variation of current density as a function of the applied

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electric field for samples sintered at 1100℃ for 2h is shown in Fig.7 (b). The breakdown

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voltage is calculated about 328.5 Vmm-1 which is higher than that of conventional prepared varistors [29]. The nonlinear coefficient (α) of the varistor disc is calculated about 5.5.

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However, the nonlinear coefficient level is lower than that of some alternative varistor ceramic materials [30~31]. Further investigations are required to increase the nonlinear coefficient of the ceramics for further application in lightning arresters and surge absorbers.

Fig.6 EDS spectra of ZnO varistor disc sintered at 1100℃. Fig.7 (a) SEM image and (b) the nonlinear I-V characteristics of the ZnO varistors sintered at 1100℃ for 2h.

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ACCEPTED MANUSCRIPT 4. Conclusion In summary, the ZnO@CBMO core-shell nanostructure was fabricated successfully

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though a solution–coating route. A surface reaction and nucleation model is proposed for the

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growth of this structure. The ZnO nanoparticles were fully covered with additional metal oxides and Bi12Mn0.5Co0.5O20 phase was observed at the as-prepared ZnO nanopowders after calcined at 700℃. The metal elements (Co, Bi, Mn) were evenly distributed in the ZnO grain

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after calcined at 1100℃ for 2h. The varistor sample has a sintered density of 5.56g/ cm3

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corresponding to 96.2% of the theoretical density, with breakdown voltage of 3285V/cm and nonlinear coefficient of ~5.5.

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Acknowledgement

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This work was supported by Changzhou Science and Technology Innovation Project

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(CC20120031, CC20110048) and 2012 Research and Innovation Project for College

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Graduates of Jiangsu Province.

References

[1] H.W. Kim, H.S. Kim, H.G. Na, J.C. Yang, Vacuum 86 (2012) 789-793. [2] L.T. Tsai, S.P. Chiu, J.G. Lu, J.J. Lin, Nanotechnology 21 (2010) 145202-145208. [3] S.S. Lin, J.H. Song, Y.F. Lu, Z.L. Wang, Nanotechnology 20 (2009) 365703-365710. [4] C.W. Fang, J.M. Wu, L.T. Lee, H.H. Yeh, W.T. Wu, Y.H. Lin, et al, Electrochem Sol Stat Lett 13 (2010) K63-69. [5] C.C. Zhang, Y.X. Hu, W.Z. Lu, M.H. Cao, D.X. Zhou, J. Eur.Ceram. Soc. 22(2002) 61-65.

9

ACCEPTED MANUSCRIPT [6] J.C. Zhang, S.X. Cao, R.Y. Zhang, L.M. Yu, C. Jing, Curr. Appl. Phys. 5(2005) 381-386.

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[8] C.J. Zhong, M.M. Maye, Adv. Mater. 13 (2001) 1507-1511.

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[7] Y.K. Li, G.R. Li, Q.R. Yin, Mate. Sci. Eng., B 130 (2006) 264-268.

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[9] O. Masala, R. Seshadri, J. Am. Chem. Soc. 127 (2005) 9354-9355.

[10] O. Lehmann, K. Kompe, M. Haase, J. Am. Chem. Soc. 126 (2004) 14935-14942. [11] S.R. Hall, S.A. Davis, S. Mann, Langmuir 16 (2000) 1454-1458.

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[12] C.R.Vestal,Z.J.Zhang, J. Am. Chem. Soc. 124 (2002) 14312-14315.

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[13] B.Schreder,T.Schmidt,V.Ptatschek,U.Winkler,A.Materny,E.Umbach, M.Lerch, G.Mu1ller, W.Kiefer, L.Spanhel, J. Phys. Chem. B 104 (2000) 1677-1681.

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[14] Y.W. Cao, U. Banin, J. Am. Chem. Soc. 122 (2000)9692-9597.

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[15] J. I. Park, J. Cheon, J. Am. Chem. Soc. 123 (2001) 5743-5746.

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[16] S.H. Elder, F.M. Cot, Y. Su, et al., J. Am. Chem. Soc. 122 (2000) 5138-5146. [17] X. M. Sun, Y.D. Li, Angew. Chem. Int. Ed. 43 (2004) 597-601.

2013.

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[18] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998)

[19] P. Viravathana, D.W.M. Marr, J. Colloid Interface Sci. 221 (2000)301. [20] H. Tamai, T. Ikeya, F. Nishiyama, H. Yasuda, K. Iida, S. Nojima, J.Mater. Sci. 35 (2000) 4945. [21] J.J. Schneider, Adv. Mater. 13 (2001) 529. [22] W.J. Walker Jr., M.C. Brown, V.R.W. Amarakoon, J. Eur. Ceram. Soc.21 (2001) 2031. [23] R.L. DeColibus, U.S. Patent 3,928,057. [24] M.H. Wang, F.Zhou, B.Zhang, C.Yao, J. Alloys Compd. 581(2013) 308-312.

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[25] W.Onreabroy, T.Tunkasiri, N.Sirikulrat, Mater.Lett.59 (2005) 283-287.

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S. Shin, Biomass Bioenergy 39 (2012) 227-236.

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[26] R. Wahab, A. Mishra, S. Yun, I.H.Hwang, J. Mussarat, A. A.Al-Khedhairy, Y. S. Kim, H.

[27] W Li, H Cheng, J. Alloys Compd. 448 (2008) 287292-287297. [28] C.W. Nahm, Ceram. Int. 39 (2013) 2177-2121.

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[29] W.Onreabroy, N.Sirikulrat, A.P.Brown, C.Hammond, S.J.Miline, Solid State Ionics 177

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(2006) 411-416.

[30] Y.K. Li, G.R. Li, Q.R. Yin, Mater. Sci. Eng., B. 130 (2006) 264-268.

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AC

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(2008) 1697-1703.

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[31] Q.Wang, Y.Qin, G.J. Xu, L.Chen, Y.Li, L.Duan, Z.X. Li, Y.L. Li, P.Cui, Ceram. Int. 34

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ACCEPTED MANUSCRIPT Graphical abstract:

After coated

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Before coated

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It can be seen clearly that the surface of coated ZnO nanoparticles become more rough than that of bare ZnO nanoparticles.

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ACCEPTED MANUSCRIPT Highlights:  Monodisperse ZnO nanoparticles were first prepared by a direct precipitation method.  Core-shell nanostructure was fabricated successfully through solution–coating route.

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 The metal elements were evenly distributed in the ZnO grain after calcinations.

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 The breakdown voltage is higher than that of conventional prepared varistors.

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