ZnO nanoparticles' decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity

ZnO nanoparticles' decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity

    ZnO nanoparticles’ decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity Rama Krishn...

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    ZnO nanoparticles’ decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity Rama Krishna Jammula, Vadali V.S.S. Srikanth, Binoy Krishna Hazra, S. Srinath PII: DOI: Reference:

S0264-1275(16)31057-7 doi: 10.1016/j.matdes.2016.08.001 JMADE 2154

To appear in: Received date: Revised date: Accepted date:

6 May 2016 23 July 2016 1 August 2016

Please cite this article as: Rama Krishna Jammula, Vadali V.S.S. Srikanth, Binoy Krishna Hazra, S. Srinath, ZnO nanoparticles’ decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity, (2016), doi: 10.1016/j.matdes.2016.08.001

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ACCEPTED MANUSCRIPT ZnO Nanoparticles’ Decorated Reduced-Graphene Oxide: Easy Synthesis, Unique Polarization Behavior, and Ionic Conductivity

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Rama Krishna Jammulaa, Vadali V. S. S. Srikantha,, Binoy Krishna Hazrab, S. Srinathb School of Engineering Sciences and Technology (SEST), University of Hyderabad,

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Gachibowli, Hyderabad 500046, India

School of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, India

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Abstract

This work’s focus is on the easy synthesis, elucidation of material characteristics and

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unique polarization behavior of ZnO nanoparticles’ decorated r-GO. An optimized molecular-level mixing technique is used to synthesize ZnO decorated r-GO. The

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composite is found to exhibit high remnant polarization while its ionic conductivity

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follows the Arrhenius law. The observed polarization behavior is attributed to the lattice

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parameter disorder in ZnO combined with the presence of an appropriate number of functional groups in the composites. Moreover, a slight increase in r-GO content in the composites can increase the remnant polarization by one order of magnitude (i.e., from

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0.062 µC/cm2 to 0.232 µC/cm2) under the influence of an external electric field (~1.6 kV/cm). Experimental observations clearly indicate that the functionalized r-GO plays an important role (i.e., it facilitates easy dipolar alignment and reorientation) in polarization and increasing the number of stable polarization states with respect to the applied field direction. Keywords: Nanocomposites; Interfaces; Dielectrics; Functional; Surfaces; ZnO; Graphene. 

Author to whom correspondence should be addressed. Electronic mail:

[email protected] ; Phone No.: +91 40 23134453

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ACCEPTED MANUSCRIPT 1. Introduction One way to meet the ever growing demand for charge storage materials is to develop

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materials which exhibit unique polarization behavior [1-5]. In this context, excellent

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graphene-based heterostructured [1,2] and graphene oxide (GO) based hybrid [3-5]

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materials are developed. Graphene was used as an electrode material in a heterostructured junction that facilitated polarization stability and resistive switching by

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virtue of the molecular layer’s (here NH3 and H2O are tried) nature and behavior at the graphene/BaTiO3 interface [2]. GO and cellulose containing hybrid showed a great

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enhancement in polarization with the addition of GO [5]; this result was attributed to the optimal functionalization of GO, which in turn enabled its appropriate interactions with

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the cellulose matrix. In this context, other GO-based hybrids or composites can be

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studied. One such material is ZnO decorated reduced-graphene oxide (r-GO) composite

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[6] which showed an excellent dielectric behavior. The orientation polarization of the dipoles (in terms of relaxation times) on reduced graphene oxide (r-GO) surfaces as a function of its content and temperature was studied by fitting the experimental

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polarization data with Debye theory [7]; this is plausibly the reason to develop GO containing materials to attain control on the overall polarization behaviour through GO. Besides materials containing GO and ZnO, materials such as ZnO/MWCNTs composite [8] and nanoneedle-like ZnO [9] have also exhibited unique dielectric behavior

and

as

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consequence

exhibited

excellent

microwave

absorption

characteristics. Recently it was demonstrated by preparing small magnetic nanoparticles’ decorated r-GO that electromagnetic attenuation can be tuned through the synergistic effect of dielectric and magnetic losses [9]. However, the advantage of combining ZnO with r-GO is the possibility it gives to tune the dielectric behavior of

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ACCEPTED MANUSCRIPT the composite through the control over oxygen containing PFG in r-GO. PFG in r-GO could also help in controlling the morphology and crystal structure of ZnO, which is a

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poor polarizable material. In this work, it will be shown that the presence of an

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appropriate number of polar functional groups (PFG) along with defects and a slightly

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distorted ZnO lattice (owing to minuscule changes in synthesis conditions) help in tuning the polarization behavior of ZnO decorated r-GO composites. Also, the enhanced

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polarization and temperature-dependent ionic conductivity of ZnO decorated r-GO will be systematically explained.

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2. Synthesis procedure

ZnO decorated r-GO composites were synthesized by molecular-level mixing technique

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[6,11] using zinc acetate dehydrate (Zn(CH3COO)2.2H2O) and r-GO as starting

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materials. However, minuscule variations in synthesis conditions [6] are introduced in

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this work so as to tune the polarization behavior. In the 1st step, 0.1 g of r-GO powder was dispersed in 250 ml ethanol by sonication for 30 min to obtain a stable suspension. In the 2nd step, 0.9 g of zinc acetate dehydrate (Zn(CH3COO)2·2H2O) was added to the

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r-GO suspension and then ultrasonicated (at 700 W) for 3 h instead of 2 h as in [6]. In the 3rd step, the resultant solution was heated at 100 °C in air. In the final step which involves the calcination process, the product was heated at 400 °C (instead of 500 °C as in [6]) for 8 h in ambient atmosphere to obtain ZnO decorated r-GO named as ZnO0.1G. Two other composites named as ZnO-0.2G and ZnO-0.3G are also synthesized using 0.2 g of r-GO and 0.8 g of Zn(CH3COO)2·2H2O and 0.3 g of r-GO and 0.7 g Zn(CH3COO)2·2H2O, respectively. For comparison purposes, ZnO similar to that in the composites was also synthesized (please see supplementary material (SM)). Synthesis procedure of ZnO nanoparticles, reaction pathways leading to ZnO decorated r-GO

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ACCEPTED MANUSCRIPT formation, and other experimental details are included in SM. In the synthesis method mentioned above, an increase in the time of ultrasonication

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and the 2nd step were aimed to disperse Zn ions among the suspended r-GO sheets in

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ethanol and to promote the effective reaction between Zn ions and functional groups on

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r-GO sheets. This type of sonochemical synthesis of nanomaterials has been previously reported [12-14]. The enchantment is mainly attributed to the improved thermolysis of

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the solute and the formation of highly reactive radicals such as hydroxyl radicals, which create extreme reaction conditions in media such as ethanol. Also, the growth of the

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nucleated solid in the solution is inhibited, thereby increasing the total solid surface in contact with the solvent. The increase in the time of ultrasonication in the 2nd step also

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3. Results and discussion

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helped in decreasing the calcination temperature by 100 °C.

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XRD profiles (observed as well as calculated) of different samples are shown in Fig. 1. Except for (002) reflection at 26.4o, which corresponds to r-GO in composites (except in ZnO-0.1G, which plausibly did not have enough r-GO content that could diffract), all

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other diffraction peaks are indexed to hexagonal ZnO crystal structure. The calculated lattice parameters are a = 3.2523 Å, c = 5.2022 Å, and therefore c/a = 1.5995 in the case of ZnO-0.2G whilst a = 3.2514 Å, c= 5.2010 Å, and therefore c/a = 1.5996 in case of ZnO-0.3G (1.6008 in case of ZnO-0.1G). This confirms that ZnO lattice in composites is ‘squeezed’ along c-axis in comparison to ZnO nanoparticles (c/a ratio is ~1.602) [15]. Figure 2 shows the composites’ Raman spectra, the features in which are similar to those observed previously [10]. The features indicate the presence of partially reduced GO and ZnO in the composites. For convenience, r-GO’s Raman and FTIR spectra are shown in Fig. SM1 and Fig. SM2, respectively, along with the appropriate discussion.

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Figure 1. XRD profiles of (a) ZnO, (b) ZnO-0.1G (c) ZnO-0.2G and (b) ZnO-0.3G

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samples and the corresponding Rietveld refined profiles. Comparison of lattice

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parameters for different samples is given in Table SM1.

Figure 2. Raman spectra of (a) ZnO, (b) ZnO-0.1G (c) ZnO-0.2G and (b) ZnO-0.3G samples. 5

ACCEPTED MANUSCRIPT Raman band at 432 cm-1 (Fig. 2) corresponds to non-polar optical phonon E2 (high) while the band at 573 cm-1 is a typical band positioned between A1 (LO) + E1 (LO) of

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ZnO. The bands at 324 and 657 cm-1 are due to surface phonon scattering by ZnO. The

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band at 1125 cm-1 is a typical second order scattering mode of ZnO. The typical D

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(1341 cm-1), G (1565 cm-1) and 2D (2673 cm-1) bands corresponding to the presence of partially reduced GO in the composites are also observed. Further, electron microscope

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images (both TEM and FESEM images, Figs. SM3 and SM4, respectively) shows clearly the excellent decoration of ZnO nanoparticles on r-GO sheets. TEM micrographs

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(including the high-resolution micrograph) and a representative electron diffraction pattern of ZnO-0.3G sample are shown in Fig. SM3. From Figs. SM3(a) and SM3(b)

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the decoration of ZnO particles on r-GO can be clearly observed. It is also clear from

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these images that the sizes of ZnO nanoparticles decorating r-GO sheets are in the range

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of 10–20 nm with a narrow size distribution. In Fig. SM3(d), the lattice spacing of 0.24 nm that corresponds to (101) ZnO in ZnO-0.3G sample is marked. In Fig. SM3(c), the diffraction spots corresponding to the presence of r-GO in the composite are faintly

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visible and hence are not indexed. Good homogeneity of the ZnO nanoparticles’ decoration is clearly observed from low magnification electron micrographs as shown in Fig. SM4. Similar observations have also been made in our previous work [10]. It is now known that r-GO can be decked with several PFG such as -OH, -CHO, CO, -COOH, etc., and that C in r-GO will be in different oxidation state depending on the bond it makes (with H or O) [16]. In this condition, the ZnO nanoparticles decorating the r-GO sheets (as shown in electron micrographs) make contacts with different functional groups typically in the form of ZnO/PFG/GO which gives an opportunity for any free charge carriers from r-GO side to tunnel into ZnO lattice during

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ACCEPTED MANUSCRIPT the formation of the composite and push the ZnO lattice into a distorted condition, thereby the changes in c/a ratios of ZnO lattice in the case of composites as revealed by

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XRD Full-Prof fit shown in Fig. 1. In general, the lattice parameters were mainly

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influenced by Coulomb interactions between atoms or ionic species, oxidation states,

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presence of foreign atoms/ions or charge carriers, and ionic radius [17,18]. In the present study, since ZnO nanoparticles are decorated on functionalized r-GO sheets, a

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charge transfer might have occurred between ZnO and r-GO and thereby a distorted ZnO lattice was observed (as confirmed by XRD analysis). A similar observation was

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previously made in the case of doped ZnO [19-21]. It has been observed that, when the ionic radius of the dopant (for example, 0.60 Å for Li+1) is smaller than the host (0.74 Å

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for Zn+2), the substituents occupy off-centered positions causing distortion in the lattice

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and locally induce electric dipoles which eventually influence the polarization behavior.

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By this, in the present case, one can anticipate that ZnO/PFG/GO arrangement may exhibit unique polarization behavior in the presence of an electric field. To understand this, polarization (P) versus (Vs) electric field (E) measurements were carried out.

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The P Vs E of different samples is shown in Fig. 3 which clearly shows that pure ZnO exhibits poor polarization characteristics. P Vs E curve of ZnO sample indicates that the current and voltage are in-phase and therefore no capability of charge storage. On the contrary, the composites clearly exhibit P-E hysteresis revealing their charge storage capability. It is observed that the area of the hysteresis loop is significantly enhanced with increasing r-GO content in the composites. However, the slightly elliptical shape of the loops indicates that the resistive component of the nanocomposite is greater than the capacitive component. Careful observation of the P-E hysteresis loops of the composites at same field strength (~1.6 kV/cm) clearly shows that an increase in r-GO

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ACCEPTED MANUSCRIPT content in the composites resulted in the increase of remnant polarization (Pr) and maximum polarization (Pmax) by at least one order. These values are tabulated in Table

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SM2. In the case of ZnO-0.3G, Pr value increased to 2.402 µC/cm2 (Fig. SM5) at 11

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kV/cm. These results are consistent with the previous works [10,14] wherein materials

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showed high dielectric permittivity due to enhancement in polarization with increasing r-GO content. However, here the composites did not exhibit any discernible saturation

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polarization (i.e., the absence of ferroelectric behavior) [22] most plausibly owing to

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heat or leakage current losses.

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Figure 3. P-E hysteresis curves of ZnO and ZnO decorated r-GO composites. The above-presented observations strongly indicate that the presence of r-GO (and through which the presence of PFG) plays an important role in pushing the system into an easy polarization direction. Here it should be noted that the PFG [21,23] at appropriate positions on r-GO are also in contact with ZnO nanoparticles (i.e., ZnO/PFG/GO) and as r-GO content increases in the composites, the number of available functional groups will also increase. In the composite, owing to ZnO/PFG/GO arrangement at numerous locations, the electrical contacts between ZnO and r-GO are expected to be robust in nature ensuring a significant increase in the relative concentration of the dipole moments which in turn lead to easy dipolar alignment and 8

ACCEPTED MANUSCRIPT reorientation [24] and thereby ultimately increasing the number of stable polarization states with respect to the applied field direction as observed in Fig. 3. This can be

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understood on the basis of Valence Shell Electron-Pair Repulsion (VSEPR) theory [2]

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which, when applied to the present situation indicates that there can be different types of

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local bonding configurations that can induce dipole moments. The polarization behavior of ZnO decorated r-GO composites is similar to that of graphene-based heterostructured

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tunnel junction [2]. Hence the enhanced polarization in ZnO/r-GO composites is due to both distorted ZnO lattice and the orientation of PFG.

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In a previous report, it was shown that the conductivity (product of mobility and charge carrier density) of the composites can be enhanced by polarization [1]. In another

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report it was shown that the proton conductivity in GO along its surface is very high due

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to the presence of -OH, -COOH, and -CO functional groups [24]. These reports [1,24]

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indicate that there is a relation between conductivity and PFG. In view of this, it is quite interesting to observe the ionic conductivity (Fig. 4) in ZnO decorated r-GO composites in the temperature range 398-523 K. In general, the total conductivity (σ(ω)) is the sum

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of dc and ac conductivities [25], i.e., σ(ω) = σdc + Aωs, where A is the pre-exponential factor which depends on temperature, ω is the frequency, and s is a dimensionless exponent. The thermally activated hopping ions aid in dc conduction which is frequency independent and has long range order. Here, all the samples exhibit high-frequency dispersion with an essential part of frequency independent conductivity at low frequencies. The samples are found to switch over from frequency independent region to frequency dependent region showing an onset of conductivity relaxation. The temperature dependent conductivity followed the Arrhenius law [26] given by:

𝜎𝑑𝑐 𝛼 𝑇 −1 exp(

−𝐸𝑎 𝑘ᵦ 𝑇

) where T is temperature in Kelvin, 𝑘ᵦ is Boltzmann constant

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ACCEPTED MANUSCRIPT and Ea activation energy for dc conduction. The conductivity (σdc) plots (Fig. 5) i.e., ln(𝜎𝑑𝑐 𝑇) Vs 1000/T plots are fitted with linear function. σdc values were found to

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increase with temperature, and the linear fits of the plots of ln(σT) Vs 1000/T (shown in Fig. 5) revealed dc activation energies (Ea) of 0.581, 0.560 and 0.538 eV in the case of

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ZnO-0.3G, ZnO-0.2G and ZnO-0.1G composites, respectively. It is well-known that reduction of functional groups increases the fraction of sp2 carbon atoms and as a result

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conductivity increased in all the composites in comparison with ZnO [27]. Also, here the conductivity is depends on the temperature [27] and values are increased from ~10-3

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-1 cm-1 to 10-2 -1cm-1 (Table 1) with increasing temperature. These observations were consistent with previous reports on the conductivity studies of r-GO [25,28-30]. From

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Fig. 4 it can be clearly observed that in the frequency region 103-106 Hz, the

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conductivity values have uniformly increased with the temperature indicating that the

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conductivity is thermally activated.

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Figure 4. Conductivity Vs frequency plots of (a) ZnO-0.1G, (b) ZnO-0.2G and (c) ZnO-0.3G samples fitted with an appropriate power law (Figs. SM6 and SM7). The straight line in the figures indicates that the conductivity is thermally activated.

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Figure 5. ln(σT) Vs 1000/T for ZnO-0.1G, ZnO-2G, and ZnO-0.3G samples.

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Table 1. Conductivity (σdc) values of ZnO/r-GO composites at different temperatures. Conductivity (-1cm-1)

Temperature

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(Kelvin)

ZnO-0.2G

ZnO-0.3G

0.40 ×10-3

0.37×10-3

0.26 ×10-3

0.18×10-2

0.17×10-2

0.11×10-2

448

0.39×10-2

0.35×10-2

0.23×10-2

473

0.89×10-2

0.56×10-2

0.50×10-2

498

1.57×10-2

0.77×10-2

0.72×10-2

1.82×10-2

1.47×10-2

0.98×10-2

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423

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393

523

ZnO-0.1G

From Table 1 it can be easily noticed that the conductivity values showed a slightly decreasing trend with increasing r-GO concentration as a consequence of varied dc activation energy values. Here it should be noted that low activation energy leads to high σdc and vice versa. This implies that r-GO in the composite is only partially reduced as indicated by Raman scattering results If it is completely reduced, higher content of r-GO should result in higher σdc of the composite because σdc of r-GO is much higher than that of ZnO. On the contrary to the composites, ZnO did not exhibit

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ACCEPTED MANUSCRIPT the Arrhenius behavior (as shown in Fig. 6) while the conductivity values are found to

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be consistent with those reported in the literature [31,32].

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Figure 6. Conductivity Vs frequency plot of ZnO.

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The conductivity of the samples can be understood on the basis of scattering mechanism as reported in the literature [33,34] according to which the conductivity is eτ

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given by 𝜎 = 𝑛𝑒µ where µ = m∗ is mobility, a function of effective mass (𝑚∗ ) and relation time (𝜏) of charge carriers. 𝜏 is related to the scattering mechanism (SM) in 3𝑘𝛽 𝑇

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materials. It is known that τ is a function of vacancies and velocity (v) =√

𝑚∗

of the

charge carriers [33,35]. For normal semiconductors, the scattering possibility decreases with increasing temperature (directional mobility of charge carriers increases and thereby the conductivity increases). In polar semiconductors, the charge carriers might be scattered by long optical wave. Therefore, in polar semiconductors both intrinsic carrier concentration and mobility are responsible for conductivity, which increases with temperature as in the present case of the composites. With increasing r-GO the charge carrier concentration might be increased but the ions’ mobility (which depends on 𝜏) might be constrained on account of polarization. 13

ACCEPTED MANUSCRIPT Based on the above observations, the conductivity behavior of composites w.r.t increase in r-GO content in composites is justified in the following manner: Generally,

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conductivity (σ) depends on both mobility (µ) and charge carrier (ions) density (n) as it

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is the product of µ and n. In the case of the ZnO-0.1G sample, the conductivity is

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slightly high (low r-GO content) in comparison to other two samples. This implies that the mobility of ions is more elevated in the case of the ZnO-0.1G sample as reflected in

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the measured low activation energy (0.538 eV) in comparison to ZnO-0.2G (0.560 eV) and ZnO-0.3G (0.581 eV) samples which contain more functional groups (owing high r-

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GO content). This clearly indicates the ions or charge carriers in ZnO-0.3G and ZnO0.2G samples are constrained and unable to wander throughout the material (reflecting

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the charge carriers’ bound nature). This in turn has been reflected as high polarization

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(easy alignment and re-orientation) in these samples in comparison to ZnO-0.1G

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sample. 4. Conclusions

ZnO decorated r-GO composites are synthesized using a simple MLM technique.

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Structural and morphological analyses confirmed the formation of the composite. The calculated lattice parameters clearly showed that ZnO in the composites experiences a ‘squeezing effect’ which brings in the structural changes. Functional groups attached to r-GO facilitate the ‘squeezing effect’ and thereby the structural changes which are responsible for the unique polarization behavior of ZnO decorated r-GO composites. With the increase in the r-GO content in the composites, maximum polarization increased owing to easy dipolar alignment, reorientation, and stable polarization states. In all the composites ionic conductivity was measured in the order of 10-2 -1cm-1 and

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ACCEPTED MANUSCRIPT the calculated activation energies are consistent and support the polarization behavior in the ZnO decorated r-GO composites.

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Acknowledgements

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The authors sincerely thank Centre of Nanotechnology for allowing us to use the TEM

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facility. VVSSS thanks, SERB, DST, Government of India for providing moral support through the Fast Track Scheme (SERB/F/3487/2012-2013) for Young Scientists.

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References

[1] X. Hong, A. Posadas, K. Zou, C. Ahn, J. Zhu, High-mobility few-layer graphene

MA

field effect transistors fabricated on epitaxial ferroelectric gate oxides, Phys. Rev. Lett. 102 (2009) 136808-4.

D

[2] H. Lu, A. Lipatov, S. Ryu, D.J. Kim, H. Lee, M.Y. Zhuravlev, C.B. Eom, E.Y.

TE

Tsymbal, A. Sinitskii, A. Gruverman, Ferroelectric tunnel junctions with graphene

CE P

electrodes, Nat. Commun. 5 (2014) 5518. [3] G. Ni, Y. Zheng, S. Bae, C.Y. Tan, O. Kahya, J. Wu, B.H. Hong, K. Yao, B. Ozyilmaz, Graphene-ferroelectric hybrid structure for flexible transparent electrodes,

AC

ACS Nano 6 (2012) 3935-3942. [4] W. Jie, J. Hao, Graphene-based hybrid structures combined with functional materials of ferroelectrics and semiconductors, Nanoscale 6 (2014) 6346-6362. [5] A. Kafy, K.K. Sadasivuni, H.C. Kim, A. Akther, J. Kim, Designing flexible energy and

memory

storage

materials

using

cellulose

modified

graphene

oxide

nanocomposites, Phys. Chem. Chem. Phys. 17 (2015) 5923-5931. [6] R.K. Jammula, S. Pittala, S. Srinath, V.V.S.S. Srikanth, Strong interfacial polarization in ZnO decorated reduced-graphene oxide synthesized by molecular level mixing, Phys. Chem. Chem. Phys. 17 (2015) 17237-17245.

15

ACCEPTED MANUSCRIPT [7] B. Wen, M. Cao, M. Lu, W. Cao, H. Shi, J. Liu, X. Wang, H. Jin, X. Fang, W. Wang, J. Yuan, Reduced graphene oxides: Light-weight and high efficiency

T

electromagnetic interference shielding at elevated temperatures, Adv. Mater. 26 (2014)

IP

3484-3489.

SC R

[8] M.M. Lu, W.Q. Cao, H.L. Shi, X.Y. Fang, J. Yang, Z.L. Hou, H.B. Jin, W.Z. Wang, J. Yuan, M.S. Cao, Multi-wall carbon nanotubes decorated with ZnO nanocrystals: mild

NU

solution-process synthesis and highly efficient microwave absorption properties at elevated temperature, J. Mater. Chem. A 2 (2014) 10540-10547.

MA

[9] J. Liu, W.Q. Cao, H.B. Jin, J. Yuan, D.Q. Zhang, M.S. Cao, Enhanced permittivity and multi-region microwave absorption of nanoneedle-like ZnO in the X-band at

D

elevated temperature, J. Mater. Chem. C 3 (2015) 4670-4677.

TE

[10] J.Z. He, X.X. Wang, Y.L. Zhang, M.S. Cao, Small magnetic nanoparticles

CE P

decorating reduced graphene oxides to tune electromagnetic attenuation capacity, J. Mater. Chem. C, 2016, DOI: 10.1039/C6TC02020H. [11] R.K. Jammula, A. Tumuluri, N.K. Rotte, K.C. James Raju, V.V.S.S. Srikanth,

AC

Cupric oxide decked few-layered graphene: Synthesis and dielectric behaviour, Carbon 78 (2014) 374-383. [12] N. Mahmood, C. Zhang, F. Liu, J. Zhu, Y. Hou, Hybrid of [email protected] nanoparticles and nitrogen-doped graphene as a lithium ion battery anode, ACS Nano 7 (2013) 10307-10318. [13] H. Yin, C. Zhang, F. Liu and Y. Hou, Doped graphene: hybrid of iron nitride and nitrogen‐doped graphene aerogel as synergistic catalyst for oxygen reduction reaction, Adv. Funct. Mater. 24 (2014) 2929-2929.

16

ACCEPTED MANUSCRIPT [14] N. Mahmood, Y. Hou, Electrode nanostructures in lithium-based batteries, Adv. Sci. 1 (2014) 1400012.

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[15] R.R. Reeber, Lattice parameters of ZnO from 4.2 to 296 oK, J. Appl. Phys. 41

IP

(1970) 5063-5066.

SC R

[16] D.R. Dreyer, H.P. Jia, C.W. Bielawski, Graphene oxide: A convenient carbocatalyst for facilitating oxidation and hydration reactions, Angew. Chem. 122

NU

(2010) 6965-6968.

[17] D.J. Kim, Lattice parameters, ionic conductivities, and solubility limits in fluorite-

MA

structure MO2 oxide [M = Hf4+, Zr4+, Ce4+, Th4+, U4+] solid solutions, J. Am. Ceram. Soc. 72 (1989) 1415-1421.

D

[18] U. Pietsch, K. Unger, The influence of free carriers on the equilibrium lattice

TE

parameter of semiconductor materials, Physica status solidi (a) 80 (1983) 165-172.

CE P

[19] M.K. Gupta, B. Kumar, High Tc ferroelectricity in V-doped ZnO nanorods, J. Mater. Chem. 21 (2011) 14559-14562. [20] A. Onodera, N. Tamaki, K. Jini, H. Yamashita, Ferroelectric properties in

AC

piezoelectric semiconductor Zn1-x Mx O (M= Li, Mg), Jpn. J. Appl. Phys. 36 (1997) 6008-6011.

[21] M.K. Gupta, N. Sinha, B.K. Singh, B. Kumar, Synthesis of K-doped p-type ZnO nanorods along (100) for ferroelectric and dielectric applications, Mater. Lett. 64 (2010) 1825-1828. [22] J.F. Scott, Ferroelectrics go bananas, J. Phys.: Condens. Matt. 20 (2008) 021001. [23] G.I. Titelman, V. Gelman, S. Bron, R.L. Khaflin, Y. Cohen, H.B. Peled, Characteristics and microstructure of aqueous colloidal dispersions of graphite oxide, Carbon 43 (2005) 641-649.

17

ACCEPTED MANUSCRIPT [24] G. Singh, A. Choudhary, D. Haranath, A.G. Joshi, N. Singh, S. Singh, R. Pasricha, ZnO decorated luminescent graphene as a potential gas sensor at room temperature,

T

Carbon 50 (2012) 385-394.

IP

[25] M.R. Karim, K. Hatakeyama, T. Matsui, H. Takehira, T. Taniguchi, M. Koinuma,

SC R

Y. Matsumoto, T. Akutagawa, T. Nakamura, S. Noro, T. Yamada, H. Kitagawa, S. Hayami, Graphene oxide nanosheet with proton conductivity, J. Am. Chem. Soc. 135

NU

(2013) 8097-8100.

[26] A.K. Jonscher, The ‘universal’ dielectric response, Nature 267 (1977) 673-679.

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[27] S. Pei, H.M. Cheng, The reduction of graphene oxide, Carbon 50 (2012) 32103228.

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(1960) 505-507.

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[28] A.R. Hutson, Piezoelectricity and conductivity in ZnO and CdS, Phys. Rev. Lett. 4

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[29] J.H. Jung, J.H. Jeon, V. Sridhar, I.K Oh, Electro-active graphene-nafion actuators, Carbon 49 (2011) 1279-1289. [30] H. Zarrin, D. Higgins, Y. Jun, Z. Chen, M. Fowler, Functionalized graphene oxide

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nanocomposite membrane for low humidity and high temperature proton exchange membrane fuel cells, J. Phys. Chem. C 115 (2011) 20774-20781. [31] Y.H. Lin, M. Li, C.W. Nan, J. Le, Grain and grain boundary effects in highpermittivity dielectric NiO-based ceramics, Appl. Phys. Lett. 89 (2006) 032907-3. [32] J. Jose, M.A. Khadar, Role of grain boundaries on the electrical conductivity of nanophase zinc oxide, Mater. Sci. Eng. A 304-306 (2001) 810-813. [33] J. Yuan, W.L. Song, X.Y. Fang, X.L. Shi, Z.L. Hou, M.S. Cao, Tetra-needle zinc oxide/silica composites: High-temperature dielectric properties at X-band, Solid State Commun. 154 (2013) 64-68.

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ACCEPTED MANUSCRIPT [34] J.J. Wang, X.Y. Fang, G.Y. Feng, W.L. Song, Z.L. Hou, H.B. Jin, J. Yuan, M.S. Cao, Scattering mechanisms and anomalous conductivity of heavily N-doped 3C–SiC in

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ultraviolet region, Phys. Lett. A 374 (2010) 2286-2289.

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[35] M. Laube, F. Schmid, G. Pensl, G. Wagner, M. Linnarsson, M. Maier, Electrical

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activation of high concentrations of N+ and P+ ions implanted into 4H-SiC, J. Appl.

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Phys. 92 (2002) 549-554.

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Graphical abstract

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ACCEPTED MANUSCRIPT

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ACCEPTED MANUSCRIPT

Highlights  ZnO nanoparticles decorated r-GO exhibits unique polarization behavior and ionic conductivity

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 Squeezing effect in ZnO and functional groups in the composites contribute to the polarization behavior

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 r-GO in the composite facilitates easy dipolar alignment and reorientation and increases the number of stable polarization states

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 The composites exhibit ionic conductivities (~10-2 Ω-1cm-1) that follow Arrhenius law

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 dc activation energy of composite with more r-GO is high  dc conductivity of composite with more r-GO is low

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 With increasing r-GO the charge carrier concentration might be increased but the mobility of ions were constrained on account of polarisation

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