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AC conductivity studies in copper decorated and zinc oxide embedded polypyrrole composite nanorods: Interfacial effects
Materials Science in Semiconductor Processing 110 (2020) 104963
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AC conductivity studies in copper decorated and zinc oxide embedded polypyrrole composite nanorods: Interfacial effects R. Megha a, b, Y.T. Ravikiran a, b, *, S.C. Vijaya Kumari c, H.G. Rajprakash d, S. Manjunatha e, M. Revanasiddappa f, M. Prashantkumar g, S. Thomas h a
Department of PG Studies and Research in Physics, Government Science College, Chitradurga, 577 501, Karnataka, India Department of Physics, Visvesvaraya Technological University - Research Resource Centre, Belagavi, 590 018, Karnataka, India Department of Physics, SJM College of Arts, Science and Commerce, Chitradurga, 577 501, Karnataka, India d Department of of Physics, JNN College of Engineering, Shivamogga, 577 204, Karnataka, India e Department of Physics, Cambridge Institute of Technology, Bengaluru, 560 036, Karnataka, India f Department of Chemistry, PES University- South Campus, Bangalore, 560100, Karnataka, India g Department of Physics, Government College (Autonomous), Kalaburagi, 585105, Karnataka, India h International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, 686560, India b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Polypyrrole Optimization Composite nanorods AC conductivity Correlated barrier hopping
In this work, synthesis, characterization and temperature dependent alternating current (AC) conductivity studies on copper decorated and zinc oxide embedded polypyrrole (PPy/Cu–ZnO) ternary composite nanorods are reported. The composite nanorods were prepared by mechanical mixing of chemically synthesized copper decorated polypyrrole (PPy/Cu composite) with zinc oxide (ZnO). The so prepared nanorods were thoroughly characterized using TEM, SEM, FTIR and XRD techniques. Significant morphological change in the form of solid nanorods of the ternary composite was confirmed from its TEM image. The surface morphology of the composite nanorods was shown to be porous hinting that these nanorods have good prospects for gas sensing applications. The AC conductivities of PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods were studied in the frequency range 100 Hz to 5 MHz over a temperature range 303 K–373 K. The increased AC conductivity of the composite nanorods as compared to those of PPy/Cu composite and PPy is discussed on the basis of interfacial effects and supported theoretically on the basis of correlated barrier hopping (CBH) model.
1. Introduction Conducting polymers like polyaniline (PANI), polypyrrole (PPy), polyacetylene (PAc) etc., offer flexibility in favorably altering their many special properties especially in tuning of their electrical conduc tivity either by doping or by preparing their composites [1–3]. It has been established that the addition of metal/metal oxide/carbon nano tubes/biopolymers into the conducting polymer matrix facilitates modification in their physical, chemical and electrical properties mainly due to synergistic effects [4,5]. Such composites called hybrids also aid in overcoming the drawbacks of conducting polymers such as poor chemical stability and poor mechanical strength [6,7]. Because of these multiple advantages, they are sought after by many researchers and not surprisingly, they have found many technological applications in many fields [8,9]. Recently, detailed studies revealing enhanced electrical
conductivity of some conducting polymer based binary composites have been reported by us [3,10–12]. Encouraged by the results of these studies, in order to further extend the scope of applications of these composites, in the present work, we have embarked upon enhancing AC conductivity of conducting polymer based ternary composites by achieving best synergistic effects among the components of the com posites. A survey on the recent research also reveals that such ternary composites have found wide applications in supercapacitors and photo catalysts [13,14]. With these perspectives, in this work, PPy/Cu–ZnO ternary com posite nanorods were prepared by simple mechanical mixing method. To the best of our knowledge, AC conductivity of these nanorods has not been reported elsewhere. We considered PPy as a conducting polymer because of its controllable electrical and chemical properties, ease of synthesis and low cost and has found wide applications in fuel cells,
* Corresponding author. Department of PG Studies and Research in Physics, Government Science College, Chitradurga, Karnataka, India. E-mail address: [email protected] (Y.T. Ravikiran). https://doi.org/10.1016/j.mssp.2020.104963 Received 9 July 2019; Received in revised form 20 December 2019; Accepted 22 January 2020 Available online 5 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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sensors, supercapacitors etc. [15]. We preferred Cu nanoparticles as metal component as they are one of the extensively studied nano particles due to their unique physical, chemical, optical and biological properties [16]. As a third component, we preferred ZnO because it is a semiconducting metal oxide with a direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [17]. Selection of suitable novel materials apart, cost effective and ecofriendly method of synthesis is the major concern of every research. So, extraction of Cu nanoparticles from green tea was contemplated because this method is very simple as it does not require high pressure, temperature or harsh and toxic chemical reagents [18]. The Cu nano particles so extracted are then composited with PPy by in-situ poly merization to form PPy/Cu composite which is further mechanically mixed with commercially available ZnO to form PPy/Cu–ZnO ternary composite nanorods. Also, the concentrations of Cu and ZnO in the ternary composite nanorods were optimized to achieve maximum syn ergistic effects to enhance its AC conductivity [11]. The prepared nanorods were structurally and morphologically characterized using TEM, SEM, FTIR and XRD techniques and comparatively analysed with those of PPy and PPy/Cu composite. As a first ever attempt, the mech anism for increased AC conductivity of the composite nanorods has been discussed in terms of interfacial effects and supported theoretically by fitting the experimentally measured AC conductivity data to CBH model. It is worth noting that such an understanding based on CBH model would provide an insight into the design of new materials and also would be beneficial in the selection of materials in multifunctional de vice technology [19,20].
2.4. Preparation of PPy/Cu–ZnO composite nanorods The PPy/Cu–ZnO ternary composite nanorods were prepared by mechanical mixing method because of its many advantages: (i) the method is simple and cost effective, (ii) facilitates best synergistic ef fects, (iii) would preserve sensitivity of the interfaces and hence offer better chances of enhancing electrical conductivity and (iv) its economic viability for industrial production [21]. So, previously prepared PPy/Cu composite was mechanically mixed with 10 wt% of ZnO in a vibration mill (Make: Techno search Instruments, Mumbai, India) for 10–15 min to obtain a homogeneous mixture of PPy/Cu–ZnO composite nanorods. 2.5. Characterization The morphologies of the samples were recorded on a JEM-2100 transmission electron microscope (TEM) at an acceleration voltage of 100 kV and Hitachi S-520 scanning electron microscope (SEM). The FTIR spectra of the samples were obtained using Nicolet 750 FT-IR spectrometer in the wavelength range 400–4000 cm 1. Siemens D5000 powder X-ray diffractometer (Germany) with CuKα source radia tion (λ¼ 1.54 � 10 10 m) was used to obtain the XRD patterns of the samples in the range 2θ ¼ 10–80� with a scanning rate of 2� min 1. 2.6. AC conductivity measurements The AC conductivity measurements of PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods in the form of pellets of about 0.8 cm in diameter and about 1 mm in thickness were carried out using a Hioki model 3532–50 (Japan) digital LCR meter in the frequency range 100 Hz-5 MHz. The real parts of complex conductivity, complex permittivity, com plex impedance and the imaginary part of complex impedance for the prepared samples were calculated using the formulae as reported in our earlier literature [22,23].
2. Experimental 2.1. Materials In order to synthesize PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods, analytical grade reagents such as Pyrrole [C4H4NH], Ammonium persulphate [(NH4)2S2O8], Copper Sulphate [CuSO4] and Zinc oxide (ZnO) were procured from SD Fine Chemicals, Mumbai, India.
3. Results and discussion 3.1. Transmission electron microscopy
2.2. Synthesis of Cu nanoparticles
The TEM images and SAED patterns of Cu nanoparticles, PPy/Cu composite and PPy/Cu–ZnO composite nanorods are depicted in Fig. 1. Spherical shaped Cu nanoparticles and Cu particles embedded in PPy matrix are shown in Fig. 1a and b respectively. Fig. 1c shows PPy/ Cu–ZnO composite nanorods formed by simple mechanical mixing of PPy/Cu composite with ZnO. The Image J software was used to deter mine average aspect ratio of the nanorods and was found to be 6.1 nm. Such nanorods having high aspect ratio are specially preferred for hu midity sensing applications [24]. The crystalline nature of Cu and Cu decorated PPy and polycrystalline nature of composite nanorods are shown in their respective SAED patterns (Fig. 1d–f).
In a typical procedure to extract Cu nanoparticles from green tea, a solution of 0.1 M CuCl2⋅4H2O was prepared by adding 2.4 g of solid CuCl2⋅4H2O to 100 mL of de-ionized water. Meanwhile, 60 g of green tea in 1 L of water was heated till boiling and allowed to settle for 1 h. Then pure filtrate was obtained by vacuum-filtration to which 0.1 M CuCl2⋅4H2O was added in 3:2 vol ratio. Following this, 1 M NaOH so lution was added dropwise with constant stirring into the above solution till the pH becomes 6 and then allowed to remain undisturbed for about 8 h during which formation of intense black precipitate of Cu nano particles takes place. The copper nanoparticles were then dried in a vacuum oven to remove moisture content in them.
3.2. Scanning electron microscopy
2.3. Synthesis of PPy/Cu composite
SEM images of PPy, Cu nanoparticles, ZnO, PPy/Cu composite and PPy/Cu–ZnO ternary composite are shown in Fig. 2. Loosely agglom erated spherical grains of PPy and thick spherical dispersed grains of Cu nanoparticles are shown in Fig. 2a and d respectively. Nearly spherical grains of PPy/Cu composite in which Cu nanoparticles are embedded in PPy matrix are depicted in Fig. 2b. Nearly spherical grains of ZnO (Fig. 2e) covered by PPy/Cu composite forming irregularly shaped clusters of grains of PPy/Cu–ZnO ternary composite with porous surface are shown in Fig. 2c. Such porous structured surfaces are reported to be conducive for gas sensing applications [25]. Also, inter-granular dis tance in PPy/Cu–ZnO ternary composite forming nanorods has decreased as compared to those of PPy/Cu composite and PPy which implies easier charge transport through the carbon back bone of its
PPy/Cu composite was synthesized by following chemical polymer ization technique. To start with, 0.001 M of pyrrole monomer was mixed with 0.002 M of ammonium persulphate solution to initiate polymeri zation. During this process, the previously prepared Cu nanopaticles (10 wt%) were added into the above mixture with constant stirring and allowed for polymerization for about 6 h when deep black PPy/Cu composite precipitate was observed. Then the composite precipitate was suction filtered and then washed repeatedly with distilled water and acetone to remove impurities if any in it. The Cu decorated PPy com posite so obtained was dried at 100 � C using a vacuum oven to remove residual moisture in it. Pure PPy was similarly synthesized without adding Cu nanoparticles. 2
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Fig. 1. TEM images of (a) Cu nanoparticles, (b) PPy/Cu composite, (c) PPy/Cu–ZnO composite nanorods; (d) SAED patterns of green tea extracted Cu nanoparticles, (e) PPy/Cu composite, (f) PPy/Cu–ZnO composite nanorods.
Fig. 2. SEM images of (a) PPy, (b) PPy/Cu composite, (c) PPy/Cu–ZnO composite nanorods, (d) Cu nanoparticles and (e) ZnO.
FTIR spectrum of nanorods (Fig. 3c) the presence of characteristic bands of PPy, some bands of Cu with slight shifts are observed but the char acteristic bands of ZnO (Fig. 3d) are not visible probably because the bands of PPy and Cu are superimposed on them but however, the appeared features confirm interaction occurring among all the three components.
polymer chains [10]. The average grain size of PPy/Cu composite and PPy/Cu–ZnO nanorods determined using image J software were found to be 0.27 μm and 0.31 μm respectively. 3.3. Fourier transform infrared spectroscopy The FTIR spectra PPy, Cu nanoparticles, ZnO, PPy/Cu composite and PPy/Cu–ZnO ternary composite are shown in Fig. 3. The characteristic absorption bands and corresponding stretching vibration of PPy, Cu nanoparticles and ZnO are given in Table 1. All these characteristic bands are in good agreement with those reported in earlier literature [16,26–28]. The appearance of characteristics bands of PPy and Cu nanoparticles in their respective spectra (Fig. 3a and e) and their presence is shown in the FTIR spectrum of PPy/Cu composite with small shifts (Fig. 3b) confirms strong interaction between PPy and Cu in the composite. In the
3.4. X-ray diffraction The XRD patterns of ZnO and PPy/Cu–ZnO composite nanorods are shown in Fig. 4. The typical peaks of ZnO (Fig. 4a) at 2θ ¼ 31.7� , 34.4� , 36.2� , 47.5� , 56.6� , 62.8� , 66.3� , 67.9� and 69� attributed respectively to the planes (100), (002), (101), (102), (110), (103), (200), (112) and (004) confirm its hexagonal structure (wurtzite lattice) and the planes also match very well with the standard JCPDS file NO. 36–1451 [26,29]. In the XRD pattern of ternary composite nanorods (Fig. 4b), only typical 3
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Fig. 4. XRD patterns of a) ZnO and b) PPy/Cu–ZnO composite nanorods. inset: shifts in peak of ZnO.
3.5. AC conductivity studies AC conductivity of each sample (100 Hz - 5 MHz) was measured at room temperature and the related plots are shown in Fig. 5. It can be seen that the AC conductivity of PPy/Cu composite has increased when compared to that of PPy. This may be because of creation of additional defects due to formation of π-π co-ordination bond between PPy and Cu in the composite and the resulting rearrangement of polymer chains leading to decrease in inter-chain separation causing hopping of charge carriers to a longer distance [31]. In the composite nanorods, inter-chain separation is further decreased due to the formation of interfaces and interaction at the interfaces between NH of PPy/Cu composite and ox ygen atoms of ZnO through hydrogen bonding which has encouraged the charge carriers to hop to still longer distances at the interface causing further increase in its electrical conductivity [10]. For theoretical study of the observed AC conducting behavior of the
Fig. 3. FTIR spectra of a) PPy, b) PPy/Cu composite, c) PPy/Cu–ZnO composite nanorods, d) ZnO and e) Cu nanoparticles.
peaks of ZnO are visible but peaks of PPy and Cu are not visible because Cu decorated PPy is deposited on ZnO and ZnO has retained its hexag onal structure [30]. Also, the characteristic peaks of ZnO in the com posite nanorods are displaced slightly towards left and occur with decreased intensity (Fig. 4c (inset)) thus inferring that the composite nanorods have acquired additional defective sites and vacancies which promote conductivity in it.
Table 1 FTIR absorption bands and modes of stretching vibrations in PPy, in green tea extracted Cu nanoparticles and in ZnO. PPy
Functionality
Wave number (cm 1) 619 797 927 1049
Cu nanoparticles
Functionality
Wave number (cm 1)
ZnO
Functionality
Wave number (cm 1)
C–H Wagging
1121
C–O stretching in amino acids
831 948
Zn–O stretching vibration
1450
C–O–H bending vibration
1043
1109
¼ C–H in plane deformation and N–H of pyrrole ring C–H in and out of plane deformation
1617
1381
1190
N–C stretching band
1321
¼ C–H in plane vibration
2851 2851 3272
Amide I band from protein carbonyl stretching vibration C–H stretching modes from hydrocarbon chains -O-H stretch from hydroxyl groups
1476 1555 1705
Vibration of pyrrole ring Ring stretching mode of pyrrole ring C–N stretching vibration
C¼O stretching vibration C–O stretching vibration C–H stretching vibration O–H stretching vibration
4
1509 3378
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Fig. 6. Temperature dependence of electrical conductivity as a function of frequency in the temperature range 303 K–373 K of PPy/Cu composite.
Fig. 5. Frequency dependence of real part of AC conductivity of PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods at 303 K. Solid lines are the best fits of equation (1).
samples, the real part of AC conductivity given by,
σ ðωÞ ¼ σ dc þ Aωs
(1)
Where the term σ dc represents DC conductivity which is frequency in dependent, attributed to long range motion of charge carriers which becomes frequency dependent above a particular frequency called crit ical angular frequency, ωc due to short range back and forth motion of trapped charge carriers following the exponential power law [32,33] represented by the termAωs in equation (1) wherein A is a temperature dependent constant which determines the strength of polarizability [34] and s is an exponent, the value of which lies between 0 and 1, represents many body interactions i.e., interactions among electrons, charges and impurities in the system [35]. To understand the mechanism of AC conduction in terms of charge transport, it is essential to study the variation of s with temperature which is a determining factor to select an appropriate model [33]. So, AC conductivity as a function of frequency at various temperatures for PPy/Cu composite and PPy/Cu–ZnO com posite nanorods were studied and plotted. The respective plots are shown in Figs. 6 and 7. The plots of variation of s with temperature are shown in Fig. 8. It can be seen from Fig. 8 that s is varying inversely with temperature. This observation confirms that CBH model is the appro priate model to understand charge transport mechanism in these sam ples [36]. So, as per this model, AC conductivity in all the samples is due to the hopping of charge carriers between the two sites over the po tential barrier separating them because of thermal activation [37]. As per CBH model, expression for s is given by, s¼1
½WM
6kB T kB T lnð1=ωτo Þ�
Fig. 7. Temperature dependence of electrical conductivity as a function of frequency in the temperature range 303 K–373 K of PPy/Cu–ZnO compos ite nanorods.
Rω ¼
(2)
s¼
6kB T WM
e2 kB T lnð1=ωτo Þ�
(4)
where e is the electron charge, ε0 is the permittivity of free space and ε is the permittivity (real part of complex permittivity) of the medium at a particular temperature and at a fixed frequency which was obtained experimentally for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods at 1 MHz and at 303 K and the related plots are shown in Fig. 10. The plots show that the permittivity of the samples decreases with increase in frequency due to interfacial Maxwell-Wagner polari zation which may be explained as follows: the PPy/Cu composite and PPy/Cu–ZnO composite nanorods are heterogeneous structures con sisting of many interfaces between the filler and the polymer matrix thus
where WM is the binding energy and τ0 is the characteristic relaxation time. For higher values of WM, equation (2) can be written as 1
πεε0 ½WM
(3)
Binding energy of each sample is determined as the slope of 1-s versus T plot (Fig. 9) [7] obtained using equation (3). Then the hop ping distance Rω at a particular frequency ω and temperature T is calculated by, 5
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Fig. 8. Temperature dependence of frequency exponent s for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods. Fig. 10. Frequency dependence of real part of dielectric constant of PPy, PPy/ Cu composite and PPy/Cu–ZnO composite nanorods.
respective complex plane impedance plot (Fig. 11) which was used to calculate Rω [12]. The plot of each sample is nearly a semicircle indi cating absence of contact effects and resembles simple Debye model for non-interacting dipoles. The semicircle plot also signifies that the contribution to conductivity from the grain boundaries is higher than that from the grains [39]. The radius of the semicircles of the plots for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods showing a decreasing trend indicates decreasing resistance and increasing con ductivity of the samples in that order [23]. Using the values of Rω, the values of density of states N(EF) at Fermi level were estimated using the equation,
σ ðωÞ ¼
1 3 2 π N εε0 ωR6ω 24
(5)
where N ¼ kTN(EF). The values of σ(ω) for each sample at 1 MHz was obtained from the respective plot shown in Fig. 5. The values of polaron binding energy WM, hopping distance Rω and the density of states N(EF) determined for PPy, PPy/Cu composite and the composite nanorods at 303 K and at 1 MHz are shown in Table 2. It is clear from Table 2 that the binding energy for each sample is less than unity and hence as per CBH model polarons are the major charge carriers [40,41]. Other highlights in relation to the AC conductivity of the ternary composite as compared to those of PPy and PPy/Cu com posite are: (i) the binding energy of PPy/Cu–ZnO composite nanorods is lowest. (ii) The hopping distance of polarons of the composite nanorods is maximum. (iii) The density of states at Fermi level for PPy/Cu–ZnO composite nanorods is minimum. So the significant conclusions from these observations as compared to those of PPy and PPy/Cu composite are: hopping length of polarons of the composite nanorods has become maximum implying decreased polymer inter-chain separation, density of states at Fermi level for PPy/Cu–ZnO nanorods is minimum indicating increase in delocalization of electronic states in its band gap [42,43]. So, in the light of the above discussions, it is worth noting that the formation of PPy/Cu–ZnO composite nanorods by mechanical mixing method has resulted in furthering enhancement in its AC electrical conductivity as
Fig. 9. Plot of 1-s versus T for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods.
allowing for accumulation of charge carriers causing interfacial polari zation. At lower frequencies, electric dipoles can align themselves with the applied electric field and so the dielectric constant is high. But at higher frequencies, dipoles cannot align themselves with the rapidly changing electric field and hence accumulation of charge carriers and hence polarization at the interfaces is diminished. So dielectric constant decreases at higher frequencies and remains constant at still higher frequencies [38]. The characteristic relaxation time τ0 in each sample was obtained experimentally from the relaxation frequency peak ωmax (꞊1/τ0) from the 6
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exciting opportunities for their applications in chemical-sensors and in photo luminescent devices. Acknowledgements The authors are thankful to the University Grants Commission, New Delhi, for granting the financial support under major research project (41–917/2012 (SR) dated: 23/07/2012). This paper is a collaborative effort between Government Science College, Chitradurga and Mahatma Gandhi University, Kottayam-686 560, India. The authors also acknowledge Visvesvaraya Technological University - Research Resource Centre, Belagavi 590 018, Karnataka, India for their support and encouragement in carrying out research activities. References [1] S. Dey, A.K. Kar, Enhanced photoluminescence through foster resonance energy transfer in polypyrrole-PPMA blends for application in optoelectronic devices, Mater. Sci. Semicond. Process. 103 (2019) 104644. [2] Y.Z. Long, M.M. Li, C. Gu, M. Wan, J.L. Duvail, Z. Liu, Z. 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Fig. 11. Complex plane impedance plots of PPy, PPy/Cu composite and PPy/ Cu–ZnO composite nanorods. Table 2 Binding energy (WM), hopping distance (Rω) and density of states N(EF) of PPy and the composites at 1 MHz and at 303 K. Samples PPy PPy/Cu composite PPy/Cu–ZnO ternaraty nanorods
WM (eV) 0.50 0.23 0.11
Rω (Å ) 0.20 1.34 3.08
N(EF) eV
1
cm
3
26
6.45 � 10 2.59 � 1025 2.74 � 1024
established experimentally and theoretically. This type of study in which decrease in polymer inter-chain separation is identified by increase in hopping length of polarons offers significant advantage for its applica tions because such structural identification of polymer composites is known to facilitate quick diffusion of gases, enhanced sensitivity, low detection limit in case of chemical sensors and also can favorably in fluence the spectral properties of the polymers for photo luminescence characteristics [31]. 4. Conclusion PPy/Cu–ZnO composite nanorods of enhanced AC conductivity were successfully prepared by mechanical mixing of chemically synthesized PPy/Cu composite with ZnO. Morphological change in the ternary composite in the form of nanorods with high aspect ratio was brought about by this method of preparation and so is very useful for their bulk production. The enhancement in AC conductivity of the so prepared composite nanorods as compared to those of PPy and PPy/Cu composite was confirmed experimentally and discussed in terms of interfacial ef fects and understood in the light of CBH model. Prediction of contraction of polymer inter-chain separation due to enhanced hopping length of the polarons in the nanorods arrived at on the basis of CBH model is worth emulating because such prior prediction in these nanorods offers 7
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
AC conductivity studies in copper decorated and zinc oxide embedded polypyrrole composite nanorods: Interfacial effects R. Megha a, b, Y.T. Ravikiran a, b, *, S.C. Vijaya Kumari c, H.G. Rajprakash d, S. Manjunatha e, M. Revanasiddappa f, M. Prashantkumar g, S. Thomas h a
Department of PG Studies and Research in Physics, Government Science College, Chitradurga, 577 501, Karnataka, India Department of Physics, Visvesvaraya Technological University - Research Resource Centre, Belagavi, 590 018, Karnataka, India Department of Physics, SJM College of Arts, Science and Commerce, Chitradurga, 577 501, Karnataka, India d Department of of Physics, JNN College of Engineering, Shivamogga, 577 204, Karnataka, India e Department of Physics, Cambridge Institute of Technology, Bengaluru, 560 036, Karnataka, India f Department of Chemistry, PES University- South Campus, Bangalore, 560100, Karnataka, India g Department of Physics, Government College (Autonomous), Kalaburagi, 585105, Karnataka, India h International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, 686560, India b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Polypyrrole Optimization Composite nanorods AC conductivity Correlated barrier hopping
In this work, synthesis, characterization and temperature dependent alternating current (AC) conductivity studies on copper decorated and zinc oxide embedded polypyrrole (PPy/Cu–ZnO) ternary composite nanorods are reported. The composite nanorods were prepared by mechanical mixing of chemically synthesized copper decorated polypyrrole (PPy/Cu composite) with zinc oxide (ZnO). The so prepared nanorods were thoroughly characterized using TEM, SEM, FTIR and XRD techniques. Significant morphological change in the form of solid nanorods of the ternary composite was confirmed from its TEM image. The surface morphology of the composite nanorods was shown to be porous hinting that these nanorods have good prospects for gas sensing applications. The AC conductivities of PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods were studied in the frequency range 100 Hz to 5 MHz over a temperature range 303 K–373 K. The increased AC conductivity of the composite nanorods as compared to those of PPy/Cu composite and PPy is discussed on the basis of interfacial effects and supported theoretically on the basis of correlated barrier hopping (CBH) model.
1. Introduction Conducting polymers like polyaniline (PANI), polypyrrole (PPy), polyacetylene (PAc) etc., offer flexibility in favorably altering their many special properties especially in tuning of their electrical conduc tivity either by doping or by preparing their composites [1–3]. It has been established that the addition of metal/metal oxide/carbon nano tubes/biopolymers into the conducting polymer matrix facilitates modification in their physical, chemical and electrical properties mainly due to synergistic effects [4,5]. Such composites called hybrids also aid in overcoming the drawbacks of conducting polymers such as poor chemical stability and poor mechanical strength [6,7]. Because of these multiple advantages, they are sought after by many researchers and not surprisingly, they have found many technological applications in many fields [8,9]. Recently, detailed studies revealing enhanced electrical
conductivity of some conducting polymer based binary composites have been reported by us [3,10–12]. Encouraged by the results of these studies, in order to further extend the scope of applications of these composites, in the present work, we have embarked upon enhancing AC conductivity of conducting polymer based ternary composites by achieving best synergistic effects among the components of the com posites. A survey on the recent research also reveals that such ternary composites have found wide applications in supercapacitors and photo catalysts [13,14]. With these perspectives, in this work, PPy/Cu–ZnO ternary com posite nanorods were prepared by simple mechanical mixing method. To the best of our knowledge, AC conductivity of these nanorods has not been reported elsewhere. We considered PPy as a conducting polymer because of its controllable electrical and chemical properties, ease of synthesis and low cost and has found wide applications in fuel cells,
* Corresponding author. Department of PG Studies and Research in Physics, Government Science College, Chitradurga, Karnataka, India. E-mail address: [email protected] (Y.T. Ravikiran). https://doi.org/10.1016/j.mssp.2020.104963 Received 9 July 2019; Received in revised form 20 December 2019; Accepted 22 January 2020 Available online 5 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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sensors, supercapacitors etc. [15]. We preferred Cu nanoparticles as metal component as they are one of the extensively studied nano particles due to their unique physical, chemical, optical and biological properties [16]. As a third component, we preferred ZnO because it is a semiconducting metal oxide with a direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [17]. Selection of suitable novel materials apart, cost effective and ecofriendly method of synthesis is the major concern of every research. So, extraction of Cu nanoparticles from green tea was contemplated because this method is very simple as it does not require high pressure, temperature or harsh and toxic chemical reagents [18]. The Cu nano particles so extracted are then composited with PPy by in-situ poly merization to form PPy/Cu composite which is further mechanically mixed with commercially available ZnO to form PPy/Cu–ZnO ternary composite nanorods. Also, the concentrations of Cu and ZnO in the ternary composite nanorods were optimized to achieve maximum syn ergistic effects to enhance its AC conductivity [11]. The prepared nanorods were structurally and morphologically characterized using TEM, SEM, FTIR and XRD techniques and comparatively analysed with those of PPy and PPy/Cu composite. As a first ever attempt, the mech anism for increased AC conductivity of the composite nanorods has been discussed in terms of interfacial effects and supported theoretically by fitting the experimentally measured AC conductivity data to CBH model. It is worth noting that such an understanding based on CBH model would provide an insight into the design of new materials and also would be beneficial in the selection of materials in multifunctional de vice technology [19,20].
2.4. Preparation of PPy/Cu–ZnO composite nanorods The PPy/Cu–ZnO ternary composite nanorods were prepared by mechanical mixing method because of its many advantages: (i) the method is simple and cost effective, (ii) facilitates best synergistic ef fects, (iii) would preserve sensitivity of the interfaces and hence offer better chances of enhancing electrical conductivity and (iv) its economic viability for industrial production [21]. So, previously prepared PPy/Cu composite was mechanically mixed with 10 wt% of ZnO in a vibration mill (Make: Techno search Instruments, Mumbai, India) for 10–15 min to obtain a homogeneous mixture of PPy/Cu–ZnO composite nanorods. 2.5. Characterization The morphologies of the samples were recorded on a JEM-2100 transmission electron microscope (TEM) at an acceleration voltage of 100 kV and Hitachi S-520 scanning electron microscope (SEM). The FTIR spectra of the samples were obtained using Nicolet 750 FT-IR spectrometer in the wavelength range 400–4000 cm 1. Siemens D5000 powder X-ray diffractometer (Germany) with CuKα source radia tion (λ¼ 1.54 � 10 10 m) was used to obtain the XRD patterns of the samples in the range 2θ ¼ 10–80� with a scanning rate of 2� min 1. 2.6. AC conductivity measurements The AC conductivity measurements of PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods in the form of pellets of about 0.8 cm in diameter and about 1 mm in thickness were carried out using a Hioki model 3532–50 (Japan) digital LCR meter in the frequency range 100 Hz-5 MHz. The real parts of complex conductivity, complex permittivity, com plex impedance and the imaginary part of complex impedance for the prepared samples were calculated using the formulae as reported in our earlier literature [22,23].
2. Experimental 2.1. Materials In order to synthesize PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods, analytical grade reagents such as Pyrrole [C4H4NH], Ammonium persulphate [(NH4)2S2O8], Copper Sulphate [CuSO4] and Zinc oxide (ZnO) were procured from SD Fine Chemicals, Mumbai, India.
3. Results and discussion 3.1. Transmission electron microscopy
2.2. Synthesis of Cu nanoparticles
The TEM images and SAED patterns of Cu nanoparticles, PPy/Cu composite and PPy/Cu–ZnO composite nanorods are depicted in Fig. 1. Spherical shaped Cu nanoparticles and Cu particles embedded in PPy matrix are shown in Fig. 1a and b respectively. Fig. 1c shows PPy/ Cu–ZnO composite nanorods formed by simple mechanical mixing of PPy/Cu composite with ZnO. The Image J software was used to deter mine average aspect ratio of the nanorods and was found to be 6.1 nm. Such nanorods having high aspect ratio are specially preferred for hu midity sensing applications [24]. The crystalline nature of Cu and Cu decorated PPy and polycrystalline nature of composite nanorods are shown in their respective SAED patterns (Fig. 1d–f).
In a typical procedure to extract Cu nanoparticles from green tea, a solution of 0.1 M CuCl2⋅4H2O was prepared by adding 2.4 g of solid CuCl2⋅4H2O to 100 mL of de-ionized water. Meanwhile, 60 g of green tea in 1 L of water was heated till boiling and allowed to settle for 1 h. Then pure filtrate was obtained by vacuum-filtration to which 0.1 M CuCl2⋅4H2O was added in 3:2 vol ratio. Following this, 1 M NaOH so lution was added dropwise with constant stirring into the above solution till the pH becomes 6 and then allowed to remain undisturbed for about 8 h during which formation of intense black precipitate of Cu nano particles takes place. The copper nanoparticles were then dried in a vacuum oven to remove moisture content in them.
3.2. Scanning electron microscopy
2.3. Synthesis of PPy/Cu composite
SEM images of PPy, Cu nanoparticles, ZnO, PPy/Cu composite and PPy/Cu–ZnO ternary composite are shown in Fig. 2. Loosely agglom erated spherical grains of PPy and thick spherical dispersed grains of Cu nanoparticles are shown in Fig. 2a and d respectively. Nearly spherical grains of PPy/Cu composite in which Cu nanoparticles are embedded in PPy matrix are depicted in Fig. 2b. Nearly spherical grains of ZnO (Fig. 2e) covered by PPy/Cu composite forming irregularly shaped clusters of grains of PPy/Cu–ZnO ternary composite with porous surface are shown in Fig. 2c. Such porous structured surfaces are reported to be conducive for gas sensing applications [25]. Also, inter-granular dis tance in PPy/Cu–ZnO ternary composite forming nanorods has decreased as compared to those of PPy/Cu composite and PPy which implies easier charge transport through the carbon back bone of its
PPy/Cu composite was synthesized by following chemical polymer ization technique. To start with, 0.001 M of pyrrole monomer was mixed with 0.002 M of ammonium persulphate solution to initiate polymeri zation. During this process, the previously prepared Cu nanopaticles (10 wt%) were added into the above mixture with constant stirring and allowed for polymerization for about 6 h when deep black PPy/Cu composite precipitate was observed. Then the composite precipitate was suction filtered and then washed repeatedly with distilled water and acetone to remove impurities if any in it. The Cu decorated PPy com posite so obtained was dried at 100 � C using a vacuum oven to remove residual moisture in it. Pure PPy was similarly synthesized without adding Cu nanoparticles. 2
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Fig. 1. TEM images of (a) Cu nanoparticles, (b) PPy/Cu composite, (c) PPy/Cu–ZnO composite nanorods; (d) SAED patterns of green tea extracted Cu nanoparticles, (e) PPy/Cu composite, (f) PPy/Cu–ZnO composite nanorods.
Fig. 2. SEM images of (a) PPy, (b) PPy/Cu composite, (c) PPy/Cu–ZnO composite nanorods, (d) Cu nanoparticles and (e) ZnO.
FTIR spectrum of nanorods (Fig. 3c) the presence of characteristic bands of PPy, some bands of Cu with slight shifts are observed but the char acteristic bands of ZnO (Fig. 3d) are not visible probably because the bands of PPy and Cu are superimposed on them but however, the appeared features confirm interaction occurring among all the three components.
polymer chains [10]. The average grain size of PPy/Cu composite and PPy/Cu–ZnO nanorods determined using image J software were found to be 0.27 μm and 0.31 μm respectively. 3.3. Fourier transform infrared spectroscopy The FTIR spectra PPy, Cu nanoparticles, ZnO, PPy/Cu composite and PPy/Cu–ZnO ternary composite are shown in Fig. 3. The characteristic absorption bands and corresponding stretching vibration of PPy, Cu nanoparticles and ZnO are given in Table 1. All these characteristic bands are in good agreement with those reported in earlier literature [16,26–28]. The appearance of characteristics bands of PPy and Cu nanoparticles in their respective spectra (Fig. 3a and e) and their presence is shown in the FTIR spectrum of PPy/Cu composite with small shifts (Fig. 3b) confirms strong interaction between PPy and Cu in the composite. In the
3.4. X-ray diffraction The XRD patterns of ZnO and PPy/Cu–ZnO composite nanorods are shown in Fig. 4. The typical peaks of ZnO (Fig. 4a) at 2θ ¼ 31.7� , 34.4� , 36.2� , 47.5� , 56.6� , 62.8� , 66.3� , 67.9� and 69� attributed respectively to the planes (100), (002), (101), (102), (110), (103), (200), (112) and (004) confirm its hexagonal structure (wurtzite lattice) and the planes also match very well with the standard JCPDS file NO. 36–1451 [26,29]. In the XRD pattern of ternary composite nanorods (Fig. 4b), only typical 3
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Fig. 4. XRD patterns of a) ZnO and b) PPy/Cu–ZnO composite nanorods. inset: shifts in peak of ZnO.
3.5. AC conductivity studies AC conductivity of each sample (100 Hz - 5 MHz) was measured at room temperature and the related plots are shown in Fig. 5. It can be seen that the AC conductivity of PPy/Cu composite has increased when compared to that of PPy. This may be because of creation of additional defects due to formation of π-π co-ordination bond between PPy and Cu in the composite and the resulting rearrangement of polymer chains leading to decrease in inter-chain separation causing hopping of charge carriers to a longer distance [31]. In the composite nanorods, inter-chain separation is further decreased due to the formation of interfaces and interaction at the interfaces between NH of PPy/Cu composite and ox ygen atoms of ZnO through hydrogen bonding which has encouraged the charge carriers to hop to still longer distances at the interface causing further increase in its electrical conductivity [10]. For theoretical study of the observed AC conducting behavior of the
Fig. 3. FTIR spectra of a) PPy, b) PPy/Cu composite, c) PPy/Cu–ZnO composite nanorods, d) ZnO and e) Cu nanoparticles.
peaks of ZnO are visible but peaks of PPy and Cu are not visible because Cu decorated PPy is deposited on ZnO and ZnO has retained its hexag onal structure [30]. Also, the characteristic peaks of ZnO in the com posite nanorods are displaced slightly towards left and occur with decreased intensity (Fig. 4c (inset)) thus inferring that the composite nanorods have acquired additional defective sites and vacancies which promote conductivity in it.
Table 1 FTIR absorption bands and modes of stretching vibrations in PPy, in green tea extracted Cu nanoparticles and in ZnO. PPy
Functionality
Wave number (cm 1) 619 797 927 1049
Cu nanoparticles
Functionality
Wave number (cm 1)
ZnO
Functionality
Wave number (cm 1)
C–H Wagging
1121
C–O stretching in amino acids
831 948
Zn–O stretching vibration
1450
C–O–H bending vibration
1043
1109
¼ C–H in plane deformation and N–H of pyrrole ring C–H in and out of plane deformation
1617
1381
1190
N–C stretching band
1321
¼ C–H in plane vibration
2851 2851 3272
Amide I band from protein carbonyl stretching vibration C–H stretching modes from hydrocarbon chains -O-H stretch from hydroxyl groups
1476 1555 1705
Vibration of pyrrole ring Ring stretching mode of pyrrole ring C–N stretching vibration
C¼O stretching vibration C–O stretching vibration C–H stretching vibration O–H stretching vibration
4
1509 3378
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Fig. 6. Temperature dependence of electrical conductivity as a function of frequency in the temperature range 303 K–373 K of PPy/Cu composite.
Fig. 5. Frequency dependence of real part of AC conductivity of PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods at 303 K. Solid lines are the best fits of equation (1).
samples, the real part of AC conductivity given by,
σ ðωÞ ¼ σ dc þ Aωs
(1)
Where the term σ dc represents DC conductivity which is frequency in dependent, attributed to long range motion of charge carriers which becomes frequency dependent above a particular frequency called crit ical angular frequency, ωc due to short range back and forth motion of trapped charge carriers following the exponential power law [32,33] represented by the termAωs in equation (1) wherein A is a temperature dependent constant which determines the strength of polarizability [34] and s is an exponent, the value of which lies between 0 and 1, represents many body interactions i.e., interactions among electrons, charges and impurities in the system [35]. To understand the mechanism of AC conduction in terms of charge transport, it is essential to study the variation of s with temperature which is a determining factor to select an appropriate model [33]. So, AC conductivity as a function of frequency at various temperatures for PPy/Cu composite and PPy/Cu–ZnO com posite nanorods were studied and plotted. The respective plots are shown in Figs. 6 and 7. The plots of variation of s with temperature are shown in Fig. 8. It can be seen from Fig. 8 that s is varying inversely with temperature. This observation confirms that CBH model is the appro priate model to understand charge transport mechanism in these sam ples [36]. So, as per this model, AC conductivity in all the samples is due to the hopping of charge carriers between the two sites over the po tential barrier separating them because of thermal activation [37]. As per CBH model, expression for s is given by, s¼1
½WM
6kB T kB T lnð1=ωτo Þ�
Fig. 7. Temperature dependence of electrical conductivity as a function of frequency in the temperature range 303 K–373 K of PPy/Cu–ZnO compos ite nanorods.
Rω ¼
(2)
s¼
6kB T WM
e2 kB T lnð1=ωτo Þ�
(4)
where e is the electron charge, ε0 is the permittivity of free space and ε is the permittivity (real part of complex permittivity) of the medium at a particular temperature and at a fixed frequency which was obtained experimentally for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods at 1 MHz and at 303 K and the related plots are shown in Fig. 10. The plots show that the permittivity of the samples decreases with increase in frequency due to interfacial Maxwell-Wagner polari zation which may be explained as follows: the PPy/Cu composite and PPy/Cu–ZnO composite nanorods are heterogeneous structures con sisting of many interfaces between the filler and the polymer matrix thus
where WM is the binding energy and τ0 is the characteristic relaxation time. For higher values of WM, equation (2) can be written as 1
πεε0 ½WM
(3)
Binding energy of each sample is determined as the slope of 1-s versus T plot (Fig. 9) [7] obtained using equation (3). Then the hop ping distance Rω at a particular frequency ω and temperature T is calculated by, 5
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Fig. 8. Temperature dependence of frequency exponent s for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods. Fig. 10. Frequency dependence of real part of dielectric constant of PPy, PPy/ Cu composite and PPy/Cu–ZnO composite nanorods.
respective complex plane impedance plot (Fig. 11) which was used to calculate Rω [12]. The plot of each sample is nearly a semicircle indi cating absence of contact effects and resembles simple Debye model for non-interacting dipoles. The semicircle plot also signifies that the contribution to conductivity from the grain boundaries is higher than that from the grains [39]. The radius of the semicircles of the plots for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods showing a decreasing trend indicates decreasing resistance and increasing con ductivity of the samples in that order [23]. Using the values of Rω, the values of density of states N(EF) at Fermi level were estimated using the equation,
σ ðωÞ ¼
1 3 2 π N εε0 ωR6ω 24
(5)
where N ¼ kTN(EF). The values of σ(ω) for each sample at 1 MHz was obtained from the respective plot shown in Fig. 5. The values of polaron binding energy WM, hopping distance Rω and the density of states N(EF) determined for PPy, PPy/Cu composite and the composite nanorods at 303 K and at 1 MHz are shown in Table 2. It is clear from Table 2 that the binding energy for each sample is less than unity and hence as per CBH model polarons are the major charge carriers [40,41]. Other highlights in relation to the AC conductivity of the ternary composite as compared to those of PPy and PPy/Cu com posite are: (i) the binding energy of PPy/Cu–ZnO composite nanorods is lowest. (ii) The hopping distance of polarons of the composite nanorods is maximum. (iii) The density of states at Fermi level for PPy/Cu–ZnO composite nanorods is minimum. So the significant conclusions from these observations as compared to those of PPy and PPy/Cu composite are: hopping length of polarons of the composite nanorods has become maximum implying decreased polymer inter-chain separation, density of states at Fermi level for PPy/Cu–ZnO nanorods is minimum indicating increase in delocalization of electronic states in its band gap [42,43]. So, in the light of the above discussions, it is worth noting that the formation of PPy/Cu–ZnO composite nanorods by mechanical mixing method has resulted in furthering enhancement in its AC electrical conductivity as
Fig. 9. Plot of 1-s versus T for PPy, PPy/Cu composite and PPy/Cu–ZnO composite nanorods.
allowing for accumulation of charge carriers causing interfacial polari zation. At lower frequencies, electric dipoles can align themselves with the applied electric field and so the dielectric constant is high. But at higher frequencies, dipoles cannot align themselves with the rapidly changing electric field and hence accumulation of charge carriers and hence polarization at the interfaces is diminished. So dielectric constant decreases at higher frequencies and remains constant at still higher frequencies [38]. The characteristic relaxation time τ0 in each sample was obtained experimentally from the relaxation frequency peak ωmax (꞊1/τ0) from the 6
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exciting opportunities for their applications in chemical-sensors and in photo luminescent devices. Acknowledgements The authors are thankful to the University Grants Commission, New Delhi, for granting the financial support under major research project (41–917/2012 (SR) dated: 23/07/2012). This paper is a collaborative effort between Government Science College, Chitradurga and Mahatma Gandhi University, Kottayam-686 560, India. The authors also acknowledge Visvesvaraya Technological University - Research Resource Centre, Belagavi 590 018, Karnataka, India for their support and encouragement in carrying out research activities. References [1] S. Dey, A.K. Kar, Enhanced photoluminescence through foster resonance energy transfer in polypyrrole-PPMA blends for application in optoelectronic devices, Mater. Sci. Semicond. Process. 103 (2019) 104644. [2] Y.Z. Long, M.M. Li, C. Gu, M. Wan, J.L. Duvail, Z. Liu, Z. 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Fig. 11. Complex plane impedance plots of PPy, PPy/Cu composite and PPy/ Cu–ZnO composite nanorods. Table 2 Binding energy (WM), hopping distance (Rω) and density of states N(EF) of PPy and the composites at 1 MHz and at 303 K. Samples PPy PPy/Cu composite PPy/Cu–ZnO ternaraty nanorods
WM (eV) 0.50 0.23 0.11
Rω (Å ) 0.20 1.34 3.08
N(EF) eV
1
cm
3
26
6.45 � 10 2.59 � 1025 2.74 � 1024
established experimentally and theoretically. This type of study in which decrease in polymer inter-chain separation is identified by increase in hopping length of polarons offers significant advantage for its applica tions because such structural identification of polymer composites is known to facilitate quick diffusion of gases, enhanced sensitivity, low detection limit in case of chemical sensors and also can favorably in fluence the spectral properties of the polymers for photo luminescence characteristics [31]. 4. Conclusion PPy/Cu–ZnO composite nanorods of enhanced AC conductivity were successfully prepared by mechanical mixing of chemically synthesized PPy/Cu composite with ZnO. Morphological change in the ternary composite in the form of nanorods with high aspect ratio was brought about by this method of preparation and so is very useful for their bulk production. The enhancement in AC conductivity of the so prepared composite nanorods as compared to those of PPy and PPy/Cu composite was confirmed experimentally and discussed in terms of interfacial ef fects and understood in the light of CBH model. Prediction of contraction of polymer inter-chain separation due to enhanced hopping length of the polarons in the nanorods arrived at on the basis of CBH model is worth emulating because such prior prediction in these nanorods offers 7
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