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Conductivity of Cu+2 ion-conducting glassy nanocomposites
Materials Science and Engineering B 189 (2014) 21–26
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Conductivity of Cu+2 ion-conducting glassy nanocomposites Arun Kr. Bar a,b , Debasish Roy b , Ranadip Kundu b,c , M.P.F. Graca d , M.A. Valente d , Sanjib Bhattacharya c,∗ a
Department of Basic Science and Humanities, Institute of Engineering and Management, Kolkata 700091, India Department of Mechanical Engineering, Jadavpur University, Jadavpur, Kolkata 700032, India Department of Engineering Sciences and Humanities, Siliguri Institute of Technology, Darjeeling 734009, West Bengal, India d Department of Physics (I3N), Aveiro University, Portugal b c
a r t i c l e
i n f o
Article history: Received 20 March 2014 Received in revised form 10 June 2014 Accepted 3 July 2014 Available online 16 July 2014 PACS: 81.07.−b 81.07.Bc Keywords: Ionic conductivity Nanocomposites Conductivity spectra
a b s t r a c t We have studied the ionic conductivity of CuI doped molybdate glass-nanocomposite systems. X-ray diffraction (XRD) and high-resolution transmission electron microscopic (HR-TEM) studies have been carried out to obtain the particle size and the distribution of coppermolybdate (CuMoO4 ) nanoparticles in glass matrix. We have investigated the electrical conductivity of these glass-nanocomposites in a wide frequency and temperature range. We have analyzed the ac conductivity data using a power law model. Dc conductivity and hopping frequency show thermally activated nature. The power law exponent is almost same for each composition. It has been observed that mobile ion concentration is slightly dependent upon composition, but independent of temperature. The scaling of the conductivity spectra shows temperature-independent electrical relaxation process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, glassy materials and their nanocomposites have been paid much attention for their use as solid electrolytes in the fabrication of solid state batteries, sensors, etc. [1,2]. These glassy systems and their nanocomposites have several advantages such as absence of grain boundaries, isotropic properties, ease of thin film formation and greater stability to moisture and iodine diffusion [3]. A lot of ion-conducting glasses and their nanocomposites consisting of a glass former (B2 O3 , P2 O5 , V2 O5 , MoO3 , etc), a metal oxide M2 O (M = Ag, Li, Na and K) and a doping salt MX (X = I, Br, Cl and F) have been investigated [4–6]. Attempts were made to modify the glass network by mixing a second glass former with the ternary glassy system [7]. In these glassy system the ionic conductivity was found to be more when compared to ternary glasses with an increase of glass forming region and higher dopant salt concentration. But the preparation and study on electrical property of non-conventional copper iodide (CuI) doped glass-nanocomposites is completely new.
∗ Corresponding author. Tel.: +91 03561 224151. E-mail address: sanjib [email protected] (S. Bhattacharya). http://dx.doi.org/10.1016/j.mseb.2014.07.002 0921-5107/© 2014 Elsevier B.V. All rights reserved.
In general, a large number of studies on ionic conductivity and ionic relaxation process in alkali oxide glasses and their nanocomposites have been reported in various journals [8–14]. However, no clear explanation on these processes has emerged and it is still under research. To understand electrical transport phenomena in these glasses and their nanocomposites, it is necessary to find out ionic-concentration and ionic-mobility [15]. A few methods have been suggested [16–19] to do so, but it has not been possible successfully. A few studies on the electrical properties of CuI doped borovanadate system have also been reported [20]. However, studies on the electrical properties and relaxation of CuI doped molybdate glass-nanocomposites in details are very much interesting not only from their technical applications but also from the academic point of view. In this paper, a comprehensive study on the dc and ac conductivity has been made for CuI doped copper molybdate glassnanocomposites over the wide frequency and temperature range. Formation of coppermolybdate (CuMoO4 ) nanoparticles has been confirmed from XRD and HRTEM studies. Ac conductivity data have been analyzed in a wide frequency and temperature range. Interestingly, it has been observed that the mobile Cu+2 ion concentration is less than the total Cu+2 ion concentration and it is almost independent of temperature, but it depends slightly on composition.
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2. Experimental Glass-nanocomposites xCuI-(1 − x)(0.5CuO–0.5MoO3 ) where x = 0.1, 0.2, 0.3 and 0.4 have been prepared from the reagent grade chemicals CuI, CuO and MoO3 . Here, x stands for molar fraction. We have taken 5 g batch for each sample. For x = 0.1, 0.79605 g CuI powder, 1.49631 g CuO powder and 2.70757 g MoO3 powder have been weighted using a high precision balance (Dhona made, 200D). Similarly, for x = 0.2, 0.3 and 0.4 we have taken appropriate amount of precursors. This appropriate amounts of CuI, CuO and MoO3 powders have been thoroughly mixed and preheated in an alumina crucible and the mixtures were then melted in an electric furnace in the temperature range from 700 ◦ C to 800 ◦ C depending upon the composition. The melts have been equilibrated for 30 min and quenched between two aluminum plates. Partially transparent glass-nanocomposites of thickness ∼1 mm have been obtained for x = 0.1–0.4. X-ray diffraction (XRD) patterns of the samples were recorded using an X-ray diffractometer. The distribution of copermolybdate nanoparticles has been confirmed from high-resolution transmission electron microscopic studies and ASTM data sheet [21]. The electrical conductivity measurements of the as-prepared samples have been carried out at various temperatures by complex impedance method. For this, the samples of about 1 mm thickness has been used and the measurements were made by the twoprobe method. The sample inside the sample holder has been kept in contact with two polished, cleaned and spring-loaded copper electrodes (Joy-Crucible made). The complex impedance measurements were carried out using Hioki LCR tester (Model No. 3532-50) in the frequency range 42 Hz–5 MHz at various temperatures.
3. Results and discussions The XRD patterns of the xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glassnanocomposites with different values of x are presented in Fig. 1. In these diffractograms, we have observed a broad diffuse scattering at low angles instead of any crystalline peaks. It indicates a long-range structural disorder, which is characteristic of amorphous network. The presence of coppermolybdate (CuMoO4 ) nanoparticles [21,22] has been confirmed from several peaks, whose change in heights indicate the variation of polycrystalline nature of the samples. The interplaner spacings between two successive planes (d-values) have been computed from XRD analysis. These d-values are also good agreement with those received from ASTM data sheet [21] and are presented in Table 1. The grain sizes of CuMoO4 nanoparticles of all the glass-nanocomposites have been determined from Debye-Scherer formula [23] t = 0.89/(ˇ cos), where t denotes the average grain size of the particles, stands for the X-ray wavelength ˚ for the Bragg diffraction angle and ˇ for the peak width in (1.54 A), radians at half-height. It may be concluded from the above fact that CuMoO4 nanoparticles of different sizes are uniformly distributed in the glass matrices. The distribution of dispersed CuMoO4 nanoparticles at room temperature has been confirmed from the transmission electron micrograph (TEM) study as illustrated in Fig. 2, for a particular sample x = 0.2. The average grain size ∼20 nm of CuMoO4 nanoparticles has been estimated from TEM study. It is also observed that
Fig. 1. X-ray diffractograms of xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glassnanocomposites with different values of x: from lower to upper direction x = 0.1, 0.2, 0.3, 0.4.
the density of distribution of CuMoO4 nanoparticles is found to increase with the increase of CuI content. Selected area electron diffraction (SAED) patterns and high resolution transmission electron micrographs (HRTEM) of them are presented in the inset of Fig. 2. The interplaner spacings between two successive lattice planes (d-values) have also been computed from these SAED patterns and HRTEM micrographs. These d-values corresponding to CuMoO4 nanoparticles are in agreement with those values obtained from ASTM data sheet [21] and also presented in Table 1 for comparison. The dc electrical conductivity ( dc ) has been computed from the Cole–Cole plots of resistivity. Fig. 3 shows the temperature dependence of the dc conductivity. The dc conductivity for all the samples is found to increase with the increase in temperature. It follows the Arrhenius type of variation dc T = 0 exp(−E /kT), where E is the dc activation energy for glass-nanocomposites under investigation, T is the absolute temperature and k is the Boltzman constant. The solid lines in Fig. 3 indicate the best fitted straight lines of the dc conductivity data. The dc activation energy (E ) has been computed from the slopes of the straight-line fits. The dc conductivity at 303 K and corresponding activation energy of CuI doped molybdate glass-nanocomposites have been presented in Fig. 4(a) and (b) respectively. The activation energy for conduction follows opposite behavior [Fig. 4(b)], i.e., the system possesses lowest activation energy. In Fig. 4(a) the dc conductivity data of mixed CuI and AgI doped vanadate glass-nanocomposites [24] are included for comparison. Here the conductivity level is found to increase slightly due to mixed mobile ions Ag+ and Cu+2 compared to the present work. The dc conductivity of these glass-nanocomposites arises due
Table 1 Compositions and d-values obtained from TEM and SAED patterns and ASTM data sheet for the xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites. 2 values and average grain size (nm) from XRD are also included. x
2 (Degree)
d-value (nm) from XRD
d-value (nm) from TEM
d-value (nm) from ASTM data sheet
Average grain size (nm) (±2 nm)
0.1 0.2 0.3 0.4
22 24 25 31
0.35 0.34 0.34 0.35
0.33 0.34 0.33 0.35
0.34 0.34 0.33 0.35
20 20 25 30
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23
Fig. 2. (i) TEM micrograph, displaying the distribution of frozen CuMoO4 nanoparticles for x = 0.2; (j) SAED pattern for x = 0.2; (k) HR-TEM for x = 0.2 and (l) HR-TEM for x = 0.4.
Fig. 3. (a) Variation of dc conductivity with reciprocal temperature of xCuI(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites. Different x values are shown. Straight lines indicate the best fitted lines.
to random motion of ion diffusion throughout the network. This process has been performed due to repeated hops between charge compensating sites. The conductivity spectra for a particular composition x = 0.3 under investigation at various temperatures have been presented in Fig. 5. It has been observed that at low frequency, the conductivity becomes flat. This frequency independent conductivity corresponds to the dc conductivity. The reason of this type of conductivity independent behavior in the low frequency regime may
Fig. 4. (a) Variation of room temperature dc conductivity and (b) corresponding activation energy with compositions.
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The crossover frequency is found to increase as the temperature is increased, in agreement with a thermally activated behavior. The ion hopping behavior above the crossover frequency of the present glass-nanocomposites may be ascribed due to localized hopping motion of Cu+2 ions, occurring on shorter timescales and shorter length scales. This correlated motion may indicate a preference on the part of ions that has hoped away to return its original position [15]. Different models have been proposed to shed some light in the mechanism responsible for electrical relaxation in crystalline and glassy materials [13,25–29]. According to Ngai [15,28], some form of inter ionic-interaction (ion coupling) offers correlated and subdiffusive motion, leading to systematic change of observed power law exponent with ion concentration. The ac conductivity data of the present glass-nanocomposites exhibit a power law [19] nature as shown in Fig. 5. These data have been analyzed using the conductivity model [19],
(ω) = dc Fig. 5. Conductivity spectra at various temperatures for x = 0.3. Solid curves indicate the best fitted curves (power law model).
be due to the diffusion of Cu+2 ions. This may be occurred owing to low frequency ion diffusion on longer timescales and longer length scales. At higher frequencies, the ac conductivity shows dispersion and follows a power law nature. This dispersion in the higher frequency region (above crossover frequency) indicates a nonrandom motion of Cu+2 ions and the motion is correlated and sub-diffusive.
1+
ω n ωH
,
(1)
which is the sum of the dc conductivity ( dc ), hopping frequency (ωH ) and a fractional power law exponent (n). This model has been used by many researchers [30] to get sufficient information on ion transport in ion-conducting system. The experimental data are fitted to Eq. (1) as presented in Fig. 5 and three parameters dc , ωH and n are obtained from the fitting. It is noted that the value of the dc conductivity is almost same as that obtained from the Cole–Cole plot of resistivity. Fig. 6(a) illustrates
Fig. 6. (a) Variation of hopping frequency with reciprocal temperature. (b) Variation of hopping frequency with compositions. (c) Variation of activation energy corresponding to hopping frequency with compositions and (d) Variation of frequency exponent with compositions of xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites. Error bars are shown in the figures.
A.Kr. Bar et al. / Materials Science and Engineering B 189 (2014) 21–26
25
Fig. 7. Variation of mobile ion concentration with reciprocal temperatures.
the variation of ωH with temperature, which shows Arrhenius nature. Composition dependence of the hopping frequency and the corresponding activation energy of all the glass-nanocomposites under investigation have been shown respectively in Fig. 6(b) and (c). Fig. 6(d) shows that the value of the frequency exponent n is less than unity and does not depend significantly on composition. It is clear from the literature [33] that the exponent n is independent of both ion concentration and temperature and it is also related to the dimensionality of the free ion-conduction space. The calculated value of n = 0.67 for the present system indicates three dimensional [33] Cu+2 ion motion in the glassy nanocomposites. It is also noted that the activation energy for dc and ωH are almost same. The mobile Cu+2 ion-concentration has been evaluated from the Nernst-Einstein relation [33]: dc =
q2 da2 Nc ωH , 12kT
(2)
where Nc is the mobile ion concentration, q is the charge, da is the average jump distance and ωH is the hopping frequency of charge carriers. Here, the average distance between Cu+2 ions (da ) is assumed to be 3 × 10−8 cm. The concentration of mobile Cu+2 ions (Nc ) with temperature has been presented in Fig. 7, which shows temperature-independent behavior. Total concentration (N) of Cu+2 ions has been estimated from the glass composition and density. It is also noted that the values of Nc is only 15–20% of the total Cu+2 ion-concentration, contributing to the electrical conduction processes. Previously, it is observed that activation energies corresponding to dc conductivity and hopping frequency respectively are almost same. This implies that the mobile ion concentration is independent of temperature and the conductivity is mainly depends upon mobility of charge carriers [31]. Also we know that = Nc q, where is the mobility of the charge carrier. This statement suggests that mobility is the main important parameter for ion conduction process in the glass matrix, since conductivity increases with temperature. It has been observed from the literature [32,34–37] that many ion-conducting systems follow time-temperature superposition principles and their ac conductivity data can be scaled into a single master curve. The temperatures-scaling for all the glassnanocomposites is presented in Fig. 8(a). The perfect overlap of the spectra at different temperatures follows the time-temperature superposition principle. The composition of scaling at a particular
Fig. 8. (a) Temperature scaling of conductivity spectra for x = 0.1: various temperatures are shown; (b) composition scaling of conductivity spectra of xCuI(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites at T = 240 K.
temperature is presented in Fig. 8(b) where, it has been noted that all the conductivity spectra for all compositions are not properly scaled. It directly indicates that the relaxation process of Cu+2 ions is independent of temperature, but dependent on CuI content.
4. Conclusions The ionic conductivity of CuI doped copper molybdate glassnanocomposites has been studied, compared with others works and presented. The coppermolybdate (CuMoO4 ) nanoparticles have been identified from XRD and HRTEM studies. Ac conductivity data have been analyzed in a wide frequency and temperature range on the frame work of power law model. The dc conductivity and the hopping frequency show thermally activated behavior. The power law exponent is found to be almost independent of CuI doping content. The mobile Cu+2 ion concentration is independent of temperature, but slightly depends upon compositions. The scaling behavior of the conductivity spectra is well described. This
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indicates the temperature-independent relaxation process of Cu+2 ions. Acknowledgements The financial assistance for the work by the Council of Scientific and Industrial Research (CSIR), India via Sanction No. 03 (1286)/13/EMR-II is thankfully acknowledged. References [1] [2] [3] [4] [5] [6]
[7] [8]
[9] [10] [11] [12]
C.A. Angell, Annu. Rev. Phys. Chem. 43 (1992) 693. T. Minami, J. Non-Cryst. Solids 56 (1983) 15. T. Minami, Y. Takuama, M. Tanaka, J. Electrochem. Soc. 124 (1977) 1659. D. Kunze, in: W. Van Gool (Ed.), Fast Ion Transport in Solids, North Holland, Amsterdam, 1973, p. 495. J.E. Shelby, R.L. Ortolana, Phys. Chem. Glasses 31 (1990) 25. F.A. Fusco, H.L. Tuller, D.P. Button, Mater. Sci. Eng. B 13 (1992) 157; T. Minami, in: B.V.R. Chowdari, S. Radhakrishna (Eds.), Materials for Solid State Batteries, World Scientific, Singapore, 1986, p. 169. M.M.H. Hassan, H. Jain, J. Am. Ceram. Soc. 90 (2007) 1934. T. Minami, J. Non-Cryst. Solids 73 (1985) 273; T. Minami, M. Tanaka, J. Non-Cryst. Solids 38/39 (1980) 289; T. Minami, T. Katsuda, M. Tanaka, J. Non-Cryst. Solids 29 (1978) 389. C.A. Angell, Chem. Rev. (Washington, D.C.) 90 (1990) 523. S.R. Elliott, A.P. Owens, Ber. Bunsenges. Phys. Chem B95 (1991) 987. A. Bunde, M.D. Ingram, P. Maass, J. Non-Cryst. Solids 172–174 (1994) 1222. D.L. Sidebottom, P.F. Green, R.K. Brow, J. Chem. Phys. 108 (1998) 5870.
[13] K. Funke, Prog. Solid State Chem. 22 (1993) 111. [14] P. Maass, M. Meyer, A. Bunde, Phys. Rev. B 51 (1995) 8164. [15] K.L. Ngai, J. Non-Cryst. Solids 203 (1996) 232; K.L. Ngai, R.W. Rendel, H. Jain, Phys. Rev. B 30 (1984) 2133. [16] M.D. Ingram, M.A. Mackenzee, W. Muller, M. Torse, Solid State Ionics 28–30 (1988) 677. [17] Y. Moyamoto, M. Itoh, K. Tanaka, Solid State Commun. 92 (1994) 895. [18] H.J. Scutt, Solid State Ionics 70–71 (1996) 505. [19] D.P. Almond, G.K. Duncan, A.R. West, Solid State Ionics 8 (1983) 159; Solid State Ionics 9–10 (1983) 277; D.L. Sidebottom, Phys. Rev. B 61 (2000) 14507. [20] B.S. Reddy, S. Buddhudu, Bull. Mater. Sci. 30 (2007) 481. [21] American Society for Testing Materials data sheet, card no. 83-0672; T.K. Ghorai, Open J. Phys. Chem. 1 (2011) 28. [22] T.K. Ghorai, D. Dhak, S. Dalai, P. Pramanik, Mat. Res. Bull. 43 (2008) 1770. [23] H.M. Xiong, X. Zhao, J.S. Chen, J. Phys. Chem. B 105 (2001) 10169. [24] N. Gupta, A. Dalvi, Indian J. Pure Appl. Phys. 51 (2013) 328. [25] J.C. Dyre, J. Non-Cryst. Solids 88 (1986) 271. [26] S.R. Elliot, A.P. Owens, Philos. Mag. B 60 (1989) 777. [27] A. Hunt, J. Non-Cryst. Solids 175 (1994) 59. [28] K.L. Ngai, Comments Solid State Phys. 9 (1979) 127. [29] K. Funke, B. Rolling, M. Lange, Solid State Ionics 105 (1998) 195. [30] M.M. Ahmad, K. Yamada, T. Okuda, Solid State Ionics 167 (2004) 285. [31] E.F. Hairetdinov, N.F. Uvarov, H.K. Patel, S.W. Martin, Phys. Rev. B 50 (1994) 13259. [32] M.M. Ahmad, Phys. Rev. B 72 (2005) 174303. [33] D.L. Sidebottom, Phys. Rev. Lett. 83 (1999) 983. [34] B. Rolling, A. Happe, K. Funke, m.D. Ingram, Phys. Rev. Lett. 78 (1997) 2160. [35] D.L. Sidebottom, Phys. Rev. Lett. 82 (1999) 3653. [36] T.B. Schrolder, J.C. Dyre, Phys. Rev. Lett. 84 (2000) 310. [37] A. Ghosh, A. Pan, Phys. Rev. Lett. 84 (2000) 2188.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Conductivity of Cu+2 ion-conducting glassy nanocomposites Arun Kr. Bar a,b , Debasish Roy b , Ranadip Kundu b,c , M.P.F. Graca d , M.A. Valente d , Sanjib Bhattacharya c,∗ a
Department of Basic Science and Humanities, Institute of Engineering and Management, Kolkata 700091, India Department of Mechanical Engineering, Jadavpur University, Jadavpur, Kolkata 700032, India Department of Engineering Sciences and Humanities, Siliguri Institute of Technology, Darjeeling 734009, West Bengal, India d Department of Physics (I3N), Aveiro University, Portugal b c
a r t i c l e
i n f o
Article history: Received 20 March 2014 Received in revised form 10 June 2014 Accepted 3 July 2014 Available online 16 July 2014 PACS: 81.07.−b 81.07.Bc Keywords: Ionic conductivity Nanocomposites Conductivity spectra
a b s t r a c t We have studied the ionic conductivity of CuI doped molybdate glass-nanocomposite systems. X-ray diffraction (XRD) and high-resolution transmission electron microscopic (HR-TEM) studies have been carried out to obtain the particle size and the distribution of coppermolybdate (CuMoO4 ) nanoparticles in glass matrix. We have investigated the electrical conductivity of these glass-nanocomposites in a wide frequency and temperature range. We have analyzed the ac conductivity data using a power law model. Dc conductivity and hopping frequency show thermally activated nature. The power law exponent is almost same for each composition. It has been observed that mobile ion concentration is slightly dependent upon composition, but independent of temperature. The scaling of the conductivity spectra shows temperature-independent electrical relaxation process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, glassy materials and their nanocomposites have been paid much attention for their use as solid electrolytes in the fabrication of solid state batteries, sensors, etc. [1,2]. These glassy systems and their nanocomposites have several advantages such as absence of grain boundaries, isotropic properties, ease of thin film formation and greater stability to moisture and iodine diffusion [3]. A lot of ion-conducting glasses and their nanocomposites consisting of a glass former (B2 O3 , P2 O5 , V2 O5 , MoO3 , etc), a metal oxide M2 O (M = Ag, Li, Na and K) and a doping salt MX (X = I, Br, Cl and F) have been investigated [4–6]. Attempts were made to modify the glass network by mixing a second glass former with the ternary glassy system [7]. In these glassy system the ionic conductivity was found to be more when compared to ternary glasses with an increase of glass forming region and higher dopant salt concentration. But the preparation and study on electrical property of non-conventional copper iodide (CuI) doped glass-nanocomposites is completely new.
∗ Corresponding author. Tel.: +91 03561 224151. E-mail address: sanjib [email protected] (S. Bhattacharya). http://dx.doi.org/10.1016/j.mseb.2014.07.002 0921-5107/© 2014 Elsevier B.V. All rights reserved.
In general, a large number of studies on ionic conductivity and ionic relaxation process in alkali oxide glasses and their nanocomposites have been reported in various journals [8–14]. However, no clear explanation on these processes has emerged and it is still under research. To understand electrical transport phenomena in these glasses and their nanocomposites, it is necessary to find out ionic-concentration and ionic-mobility [15]. A few methods have been suggested [16–19] to do so, but it has not been possible successfully. A few studies on the electrical properties of CuI doped borovanadate system have also been reported [20]. However, studies on the electrical properties and relaxation of CuI doped molybdate glass-nanocomposites in details are very much interesting not only from their technical applications but also from the academic point of view. In this paper, a comprehensive study on the dc and ac conductivity has been made for CuI doped copper molybdate glassnanocomposites over the wide frequency and temperature range. Formation of coppermolybdate (CuMoO4 ) nanoparticles has been confirmed from XRD and HRTEM studies. Ac conductivity data have been analyzed in a wide frequency and temperature range. Interestingly, it has been observed that the mobile Cu+2 ion concentration is less than the total Cu+2 ion concentration and it is almost independent of temperature, but it depends slightly on composition.
22
A.Kr. Bar et al. / Materials Science and Engineering B 189 (2014) 21–26
2. Experimental Glass-nanocomposites xCuI-(1 − x)(0.5CuO–0.5MoO3 ) where x = 0.1, 0.2, 0.3 and 0.4 have been prepared from the reagent grade chemicals CuI, CuO and MoO3 . Here, x stands for molar fraction. We have taken 5 g batch for each sample. For x = 0.1, 0.79605 g CuI powder, 1.49631 g CuO powder and 2.70757 g MoO3 powder have been weighted using a high precision balance (Dhona made, 200D). Similarly, for x = 0.2, 0.3 and 0.4 we have taken appropriate amount of precursors. This appropriate amounts of CuI, CuO and MoO3 powders have been thoroughly mixed and preheated in an alumina crucible and the mixtures were then melted in an electric furnace in the temperature range from 700 ◦ C to 800 ◦ C depending upon the composition. The melts have been equilibrated for 30 min and quenched between two aluminum plates. Partially transparent glass-nanocomposites of thickness ∼1 mm have been obtained for x = 0.1–0.4. X-ray diffraction (XRD) patterns of the samples were recorded using an X-ray diffractometer. The distribution of copermolybdate nanoparticles has been confirmed from high-resolution transmission electron microscopic studies and ASTM data sheet [21]. The electrical conductivity measurements of the as-prepared samples have been carried out at various temperatures by complex impedance method. For this, the samples of about 1 mm thickness has been used and the measurements were made by the twoprobe method. The sample inside the sample holder has been kept in contact with two polished, cleaned and spring-loaded copper electrodes (Joy-Crucible made). The complex impedance measurements were carried out using Hioki LCR tester (Model No. 3532-50) in the frequency range 42 Hz–5 MHz at various temperatures.
3. Results and discussions The XRD patterns of the xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glassnanocomposites with different values of x are presented in Fig. 1. In these diffractograms, we have observed a broad diffuse scattering at low angles instead of any crystalline peaks. It indicates a long-range structural disorder, which is characteristic of amorphous network. The presence of coppermolybdate (CuMoO4 ) nanoparticles [21,22] has been confirmed from several peaks, whose change in heights indicate the variation of polycrystalline nature of the samples. The interplaner spacings between two successive planes (d-values) have been computed from XRD analysis. These d-values are also good agreement with those received from ASTM data sheet [21] and are presented in Table 1. The grain sizes of CuMoO4 nanoparticles of all the glass-nanocomposites have been determined from Debye-Scherer formula [23] t = 0.89/(ˇ cos), where t denotes the average grain size of the particles, stands for the X-ray wavelength ˚ for the Bragg diffraction angle and ˇ for the peak width in (1.54 A), radians at half-height. It may be concluded from the above fact that CuMoO4 nanoparticles of different sizes are uniformly distributed in the glass matrices. The distribution of dispersed CuMoO4 nanoparticles at room temperature has been confirmed from the transmission electron micrograph (TEM) study as illustrated in Fig. 2, for a particular sample x = 0.2. The average grain size ∼20 nm of CuMoO4 nanoparticles has been estimated from TEM study. It is also observed that
Fig. 1. X-ray diffractograms of xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glassnanocomposites with different values of x: from lower to upper direction x = 0.1, 0.2, 0.3, 0.4.
the density of distribution of CuMoO4 nanoparticles is found to increase with the increase of CuI content. Selected area electron diffraction (SAED) patterns and high resolution transmission electron micrographs (HRTEM) of them are presented in the inset of Fig. 2. The interplaner spacings between two successive lattice planes (d-values) have also been computed from these SAED patterns and HRTEM micrographs. These d-values corresponding to CuMoO4 nanoparticles are in agreement with those values obtained from ASTM data sheet [21] and also presented in Table 1 for comparison. The dc electrical conductivity ( dc ) has been computed from the Cole–Cole plots of resistivity. Fig. 3 shows the temperature dependence of the dc conductivity. The dc conductivity for all the samples is found to increase with the increase in temperature. It follows the Arrhenius type of variation dc T = 0 exp(−E /kT), where E is the dc activation energy for glass-nanocomposites under investigation, T is the absolute temperature and k is the Boltzman constant. The solid lines in Fig. 3 indicate the best fitted straight lines of the dc conductivity data. The dc activation energy (E ) has been computed from the slopes of the straight-line fits. The dc conductivity at 303 K and corresponding activation energy of CuI doped molybdate glass-nanocomposites have been presented in Fig. 4(a) and (b) respectively. The activation energy for conduction follows opposite behavior [Fig. 4(b)], i.e., the system possesses lowest activation energy. In Fig. 4(a) the dc conductivity data of mixed CuI and AgI doped vanadate glass-nanocomposites [24] are included for comparison. Here the conductivity level is found to increase slightly due to mixed mobile ions Ag+ and Cu+2 compared to the present work. The dc conductivity of these glass-nanocomposites arises due
Table 1 Compositions and d-values obtained from TEM and SAED patterns and ASTM data sheet for the xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites. 2 values and average grain size (nm) from XRD are also included. x
2 (Degree)
d-value (nm) from XRD
d-value (nm) from TEM
d-value (nm) from ASTM data sheet
Average grain size (nm) (±2 nm)
0.1 0.2 0.3 0.4
22 24 25 31
0.35 0.34 0.34 0.35
0.33 0.34 0.33 0.35
0.34 0.34 0.33 0.35
20 20 25 30
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Fig. 2. (i) TEM micrograph, displaying the distribution of frozen CuMoO4 nanoparticles for x = 0.2; (j) SAED pattern for x = 0.2; (k) HR-TEM for x = 0.2 and (l) HR-TEM for x = 0.4.
Fig. 3. (a) Variation of dc conductivity with reciprocal temperature of xCuI(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites. Different x values are shown. Straight lines indicate the best fitted lines.
to random motion of ion diffusion throughout the network. This process has been performed due to repeated hops between charge compensating sites. The conductivity spectra for a particular composition x = 0.3 under investigation at various temperatures have been presented in Fig. 5. It has been observed that at low frequency, the conductivity becomes flat. This frequency independent conductivity corresponds to the dc conductivity. The reason of this type of conductivity independent behavior in the low frequency regime may
Fig. 4. (a) Variation of room temperature dc conductivity and (b) corresponding activation energy with compositions.
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The crossover frequency is found to increase as the temperature is increased, in agreement with a thermally activated behavior. The ion hopping behavior above the crossover frequency of the present glass-nanocomposites may be ascribed due to localized hopping motion of Cu+2 ions, occurring on shorter timescales and shorter length scales. This correlated motion may indicate a preference on the part of ions that has hoped away to return its original position [15]. Different models have been proposed to shed some light in the mechanism responsible for electrical relaxation in crystalline and glassy materials [13,25–29]. According to Ngai [15,28], some form of inter ionic-interaction (ion coupling) offers correlated and subdiffusive motion, leading to systematic change of observed power law exponent with ion concentration. The ac conductivity data of the present glass-nanocomposites exhibit a power law [19] nature as shown in Fig. 5. These data have been analyzed using the conductivity model [19],
(ω) = dc Fig. 5. Conductivity spectra at various temperatures for x = 0.3. Solid curves indicate the best fitted curves (power law model).
be due to the diffusion of Cu+2 ions. This may be occurred owing to low frequency ion diffusion on longer timescales and longer length scales. At higher frequencies, the ac conductivity shows dispersion and follows a power law nature. This dispersion in the higher frequency region (above crossover frequency) indicates a nonrandom motion of Cu+2 ions and the motion is correlated and sub-diffusive.
1+
ω n ωH
,
(1)
which is the sum of the dc conductivity ( dc ), hopping frequency (ωH ) and a fractional power law exponent (n). This model has been used by many researchers [30] to get sufficient information on ion transport in ion-conducting system. The experimental data are fitted to Eq. (1) as presented in Fig. 5 and three parameters dc , ωH and n are obtained from the fitting. It is noted that the value of the dc conductivity is almost same as that obtained from the Cole–Cole plot of resistivity. Fig. 6(a) illustrates
Fig. 6. (a) Variation of hopping frequency with reciprocal temperature. (b) Variation of hopping frequency with compositions. (c) Variation of activation energy corresponding to hopping frequency with compositions and (d) Variation of frequency exponent with compositions of xCuI-(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites. Error bars are shown in the figures.
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Fig. 7. Variation of mobile ion concentration with reciprocal temperatures.
the variation of ωH with temperature, which shows Arrhenius nature. Composition dependence of the hopping frequency and the corresponding activation energy of all the glass-nanocomposites under investigation have been shown respectively in Fig. 6(b) and (c). Fig. 6(d) shows that the value of the frequency exponent n is less than unity and does not depend significantly on composition. It is clear from the literature [33] that the exponent n is independent of both ion concentration and temperature and it is also related to the dimensionality of the free ion-conduction space. The calculated value of n = 0.67 for the present system indicates three dimensional [33] Cu+2 ion motion in the glassy nanocomposites. It is also noted that the activation energy for dc and ωH are almost same. The mobile Cu+2 ion-concentration has been evaluated from the Nernst-Einstein relation [33]: dc =
q2 da2 Nc ωH , 12kT
(2)
where Nc is the mobile ion concentration, q is the charge, da is the average jump distance and ωH is the hopping frequency of charge carriers. Here, the average distance between Cu+2 ions (da ) is assumed to be 3 × 10−8 cm. The concentration of mobile Cu+2 ions (Nc ) with temperature has been presented in Fig. 7, which shows temperature-independent behavior. Total concentration (N) of Cu+2 ions has been estimated from the glass composition and density. It is also noted that the values of Nc is only 15–20% of the total Cu+2 ion-concentration, contributing to the electrical conduction processes. Previously, it is observed that activation energies corresponding to dc conductivity and hopping frequency respectively are almost same. This implies that the mobile ion concentration is independent of temperature and the conductivity is mainly depends upon mobility of charge carriers [31]. Also we know that = Nc q, where is the mobility of the charge carrier. This statement suggests that mobility is the main important parameter for ion conduction process in the glass matrix, since conductivity increases with temperature. It has been observed from the literature [32,34–37] that many ion-conducting systems follow time-temperature superposition principles and their ac conductivity data can be scaled into a single master curve. The temperatures-scaling for all the glassnanocomposites is presented in Fig. 8(a). The perfect overlap of the spectra at different temperatures follows the time-temperature superposition principle. The composition of scaling at a particular
Fig. 8. (a) Temperature scaling of conductivity spectra for x = 0.1: various temperatures are shown; (b) composition scaling of conductivity spectra of xCuI(1 − x)(0.5CuO–0.5MoO3 ) glass-nanocomposites at T = 240 K.
temperature is presented in Fig. 8(b) where, it has been noted that all the conductivity spectra for all compositions are not properly scaled. It directly indicates that the relaxation process of Cu+2 ions is independent of temperature, but dependent on CuI content.
4. Conclusions The ionic conductivity of CuI doped copper molybdate glassnanocomposites has been studied, compared with others works and presented. The coppermolybdate (CuMoO4 ) nanoparticles have been identified from XRD and HRTEM studies. Ac conductivity data have been analyzed in a wide frequency and temperature range on the frame work of power law model. The dc conductivity and the hopping frequency show thermally activated behavior. The power law exponent is found to be almost independent of CuI doping content. The mobile Cu+2 ion concentration is independent of temperature, but slightly depends upon compositions. The scaling behavior of the conductivity spectra is well described. This
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indicates the temperature-independent relaxation process of Cu+2 ions. Acknowledgements The financial assistance for the work by the Council of Scientific and Industrial Research (CSIR), India via Sanction No. 03 (1286)/13/EMR-II is thankfully acknowledged. References [1] [2] [3] [4] [5] [6]
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