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Spectrochimica Acta Part A 83 (2011) 384– 391

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

jou rn al hom epa ge: www.elsev ier .com/ locate /saa

ibrational spectroscopy and ionic conductivity of polyethylenexide–NaClO4–CuO nanocomposite

rup Dey, Kajari Das, S. Karan, S.K. De ∗

epartment of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 3 June 2011

a b s t r a c t

Structure, morphology and thermal properties of polyethylene oxide (PEO) with sodium perchlorate(NaClO4) as electrolytic salt have been investigated by incorporating cupric monoxide (CuO) nanopar-

eceived in revised form 14 August 2011ccepted 24 August 2011

eywords:olymer electrolyteanocomposite

ticles. Monoclinic CuO affects melting and glass transition temperatures of PEO–NaClO4. Crystallinityand free ion concentration change with the variation of CuO concentration. The maximum ionic conduc-tivity is observed for 10 wt.% CuO. Ionic conductivity follows Arrhenius type behavior as a function oftemperature.

© 2011 Elsevier B.V. All rights reserved.

onic conductivity

. Introduction

Poly(ethylene) oxide (PEO) based solid polymer electrolytes,ecause of their possessions of a wide range of salt-compositionalariability, chemical compatibility with a variety of electrode cou-les, good electrode–electrolyte contacts and favorable mechanicalroperties to fabricate thin film devices of desirable shapes andizes, have received much technological interest in the recent pasto develop compact, light weight, high energy density recharge-ble batteries, fuel cells, supercapacitors, electrochromic displayevices, etc. [1–4]. However, the prototypical polymer electrolytes,

solid solution composed of some salts and PEO with higholecular weight, usually exhibit low ionic conductivity at room

emperature. Attempts were made to increase the conductivity ofhese conventional PEO based electrolytes further by lowering theperational temperature. Among many strategies suggested by theesearchers, incorporation of a variety of nanoscale inorganic fillers,uch as SiO2, Al2O3, TiO2, ZnO, ZrO2 and CeO2 has been accepteds the most promising technique to improve the ionic conductivityhile retaining the other physical properties, namely mechanical

tability, electrolyte reactivity towards electrodes, etc. [5–14]. Its observed that the nanosized fillers play a role in inhibiting theEO chain crystallization kinetics, enlargement of the amorphoushase region in the polymer matrix and providing specific surface

nteractions with the electrolyte components. The basic assump-ion in several investigations has been that the Lewis acid–baseype surface groups of inorganic fillers interact with cations and

∗ Corresponding author. Tel.: +91 33 2473 3073; fax: +91 33 2473 2805.E-mail address: msskd@iacs.res.in (S.K. De).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.08.050

anions and promote the disassociation of the ion pairs so that theelectrolytes can release more free carriers and remain amorphousstate for charge carriers to transfer [15–20]. It is also found thatphysical properties of polymer electrolyte composites depend onthe filler concentration as well as nature of the filler. However, themechanism of ionic conductivity enhancement and the role playedby the nanosized fillers is still not well understood.

Most of the composite polymer electrolytes, in the past threedecades, have been mainly based on alkali metal salt systems, withparticular attention given to lithium. There have been a few lit-eratures on solid polymer electrolytes based on sodium complexsystems though it has lower lattice energy (ionic radius of Na+ is0.95 A). Apart from the scientific interest, sodium is much moreabundant and lower in price than lithium. Moreover, the softness ofthis metal makes it easier to achieve and maintain contact to othercomponents in the electrochemical devices. Keeping the above inview, the authors report in this paper a polymer electrolyte sys-tem composed of sodium perchlorate (NaClO4) as salt and CuO asnanofiller. The electrical conductivity measurements as a functionof CuO concentration have been carried out to identify the optimumconducting composition of the composite polymer electrolyte. It isexpected that such a study will provide useful information aboutthe structure and properties of the PEO–NaClO4–CuO electrolytes.

2. Experimental

2.1. Preparation of CuO

All the chemical reagents used in this experiment were of ana-lytical grade and were used without further purification. In a typicalprocedure, 0.4 g of cupric acetate monohydrate (CuAc2) was mixed

a Acta Part A 83 (2011) 384– 391 385

wftcarawfpe

aidOt

C

C

2

pinwtwuumvavdcaSb

iPtTrHtNcDeo1wt

meivnait

Fig. 1. X-ray diffraction pattern and (inset) high resolution TEM picture of mono-

The X-ray diffraction spectrum of the sample shown in Fig. 1indicate the formation of the monoclinic cupric oxide (JCPDS fileNo. 41-0254). The other impurities like Cu2O and Cu have not been

(a)

(b)

Inte

nsity

(a.u

.)

(c)

A. Dey et al. / Spectrochimic

ith 48 mL ethylene glycol (EG) under constant magnetic stirringor 1 h. A homogeneous solution was appeared, which was thenransferred to a teflon-lined stainless steel autoclave with 60 mLapacity and heat treated at the temperature 160 ◦C for 2 h. Theutoclave chamber was air-cooled to room temperature after theeaction. The resulting precipitate was recovered by centrifugationnd washed several times with distilled water, and the precipitateas finally vacuum dried for 4 h. The monoclinic cupric oxide (CuO)

ormation has been confirmed by XRD spectrum and the meanarticle diameter was determined by high resolution transmissionlectron microscopy (HRTEM) of the powder sample.

Cupric acetate monohydrate dissolves in the EG at high temper-ture and pressure in the autoclave and produces OAc− and Cu2+

ons. Then esterification was carried out by EG with OAc− to pro-uce ethylene glycol mono-acetate ester and strong hydroxyl baseH−. Finally Cu2+ and OH− ions combine in solution and dehydrate

o form CuO. The reaction process can be written as follows:

H3COO− + HOCH2CH2OH = CH3COOCH2CH2OH + OH− (1)

u2+ + 2OH− = CuO + 2H2O (2)

.2. Preparation of PEO-based polymer electrolyte

The PEO and NaClO4 molar ratio 25:1 was considered from thehase diagram of PEO–NaClO4 system [21]. The ionic conductiv-

ty was also studied at PEO:NaClO4::25:1 in polymer electrolyteanocomposite [22,23]. The standard solvent casting techniqueas used to prepare PEO25–NaClO4 and composite polymer elec-

rolyte films using methanol as the common solvent. PEO (Aldrich)ith molecular weight, MW = 106 and NaClO4 (Fluka) were driednder vacuum at 50 ◦C and 120 ◦C, respectively, for 48 h prior tose. First, weighted quantity of PEO and NaClO4 were dissolved inethanol and stirred thoroughly for 4 h at room temperature. The

iscous solution was then cast onto a teflon plate and allowed to dryt room temperature over night. Finally, the film was dried underacuum at 50 ◦C for 48–50 h to evaporate any residual solvent. CuOispersed composite polymer electrolytes in different weight per-ent were also prepared following the same procedure mentionedbove. The thin films were then preserved in a vacuum desiccator.table films up to 15 wt.% CuO were prepared. Films are very brittleeyond this concentration.

X-ray diffraction patterns of the pure CuO and the compos-te films were recorded by means of a high resolution X’PertRO Panalytical X-ray diffractometer with Cu K� radiation overhe Bragg angle (2�) range of 20–80◦ and 10–80◦, respectively.he average size of the pure CuO was examined using a highesolution transmission electron micrograph (TEM) (JEOL Model:RTEM JEM 2010). Morphological and microstructural features of

he membranes were examined by Autoprobe CP Base Unit, Modelo. AP0100 atomic force microscope (AFM). Differential scanningalorimetric traces were recorded using a PerkinElmer DiamondSC instrument. The thermal stabilities of composite polymerlectrolytes were recorded under a constant flow (100 mL min−1)f nitrogen gas between −60 and 100 ◦C at a heating rate of0 ◦C min−1. Fourier transform infra-red (FTIR) spectra were takenith a computer interfaced Shimadzu FTIR-8300 spectrometer in

he frequency range of 1800–400 cm−1.Electrical conductivity of polymer electrolytes arises from the

igration of ions. The measurement of ionic conductivity is differ-nt from electronic conductivity due to large electrolyte–electrodenterface resistance. Electrical properties of electrolytes are con-entionally characterized by the build-up of space charge region

ear the electrodes [24,25]. In this process mobile ions are blockedt the electrodes to generate space charge region. Stainless steels electrochemically inert and is commonly used as electrodeso block the ions. Electrical conductivity is determined by the

clinic CuO powder.

movement of ionic space charge between the blocking electrodesunder the application of alternating electric field. The electricalconductivity of the composite polymer electrolyte films were eval-uated by means of an ac impedance technique using an Agilent4192A frequency response impedance analyser in the tempera-ture range 273–333 K. For this, the nanocomposite electrolyte filmswere sandwiched between two polished stainless steel blockingelectrodes and temperature was monitored by Eurotherm temper-ature controller (Model No. 2404) using thermocouple sensor.

The total ionic transport number (tion) was measured by thepolarization technique [26–28]. In this technique, a cell configu-ration of stainless steel |nanocomposite electrolyte| stainless steelwas established and then the cell was polarized by applying a steppotential of 0.8 V. The resulting potentiostatic current was mon-itored as a function of time by Keithley Electrometer, Model No.6517A. The tion was evaluated using the formula:

tion = I0 − ISI0

(3)

where I0 and IS are initial and saturation current, respectively.

3. Results and discussion

10 20 30 40 50 60 70 80

2θ (Degrees)

Fig. 2. X-ray diffraction patterns of (a) pure PEO; (b) PEO25–NaClO4 and (c) 15 wt.%CuO of PEO25–NaClO4–CuO.

386 A. Dey et al. / Spectrochimica Acta Part A 83 (2011) 384– 391

ClO4 a

fthbt

d

FoC

Fig. 3. AFM topoghaphic images of (a) pure PEO; (b) PEO25–Na

ound from the spectrum. From the high resolution TEM image ofhe sample shown in Fig. 1(inset), it is also clear that the particles areighly crystalline with a diameter of ∼2–3 nm. The 0.23 nm spacing

etween two adjacent lattice planes of a nanoparticle correspondso the separation of the (1 1 1/2 0 0) lattice planes of CuO.

X-ray diffraction patterns of pure PEO, PEO25–NaClO4 and CuOispersed composite polymer electrolytes are presented in Fig. 2.

-60 -50 -4 0 -30 -2 0 -10 0

Tg

(g)

(f)

(e)

(d)

(c)

(b)

(a)

Hea

t Flo

w

Hea

t Flo

w

Temperature(ºC)

A

ig. 4. (A) Glass transition temperature (Tg) plot of (a) PEO25–NaClO4 and (b) 2 wt.% CuO;

f PEO25–NaClO4–CuO. (B) DSC traces of (a) pure PEO; (b) PEO25–NaClO4 and (c) 2 wt.% CuuO of PEO25–NaClO4–CuO.

nd (c) 10 wt.% CuO and (d) 15 wt.% CuO of PEO25–NaClO4–CuO.

In Fig. 2(a) the peaks at 2� = 19.1◦ and 23.5◦ are correspond to(1 2 0) and (1 1 2) crystalline peaks of PEO. The characteristic peaksof pure PEO in PEO25–NaClO4 complex (Fig. 2(b)) show variation

in intensity suggesting that the ordering of the PEO polymer isdisturbed due to coordination interactions between the Na+ ionsand ether oxygens. After the addition of CuO nanofiller in poly-mer electrolytes, some new peaks appear which correspond to the

(h)

(g)

(f)

(e)

(d)

(c)

(b)

Temperature(ºC)

(a)

30 40 50 60 70 80 90

B

(c) 5 wt.% CuO; (d) 8 wt.% CuO; (e) 10 wt.% CuO; (f) 12 wt.% CuO and (g) 15 wt.% CuOO; (d) 5 wt.% CuO; (e) 8 wt.% CuO; (f) 10 wt.% CuO; (g) 12 wt.% CuO and (h) 15 wt.%

A. Dey et al. / Spectrochimica Acta Part A 83 (2011) 384– 391 387

Table 1Composition and glass transition temperature (Tg), melting temperature (Tm), change of enthalpy of melting (�H) and percentage of crystalline (�) of pure PEO, PEO25–NaClO4

and different composite polymer electrolytes.

Sample Tg

(◦C)Tm

(◦C)�H(J/g)

�(%)

Pure PEO −66 71.1 155 72.53PEO25–NaClO4 −37.8 63.5 100.37 46.97PEO25–NaClO4–CuO 2 wt.% −44.1 61.5 90.93 42.55PEO25–NaClO4–CuO 5 wt.% −45.3 61.2 78.49 36.73PEO25–NaClO4–CuO 8 wt.% −45.6 61.1 85.27 39.90PEO25–NaClO4–CuO 10 wt.% −44.9 60.5 75.09 35.14PEO25–NaClO4–CuO 12 wt.% −45.2 60.2 79.83 37.36

59.

cirnb

t

wmncn

cfpwdsists1FpmtifiMttf

pwFtwtuTwehcawc

modes at 1281 and 1242 cm−1. The characteristic C–O–C stretch-ing triplet peaks at 1147, 1110 and 1061 cm−1 with a maximumat 1110 cm−1 are also identified in the pure PEO. All these bands

36

38

40

42

44

46

48

Per

cen

tag

e o

f C

ryst

allin

e (χ

)

PEO25–NaClO4–CuO 15 wt.% −46.8

haracteristic peaks of monoclinic CuO as depicted in Fig. 2(c). Its also evident from Fig. 2(c) that CuO nanoparticles are incorpo-ated in PEO25–NaClO4. The average crystallite size (t) of the CuOanocrystallites has been estimated by Scherrer formula [29] giveny,

= 0.9�

cos�(4)

here � is the wavelength of X-rays and is the full-width at halfaxima of XRD peaks related to CuO. The estimated size of CuO

anparticles varies from 3 nm to 12 nm. The size of CuO nanoparti-les increase with increase of concentration due to aggregation inanocomposite.

The morphology of pure PEO, PEO25–NaClO4 and CuO dispersedomposite polymer electrolyte films has been examined by atomicorce microscope and two-dimensional topoghaphic images areresented in Fig. 3. The image of pure PEO is seen to have a net-ork of regular spherulites producing spirals and branches of wellistributed surface contours (Fig. 3(a)). This is an evidence of theemi-crystalline nature of pure PEO. The morphology of neat PEOs in agreement with earlier studies [30,31]. In Fig. 3(b), a certainurface morphological change has been observed due to the addi-ion of NaClO4. Fig. 3(c) shows that the surface appears to be muchmoother and spirals and branches are completely disappeared in0 wt.% of CuO added composite polymer electrolyte. In contrast,ig. 3(d) shows aggregated features in 15 wt.% of CuO added com-osite polymer electrolyte which provides a new type of surfaceorphology. Insertion of CuO nanoparticles influences the crys-

alline network of PEO chain. The agglomeration of nanoparticlesn PEO matrix occurs at higher concentration of CuO. This is con-rmed from size determination using Eq. (4) based on XRD data.oreover, PEO molecules are deposited on the surface of larger par-

icles due to interaction between PEO and CuO. As a result of these,he morphology of electrolyte with 15 wt.% is completely differentrom that with 10 wt.% CuO.

The typical differential scanning calorimetric (DSC) curves ofure PEO and PEO25–NaClO4 and composite polymer electrolyteith different CuO contents during heating scans are displayed in

ig. 4 and are listed in Table 1. In each curve, the melting tempera-ure is identified at the peak minimum of the endothermic processhich is attributed to the melting of crystalline phase. The melting

emperature, Tm of crystalline PEO decreases from 71 ◦C to 63.5 ◦Cpon the addition of NaClO4 into the polymer matrix. The value ofm of composite polymer electrolyte decreases further to 59.1 ◦Chen the concentration of CuO increases from 0 to 15 wt.%. The

nthalpy of melting (�H) is represented as energy in the form ofeat absorbed per unit weight of the polymeric sample. The per-

entage of crystallinity (�) has been calculated by taking pure PEOs completely crystalline and using the relation � = �H/(�H(PEO)),here �H(PEO) = 213.7 J/g is the melting enthalpy of a completely

rystalline PEO sample [32]. The values of �H and � are shown

1 81.63 38.20

in Table 1. The variation of � with CuO concentration is shown inFig. 5. It is noticed that the decrease in degree of crystallinity is upto ∼5% on dispersing the CuO nanoparticle. Throughout the CuOconcentration, � shows two minima with a maximum at 8 wt.%.

The glass transition temperature Tg of pure PEO is −66 ◦C [33].In Table 1, the glass transition temperature Tg, at which a glassyphase transforms into a rubbery amorphous phase, clearly indi-cates that the value of Tg decreases from −37.8 ◦C to −44.1 ◦Cwhen even a small amount (2 wt.%) of CuO is incorporated intothe polymer complex. The segmental motion of polymer chain isa cooperative process and is correlated with Tg. Decrease in Tg

enhances the mobility of the polymer chain. This result suggeststhat the segmental motion of the composite polymer electrolytehas been increased appreciably. However, the values of Tg havenot changed appreciably for further doping of the nanofiller. Sur-faces of CuO nanoparticles play important roles to interact withPEO25–NaClO4 salt complex. Grain boundary effect of nanofillersand strong Lewis acid–base interaction between Cu2+ and etheroxygen of PEO significantly affect thermal and crystalline proper-ties of nanocomposites. Similar variation of Tg, Tm and crystallinitywith increase of filler concentration has also been observed by sev-eral authors [4,5,34,35].

Fourier transform infra-red spectroscopic studies have beenconducted to investigate the ion–ion interactions at the micro-scopic level among the various components of the compositepolymer electrolyte. Fig. 6 shows the comparative FTIR spectraof pure PEO, PEO25–NaClO4 and composite polymer electrolytewith different CuO contents. The most important absorption fea-tures present in pure PEO are CH2 bending mode at 1466 cm−1,the CH2 wagging doublet at 1360 and 1342 cm−1, the CH2 twisting

86420 10 12 14 1634

CuO concentration (wt.%)

Fig. 5. Variation of percentage of crystalline (�) with CuO concentration.

388 A. Dey et al. / Spectrochimica Acta Part A 83 (2011) 384– 391

1800 1600 1400 1200 1000 800 600 400

(a)

Ab

sorb

ance

(a.

u)

Wavenumber (cm-1)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 6. FTIR spectra of (a) pure PEO; (b) PEO25–NaClO4 and (c) 2 wt.% CuO; (d)5o

ae9itcijt9tPtwtpacacebtv

FC

Table 2Composition and peak positions and percentage of free ClO4

− anions ofPEO25–NaClO4 and different composite polymer electrolytes.

Sample Peak position Free ClO4− ions

(%)Free anions Contact–ion

pairs

PEO25–NaClO4 622.83 631.31 68.00PEO25–NaClO4–CuO 2 wt.% 623.39 632.42 72.75PEO25–NaClO4–CuO 5 wt.% 624.06 632.23 76.78PEO25–NaClO4–CuO 8 wt.% 623.18 631.97 79.73PEO25–NaClO4–CuO 10 wt.% 623.93 632.36 86.98PEO25–NaClO4–CuO 12 wt.% 623.62 631.99 82.85

wt.% CuO; (e) 8 wt.% CuO; (f) 10 wt.% CuO; (g) 12 wt.% CuO and (h) 15 wt.% CuOf PEO25–NaClO4–CuO.

re very sensitive to macromolecular conformations and providevidence of crystalline phase in pure PEO. The bands at 962 and49 cm−1 are assigned to the symmetric and asymmetric CH2 rock-

ng modes, respectively and the band at 843 cm−1 is ascribed tohe CH2 asymmetric rocking mode. The conformational changes inrystalline phase are noticed with the introduction of NaClO4 saltnto PEO. The bands at 1360, 1342 cm−1 and 962 and 949 cm−1 areoined together to form a broad band at 1352 and 953 cm−1, respec-ively. The shifting of the peak position of CH2 stretching mode at62 cm−1 to a lower wave number and change in shape and posi-ion of the C–O–C stretching band at 1200–1000 cm−1 region inEO25–NaClO4 complex suggests the co-ordination of Na+ ions tohe ether oxygen of PEO. Moreover, the single peak at 1352 cm−1

hich is related to the swinging vibration of C–H in CH2 group inhe amorphous phase of PEO, is the evidence of increase in amor-hous phase of the polymer–salt complex. The vibrational bandsppear at 843, 952, 1281 and 1242 cm−1, are also observed in theomplex and in all the composite polymer electrolytes. A new peakt 625 cm−1 is appeared which can be assigned to spectroscopi-ally free � (ClO4

−) anions. This characteristic peak is also found toxist in all the composite polymer electrolytes. Another peak has

−1

een observed to appear at 416 cm for composite polymer elec-rolytes with 5 wt.% CuO and above. This peak corresponds to Cu–Oibration band of CuO.

(a)

(b)

680 660 640 620 600 580

No

rmal

ized

Inte

nsi

ty

Wavenumber (cm-1)

ig. 7. Peak fitting of FTIR spectra for � (ClO4−) of (a) PEO25–NaClO4 and (b) 10 wt.%

uO of PEO25–NaClO4–CuO.

PEO25–NaClO4–CuO 15 wt.% 624.61 632.16 82.37

The ion–ion interactions at the molecular level can be under-stood by analyzing the position and shape of the � (ClO4

−) bandcentered at 625 cm−1 in more detail. For all the composite polymerelectrolytes, peak maximum of the � (ClO4

−) band in FTIR spec-tra has been normalized to unity and fitted to Gaussian–Lorentzianproduct function with a straight base line [36]. Fig. 7 shows the fit-ted Gaussian–Lorentzian peak to the experimental FTIR data in the� (ClO4

−) region for PEO25–NaClO4 and 10 wt.% CuO doped compos-ite polymer electrolyte. For both the spectra, the � (ClO4

−) band hasbeen well separated into two maxima centered at around 624 and632 cm−1. It is suggested that the � (ClO4

−) band within the ranges620 and 625 cm−1 can be attributed to spectroscopically free ClO4

−

anions whereas the band centered between 630 and 635 cm−1 isrelated to the contact–ion pairs [37,38]. The fraction of free ClO4

−

anions and Na+–ClO4− contact–ion pairs have been evaluated by

taking the ratio of the integral area of each peak to the total area of� (ClO4

−) vibration. It is observed that the peak characteristic forfree anions is much larger than that representing contact–ion pairs.In Table 2, the fraction of free anions evaluated on the basis of fittedFTIR data is presented for different composite polymer electrolytes.The percentage of free anions in composite polymer electrolytes ishigher than that of the PEO25–NaClO4. This result suggests that theLewis acid–base interaction of CuO nanoparticles favors the dis-solution of NaClO4 salt in the composite polymer electrolyte andpromotes more free Na+ ions. The free ion and contact ion pairsas a function of CuO concentration are shown in Fig. 8(a and b),respectively. Free ion percentage increases in the lower concentra-

tion region of added CuO nanoparticles. The maximum value of freeion percentage is found around 10 wt.% of CuO. Fig. 8 shows that foreach maximum in free ion matches with each minimum of contaction pairs with the variation of CuO content.

-2 86420 10 12 14 160.64

0.68

0.72

0.76

0.80

0.84

0.88

(b)

(a)

Co

ntact-io

n p

airFre

e C

lO4-1

ion

CuO concentration (wt.%)

0.12

0.16

0.20

0.24

0.28

0.32

0.36

Fig. 8. Variation of (a) free ClO4− ion percentages and (b) contact–ion pair percent-

ages with CuO concentration.

A. Dey et al. / Spectrochimica Acta Part A 83 (2011) 384– 391 389

1250 1200 1150 1100 1050 1000 950

No

rmal

ized

Inte

nsi

ty

Wavenumber (cm-1)

F1

Cioamt1mmrrptaC

F2o

86420 10 12 14 160.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

CuO concentration (wt.%)

1107-1114 cm-1

1140-1150 cm-1 1026-1033 cm-1

1068-1075 cm-1

Are

a R

atio

ig. 9. Deconvolution of FTIR spectra for C–O–C band in pure PEO in the region of250–950 cm−1.

The most noticeable changes in FTIR spectra are observed in–O–C vibrational modes in the region of 1200–1000 cm−1, which

nvolve various interactions among PEO, NaClO4 and CuO moleculesf the composite polymer electrolytes. A quantitative analysis haslso been performed for the FTIR spectra of C–O–C vibrationalodes in the region of 1200–1000 cm−1. The fitted FTIR spectrum of

his regime consists of six bands at 1014, 1033, 1059, 1094, 1112,148 cm−1 which are attributed to C–O–C symmetric and asym-etric stretching and deformation modes as shown in Fig. 9. Theaximum area is found for the peak at 1112 cm−1 and the cor-

esponding full-width at half maximum (FWHM) is 29 cm−1. Theelative area with respect to the maximum area for the other three

−1

eaks at 1059, 1094 and 1148 cm are 29%, 37% and 20%, respec-ively. The results of deconvoluted FTIR spectra of PEO25–NaClO4nd composite polymer electrolytes of 2 wt.%, 5 wt.% and 15 wt.%uO are presented in Fig. 10(a–d). The major absorption band of

(a)

Wavenumber (cm-1)

(b)

(c)

No

rmal

ized

Inte

nsi

ty (

a.u

)

(d)

1250 1200 1150 1100 1050 1000 950

ig. 10. Deconvolution of FTIR spectra for C–O–C band in (a) PEO25–NaClO4 and (b) wt.% CuO; (c) 5 wt.% CuO and (d) 15 wt.% CuO of PEO25–NaClO4–CuO in the regionf 1250–950 cm−1.

Fig. 11. Variation of relative integrated intensity with CuO concentration for fourpeaks in the region of 1150–1026 cm−1.

PEO in PEO–NaClO4 shifts to a lower wave number at 1108 cm−1,the value of FWHM becomes broadened to 53 cm−1 and the bandat 1059 associated with crystalline PEO vanishes completely. Theseresults clearly indicate the complex formation of Na+ of NaClO4with ether oxygen of PEO and decrease in crystallinity. The cal-culated area ratio as a function of CuO concentration is presentedin Fig. 11. The position of the peak maximum gradually increaseswith the CuO concentration and attains a value of 1114 cm−1 forthe highest content of the nanoparticle. The area ratios of peaksat 1026–1033 cm−1 and 1068–1075 cm−1 show maxima at 5 and10 wt.% CuO concentration. So it may be concluded that the interac-tions among PEO, NaClO4 and CuO play an important role to changein intensity, shape and position of these stretching modes.

Fig. 12 shows room temperature impedance plots at differentCuO concentrations. The spectrum consists of depressed semicir-cle at high frequency and inclined spikes at low frequency. The lowfrequency inclined spikes actually represent blocking electrodeseffect. The resistance of the electrolyte is evaluated from measuredimpedance data simulated by the equivalent circuit as presentedin Fig. 12. The circuit consists of parallel combination of bulk resis-tance (Rb) and the nonideal capacitor usually known as constantphase element (CPE) and a series capacitor C1. The pure capacitoris replaced by a CPE capacitor C(ω) = B(iω)l−1, which is assumed todescribe the depressed semicircles and also the nonideal electrodegeometry. The parameter B is a constant for a given set of experi-ment and the exponent l varies between 0 and 1. CPE behaves as anideal capacitor for l = 1 and ideal resistor for l = 0. C1 is the double-layer capacitance that represents the capacitive coupling across theinterface between electrolyte and blocking electrodes. The valuesof equivalent circuit elements have been evaluated by fitting theimpedance plots using nonlinear least square fitting program. Thesolid lines in Fig. 12 represent the best fitted calculated values. Theconductivity has been estimated from the fitted value of bulk resis-tance Rb and the dimensions of the polymer electrolyte at eachtemperature for different concentrations of CuO nanoparticle.

The estimated conductivity variation with CuO concentrationsat three different temperatures (0 ◦C, 30 ◦C and 60 ◦C) are depictedin Fig. 13. The ionic conductivity at each temperature increases withthe CuO content and reaches to an optimum value at about 10 wt.%of the nanoparticle. The room temperature conductivity at 10 wt.%CuO is 1.15 × 10−4 S cm−1 which is almost two order higher thanthat of PEO–NaClO4 system. The addition of nanoparticles into thepolymer–salt complex creates an interface surrounding CuO. Lewis

acid–base interaction between the polar surface groups of CuO andthe electrolyte ions favors the dissolution of NaClO4 salt and createsmore conducting pathways. Hence an overall increase in conductiv-ity is expected to occur. Similar ion conduction mechanisms have

390 A. Dey et al. / Spectrochimica Acta Part A 83 (2011) 384– 391

141210864200

2

4

6

8

10

12

14

A

(a)

(b)

C1

Rb

CPE

Z1(kΩ)

Z2(

kΩ)

32100

1

2

3

B

(a)

(b)

(c)

(d)

(e)

F .% CuO of PEO25–NaClO4–CuO and (B) (a) 5 wt.% CuO; (b) 8 wt.% CuO; (c) 10 wt.% CuO; (d)1

bficdthddaiaa

Petwsb

Ft

3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7

-6

-5

-4

-3

-2 (a) (b) (c) (d) (e) (f) (g)

1000/T (K-1)

log

(σ)

(S-c

m-1

)

ig. 12. Impedance plot at room temperature of (A) (a) PEO25–NaClO4 and (b) 2 wt2 wt.% CuO and (e) 15 wt.% CuO of PEO25–NaClO4–CuO.

een reported earlier in the case of other nanosized materials as thellers in composite polymer electrolytes [6,12,39]. As the CuO con-entration is increased further, the conductivity shows a drop. Thisrop in ionic conductivity can be attributed to the blocking effect onhe transport of charge carriers resulting from the aggregation of ofigh concentrations of Cu2+–Na+ rich domains. Ionic conductivityepends on the degree of crystallinity. Conductivity increases withecrease of crystallinity. Percentage of crystallinity is minimum atbout 10 wt.% CuO as evidence from Fig. 5. Moreover, Fig. 8 clearlyndicates that the highest value of free ion concentration appearst about 10 wt.% CuO. These correspond to maximum conductivityround 10 wt.% CuO.

The temperature dependence of ionic conductivity ofEO25–NaClO4 and composite polymer electrolytes for differ-nt concentrations of CuO are presented in Fig. 14. It is observedhat the conductivity plots have positive temperature coefficientsithin the measured temperature range. Log(�) vs. 1/T plot

hows linear behavior in two temperature regions (above andelow the melting temperature). These behaviors are consistent

86420 10 12 14 16

-6

-5

-4

-3

-2

0ºC 30ºC 60ºC

log

(σ)

(S-c

m-1

)

CuO concentration (wt.%)

ig. 13. Conductivity variation with CuO concentration at three different tempera-ures.

Fig. 14. Temperature dependent conductivity plots of (a) PEO25–NaClO4 and (b)2 wt.% CuO; (c) 5 wt.% CuO; (d) 8 wt.% CuO; (e) 10 wt.% CuO; (f) 12 wt.% CuO and (g)15 wt.% CuO of PEO25–NaClO4–CuO.

86420 10 12 14 16

0.08

0.12

0.16

0.20

0.24

0.28

0.36

0.40

0.44

0.48

(a)

(b)

Ea

(eV

)

CuO concentration (wt.%)

Fig. 15. Variation of activation energy (Ea) for (a) above the melting temperatureand (b) below the melting temperature with CuO concentration.

A. Dey et al. / Spectrochimica Acta

0 100 200 300 400 500

0.0

0.1

0.2

0.3

IS

I0C

urr

ent

(mA

)

F

wn

�

wafttaFp

uicsicoewefi1vcc

4

osCb

[

[

[[[[[

[

[

[

[

[

[[

[[[[[[

[[

[[[

[[[37] M. Salomon, M.Z. Xu, E.M. Eyring, S. Petrucci, J. Phys. Chem. 98 (1994)

Time (min.)

ig. 16. Polarization current versus time plot for 10 wt.% CuO of PEO25–NaClO4–CuO.

ith Arrhenius type charge conduction in polymer electrolyteanocomposites which can be expressed as,

= �0exp(−Ea

kT

)(5)

here �0 is the pre-exponential factor, Ea is the activation energynd k is the Boltzmann’s constant. The variation of activation energyor above and bellow the melting temperature with CuO concen-ration is shown in Fig. 15(a and b), respectively. It is evident fromhe plots that the change in activation energy for both the temper-ture ranges is consistent with that of conductivity as exhibited inig. 13. The plots in Fig. 15 also indicate that the ionic conductionrocess depends on the CuO concentration.

The total ion transference number (tion) has been estimatedsing polarization method. Fig. 16 shows a typical plot of polar-

zation current as a function of time for 10 wt.% of CuO dispersedomposite polymer electrolyte on applying a voltage of 0.8 V. Thealt NaClO4 complexed with PEO dissociates into Na+ and ClO4

−

ons which move towards the electrodes under the application ofonstant voltage. Initially current decreases until a steady state isbtained as depicted in Fig. 16. Initial fall in current is due to thestablishment of concentration gradient in the polarization processhich affects the motion of ions. The value of the ion transfer-

nce number is determined using Eq. (3) and it is found to be 0.93or PEO25–NaClO4. The value of tion gradually increases with thencrease of CuO content and finally reaches to a value of 0.98 for0 wt.% of CuO added composite polymer electrolyte. However, thealue of tion slightly decreases for the higher contents of CuO. Thehange of tion with CuO concentration is consistent with that ofonductivity.

. Conclusion

The addition of CuO nanoparticles influences the crystallinity

f PEO–NaClO4 salt complex. Quantitative analysis of FTIR spectrauggests that the free ions percentage have a strong dependence ofuO content. Ionic conductivity at room temperature is improvedy two order of magnitude by incorporating CuO nanoparticles.

[

[

Part A 83 (2011) 384– 391 391

Lowering of crystallinity, increase of free ion concentration andprominent acid–base interaction with CuO nanoparticle enhanceionic conductivity of PEO–NaClO4 electrolyte nanocomposite.

Acknowledgements

This work is funded by the Council of Scientific and IndustrialResearch, Government of India, Scheme No: 03(1046)/05/EMR-II.

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