a novel ion-selective electrode of polythiophene based on...

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Copyright © 2014 American Scientific PublishersAll rights reservedPrinted in the United States of America

SENSOR LETTERSVol. 12, 1–8, 2014

A Novel Ion-Selective Electrode of PolythiopheneBased on Surfactant-Assisted Dilute

Polymerization Method

Tabassum Akhtar∗ and Masood AlamEnvironmental Science Laboratory, Department of Applied Sciences and Humanities,Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi 110025, India

(Received: xx Xxxx xxxx. Accepted: xx Xxxx xxxx)

Polythiophene (PTh) nanoparticles were synthesized by cationic surfactant assisted dilute polymer-ization method using FeCl3 as oxidant in an anhydrous medium. The physical characterizations ofthe synthesized PTh nanoparticles were studied by FT-IR and UV-Visible spectroscopy, XRD, SEMand EDX. The synthesized PTh nanoparticles were used for the first time to fabricate ion-selectivemembrane electrode. The experimental results herein concern a novel ion-selective electrode man-ufactured with an inner solid contact based on PTh nanoparticles. The electrode exhibited a goodNernstian response of 30.2 mV/decade of Mn2+ concentration over an ample concentration range(1.0×10−6−1.0×10−1 mol L−1�, with a detection limit of 1.0×10−6 mol L−1. The membrane elec-trode showed a very fast response time of 45 s and could be operated well in the pH range 3–7.The selectivity coefficients were determined by the mixed-solution method which revealed that theelectrode was selective in the presence of interfering cations. The material was used as elec-troactive component for the construction of an ion-selective membrane electrode. The membraneelectrode was mechanically stable, having wide dynamic range, with quick response time and couldbe operated for at least 5 months without any considerable divergence in the potential responsecharacteristics. The electrode was used satisfactorily act as indicator electrode during the potentio-metric titration of Mn2+ ions with EDTA.

Keywords: Polymer, Polythiophene Particles, Ion-Selective Membrane Electrode, ManganeseIon-Selective Electrode.

1. INTRODUCTIONAfter 30 years of development, the world of conductingpolymers is established as an important branch of mate-rial science with many opportunities for applications inelectronics and photonics. The term “conducting polymer”covers several types of polymeric materials with electronicand/or ionic conductivity, including (i) doped conjugatedpolymers, (ii) redox polymers, (iii) polymer composites,and (iv) polymer electrolytes. Conducting polymers arepromising materials for a large variety of chemical sen-sors, as can be seen from several reviews.1–11 However, thepresent review deals only with conducting polymer-basedpotentiometric ion sensors (ion-selective electrodes, ISEs).

Ion selective electrode is an important tool allowingthe sensitive and selective determination of various ions

∗Corresponding author; E-mail:

in the wide range of concentration. Precipitate basedion selective membrane electrodes are well known asthey are successfully employed for the determination ofseveral anions and cations. The heterogeneous precip-itate ion exchange membranes consist of suitable col-loidal ion-exchanger particles as electro-active materialsembedded in polymer (binder) like poly(vinyl chloride),epoxy resin (Araldite), poly styrene, polyethylene, nylonetc., by physically mixing or chemical reaction and havebeen extensively studied as potentiometer sensors.13–19 Thepotentiometric sensors offer several advantages such as:low cost, ease of fabrication, low detection limits, wideresponse range, reasonable selectivity, among others. Ionexchange membrane also finds application in diverse pro-cesses such as electrodialysis, diffusion dialysis, electro-de-ionization, membrane electrolysis, fuel cells, storagebatteries, electro-chemical synthesis etc., which are energy

Sensor Lett. 2014, Vol. 12, No. xx 1546-198X/2014/12/001/008 doi:10.1166/sl.2014.3376 1

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A Novel Ion-Selective Electrode of Polythiophene Based on Surfactant-Assisted Dilute Polymerization Method Akhtar and Alam

sources and environmentally saving. Ion selective elec-trode is an important tool allowing the sensitive and selec-tive determination of various ions in the wide range ofconcentrations. However, conducting polymers potentiallyfall victim to their poor mechanical stability and nar-row operating electrochemical windows. These demeritsof conducting polymers restrain their practical applicationsin SCs. A general strategy to overcome the above draw-backs is to introduce conducting polymers into the matrixof other active electrode materials.20�21 The correspondingcomposites such as manganese oxide/conducting polymerand carbon nanotube/conducting polymer are expected toconvey enhanced electrochemical properties if each com-ponent can retain its merits and both individuals can com-pensate their shortcomings synergistically. Moreover, someprevious studies have just focused on controllable forma-tion of nanostructured oxides and conducting agents sepa-rately, rather than combining both together.This research work presents the potentiometric studies

for Mn(II) detection on a new ion-selective electrode ofpolythiophene nanoparticles, prepared by cationic surfac-tant assisted dilute polymerization method. The synthe-sized PTh nanopaticles were characterized by XRD, SEM,FTIR, UV-Visible spectroscopy and EDX. In view of theaforementioned, in this work a novel ISE electrode forthe selective and quantitative determination of Mn(II) ispresented.

2. EXPERIMENTAL DETAILS2.1. Reagents and InstrumentsThe main reagents or chemicals used for the synthesisof the material were obtained from CDH, Loba Chemie,E-merck and Qualigens (India Ltd., used as received). Allother reagents and chemicals were of analytical grade.A digital pH-meter (Elico Li-10, India), a digital poten-

tiometer (Equiptronics EQ 609, India; accuracy ±0.1 mVwith a saturated calomel electrode as reference electrode,a digital electronic balance (Sartorius 21OS, Japan) and anagate mortar pastel were used.The stock solution of 1×10−1 M Mn(II) was prepared in

double distilled water (DMW). The solution was standard-ized by complexometric titration.22 The working standardsolutions (1× 10−1 M to 1× 10−10 M) were prepared byproper dilution of the stock solution.

2.2. Synthesis of Polythiophene with SurfactantAbout 1 ml amount of thiophene was mixed with 50 mlof chloroform in a reaction flask 0.0034 mol of surfac-tant (CTAB) dissolved in 30 ml of CHCl3 was added tothe monomer solution and stirred on a magnetic stirrer for15 min. The molar ratio of monomer to surfactant was 7:1.Finally, 0.055 mol of solid anhydrous FeCl3 was added tothe above monomer solution. It is important that FeCl3 isadded in the solid state and not as a solution.23 The molar

ratio of FeCl3 to thiophene was 2:3 and the polymeriza-tion was carried out for 24 h at room temperature. Thedark-brown precipitate of Polythiophene (PTh) was col-lected by filtration and washed with CHCl3 three times.The PTh was further washed with methanol to remove theresidual oxidant. During this procedure, the colour of PThwas changed from dark-brown to brown. The prepared PThpowder was dried in a vacuum oven at 80 �C for 6 h.The flow chart and reaction scheme for the preparation ofPolythiophene are shown as Figure 1.

2.3. X-Ray AnalysisPowder X-ray diffraction (XRD) pattern was obtained inan aluminum sample holder for the Polythiophene withsurfactant in the original form using a PW, 1148/89 baseddiffractometer with Cu K� radiations.

2.4. Scanning Electron Microscopy (SEM) StudiesScanning Electron Microscopy (SEM) microphotographsof the original form of Polythiophene with surfactant wereobtained at various magnifications.

2.5. Fourier Transform Infra Red (FTIR) StudiesThe FTIR spectrum of Polythiophene with surfactant inthe original form dried at 40 �C were taken by KBr discmethod at room temperature.

3. PREPARATION OF ION-EXCHANGEMEMBRANE

The ion-exchange membrane of Polythiophene was pre-pared by following the procedure of Coetzee and Benson.24

Polythiophene (PTh) powder (100 mg) as electroactive

Fig. 1. Flow chart of the synthesis of polythiophene nanoparticles.

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Akhtar and Alam A Novel Ion-Selective Electrode of Polythiophene Based on Surfactant-Assisted Dilute Polymerization Method

material was ground to fine powder, and was mixed thor-oughly with Araldite25 (Ciba-Geigy, India Ltd.) (100 mg)in 1:1 (w/w) ratio to make a homogeneous paste whichwas then spread between the folds of Whatman’s filterpaper No. 42. Glass plates were kept below and above thefilter paper folds as support. The phase of the exchangerand Araldite was kept under pressure of 2 kg cm2 for24 h and left to dry. Two sheets of different thickness of0.41 and 0.43 mm of master membranes were prepared.These sheets were dipped in distilled water to remove filterpaper. After drying, the membrane sheets were cut in theshapes of discs using a sharp edge blade. The membranethickness of 0.41 mm exhibited good surface qualities, likeporosity, thickness, swelling etc. were selected for furtherinvestigations.

3.1. Characterization of MembranePhysico-chemical characterization is important to under-stand the performance of membrane. Thus some param-eters such as porosity, water content, swelling, thickness,etc. were determined as described elsewhere.26–28

3.1.1. Water Content �%Total Wet Weight�First, the membranes were soaked into water to elute dif-fusible salt, blotted quickly with Whatmann filter paper toremove surface moisture and immediately weighed. Thesewere further dried to a constant weight in a vacuum overP2O5 for 24 hr. The water content (total wet weight) wascalculated as:

% Total wet weight= Ww −Wd

Ww

×100

Where Ww = weight of the soaked/wet membrane andWd = weight of the dry membrane.

3.1.2. PorosityThe thickness of the membrane was measured by takingthe average thickness of the membrane by using screwgauze. Swelling is measured as the difference betweenthe average thicknesses of the membrane equilibrated with1 M NaCl for 24 hrs and the dry membrane.

Porosity= Ww−Wd

Ww�oL

Where Ww and Wd are weight of wet and soaked (inNaCl) membrane while �0 and L are the density of waterand thickness of the membrane.

3.1.3. Fabrication of Ion-Selective Membrane ElectrodeThe membrane sheet of 0.41 mm thickness as obtainedby the above procedure was cut in the shape of disc andmounted at the lower end of a Pyrex glass tube (o.d.0.8 cm, i.d. 0.6) with araldite. Finally the assembly wasallowed to dry in air for 24 hr. The glass tube was filled

with solution of the ion (as reference) towards which themembrane is selective and it was kept dipped in an iden-tical solution of the same ion at room temperature. Incase of polythiophene ion-selective membrane electrode,the glass tube was filled with 0.1 M Manganese nitratesolution. A saturated calomel electrode was inserted in thetube for electrical contact and another saturated calomelelectrode was used as external reference electrode. Thewhole arrangement can be shown as:

Internalreferenceelectrode(SCE)

Internalelectrolyte0.1 MMn2+

Membrane Samplesolution

Externalreferenceelectrode(SCE)

Following parameters were evaluated to study the char-acteristics of the electrode such as lower detection limit,electrode response curve, response time and working pHrange.

3.1.4. Electrode Response or Membrane PotentialTo determine the electrode response, a series of standardsolutions of varying concentration which were to be stud-ied were prepared. External electrode and ion selectivemembrane electrode are plugged in digital potentiometerand the potentials were recorded.For the determination of electrode potentials the mem-

brane of the electrode was conditioned by soaking in 0.1 MMn(NO3�2 solution for 5–7 days and for 1 h before use.When electrode was not in use electrode was in 0.1 Mselective ion solution. Potential measurement was plot-ted against selected concentration of the respective ion inaqueous solution as given in Figure 6.

3.1.5. Effect of pHpH solutions ranging from 1–13 were prepared at constantion concentration i.e., (1×10−2 M). The value of electrodepotential at each pH was recorded and plotted against pHas shown in Figure 7.

3.1.6. Response TimeThe method for determining response time in the presentwork is being outlined as follows: The electrode is firstdipped in a 1×10−3 M solution of the ion concerned andimmediately shifted to another solution of 1×10−2 M ionconcentration of the same ion (10 fold higher concentra-tion). The potential of the solution was read at zero secondi.e., just after dipping the electrode in the second solu-tion and subsequently recorded at the intervals of 5 s. Thepotentials were then plotted versus time as shown Figure 8.

3.1.7. SelectivityThe most important characteristic of an ISE is its selec-tive response capacity to a primary ion in the presenceof other ions. This parameter is directly related to the

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thermodynamic equilibrium at the surface/solution inter-phase. The selectivity of the electrode containing the lig-and PTh/MnO2 was tested by potentiometry with differentdivalent cations like: Ca(II), Ni(II), Cd(II), Mg(II), Cu(II),Co(II), Ba(II) and Hg(II). The slope assessment for themajority of cations tested showed values much lower thanexpected from Nernstian behavior. However, for Mn(II), alinear potentiometric response was observed with a valuecloser to such behavior, within a wide concentration rangeand a low detection limit.The potentiometric selectivity coefficient (as measured

in terms of the potentiometric selectivity constant KPoti� j )

that describes the preference of the electrode suggestedto an interfering ion in the presence of Mn(II), wasdetermined by means of the fixed interference method,which is based on the Nikolsky-Eisenman semiempiricequation,29�30 as follows:

EISE = Eo +RT /Zif ln�ai+Kpotij a

Zi/Zj

j � (1)

where EISE is the measured potential, Eo is the stan-dard potential, ai and aj are the activities of the primaryand interfering ions, respectively, zi is the charge of pri-mary ion, zj is the charge of the interfering ion, F isthe Faraday constant, R is the universal gas constant andT is the absolute temperature. In this method, the pri-mary ion concentration is varied at 1.0 mol L−1 solutionwhere as the secondary ion concentration is kept constantat 1�0×10−3 mol L−1. The equations for each ion can bewritten as

Ei = Eo +RT /Zif ln�ai (2)

Ej = Eo +RT /Zif ln�Kpotij aj (3)

Equalizing the potentials one gets

logKpotij = aj

�aj�zi/zj

(4)

Using the fixed interference method (FIM), the selectiv-ity coefficients were calculated and are shown in Table III.

3.1.8. Storage of ElectrodesThe polythiophene electrode was stored in distilled waterwhen not in use for more than one day. It was activatedwith (0.1 M) Mn(II) solution by keeping immersed in itfor 2 h, before use, to compensate for any loss of metalions in the membrane phase that might have taken placedue to a long storage in distilled water. Electrode was thenwashed thoroughly with DMW before use.

3.1.9. Analytical Application of the ElectrodeThe analytical utility of this membrane electrode has beenestablished by employing it as an indicator electrode inthe potentiometric titration of a 0.01 M Mn(NO3�2 solu-tion against 0.005 M EDTA solution. Potential values areplotted against the volume of EDTA.

Fig. 2. Powder X-ray diffraction pattern of polythiophene (as prepared).

4. RESULTS AND DISCUSSIONX-ray diffraction study showed the crystalline nature ofPTh nanoparticles (Fig. 2). There was a partially crys-talline broad peak centered at near 2 value of 24.2�. Thestrong diffraction peak associated with the chain-to-chainstacking distance of about 2 = 24�2� is due to the amor-phously packed polythiophene main chain.31�32

Fig. 3. SEM photographs of polythiophene composite.


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Fig. 4. EDX spectrum of polythiophene.

The SEM image of polythiophene nanopowder is shownas Figure 3. The image appears uniformly microporous onthe surface and the particles were in nanometer scale.

The EDS spectra of pure polythiophene is shown inFigure 4. It can be seen that there are only two elementsC and S and traces of Fe were also found in Polythio-phene. It further proves that the polythiophene is synthe-sized successfully, as FeCl3 is used as an oxidant duringpolymerization. Table I gives the elemental compositionsof Polythiophene.

The FTIR spectrum of polythiophene is shown asFigure 5. In the spectrum of Polythiophene, there are sev-eral low-intensity peaks in the range of 2,000–3,000 cm−1

which can be attributed to the aromatic C–H stretchingvibrations and C C characteristic band.33�34 The absorp-tion in this region is obscured by the bipolaron absorptionof the doped PTh. The range of 5,00–1,500 cm−1 is thefingerprint region of PTh. The peak at 789 cm−1 is usu-ally ascribed to the C–H out-of-plane deformation mode,whereas other peaks in this region are attributed to the ringstretching and C–H in-plane deformation modes.35 TheC–S bending mode has been identified at approximately690 cm−1 which indicates the presence of a thiophenemonomer.36 These results support to the polymerization.

4.1. Characterization of Composite MembraneSensitivity and selectivity of the ion-selective electrodesdepend upon the nature of electro-active material, mem-brane composition and physico-chemical properties of themembranes employed. A number of samples of the poly-thiophene membrane were prepared with different amountof composite and Araldite and checked for the mechanicalstability, surface uniformity, materials distribution, cracksand thickness, etc. The membranes obtained with 0.41 mmwere found to be good.

Table I. EDX element composition of polythiophene.

Elements Wt% At%

CK 62�03 81�09SK 36�19 17�87FeK 1�78 1�04

Fig. 5. FTIR spectra of as prepared polythiophene composite material.

The results of thickness, swelling, porosity and watercontent capacity of polythiophene membrane are summa-rized in Table II. The membrane sample M-1 of thickness0.41 mm was selected for further studies. The low order ofwater contents, swelling and porosity with less thicknessof the membrane suggest that interstices are negligible anddiffusion across the membrane occur mainly through theexchanger sites.

4.2. Potentiometeric Studies of HeterogeneousPolythiophene Membrane Electrode

Polythiophene membrane (M-1) was fabricated into ion-selective electrode. The membrane electrode was placed in0.1 M Mn(NO3�2 solution for 7 days to get it conditioned.After conditioning the electrode, the potentials for a seriesof standard solution of the Mn(NO3�2 in the range 10−10 Mto 10−1 M were measured at a fixed concentration of Mn2+

ion as internal reference solution.Potentials of the membrane electrode were plotted

against the selected concentration of Mn2+ ions (Fig. 6).It gives linear response in the range 1× 10−1 M and1×10−6 M. Suitable concentrations were chosen for slop-ing portion of the linear curve. The limit of detection, isdetermined from the intersection of the two extrapolatedsegments of the calibration graph37 which was found tobe 1× 10−6 M. It shows a good Nernstian response of30.2 mV/decade of Mn2+ concentration and the wide linearconcentration range from 1.0×10−6–1.0×10−1 M. Below1×10−6 M non linear response was observed which could

Table II. Characterization of ion-exchange membrane.

Thickness Water content Swelling of %Polythiophene of the as % weight of weight of wetcomposite membrane (mm) wet membrane Porosity membrane

M-1 0.41 2.012 0.0016 2.122M-2 0.43 2.233 0.0018 3.160


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Fig. 6. Calibration curve of polythiophene membrane electrode in aque-ous solutions of Mn(NO3�2.

be used for analytical applications.38 Experiments wereconducted a number of times to check the reproducibilityof the results. EMFs were plotted against log of activi-ties of manganese ions and calibration curves were drawnfor three sets of experiments and a standard deviation of±0.2 mV was observed.The influence of the pH on the response of Mn2+-ISE

based on ionophore was studied. The pH effect is illus-trated in Figure 7 by varying the pH of test solutions with0.1 M HNO3 and 0.1 M NaOH for 1.0×10−2 M and 1.0×10−3 M Mn(NO3)2, respectively. The results were recordedonly for 1×10−2 M ion solutions. It is clear from Figure 7that the operational pH range was 3.0–7.0 for the electrodebased on ionophore. At higher pH, Mn2+ will form precip-itate with OH−, thus causing the potential reduction. Whenthe pH was low, the concentration of H+, shows obviousdisturbance on the potentiometric response of Mn2+-ISE.

200

220

240

260

280

300

320

340

360

380

1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH

-Ele

ctro

de

Po

ten

tial

(m

V)

Fig. 7. Effect of pH on the potential response of the polythiophenemembrane electrode at 1×10−2 M Mn2+ concentration.

Another important factor is the promptness of theresponse of the ion-selective electrode. The averageresponse time is defined39 as the time required for the elec-trode to reach a stable potential after successive immersionof the electrode in different Mn2+ ion solutions, each hav-ing a 10-fold difference in concentration from 10−10 Mto 10−1 M was investigated. In this work, the practicalresponse time was recorded by immediate and successivechanging of Mn2+ concentration from 1.0×10−10 to 1.0×10−1 M. The results are shown in Figure 8. As it canbe seen that in the whole concentration range, the Mn2+-ISE electrode showed reasonably fast and stable potentialwithin 45 s and no change was normally observed up to5 min after which it started deviating.It is very important that the performance of any ion-

selective electrode should be checked soon every timebefore using it for any analytical purpose. For the presentpolythiophene membrane electrode, it was observed thatthe measured potential of Mn2+ ions in a given concen-tration range of 10−1–10−10 M was reproducible within±2 mV, and there was no significant change in the slopeof the Nernst plot during the experiment over a time periodof 5 months. This suggests a longer electrode life and astable electrode performance.The selectivity coefficients, K

potMn�M of various differ-

ing cations for Mn(II) ion selective polythiophene mem-brane electrode were determined using the mixed solutionmethod.40 The selectivity coefficient indicates the extent towhich a foreign ion (Mn+� interferes with the response ofthe electrode towards its primary ions (Mn2+�. By exam-ining the selectivity coefficient data given in Table III it isclear that that the electrode is selective for Mn(II) in thepresence of interfering cations.

Fig. 8. Time response curve of polythiophene membrane electrode.


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Table III. The selectivity coefficients (KPOTMn�M) of various interfering

cations (Mn+).

Na+ 0.002Mg2+ 0.002Sr2+ 0.005Al3+ 0.008Zn2+ 0.004Mn2+ 0.007Pb2+ 0.006Cu2+ 0.001Fe3+ 0.005Ca2+ 0.003K+ 0.004

5. ANALYTICAL APPLICATION5.1. Potentiometer TitrationThe study suggests that this newly characterized Mn2+-ISEcould work well under the laboratory conditions. It hadbeen effectively applied as indicator electrode to the poten-tiometric titration of a spiked 1.0× 10−2 M Mn(NO3)2solution with 0.01 M EDTA as titration reagent. The addi-tion of EDTA causes a decrease in potential as a resultof the decrease in the free Mn(II) ions concentration dueto its complexation with EDTA (Fig. 9). The amount ofMn(II) ions in solution can be accurately determined fromthe resulting neat titration curve providing a sharp rise inthe titration curve at the equivalence points. The resultsdemonstrate that the Mn2+ amount in this solution can bedetermined with good accuracy.

Mn2+ ion-selective membrane electrode was alsoapplied to direct measurements of Mn2+ in the drainwater collected from Department of Applied Sciences andHumanities, Jamia Millia Islamia, New Delhi. The sam-ples were collected from three different locations of drainsand preserved with HNO3, stored in glass bottles and ana-lyzed within 12 h after collection. Since the samples con-tained particulate matters, they were centrifuged, and thepotentials were measured. Three replicate measurementswere made to obtain the Mn(II) contents in three samples

150

170

190

210

230

250

270

290

310

330

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 6 6.5 7

Volume of EDTA (ml)

-Ele

ctro

de p

oten

tial (

mV)

Fig. 9. Potentiometric titration of Mn(II) against EDTA solution usingpolythiophene membrane electrode.

with this electrode using the membrane sensor’s calibrationgraph. The concentration of manganese in the drain watersamples were in the range of 1.0×10−7 M to 6.5×10−8 M,and the reproducibility of the results was checked up tothree times.

6. CONCLUSIONPolymer Polythiophene based Araldite membrane elec-trode was found to be selective and sensitive for Mn(II)ions. The membrane electrode possessed a quick responsetime of only 45 s. The functional pH range of the elec-trode was 3–7. A Mn2+-ISE based on a newly synthesizedionophore was prepared. The results show that this sensorshowed a wide linear range of 1.0× 10−5–1.0× 10−1 Mwith a detection limit of 1.0×10−5 M Mn2+. The selectiv-ity of the ion-selective electrode showed good results overa wide range of interfering metal ions. These character-istics and the typical applications presented in this papermake the sensor a suitable for measuring Mn(II) content inreal samples without a significant interaction from cationicor anionic species. It could be successfully applied in thepotentiometric titration of Mn(II) ions in solution withEDTA in the presence of metal nitrate solutions.

Acknowledgments: The authors are highly thankful toHead, Department of Applied Physics, AMU, Aligarhfor providing FTIR, XRD facilities and Department ofApplied Sciences and Humanities, Jamia Millia Islamia,New Delhi for providing SEM and EDX facilities.Tabassum Akhtar is thankful for the financial support (Fel-lowship) to UGC.

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