J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 1, 2 0 0 2, 1393 – 1395

Polymer electrolyte composites with dispersed semiconductors

P. K. SINGH, S. CHANDRADepartment of Physics, Banaras Hindu University, Varanasi 221005, India

A. CHANDRADepartment of Physics, Panjab University, Chandigarh 160014, India

Electron and ion conducting polymers have recentlybeen suggested for applications in polymeric electronicor ionic devices. Familiar common examples of elec-tron conducting polymers are polypyrrole, polyaniline,polythiophene etc. while salt complexed polar poly-mers (e.g. PEO, PEG, PPO etc.) and ion exchangednafion are the better known ion conducting polymers.Since the first report [1–3] on the ion conducting poly-mers, the interest has become focused on Li+ andH+ ion conducting polymers for battery and fuel cellapplications respectively. The primary motivation forthe present study was to obtain a mixed ion + electronconducting polymer by following a strategy of the dis-persal of different amounts of semiconductors withdifferent band gaps in a known polymer electrolyte.For the present study, polyethylene oxide (PEO) com-plexed with NH4I (80 : 20 wt%), was used as the sup-porting ion conducting membrane [4] in which CdSwas dispersed for developing mixed ion + electronconducting polymeric membranes. In all our studiesthe PEO : NH4I ratio was kept at 80 : 20 because forhigher NH4I contents (say 70 : 30) we [5] have pre-viously reported growth of fractals and for still higherNH4I contents the membranes were “jelly” like and notrigid. To prepare the semiconductor-dispersed polymerelectrolyte (SDPE) films, pure PEO (molecular weight∼6 × 105, Aldrich) and NH4I (Aldrich) were mixedand dissolved in dehydrated methanol and stirred for3–4 h to form a viscous solution. To this, Cd-acetatesolution was added in the desired weight ratio (1, 5,10 wt%) and H2S was bubbled through it. Sulfurationof the Cd-acetate by H2S results in the formation ofCdS which was confirmed by the appearance of promi-nent XRD peaks of CdS at 2θ = 26.5◦, 44◦ and 55◦. Therate of bubbling of H2S gas in the solution was variedto control the rate of formation of the semiconductingparticulates. The final viscous mixture was poured intopolypropylene petri dishes for “solution casting”of thepolymeric membrane. It was dried first under ambientroom conditions and then under 10−6 Torr vacuum toeliminate all traces of the solvent.

Films of PEO : NH4I polymer electrolytes were tran-sparent and colorless but the (polymer electrolyte +CdS) films were of different colors. The color of a ma-terial is directly linked with its optical absorption andthe band gap. Freshly prepared viscous polymeric so-lution with dispersed CdS poured into the petri dishwas uniformly yellowish. After 4–5 days, the heav-ier particles collect near the center portion giving this

portion a more yellowish color while the smaller par-ticles collect near the edge giving a different color(yellowish-greenish). This suggests that a reasonableproportion of the nano-particles of CdS are formedleading to a change in the band gap (and hence color)due to the well known quantum-size effect. The valueof band gap (Eg) can be evaluated from a plot of (αhν)2

vs. hν since the absorption coefficient (α) at a frequencyν for a direct band gap material is given by

α = K · (Eg − hν)1/2/hν

Fig. 1 shows the optical absorption spectra of the centerand edge portions of a typical film giving respectiveEg values of 2.4 eV (same as bulk crystalline CdS)and 3.44 eV (more than bulk Eg) indicating that theformer has large particles with bulk-like Eg while thelatter has some nano-particles (showing quantum sizeeffect). From the measured values of Eg for differentSDPE’s the following important general conclusionscan be drawn:

(i) For the same H2S bubbling rate, the CdS particlesize increases as the amount of Cd-acetate in thereaction vessel increases from 1 to 10 wt%.

(ii) For the same Cd-acetate in the reaction vessel, theparticle size was reduced for the lower H2S bub-bling rates.

Figure 1 Absorption spectra of the center and edge portions of the(PEO : NH4I) + 1 wt% CdS membrane prepared at H2S bubbling rateof 200 bubbles/min. The values of absorbance on the Y -axis is given asthe logarithm of ratio of the intensity of the incident beam I0 to that ofthe transmitted beam I .

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(a)

(b)

Figure 2 TEM micrographs showing CdS particle size distribution inthe (PEO : NH4I) + x wt% CdS films (a) x = 1 wt% (b) x = 10 wt% ofCdS. The H2S bubbling rate during formation of CdS was maintained at50 bubbles/min.

The TEM micrographs of two typical (PEO : NH4I) +CdS composite polymeric membranes are shown inFig. 2 which clearly indicates a particle size distribu-tion in the range of ∼8–50 nm. The samples preparedfrom the reaction vessel containing a low Cd-salt con-centration (1 wt%) contained more smaller particles incomparison to that with higher Cd-salt concentration(10 wt%). Interestingly, the TEM shows, apart fromthe spherical/clusters, regions with “pancake-like” par-ticles. Non-spherical shapes of nano-particles are ex-pected due to the enhanced surface-to-volume ratio [6].

Measurement of the ionic transference number tion(and hence electronic transference number te = 1 − tion)by Wagner’s polarization method [7, 8] shows that thedispersal of CdS in the polymer electrolyte membraneintroduces partial electronic conductivity. The values oftion evaluated for different SDPE- are shown in Table I.

It is obvious that the relative electronic transfer-ence number (te = 1 − tion) is greater for the membranes

T ABL E I Calculated values of tion for (PEO : NH4I) + CdS mem-branes using Wagner’s polarization method

tion for the films prepared withH2S bubbling rate

Composition (wt%)of Cd-salt 200 b/m 100 b/m 50 b/m

1 0.87 0.91 0.925 0.80 0.85 0.86

10 0.79 0.84 0.85

richer in CdS as expected. Another feature which comesout from Table I is that the membranes with the sameamount of CdS (1, 5 or 10 wt%) but prepared at lowH2S sulfuration rate ( say 50 bubbles/min ≈30 mL/min)have lower electronic conductivity than those preparedat 200 bubbles/min (≈120 mL/min) since the CdS par-ticles obtained at low bubbling rate are smaller withlarger Eg, giving a relatively smaller electronic contri-bution.

The dispersal of semiconductor CdS also changes theoverall total conductivity (σT) of the membranes inclu-sive of ionic (σi = tion · σT) and electronic contributions(σe = te · σT). The true bulk conductivity was evaluatedby using complex impedance spectroscopy in the fre-quency range 40 Hz–100 KHz with the help of a HIOKI3520 LCR HI TESTER. Fig. 3 shows the variation ofelectrical conductivity of (PEO : NH4I) + CdS mem-branes with different amounts of dispersed CdS pre-pared at different H2S bubbling rates. For the same H2Sbubbling rate, the conductivity first increases from 1–5 wt% of CdS and then decreases. Peaking in σ vs. com-position plots is a common feature for the “ionic con-ductivity” of the dispersed phase composites [9, 10] e.g.AgBr : Al2O3, AgI : Al2O3. The increase in the ionicconductivity of the “ionic conductor-insulating com-posites” has been explained on the basis of an interfa-cial space-charge model due to Maier [11a, b] and apercolation threshold model given by Bunde et al. [12]The percolation threshold for σ in our “semiconductor-ionic conductor composites” is at ∼5 wt% while it isgenerally found to be in the 40–60 wt% range for theinsulator-ionic conductor composites. The occurrenceof maxima in σ at a lower dispersoid concentration ispossibly linked with our dispersoid being electronic innature. The higher concentration of CdS has a “killingeffect” on the conductivity enhancement, probably

Figure 3 The variation of total conductivity with the amount of dis-persed CdS in (PEO : NH4I) + CdS composite polymeric membranesprepared at different H2S bubbling rates of 50, 100 and 200 bubbles/min.

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because of the immobilization (or charge neutraliza-tion) of the ionic carriers at the interface of the elec-tronic conductor CdS.

To summarise, we have successfully prepared semi-conductor dispersed polymer electrolyte (SDPE) mem-branes by in situ dispersal and formation of CdSparticulates of different sizes in the (PEO : NH4I)matrix. SEM/TEM shows the presence of nano-CdSwith a particle size distribution. Our SDPE’s are (mixedionic + electronic) conductors while the PEO : NH4Ipolymeric film with no dispersed CdS was almost ionicin nature (tion ∼ 0.99).

AcknowledgmentThe financial assistance from C.S.I.R. (Govt. of India)in the form of an Emeritus Scientist project is gratefullyacknowledged.

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Received 3 Apriland accepted 28 May 2002

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