OPTICAL PROPERTIES OF PVP BASED POLYMER ELECTROLYTE FILMS

Transcription

1 International Journal of Physics and Research (IJPR) ISSN Vol. 3, Issue 3, Aug 2013, 7-12 TJPRC Pvt. Ltd. OPTICAL PROPERTIES OF PVP BASED POLYMER ELECTROLYTE FILMS G. NAGA SUDHA VANI 1, V. RAJA 2, A. K. SHARMA 3, V. V. R. NARASIMHA RAO 4 1,3,4 Department of Physics, Sri Venkateswara University, Tirupati, Andhra Pradesh, India 2 Department of Physics, N.B.K.R. Science & Arts College, Vidyanagar, Hyderabad, Andhra Pradesh, India ABSTRACT Poly vinyl pyrrolidone (PVP) based polymer electrolyte films complexed with potassium chloride (KBr) salt were prepared using solution casting technique. Structural and complexation of the polymer was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques.the temperature dependence of conductivity was performed using AC Impedance Analyzer in the frequency range khz and temperature range K. The transference numbers were determined using Wagner Polarization technique and dominant conducting species were found to be ions rather than electrons. Optical absorption spectra were recorded at room temperature in the wavelength range nm using Perkin Elmer UV-VIS-NIR(Model-UV-3100) spectrophotometer. KEYWORDS: Polymer Electrolytes, XRD, FTIR, Conductivity, Transference Numbers, Optical Band Gaps INTRODUCTION Studies on optical properties of polymer electrolyte have attracted much attention in view of their application in electronic and optical devices like electro chromic display devices, super capacitors, fuel cells, gas sensors, solid state batteries etc. Electrical conduction in polymers has been studied aiming to understand the nature of the charge transport prevalent in these materials while the optical properties are aimed at achieving better reflection, antireflection, interface and polarization properties. The optical properties of polymer can be suitably modified by the addition of dopants. Although some work has been reported on the charge transport and optical properties of doped polymer electrolyte films [1 3].. Since K + is a fast conducting ion in a number of crystalline and amorphous materials, its incorporation in a polymer system may be expected to enhance its optical performance. This article presents the results of such investigations on the optical properties of KBr doped PVP films. Lithium-ion conducting polymer electrolytes are of great research interest owing to their possible application to lithium-polymer batteries with high-energy density.a few attempts have tried polymer electrolytes based on potassium ion complexed films [4-5]. The main advantage of using potassium metal ion is its availability in abundance at a cheaper cost than lithium. Further, more softness of the material makes it easier to achieve good contact with other components. In the present investigations, our intension is mainly focused on the preparation and characterization of PVP based polymer electrolytes using potassium chloride as dopant. EXPERIMENTAL Films of pure PVP and KBr salt complexed were prepared with weight percent ratios (95:05), (90:10) and (85:15) by solution cast technique using triple distilled water as solvent. The solutions were stirred for hours to get a homogenous mixture and then casted on to polypropylene dishes and allowed to evaporate slowly at room temperature. The final product was vacuum dried thoroughly. The XRD patterns of the films were recorded with a Seifert-3003 TT X- ray diffract meter. FTIR spectra of these films were recorded using EO-SXB IR spectrometer with a resolution of 4 cm -1. The measurements were taken over a wave number range of cm -1. Optical absorption spectra were recorded at

2 8 G. Naga Sudha Vani, V. Raja, A. K. Sharma, V. V. R. Narasimha Rao room temperature in the wavelength range nm using Perkin Elmer UV-VIS-NIR (Model-UV-3100) spectrophotometer. RESULTS AND DISCUSSIONS XRD Analysis X-ray diffraction pattern is essential to obtain the detailed structure, knowledge on the given material. If the material under investigation is crystalline, well defined peaks are observed, while non-crystalline or amorphous systems show a broad instead of well defined peaks.the X-ray diffraction pattern of pure PVP, potassium chloride salt and PVP complexed with KBr were shown in Figure 1. In the figure, the pure PVP shows a characteristic peak at14 0 indicating its semi crystalline nature. The intensity of this peak gradually decreases with the increase of potassium chloride salt. This could be due to the disruption of the PVP crystalline structure by KBr. This shows a decrease in the degree of crystallanity of polymer after the addition of salt. No sharp peaks were observed for higher concentrations of KBr salt in the polymer, suggesting the dominant presence of amorphous phase. Figure 1: XRD Analysis of PVP, KBr and KBr Doped PVP Polymer Electrolyte Films FTIR STUDIES FTIR spectroscopy is important tool to investigate the information about the complexation and interactions between the various constituents in the polymer electrolyte. Figure 2 shows the FTIR spectra of pure PVP, KBr complexed PVP and KBr salt. The absorption band in the region 2874cm -1 is due to the intermolecular hydrogen bonded with carbon C-H stretching frequency of pure PVP which is shifted to 2922 cm -1,2939 cm -1 and 2952 cm -1 in the 05%, 10%, and 15% salt complexed PVP films respectively. In addition to this, the C-N stretching showed an absorption band at 2151 cm -1 in pure PVP and is shifted to 2137 cm -1, 2131 cm 1 and 1878 cm -1 for 05%, 10% and 15% salt complexed PVP films respectively. Similarly for C=O stretching for pure PVP is 1750 cm -1 and is shifted 1752 cm -1, 1755 cm -1 and 1760 cm -1 for 05%, 10% and 15% salt complexed PVP films The C-C stretching for pure PVP is 1704 cm -1 for Pure PVP and is shifted to 1710 cm cm -1 and 1716 cm - 1 for 05%, 10% and 15% salt complexed PVP films. The C-H bending in pure PVP exhibited absorption at 1465 cm -1 is also shifted to1459 cm -1, 1453 cm -1, and 1439 cm -1 for 05%, 10% and 15% salt complexed PVP films.finally the deformation is coupled to C-H wagging and gives rise to a peak at 1382 cm -1 in pure PVP which is shifted to 1374 cm -1, 1370 cm -1 and 1368 cm -1 in the complexed films due to complexation of salt. All these changes in the FTIR spectra are clear indications for the complexation of PVP with KBr salt.

3 Optical Properties of PVP Based Polymer Electrolyte Films 9 Figure 2: Fourier Infrared Spectra of KBr Complexed PVP Polymer Electrolyte Films TRANSFERENCE NUMBER MEASUREMENTS In order to verify the dominant conducting species in the present electrolyte system, Transference number measurements were carried out using well known Wagner s polarization technique [6]. In this method, the dc current was monitored as a function of time on application of a fixed dc voltage of 1.5 V across the cell K/(PVP + KBr)/C. The current vs. time plot of (PVP + KBr) is shown in Figure 3. From the figure, the transference numbers (t ion and t ele ) have been evaluated using the formula t ion =1 I f /I i t ele = 1- t ion where I i and I f are the initial and final currents, respectively. The resulting data are shown in table 1 for all the compositions of the (PVP: KBr) electrolyte system, the values of the ionic transference number are close to unity. This suggests that the charge transport in these polymer electrolyte films is predominantly to ions rather than electrons. Figure 3: Transference Number Measurements for PVP: KBr Doped Polymer Electrolyte Films OPTICAL PROPERTIES Optical absorption studies on pure PVP and KBr doped films with weight ratios (95:05), (90:10) and (85:15) were carried out over a wave length range nm to determine the optical constants such as optical band gap (both direct and indirect) and absorption edge. The absorption coefficient was determined from the spectra using the formula

4 10 G. Naga Sudha Vani, V. Raja, A. K. Sharma, V. V. R. Narasimha Rao I=I o exp (-αx) Hence, α =2.303/x log(i/i o ) = 2.303/ x A where A is the absorption, x is the thickness of the film. The absorption edge values were evaluated by extrapolating the linear portions of the α versus hυ as shown in Figure 4 to zero absorption value. It was observed that absorption edge for pure film lies at 5.29eV and while for doped films the values decreases as seen from the table 2. Figure 4: α vs. hν Plots of PVP and KBr Doped Polymer Electrolyte Films When the direct band gap exists, the absorption coefficient has the following dependence on the energy of the incident photon [7-8]. αhν = C(hν-E g ) 1/2 where Eg is the optical band gap, C (= 4πσ o /ncδe) a constant dependent on specimen structure, υ frequency of the incident light and h, the Planck s constant. Thus, a plot of (αhυ) 2 versus photon energy for all the samples is as shown in Figure 5. The intercept on the energy axis on extrapolating the linear portion of the curves to zero absorption value may be interpreted as the value of the band gap. For pure film the direct band gap lies at 5.25eV, while for doped films the values decreases as seen from the table 2. Figure 5: (αhυ) 2 vs. hν Plots of PVP and KBr Doped Polymer Electrolyte Films

5 Optical Properties of PVP Based Polymer Electrolyte Films 11 For indirect transitions, which require phonon assistance, the absorption coefficient has the following dependence on the photon energy [9-10]. h = A(h - E g +E p 2 ) +B(h - E g where E p is the energy of the photon associated with transition, and A and B are constants depending on the band structure. The two terms here represent contributions from transitions involving phonon absorption and emission respectively and will have different coefficients of probability and temperature dependences. The indirect band gaps were obtained from the plots of (αhυ) 1/ versus photon energy as shown in Figure 6 and are given in the table 2. - E p ) 2 Figure 6: (αhυ) 1/ vs. hν Plots of PVP and KBr Doped Polymer Electrolyte Films It is clear from the table that the band edge, direct band gap and indirect band gap showed a decreasing trend on doping with KBr salt up to a dopant concentration 15%wt. This type of behaviour was also observed in many polymer films [11]. The magnitudes of the activation energies obtained from conductivity data are small in comparison with optical band gap energies. This is due to the fact that their nature is different. While the activation energy corresponds to the energy required for conduction from one site to another, the optical band gap corresponds to interband transition [12]. CONCLUSIONS Polymer electrolytes based on PVP complexed with KBr were prepared by solution cast technique.the XRD pattern reveals the increase in amorphous nature of the film with the addition of salt. FTIR spectra of these films indicating the complexation of PVP polymer with the salt. The electrical conductivity was found to increase with the increase of temperature. From transference measurements it is clear that, the conduction mechanism in these electrolyte systems is predominantly due to ions rather than electrons. Optical absorption edge, direct and indirect optical energy gap values were evaluated from the optical absorption data, showed a decrease in their value on the addition of salt REFERENCES 1. P.V. Wright, Br. Polym. J. 7 (1975) M.B.Armand, J.M.Chabagno, M. Duclot In: Vashishta P, Murthy JN, Shenoy GK (eds.) Fast ion transport in solids. North Holland, Amsterdam (1979) 3. H. Akashi, K. Tanaka, K. Sekai, J. Electrochem. Soc. 145 (1998) V.Raja, A.K.Sharma, V.V.R.Narasimha Rao, Mater.Lett.58 (2004) 3242

6 12 G. Naga Sudha Vani, V. Raja, A. K. Sharma, V. V. R. Narasimha Rao 5. V.M.Mohan, V.Raja, A.K.Sharma, V.V.R.Narasimha Rao, Ionics12 (2006) J.B.Wagner, C.Wagner Electrical conductivity measurements on Cuprous Halides J Chem Phys 26 (1957) G.K.M. Thutupalli, S.G. Tomlin, J. Phys. D 9 (1976) E. Khawaja, S.G. Tomlin, Phys. D 8 (1975) J.I. Pankove, In Optical Processes in Semiconductors Dover Publications, New York (1971) 10. J.R.Chetia,M.Maullick,A.Datta,NN.Dass,Mater.Sci.Eng.B107(2004) T.Miyamoto,K.Shibayana,J.Appl.Phys. 44 (1973) V.Raja, A.K.Sharma, V.V.R.Narasimha Rao, Mater. Lett. 57 (2003) 4678 APPENDICES Table: 1 Transference Numbers of KBr Doped PVP Polymer Electrolyte Films Polymer Electrolyte Composition Transference Numbers t ion t ele PVP -- PVP+KBr (95:05) PVP+KBr (90:10) PVP+KBr (85:15) Table 2: Optical Properties of KBr Doped PVP Polymer Electrolyte Films Polymer Electrolyte Composition Absorption Edge (ev) Band Gap (ev) Direct Indirect PVP PVP+KBr (95:05) PVP+KBr (90:10) PVP+KBr (85:15)