Electrical and Optical Properties of PVA/LiI Polymer Electrolyte Films

Transcription

1 Asian Transactions on Science & Technology (ATST ISSN: ) Volume 1 Issue 6 Electrical and Optical Properties of PVA/LiI Polymer Electrolyte Films Hamed M. Ahmad *, Sabah H. Sabeeh **, Sarkawt A. Hussen * *Physics Department, Faculty of Science and Science Education, University of Sulaimani,Sulaimani-Iraq, ** Department of applied sciences, University of Technology, Bagdad-Iraq. Abstract- PVA has been doped by different percentage of Lithium Iodide (LiI), electrical and optical properties of polymer electrolytes have been investigated. At low frequency, the variation of dielectric constant and dielectric loss with frequency shows the presence of material electrode inter-face polarization processes. The exponent factor found is between.98 and.442 and obeys the universal power law. The absorption of pure and doped films have been studied in the visible and ultra-violet wavelength regions. It has been observed that the new absorption peaks at 29 and 375 nm are due to the formation of charge transfer complex. From direct allowed transition the optical energy gap decreases from 5.56 ev (for pure PVA) to 4.95 ev (for PVA+). Key words: Polymer electrolytes, ac conductivity, complex dielectric permittivity, optical band gap. I. INTRODUCTION Polymer electrolytes [PE] are among the important classes of macromolecules. These polyelectrolytes are charged macromolecules containing a large number of ionizable or ionic groups [1]. The conduction in PEs takes place through two distinct events: the first is due to the charge migration of ions between the coordinate sites of the host polymer and the s * HMA (hamed946@yahoo.com). second is associated with the polymeric chain segmental motion [2]. The ionic conductivity of PEs is strongly affected by various factors such as (i) crystalline of the material, (ii) simultaneous cation and anion motions and (iii) the ion pair formation. These factors reduce the cationic conductivity and; therefore, act as a barrier for potential applications [3]. In the recent years, studies on the electrical and optical properties of polymers have attracted much attention in view of their application in electronic and optical devices. 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, interference and polarization properties [4]. One of the important classes of polymer electrolytes is polar polymer {like Polyethylene oxide (PEO), Polypropylene oxide (PPO), polyvinyl alcohol (PVA), etc}. In this paper, we have used PVA as a host polymer because PVA is semi-crystalline polymer and has very important applications due to the role of OH group and hydrogen bonds [5]. The present study will help in understanding the effect of different concentrations of lithium salt on the AC conductivity, dielectric property, energy band diagram and optical parameters. ** SHS (sabah_habeeb@yahoo.com). * SAH (sarkawtah@gmail.com). II.EXPERIMENTAL METHODS Films Preparation Polyvinyl-alcohol (PVA) was mixed with different proportions of LiI, i.e. (5, 1, 15 and Jan 212 ATST Asian-Transactions 16

2 s ac (S/m) Asian Transactions on Science & Technology (ATST ISSN: ) Volume 1 Issue 6 2 % by weight). Two grams of PVA, supplied by Merck Company were dissolved in 4ml of distilled water in order to obtain pure PVA film. LiI with amount of 1,2, 3, and 4 gms were added to the PVA solution. A magnetic stirrer was used for one hour to make the solution highly homogeneous. Each solution was placed in a Petri dish and then placed in a dust free chamber to evaporate the solvent slowly in air at room temperature for seven days to make cast films with different proportions of LiI. The average thickness of these films was found to be in the range of (.2.3) mm. Electrical measurement The electrical measurement (dielectric permittivity, dielectric loss (, loss tangent and conductivity ( ) on PVA/LiI polymer electrolyte of different LiI percentages were obtained at frequency range from 1kHz to 1MHz and a temperature range from 3 to 37 K. Using the values of the equivalent parallel capacitance,, and parallel equivalent resistance,, recorded by the LCR meter type PM636 at a selected frequency,, dielectric and conductivity parameters have been calculated using the following equations [6]: (1) = (2) (3) (4) (5) Where ( ) is the geometrical capacitance of vacuum of the same dimensions as that of the sample; and the area and thickness of the sample, respectively; is the capacitance measured in : and being the phase angle. The and are, respectively, the real and imaginary parts of the complex dielectric constant. Similarly, and are, respectively, the real and imaginary parts of the AC conductivity,. The real part of conductivity shows the features of AC conductivity in disordered materials. Optical Measurement The absorption spectra of PVA/LiI films in the UV-visible region were measured using UV-VIS double beam spectrometer (model: Lambda 25) in the wavelength range 19-8 nm. The optical absorbance ( ) spectra of PVA/LiI films were collected at room temperature. The absorption coefficient was calculated from the optical absorbance ( ). After correction for reflection, was calculated using the following relation [7]: (6) (7) Where and are the incident and transmitted intensity, respectively, and is the sample thickness. III.RESULTS AND DISCUSSION Electrical results The frequency dependent ac conductivity for pure PVA and PVA/LiI doped polymer electrolyte obtained at room temperature are shown in Fig (1). 8.E-5 7.E-5 6.E-5 5.E-5 4.E-5 3.E-5 2.E-5 1.E-5.E+ %LiI 1%LiI Fig.1: Plot of ac conductivity as a function of frequency of PVA and PVA/LiI at room temperature. Jan 212 ATST Asian-Transactions 17

3 Dielevtric loss Dielectric permittivity Asian Transactions on Science & Technology (ATST ISSN: ) Volume 1 Issue 6 The ac conductivity pattern shows a frequency independent plateau in the low frequency region and exhibits dispersion at higher frequencies. This behavior obeys the universal power law, [8], where is the dc conductivity, is the preexponential factor and is the fractional exponent which is lies between and 1. According to jump relaxation model, at very low frequencies, an ion can jump from one site to its neighboring vacant site successfully contributing to the dc conductivity. At higher frequencies, however the probability for the ion to go back again to its initial site increases due to the short time periods available [2]. This high probability for the correlated forward backward hopping at higher frequencies together with the relaxation of the dynamic cage potential are responsible for the observed high frequency conductivity dispersion [9]. In the present work, has been obtained from the slope of versus plot, Table (I) represents the values of exponent and optical energy gap. As we can see that the values of lies between and 1. It is clear that the values of decrease with increasing the concentration of LiI which means that the increases of the salt concentration give rise to the increase of the dc conductivity according to the equation of universal power law. TABLE I: Values of the exponential factor ( ) and optical energy gap for PVA-LiI. Samples Optical energy PVA-%LiI PVA- PVA-1%LiI PVA- PVA gap (ev) Figures (2, 3) show the variation of dielectric permittivity and dielectric loss of all samples investigated as a function of frequency at a room temperature. As it can be seen in all the cases a strong frequency dispersion of permittivity and dielectric loss was observed. It is also clear that the increase of salt concentration in the PVA has a small and anomalous increase of and values over the frequency range from 1 khz-1 MHz [1]. Furthermore it can be noted that the values of dielectric parameters decrease by increasing the applied frequency. This behavior is may be the fact that at low frequencies the dipoles or ionic charges have sufficient time to align with the field before it changes its direction and consequently, the dielectric permittivity is high. While the decreases of permittivity value with increasing of applied frequency toward higher values is attributed the insufficient time for dipoles to align before the field changes direction [11] %LiI 1%LiI Fig.2: Variation of dielectric permittivity of PVA with frequency for different concentration of lithium salt at room temperature. 8 %LiI Fig.3: Variation of dielectric loss of PVA with frequency for different concentration of lithium salt at room temperature. 1%LiI The dissipation factor,, recorded as a function of frequency for PVA/LiI films, is shown in the Fig (4). The loss tangent for all films decreases with increasing frequency.the peak of the loss tangent curve has not been observed in the measured frequency range 1 khz-1mhz, i.e, high loss at low frequency in all films. We note that increases with the Jan 212 ATST Asian-Transactions 18

4 (ahv)^2 Absorbance (a.u.) (ahv)^2 tan(d) Asian Transactions on Science & Technology (ATST ISSN: ) Volume 1 Issue 6 increase in wt% of LiI. This is expected because conductivity increases with increase in wt% of LiI [6]. incident photon according to Mott, Davis and Tauc formula [12]: (8) %LiI 1%LiI Where is constant, is the optical energy gap and is an index. The value determines the type of electronic transitions causing the optical absorption; it can take values 1/2, 3/2, 2 and 3 for direct-allowed, direct-forbidden, indirect-allowed, and indirect-forbidden transitions, respectively..5 Fig.4: Variation of loss tangent of PVA with frequency for different concentration of lithium salt at room temperature. Fig (6) shows the relation between versus plot according to Eq. (8). The direct optical energy gap can be obtained from the intercept of the resulting straight lines with the energy axis at. The optical energy gap decreases from 5.56 ev (for pure PVA) to 4.95 ev as LiI concentration 2%. Optical results The absorbance spectra for PVA/LiI doped films is shown in Fig (5). As indicated in figure that LiI salt enhances the absorbance of the PVA host %LiI %LiI 1%LiI hv (ev) Wavelength (nm) %LiI Fig 5: Optical absorption as a function of wavelength for PVA at different Lithium Iodide content and at room temperature. The doped of LiI in PVA introduces a new absorption peaks at 29 and 375 nm, these new peaks may be attributed to the formation of charge transfer complex [4]. At high absorption coefficient levels the absorption coefficient for amorphous materials can be related to the energy of the hv (ev) Fig 6: versus photon energy for pure PVA and (5, 1, 15 and 2) of LiI. Jan 212 ATST Asian-Transactions 19

5 Asian Transactions on Science & Technology (ATST ISSN: ) Volume 1 Issue 6 According to Mott and Devis, the width of mobility edge depends on the degree of disorder and defects present in the amorphous structure. Such defect produces localized states in the forbidden gap [13]. So the increase of percentage of LiI salt to PVA host increases the localized states which directly affects the decrease in the optical energy gap of the PVA as shown in Table (I). IV.CONCLUSION Polymer electrolyte films of poly-vinyl alcohol with different percentages of Lithium Iodide have been prepared by casting method. The frequency dependence of permittivity, dielectric loss, loss tangent and ac conductivity of films are all studied as a result both of them increases with salt percentage increase. The ac conductivity obeys the universal power law, and the frequency exponent (s) decreases with increases of doping content. Optical absorption studies showed additional peaks on doping with LiI which is characteristic of formation of charge transfer complexes. Optical energy gap showed a decrease versus the increase of doping concentration. REFERENCES [1] Yeong-Soon Gel, and Sung-Ho Jin, ''A self-doped Ionic Conjugated Polymer: Poly(2-ethynylpyridinium-Nbenzoylsulfonate) by the Axtivated Polymerization of 2- Ethynylpyridine with Ring-Opening of 2-Sulfobenzoic Acid Cyclic Anhydride", Bull. Korean Chem. Soc. 25, 6, (24). [2] Dilip K. Pradhan, R.N.P. Choudhary, and B.K. Samantaray," Studies of dielectric and electrical properties of plastized polymer nanocomposite electrolytes", Materials Chemstry and Physics 115, (29). [4] C. Uma Devi, A.K. Sharma, and V.V.R.N. Rao, "Electrical and optical properties of pure and silver nitratedoped polyvinyl alcohol films", Materials Letters, 56, (22). [5] A. Tawansi, A. El-Khodary, and M.M. Abdelnaby, "A study of the physical properties of FeCl 3 filled PVA", Current Applied Physics, 5, (25). [6] Y.T. Ravikiran, M.T. Lagare, M. Sairam, N.N. Mallikajuna, B. Sreebhar, S. Manohar, A.G. MacDiarmid, and T.M. Aminabhavi, "Synthesis, characterization and low frequency AC conductivity of polyaniline/niobium pentoxide composites", Synthetic Metals 156, (26). [7] Ayman S. Ayesh, " ELECTRICAL AND OPTICAL CHARACTERIZATION OF PMMA DOPED WITH Y.25Si.25Ba.9725 (Ti (.9)Sn.1)O 3 CERAMIC", Chinese Jornal of Polymer Science 28, 4, (21). [8] Dilip K. Pradhan, R.N.P. Choudhary, and B.K. Samantaray, "Studies of structural, thermal and electrical behavior of polymer nanocomposite electrolytes", Polymer Letters, 2, 9, (28). [9] P.S. Anantha and K. Hariharan, " Structure and ionic transport studies of sodium borophosphate glassy system" Materials Chemistry and Physics, 89, (25). [1] R.J. Sengwa, Shobhna Choudhary, and Sonu Sankhla, " Dielectric dispersion and ionic conduction in hydrocolloids of poly (vinyl alcohol)-poly (ethylene oxide) blend-montmorillonite clay nanocomposites", Indian Jornal of Engineering and Materials Sciences, 16, (29). [11] Aymen S. Ayesh and Rami A. Abdel-Rahem, "Optical and Electrical Properties of Polycarbonate/MnCl 2 composite films", Jornal of Plastic film and sheeting, 24, (28). [12] S.M. Alniami, and M.N. AL-Dileamy, "Determination of the Optical Constants of Cadmium Stannate (Cd 2SnO 4) Films" International Jornal of pure and Applied Physics, 3, 1, 3-39 (27). [13] P. Sharma, V. Sharma, and S.C. Katyal, " VARIATION OF OPTICAL CONSTANTS IN GE 1SE 6TE 3 THIN FILM", Chalcogenide Letters, 3, 1, (26). [3] Dilip K. Pradhan, R.N.P. Choudhary, and B.K. Samantaray" Studies of Dielectric Relaxation and AC Conductivity Behavior of Plasticized Polymer Nanocomposite Electrolytes", Int. J. Electrochem..Sci., 3, (28). Jan 212 ATST Asian-Transactions 2