The values of the optical constants like the absorption edge, the direct band gap and the indirect band gap decrease due to the formation of the charge transfer complexes between the hos[r]
(1)Original Article
Optical and electrical conductivity studies of VO2ỵ doped polyvinyl
pyrrolidone (PVP) polymer electrolytes
K Sreekanth, T Siddaiah, N.O Gopal, Y Madhava Kumar, Ch Ramu*
Department of Physics, Vikrama Simhapuri University PG Centre, Kavali, 524201, India
a r t i c l e i n f o
Article history: Received January 2019 Received in revised form June 2019
Accepted June 2019 Available online June 2019 Keywords:
Polymer electrolytes Polyvinyl pyrrolidone FTIR
UV - Visible
Polymer electrical conductivity Optical energy band gaps
a b s t r a c t
Polymer electrolytefilms of polyvinyl pyrrolidone (PVP) complexed with different concentrations (1, 2, 3, and mol%) of VO2ỵions were prepared by a solution casting technique The formation of complexes between the VO2ỵions and the polymer was conrmed by the Fourier transform infrared spectroscopy (FTIR) and the UV-Vis spectroscopy Room temperature impedance measurements in the frequency range 42 Hz to MHz revealed that the ionic conductivity increased with the increasing the VO2ỵion con-centration The maximum ionic conductivity of 5.39 108Scm1at 303 K was observed for the mol%
VO2ỵions doped PVP polymer electrolytefilm From the UV-Visible absorption spectra in the wavelength range of 200e800 nm the direct and indirect optical energy band gaps and optical absorption edges were found decreased with the increase in the VO2ỵion concentration FTIR studies on pure and VO2ỵdoped PVP polymerlms revealed the vibrational changes to occur due to the effect of the dopant VO2ỵions in
the polymer It is suggested that VO2ỵ, as a dopant, is a good choice to improve the electrical properties of the PVP polymer electrolyte
© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Polymer electrolytes have been a subject of great interest in the past few decades due to their importance in theoretical studies as well as their potential in practical applications such as sensors, electronic devices and components like light-emitting diodes, solar cells, thinfilm transistors, in micro electronics, batteries and linear or non-linear optics, photonics devices etc.[1e3] Solid polymer electrolytes, compared to conventional liquid onces, can be easily fabricated with several advantages like safety, leakage-free nature, low cost and easy fabrication intoflexible geometries[4,5] Solid polymer electrolytes exhibit a reasonable ionic conductivity, the dimensional stability, processability andflexibility under ambient conditions These features have been achieved by a variety of polymer electrolytes such as polymeresalt complexes, gel polymer electrolytes, composite polymer electrolytes and blend based polymer electrolytes
PVP is one of the attractive polymers due to the strong affinity of the pyridine group and its ability to undergo the hydrogen bonding
with polar species[6,7] The chemical structure of PVP is shown in
Fig As PVP binds to polar molecules exceptionally well, it is used as a binder in many pharmaceutical industrial process for the manufaction of drugs This binding nature leads to the application of PVP in coatings for photo-quality transparencies PVP can be thermally cross-linked, thus ensuring a good thermal stability and mechanical strength and also because of the stability of PVP in water[8,9] It should be noted that the formation of the transition metal particles inside the polymer system is also of interest for some potential applications, including the storage of optical data, shielding of the electromagnetic radiations, asflexible elements for resistive heating, laser systems, optical lenses and integrated waveguides[10,11] The interactions between water-soluble poly-mers and metal ions have a significant influence on the polymer physical properties[12,13] The PVP polymerfilm has a potential capacity to store the charges responding to the dopant dependent electrical and optical properties It has a strong tendency for com-plex formation with a variety of molecules[14]
Transition metal ions can be used to probe the polymer struc-ture because their outer d- orbital electron functions have broad distributions and their response to the surrounding cations is very sensitive[15] Also, the study of transition metal ions in amorphous materials is one of the interesting research subjects both from the theoretical and experimental points of view Among the transition
* Corresponding author
E-mail address:chramu8@gmail.com(Ch Ramu)
Peer review under responsibility of Vietnam National University, Hanoi
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.06.002
(2)metal oxides, V2O5is of much interest for the contemporary and
emerging technology, i.e applications in microelectronics, solid state ionics and optoelectronics[16e18] It is also found that the addition of transition metal ions have considerable effects on the structural, optical, electrical and magnetic properties of the poly-mer The main object of the present work is to develop a new kind of polymer electrolyte system doped with VO2ỵions, which can be used in the production of novel, solar cells, optical and electronic devices
2 Experimental
The Polyvinyl Pyrrolidone (S D Fine e Chemical Ltd., India) basedfilms were prepared by solution casting technique using the polymer solution doped with VOSO4 (MOLY CHEM LTD) one, in
which double distilled water was used as a solvent For this pur-pose, a stock solution of 5% of PVP and solutions of various VOSO4
concentrations (1, 2, 3, and mol%) were prepared separately These solutions were stirred individually at room temperature us-ing a magnetic stirrer for an hour to obtain transparent solutions The aqueous solutions of pure PVP and one of the solutions of various VOSO4 concentrations were then mixed and thoroughly
stirred to form well homogeneous and clear solutions The stirred solutions were cast onto polypropylene disks and allowed to slowly evaporate at room temperature until the polymerfilms were ob-tained on the bottom of the disks The optical absorption curves of the as preparedfilms were recorded in the range 200e800 nm at room temperature using the JASCO UVe VIS e NIR Spectrometer (of the Modele V.700, Japan) The FTIR spectra were recorded using a FTIR spectrometer (PerkineElmer model-1605, USA), over the wave number range of 500e4000 cm1 The electrical properties of the as prepared polymer electrolyte films at room temperature were studied by using (The Hioki 3532-50 LCR Hi-Tester, Hioki, Japan) over the frequency range 42 Hz - MHz
3 Results and discussion 3.1 FTIR analysis
FTIR spectroscopy is an important tool to study the polymer structure changes as well as the formation of the complexes be-tween the VO2ỵions and the host polymer in the polymer elec-trolyte system The FTIR spectra were recorded in the range 500e4000 cm1in the transmittance mode and the results for the
pure and the VO2ỵ(1e5 mol%) doped PVP polymer lms are shown inFig The FTIR spectrum of the pure PVP exhibits pronounced bands at 3643-2849, 2324, 2123, 1707-1576, 933 cm1which are assigned to the aliphatic CeH stretching, the CH2bending, the CH2
wagging, and the OeH stretching groups The FTIR peak assignment of pure and different concentration (1-5 mol%) VO2ỵions doped PVP polymer electrolytelms are listed inTable The spectrum shows a broad band at 3643-2849 cm1which is assigned to OeH stretching vibrations of alcohols and phenols [19,20] The pre-dominant OH vibrational band at 3643-2849 cm1is shifted to-wards the lower wave numbers and appears as the broad hydroxyl
band in all the dopedfilms as compared to the pure PVP[21] It gives a strong indication of specific interactions in the polymer matrices The peak corresponding to the C]O bonding at 1707-1576 cm1in the purefilm is shifted to 1707-1567 cm1forfilms of
1 and mol % of VO2ỵions doping[22] It is also found that this peak disappears in the spectrum for thelms of 3e5 mol % VO2ỵ doping In other words when VO2ỵ is added to PVP, there is a broadening of the C]O combination peak, its intensity reduces and the peaks finally disappears, which results from the induced structural rearrangement by adding the dopant to the polymer matrix[23]
The characteristic vibrational peaks at 2324, 2123 and 933 cm1 are assigned to the CH2deformation, the C]O combination and the
CeO stretching of PVP respectively They are shifted to 2324-2300 cm1, 2116-2147 cm1and 933-931 cm1respectively in the cases of the PVP polymerlms doped with VO2ỵions The inter-action of the dopant ions with the polymer matrix leads to the shift of these bands The above results confirm the formation of the complexes of the polymers with the VO2ỵions[24] This influences the local structure of the polymer backbones and significantly af-fects their mobility The compatibility between the polymer matrix and the inorganic dopants influences the optical and ionic con-ductivity of the PVP polymer electrolytes
3.2 Optical absorption studies
The measurement of the absorption spectrum is the most direct and the simplest method for investigating the band structure of materials In the absorption process, an electron gets excited from a lower to a higher energy state by absorbing a photon of known energy in the transmitted radiation The changes in the transmitted radiation gives information on the types of the possible electron transitions The fundamental absorption refers to band-to-band or the exciton transition Furthermore, the fundamental absorption manifests itself by a rapid rise in the absorption known as ab-sorption edge The abab-sorption edge can be used to determine the optical band gap, (Eg ¼ hc/l) as insulator/semiconductors are
generally classified into two types (a) direct band gap and (b) in-direct band gap materials In in-direct band gap semiconductors, the
Fig The chemical structure of PVP
(3)films are shown in
pure PVPfilm shows a peak at 220 nm, which is assigned to the carbonyl group of PVP For the dopedfilm, the intensity of this peak is increased as the absorption coefficient increases and the doping level increases This may be due to the strong interaction between the PVP and the VO2ỵions[25] The absorption coefficientacan be determined from the spectra using the formula
a¼ 2:303 A
d (1)
where A is the absorbance and d is the thickness of thefilm When a direct band gap exists, the absorption coefficient has the following dependence on the energy of the incident photon[26]
a:h:y¼ c h:y Eg1=2 (2)
where Egis the band gap energy, c is constant which is dependent
on the specimen structure,yis the frequency of the incident light and h is the Planck's constant.Fig 4shows the variation of (a.h.y)2 vs h.y, (photon energy) The intercept on the energy axis up on extrapolating the linear portion of the curve to the zero absorption value may be taken as the value of the band gap The optical band gap energy is obtained by extrapolating the linear region of the curve to the h.yaxis (the X-axis) using the origin software In the present work, the direct band gap was determined as 3.88 eV for the pure PVP polymer electrolyte, while for the VO2ỵdopedlms, values were found to decrease from 3.71 to 3.00 eV For the indirect transitions which require a photon assistance, the absorption co-efficient has the following dependence on the photon energy[26]
a:h:yẳ A h:y Egỵ EP
2
ỵ B h:y Eg EP
2
(3)
where EPis the energy of the phonon associated with the transition
and A and B are constants depending on the band structure The indirect band gaps were obtained from the plots of (a.h.y)1/2vs h.y as shown inFig For the pure PVP polymer electrolyte, the indi-rect band gap was 3.48 eV while for the VO2ỵdopedlms, the values were found to decrease from 3.30 to 2.43 eV, which are presented in Table The position of the absorption edge was determined by extrapolating the linear portions of theavs h.yplots inFig 6to zero absorption value For the purefilm the absorption edge was at 3.70 eV and for the dopedfilms the values were found to decrease from 3.43 to 2.86 eV Similar behavior was observed by Morsi et al.[23]and Zidan et al.[27]for the PEO/PVP blended with gold nano particles and the PVA/PVP blended with methylene blue films The defects in the polymeric matrix might be the reason for the decrease in the band gap values with dopant concentrations, which produce the localized states in the optical band gap, and
which overlaps with the band system These localized states, are responsible for the decrease in the band gap energy This indicates that with the increase in the concentration of the VO2ỵions, PVP polymerlms exhibits more semiconducting nature [28,29] The process of doping introduces additional defect states in the poly-meric matrix The density of localized the states was found to be proportional to the concentration of these defects[30]and conse-quently, to the VO2ỵcontent The values of the optical constants like the absorption edge, the direct band gap and the indirect band gap decrease due to the formation of the charge transfer complexes between the host polymer matrix and the dopant ions
3.3 Impedance analysis
Fig 7shows the plots of the impedance Z00as a function of Z0, of the pure and the VO2ỵions doped PVP polymer electrolytes at room temperature These plots known as the Cole - Cole plots, and their equivalent circuits are shown in Fig The analysis of the ColeeCole plots has been carried out for the polymer electrolytes using the impedance spectroscopy A small amount of ac current
Table
FTIR peak assignment of the pure and the VO2ỵ(1e5 mol%) doped PVP polymer electrolytes
Wave number (cm1) Band assignment
3643e2833 Oe H stretching frequency
2300e2324 CH2deformation
2116 CH2Symmetric stretching
1707e1576 Symmetric stretching of C]O
933 Ce O stretching
Fig Absorption spectra of the pure and the VO2ỵ(1, 2, 3, 4, and mol%) doped PVP polymer electrolytefilms
(4)was applied across the sample and the variation of imaginary (Z00) and real part (Z0) were measured using the 3532-50 LCR Hi-Tester in the frequency range from 42 Hz to MHz The Nyquist plot (also called ColeeCole) generally consists of a semicircular arc at higher frequencies and a spike is formed at lower frequencies for the bulk resistance In the obtained plots, the spike corresponds to the ionic conduction [31] and the semicircle is due to the space charge polarization
The obtained conductivity values are shown inTable The ionic conductivities of the pure and VO2ỵdoped PVP polymer electro-lytes were calculated from the following relation
s¼Rt
b:A
(4)
wheresis the ionic conductivity, Rbis the bulk resistance, t is the
thickness of the polymer electrolyte and A is the area of an electrode
From the ColeeCole plots, it can be seen that as the dopant concentration increases the diameter of the semicircle decreases implying the decrease of the bulk resistance (Rb) The depressed
semicircle indicates that the semi-crystalline nature in the polymer is converted to the amorphous nature where the transfer of the VO2ỵions takes place in the polymer matrix This amorphous phase produces a free space volume[32] The calculated values of the ionic conductivity for the pure and VO2ỵ doped PVP polymer electrolytes are listed inTable When compared to other dopant concentrations, the mol% VO2ỵdoped PVP polymer electrolyte shows the highest ionic conductivity The increment in the ionic
conductivity with VO2ỵion concentration is attributed as due to the rise in the number of charge carriers as shown inFig
3.4 Dielectric properties
The dielectric spectroscopy and impedance analysis were car-ried out using the HIOKI 3532-50 LCR Hi-Tester in the frequency range from 42 Hz to MHz at room temperature The dielectric constantε0and tandof the present polymer samples were evaluated Fig (a.h.y)1/2vs h.ycurves of the pure and the VO2ỵ(1, 2, 3, 4, and mol %) doped
polymer electrolytefilms
Table
Optical energy band gap values of the pure and VO2ỵdoped PVP polymer electrolyteslms
Concentration (mol%) VO2ỵ:PVP Direct band gap (eV) Indirect band gap (eV) Absorption edge (eV)
Pure PVP 3.88 3.48 3.70
1 3.71 3.30 3.43
2 3.56 3.17 3.26
3 3.49 2.76 3.15
4 3.36 2.54 3.02
5 3.00 2.43 2.86
Fig (a.h.y)1/2vs hycurves of the pure and the VO2ỵ(1, 2, 3, 4, and mol %) doped polymer electrolytefilms
(5)from the capacitance measurements.Fig 10shows the variation of ε׀as a function of frequency for the pure and the VO2ỵdoped PVP
polymer samples at room temperature From the Fig 10, it is observed that by increasing the frequency the dielectric constant (ε׀) decreases sharply The decrease in the dielectric constant with
the increase in the frequency takes place due to the polarization at the electrodeeelectrolyte interface and is found to be high for the sample of mol% VO2ỵdopant This may be due to the drifting of the ions, resulting in a high conductivity This reveals that the VO2ỵ
ions are completely dissolved in the polymer chains giving raise to the mobile ions Due to the formation of the space charge region at the electrodeeelectrolyte interface a variation is observed in be-tween dielectric constant (ε׀) and the frequency which is dependent on the electrolytes, indicating the non-Debye behavior[33e37]
Fig 11shows the variation of the dielectric loss factor tandwith the frequency (f) at room temperature for all the samples The decrease in the loss factor tandalong with the increasing frequency may be due to the reduced proportion of the amorphous nature leading to the reduction in the dispersion magnitude The appear-ance of peaks suggests the presence of the relaxing dipoles in the samples and associated with the electrical relaxation process or the inability of dipoles[38]
Fig 12shows the variation of the ionic conductivity with respect to the frequency It explains the conductivity dependence on the relaxation process that is non-exponential in time The plateau region describes the space charge polarization at the blocking electrode and is associated with ac conductivity (sac) of the
com-plexed polymer electrolyte As shown inFig 11, in the high fre-quency dispersion region, the ionic conductivity remains nearly constant However, at high frequencies the conductivity variation at
Fig The impedance plots for series and parallel combinations of R and C
Table
The conductivity values of the pure and the VO2ỵ(1, 2, 3, and mol%) doped PVP polymer electrolytefilms at room temperature
Concentration (mol %) VO2ỵ: PVP Conductivity at 303 K (S Cm1)
Pure PVP 0.96 108
1 1.55 108
2 1.96 108
3 3.02 108
4 3.60 108
5 5.39 108
Fig The dependence of the ionic conductivity of the PVP doped polymer electrolyte films at room temperature for the VO2ỵdoping concentrations of 1, 2, 3, and mol%.
Fig 10 Typical plots of the variation of the imaginary real (ε') part of the dielectric constant the frequency for the pure and the VO2ỵ(1, 2, 3, and mol%) doped PVP polymer electrolytefilms at room temperature
(6)room temperature is much less than that at lower frequencies
[39,40] Conclusion
Solid polymer electrolytefilms of polyvinyl pyrrollidone (PVP) complexed with different (1e5 mol%) concentrations of VO2ỵions were prepared by the solution cast method UV-Visible analysis revealed that the value of the optical band gap decreases as the VO2ỵcontent increases in the polymer electrolyte This indicates the formation of the charge transfer complexes between the polymer and the dopant FTIR spectra show shifts in some of the band positions with a change in their intensities This indicates the considerable interaction between the polymer and the VO2ỵions The maximum ionic conductivity of 5.39 108Scm1at 303 K was
observed for the mol% VO2ỵ doped PVP polymer electrolyte sample From all the characterization results, the mol% VO2ỵ doped PVP polymer electrolyte system exhibits the better amor-phous nature and a high ionic conductivity Hence the VO2ỵdoped polymer electrolytes are considered as the potential materials for the fabrication of solid state batteries and other electrochemical devices
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