- Email: [email protected]
Effect of potassium chromate nanoparticles on the optical properties of poly (methyl methacrylate) (PMMA) films
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 10 (2019) 38–45
www.materialstoday.com/proceedings
INTENSE_2017
Effect of potassium chromate nanoparticles on the optical properties of poly (methyl methacrylate) (PMMA) films Minal Bafnaa*, Ankit Kumar Guptaa,b, R.K. Khannab a
b
Department of Physics, Agrawal P.G. College, Jaipur-302004, Rajasthan, India Department of Physics, Vivekananda Global University, Jagatpura, Jaipur-303012, Rajasthan, India
Abstract In this work, the authors have presented a study on the variation in optical parameters that occur on doping Poly-methyl-metha crylate (PMMA) thin films with Potassium chromate (K2CrO4) by different weight percentages (1%,2%, 5% and 10%). These pure PMMA and its composite films are of thickness ~100m and have been synthesized using the solution cast technique. The absorbance (A) and transmittance (T %) spectra of these films were recorded in the wavelength range (300-1100) nm using EI 2375 UV-VIS double beam spectrophotometer. From the obtained data, the optical parameters viz. absorption coefficient (), extinction coefficient (), finesse coefficient (F), refractive index (), real and imaginary parts of dielectric constant (randi), optical conductivity () and optical energy gap for indirect transition were evaluated and are found to show variation with increase of K2CrO4 concentration in the mentioned wavelength range. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International conference on NanoTechnology in Energy, Nano-bio interface & Sustainable Environment (INTENSE). Keywords: PMMA-K2Cr2O4 Composite films; Optical properties; Energy band gap; dielectric constants.
* Corresponding author. Tel.: +91 8890650244. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International conference on NanoTechnology in Energy, Nano-bio interface & Sustainable Environment (INTENSE).
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
39
1. Introduction The polymeric materials offer to be quite promising potential materials for various applications as these are light weight, easily moldable, amenable to be casted in various configurations and shapes and are economical. Although these polymeric substances may not have the desired physical property but by addition of filler material their properties can be dramatically obtained as desired for development of novel materials. The addition of nano dimension low weight filler material in to polymer produces a refinement of the properties of polymeric material [15]. In their quest to design potential electrically conductive composite material, to be used as an effective EMI shielding substance, the authors have synthesized few potassium metal oxide polymeric composite films as explained in reference [6-8]. During the characterization and analysis of physical properties of these synthesized composites, we found that the optical properties of these potassium compound doped PMMA composites were interestingly dopant-dependent and of good use in applications like electrolytes, filters of light etc. [7-8]. On conducting a vast literature survey on these polymeric metal composites, it is found that although a sufficient amount of reports are available on optical properties of filler dispersed in PMMA yet there is no report about the variation of optical properties of K2CrO4 doped PMMA composite films. So, the study of variation of optical parameters like optical band gap, refractive index, extinction coefficient, dielectric constant, optical conductivity etc. with wavelength of incident radiation and concentration of the dopant material seems to be meaningful. So, in the present work, we have measured the absorbance (A) spectra and transmission (T%) spectra of these potassium chromate doped PMMA films using UV-Visible absorption spectroscopy and from this data we have then calculated the above-mentioned optical parameters, which provide a concise understanding of the effect of potassium chromate nanoparticles on the optical properties of PMMA films. 2. Materials and methods 2.1. Chemicals and materials The chemicals used are Poly methyl methacrylate (PMMA) granules from M/s Gadra Chemicals, Bharuch; potassium chromate (K2CrO4) (purity 98.5%) and dichloromethane (purity 99.8%) purchased from Merch Specialties Private Limited, Mumbai. These were used as received to prepare the films of pure PMMA and its composites with different weight percent of K2CrO4using the solution cast method. 2.2. Synthesis of K2CrO4-PMMA Composite films The polymer PMMA granules are dissolved uniformly in dichloromethane using a magnetic stirrer. This solution was taken in five separate beakers to which different concentration i.e. 1%, 2%, 5% and 10% by weight of potassium chromate were dispersed separately and stirred on a magnetic stirrer for six hours at 600 C to obtain homogeneous solution. These prepared solutions were cooled to ambient temperature and then poured into flat bottom petri dishes of diameter 7.62 cm floating over mercury and covered by foil. These were then left for 24 hours so that the solvent evaporates gradually and leaves the dried translucent yellow sample films shown in Figure1.
0%
1%
2%
5%
10%
Figure 1. Sample films of pure PMMA and its composites with different weight percent of K2CrO4
40
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
2.3. Optical Characterization of prepared films The EI 2375 UV-VIS double beam spectrophotometer was used to measure the absorbance and transmission spectra of the prepared sample films in the wavelength range (300-1100) nm. The data obtained was then used to generate the optical parameters using the theoretical considerations explained below. The optical absorbance (A) of a material measured using double beam spectrophotometer is defined as the ratio of logarithm of absorbed light intensity (I0) to the incident light intensity (I ) A log T = log I0 / I where T is the transmission coefficient defined as I/I0.
(1)
The absorption coefficient which gives the measure of the ability of a material of thickness (t) to absorb light is measured in cm-1 and expressed as 2.303 (A/ t) (2) Thus for wavelength of the incident ray the extinction coefficient () is calculated using the relation:
(3) The value of optical energy gap is evaluated using the below written Mott and Davis relation: hC (hEg)m (4) where his the photon energy, C is the proportional constant depending on the specimen structure, Eg is the allowed or forbidden energy gap of transition. The exponent m determines the type of electronic transition responsible for absorption and can take values 1/2, 3/2 for direct and 2, 3 for indirect allowed and forbidden transitions respectively. It is well established in literature [3-5] that when the values of α >104 cm-1, then the electron transitions are direct while when values of α < 104 cm-1 indirect transition of electrons are expected to occur. If the plot of √ vs hforms a straight line, then we infer it as a indirect band gap, measured equal to the intercept of the extrapolation of the straight line to the =0 on h axis . Further, we calculate the reflectance R as R = 1-(A+T) using which the finesse coefficient F given by equation (5) is calculated. F R R5) The fundamental optical parameter refractive index ) for these sample films is then calculated using the below expression: = (4R/(R-1)2 - k2)½ - (R+1/R-1) (6) Then we calculate the optical conductivity which is related to refractive index and absorption coefficient as given below: opt = c47) From these values of and , the real (r) and imaginary (i) parts of the dielectric constant are obtained accordingly: r22 and i2(8)
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
(a)
41
(b)
Figure 2. Variation of (a) Absorbance and (b) Transmittance with wavelength for PMMA films with different concentration of K2CrO4
3. Results and discussions We have used EI 2375UV VIS Double beam spectrophotometer to record the absorbance and transmission spectra in the wavelength range (300-1100) nm of the prepared composite films and to determine the electronic transition and estimate the band gap energy of these composite films. 3.1 The Absorption and Transmission Data The measured absorbance spectra and transmittance spectra using double beam spectrophotometer in the wavelength region 300-1100 nm for PMMA film with different concentration of K2CrO4are as shown in Figure 2 (a) and (b) respectively. From these figures it is clear that absorption increases whereby transmission decreases with increase in concentration of K2CrO4. For each sample in the region 330-430 nm, the curves in figure 2(a) depict an absorption peak analogous to a dip in transmission curves in Figure 2(b) and then become almost constant in the wavelength range higher than 500 nm. An increase in absorption peak with increase in K2CrO4 concentration is also observed which indicates about the existence of chemical interaction between the filler and host matrix. Our analysis of optical properties of potassium permanganate doped PMMA [7] and other reports in literature for different salts like methyl blue and methyl red [1], or CrCl2 [2], or the couramine-102[4], or iron chromate [11] or diarylethen compound [12] doped in PMMA also depict similar behaviour. Interestingly the absorption and transmittance spectra reported for K2CrO4 doped PVA composite films by Al-Sulaimawi [10] resemble such response. It can be noted that absorption is relatively low at high wavelengths indicating lesser probability of electron transition to higher energy band. On the contrary, at low wavelengths i.e at high energies, absorption is high, indicating higher possibility for electron transitions. It is a familiar fact that in pure PMMA the electrons are strongly linked to their atoms through covalent bonds and hence there are no free electrons, thus attributing it the property of high transmittance. The potassium chromate molecules have free electrons which absorb photons of the incident light and transit to the higher energy levels. Thus, for composite films the absorption of the incident light increases. 3.2 Absorption Coefficient Study Figure 3(a) depicts the values of absorption coefficient (α) which have been calculated using equation (2). We assess the value of optical band gap and the type of transition from the values of as described in the subsequent section.
42
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
(a)
(b)
Figure 3. (a) Variation of absorption coefficient () in cm-1with photon energy for PMMA films with different concentration of K2CrO4. (b)Absorption edges (αhυ)1/2 for composite films as a function of photon energy. At extension of the curve to the values of (αhυ)1/2 = 0 ,we get indirect allowed gap transition.
3.2.1 Band Gap Analysis The observed value for absorption coefficient (α < 104 cm-1) are indicative of indirect band gap for the K2CrO4 doped PMMA films. For all the composite films, we plot (αhυ)1/2 as a function of photon energy (hυand extrapolate the linear portion of curves in Figure 3(b) to the values of (αhυ)1/2 = 0. The value of intercepts gives the value of optical indirect allowed band gap energy Eg which are listed in Table 1. It is clear from these values that as the concentration of K2CrO4 increases the values of energy gap Eg decreases. Such behaviour of decrease in optical band energy is in agreement with our results reported for potassium permanganate doped PMMA [7] and for K2CrO4 doped PVA composite films by Al-Sulaimawi [10]. Due to doping there is a disorder in the composite film which gives rise to generation of new vacant energy levels between the HOMO and LUMO by decreasing the value of band gap. 3.2.2 Extinction Coefficient From equation (3) it is evident that extinction coefficient is directly proportional to optical absorption coefficient and the incident wavelength, so we can ascribe the variations of as a function of wavelength to the variation of the absorption coefficient Figure (4) depict an increase in values of with increasing K2CrO4 content and wavelength of incident radiation similar to the one reported by Abdullallah M H et. al [11] when Table 1:Value of optical energy band gap for PMMA: K2CrO4 composite films
Samples Pure PMMA PMMA doped with 1% K2CrO4 PMMA doped with 2% K2CrO4 PMMA doped with 5 % K2CrO4 PMMA doped with 10 % K2CrO4
Figure 4. Variation of with for PMMA films with different concentration of K2CrO4
Band Gap (eV) 4.2 3.56 3.43 3.12 3.02
Eg
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
43
PMMA is doped with iron chromate or as noted by Najeeb H N et.al [12] when PMMA is doped with di-arylethen compound. The value of decrease in value of with increasing wavelength has been previously observed on doping PMMA with CrCl2 in reference [2] or on doping PMMA with methyl blue or red [1]. Thus, we conclude that the values of for pure PMMA are observed to be constant for the entire wavelength range as reported by others too [1, 2, 5, 11], but the response of extinction coefficient for composite depends on the nature of the dopant. 3.3 Fundamental Optical Parameters To understand the optical characteristics of a material, the fundamental optical parameters like the refractive index, finesse coefficient, dielectric constants and optical conductivity are necessary to be evaluated. It is evident from equation (5) and (6) that the finesse coefficient F and refractive index are dependent on reflectance R so their observed variations with concentration and wavelength bear resemblance. Figure 5 (a) and (b) shows that the values of finesse coefficient F and refractive index are higher for doped composites than those of pure polymer. This observed increase in the refractive index on doping the polymer with different salts has been reported previously too [1,2,5,9,10,12] and can be attributed to the intermolecular hydrogen bonding between K+ ions and the adjacent ion of PMMA chain making the polymer film optically dense. It is also observed that although the values of F and in figure 5 show increase relative to pure polymer but the trends of these shows an initial increase and then decrease with increase in concentration of dopant. The refractive index which indicates not only the optical density of a material but also explains the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance suggests that the increase in the dopant potassium chromate molecules speeds up the light through the material. The values of optical conductivity ( for pure PMMA and K2CrO4 doped PMMA samples were computed in accordance to equation (7) using the absorption coefficient and the refractive index data. It can be easily observed from Figure 6 that the optical conductivity increases on doping and decreases with incident wavelength similar to the one reported in reference [11] where K2CrO4 has been doped in PVA. This suggests that increase in doping of (K2CrO4) ions, increase the contribution of electron transitions between the valence and conduction bands, leading to reduction of energy gap. Equation (8) relates the real (r) and imaginary (i) parts of the dielectric constant to refractive index n and extinction coefficient It can be seen in Figure 7(a) that the variation of r with wavelength follows the variation of refractive index n and the values of it varies from 2-7 for our samples. Reports in literature show its variation between 4-15 on addition of methyl red and blue in PMMA in different concentration [1], while between 2-8 on addition of CrCl2 in PMMA [2], and a drastic change from 20-100 on doping PMMA with different concentration of
(a) (b) Figure 5. (a) Variation of Finesse coefficient (F) and (b) refractive index () with wavelength for PMMA films with different concentration of K2CrO4.
44
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
Figure 6. Variation of optical conductivity () as function of wavelength for PMMA films with different concentration of K2CrO4
diarylethen compound [12]. The imaginary part of dielectric constant for our samples increases with increase in wavelength similar to the behaviour reported in references [11-12]. 4. Conclusion In this work we have analysed the optical parameters of K2CrO4 doped PMMA composite films prepared using solution casting technique. Determination of various optical parameters has been done from the absorbance and transmittance (%T) spectra obtained using UV-Visible spectra analysis. It has been observed that the absorption coefficient increases on addition of potassium chromate in PMMA matrix and its values are indicative of indirect electronic transitions. The decrease in indirect optical band gap energy on increasing the dopant chromate molecules indicate the possibility of tuning the band gap as desired and making it useful for optoelectronics. All the fundamental optical parameters like refractive index, dielectric constant and optical conductivity are significantly affected in the doped samples.
(a)
(b)
Figure 7. Variation of (a) real and (b) imaginary part of dielectric constant with wavelength for PMMA films with different concentration of K2CrO4.
References [1] Z .T. Khodair, M.H. Saeed, M. H. Abdulallah, Iraqi Journal of Physics, 12(24) (2014) 47-51. [2] K. Al-Ammar , A. Hashim , M.Husaien, Chemical and Materials Engineering,1(3) (2013) 85-87.
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
45
[3] S.H. Deshmukh, D.K. Burghate, S.N. Shilaskar, G.N. Chaudhari, P.T. Deshmukh, Indian Journal of Pure and Applied Physics,46 (2008) 344-348. [4] B. R. Ali, F.N. Kadhem, International Journal of Applied or Innovation in Engineering and Management, 2(4) (2013) 564-571. [5] O.G.Abdullah, B. A. Shujahadeen , M. A. Rasheed, Results in Physics, Elsevier Publication, 6 (2016 ) 11031108. [6] A.K.Gupta, M. Bafna, R. K. Khanna,Y.K.Vijay, Proceedings of 11 th BICON International Conference on Advanced Material Science and Technology, 1 (2016) 113-116. [7] M. Bafna, A.K.Gupta, Y.K.Vijay, Accepted in Bulletin of Material Science, (2018) [8] M. Bafna, N.Garg, A.K. Gupta, Journal of Emerging Technology and Innovation Research, 5(2) (2017) 433-36. [9] M R Ranganath, B. Lobo, Proceedings of International Conference on Material Science Research and Nano technology, (2008) 194-195. [10] I. F. H. Al-Sulaimawi, Journal for Pure and Applied Science, 28 (2) (2015) 32-39. [11] M. H. Abdulallah, S. S. Chiad, N. F. Habubi, Diyala Journal for Pure Science, 6 (2) (2010) 161-169. [12] H. N. Najeeb, A.A. Balakit, G. A. Wahab, A. K. Kodeary, Acadmic Research Institute, 5 (1)( 2014) 48-56. [13] C. Kittel, Introduction to Solid State Physics, John Wiley & Sons, New York, 2005 (8th ed) .
ScienceDirect Materials Today: Proceedings 10 (2019) 38–45
www.materialstoday.com/proceedings
INTENSE_2017
Effect of potassium chromate nanoparticles on the optical properties of poly (methyl methacrylate) (PMMA) films Minal Bafnaa*, Ankit Kumar Guptaa,b, R.K. Khannab a
b
Department of Physics, Agrawal P.G. College, Jaipur-302004, Rajasthan, India Department of Physics, Vivekananda Global University, Jagatpura, Jaipur-303012, Rajasthan, India
Abstract In this work, the authors have presented a study on the variation in optical parameters that occur on doping Poly-methyl-metha crylate (PMMA) thin films with Potassium chromate (K2CrO4) by different weight percentages (1%,2%, 5% and 10%). These pure PMMA and its composite films are of thickness ~100m and have been synthesized using the solution cast technique. The absorbance (A) and transmittance (T %) spectra of these films were recorded in the wavelength range (300-1100) nm using EI 2375 UV-VIS double beam spectrophotometer. From the obtained data, the optical parameters viz. absorption coefficient (), extinction coefficient (), finesse coefficient (F), refractive index (), real and imaginary parts of dielectric constant (randi), optical conductivity () and optical energy gap for indirect transition were evaluated and are found to show variation with increase of K2CrO4 concentration in the mentioned wavelength range. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International conference on NanoTechnology in Energy, Nano-bio interface & Sustainable Environment (INTENSE). Keywords: PMMA-K2Cr2O4 Composite films; Optical properties; Energy band gap; dielectric constants.
* Corresponding author. Tel.: +91 8890650244. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International conference on NanoTechnology in Energy, Nano-bio interface & Sustainable Environment (INTENSE).
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
39
1. Introduction The polymeric materials offer to be quite promising potential materials for various applications as these are light weight, easily moldable, amenable to be casted in various configurations and shapes and are economical. Although these polymeric substances may not have the desired physical property but by addition of filler material their properties can be dramatically obtained as desired for development of novel materials. The addition of nano dimension low weight filler material in to polymer produces a refinement of the properties of polymeric material [15]. In their quest to design potential electrically conductive composite material, to be used as an effective EMI shielding substance, the authors have synthesized few potassium metal oxide polymeric composite films as explained in reference [6-8]. During the characterization and analysis of physical properties of these synthesized composites, we found that the optical properties of these potassium compound doped PMMA composites were interestingly dopant-dependent and of good use in applications like electrolytes, filters of light etc. [7-8]. On conducting a vast literature survey on these polymeric metal composites, it is found that although a sufficient amount of reports are available on optical properties of filler dispersed in PMMA yet there is no report about the variation of optical properties of K2CrO4 doped PMMA composite films. So, the study of variation of optical parameters like optical band gap, refractive index, extinction coefficient, dielectric constant, optical conductivity etc. with wavelength of incident radiation and concentration of the dopant material seems to be meaningful. So, in the present work, we have measured the absorbance (A) spectra and transmission (T%) spectra of these potassium chromate doped PMMA films using UV-Visible absorption spectroscopy and from this data we have then calculated the above-mentioned optical parameters, which provide a concise understanding of the effect of potassium chromate nanoparticles on the optical properties of PMMA films. 2. Materials and methods 2.1. Chemicals and materials The chemicals used are Poly methyl methacrylate (PMMA) granules from M/s Gadra Chemicals, Bharuch; potassium chromate (K2CrO4) (purity 98.5%) and dichloromethane (purity 99.8%) purchased from Merch Specialties Private Limited, Mumbai. These were used as received to prepare the films of pure PMMA and its composites with different weight percent of K2CrO4using the solution cast method. 2.2. Synthesis of K2CrO4-PMMA Composite films The polymer PMMA granules are dissolved uniformly in dichloromethane using a magnetic stirrer. This solution was taken in five separate beakers to which different concentration i.e. 1%, 2%, 5% and 10% by weight of potassium chromate were dispersed separately and stirred on a magnetic stirrer for six hours at 600 C to obtain homogeneous solution. These prepared solutions were cooled to ambient temperature and then poured into flat bottom petri dishes of diameter 7.62 cm floating over mercury and covered by foil. These were then left for 24 hours so that the solvent evaporates gradually and leaves the dried translucent yellow sample films shown in Figure1.
0%
1%
2%
5%
10%
Figure 1. Sample films of pure PMMA and its composites with different weight percent of K2CrO4
40
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
2.3. Optical Characterization of prepared films The EI 2375 UV-VIS double beam spectrophotometer was used to measure the absorbance and transmission spectra of the prepared sample films in the wavelength range (300-1100) nm. The data obtained was then used to generate the optical parameters using the theoretical considerations explained below. The optical absorbance (A) of a material measured using double beam spectrophotometer is defined as the ratio of logarithm of absorbed light intensity (I0) to the incident light intensity (I ) A log T = log I0 / I where T is the transmission coefficient defined as I/I0.
(1)
The absorption coefficient which gives the measure of the ability of a material of thickness (t) to absorb light is measured in cm-1 and expressed as 2.303 (A/ t) (2) Thus for wavelength of the incident ray the extinction coefficient () is calculated using the relation:
(3) The value of optical energy gap is evaluated using the below written Mott and Davis relation: hC (hEg)m (4) where his the photon energy, C is the proportional constant depending on the specimen structure, Eg is the allowed or forbidden energy gap of transition. The exponent m determines the type of electronic transition responsible for absorption and can take values 1/2, 3/2 for direct and 2, 3 for indirect allowed and forbidden transitions respectively. It is well established in literature [3-5] that when the values of α >104 cm-1, then the electron transitions are direct while when values of α < 104 cm-1 indirect transition of electrons are expected to occur. If the plot of √ vs hforms a straight line, then we infer it as a indirect band gap, measured equal to the intercept of the extrapolation of the straight line to the =0 on h axis . Further, we calculate the reflectance R as R = 1-(A+T) using which the finesse coefficient F given by equation (5) is calculated. F R R5) The fundamental optical parameter refractive index ) for these sample films is then calculated using the below expression: = (4R/(R-1)2 - k2)½ - (R+1/R-1) (6) Then we calculate the optical conductivity which is related to refractive index and absorption coefficient as given below: opt = c47) From these values of and , the real (r) and imaginary (i) parts of the dielectric constant are obtained accordingly: r22 and i2(8)
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
(a)
41
(b)
Figure 2. Variation of (a) Absorbance and (b) Transmittance with wavelength for PMMA films with different concentration of K2CrO4
3. Results and discussions We have used EI 2375UV VIS Double beam spectrophotometer to record the absorbance and transmission spectra in the wavelength range (300-1100) nm of the prepared composite films and to determine the electronic transition and estimate the band gap energy of these composite films. 3.1 The Absorption and Transmission Data The measured absorbance spectra and transmittance spectra using double beam spectrophotometer in the wavelength region 300-1100 nm for PMMA film with different concentration of K2CrO4are as shown in Figure 2 (a) and (b) respectively. From these figures it is clear that absorption increases whereby transmission decreases with increase in concentration of K2CrO4. For each sample in the region 330-430 nm, the curves in figure 2(a) depict an absorption peak analogous to a dip in transmission curves in Figure 2(b) and then become almost constant in the wavelength range higher than 500 nm. An increase in absorption peak with increase in K2CrO4 concentration is also observed which indicates about the existence of chemical interaction between the filler and host matrix. Our analysis of optical properties of potassium permanganate doped PMMA [7] and other reports in literature for different salts like methyl blue and methyl red [1], or CrCl2 [2], or the couramine-102[4], or iron chromate [11] or diarylethen compound [12] doped in PMMA also depict similar behaviour. Interestingly the absorption and transmittance spectra reported for K2CrO4 doped PVA composite films by Al-Sulaimawi [10] resemble such response. It can be noted that absorption is relatively low at high wavelengths indicating lesser probability of electron transition to higher energy band. On the contrary, at low wavelengths i.e at high energies, absorption is high, indicating higher possibility for electron transitions. It is a familiar fact that in pure PMMA the electrons are strongly linked to their atoms through covalent bonds and hence there are no free electrons, thus attributing it the property of high transmittance. The potassium chromate molecules have free electrons which absorb photons of the incident light and transit to the higher energy levels. Thus, for composite films the absorption of the incident light increases. 3.2 Absorption Coefficient Study Figure 3(a) depicts the values of absorption coefficient (α) which have been calculated using equation (2). We assess the value of optical band gap and the type of transition from the values of as described in the subsequent section.
42
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
(a)
(b)
Figure 3. (a) Variation of absorption coefficient () in cm-1with photon energy for PMMA films with different concentration of K2CrO4. (b)Absorption edges (αhυ)1/2 for composite films as a function of photon energy. At extension of the curve to the values of (αhυ)1/2 = 0 ,we get indirect allowed gap transition.
3.2.1 Band Gap Analysis The observed value for absorption coefficient (α < 104 cm-1) are indicative of indirect band gap for the K2CrO4 doped PMMA films. For all the composite films, we plot (αhυ)1/2 as a function of photon energy (hυand extrapolate the linear portion of curves in Figure 3(b) to the values of (αhυ)1/2 = 0. The value of intercepts gives the value of optical indirect allowed band gap energy Eg which are listed in Table 1. It is clear from these values that as the concentration of K2CrO4 increases the values of energy gap Eg decreases. Such behaviour of decrease in optical band energy is in agreement with our results reported for potassium permanganate doped PMMA [7] and for K2CrO4 doped PVA composite films by Al-Sulaimawi [10]. Due to doping there is a disorder in the composite film which gives rise to generation of new vacant energy levels between the HOMO and LUMO by decreasing the value of band gap. 3.2.2 Extinction Coefficient From equation (3) it is evident that extinction coefficient is directly proportional to optical absorption coefficient and the incident wavelength, so we can ascribe the variations of as a function of wavelength to the variation of the absorption coefficient Figure (4) depict an increase in values of with increasing K2CrO4 content and wavelength of incident radiation similar to the one reported by Abdullallah M H et. al [11] when Table 1:Value of optical energy band gap for PMMA: K2CrO4 composite films
Samples Pure PMMA PMMA doped with 1% K2CrO4 PMMA doped with 2% K2CrO4 PMMA doped with 5 % K2CrO4 PMMA doped with 10 % K2CrO4
Figure 4. Variation of with for PMMA films with different concentration of K2CrO4
Band Gap (eV) 4.2 3.56 3.43 3.12 3.02
Eg
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
43
PMMA is doped with iron chromate or as noted by Najeeb H N et.al [12] when PMMA is doped with di-arylethen compound. The value of decrease in value of with increasing wavelength has been previously observed on doping PMMA with CrCl2 in reference [2] or on doping PMMA with methyl blue or red [1]. Thus, we conclude that the values of for pure PMMA are observed to be constant for the entire wavelength range as reported by others too [1, 2, 5, 11], but the response of extinction coefficient for composite depends on the nature of the dopant. 3.3 Fundamental Optical Parameters To understand the optical characteristics of a material, the fundamental optical parameters like the refractive index, finesse coefficient, dielectric constants and optical conductivity are necessary to be evaluated. It is evident from equation (5) and (6) that the finesse coefficient F and refractive index are dependent on reflectance R so their observed variations with concentration and wavelength bear resemblance. Figure 5 (a) and (b) shows that the values of finesse coefficient F and refractive index are higher for doped composites than those of pure polymer. This observed increase in the refractive index on doping the polymer with different salts has been reported previously too [1,2,5,9,10,12] and can be attributed to the intermolecular hydrogen bonding between K+ ions and the adjacent ion of PMMA chain making the polymer film optically dense. It is also observed that although the values of F and in figure 5 show increase relative to pure polymer but the trends of these shows an initial increase and then decrease with increase in concentration of dopant. The refractive index which indicates not only the optical density of a material but also explains the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance suggests that the increase in the dopant potassium chromate molecules speeds up the light through the material. The values of optical conductivity ( for pure PMMA and K2CrO4 doped PMMA samples were computed in accordance to equation (7) using the absorption coefficient and the refractive index data. It can be easily observed from Figure 6 that the optical conductivity increases on doping and decreases with incident wavelength similar to the one reported in reference [11] where K2CrO4 has been doped in PVA. This suggests that increase in doping of (K2CrO4) ions, increase the contribution of electron transitions between the valence and conduction bands, leading to reduction of energy gap. Equation (8) relates the real (r) and imaginary (i) parts of the dielectric constant to refractive index n and extinction coefficient It can be seen in Figure 7(a) that the variation of r with wavelength follows the variation of refractive index n and the values of it varies from 2-7 for our samples. Reports in literature show its variation between 4-15 on addition of methyl red and blue in PMMA in different concentration [1], while between 2-8 on addition of CrCl2 in PMMA [2], and a drastic change from 20-100 on doping PMMA with different concentration of
(a) (b) Figure 5. (a) Variation of Finesse coefficient (F) and (b) refractive index () with wavelength for PMMA films with different concentration of K2CrO4.
44
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
Figure 6. Variation of optical conductivity () as function of wavelength for PMMA films with different concentration of K2CrO4
diarylethen compound [12]. The imaginary part of dielectric constant for our samples increases with increase in wavelength similar to the behaviour reported in references [11-12]. 4. Conclusion In this work we have analysed the optical parameters of K2CrO4 doped PMMA composite films prepared using solution casting technique. Determination of various optical parameters has been done from the absorbance and transmittance (%T) spectra obtained using UV-Visible spectra analysis. It has been observed that the absorption coefficient increases on addition of potassium chromate in PMMA matrix and its values are indicative of indirect electronic transitions. The decrease in indirect optical band gap energy on increasing the dopant chromate molecules indicate the possibility of tuning the band gap as desired and making it useful for optoelectronics. All the fundamental optical parameters like refractive index, dielectric constant and optical conductivity are significantly affected in the doped samples.
(a)
(b)
Figure 7. Variation of (a) real and (b) imaginary part of dielectric constant with wavelength for PMMA films with different concentration of K2CrO4.
References [1] Z .T. Khodair, M.H. Saeed, M. H. Abdulallah, Iraqi Journal of Physics, 12(24) (2014) 47-51. [2] K. Al-Ammar , A. Hashim , M.Husaien, Chemical and Materials Engineering,1(3) (2013) 85-87.
Bafna M et al. / Materials Today: Proceedings 10 (2019) 38–45
45
[3] S.H. Deshmukh, D.K. Burghate, S.N. Shilaskar, G.N. Chaudhari, P.T. Deshmukh, Indian Journal of Pure and Applied Physics,46 (2008) 344-348. [4] B. R. Ali, F.N. Kadhem, International Journal of Applied or Innovation in Engineering and Management, 2(4) (2013) 564-571. [5] O.G.Abdullah, B. A. Shujahadeen , M. A. Rasheed, Results in Physics, Elsevier Publication, 6 (2016 ) 11031108. [6] A.K.Gupta, M. Bafna, R. K. Khanna,Y.K.Vijay, Proceedings of 11 th BICON International Conference on Advanced Material Science and Technology, 1 (2016) 113-116. [7] M. Bafna, A.K.Gupta, Y.K.Vijay, Accepted in Bulletin of Material Science, (2018) [8] M. Bafna, N.Garg, A.K. Gupta, Journal of Emerging Technology and Innovation Research, 5(2) (2017) 433-36. [9] M R Ranganath, B. Lobo, Proceedings of International Conference on Material Science Research and Nano technology, (2008) 194-195. [10] I. F. H. Al-Sulaimawi, Journal for Pure and Applied Science, 28 (2) (2015) 32-39. [11] M. H. Abdulallah, S. S. Chiad, N. F. Habubi, Diyala Journal for Pure Science, 6 (2) (2010) 161-169. [12] H. N. Najeeb, A.A. Balakit, G. A. Wahab, A. K. Kodeary, Acadmic Research Institute, 5 (1)( 2014) 48-56. [13] C. Kittel, Introduction to Solid State Physics, John Wiley & Sons, New York, 2005 (8th ed) .