Influence of KI Salt Concentration on The Hydroxypropyl Methylcellulose Films: Optical Study


 Herein the optical properties of Hydroxypropylmethyl cellulose/Potassium iodide (HPMC/KI) composite films were determined. Casting technique was introduced to make HPMC/KI films with different KI salt concentrations (0.1–1) wt%. The absorbance model was used to determine parameters like absorption edge, refractive index, real and imaginary sections of dielectric function, extinction coefficient, Urbach energy, band gap and optical conductivity in the spectral range 200–800 nm. As shown by study, KI salt doping affects the optical properties of HPMC. The absorption edge (\({E}_{e}\)) was widely displaced towards a region of lower photonic energy. For the 1 wt% KI/HPMC film, the direct and indirect optical bandwidth gaps of pure HPMC were reduced from 5.6 eV to 2.56 eV and 5.86 to 2.5 eV respectively. The optical dielectric loss method was effectively employed as an alternate method for estimating the optical bandgap. In addition, the Tauc’s extrapolation method identified the kind of electronic transition. The variation of optical energy band gap and dielectric constant based on KI salt concentration was used to investigate the credibility of the Penn’s model. In salt-composite films, an increase in Urbach energy and optical conductivity were observed which may be evidence of large shift from tail-to-tail and band to tail. Meanwhile, X-ray diffraction (XRD) examination revealed that the KI salts damaged the HPMC polymer's crystalline phase. Lastly, the films were also subjected to Fourier transform infrared spectroscopy (FTIR). The considerable variation in transmittance and band change in FTIR spectra was exposed for doped films.


Introduction
Doping is the process of adding impurities into a host polymer to modulate its physical properties like electrical, optical, thermal, mechanical, and structural properties. The comprehensive investigation of the doped polymer with various concentrations of dopant enables the selection of the desired properties [1,2]. Depending on their reactivity with the host matrix, dopants can be used to modify the electrical and optical properties of polymers [3].
The effect of doping concentration on optical, morphological and dielectric properties of PMMA films has been studied by L. N. Ismail et al [4]. Ranganath M R et al [5,6] studied optical properties of iodine and ferric chloride FeCl3 doped poly(vinyl alcohol)poly(vinyl pyrrolidone) ( PVA/PVP) blend films. The doping of HPMC with FeCl3 in aqueous dispersions has been studied by Y Prakash et al [7], which showed that doping of FeCl3 caused a significant change of Physico-mechanical, AC-conductivity, and microstructural properties. N Sandhya Rani et al [8][9][10] studied the effect of CdCl2 and NaI on the functional, structural, electrical conductivity and thermal properties of HPMC polymer electrolyte films. Biodegradable polymers are raising prominence as a green alternative to commercially available non-degradable polymers. They are becoming a growing discipline of research and have attracted many industries as they are plentiful and renewable [11,12]. The majority of conjugated polymers with intramolecular charge transfer (ICT) properties have been reported to be narrow bandgap polymers  HPMC contains a slight hump at 275 nm that can be seen due to the n→π* transition in the methyl groups [32].
The shift in the humps for the samples once doping with KI specify a significant interaction between HPMC and KI [33]. The absorption spectra declined with rising wavelength for Pure and doped samples. According to Beer's Law, absorption increased as the KI doping concentration increased [33,34,35]. The quantity of absorbing molecules determines the degree of absorption. This is consistent with prior reports and findings [36]. (1) Where, A is the absorbance, T is the transmittance and the thickness of the film is d. The photon energy hν dependent optical absorption coefficient was revealed in Fig. 3 for pure and doped HPMC samples. The basic absorption edge is thought to be the really important properties of the absorption spectra of crystalline and noncrystalline substances [38]. Extrapolating the linear section of the curves to zero absorption (α=0) yielded the position of the absorption edge. The absorption coefficient increased largely near the edge due excitation of electrons from the lower energy band to higher energy [39]. The virgin HPMC's absorption edge ( ) was discovered to be 5.09 eV and reduced to 2.57 eV for 1 wt% of KI fixed HPMC films (reported in Table 1). The establishment of intramolecular charge transfer (ICT) and enrichment in intermolecular stacking is responsible for such a large change in the absorption edge in the doped HPMC polymer films [40]. The relocation of the absorption edge ( ) to smaller photon energy indicates that the optical bandgap has shortened in the doped samples [41]. The lowest absorption edge of 2.57 eV was found in the highest doped polymer indicating that it is the most suitable conduction among the other films.

Tail study
The feature of the primed polymer is predicted through Urbach energy. In the shorter absorption region, the absorption coefficient (α) has an exponential effect on photon energy (hν) and complies the Urbach relation (2) [42]: The Urbach energy is also known as band tail energy, is denoted by and denotes a constant [43].  Table 1. The optical bandgap is inversely proportional to the Urbach energy. It has been proven that the increase of Urbach energy leads to an increase of structural disorder in the polymer complexes. As a result, the states will be redistributed from band to tail, allowing a larger number of states to participate in band-to-tail and tail-to-tail transfers [44]. The measured Urbach energy ( ) for the pure HPMC sample was 556 meV and improved to 874 meV for the doped HPMC film with 1 wt% of the KI. Early studies have shown that a decrease in the crystalline province may increase Urbach energy. This confirms that the trap states are increasing inside the optical energy bandgap and a reduction in the optical bandgap is possible [43,45,46].

Fig. 4
Urbach plots for pure HPMC and doped HPMC samples.

Tauc and Optical dielectric models
The optical absorption of the films to be learned at a great deal to gain more knowledge about the band structure [47]. The fundamental absorption due to transition of electrons from the valence band to the conduction band can be utilised to fix the bandgap of a substance [48]. The optical energy band gap of HPMC and HPMC/KI composites were assessed using Davis and Mott solution, which gives the connection between the incident photon energy (hʋ) and absorption coefficient (α) by the Tauc plot equation (3) [49].

(3)
Where, A is the slope of the Tauc edge known as band tail parameter, denotes the optical energy bandgap and r denotes the exponent factor. The exponent r gives the nature of electronic transition accountable for absorption and it follows the values of 1/2, 2, 3/2 and 3 for permitted direct, allowed indirect, forbidden direct and forbidden indirect electron excitations respectively [48]. diodes (OLED), photonics, and optoelectronic products [23][24][25]. The HPMC/KI films fine optical bandgap shows their suitability for optoelectronics, solar cells and photonics applications.

Refractive Index Study (n):
Another important optical constant is the refractive index (n), which offers knowledge about local fields, polarisation, and phase velocity of light in propagating substance and is used to design optical devices. The following equation (4) can be used to calculate it [54].
Where T S is the percentage transmission coefficient. Figure 7 illustrates that as the wavelength increases and refractive index drops, suggesting proper dispersion behavior [55]. While this aspect of dispersion is critical for practical drives and optical system design [30]. The Refractive index for complex polymer shows a small hump around 370 nm and nearly flat behavior between 400-700 nm. The observable humps may be attributable to the formation of KI in the host polymer matrix. In the visible region, n does not equal zero indicating that some incident photon light may be absorbed.

Extinction coefficient (K)
The extinction coefficient (K) determines how much light is reduced due to scattering and absorption per unit distance of penetration medium.
The extinction coefficient K of virgin and doped samples varies with the wavelength in the 200-600 nm region as seen in Fig. 8(a). Extinction coefficient (K) values are higher at shorter wavelengths (200-400 nm) indicating considerable absorption in this region. It is undeniable that the extinction coefficient for pure HPMC and doped samples decreases with an increase in wavelengths. Because the following equation (5) [29,30] shows that the extinction coefficient is directly proportional to the absorption coefficient. Fig. 1 for α and Fig. 4 for K follow the same pattern.
Where, is the absorption coefficient. Figure 8(a) and 8(b) represents the dependence of extinction coefficient on wavelength and photon energy respectively. The increase in extinction coefficient with a raise in photon energy (decrease wavelength) stating that a fraction of light reduced due to scattering, and absorbance raises [56].

Fig. 8
Extinction coefficient variation as a function of (a) wavelength and (b) energy.

The real ( ) and imaginary ( ) parts of the dielectric function (ε*)
The linear dependence of the substance to electromagnetic radiation can be understood with a complex dielectric function (ε* = ε 1 -iε 2 ), which will give you a better knowledge of the solid's optical features. Equations (6) and (7) can be used to compute the real and imaginary parts of dielectric constants. The most significant physical variable that hangs on the bandgap is the dielectric function's real part, that relates to an electronic part [57]. The effect of the real component of the dielectric constant on wavelength is seen in Fig. 9 for both pure and doped films. With rising wavelength and impurity concentration, the real part of the curve grows and shifts the curve's vertex towards higher wavelengths may be credited to equation (3) by the real part of the dielectric constant with refractive index [58]. The largest value of optical dielectric constant 1 for the smallest optical band gap justifies the well-known Penn's model [59].
The imaginary part ε 2 is directly linked to the valence and conduction band, and it refers to the material's optical absorption which is given by (4). The optical dielectric loss versus directed photon energy and wavelength for all films is shown in Figures 10(a) and 10(b) respectively. In low photon energy region, the dielectric loss was found to be relatively small but exponentially raised at high photon energy (i.e at lower wavelength) for the pure and doped HPMC film. On the other hand, the absorption edge in doped films was noticed to be reallocated to the lower photon energy. It has been proved that the absorption edge resulting from dielectric loss should be highly comparable to the Tauc's relation anticipated values [60]. Table 1 shows the calculated values of the bandgap using both approaches (optical dielectric loss and Tauc).
The optical bandgap results obtained in Fig. 10(a) are similar to those obtained in Fig. 6. Therefore, the optical dielectric function can be utilized to calculate the optical bandgap and analyse the band structure [30,41].
Based on the preceding analysis, it is evident that the Tauc's model can be used to fix the mode of electronic transition and optical dielectric loss can be utilised to analyse the bandgap and [41,43].

Fig. 10
Optical dielectric loss vs photon energy (hυ) for pure and doped HPMC samples.

Optical Conductivity:
The optical conductivity of a material is one of the most essential characteristics that affect its optical properties. It is used to expose the material's interband allowed optical transitions. The optical conductivity σ determined by relation (8) using the absorption coefficient (α) and refractive index (n) of the material [49,61]. . 11 The optical conductivity as a function of photon energy (hυ) for pure and doped HPMC.
For pure and doped films, Fig. 11 displays the optical conductivity versus photon energy. The optical conductivity increases as photon energy increases. The increase in absorption coefficient is responsible for the excitation of carriers from the valence to conduction band due to the absorption of photon energy given by equation 8. The absorption coefficient and refractive index have a direct relationship with optical conductivity and shadows the same fashion of .

XRD Study
The XRD patterns of pure HPMC and doped HPMC samples are shown in Fig. 12. It is known that a large crystalline hump appears at around 2θ = 20.88⁰ for pure HPMC polymer [2]. KI also showed its peaks (PCPDF file no. 780750). The intensity of broad peak suggestively declines for all doped film samples (0.1 wt% to 1 wt% KI) as shown in Fig. 12. The calculated crystallinity percentage was also decreased with dopant concentration (given in table 2) which signifies the distribution of KI in the host polymer's matrix [27,28]. According to the XRD results, the amorphous segment increases in the salt complex samples implying that disorder is more prevalent in doped samples. Previous studies have shown that an increase in the crystalline region in a sample can indicate a drop in the Urbach energy, whereas an increase in the amorphous area is indirectly linked to an increase in the Urbach energy [30,43,45,46]. As a consequence, both amorphous percentage and Urbach energy improved with dopant concentration. XRD measurements found in this mixture will support the Urbach energy calculation (Eu).   bonding due to KI salt interaction in the host polymer. An auxiliary broad peak for HPMC was observed at 2908 −1 , which may be endorsed to symmetric stretching of methyl (-CH3) and hydroxypropyl groups (-CH2 CH (CH3) OH). A sequential change in the peak was observed for complex films, which is the product of the formation of complexes in the polymer film, which causes band length variation. The C-O bond stretching within the sixmember cyclic ring, asymmetric and symmetric bending vibrations of a methoxy group (O-CH3) will trigger the peaks at 1663 −1 , 1455 −1 , and 1349 −1 , respectively. The C-O-C stretching vibrations can be attributed to the characteristic peak at 1046 −1 (Fig. B). The asymmetric pyranose ring may be responsible for the peak at 948 −1 , and the change in the intensity of the peak was observed for the complex films (B2 and B4) as shown in Fig. 13 [7][8][9].
The effect of dopant salt on the modes of vibrations was seen in terms of a decrease in amplitude, broadening of the bands, and shifting of the bands to lower wavenumbers, as well as the emergence of new peaks in the IR spectra. All of which directly implies the complexation between potassium conducting salt and the host polymer [7][8][9]. The OH groups play an important role in directing the crystalline packing [63].