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Mar 17, 2016 - (15) Calcium zincate (Ca0.2Zn0.8O) semiconductors with its wide band gap energies are totally transparent in the visible regions and ar...
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Highly Flexible and Visibly Transparent Poly(vinyl alcohol)/Calcium Zincate Nanocomposite Films for UVA Shielding Applications As Assessed by Novel Ultraviolet Photon Induced Fluorescence Quenching Nithin Kundachira Subramani,†,‡ Shilpa Kasargod Nagaraj,‡ Sachhidananda Shivanna,‡ and Hatna Siddaramaiah*,‡ †

Postgraduate Department of Chemistry, JSS College, Mysuru 570 025, India Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysuru 570 006, India

‡

S Supporting Information *

ABSTRACT: Herein we report the successful fabrication of highly flexible, reversibly stretchable, transparent, and conductive poly(vinyl alcohol) (PVA) nanocomposite (NC) films with a hydrophobic surface by reinforcing varying amounts, viz., 0, 0.5, 1, 2, and 4 wt %, of calcium zincate (Ca0.2Zn0.8O) nanofillers. The developed nanocomposite films show appreciable UVA screening efficacies as established by a novel (UV-transillumination studies) method. The Fourier transform infrared (FTIR) studies reveal a positive interaction between PVA matrix and incorporated nanofiller, while scanning electron microscopic (SEM) studies support uniform filler dispersions. The electronic spectral studies substantiate the changes in electronic band structure of composite films leading to appreciable changes in the optoelectronic properties. The fluorescent emission studies reveal dopantdependent photonic emissions, while the dielectric properties, such as dielectric constant (ε′) and dielectric loss (ε″), increase with an increase in filler volumes up to an optimal filler fraction (2 wt % of Ca0.2Zn0.8O) owing to the segmental motion of polymer chains in addition to interfacial polarization associated with multicomponent systems. The developed films with excellent optoelectronic properties alongside appreciable flexibilities and stretchabilities aid their applications as multifunctional UVA shielding polymeric composites with enhanced photoconductivities. tions.5 Furthermore, the high surface reactivity attributed to large surface-to-volume ratio of nanofiller intercalations may induce totally new optoelectronic properties to the polymeric matrix.6 The possible electronic energy transfer between the inorganic filler and organic matrix, widens the application window of resultant composites. Polymer nanocomposite films are also attractive as flexible luminescent semiconductor films analogous to dye doped polymers.7,8 The higher emission intensity and greater photostability of semiconductor based nanofillers combined with excellent processability of their dispersions in polymers offer a new range of promising

1. INTRODUCTION The exponential intensification of optical material applications has opened up the technological gateway toward the design and development of novel high performance optically functional materials for future use in optical computing,1 to hard transparent coatings as protective or barrier layers, solar cells for photoelectric transformations, transistors for electronic switches, and so forth.2−4 Polymer encapsulated semiconductor nanocomposite films show great promise toward optoelectronics with necessary stability and excellent processability. The ease of polymer nanocomposite (NC) based device fabrication and availability of novel morphology dependent material properties have influenced the scientific community to fabricate and evaluate the efficacy of various transparent polymer/ semiconducting filler combination for electro-optic applica© XXXX American Chemical Society

Received: October 16, 2015 Revised: January 14, 2016

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Figure 1. (a) FTIR spectra of PVA/Ca0.2Zn0.8O nanocomposite films. (b) X-ray diffraction spectra of PVA/Ca0.2Zn0.8O nanocomposite films.

stretchable, and polar nature renders PVA as ideal polymeric hosts for inorganic semiconductors in contrast to low impact resistance and relatively brittle acrylic plastics such as poly(methyl methacrylate) (PMMA). However, PVA is also known to undergo photochemical transformations under the influence of high energy radiations.14 This distinctive feature of PVA can also affect the properties of PVA/nanofiller composites, since material characteristics of semiconductor, especially optical absorption, emission, and electronic conduction, are known to be influenced significantly by the extent of passivation of under coordinated surface atoms of the semiconductor by organic ligands in addition to the chemical nature of passivating entities.15 Calcium zincate (Ca0.2Zn0.8O) semiconductors with its wide band gap energies are totally transparent in the visible regions and are excellent UVA shielding materials owing to their direct band gap absorptions.16 Furthermore, alkali and alkaline earth metal doped

applications in display devices and light-emitting diodes (LEDs). Previous studies have established that the integration of semiconductor nanofillers embedded in polymeric matrix can enhance its transport characteristics, thereby increasing the efficiency of polymer based solar cells and LEDs.9−11 Colloidal quantum dots based polymer devices have emerged as a competitive choice in thin film displays with improved color saturation and white lighting with a high color rendering index.12 Poly(vinyl alcohol) (PVA) with hydroxy functional backbone is known to offer excellent filler dispersions owing to its excellent particle stabilizing nature. Furthermore, the ease of film casting, environmental benign nature, high tensile strength, and flexibilities in addition to high gas barrier properties of PVA matrix account for its applications as optically clear soft contact lenses and its filler dependent material properties to be investigated as a potential polymeric gate insulator for organic thin-film transistors (OTFTs).13 The highly flexible, reversibly B

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Figure 2. (a) SEM images of PVA nanocomposite films with (a) 0, (b) 1.0, (c) 2.0, and (d) 4.0 wt % of Ca0.2Zn0.8O. (b) EDS profile of PVA/4.0 wt % of Ca0.2Zn0.8O nanocomposite film.

valuable particle stabilizing PVA encapsulation of Ca0.2Zn0.8O nanofillers to address the key issues in the development of PVA/Ca0.2Zn0.8O NC films with high flexibility, visible luminescence, optical transparency, and filler homogeneity in addition to UVA shielding abilities. The central theme of the study is to establish the UVA shielding efficacies of fabricated films with special emphasis on tailorable engineering properties such as optical energy gaps, refractive index (RI) dispersions, fluorescent emission, dielectric response, and frequency

ZnO are known to emit visible luminescence arising from intrinsic structural characteristics besides exciton emission.17 However, the destruction of luminescence centers due to intense aggregations and meager stability are the important factors that needs to be addressed toward the design and development of visible luminescent films. An interesting solution is the appropriate surface modification that leads to bright size-dependent photoluminescence (PL) and long-term physicochemical stability.18 Herein, author’s demonstrate a C

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Macromolecules dependent charge conduction. The developed films with considerably high optical clarity, mechanical strength, and dielectric response along with UVA shielding efficacies may give a basis for these nanocomposite films to be employed as multifunctional transparent conducting films (TCFs) with appreciable UVA shielding efficacies, in addition to be used in power integrated stretchable optoelectronic systems.

and its nanocomposites with Ca0.2Zn0.8O filler (Figure 1a) reveals a valley around 932 cm−1, symbolizing the syndiotactic structure of PVA19 with a sequential distribution of −OH groups to planarize the polymeric backbone.20 However, a blueshift of this valley (932−920 cm−1) with the nanofiller loadings ascertains a loss in planarity of resultant composite films, thereby inducing entirely new material properties unattainable by dopant free PVA films.21 Furthermore, the decrease in intensity of −OH stretching vibration band (3259 cm−1) and a blue-shifting of −CH asymmetric vibration bands (2920 cm−1) indicates the existence of electrostatic interactions between the −OH groups of PVA and the integrated fillers. The increased intensity at around 2145 cm−1 and its relative broadening correlates the formation of defects arising from the charge transfer interaction between the nanointegration and the PVA matrix, which in turn supports the novel luminescent properties of the NC films. Furthermore, the integrated fillers brought about a continuous decrease in reflectance intensities in the range 1500−1075 cm−1 owing to the decoupled −OH and −CH vibrations due to the electrostatic interactions between the integrated filler and the PVA matrix. The induced structural change in the polymeric host elucidates the enhanced physicochemical properties of nanocomposite films. The powder XRD diffraction patterns of Ca0.2Zn0.8O nanofiller fabricated PVA films shown in Figure 1b reveals a number of strong reflections, giving a clear picture of crystal orientations in the three-dimensional array with all nanocomposite films exhibiting a sharp diffraction peak at 2θ = 19.4° corresponding to (101) γ-crystalline phase of PVA indicating a semicrystalline nature.22 However, the nanofiller integrated films showed additional peaks at 2θ values of 31.6°, 34.5°, and 36.1° corresponding to (100), (002), and (101) planes of Ca0.2Zn0.8O,23 substantiating the presence of Ca0.2Zn0.8O in PVA matrix. However, there is no much variation in the peak positions corresponding to 2θ = 19.4°, validating the effective particle stabilizing nature of PVA, which effects an uniform filler dispersion24 as evidenced by SEM studies discussed in subsequent sections. The morphological features of 0, 1, 2, and 4 wt % Ca0.2Zn0.8O integrated PVA films were analyzed by scanning electron microscopy (SEM) (Figure 2a) coupled with energy dispersive spectroscopy (EDS) (Figure 2b) to establish polymer−filler compatibilities. The SEM studies reveal the successful incorporation of Ca0.2Zn0.8O fillers into the polymeric matrix with uniform dispersions. Moreover, the filler introduced composite films exhibited increased whitening and roughening with increasing filler content. However, the effect of increasing the filler above the threshold levels is displayed in Figure 2a with the nanofillers aggregating due to increased filler content, which in turn influences the material properties of resultant films. Furthermore, the presence of inorganic nanoceramics in the organic host was assessed by EDS analysis of PVA/4 wt % Ca0.2Zn0.8O film (Figure 2b), which distinctly identifies the elemental components of the filler (Ca, Zn, and O) in the host polymer supporting the possible structural changes in the polymeric host upon filler intercalations. Effect of Ca0.2Zn0.8O Nanofillers on UV−Vis Absorption, Transmission, and Reflectance Behaviors of PVA Films. The profound increase in UV radiation dosage in the past few decades has imposed the scientific community to design and develop wavelength selective photoprotective materials with added flexibilities that effectively shield UVA radiations, a major constituent of terrestrial sunlight. With this

2. EXPERIMENTAL SECTION Materials. Zinc nitrate hexahydrate [Zn(NO3)2·6H2O], calcium nitrate tetrahydrate [Ca(NO3)2·4H2O], and hexamethylenetetramine (HMTA) were procured from Sigma-Aldrich, India, for nanofiller synthesis. The PVA with a weight-average molecular weight of 124 000 (86−89% hydrolyzed) was obtained in the powder form from M/s SD Fine Chemicals, India. Double distilled water was used for dissolution and film casting of PVA nanocomposite while other chemicals of AR grades were employed without further purification. Synthesis of Ca0.2Zn0.8O Nanoparticles. 1.18 g of Zn(NO3)2· 6H2O and 0.56 g of HMTA were mixed in a beaker containing 25 mL of deionized water. The calcium doping reagent, aqueous solution of Ca(NO3)2·4H2O, was added dropwise to the above mixture so as to obtain Ca(NO3)2·4H2O:Zn(NO3)2·6H2O in the molar ratios of 0.2:0.8. The solutions were then stirred on a hot plate at 70 °C for 3 h. The resultant mixture was then autoclaved at a constant temperature of 95 °C for 8 h to achieve hydrothermal growth of desired nanofillers; after the growth process, the obtained powder was subsequently washed with distilled water followed by ethanol and dried. The powders so obtained were finally annealed in a furnace at 500 °C in oxygen atmosphere for 2 h to obtain the desired nanofillers. Fabrication of PVA/Ca 0.2 Zn 0.8 O NC Films. The PVA/ Ca0.2Zn0.8O NC films were fabricated by the solution intercalation technique. Films were cast from Ca0.2Zn0.8O−water suspension in which PVA was dissolved by heating at 95 °C for 3 h. The solid content of the polymer solution was optimized at 7.5 wt % of PVA. The PVA solution containing nano-Ca0.2Zn0.8O was ultrasonicated for 30 min to achieve homogeneous particle dispersion. The homogenized solution was then casted onto a clean glass mold and dried at room temperature. The PVA films were casted with varying amounts, viz., 0.5, 1.0, 2.0, and 4.0 wt %, of nano-Ca0.2Zn0.8O filler. The fabricated films were free from air bubbles and exhibited uniform filler dispersion. Characterizations. The FTIR spectra of the fabricated films were recorded in the ATR mode in the spectral range 4000−400 cm−1 using JASCO 4100 spectrometer, Japan, with 4 cm−1 resolution. The morphological behaviors of the PVA/Ca0.2Zn0.8O nanocomposite films were recorded by a Hitachi 3400, Japan, scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). The X-ray powder diffraction (XRD) patterns were recorded using a Bruker diffractometer (Germany) in the scanning range of 4°−40° (λ = 1.54 Å). The optical absorption characteristics of the developed films were established by a Schimadzu-1800 spectrophotometer (Japan) in the spectral range of 220−800 nm. The fluorescent emission features have been established by a Hitachi F-4600, Japan, spectrofluorometer in the wavelength range 200−700 nm. The water contact angle measurement was accomplished using a Holmarc HO:IAD-CAM-018 contact angle meter, India. The UVA shielding properties were established by employing a Bio-Rad trans-illuminator (USA). The dielectric properties of the films were probed as per ASTM-150 standards using a Hioki LCR HiTESTER 3532-50, Japan, in a frequency range of 100 Hz−5 MHz at a constant voltage of 1 V at room temperature.

3. RESULTS AND DISCUSSION Structural and Gross Morphological Behaviors of PVA/Ca0.2Zn0.8O NC Films. The effect of Ca0.2Zn0.8O nanofiller introduction on the gross structural properties of PVA matrix was established by Fourier transform infrared (FTIR) spectroscopic studies in the attenuated total reflectance (ATR) mode. The FTIR spectrum recorded for pristine PVA D

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Figure 3. UV−vis absorption spectra of PVA/Ca0.2Zn0.8O nanocomposite films: (a) absorbance and (b) absorption coefficient.

Figure 4. UV−vis transmittance spectra of PVA/Ca0.2Zn0.8O nanocomposite films. Background: digital photographs of 0.3 ± 0.03 mm thick PVA nanocomposite films depicting high visible transparencies (a) 2 wt % PVA/Ca0.2Zn0.8O and (b) 4 wt % PVA/Ca0.2Zn0.8O.

intent, the various formulations of Ca0.2Zn0.8O fillers integrated PVA films were subjected to electronic spectral studies, which is a useful tool to establish the energy band structure of solids. Furthermore, the measured values of absorption, transmission, reflectance, and so forth are efficiently employed to deduce the dielectric functions and optical conductivities along with the energies associated with electronic transitions. The absorbance spectra recorded in the UV−vis region (220−800 nm) is depicted in Figure 3a, which reveals a broad shouldered valley at 260−280 nm substantiating the characteristics of PVA. The appearance of new peak at 355−360 nm with nanoinclusions and there relative broadening support the possible interaction between the PVA matrix and Ca0.2Zn0.8O dopant owing to the absorption edge of polymer and metal fractals in the NC film.25 The increased absorption with increasing filler content (0.5, 1, 2, and 4 wt % Ca0.2Zn0.8O) is in good agreement with the values reported by earlier researchers,26,27 in accordance with Beer’s law. The increase in the filler content increases the number of light absorbing molecules, which in turn accounts for the increased absorbance. The nanocomposite films exhibit an absorption maximum in the UVA regions with photonic energies ranging from 3.35 to 3.44 eV. Furthermore, the nanocomposite films exhibit dopant content dependent

photonic absorption, with the absorption maximum red-shifting on increasing the filler content. The observed steep absorption coefficients of nanocomposite films (Figure 3b) at relatively higher photon energies (>3.1 eV) support their strong UV light absorbing tendencies in contrast to visible. The UV−vis transmittance spectrum recorded as a function of applied wavelengths (Figure 4) reveals the effect of filler content on the optical clarity of PVA films with UV region (400 nm) the transmittance intensities of filler introduced PVA films were on par with transmittance of undoped PVA film (Figure 4). As shown in the spectral background, the NC films were highly transparent, and there was no visible clouding. The excellent transparency of PVA/Ca0.2Zn0.8O NC films may be owed to the near matching refractive indices (RIs) of the PVA matrix and Ca0.2Zn0.8O filler and a very low absorption coefficient in the visible spectrum.28 An additional factor is the excellent dispersion of Ca0.2Zn0.8O nanofillers within the particle stabilizing PVA matrix that nearly nullifies the Rayleigh scattering losses especially in the visible region, thereby retaining the optical clarity.29 E

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Figure 5. Digital photographs of PVA/4 wt % Ca0.2Zn0.8O nanocomposite film: (a) bending mode, (b) relaxed mode, and (c) stretched mode.

Figure 6. Water contact angle images of PVA/Ca0.2Zn0.8O nanocomposite films with (a) 0, (b) 1.0, (c) 2.0, and (d) 4 wt % of Ca0.2Zn0.8O.

Figure 7. UVA-transilluminator images of PVA/4 wt % Ca0.2Zn0.8O nanocomposite film: (a) UVA radiation fluorescent and (b) UVA shielding barrier leading to fluorescence quenching.

Surface Wettability Studies of Highly Flexible and Reversibly Stretchable PVA/Ca0.2Zn0.8O NC Films. The highly flexible and reversibly stretchable nature of fabricated PVA/Ca0.2Zn0.8O nanocomposite films is depicted in Figure 5. As can be seen, the PVA nanocomposite films are highly flexible and reversibly stretchable owing to the presence of hydroxyl groups in the polymeric backbone. The pendant hydroxyl groups also induce hydrophilic nature to the polymeric matrix, thereby enhancing the material wettability that depends on the surface roughness in addition to chemical composition.30 The surface wettability of PVA/Ca0.2Zn0.8O nanocomposite films with water surface is established by contact angle (CA) studies (Figure 6). The solid surface under investigation is said to be hydrophilic if CA < 90° and hydrophobic if CA > 90°. The CA data revealed a continuous increase in contact angle with nanofiller loadings. The undoped PVA film shows a hydrophilic nature (CA = 81.6°), while Ca0.2Zn0.8O nanofiller introduced films exhibit a steady deviation toward hydrophobicity, with 4 wt % nanofiller integrated systems showing a minimal wettability (CA = 97.4°). The observed decrease in wettability with filler intercalations may be ascribed to the increased surface roughening (as evidenced from SEM studies (Figure 2a) of nanocomposite films with an increase in filler content. The induced roughness may facilitate air trapping within the voids on the surface leading to a heterogeneous gas/air−solid surface that in turn reduces the adhesive force between the liquid/water−solid surface leading to enhanced wetting resistance and hence, a hydrophobic nature.31−33

UVA Shielding Efficacies of NC Films Assessed through Fluorescence Quenching Studies. The UVA light harnessing properties of NC films were further explored by fluorescence quenching studies using a UV-transilluminator (for detailed procedure see Supporting Information) operating with photon energies corresponding to the UVA region. Figure 7a depicts the UVA radiation fluorescent PVA/4 wt % Cs2ZnO2 NC film, while Figure 7b sheds light on the UVA shielding nature of PVA/4 wt % Ca0.2Zn0.8O NC film that effectively shields the otherwise fluorescent PVA/4 wt % Cs2ZnO2 NC film from UVA radiations, thereby quenching the visible fluorescence.34 The observed fluorescence quenching may be attributed to the excellent UVA absorbing nature of PVA/4 wt % Ca0.2Zn0.8O NC barrier that converts the absorbed higher energy/shorter wavelength UVA radiations into lower energy or higher wavelength visible radiations as noticed by fluorescent spectroscopic studies. The observed photonic conversion (UVA absorption → visible emission) by PVA/Ca0.2Zn0.8O NC films leads to a decrease in photonic excitation energies available for interband electronic transitions in Cs2ZnO2 introduced PVA films, which in turn leads to fluorescence quenching. The excellent UVA shielding properties coupled with high visible transparencies support the possible application of PVA/ Ca0.2Zn0.8O NC films as transparent UVA filters35 for UVsensitive device fabrications. Effect of Ca0.2Zn0.8O Nanofillers on Optical Constants of PVA Films. The energies associated with interband electronic transitions were deduced from the plots of (αhv)1/n F

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Table 1. Extracted Values of Absorption Edge, Direct and Indirect Band Gaps, Refractive Index, and Abbe Number for PVA/ Ca0.2Zn0.8O Nanocomposite Films absorption edge (eV)

direct band gap (eV)

indirect band gap (eV)

refractive index

wt % of Ca0.2Zn0.8O in PVA

lower energy

higher energy

lower energy

higher energy

lower energy

higher energy

at 550 nm

Abbe number

0.5 1.0 2.0 4.0

2.87 2.62 2.50 2.25

4.50 4.00 3.75 3.10

2.75 2.55 2.45 2.40

4.55 4.45 4.30 4.15

3.30 3.20 3.17 1.15

4.97 4.55 4.37 4.30

1.72 1.97 2.30 2.58

6.36 5.35 5.08 4.18

against hν as shown in Figures S1 and S2 (see Supporting Information) looking for allowed values (1/2 and 2) of n that yields best linear fit for a given data set depending on the nature of transitions.36 For interband electronic excitations, the energy dependence of the absorption coefficient is of the form αhν = A(hν − Eg )n

(1)

When the nature of transition is direct (n = 1/2), the energy dependence may be expressed as αhν = A(hν − Egd)1/2

(2)

and ⎡ (hν − E + E )2 (hν − Eg − Ep)2 ⎤ g p ⎥ + αhν = B⎢ ⎢⎣ exp(Ep + kT ) − 1 1 − exp(Ep + kT ) ⎥⎦

Figure 8. Refractive index dispersion as a function of wavelength of PVA/Ca0.2Zn0.8O nanocomposite films. (3)

2.55, and 1.89 eV (nD, nF, and nC, respectively) (Table 1) using eq 4:

where A and B are empirical constants, Eg is the energy associated with interband electronic transition, and Ep is the energy associated with a phonon. The lower values of optical band gap with higher filler loading (Table 1) reflect the changes associated with the electronic structure of polymeric host with nanofiller inclusions. Furthermore, the integrated fillers may induce localized electronic states in the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gaps, making the lower energy transitions feasible and hence decreasing the optical band gaps. The blueshifting of interband transition energies with nanofiller loading ropes, the observed larger conductivities (discussed in subsequent sections) of nanocomposite films in contrast to the host. Additionally, the optical applications of polymers are often restrained owing to their relative lower RI’s.37 However, there exists a virtuous possibility of improvising the optical response of the polymer by suitably embedding with metallic fillers and exploring these hybrids as materials for optical applications. One such attempt is made in the current study, wherein the effect of embedding nanosized Ca0.2Zn0.8O on the RI of PVA films has been studied (Figure 8) and presented in Table 1. Figure 8 clearly perceives a continuous increment in RI with the filler loadings arising from the increased packing fraction of composite films with the filler loadings. The increased packing fraction explicates a strong intermolecular interaction between the metallic nanofiller and the polymeric host as established by FTIR studies. The optical dispersion characteristics of nanocomposite films were further established by measuring the Abbe number, a parameter characterizing the dependence of refractive index on applied wavelengths at varied filler loadings.38 The Abbe (υD) number for PVA and its NC films were deduced from their RIs39 at applied photonic energies of 2.10,

υD = (n2.10eV − 1)/(n2.55 − n1.89)

(4)

The Abbe numbers of NC films showed a sharp decrease from 51 for undoped PVA films to 8 for PVA/4 wt % Ca0.2Zn0.8O NC film explicating a higher chromatic aberration. 40 Furthermore, the linear dependence of RI of PVA/ Ca0.2Zn0.8O films with filler content (Table 1) indicates the excellent filler dispersions in the PVA matrix, which is supported by SEM studies. The observed dependence of RIs on filler content provides ample scope for RI engineering, which facilitates optical device design.41 Light-Emitting Properties of NC Films on UVA Excitation. The plasmonic PL signature of PVA and its nanocomposites with Ca0.2Zn0.8O fillers were explored by fluorescence excitation studies with intent of optimizing the energy requirements for luminous transitions as shown in Figure 9a. The excitation spectrum exhibits a peak maximum around 370 nm/3.35 eV in accordance with electronic spectral (UV−vis absorbance studies) results with narrowing of excitation bands supporting UVA shielding behaviors of NC films. The effect of Ca0.2Zn0.8O fillers on the fluorescent intensities excited at 3.35 eV (Figure 9b) revealed dopant dependent emission of PVA/Ca0.2Zn0.8O composite films exhibiting a broad (400−450 nm) blue emission corresponding to emission energy of 2.75−3.10 eV. The effect of Ca0.2Zn0.8O nanointercalations were further established by UV-transillumination studies, wherein the introduced nanofillers induced visible emission in an otherwise nonfluorescent PVA matrix (Figure 9c). The novel fluorescence emission of PVA/ Ca0.2Zn0.8O films may be attributed to the higher surface area of Ca0.2Zn0.8O nanostructures which facilitates a synergistic matrix−filler interaction inducing totally new material behavG

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Figure 9. Fluorescent spectra for PVA/Ca0.2Zn0.8O nanocomposite films: (a) excitation spectrum, (b) emission spectrum, and (c) UVtransilluminator images under UV excitation.

fluctuating macroscopic dipole of the composite. The observed variations in ε′(ω) as a function of applied frequency at room temperature for PVA/Ca0.2Zn0.8O NC films is depicted in Figure 10. The observed changes in the values of frequency

iors.42,43 The broad emission thus obtained may be accredited to the electronic transition from shallowly trapped electrons to deeply trapped holes.44 The narrow absorption in the UVA and broad emission in the visible regime formulates PVA/ Ca0.2Zn0.8O nanocomposite films as potential UV shieldants. Furthermore, the emission intensity is found to increase with the filler content up to 2 wt % in PVA matrix. However, 4 wt % Ca0.2Zn0.8O filler loaded composite exhibit a decreased emission owing to the fact that an increase in filler above the optimal range may lead to filler aggregations, leading to PL quenching at higher filler loadings.45 Dielectric Behaviors. The high electrical conductivities coupled with enhanced carrier mobilities along with moderate to high visible transmittances make alkali metal doped zinc oxide semiconductors as promising materials for design and development of transparent conducting films (TCFs). The ability of a material to exhibit charge separation under the influence of applied field may successfully be employed to interpret the dielectric properties. Dielectric studies are aimed to establish the total material polarization in a time dependent electric field; under an alternate electric field, the response is however expressed in terms of complex permittivities employing the relation ε(ω) = ε′(ω) + ε″(ω)

Figure 10. Dielectric constant as a function of frequency of PVA/ Ca0.2Zn0.8O nanocomposite films.

(5)

where ε′(ω) and ε″(ω) are the real and imaginary parts of permittivity exhibiting energy storage and energy loss in each cycle of the electric field. The real part shows energy dispersion arising from static permittivity (lower frequency region), while the imaginary part expresses the energy dissipation resulting from the coupling of time dependent electric field with a

dependent permittivity are assigned to dielectric relaxations arising from the segmental/micro-Brownian movement of the polymer chain. An additional factor is the phenomenon of interfacial polarization associated with multicomponent systems such as nanocomposite dielectrics owing to charge migrations.46,47 The permittivities of PVA/Ca0.2Zn0.8O nanocompoH

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frequency independent region followed by (ii) a higher frequency dependent region. The observed trend may, however, be explained on the basis of electrode polarization effects referring to the emblematic behavior of ionic conductors. In the lower frequency region, a greater volume of charges accumulates between the filler−matrix interfaces causing a drop in conductivities. However, at higher frequencies, a strong frequency dependent conductance is observed. The frequency dispersion of conductivities reveals a nonrandom ionic diffusion type of ion transport. Furthermore, the conductivity of NC films increase with increasing filler content owing to the electronic interactions between PVA matrix and nanostructured fillers. The ac conductivity has been observed to be the highest for 2 wt % Ca0.2Zn0.8O loaded PVA nanocomposite due to the increasing in the number of charge carriers. However, at higher filler loadings leads to filler clustering which inturn entraps the charge carriers creating isolated pockets of ion pairs in the matrix leading to a decline in the fraction of free mobile carriers and a decrease in conductivity.51 The same is true for dielectric permittivity because the origin of dielectric response in such systems is a function of transient dipoles formed due to ion-pairing. The segregation of filler also slows down the ion dynamics, commonly expressed as rates of hopping/mobility.

site systems are found to be dependent on the number of orientable dipoles and their relative aptitudes to orient under the influence of applied field.48 Universally, the dipoles are found to follow the applied field variations at relatively low frequencies (ω ≪ 1/τ, where τ is the relaxation time) resulting in higher values of permittivity, thereby verifying the fact that initially (limω→0) polar materials exhibit higher values of dielectric permittivity. However, with an increase in frequency, the dipolar groups are not capable of following the field variations and are enable to orient at same pace as the alternating field.49 The dielectric constant of nanocomposite films is found to exhibit a monotonic increase with filler content. The observed trend may, however, be attributed to the formation of charge transfer complexes (CTC’s) creating channels for charge migration, eventually leading to accumulation of space−charges, a greater average polarization, and hence a greater contribution to the dielectric constant. An additional factor is the formation of mini-capacitor networks in the PVA/Ca0.2Zn0.8O nanocomposites with an increasing nanofiller content. The imaginary part of permittivity (ε″) reveals the energy dissipation of a material in the presence of an external field (see Supporting Information Figure S3). From the figure, it is quite evident that the dielectric loss increases with an increase in filler content (up to 2 wt % of Ca0.2Zn0.8O) owing to an increase in the number of charge carriers with optimal polymer−filler interactions. However, further increase in filler content (above 4 wt %) brings about a decreased dissipation due to the agglomeration of fillers at higher concentrations.50 In contrast to the filler effect, the variation of frequency has a profound influence on energy dissipation, which decreases sharply with an increase in frequency. The observed higher values of dielectric loss at lower frequency regimes could be due to mobility of charges within the polymer matrix. Whereas at high frequency, the periodic reversal of the field is so rapid that there is no excess ion diffusion in the direction of electric field, resulting in a decreased polarization due to charge accumulation leading to a declined loss factor. The ac conductivity of the PVA nanocomposite films arises from the motion of charge carriers through the polymer matrix. Figure 11 shows the logarithmic plots of conductivity as a function of frequency for PVA/Ca0.2Zn0.8O films. As can be seen, the conductivity regime indicates two regions: (i) a lower

4. SUMMARY To sum up, we have fabricated a series of highly flexible and reversibly stretchable transparent conducting films with appreciable UVA shielding properties by suitably decorating PVA matrix with Ca0.2Zn0.8O nanofillers. The fabricated films exhibits excellent transparencies in the visible regime coupled with steep UVA absorptions. The UVA shielding efficacies of PVA/Ca0.2Zn0.8O NC films were established by a novel fluorescence quenching studies. The UVA screening abilities of solution casted nanocomposite films were monotonic with filler content and the PVA/4 wt % Ca0.2Zn0.8O nanocomposite exhibit a maximum shielding. The fluorescence emission studies revealed a sharp UVA excitation peak (≅ 360 nm) suggesting the possible application of fabricated films as UVA filters. The water contact angle studies reveal a hydrophilic−near-hydrophobic transition in surface wettabilities. The dielectric constant and ac conductivity increase up to an optimal filler content (2 wt % Ca0.2Zn0.8O) and then diminish, owing to filler clustering. Overall, the newly developed series of UVA absorbing, nearhydrophobic NCs with excellent optoelectronic properties and appreciable stretchabilities are very promising as UVA filters toward the fabrication of UVA-sensitive optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02282. Methodology employed for quenching studies, optical energy gap measurements, fluorescent quenching profiles via UV-transilluminator measurements, and DLS profile of Ca0.2Zn0.8O nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +91-9972095262 (H.S.).

Figure 11. The ac conductivity as a function of frequency of PVA/ Ca0.2Zn0.8O nanocomposite films. I

DOI: 10.1021/acs.macromol.5b02282 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Notes

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The authors declare no competing financial interest.

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DOI: 10.1021/acs.macromol.5b02282 Macromolecules XXXX, XXX, XXX−XXX