Preparation of Polyvinyl AlcoholâLithium Zirconate ... - ACS Publications

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Preparation of Polyvinyl Alcohol−Lithium Zirconate Nanocomposite Films and Analysis of Transmission, Absorption, Emission Features, and Electrical Properties. H. N. Chandrakala,†,∥ B. Ramaraj,‡ Shivakumaraiah,† G. M. Madhu,§ and Siddaramaiah*,∥ †

Department of Chemistry, Siddaganga Institute of Technology, Tumkur-572 103, India Central Institute of Plastics Engineering and Technology (CIPET), Ahmedabad-382445, India § Department of Chemical Engineering, M.S.Ramaiah Institute of Technology, Bangalore-560 054, India ∥ Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore-570 006, India ‡

S Supporting Information *

ABSTRACT: Polymeric nanocomposites were reported to enhance the barrier properties and lifetime of organic solar cells. In this investigation, we are reporting the development of one such nanocomposites by dispersing the lithium zirconate (Li2ZrO3) nanoparticles in polyvinyl alcohol (PVA) matrix. It was found that the incorporation of Li2ZrO3 nanoparticles has not affected the transparency of the PVA matrix in the visible region at the same time it generates fluorescence emission. The emission intensity increases with the nanoparticles concentration. Similarly microwave irradiation also does not show any transmittance peaks in the visible region even though it had a positive effect in the UV region. The dielectric properties (dielectric constant and dielectric loss) of PVA/Li2ZrO3 nanocomposites increase with increase in the nanoparticles concentration, while it decreases with an increase in frequency. The alternating current conductivity increases with the increase in filler loading and frequency. The dissipation factor also increases with nanoparticles addition and decreases with frequency.

1. INTRODUCTION Composite materials consisting of metal nanoparticles embedded in a transparent host matrix have attracted the attention of researchers recently as advanced technological materials such as solar cells for photoelectric conversation, optical switching, and transistors for electronic switches,1−5 because of their unique optical, electronic,6 mechanical,7 and structural8 properties. Such nanocomposites can be used as a promising material for novel functional applications in optoelectronics,9 magnetics,10 medicine,11 and so forth. Now, it is well established that the polymers, as dielectric materials, are excellent host matrices for metal nanoparticles and semiconductors.12−16 When the nanoparticles are embedded or encapsulated in a polymer matrix, the polymer matrix acts as surface capping agent. In addition, dispersing the metallic nanoparticles in the polymer is much easier, because some of the hydrophilic polymers act as particle stabilizers, so that film casting becomes easier. For applications in optoelectronics and electronics, the particle size control and their uniform distribution within the polymer matrix is the key to the © 2013 American Chemical Society

nanocomposite technology. Polyvinyl alcohol (PVA) has been used widely for preparation of metallic nanocomposites due to its easy processability and optical clarity. In addition, due to its water solubility the nanoparticles can be easily dispersed in an aqueous medium thus making the preparation virtually nontoxic. The polymer nanocomposites are known to possess good barrier properties apart from the flexibility and transparency. The essentiality of the barrier material has to be stable under direct exposure to sunlight. Organic solar cells (OSCs) have shown a low resistance to light and environmental stresses.17 Basically, the OSCs need longer lifetimes, so OSCs exposed to sunlight require protection against weathering factors to achieve long lifetimes. An interesting solution may be the encapsulation by transparent and flexible coatings with high gas barrier properties and long durability. A viable option is the Received: December 1, 2012 Revised: January 30, 2013 Published: February 7, 2013 4771

dx.doi.org/10.1021/jp311828n | J. Phys. Chem. C 2013, 117, 4771−4781

The Journal of Physical Chemistry C

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glycine were obtained from M/s. Merck India, M/s. Himedia India and M/s SD Fine chemicals, respectively. PVA was obtained in the powder form from M/s SD.Fine Chemicals, India with the weight average molecular weight of 124 000 (86−89% hydrolyzed). Doubly distilled (DD) water was used for dissolution and PVA film casting. All other chemicals and reagents used were of analytical grade. 2.2. Synthesis and Characterization of Li2ZrO3 Nanoparticles. Synthesis of Li2ZrO3 was carried out by solution combustion (SC) technique. The SC technique is a fairly recent method compared to solid-state combustion (SSC) or selfpropagating high-temperature synthesis (SHS) method.20 SC synthesis is a versatile, simple, and rapid process, which allows effective synthesis of a variety of nanosize materials. This process involves a self-sustained reaction in homogeneous solution of different oxidizers (e.g., metal nitrates) and fuels (e.g., urea, glycine, hydrazides). This process not only yields nanostructured oxide materials but also allows uniform (homogeneous) dispersion of trace amounts of rare-earth impurity ions in a single step.21 In a typical experimental procedure, zirconyl nitrate and lithium nitrate were used with glycine as fuel. Sample sizes of 4.14 g of zirconyl nitrate, 5.791 g of lithium nitrate, and 2.773 g of glycine were dissolved in 125 mL of distilled water. The reaction mixture was placed on a hot plate. As the heating progressed, water vapor and nitrates (nitric gases) were released, resulting in the formation of a gel. The reaction goes to completion by self-ignition (combustion), minutes after the formation of the gel, leaving behind fine powder. The catalyst was then calcined at 400 °C to produce nanopowder (Li2ZrO3) of size 23.72 nm and characterized by UV, XRD, and SEM.

preparation of PVA and lithium zirconate (Li2ZrO3) nanocomposites. PVA is a semicrystalline polymer with 1,3-glycol structure. It is highly transparent polymer in the visible spectral domain and has a wide range of potential applications in optical, pharmaceutical, medical, and membrane fields. In fact, it is a water-soluble polymer and allows the development of environment-friendly material processes. PVA has been used extensively for its oxygen barrier effect in various applications.18 PVA has also been used in numerous fields because of its biodegradability and environmentally friendly processing.18,19 Lithium zirconate is a cream colored powder. It has been proposed as a medium to store heat energy from sunlight in solar cooker applications. It is being used as solid carbon dioxide sorbents and is used in the manufacture of dielectric bodies based on zircon to obtain capacitors with special electrical properties. Also, it is an insulator and is used to improve thermal stability. The hydrophilic character of lithium zirconate is also expected to promote the dispersion of inorganic crystalline layers in water-soluble polymer such as PVA. The representative molecular structure of PVA, Li2ZrO3 nanoparticles and the possible interaction between them are shown in Figure 1.

2LiNO3(aq) + ZrO(NO3)2(aq) + 2CH 2NH 2COOH(aq) → Li 2ZrO3(s) + 4CO2(g) + 3N2(g) + 5H 2O(g)

2.3. Fabrication of PVA/Li2ZrO3 Nanocomposite. PVA/ Li2ZrO3 nanocomposite samples were prepared by solution intercalation film casting method. Films were cast from Li2ZrO3 water suspension in which PVA was dissolved by heating at 95 °C for 3 h with constant stirring. A water bath was used to monitor the temperature of the PVA solution to prevent thermal decomposition of the polymer. The solid content of the solution was maintained at 7.5 wt %. The solution was ultrasonicated at 80−90 °C for 30 min. The solution was then poured into a clean glass mold and dried. The PVA nanocomposite films have been cast with various amounts; viz. 0.5, 1.0, and 2.0 wt % of lithium zirconate. The prepared films were free from air bubbles and with uniformly dispersed Li2ZrO3 particles in PVA matrix. The thickness of the transparent film samples ranges from 0.10 to 0.13 mm. 2.4. Techniques and Instrumentation. UV−vis measurements were done on a Hitaichi Spectrophotometer model U3210, and photoluminescence spectrum was obtained on Hitachi F-2500. PVA and PVA/Li2ZrO3 nanocomposites were subjected to microwave irradiation (MW) using a microwave oven (MS-194W AT 160 micro). FTIR spectra of the samples were measured in the spectral range of 4000−400 cm−1 using Perkin-Elmer Fourier transform-infrared spectrometer (FT-IR) with a resolution of 4 cm−1. Annealing of polymer compositeswas done in a hot air oven. The sample was taken between the glass plate and heated above 100 °C. It was kept at that temperature for 5 min to destroy the previous thermal history before cooling it down to room

Figure 1. Molecular representation of polyvinyl alcohol and lithium zirconate.

In this work, we report the preparation and the optical properties of new luminescent nanocomposites based on the dispersion of Li2ZrO3 nanoparticles within PVA polymer matrices. The influence of Li2ZrO3 nanoparticles addition on optical and structural characteristics of PVA film was analyzed by ultraviolet (UV) and Fourier transform infrared spectroscopy (FT-IR) techniques. The effect of annealing on structural and morphological characteristics of PVA/Li2ZrO3 composite was analyzed by X-ray diffraction analysis (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The dielectric properties of PVA/Li2ZrO3 nanocomposites such as dielectric constant (ε′), dielectric loss (ε″), electrical conductivity (σ), real and imaginary modulus (M′, M″) and dissipation factor (tan δ) were evaluated to analyze the effect of nanoparticles addition on the dielectric properties of the PVA host.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. The following materials were used for the preparation of lithium zirconate nanoparticles. Lithium nitrate (≥90%), zirconyl nitrate (≥99%), and 4772



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except the peak at 253 nm. Analyzed results are shown in Table 1 and it shows that the peak observed at 253 nm shows percent

temperature. After crystallization, it was again heated to the predetermined annealing temperature, 90, 120, and 150 °C, and annealed for 2 h; it was then brought to room temperature. Then the annealed samples were analyzed by DSC and XRD. XRD patterns recorded using Bruker diffractometer (Radiation, LAMBDA = 1.54 Å; θ scanning range 0−40°; Stepwise, 0.020; Voltage = 40.0 KV). Solution-cast nanocomposite films were analyzed by differential scanning calorimetry (DSC; DSC 2010, TA Instruments, New castle, DE) to determine the PVA crystal properties from ambient to 250 °C in a nitrogen atmosphere at the heating rate of 10 °C/min. Surface images were recorded by scanning electron microscopy (SEM, Model ESEM Quanta 200, FEI) at an operating voltage of 10.00 KV. For the electrical characterization of the composites, high-frequency LCR meter (Wayne KERR model: 6500P) is employed in the frequency range of 20 Hz to 10 MHz. The test is carried out at a constant voltage of 1 V and at room temperature.

Table 1. Influence of Li2ZrO3 Nanoparticles Addition on the Peak Intensity of UV Transmittance Spectra of PVA Matrix weight percentage of Li2ZrO3

peak intensity at 248 nm




0.0 0.5 1.0 2.0

32 32 40 41

100 100 100 100

67 85 74 88

52 48 39 40

transmittance and does not show any reduction in transmittance with an increase in the Li2ZrO3 nanoparticles concentration, But the transmittance peak intensity at 248 and 255 nm in fact increases with increase in nanoparticles content. However, the peak observed at 277 nm is decreased in its percentage of transmittance but is split into two peaks; the new peak appeared after splitting shows increase in intensity. This leads to the presumption that the new peak corresponds to the formation of charge transfer complexes between Li2ZrO3 nanoparticles and hydroxyl groups of PVA. These results clearly indicate the influence of Li2ZrO3 nanoparticles on the conjugated carbonyl groups of PVA chain. However, the transmittance spectra of PVA films does not show any peaks in the visible region (400−700 nm) (see Figure S1 in the Supporting Information). This reveals that the transparency was not affected due to the addition of nanoparticles, especially in the visible region. Indeed the UV−vis spectra of Li2ZrO3 nanoparticles alone show only very weak transmittance peak at 226 nm (see Figure S2 in the Supporting Information). In addition to the UV transmittance analysis, PVA/Li2ZrO3 nanocomposites films are subjected to UV absorbance analysis also. The UV absorbance spectra (Figure 3) show that PVA and PVA/Li2ZrO3 nanocomposites films are good UV absorbent material and its absorbance increases with nanofiller loading. The prominent absorbance peaks appearing at 263, 270, and 286 nm does not show either red shift or blue shift but shows an increase in absorbance intensity with the increase in nanoparticles concentration. The absorption pattern is relatively broad indicating uneven particle size distribution.

3. RESULTS AND DISCUSSION 3.1. Influence of Li2ZrO3 Nanoparticle Addition on Optical Transmittance, Absorbance, Reflectance and Emission Characteristics of PVA Matrix. Flexible polymer films containing nanoparticles have been considered as potential materials for applications in electronic and optoelectronic devices. Further, it is being used as a coating material in OSCs. In OSC applications, the incorporation of fillers should not affect the transparency of polymer films to avoid any reduction in light penetration. In this work, Li2ZrO3 nanoparticles are dispersed in PVA solution and cast into films in an environmental friendly process, i.e., in aqueous solution. The films have been released by subjecting the solutions to slow evaporation of water in the air. There is no agglomeration of nanoparticles because PVA is one of the well-known particle stabilizer in aqueous solutions.22 To analyze the influence of incorporation of Li2ZrO3 nanoparticles on the transparency and optical properties of PVA matrix, PVA films containing 0, 0.5, 1.0, and 2.0 wt % of Li2ZrO3 nanoparticles were subjected to UV−vis analysis in the transmittance as well as in absorbance mode. The UV spectra recorded in the transmittance mode are shown in Figure 2. In the transmittance spectra, there are four prominent peaks appearing at 248, 253, 255, and 277 nm, respectively in the UV (220−300 nm) region. As illustrated in the Figure 2, the transmittance intensity of the peaks at 248, 253, 255, and 277 nm varies almost linearly with nanoparticles concentration

Figure 3. UV absorbance spectra of PVA/Li2ZrO3 nanocomposite film samples containing 0, 0.5, 1.0, and 2.0 wt % of Li2ZrO3 nanoparticles from (a) 220−300 nm and (b) 260−300 nm.

Figure 2. UV transmittance spectra of PVA/Li2ZrO3 nanocomposite film samples containing 0, 0.5, 1.0, and 2.0 wt % of Li2ZrO3 nanoparticles. 4773



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The PVA/Li2ZrO3 nanocomposite films have high transparency in the UV−visible region before the microwave irradiation and retained its transparency after microwave irradiation also (see Figure S3 in the Supporting Information). The percentage of transmittance is recorded from 220 to 300 nm (only) is shown in Figure 4 and the intensity variation is presented in Table 2. The effect of MWI on neat PVA films is shown in Figure 4a, where neat PVA shows four prominent transmittance peaks at 248, 253, 257, and 277 nm. After MWI, there is not much variation at the peaks 248 and 253 nm. However, the peak at 257 nm shows reduction in transmittance intensity and the peak at 277 nm shows reduction in intensity as well as variation in peak width. After the incorporation of 0.5 wt % Li2ZrO3 nanoparticles but before the MWI, the peaks at 248, 253, 257, and 277 nm appear at the same position. However, after MWI the peak at 257 nm shows reduction in intensity; interestingly the peak at 277 nm splits into two peaks, one at 277 nm and another one at 279 nm. The PVA/Li2ZrO3 nanocomposite films containing 1% Li2ZrO3 nanoparticles shows entirely different results. The peak at 248 nm shifted to 242 nm (blue shift), the peak at 253 shifted to 254 nm (red shift), the peak at the 257 nm showed reduction in transmittance intensity, and the peak at 277 nm splits into two before MWI shows inward shift after MWI. For the composites containing 2 wt % nanoparticles, the peak at 248 nm shows reduction in intensity, the peak at 253 shows splitting and reduction in intensity, the peak at 257 shows reduction in intensity, and peak at 277 nm shows surprising increase in intensity. The effect of microwave irradiation on the PVA/Li2ZrO3 nanoparticles composites can lead to three possible mechanisms as given in Figure 5. These results indicate that the effect of microwave energy on the structure of PVA and PVA nanocomposites but it maintains the same 100% transmittance at 255 nm. However, it does not show any transmittance in the visible region due to microwave irradiation (see Figure S3 in the Supporting Information). The result of the fluorescence measurement performed at room temperature on PVA-capped Li2ZrO3 nanocomposite films at an excitation wavelength of 470 nm is shown in Figure 6. As seen in the figure, the curve for pure PVA means nonemission. The fluorescence spectrum of samples containing 0.5, 1.0, and 2.0 wt % Li2ZrO3 nanoparticles exhibited three emission peaks with peak maximum at 331, 346, and 362 nm, respectively. The emission peak observed in the nanocomposite samples could be assigned to the optical transmission of the first excitonic state of nanoparticles. The intensity of the band increases with an increase in the nanoparticles concentration. The enhanced emission could be due to possible chemical bonding between Li2ZrO3 nanoparticles surface and hydroxyl groups of PVA. This observation indicates that PVA/Li2ZrO3 nanostructured film has potential for applications similar to the semiconductor nanoparticles, such as photoanode in high-

However, absorption band is not affected in terms of its position thus indicating better distribution of the particles in PVA matrix. In comparison to pure PVA film, composite films showed a lower absorbance over the wavelengths ranging from 220 to 300 nm. The large peak around 270 nm is characteristic of pure PVA. The absorption band at 270 nm is assigned to π → π* transition that comes from unsaturated bonds. The absorbing groups cannot be unconjugated carbonyls and are probably of the type −(CC)n−CO−. Ultraviolet dichroism measurements tend to confirm this view. The absorption bands at 263, 270, and 286 nm have been assigned to −(CC)n− CO where n = 1, 2, and 3 respectively. The high intensities of these bands are additional evidence that the absorbing groups cannot be unconjugated carbonyl and are probably of the type −(CC)n, −CO−. This observation indicates that the PVA/Li2ZrO3 composite is useful for optical switch devices in UV and visible ranges. Optical switches are very attractive for optical communication devices. Previous studies showed that PVA undergoes degradation by exposure to heat and UV irradiation23−25 in order to analyze these sensitivities further in this work, the PVA and PVA/Li2ZrO3 nanocomposite films were exposed to microwave irradiation and the results were analyzed using UV transmittance spectra and shown in Figure 4.

Figure 4. UV transmittance spectra of PVA/Li2ZrO3 nanocomposite film samples before and after microwave irradiation containing 0, 0.5, 1.0, and 2.0 wt % of Li2ZrO3 nanoparticles from 220 to 300 nm.

Table 2. Influence of Microwave Irradiation on the Peak Intensity of the UV Transmittance Spectra of PVA/Li2ZrO3 Nanocomposite Films peak intensity at 248 nm




weight percentage of Li2ZrO3

before MWI

after MWI

before MWI

after MWI

before MWI

after MWI

before MWI

after MWI

0.0 0.5 1.0 2.0

32 32 41 42

32 32 20 27

100 100 100 100

100 100 100 95

84 85 75 89

60 82 38 82

42 47 40 41

41 34 20 60

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Figure 5. Effect of microwave irradiation on the molecular architecture of PVA/Li2ZrO3 nanoparticles.

recorded for (a) neat PVA and PVA containing (b) 0.5, (c) 1, and (d) 2 wt % Li2ZrO3 nanoparticles in transmittance mode and shown in Figure 7. Since PVA is a hydrolytic derivative of polyvinyl acetate, it shows IR peak characteristics of hydroxyl groups as well as acetate groups of unhydrolyzed polyvinyl acetate. All spectra exhibit the characteristic absorption bands of neat PVA. A broad, very strong band observed around 3549−3095 cm−1, together with the broad 642−604 cm−1 band, which arises from the O−H stretching frequency, indicates the presence of hydroxyl groups. Another strong band observed at 2930−2908 cm−1 indicates an asymmetric stretching mode of CH2 group. A peak at 1714−1723 cm−1 is due to the residual acetate groups present in the partially hydrolyzed PVA and 1423−1440 cm−1 is corresponding to a C−H bending of CH2 in PVA backbone. A strong band observed at 1427 cm−1 has been assigned to bending mode of vibration corresponding to CH2 group, while another strong band observed at 853 cm−1 has also been attributed to CH2 in stretching mode. A band at 1240−1260 cm−1 corresponds to −CH2 wagging. A band at 1092−1122 cm−1 is attributed to C−O−C stretching of the acetyl group present on PVA backbone.26,27 PVA, being a polyhydroxy polymer with one hydroxyl group with every alternate carbon, has a very wide and strong IR band at 3549−3091 cm−1 assigned to O−H stretching and, inter- and intramolecular

Figure 6. Fluorescence emission spectra of PVA/Li2ZrO3 nanocomposite film samples containing 0, 0.5, 1.0, and 2.0 wt % of Li2ZrO3 nanoparticles. Inset: Emission intensity at 331, 346, and 362 nm as a function of nanoparticles contents.

efficiency dye-sensitized solar cells, light-emitting diodes, lasers, field-emission devices, and chemical sensors. To further confirm and investigate the interaction between PVA matrix and Li2ZrO3 nanoparticles, FTIR spectra were 4775



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Figure 8. XRD pattern of PVA/Li2ZrO3 nanocomposite film samples containing (a) 0, (b) 0.5, (c) 1.0, and (d) 2.0 wt % of Li2ZrO3 nanoparticles at 90, 120, and 150 °C.

PVA shows a sharp diffraction peak at 2θ = 19.4° contributed by γ-crystalline phase of PVA. Two additional diffraction peaks, more exactly shoulders, appear in the XRD pattern of PVA after being annealed at 120 °C for 2 h. These two peaks, one at 2θ = 16.0° (α1) and another one at 2θ = 22.5° (α2), are the distinctive features of the α-crystalline phase of PVA. After being annealed at 150 °C, the diffraction peaks of the αcrystalline phase do not show any visible variation, while the γcrystalline phase is still dominant. All the figures show a peak at 2θ = ∼19.4° corresponding to (101) crystal plane for PVA, which indicates the semicrystalline nature of PVA. The crystalline nature of PVA results from the strong intermolecular interaction between PVA chains. The crystalline nature of synthesized nano Li2ZrO3 was observed by the various sharp crystalline peaks in the XRD pattern of nano Li2ZrO3 (see Figure S4 in the Supporting Information). It shows at least twelve distinct diffraction peaks at the 2θ value of 21.5, 23.5, 29.5, 30.5, 32, 34.5, 35, 36, 37, 40, 42.5, and 48.5° for the nanosized Li2ZrO3. Table 3 shows the effect of annealing and nanoparticles additions on the main crystalline peak 2θ = ∼19.4° of PVA. The peak intensity of 2θ = 19.4° increases with increase in annealing temperature and further the lithium zirconate nanoparticles addition reduces the intensity of the main peak; this makes it clear that the increase in annealing temperature increases the degree of crystallanity, whereas the nanoparticles addition decreases the degree of crystallanity. However, there is not much variation in the position of the main peak, which shows only marginal variation, it clearly shows there is no formation of new crystalline phases. Additionally the effects of annealing and lithium zirconate addition on crystallinity of the PVA were investigated by DSC measurements also. Figure 9 illustrates the DSC thermograms of the annealed PVA and its composites. The results obtained by the DSC were tabulated and shown in Table 4. Figure 9 and Table 4 reveals that the Tg, which appeared at 91 °C for PVA at annealing temperature 90 °C, increases with an increase in annealing temperature as well as increases in nanoparticles addition in general. The effect of annealing and

Figure 7. FTIR spectra of PVA/Li2ZrO3 nanocomposite film samples containing (a) 0, (b) 0.5, (c) 1.0, and (d) 2.0 wt % of Li2ZrO3 nanoparticles.

hydrogen bonds of −OH groups of PVA. The incorporated Li2ZrO3 particles with three terminal oxygen atoms can interact with PVA mainly through hydroxyl groups (see Figure 1). Since the incorporation of nanoparticles is only to the extent of 2 wt %, the shift or widening of the strong IR band at 3549−3091 cm−1 is only marginal. However, there is a considerable shift in the peak position of the band corresponding to CO stretch and C−O−C stretch vibrations of PVA indicating a positive interaction between −OH groups of PVA and nano Li2ZrO3. The hydroxyl groups of PVA have a very strong tendency to form charge-transfer-complex with Li2ZrO3 nanoparticles through chelation. 3.2. The Effect of Annealing on Structural and Morphological Characteristics of PVA Matrix. Annealing of polymers can be defined as a secondary process wherein the polymer is brought to a certain temperature, kept there for sometime, and then cooled to room temperature. The process of annealing of semicrystalline polymer was expected to favor the crystal packing, which in turn enhances the degree of crystallinity, orientation of phases, and their continuous structural morphology. Polymer crystallization behavior near an inorganic surface has been the focus of the extensive study.28−30 In most cases, the inorganic surface is shown to produce a nucleating or an epitaxial effect,31−33 which often stabilizes the bulk crystal phase, or in some cases promotes growth of a different crystal phase. X-ray diffractograms of the annealed PVA and its composites at different temperatures are shown in Figure 8. The X-ray pattern of the PVA sample showed a number of strong reflections which provide information about the crystal orientations in a three-dimensional array. The XRD pattern of 4776



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Table 3. Effect of Annealing and Nanoparticles Addition on the Main Crystalline Peak of PVA weight percentage of lithium zirconate

peak intensity at 2θ = 19.4° 90 °C

0.0 0.5 1.0 2.0

5068 4028 3923 2764

(19.4) (19.2) (19.3) (19.4)

120 °C 5611 4845 4168 4911

(19.3) (19.4) (19.2) (19.4)

150 °C 5849 4951 4209 4912

(19.2) (19.5) (19.1) (19.5)

Figure 9. DSC thermograms of PVA/Li2ZrO3 nanocomposite film samples containing (a) 0, (b) 0.5, (c) 1.0, and (d) 2.0 wt % of Li2ZrO3 nanoparticles at 90, 120, and 150 °C.

annealing temperature but decrease with nanoparticles addition. Assessment of the nanoparticles dispersion and morphology can be evaluated from the SEM images of the composites. The SEM image of pure Li2ZrO3 is shown in Figure 10. Figure 11 shows the SEM images of the PVA films with (a) 0, (b) 0.5, (c) 1.0, and (d) 2.0 wt % Li2ZrO3 nanoparticles. These images show the presence Li2ZrO3 nanoparticles in the PVA matrix. This nanocomposite film presents a low surface roughness to allow good adhesion between organic and inorganic layers in the solar sell applications. 3.3. Influence of Li2ZrO3 Nanoparticles Addition on Different Electrical Properties of PVA Matrix. The dielectric properties of polymeric films encapsulated with metal nanoparticles are now one of the main focus points of research because of their novel technological applications. It is well established that the polymers, as dielectric materials, are excellent host matrices and also provide environmental and chemical stability.34,35 Thus the obtained nanocomposites might exhibit improved electrical properties.36,37 Many reports in literature show attempts for synthesis of metal nanoparticles based polymer nanocomposites, with the possibility of variation in their electrical properties for their application in high

Table 4. Influence of Annealing and Nanoparticles Addition on the Tg, Tm, ΔH, and χc of PVA lithium zirconate (wt %) 0.0

0.5

1.0

2.0

Tg (°C) Tm (°C) ΔH (J/g) χc (%) Tg (°C) Tm (°C) ΔH (J/g) χc (%) Tg (°C) Tm (°C) ΔH (J/g) χc (%) Tg (°C) Tm (°C) ΔH (J/g) χc (%)

90 °C

120 °C

150 °C

91 187 59.79 18.90 96 188 47.03 16.92 100 188 39.50 16.81 91 187 25.90 16.68

105 186 60.80 21.44 105 185.0 59.10 20.74 105 187 55.54 18.95 97 185.4 36.82 18.29

93 187 109.74 29.96 102 186.4 99.56 26.93 95 185.1 96.34 26.85 100 186.4 84.9 23.85

nanoparticles addition to the melting temperature of PVA is quite marginal. The enthalpy of fusion (ΔH) as well as the percentage of crystallinity (χc) increase with an increase in 4777



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respectively, in each cycle of the electric field. The measured capacitance, C(ω) was used to calculate the dielectric constant, ε′(ω), using the following expression ε′(ω) =

Ct εoA

(2)

where t is sample thickness, εo is the permittivity of air (8.854 × 10−12 m), C is the capacitance, and A is the surface area of the sample. Whereas for dielectric loss, ε″(ω) is calculated as ε″(ω) = ε′(ω)tan δ(ω)

(3)

where, tan δ is tangent delta. Figure 12 shows the variation of dielectric constant with Li2ZrO3 nanoparticle concentration (Figure 12a) and frequency (Figure 12b).

Figure 10. SEM image of Li2ZrO3 nanoparticles.

performance capacitors, conductive inks, and other electronic components.38,39 PVA is currently being investigated as a potential polymeric gate insulator for organic thin-film transistors (OTFTs).40 Dielectric properties are a complex function of bulk permittivity, conductivity, size, shape, spatial arrangement of the constituents and the frequency.41 The dielectric parameter as a function of frequency is described by the complex permittivity in the form ε*(ω) = ε′(ω) − ε″(ω)

Figure 12. Dielectric constant of PVA/Li2ZrO3 nanocomposite film samples as a function of nanoparticles concentration and frequency.

(1)

where the real part ε′(ω) and imaginary part ε″(ω) are the components for the energy storage and energy loss,

Figure 11. SEM images of PVA/Li2ZrO3 nanocomposite film samples containing (a) 0, (b) 0.5, (c) 1.0, and (d) 2.0 wt % of Li2ZrO3 nanoparticles. 4778



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The dielectric constant (ε′) of the composites increased as the concentration of embedded Li2ZrO 3 nanoparticles increases, which directs toward the enhancement of the insulating capacity of PVA matrix. The enhancement in dielectric constant is attributed to the interfacial polarization effect, which is a phenomenon that appears in a heterogeneous system consisting of phases with different dielectric constants and conductivities, attributed to the accumulation of charges at the interfaces. The increase in dielectric constant with increasing nanoparticles content is attributed to the formation of clusters, which leads to the greater average polarization and thus a greater contribution to the dielectric constant. It is also observed that the dielectric constant decreases moderately with frequency. As the frequency increases, the charge carriers migrating through the dielectric get trapped against a defect site and induce an opposite charge in its vicinity, as a result of which they slow down and the value of dielectric constant decreases moderately.42 Similar behavior is also observed in a number of other polymers like PVP, PMMA, and so forth, verifying the fact that for polar materials, the value of ε′ is high for low-frequency range and begins to drop as frequency increases.43−45 Figure 13 shows the plot of dielectric loss as function of Li2ZrO3 nanoparticles concentration (Figure 13a) and frequency (Figure 13b).

Figure 14. The ac conductivity of PVA/Li2ZrO3 nanocomposite film samples as a function of nanoparticles concentration and frequency.

σac = d

Gs A

(4)

where d is the film thickness, A is the effective cross-sectional area and, Gs is measured conductance. The conductivity of the composite increases moderately with the composition of the filler; this may be due to the electronic interaction process taking place in the composites and therefore the resulted composites became more relatively conductive. In PVA, as the bond rotates with frequency, the existing flexible polar groups with polar bonds cause dielectric transition. Thus, there is a change in chemical composition of the polymer repeat unit due to the formation of charge transfer complexes within the PVA chains, which in turn makes the polymer chains more flexible and hence enhances AC electrical conductivity.48 Figures 15 and 16 show the calculated values of real and imaginary parts of electrical modulus for the nanocomposite films for different dopant concentration of Li2ZrO3 nanoparticles.

Figure 13. Dielectric loss of PVA/Li2ZrO3 nanocomposite film samples as a function of nanoparticles concentration and frequency.

With increase in nanoparticles concentration (Figure 13a), there is a very moderate increase in dielectric loss of PVA/ Li2ZrO3 composite films especially at lower frequency. The increase in dielectric loss with increasing nanoparticles content may be attributed to the interfacial polarization mechanism of the heterogeneous system. In contrast to the concentration effect, the variation of frequency has a profound effect on the dielectric loss, which decreases sharply with increase in frequency. The larger value of the loss factor or dielectric loss at low frequency could be due to the mobile charges within the polymer matrix. At high frequency, periodic field reversal is so fast that there is no excess ion diffusion in the direction of electric field.43,44 Polarization due to charge accumulation decreases, leading to the decrease in the value of loss factor. The alternating current (ac) conductivity of the composites as a function of nanoparticles concentration (Figure 14a) and frequency (Figure 14b) was studied and the obtained results are shown in Figure 14. σac is the ac conductivity of the sample that arises from the motion of charge carriers through the polymer and is measured using the equation46,47

Figure 15. Electrical modulus (M′) of PVA/Li2ZrO3 nanocomposite film samples as a function of nanoparticles concentration and frequency.

The spectrum of M″ shows an asymmetric peak approximately centered in the dispersion region at low frequency. The peak shifts to a higher frequency with increase in nanoparticles concentration. The loss tangent, tan δ, was determined from the values of dielectric constant (ε′) and dielectric loss (ε″) using the equation

ε″ = ε′tan δ

(5)

where δ is the phase angle between the electric field and the polarization of dielectric. The dissipation factor, tan δ, recorded as a function of Li2ZrO3 nanofiller loading, is shown in Figure 17. 4779



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reduces the percentage of crystallanity. The dielectric constant and dielectric loss of PVA/Li2ZrO3 nanocomposites increases with the increase in nanoparticles concentration, while it decreases with an increase in frequency. The ac conductivity increases with the increase in filler loading and frequency. The dissipation factor increases with nanoparticles addition and decreases with frequency. The present investigation has only focused on the exploring the influence of Li2ZrO3 nanoparticle on transmission, absorption, emission, structural, and dielectric characteristics of PVA matrices; other factors such as mechanical and barrier properties may also be used and explored to find its suitability in organic solar cell applications.

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Figure 16. Electrical modulus (M″) of PVA/Li2ZrO3 nanocomposite film samples as a function of nanoparticles concentration and frequency.

ASSOCIATED CONTENT

S Supporting Information *

UV transmittance spectra of PVA films containing lithium zirconate nanoparticles (Figure S1), UV transmittance spectra of lithium zirconate nanoparticles (Figure S2), UV transmittance spectra of PVA films containing 1% lithium zirconate nanoparticles before and after microwave irradiation (Figure S3), and X-ray diffraction pattern of neat lithium zirconate nanoparticles (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: 091-821-2548285. Fax: 091-821-2548290. E-mail: [email protected].

Figure 17. The tan δ of PVA/Li2ZrO3 nanocomposite film samples as a function of nanoparticles concentration and frequency.

Notes

The authors declare no competing financial interest.

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The loss tangent for all nanocomposites increased with increase in filler loading. This is expected because conductivity increases with an increase in weight percentage of Li2ZrO3 nano filler.49 The increase in loss tangent with increasing filler contents may be attributed to the interfacial polarization mechanism of the heterogeneous system. The number of nanoparticles causes an increase in electrical conductivity with filler loading that in turn influences the tan δ behavior. As the number of particles is greater with increasing nanofiller loading, the interparticle distances are less. With this effect, electrical conduction in the fraction of the first nanolayer region is enhanced and electrical conduction in the fraction of the second nanolayer is lower due to the formation of overlapping regions. Therefore, the electrical conduction taking place at the close proximity of the interface regions due to the availability of free charge carriers contribution from the second layer is less. Therefore, there might be a reduction in the possibility of defects and charge trapping sites at the interface region.

REFERENCES

(1) Zhao, H.; Douglas, E. P. Preparation of Carona-Embedded CdS Nanoparticles. Chem. Mater. 2002, 14, 1418−1423. (2) Kim, J. Y.; Kim, H. M.; Shin, D. H.; Ihn, K. J. Synthesis of CdS Nanoparticles Dispersed Within Poly(urethane acrylate-co-styrene) Films Using an Amphiphilic Urethane Acrylate Nonionomer. Macromol. Chem. Phys. 2006, 207, 925−932. (3) Pardhan, B.; Sharma, A. K.; Ray, A. K. Conduction Studies on Chemical Bath Deposited Nanocrystalline CdS Thin Films. J. Cryst. Growth 2007, 304, 388−392. (4) Pattabi, M.; Amma, B. S.; Manzoor, K.; Sanjeev, G. Effect of 8 MeV Electron Irradiation on the Optical Properties of PVP Capped CdS Nanoparticles in PVA Matrix. Sol. Eng. Mater. Sol. Cell. 2007, 91, 403−1407. (5) Wang, H.; Fang, P.; Chen, Z.; Wang, S. Synthesis and Characterization of CdS/PVA Nanocomposite Flms. Appl. Surf. Sci. 2007, 253, 8495−8499. (6) Dan, A.; Satpati, B.; Satyam, P. V.; Chakravortya, D. Diodelike Behavior in Glass−Metal Nanocomposites. J. Appl. Phys. 2003, 93, 4794−4800. (7) Gautam, A.; Ram, S. Preparation and Thermochemical Properties of Ag-PVA Nanocomposite Films. Mater. Chem. Phys. 2010, 119, 266− 271. (8) Luque, R.; Balu, A. M.; Campelo, J. M.; Arellano, C. G.; Gracia, M. J.; Luna, D.; Marinas, J. M.; Romero, A. A. Tunable Shapes in Supported Metal Nanoparticles: From Nanoflowers to Nanocubes. Mater. Chem. Phys. 2009, 117, 408−413. (9) Kamat, P. V. Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles. J. Phys. Chem. B 2002, 106, 7729− 7744. (10) Mazaleyrat, F.; Varga, L. K. Ferromagnetic Nanocomposites. J. Magn. Magn. Mater. 2000, 215, 253−259. (11) Sizovaa, E.; Glushchenkob, N.; Miroshnikova, S.; Skalny, A. Influence of Cu10x Copper Particles Intramuscular Injection on

4. CONCLUSIONS UV−vis analysis shows that the addition of Li2ZrO3 nanoparticles does not affect the absorbance or transparency in the visible region even though it had a positive interaction in the UV region. Similarly microwave irradiation also does not show any transmittance peaks in the visible region even though it had a positive effect in the UV region. PVA films show fluorescent emission for the addition on Li2ZrO3 nanoparticles and the emission intensity increases with an increase in the concentration of nanoparticles. XRD and DSC analysis shows that increase in annealing temperature increases the degree of crystallanity, whereas the incorporation of nanoparticles 4780



Article

Mineral Composition of Rat Spleen. J. Trace Elem. Med. Biol. 2011, 25S, S84−89. (12) Sun, Y.; Xia, Y. Large-Scale Synthesis of Uniform Silver Nanowires Through a Soft, Self-Seeding, Polyol Process. Adv. Mater. 2002, 14, 833−837. (13) Mbhele, Z. M.; Sakmane, M. G.; Van Sittert, C. G. C. E.; Nedeljkovic, J. M.; Djokovic, V.; Luyt, A. S. Fabrication and Characterization of Silver-Polyvinyl Alcohol Nanocomposites. Chem. Mater. 2003, 15, 5019−5024. (14) Zeng, R.; Rong, M. Z.; Zhang, M. Q.; Liang, H. C.; Zeng, H. M. Laser Ablation of the Polymer-Based Silver Nanocomposites (Academic). Appl. Surf. Sci. 2002, 187, 239−247. (15) Liu, H.; Ge, X.; Zhu, Y.; Xu, X.; Zhang, Z.; Zhan, M. Synthesis and Characterization of Polyacrylamide−Nickel Amorphous Nanocomposites by γ-Irradiation. Mater. Lett. 2000, 46, 205−208. (16) Firth, A. V.; Haggata, S. W.; Khanna, P. K.; Williams, S. J.; Allen, J. W.; Magennis, S. W.; Samuel, I. D. W.; Cole-Hamilton, D. J. Production and Luminescent Properties of CdSe and CdS Nanoparticle-Polymer Composites. J. Lumin. 2004, 109, 163−172. (17) Rivaton, A.; Chambon, S.; Manceau, M.; Gardette, J. L.; Lemaître, N.; Guillerez, S . Light-Induced Degradation of the Active Layer of Polymer-Based Solar Cells. Polym. Degrad. Stab. 2010, 95, 278−284. (18) Jayasekara, R.; Harding, I.; Bowater, I.; Christie, G. B. Y.; Lonergan, G. T. Preparation, Surface Modification and Characterisation of Solution Cast Starch PVA Blended Films. Polym. Test. 2004, 23, 17−27. (19) Kokabi, M .; Sirousazar, M.; Hassan, Z. M. PVA-Clay Nanocomposite Hydrogels for Wound Dressing. Eur. Polym. J. 2007, 43, 773−781. (20) Kashinath, C.; Patil, Aruna, S. T.; Tanu, M. Combustion Synthesis: an Update. Curr. Opin. Solid State Mater. Sci. 2002, 6, 507− 512. (21) Singanahally, T.; Aruna, Alexander, S.; Mukasyan. Combustion Synthesis and Nanomaterials. Curr. Opin. Solid State Mater. Sci. 2008, 12, 44−50. (22) Longenberger, L.; Mills, G. Formation of Metal Particles in Aqueous Solutions by Reactions of Metal Complexes with Polymers. J. Phys. Chem. 1995, 99, 475−478. (23) Celina, M.; Clough, R. L.; Jones, G. D. Initiation of Polymer Degradation via Transfer of Infectious Species. Polym. Degrad. Stab. 2006, 91, 1036−1044. (24) Ramani, R.; Ranganathaiah, C. Degradation of AcrylonitrileButadiene-Styrene and Polycarbonate by UV Irradiation. Polym. Degrad. Stab. 2000, 69, 347−354. (25) Fernandes, D. M.; Winkler Hechenleitner, A. A.; Job, A. E.; Radovanocic, E.; Gómez Pineda, E. A. Thermal and Photochemical Stability of Poly(vinyl alcohol)/Modified Lignin Blends. Polym. Degrad. Stab. 2006, 91, 1192−1201. (26) Soliman Selim, M.; Seoudi, R.; Shabaka, A. A. Polymer Based Films Embedded with High Content of ZnSe Nanoparticles. Mater. Lett. 2005, 59, 2650−2654. (27) Shin, E. J.; Lee, Y. H.; Choi, S. C. Study on the Structure and Processibility of the Iodinated Poly(vinyl alcohol). 1. Thermal Analyses of Iodinated Poly(vinyl alcohol) Films. J. Appl. Polym. Sci. 2004, 91, 2407−2415. (28) Mucha, M.; Marszalek; Fidrych, J. Crystallization of Isotactic Polypropylene Containing Carbon Black as a Filler. Polymer. 2000, 41, 4137−4142. (29) Stocker, W.; Schumacher, M.; Graff, S. Epitaxial Crystallization and AFM Investigation of a Frustrated Polymer Structure: Isotactic Poly(propylene),β phase. Macromolecules 1998, 31, 807−814. (30) Doye, J. P. K.; Frenkel, D. Crystallization of a Polymer on a Surface. J. Chem. Phys. 1998, 109, 10033−10041. (31) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T. T.; Kamigaito, O. Synthesis and Mechanical Properties of Nylon-6/Clay Hybrid. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 983−986.

(32) Janigova, I.; Chodak, I. Temperature Effect on Kinetics of Isothermal Crystallization of Crosslinked Filled LDPE-2. Particulate Silica with High Surface Area as a Filler. Eur. Polym. J. 1995, 31, 271− 274. (33) Alonso, M.; Velasco, J. I.; de Saja, J. A. Constrained Crystallization and Activity of Filler in Surface Modified Talc Polypropylene Composites. Mater. Chem. Phys. 2007, 104, 367−372. (34) Singh, N.; Khanna, P. K. In Situ Synthesis of Silver Nanoparticles in Polymethyl Methacrylate. Mater. Chem. Phys. 2007, 104, 367−372. (35) Khanna, P. K.; Gokhale, R.; Subbarao, V. V. V. S.; Vishwanath, A. K.; Das, B. K.; Satyanarayana, C. V. V. PVA Stabilized Gold Nanoparticles by use of Unexplored Albeit Convetional Reducing Agents. Mater. Chem. Phys. 2005, 92, 229−233. (36) Vodnik, V. V.; Vukuvic, J. V.; Nedeljkovic, J. M. Synthesis and Characterization of SilverPoly(methylmethacrylate) Nanocomposites. Colloid Polym. Sci. 2009, 287, 847−851. (37) Gautam, A.; Ram, S. Synthesis, Mechanical and I −V Characteristics of Ag−PVA Nanocomposite Films. Phys. Status Solidi (a) 2009, 206, 1471−1477. (38) Monti, O. L. A.; Fourkas, J. T.; Nesbitt, D. J. Diffraction-Limited Photogeneration and Characterization of Silver Nanoparticles. J. Phys. Chem. B 2004, 108, 1604−1612. (39) Kelly, K. L.; do Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668− 677. (40) Park, S. J.; Sung, J. H.; Park, H. J.; Choi, H. J.; Choi, J. S. Characteristics of Poly(vinyl acetate)/Organoclay as a Gate Insulating Material in Organic Thin Film Transistor. Curr. Appl. Phys. 2006, 6, 636−639. (41) Brosseau, C.; Beroual, A.; Boudida, A. How Shape Anisotropy and Spatial Orientation of the Constituents Affect the Permittivity of Dielectric Heterostructures. J. Appl. Phys. 2000, 88, 7278−7288. (42) Qureshi, A.; Mergen, A.; Mehmet, S. E.; Gulluoglu, A.; Singh, N. L. Dielectric Properties of Polymer Composites Filled with Different Metals. J. Macromol. Sci., Part A: Pure Appl. Chem. 2008, 45, 462−469. (43) Bhargav, P. B.; Sarada, B. A.; Sharma, A. K.; Rao, V. V. R. N. Electrical Conduction and Dielectric Relaxation Phenomena of PVA Based Polymer Electrolyte Films. J. Macromol. Sci., Part A: Pure Appl. Chem. 2010, 47, 131−137. (44) Mardare, D.; Rusu, G. I. Comparison of the Dielectric Properties for Doped and Undoped TiO2 Thin Films. J. Optoelectron. Adv. Mater. 2004, 6, 333−336. (45) Fuyuki, T.; Matsunmi, H. Electronic Properties of the Interface Between Si and TiO2 Deposited at very Low Temperatures. Jpn. J. Appl. Phys. 1986, 25, 1288−1291. (46) Dyre, j.C.; Schrøder, T B. Universality of AC Conduction in Disordered Solids. Rev. Mod. Phys. 2000, 72, 873−892. (47) Harun, M. H.; Saion, E.; Kassim, A.; Hussain, M. Y.; Mustafa, I. S.; Omer, M. A. A. Temperature Dependence of AC Electrical Conductivity of PVA-PPy-FeCl3 Composite Polymer Films. Malaysian Polym J. 2008, l3, 24−31. (48) Bhajanti, R. F.; Ravindrachary, V.; Harisha, A.; Ranganathaiah, G.; Kumaraswamy, G. N. Effect of Barium Chloride Doping on Microstructure: Position Annihilation Study. Appl. Phys. A 2007, 87, 797−805. (49) Ravikiran, Y. T.; Lagare, M. T.; Sairam, M.; Mallikajuna, N. N.; Sreedhar, B.; Manohar, S.; MacDiarmid, A. G.; Aminabhavi, T. M. Synthesis, Characterization and Low Frequency AC Conduction of Polyaniline/Niobium Pentoxide Composites. Synth. Met. 2006, 156, 1139−1147.

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