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Polymer Degradation and Stability 98 (2013) 1862e1868

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Polymer Degradation and Stability

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Thermal and photochemical effects on the structure, morphology,thermal and optical properties of PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite films

D.M. Fernandes a,*, J.L. Andrade a, M.K. Lima a, M.F. Silva a, L.H.C. Andrade b, S.M. Lima b,A.A. Winkler Hechenleitner a, E.A. Gómez Pineda a

aUniversidade Estadual de Maringá, Departamento de Química, Av. Colombo 5790, 87020-900, Zona 07, Maringá, PR, BrazilbUniversidade Estadual de Mato Grosso do Sul, Programa de Pós-Graduação em Recursos Naturais, CP 351, 79804-970 Dourados, MS, Brazil

a r t i c l e i n f o

Article history:Received 9 July 2012Received in revised form18 April 2013Accepted 3 May 2013Available online 17 May 2013

Keywords:Optical propertiesNi0.04Zn0.96OFe0.03Zn0.97OPVANanocomposite films

* Corresponding author. Tel.: þ55 44 3011 5185; faE-mail addresses: danidqi@hotmail.com.

(D.M. Fernandes).

0141-3910/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2013.05.0

a b s t r a c t

Ni0.04Zn0.96O and Fe0.03Zn0.97O with average diameter of 23 and 19 nm, respectively, have been syn-thesized by a modified solegel method to be used in the preparation of (100 � x)/x poly(vinyl alcohol)/oxide nanocomposite films, with x ¼ 0, 1, 3 and 5 (in wt.%). A 125 W-Hg vapor lamp with emission above254 nmwas used to irradiate PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O films. The effect on their structural,thermal, morphological and optical properties was studied by TG, DSC, DRX, AFM, UVevis and PLspectrophotometry. The Ni0.04Zn0.96O addition on PVA films decreases the thermal stability of thepolymer in inert and in oxidative atmosphere. In contrast, the Fe0.03Zn0.97O presence in the PVA filmsseems to increase the thermal stability of the polymer. The characteristic peak of the crystalline phase ofPVA and wurtzite phase of the zinc oxide were identified through X-ray diffraction in both films. Thecrystallinity of the PVA film increases with UV irradiation and with the presence of Ni0.04Zn0.96O andFe0.03Zn0.97O. The roughness of the PVA film was not modified by the addition of the doped oxides;however, it increases after UV irradiation, more significantly in the films containing the oxides. The PVAfilm exhibits absorption around 280 nm characteristic of pep* transitions related to carbonyl groupsfrom residuals acetate, while the 95/05 PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O nanocompositefilms show absorption at the visible region which is characteristics of the band gap reduction of thedoped oxides. The photoluminescence of PVA was modified by the presence of the oxides in the film.These nanocomposite films are interesting due to their thermal, mechanical (flexible) properties and lowcost of production. In addition they are also able to exhibit peculiar optical properties showing potentialto be used in photonic devices, gas sensors and organic solar cell applications.

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1. Introduction

In the last fifteen years many properties of nanostructuredmaterials have been studied due to their applicability in manytechnological areas [1]. The experimental conditions used for thepreparation of these materials play an important role in the prop-erties of the product. For this reason, a great variety of experimentalmethods have been used in the production of nanoparticles, such assolegel [2,3], and techniques that use liquid ammonia as solvent[4]. Among all these techniques, solegel is the simplest and themost utilized [5e7].

x: þ55 44 32614125.br, dmfoliveira2@uem.br

All rights reserved.03

It is well known that metal oxide nanocomposite materialspresent a variety of interesting magnetic, electric, and catalyticproperties [8,9]. In particular Fe2O3, NiO and ZnO are fundamentalmaterials for chemical heat pump [10], or as substrate materials forthe epitaxial growth of thin films with desirable magnetic orelectronic properties [11]. The great advantage of the syntheses ofthese oxide doped polymers is the combination of its properties forcatalysis, photocatalysis, sensors, and optoelectronic devices [12].However, many methods have been suggested for the synthesis ofthis kind of material with the disadvantage of the use of largeamounts of organic solvents [13,14]. In order to contour thisinconvenience, in our previous works were reported the synthesesof mixed Zn and Cu oxide [15] and mixed Ni0.04Zn0.96O andFe0.03Zn0.97O oxide nanoparticles [16] using a relatively simplemodified solegel method which uses only water as solvent.

D.M. Fernandes et al. / Polymer Degradation and Stability 98 (2013) 1862e1868 1863

Poly(vinyl alcohol), also known as PVA, is a polymeric materialwhich is soluble in water, has a high dielectric strength, a goodcharge storage capacity and interesting optical properties. It hascarbon chain backbonewith hydroxyl groups attached that can be asource of hydrogen bonding which assist the formation of polymercomposite [17]. This polymer is a good candidate for incorporationinto multilayer coatings of organic solar cells due to its hightransparency and ability to form a barrier to oxygen [18].

Several approaches have been employed to prepare nano-particles/polymer composites [19e21]. Zinc oxide (ZnO) is a wideband-gap semiconductor with unique physical properties for po-tential applications like sensors, transparent coating for solar cellsand acoustic devices. ZnO has also recently attract considerableinterest for efficient ultraviolet light-emitting diodes (LEDs) andlaser devices. Indeed, it is an excellent candidate for high-temperature and high-efficiency optoelectronic applications dueto its large excitation binding energy (60 meV), that is the largestvalue found among the conventional semiconductors. Whennanostructured ZnO is incorporated into polymers, it can improvetheir mechanical and optical properties due to strong interfacialinteractions between the inorganic nanoparticles and the organicgroups [22], also conferring better photochemical properties onsome polymers, such as polypropylene [23,24], polyurethane [24],among others. It is possible because nanoparticles of ZnO and someother metal oxides absorb UV radiation.

It is also important to consider the photocatalytic activity of theZnO nanoparticles with visible light. This activity of polianiline/ZnOnanocomposite was investigated by the degradation of methyleneblue (MB) and malachite green (MG) dyes in aqueous mediumunder natural sunlight and UV light irradiation [25]. The resultsindicated that the addition of the ZnO nanoparticles to the PANIhomopolymer enhances the photocatalytic efficiency under naturalsunlight irradiation for the degradation of both dyes. Anotherexample is the photocatalytic degradation of methylene blue (MB)and methyl orange (MO) in the ZnO/CuO nanocomposites presenceunder visible light irradiation. The results showed that the coupledZnO/CuO semiconductor possesses higher photocatalytic activitytoward MB and MO degradation than ZnO under visible light. Thecoupling of ZnO/CuO reduces the band gap, extending the wave-length range to visible light region leading to electronehole pairseparation under visible light irradiation and consequently,achieving a higher photocatalytic activity [26]. Numerous recentstudies have reported that ZnO can be improved as photocatalyst byvarious techniques such as doping with non-metals [27], additionof transition metals [28] as well as use of coupled semiconductors[29,30].

Nanostructured NiO is a p-type semiconductor metal oxidehaving a stable wide band gap, with potential to be used as atransparent p-type semiconductor layer [31]. Furthermore, it isutilized for applications in smart windows, electrochemical super-capacitors [31,32] and electro-optical devices [33]. However, thefunctional properties of NiO and its applicability depend signifi-cantly on the pore morphology, pore matrix-interface, and also onits porosity. In addition, an exceptionally important material is theg-Fe2O3 (maghemite) nanoparticles because of their magnetic andoptical properties [34].

The research on the preparation of nanostructured mixed oxideZnOeNiO and ZnOeFe2O3 in one phase is of great importance. Thecombination of n-type ZnO with a p-type NiO has been extensivelystudied expecting numerous heterojunctions, opening the possi-bility of sensitive gas sensors, photocatalysts and semiconductingmaterials [35]. ZnOeFe2O3 mixed nanoparticles are receiving anincreasing attention due to magnetic and luminescent propertiesexhibiting potential applications in the medical area, includingmagnetic separation and detection of cancer cells, bacteria and

viruses [36]. In addition, organic/inorganic nanocomposites areextremely promising for applications in LEDs, photodiodes,photovoltaic cells, smartmicroelectronic device, photocatalysts andgas sensors among others applications as biomedical systems [37].

In a recent work, our research group reported the thermal,photochemical, morphological behavior and the optical propertiesof PVA/ZnO nanocomposite films. It was possible to verify that thepresence of ZnO nanoparticles in the PVA film causes significantchanges in the optical properties of the polymer [38]. The aim of thepresent work was to extend the comparison to a PVA/nickelezincand PVA/ironezinc nanocomposite systems containing 1, 3 and5 wt.% of each dopping oxide.

2. Experimental

2.1. Reagents and chemicals

Poly(vinyl alcohol) 98e99% hydrolyzed, MW 13,000e23,000was purchased from Aldrich. Zn(NO3)2$6H2O, PA 96e100% waspurchased from Synth; Ni(NO3)2$6H2O, purity of 97e100%, waspurchased from Lafan, and Fe(NO3)3$9H2O, purity of 98e100%, waspurchased from Vetec. Dimethyl sulfoxide (DMSO) PA from Nuclearwas used as solvent to prepare the films. Analytical grade reagentswere used without further purification.

2.2. Ni0.04Zn0.96O and Fe0.03Zn0.97O nanoparticles preparation

The synthesis and characterization of Ni0.04Zn0.96O andFe0.03Zn0.97O nanoparticles was described in our previous work[16]. The average sizes of the obtained Ni0.04Zn0.96O andFe0.03Zn0.97O nanocrystallites were evaluated by X-ray line broad-ening using Scherrer’s equation.

2.3. PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite filmspreparation

PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite filmswere prepared by dispersing Ni0.04Zn0.96O and Fe0.03Zn0.97O in10 mL of PVA solution in DMSO (20 g/L) at room temperature usinga magnetic stirrer. The solvent was then evaporated at controlledtemperature in 313e315 K. The samples were kept in desiccator.The thickness of the films was 30e40 mm.

2.4. PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite filmsirradiation (l � 254 nm)

The PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocompositefilms containing 1, 3 and 5 wt.% of the respective oxides wereirradiated during 96 h with a 125-W Hg vapor lamp (l � 254 nm)with fluence rate of 16 J/(m2 s) at 0.15 m from the sample measuredat the film surface. The photodegradation experiments were per-formed in a rectangular dark box photoreactor. During this irradi-ation, the local temperature did not exceed 315 K.

2.5. Analyses of PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97Onanocomposite films

2.5.1. Thermogravimetric analysis (TGA)Thermogravimetric measurements were performed on a Shi-

madzu TGA 50 Instrument, operating in dynamic mode undereither flowing nitrogen or synthetic air atmosphere (50 mL/min) ata heating rate of 10 K/min. Samples weighing 6 mg were placed inplatinum pans. All the measures were performed in duplicate toestimate the average deviations that was about 5%.

D.M. Fernandes et al. / Polymer Degradation and Stability 98 (2013) 1862e18681864

2.5.2. Differential scanning calorimetry (DSC)DSC was performed in a Shimadzu DSC-50 under flowing ni-

trogen (50 mL/min) atmosphere at 10 K/min. Samples weighing6 mg were placed in aluminum pans. All the analytical measureswere performed in duplicate. The melting enthalpy (DHm) of PVA inPVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O filmswere computed fromthe peak area of the melting peak in the DSC curves.

2.5.3. X-ray diffraction (DRX)The crystalline phases were identified using a Shimadzu XRT

6000 X-ray diffractometer. The diffractogramswere obtained in thefollowing conditions: 2q ¼ 10e70�, 40 kV and 30 mA, Cu Ka radi-ation. All the analytical measures were performed in duplicate.

2.5.4. Atomic force microscopy (AFM)The film surfaces were analyzed using a Shimadzu SPM-9500 J3

in dynamic mode. Medium roughness in nm units of the films wereevaluated.

2.5.5. UVevis spectrophotometryUVevis spectrophotometry was carried out using a Perkin Elmer

Lambda 1050 spectrophotometer. Absorbance spectra were deter-mined in the 200e800 nm wavelength range, slit width of 2.0 nmand scan rate of 266,75 nm/min, using the integrating sphereaccessory.

2.5.6. Photoluminescence (PL) spectroscopyPhotoluminescence spectra were obtained with a fluorescence

spectrophotometerdCary Eclipse (Varian) using emission andexcitation slit width of 5 nm, scan rate of 120 nm/min, a data in-terval of 1 nm (average time 0.5 s), PMT voltage of 600 V, andlex ¼ 325 nm excitation wavelength. For the time resolved pho-toluminescence study, an optical parametric oscillator (OPO)coupled to the Nd3þ-YAG laser was used to excite the sample at325 nm in a frequency of 10 Hz. The sample emission was analyzedby an Oriel f-125 monochromator with a grating of 400 grooves/mm and detected by an ICCD Instaspec V ICCD detector and cali-brated with an Hg lamp. All the measures were performed with arepeat at least.

Fig. 1. TG curves of PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite films obta

3. Results and discussion

TG curves for PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nano-composite films obtained in N2 atmosphere are shown in Fig. 1(a)and (b), respectively. It is possible to observe that in the maindecomposition stage (at approximately 470e570 K) the films with1e5 wt.% of Ni0.04Zn0.96O showed lower thermal stability than purePVA, following the behavior observed with PVA/ZnO films [38].PVA/Fe0.03Zn0.97O films exhibit thermal stability relatively similarto the pure PVA in this main decomposition stage. However, insynthetic air atmosphere, the thermal stability of pure PVA is notaffected by the presence of Ni0.04Zn0.96O (Fig. 1(c)) in the degra-dation main step, while the nanostructured Fe0.03Zn0.97O increasesthe thermal stability of PVA films (Fig. 1(d)). In both atmospheres,PVA film does not show residual mass at 1000 K (not shown), whilethe nanocomposite films displayed residual mass corresponding tothe presence of the Ni0.04Zn0.96O and Fe0.03Zn0.97O nanostructuredoxides. These results suggest that Fe0.03Zn0.97O promotes a thermo-protector effect more intense than Ni0.04Zn0.96O in the PVA film,mainly in oxidative atmosphere. This effect is possibly related toelectronic structure of the oxides and specific interactions at theoxide-polymericmatrix interface. The difference behavior observedin N2 and O2 atmosphere reveals that the thermal decompositionreaction of the PVA and nanocomposite films is strongly dependenton the surrounding gas [39]. It is important to note that TG mea-sures were performed in duplicate and the average deviations wereabout 5%.

Fig. 2 presents the melting enthalpy (DHm) of PVA in PVA/Ni0.04Zn0.96O (a) and PVA/Fe0.03Zn0.97O films (b), computed fromthe peak area of the melting peak in the DSC curves. After 96 hunder UV irradiation, the melting enthalpy for pure PVA film doesnot change significantly. Both nanocomposites films show DHmalterations when the oxide content varies, which can be associatedwith nanoparticles agglomeration in some regions of the PVA film.This fact can contribute for crystallinity increase or decreasedepending on how the polymeric chains are located in the micro-domains in contact with the oxide nanoparticles. For PVA/Ni0.04Zn0.96O nanocomposite films there are ratios in which DHmdoes not changes due to the irradiation and in others it shows

ined at 10 K/min under N2 (a) and (b) and synthetic air (c) and (d) atmospheres.

Fig. 2. Melting enthalpy (DHm) in J/Kg of PVA unit of non-irradiated (0 h) and UVirradiated (96 h) PVA/Ni0.04Zn0.96O (a) and PVA/Fe0.03Zn0.97O (b) nanocomposite films.

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greater values. A similar effect is observed with PVA/Fe0.03Zn0.97Ofilms, however in this case the irradiation tends to decrease theDHm values in some PVA/oxide ratios.

The X-ray diffraction patterns of non-irradiated PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite films are pre-sented in Fig. 3(a) and (b), respectively. The PVA film diffractogram

Fig. 3. XRD patterns of PVA/Ni0.04Zn0.96O (a) and PVA/Fe0.03Zn0.97O (b) nanocompositefilms.

exhibits peaks at 2q ¼ 19.8�, 22.9�, and 40.6� (marked with *),which are characteristics of the PVA crystalline phase [40]. ThePVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O diffractograms showpeaks located at 2q ¼ 31.8�, 34.5�, 36.3�, 47.6�, 56.7�, 62.9�, and68.1� (marked with #), which correspond to the reflections plane ofthe ZnO hexagonal structure [15]. The peaks attributed to ZnO aremore evident in nanocomposite films containing 5 wt.% of thedoped oxide. Because of the relatively small amount of Ni and Fe inthe mixed oxides, the XRD diffractogram of PVA/doped oxidesshows only peaks characteristics of ZnO wurtzite single phase.

PVA and 99/01, 97/03, and 95/05 PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O films crystallinity were analyzed by obtaining the fullwidth at half maximum (FWHM) of the PVA diffraction peak in2q ¼ 19.8�. This parameter for PVA/Ni0.04Zn0.96O (Fig. 4(a)) andPVA/Fe0.03Zn0.97O (Fig. 4(b)) nanocomposite films non-irradiateddecreases due to the incorporation of the respective oxides intothe polymer, indicating that these films are more crystalline thanthe corresponding of the pure PVA. This suggests that Ni0.04Zn0.96Oand Fe0.03Zn0.97O nanostructured oxides induce an increase in PVAcrystallinity due probably to intermolecular interactions betweenhydroxyl groups of the polymer and nanostructured oxides. In aprevious work [38], an increase in crystallinity of PVA was alsoobserved due to incorporation of nanostructured ZnO in PVA films.Irradiated PVA film showed a significant decrease in the FWHMcompared to the value for non-irradiated PVA film, indicating thatthe pure PVA crystallinity increases after UV irradiation. On theother hand, there is a tendency of the peak width to increase withUV irradiation for both nanocomposite films, indicating a decreasein the PVA crystallinity during UV irradiation. Crystallinity datafromDSC and DRX analysis show similar tendencies in some extent,they do not agree totally as was also observed in our previous study[38]. This fact was attributed to the different experimental condi-tions (mainly of temperature) in these analyses.

Fig. 4. Full width at half maximum (FWHM) data of the diffraction peak in 2q ¼ 19.8�

for pure PVA, irradiated and non-irradiated PVA/Ni0.04Zn0.96O (a) and PVA/Fe0.03Zn0.97O (b) nanocomposite films.

Table 1Surface roughness data of the non-irradiated (0 h) and irradiated for 96 hours (96 h)PVA, 95/05 PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O films.

Sample Irradiation time(h)

Roughness (�2)(mm)

PVA 0 796 10

95/05 PVA/Ni0.04Zn0.96O 0 796 15

95/05 PVA/Fe0.03Zn0.97O 0 996 15

D.M. Fernandes et al. / Polymer Degradation and Stability 98 (2013) 1862e18681866

The film surface roughness data were evaluated from atomicforce micrographies of non-irradiated and irradiated PVA, 95/05PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O films. These resultsare showed in Table 1. The data reveal that the roughness of the PVAfilm is not significantly modified by the presence of Ni0.04Zn0.96Oand Fe0.03Zn0.97O in the non-irradiated films, indicating that theoxides are homogeneously distributed in the film surface,remarking also that a major part of the oxide concentrates at thebottom of the film [38]. However, the roughness of the PVA, 95/05PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O nanocompositefilms increase after UV irradiation: for the PVA film, this roughnessincrease occurs due to the photodegradation and cross-linkingexistence in the surface polymer initiated by UV irradiation,which causes the destruction of the amorphous phase of PVA[38,41]; the increase in the roughness for the nanocompositescontaining 5 wt.% of Ni0.04Zn0.96O and Fe0.03Zn0.97O may be relatedto the agglomeration of PVA crystallites on the film surface,together with some exposition of the oxide nanocrystallites similarto observed behavior for 95/05 PVA/ZnO nanocomposite film [38].

Fig. 5 shows the optical density (left axis) and the room tem-perature photoluminescence (right axis) spectra of pure PVA, 95/05PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O films. It can beobserved that pure PVA film exhibits absorption around 280 nmwhich is characteristic of pep* transitions related to the carbonylgroups (C]O) associated with ethylene unsaturation from re-siduals acetate groups remaining after the manufacture of PVAfrom hydrolysis of polyvinyl acetate [42]. The sample with Ni2þ

exhibit more intense absorption than the Fe3þ doped one, mainly at364 nm, which is characteristics of ZnO nanostructure [43]. Whenthe Ni2þ and Fe3þ doped ions enter into the ZnO crystal structure,localized states are formed at the doped sites with a reduction ofthe band gap due to the crystal lattice confinement effect [44].

It is known from previews works that PVA samples show broadviolet luminescence under UV excitation caused from hydroxyl

Fig. 5. UVevis absorption spectra (200e800 nm) of PVA, 95/05 PVA/Ni0.04Zn0.96O and95/05 PVA/Fe0.03Zn0.97O nanocomposite films and their correspondents fluorescencespectra obtained by exciting at 325 nm.

groups and that there is a slight red shift inducedbymetallic particlesdispersed in the polymer matrix [45,46]. This behavior is alsoobserved in PL spectrum shown in Fig. 5 formetal oxides-doped PVAcompared with single PVA samples. Besides, the observed emissionband around the violet region can be attributed usually to intrinsicdefect structure, in particular the oxygen vacancies originated fromthe oxygen deficiency in ZnO structure due to the nickel doped intothe ZnO crystal lattice [43]. It is possible to observe that PVA con-taining Ni0.04Zn0.96O or Fe0.03Zn0.97O have different photo-luminescence spectra than pure PVA, indicating that the dopingchanges the optical characteristics of the polymer. In addition to thecharacteristic broadband emission of these materials, there is anobserved reduction in the luminescence maximum intensity of PVAin function of the Fe and Ni addition, which is related to the phononenergyof the sample.At the literature, thephononenergy fora-Fe2O3nanoparticles and for Ni doped ZnOfilm is around 600 and 672 cm�1,respectively [47,48]. These high phonon energy values for Ni and Fedoped samples can explain the suppression observed in the lumi-nescence. Besides, the nature of interaction between nanoparticlesand the polymer chains tends to be complex and the surface passiv-ation is usually not complete, resulting in a high density of surfacetrap states on the nanoparticle surface, as indicated by low PL yieldand trap state PL in semiconductor nanoparticles [49].

Time-resolved photoluminescence for the PVA, 95/05 PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O films was performed to

Fig. 6. Time resolved photoluminescence spectra for the 95/05 PVA/Ni0.04Zn0.96O (a)and 95/05 PVA/Fe0.03Zn0.97O films (b) from 3 to 31 ns in steps of 4 ns.

D.M. Fernandes et al. / Polymer Degradation and Stability 98 (2013) 1862e1868 1867

verify any possible energy transfer between PVA and doped oxides,since both are luminescent in the UVeblue region. Fig. 6 shows thetime-resolved photoluminescence spectra for the 95/05 PVA/Ni0.04Zn0.96O (a) and 95/05 PVA/Fe0.03Zn0.97O (b) films from 3 to31 ns in steps of 4 ns. For both samples it can be noted that energytransfer process does not occur, so the energy of the maximumemission peak is constant until 31 ns. From the same Figure it canalso be observed that between the first and second acquisitionsafter the laser pulse, the emission intensity decreases considerably,indicating a low lifetime for these films (units of nanoseconds). Thetime-resolved photoluminescence data are very similar to thoseobtained with pure PVA and PVA/ZnO (95/5) samples [38].

4. Conclusions

PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite weresynthesized in a flexible and transparent films form. In inert at-mosphere, the nanostructured Ni0.04Zn0.96O decreases the thermalstability of the PVA film. However, in oxidative atmosphere thePVA/Fe0.03Zn0.97O films exhibit greater thermal stability than thepure PVA. The crystallinity of the PVA film changes with UV irra-diation performed in a dark box photoreactor during 96 h using a125W-Hg lamp (l� 254 nm) andwith the addition of Ni0.04Zn0.96Oand Fe0.03Zn0.97O. DRX data reveal that ZnOwurtzite single phase ispresent in the zinc oxides doped with Ni and Fe and the peaks arethose attributed to ZnO hexagonal structure. The roughness of thePVA film was not modified by the addition of doped oxides; how-ever, it increases after UV irradiation, this effect was greater innanocomposite films. Pure PVA film exhibited absorption around280 nm while the 95/05 PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O nanocomposite films showed absorption at the visibleregion which can be attributed to the band gap reduction of thedoped oxides. The nanocomposite films containing 5 wt.% ofNi0.04Zn0.96O and Fe0.03Zn0.97O exhibit different photo-luminescence spectra than pure PVA, indicating that the dopedoxides causes changes in optical properties of the polymer. Thethermal and optical properties of these flexible and transparentnanocomposite films together with the biocompatibility of the PVAmatrix suggest that these materials exhibit potential for variousapplications such as molecular markers, polymeric matrix for drugdelivery, and for organic solar cells.

Acknowledgments

The authors are grateful to Dr. Antônio Medina Neto forproviding UVevis spectra absorption using integrating spheremeasurements in his laboratory, to COMCAP/UEM for the equip-ment used in our study and to CNPq and CAPES for financialsupport.

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