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

https://doi.org/10.1016/j.polymdegradstab.2013.05.003Get rights and content

Highlights

  • Nanocomposite films of PVA containing Ni0.04Zn0.96O and Fe0.03Zn0.97O were prepared.

  • The Fe0.03Zn0.97O addition on PVA increases the thermal stability of the polymer.

  • The crystallinity of the PVA increases with UV irradiation and oxides addition.

  • The PVA photoluminescence was modified by the presence of the oxides in the films.

  • The oxides addition on PVA affects theirs optical, thermal and morphologic properties.

Abstract

Ni0.04Zn0.96O and Fe0.03Zn0.97O with average diameter of 23 and 19 nm, respectively, have been synthesized by a modified sol–gel method to be used in the preparation of (100 − x)/x poly(vinyl alcohol)/oxide nanocomposite films, with x = 0, 1, 3 and 5 (in wt.%). A 125 W-Hg vapor lamp with emission above 254 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, UV–vis and PL spectrophotometry. The Ni0.04Zn0.96O addition on PVA films decreases the thermal stability of the polymer in inert and in oxidative atmosphere. In contrast, the Fe0.03Zn0.97O presence in the PVA films seems to increase the thermal stability of the polymer. The characteristic peak of the crystalline phase of PVA and wurtzite phase of the zinc oxide were identified through X-ray diffraction in both films. The crystallinity of the PVA film increases with UV irradiation and with the presence of Ni0.04Zn0.96O and Fe0.03Zn0.97O. 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 PVA film exhibits absorption around 280 nm characteristic of π–π transitions related to carbonyl groups from residuals acetate, while the 95/05 PVA/Ni0.04Zn0.96O and 95/05 PVA/Fe0.03Zn0.97O nanocomposite films show absorption at the visible region which is characteristics of the band gap reduction of the doped 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 low cost of production. In addition they are also able to exhibit peculiar optical properties showing potential to be used in photonic devices, gas sensors and organic solar cell applications.

Introduction

In the last fifteen years many properties of nanostructured materials have been studied due to their applicability in many technological areas [1]. The experimental conditions used for the preparation of these materials play an important role in the properties of the product. For this reason, a great variety of experimental methods have been used in the production of nanoparticles, such as sol–gel [2], [3], and techniques that use liquid ammonia as solvent [4]. Among all these techniques, sol–gel is the simplest and the most utilized [5], [6], [7].

It is well known that metal oxide nanocomposite materials present a variety of interesting magnetic, electric, and catalytic properties [8], [9]. In particular Fe2O3, NiO and ZnO are fundamental materials for chemical heat pump [10], or as substrate materials for the epitaxial growth of thin films with desirable magnetic or electronic properties [11]. The great advantage of the syntheses of these oxide doped polymers is the combination of its properties for catalysis, photocatalysis, sensors, and optoelectronic devices [12]. However, many methods have been suggested for the synthesis of this kind of material with the disadvantage of the use of large amounts of organic solvents [13], [14]. In order to contour this inconvenience, in our previous works were reported the syntheses of mixed Zn and Cu oxide [15] and mixed Ni0.04Zn0.96O and Fe0.03Zn0.97O oxide nanoparticles [16] using a relatively simple modified sol–gel method which uses only water as solvent.

Poly(vinyl alcohol), also known as PVA, is a polymeric material which is soluble in water, has a high dielectric strength, a good charge storage capacity and interesting optical properties. It has carbon chain backbone with hydroxyl groups attached that can be a source of hydrogen bonding which assist the formation of polymer composite [17]. This polymer is a good candidate for incorporation into multilayer coatings of organic solar cells due to its high transparency and ability to form a barrier to oxygen [18].

Several approaches have been employed to prepare nanoparticles/polymer composites [19], [20], [21]. Zinc oxide (ZnO) is a wide band-gap semiconductor with unique physical properties for potential applications like sensors, transparent coating for solar cells and acoustic devices. ZnO has also recently attract considerable interest for efficient ultraviolet light-emitting diodes (LEDs) and laser devices. Indeed, it is an excellent candidate for high-temperature and high-efficiency optoelectronic applications due to its large excitation binding energy (60 meV), that is the largest value found among the conventional semiconductors. When nanostructured ZnO is incorporated into polymers, it can improve their mechanical and optical properties due to strong interfacial interactions between the inorganic nanoparticles and the organic groups [22], also conferring better photochemical properties on some polymers, such as polypropylene [23], [24], polyurethane [24], among others. It is possible because nanoparticles of ZnO and some other metal oxides absorb UV radiation.

It is also important to consider the photocatalytic activity of the ZnO nanoparticles with visible light. This activity of polianiline/ZnO nanocomposite was investigated by the degradation of methylene blue (MB) and malachite green (MG) dyes in aqueous medium under natural sunlight and UV light irradiation [25]. The results indicated that the addition of the ZnO nanoparticles to the PANI homopolymer enhances the photocatalytic efficiency under natural sunlight irradiation for the degradation of both dyes. Another example is the photocatalytic degradation of methylene blue (MB) and methyl orange (MO) in the ZnO/CuO nanocomposites presence under visible light irradiation. The results showed that the coupled ZnO/CuO semiconductor possesses higher photocatalytic activity toward MB and MO degradation than ZnO under visible light. The coupling of ZnO/CuO reduces the band gap, extending the wavelength range to visible light region leading to electron–hole pair separation under visible light irradiation and consequently, achieving a higher photocatalytic activity [26]. Numerous recent studies have reported that ZnO can be improved as photocatalyst by various techniques such as doping with non-metals [27], addition of transition metals [28] as well as use of coupled semiconductors [29], [30].

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

The research on the preparation of nanostructured mixed oxide ZnO–NiO and ZnO–Fe2O3 in one phase is of great importance. The combination of n-type ZnO with a p-type NiO has been extensively studied expecting numerous heterojunctions, opening the possibility of sensitive gas sensors, photocatalysts and semiconducting materials [35]. ZnO–Fe2O3 mixed nanoparticles are receiving an increasing attention due to magnetic and luminescent properties exhibiting potential applications in the medical area, including magnetic separation and detection of cancer cells, bacteria and viruses [36]. In addition, organic/inorganic nanocomposites are extremely promising for applications in LEDs, photodiodes, photovoltaic cells, smart microelectronic device, photocatalysts and gas sensors among others applications as biomedical systems [37].

In a recent work, our research group reported the thermal, photochemical, morphological behavior and the optical properties of PVA/ZnO nanocomposite films. It was possible to verify that the presence of ZnO nanoparticles in the PVA film causes significant changes in the optical properties of the polymer [38]. The aim of the present work was to extend the comparison to a PVA/nickel–zinc and PVA/iron–zinc nanocomposite systems containing 1, 3 and 5 wt.% of each dopping oxide.

Section snippets

Reagents and chemicals

Poly(vinyl alcohol) 98–99% hydrolyzed, MW 13,000–23,000 was purchased from Aldrich. Zn(NO3)2·6H2O, PA 96–100% was purchased from Synth; Ni(NO3)2·6H2O, purity of 97–100%, was purchased from Lafan, and Fe(NO3)3·9H2O, purity of 98–100%, was purchased from Vetec. Dimethyl sulfoxide (DMSO) PA from Nuclear was used as solvent to prepare the films. Analytical grade reagents were used without further purification.

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

The synthesis and characterization of Ni0.04Zn0.96O and Fe0.03Zn0.97O nanoparticles was

Results and discussion

TG curves for PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite films obtained in N2 atmosphere are shown in Fig. 1(a) and (b), respectively. It is possible to observe that in the main decomposition stage (at approximately 470–570 K) the films with 1–5 wt.% of Ni0.04Zn0.96O showed lower thermal stability than pure PVA, following the behavior observed with PVA/ZnO films [38]. PVA/Fe0.03Zn0.97O films exhibit thermal stability relatively similar to the pure PVA in this main decomposition

Conclusions

PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite were synthesized in a flexible and transparent films form. In inert atmosphere, the nanostructured Ni0.04Zn0.96O decreases the thermal stability of the PVA film. However, in oxidative atmosphere the PVA/Fe0.03Zn0.97O films exhibit greater thermal stability than the pure PVA. The crystallinity of the PVA film changes with UV irradiation performed in a dark box photoreactor during 96 h using a 125 W-Hg lamp (λ ≥ 254 nm) and with the addition

Acknowledgments

The authors are grateful to Dr. Antônio Medina Neto for providing UV–vis spectra absorption using integrating sphere measurements in his laboratory, to COMCAP/UEM for the equipment used in our study and to CNPq and CAPES for financial support.

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