Article pubs.acs.org/JPCC
Thin Films of Zinc Oxide Nanoparticles and Poly(acrylic acid) Fabricated by the Layer-by-Layer Technique: a Facile Platform for Outstanding Properties Mohamed Eita,*,† Lars Wågberg,‡,§ and Mamoun Muhammed† †
Division of Functional Materials (FNM), Royal Institute of Technology (KTH), SE-16440 Kista-Stockholm, Sweden Department of Polymer and Fibre Technology, Royal Institute of Technology (KTH), Teknikringen 56, SE-10044 Stockholm, Sweden § Wallenberg Wood Science Centre, Teknikringen 56, SE-10044 Stockholm, Sweden ‡
S Supporting Information *
ABSTRACT: The incorporation of nanoparticles into polyelectrolytes thin films opens the way to a broad range of applications depending on the functionality of the nanoparticles. In this work, thin films of ZnO nanoparticles and poly(acrylic acid) (PAA) were built up using the layer-by-layer technique. The thickness of a 20-bilayer film is about 120 nm with a surface roughness of 22.9 nm as measured by atomic force microscopy (AFM). Thin ZnO/PAA films block UV radiation starting at a wavelength of 361 nm due to absorption by ZnO although the films are highly transparent. Due to their high porosity, these thin films show a broadband antireflection in the visible region, and thus they provide selective opacity in the UV region and enhanced transmittance in the visible region up to the near-infrared region. They are also superhydrophilic due to their high porosity and surface roughness.
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INTRODUCTION Nanocomposites consisting of polymers and inorganic nanoparticles have attracted great attention because of their versatility for a wide range of applications.1,2 Different methodologies have been applied to introduce the nanoparticles into a polymer matrix. These methods include chemical methods such as in situ polymerization involving spontaneous nanoparticle formation and polymerization3−8 and physical methods such as solvent casting9 or spin coating.10 Over the past decade, scientists have paid attention to the optical applications of nanocomposites, this includes the use of nanocomposites to shield the UV radiation.2 A natural increase in solar UV radiation is expected due to the depletion of the ozone layer. A greater proportion of UVB radiation (315−280 nm) was shown to cause damage and rearrangements in the DNA of plants.11 Exposure to UV radiation is not only a result of climate or environmental changes, it also occurs in different aspects of life such as dealing with UV-radiant machines in industry or in medicine. It is therefore necessary to develop materials that are able to shield the UV radiation while maintaining high transparency in the visible region. The best known UV-blocking nanocomposite is that consisting of poly(methyl methacrylate) (PMMA) for its high transparency and ZnO nanoparticles for their UV absorption efficiency.2,10,12 The combination of ZnO and PMMA leads to a full absorption of the UV radiation in the range of 290−340 nm, PMMA absorbs the radiation in the range of 200−280 nm, but the © 2012 American Chemical Society
whole composite shows a reasonable transparency in the visible region.12 Thin ZnO/PMMA films fabricated by either dip- or spin-coating from a ZnO/PMMA mixture prepared by the sol− gel process provide a high degree of orientation of the ZnO nanoparticles as well as an increasing UV absorption with increasing number of layers.13 Mechanical blending has also been used to prepare ZnO/PMMA nanocomposites which show a crystalline hexagonal structure that has an increasing UV absorption with increasing ZnO content.14 Solution casting was applied to prepare a nanocomposite-containing polystyrene (PS) which as a matrix polymer yielded an ordered structure that blocked the UV radiation and gave a high degree of transparency which decreased with increasing ZnO content.15 Both PS and PMMA were used as a flowing liquid in front of a ZnO target to be sputtered by focused pulsed laser ablation to form a nanocomposite after drying off the solvent.16In another approach, radio frequency magnetron sputtering was applied to prepare a ZnO/poly(acrylic acid) (PAA) nanocomposite,17 which shows high transmittance in the visible region and also visible photoluminescence. The layer-by-layer (LbL) technique has also been used to prepare thin films of ZnO2/PAA.18 The absorption of these films in the UV region increases with Received: October 3, 2011 Revised: January 12, 2012 Published: January 19, 2012 4621
dx.doi.org/10.1021/jp2095328 | J. Phys. Chem. C 2012, 116, 4621−4627
The Journal of Physical Chemistry C
Article
Figure 1. SEM images of (a) 2 bilayers PAH-(PAA−ZnO) on a glass substrate using a cationic surfactant-stabilized ZnO dispersion in water (b) top view and (c) cross section of a 20 bilayer film, the scale bar corresponds to 100 nm. The top view image shows the porous structure of the surface, the cross section shows a film thickness of about 120 nm.
In the present work, the layer-by-layer technology has been used to fabricate thin transparent UV-blocking films. The polymer used is PAA, as an alternative for PMMA, and all of the solutions and dispersions are water-based in order to avoid the environmental hazards of organic solvents. In addition, the layer-by-layer technology would require less energy and machinery costs if applied on a large industrial scale. Thus this proposed method is facile, environmentally friendly and can be applied to a variety of substrates.31 The thin film buildup has been followed by UV spectroscopy through the absorption peak of ZnO. The film morphology has been studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the degree of hydrophilicity has been assessed by contact angle measurements. By combining the results of these techniques, a molecular level understanding of the film build-up, the interfacial interactions and the optical properties of the film can be obtained.
increasing number of bilayers and with salt (NaCl) concentration up to 0.1 M. For some optical applications, antireflective coatings are of interest in order to maximize the light transmittance and minimize the reflected light.19−24 The principle of antireflection is the destructive interference occurring between the beam reflected at the substrate−film interface and that reflected at the film−air interface, so that the reflection is minimized.25 At normal incidence, a single layer of antireflective coating must have a thickness that meets the quarter wavelength requirement for total antireflection20−26
d f = λ /4n f
(1)
where df is the thickness of the reflective coating, λ is the wavelength of the incident light and nf is the refractive index of the film. An ideal antireflective coating should have a refractive index nf given by nf = (n1n2)1/2, where n1 and n2 are the refractive indices of air and the substrate, respectively.26 Consequently, for a glass substrate with a refractive index of ∼1.5, a single antireflective coating should have a refractive index ∼1.22. However, the lowest refractive index known for a solid nonporous dielectric material is 1.35;27 according to theoretical calculations, a polymer of a refractive index of 1.29 may exist.28 In order to develop a coating with such a low refractive index, researchers have tried to introduce a controlled porosity into the coating film by, e.g., the adsorption of colloidal nanoparticles using the LbL technology.29Antireflective coatings have also been achieved by assembling hollow silica nanoparticles and poly(allylamine hydrochloride) (PAH). The antireflective properties were enhanced by the porosity and the low refractive index of the hollow silica nanoparticles.30 This example indicates that the role of the polyelectrolyte is limited, but that other key factors like porosity and refractive index may play a major role.
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EXPERIMENTAL SECTION
Materials and Film Preparation. ZnO powder nanoparticles (size
