Facile fabrication of high-efficiency near-infrared absorption film with tungsten bronze nanoparticle dense layer
© Lee et al.; licensee Springer. 2014
Received: 22 April 2014
Accepted: 27 May 2014
Published: 11 June 2014
An excellent transparent film with effective absorption property in near-infrared (NIR) region based on cesium-doped tungsten oxide nanoparticles was fabricated using a facile double layer coating method via the theoretical considerations. The optical performance was evaluated; the double layer-coated film exhibited 10% transmittance at 1,000 nm in the NIR region and over 80% transmittance at 550 nm in the visible region. To optimize the selectivity, the optical spectrum of this film was correlated with a theoretical model by combining the contributions of the Mie-Gans absorption-based localized surface plasmon resonance and reflections by the interfaces of the heterogeneous layers and the nanoparticles in the film. Through comparison of the composite and double layer coating method, the difference of the nanoscale distances between nanoparticles in each layer was significantly revealed. It is worth noting that the nanodistance between the nanoparticles decreased in the double layer film, which enhanced the optical properties of the film, yielding a haze value of 1% or less without any additional process. These results are very attractive for the nanocomposite coating process, which would lead to industrial fields of NIR shielding and thermo-medical applications.
78.67.Sc; 78.67.Bf; 81.15.-z
Tungsten bronze nanoparticles such as tungsten trioxide doped with alkali metals have selective optical absorption properties in the near-infrared region, leading to the synthesis of various morphologies and new compounds including nanorods [1, 2], nanowires , and nanosheets . Although the optical characteristics of solutions including tungsten bronze compounds have been previously analyzed , additional data are essential to fully understand the absorption and reflection-induced optical characteristics for the composite coating film application.
This study has attempted to clarify the near-infrared absorption characteristics of the film using a theoretical model that considers the localized surface plasmon resonance(LSPR)-induced absorption , scattering  caused by nanoparticles, and an interlayer refractive index-induced reflection . Absorption characteristics in the near-infrared region generally originate from the LSPR and can be predicted using the Mie-Gans theory  with the following factors proving influential: the aspect ratio , the electron deficiency [10, 11] of the tungsten bronze compounds according to nonstoichiometric compositions, the types of doped positive-ion metals [12, 13], and the purity of the tungsten bronze compounds as determined by the annealing condition . Although these parameters are well defined, they focus on rather qualitative aspects confined to the material itself. The optical characteristics based on quantitative data such as the number of nanoparticles, the interference of the medium, and the internanoparticle distance must be understood.
Therefore, this study quantitatively defined these parameters based on simulated results and plotted a spectrum ranging from the visible to the near-infrared region using correlations with a theoretical model. Because simultaneously observing the selective optical transmittance in both the visible and near-infrared regions is difficult, the two regions have been analyzed using a single index, the solar transmittance selectivity. In particular, the effects of primary factors such as the internanoparticle nanodistance have been analyzed using a theoretical model-based optical spectrum.
This investigation utilized theoretically required quantitative relations and sought ways to enhance the processability. To fabricate films with a low haze, different processing conditions were tested. For these studies, a film was fabricated from nonstoichiometric cesium-doped tungsten trioxide (Cs0.33WO3) nanoparticles synthesized using a solid reaction  and bead milling method  using a composite layer coating and a novel double layer coating. Then, the optical absorption characteristics from the visible to near-infrared regions were compared to examine the effect of distance between Cs0.33WO3 nanoparticles in each layer.
The composite layer-coated film was prepared using a mixture of dispersed sol and acrylic UV-curing binder. A rotating mixer (model MS 3basic, IKA, Nara, Japan) was used, and the polyethylene terephthalate (PET, film thickness = 186 μm) substrate was coated using the bar casting method. The coated film was dried at 80°C for 1 min in a heating chamber and illuminated using UV-curing equipment (model LZ-U1O1DCH, LICHTZEN, Gyeonggi-do, South Korea) at an intensity of 800 W/cm for 20 s. To produce the double layer-coated film, dispersed Cs0.33WO3 sol was first coated on PET substrate, and the UV binder was coated using the bar casting method.
The thickness was measured using the cross-sectional length of each film via scanning electron microscopy (SEM, JSM-6700 F, JEOL). The optical properties were examined using a UV/VIS/near-infrared (NIR) spectrophotometer (model Cary 5000, Varian Australia Pty. Ltd., Mulgrave, Australia) in the range of 300 ~ 3,300 nm. The nanodistance of the internanoparticles was measured by a transmission electron microscope (TEM, JEM-2100 F, JEOL Ltd.).
Results and discussion
Parameters used for calculating optical shielding property of the coated film
Thickness of the coated layer [nm]
Distance between nanocrystals [nm]
Mean diameter of nanocrystals [nm]
Dielectric constant of medium
Refractive index of the coating layer
Refractive index of the nanocrystals
Refractive index of PET substrate
Incident light absorption by the LSPR
Incident light reflection by the difference in refractive indices between the layers
Incident light scattering according to the size of the nanoparticles
The total light transmission and shielding functions for the tungsten bronze film
The effect of the internanoparticle distance is demonstrated in Figure 4, which shows the solar transmittance selectivity for the multiple ratios of parameters. The multiple ratio with ‘1’ of the number density of free electrons was determined from the solution-based results (i.e., ϱ = 6.3 × 1021 cm−3) . Unfortunately, the distance of nanoparticles was not reported before; we used 8 nm as the standard parameter. As the distance between nanoparticles is too small (<1 of multiple ratio), the solar transmittance selectivity is also decreased due to the loss of transmittance in visible range. According to this sensitivity analysis, we find that the distance of nanoparticles has a pronounced effect on the solar transmittance selectivity in common with those from the number density of free electrons. Moreover, one can reasonably state that the number density on the thin layers is more important than the content of the coated layer throughout the entire volume. Therefore, this study fabricated a double layer-coated film using the facile dense layer of nanoparticles  and attempted to analyze the factors that quantitatively influence its optical characteristics.
The quantitative evaluation of a novel double layer-coated film
EDS results of the coated layer in the composite layer and double layer films
Double layer-coated film
Composite layer-coated film
Carbon K shell
Oxygen K shell
Cesium L shell
Tungsten M shell
Haze values by varying the drying conditions and different coating methods
Double layer-coated film dried at 80°C
Composite layer-coated film dried at
100°C (vacuum oven)
Using a LTS model based on the Mie-Gans theory, double layer reflection, and Rayleigh scattering, this study quantitatively analyzed the contributions for high near-infrared absorption film with high transparency. After determining the effects of internanoparticle distance within the layer on the STS, a novel double layer-coated film was fabricated with a small nanodistance between Cs0.33WO3 tungsten bronze nanoparticles. Considering the total solar energy spectrum, 380 W/m2 of solar absorption energy was estimated. Moreover, the double layer-coated film has 80% visible transmittance at 550 nm, 10% near-infrared transmittance at 1,000 nm, and low haze with 1% or less. In addition, the STS of the film was 0.793, and thus, the double layer-coated film was found to have excellent near-infrared absorption compared with that of a composite layer-coated film (0.696).
This work was funded by the Ministry of Trade, Industry and Energy (K006005, ‘Development of nano-composite materials for thermal resistivity control’), Korea.
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