Effect of CsxWO3 Nanoparticles Content and Atomic Ratio for Near Infrared Cut-off Characteristics

Jae young Kim, Hyun Jin Yoon, Seung yong Jeong, Sangkug Lee, Gyo jic Shin and Kyung Ho Choi

Center for Green Materials Technology, Korea Institute of Industrial Technology, 35-3, Hongcheon-ri, Ipjang-myeon, Seobukgu, Cheonan-si, Chungnam-do 330-825, Republic of Korea

Abstract. In this work, the ceramic nanocomposites films with homogeneous dispersions of tungsten bronze nanoparticles with ternary additive cesium have been prepared by mixing Urethane acrylic/UV-coating binder and dispersed sol depending on the atomic ratio and the particle contents. Furthermore, we are synthesized the CsxWO3 with different annealing at 700, 800, and 900 ℃ in an N2 gas for 1 hr respectively. The structure and sizes of ceramic particles was observed XRD and PSA spectrometer, the heat-shielding properties of their films were investigated by NIR spectrometer. Synthesized Cs0.33WO3 particles with annealing at 800 ℃ typically formed hexagonal tungsten bronze structure for good absorption of NIR, size observed as being 30-100 nm and <150 nm. Especially, remarkable absorption of NIR wavelength ranges makes ceramic composite films good candidate for use as a heat shielding window, it clearly shows useful for applications where heat shield is required. Keywords: Caesium, Nanoparticles, NIR, Hexagonal, Heat-shielding.

1. Introduction The various nanoparticles have been investigating the continuous and new methods to reduce solar heat

as it ensures a potentially low-cost and high-productivity solution. Not only does it needs high transmittance of ultraviolet radiation but also achieves complete shielding of infrared solar radiation can be used for solar control windows. In the other word, an effective IR absorbent should have high absorbance as well as a broad working wavelength. A well known the kinds of materials to realize the purpose is the nanoparticles of transparent oxide conductors with heat-ray cut-off effect such as tin doped indium oxide (ITO) and antimony doped tin oxide (ATO). They also are well known to provide highly transparent solar filters to absorb heat-ray by the effect of the plasma vibration of the free electrons, as typically observed in gold and silver nanoparticle solution. However, ITO can only shield the IR wavelength ranges longer than 1500 nm as well as indium is an expensive metal resource. In recent years, for practical application, tungsten bronzes actively have been investigating due to their interesting electro-optics, photochromic, electrochromic, and superconducting properties. Tungsten trioxide (WO3) has a wide band gap of 2.6-2.8 eV5 and is transparent in the visible and NIR ranges. A metallic conductivity and a strong NIR wavelength absorption can be induced when free electrons are introduced into crystals by either decreasing the oxygen content or by adding ternary elements. The oxygen deficiency in tungsten oxides leads to a complex-ordered structure known as the Magneli structure, while the ternary addition of the positive ions leads to the tungsten bronze structure. In other words, tungsten bronzes MxWO3 with doping small ions such as H+, Ag+, Li+, Na+, K+ and Cs+ into WO3 have better optical and electrical properties. It has been reported that the tungsten bronzes with the hexagonal phase are of particular interest in the application of electrochromic devices owing to the relatively high diffusion coefficients of hydrogen ions and metal ions compared with those of the orthorhombic phase and pure WO3.

On the other hand, as for the Cs-doped WO3, the superconductivity of CsxWO3 prepared by reported that CsxWO3 with 0.3 ≥ x ≥ 0.19 was a 3D superconductor, a transition from a metal to an insulator occurs at x <

2012 3rd International Conference on Biology, Environment and Chemistry IPCBEE vol.46 (2012) © (2012) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2012. V46. 9

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0.19, and a 2D superconductor was formed at x=0.05. In addition to, the hexagonal tungsten bronze phase of Cs0.33WO3 was regarded as being highly attractive in solar filter applications because of its strong absorption in the NIR wavelengths.

The traditional methods for preparing the hexagonal tungsten bronze CsxWO3 usually require high temperature and harsh reaction conditions, such as heating mixtures of WO3 with K, Rb or Cs salts in reducing atmospheres at 1000 ℃. Recently, morphology and phase control of particles by soft chemical synthesis methods have attracted wide interest in the synthesis of many oxides such as ZnO, WO3, etc. It has been acknowledged that the microstructure and morphology of particles play an important role in its properties. Up to now, synthesizing pure WO3 with different morphologies and properties has been widely reported. However, there is limited work reported on the synthesis and characterization of CsxWO3 with controlled morphology by solvothermal reaction. Furthermore, the effects of ammonia annealing on the heat-shielding properties of CsxWO3 have not been reported.

In this paper, we synthesized the caesium doped tungsten tri-oxide (Cs0.33WO3) nanoparticle with the different annealing temperature, various concentration and controlled atomic ratio by using the traditional solid state reaction method and investigated the their morphological properties and the optical properties of nanoceramic composite films for solar light control applications.

2. Experimentals Synthesizing of homogenous CsxWO3 powder, Cs2CO3 of various concentration and H8MoN2O4 were

mixed in aqueous, dried at 180 ℃ for 8 hr. The precursor as dried at 180 ℃ was heated in a tubular furnace at 450 ℃ for 1 hr with H2/N2 gas flow at H2/N2=90/10, followed by annealing at 800 ℃ in an N2 gas for 0.5 hr. The as-produced powders were dispersed by turbomill with iron ball for 2hr and the zirconia bead for 1 hr respectively. In the other hand, nanocomposite films were prepared with mixing urethane acrylic/UV coating binder and well dispersed perovskites sol by using bar coater. Dispersion and coating binder were mixed well in vial with rotating mixer. The prepared films were dried at 80 ℃ for 1min in heating chamber and illuminated with a mercury UV lamp UV-curing equipment at intensity 800 W/cm for 20 sec. The structure and sizes of synthesized perovskite particles was observed XRD and PSA spectrometer, the optical properties of their composite films were investigated by UV-VIS and NIR spectrometer. The synthesis of products is summarized outlined in Scheme1.

Scheme 1. Synthetic scheme of the nanoparticles and theirs composite films.

3. Results and Discussion Figure 1 exhibits the typical XRD patterns of the synthesized Cs0.33WO3 particles with the different

annealing temperatures. All peaks could be indexed to the hexagonal cesium tungsten bronze (JCPDS No. 831334) and the only Cs0.33WO3 particle synthesized in annealing at 800 ℃ almost was identified to disappear the impurity peak. It can be seen that the nanoparticles obtained by annealing at 800 ℃ prefers the hexagonal tungsten bronze structure. It was well known that the relatively larger ions such as Ti (0.150 nm),

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Rb (0.152 nm), and Cs (0.170 nm) compared with Li, Na etc. could fit better in the hexagonal vacant tunnels (0.163 nm) in the hexagonal tungsten bronze structure rather than in the rectangular vacant tunnels in the cubic tungsten bronze structure. In the other hand, this result suggested that the typical hexagonal structure of Cs0.33WO3 with remarkable absorption of NIR was formed by annealing at 800 ℃.

2theta scale0 20 40 60 80 100

Ref.

Annealing - 900

Annealing - 800

Annealing - 700

Fig. 1: XRD patterns of CsxWO3 particles synthesized in different annealing temperature.

Fig. 2: Shows the typical SEM image of synthesized Cs0.33WO3 particle by 800 ℃ annealing temperature.

A synthesis of Cs0.33WO3 particle with annealing temperature of 800℃ was morphology as measured by SEM. The Cs0.33WO3 particle under the 100 nm can be observed in Fig. 2. Each Cs0.33WO3 particle was aggregated with the mass of the few micron size. Those clusters shatter to pieces by pulverization dispersed methods. These results agree well with the XRD profiles of the samples shown in Fig. 1.

Figure 3 exhibits the properties of NIR absorption with different annealing temperature. These results indicated that the NIR absorption properties sensitively depended on the annealing temperature of Cs0.33WO3 particle. As above mentioned, it means that the effective absorption of NIR range for Cs0.33WO3 composite films increase with increasing content of hexagonal tungsten bronze structure of Cs0.33WO3. This is consistent well with the XRD profiles of the samples shown in Fig. 1.

Fig. 3: NIR transmittance spectra of Cs0.33WO3 (1.5 wt%) particle with different annealing temperature.

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Figure 4 shows the properties of NIR absorption of CsxWO3 with annealing at 800 ℃. As the atomic ratio (from 0.33 to 1.00) of CsxWO3 increased, only NIR transmittance of Cs0.33WO3 and Cs0.55WO3

significantly was decreased while the other sample is not changes the NIR transmittances. As above mentioned, this result implied that only Cs0.33WO3 was formed the typical hexagonal tungsten bronze structure with superior absorption of NIR wavelength.

Fig. 4: NIR transmittance spectra of CsxWO3 (1.5 wt%) composite films with different atomic ratio (annealing

temperature 800 ℃).

Figure 5 represented the relation between NIR shielding effect and Cs0.33WO3 concentrations. According to the illustration, As the Cs0.33WO3 concentration increased from 0.5, 1.0, 1.5 and 2.5 wt%, NIR transmittance value commonly were declined whereas NIR transmittance increased with increasing x values of Cs0.33WO3. Particularly, in case of Cs0.33WO3 clearly indicated that the distinct absorption curve around 1000 nm. This discrepancy could be explained by an underestimation of short-range order effects in the XRD pattern whereas in IT phonon absorption short-range-order effects are seen mainly. Single-phase hexagonal tungsten bronze (x=0.33) only possess a clear increase in absorption that increases with increasing contents. As a result, this suggests that the properties of NIR absorption subtly depended on the annealing temperature, content and composition(x content) of Cs0.33WO3.

Fig. 5: NIR transmittance spectra of films containing various concentration of Cs0.33WO3 (1.5 wt%) particle (annealing

temperature 800 ℃).

Because the perovskites-based nanomaterials exhibit strong absorption of NIR wavelength, it is expected to convert the absorbed radiation to local heat directly to phototherrmal conversion properties of the nanomaterials, the prepared composite film were put on a paper and then irradiated by a 250 W IR lamp for 30 s. The temperature distribution was recorded by thermographic analysis. Figure 6 shows the image of Cs0.33WO3 composite film. The measurement was performed at an ambient temperature of 25 °C. Within only 20 s, the temperature of the composite film increased from 25 to 135 °C, whereas the temperature of the

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other around the composite film did not rise very much, indicating the quick conversion of absorbed NIR wavelength energy to local heating energy on the composite film.

Fig. 6: Thermographic images of the Cs0.33WO3 nanoparticles (irradiation by a 250 W IR lamp).

4. Conclusion In this study, we synthesized the homogeneous cesium tungsten bronze nanoparticles by using traditional

solid state reaction method with changing reaction condition and investigated the heat-shielding properties of their composite films. It can be confirmed that only the Cs0.33WO3 nanoparticles mainly has the hexagonal tungsten bronze structure by annealing at 800 ℃, size of CsxWO3 observed as being 30~100 nm and ≤ 150 nm. In the other hand, the Cs0.33WO3 nanoparticle observed at the 75 nm and was aggregated with the mass of the few micron size. All synthesized nanoparticles exhibited the properties of NIR shielding. Particularly, the nanoparticle of Cs0.33WO3 synthesized by the annealing at 800 ℃ showed a pronounced absorption of NIR light transmittance. Due to thermographic analysis of IR light, the Cs0.33WO3 nanoparticle thin film cut off the NIR radiation mainly by absorption.

5. References [1] Hiromitsu Takeda, and Kenji Adachi, J. Am. Ceramic. Soc. Near infrared absorption of tungsten oxide

nanoparticle dispersions. 2007, 90: 4059.

[2] Jing-Xiao Liu, Yoshihiko Ando, Xiao-Li Dong, Fei Shi, Shu Yin, Kenji Adachi, Takeshi Chonan, Akikazu Tanaka, and Tsugio Sato. Microstructure and electrical-optical properties of cesium tungsten oxides synthesized by solvothermal reaction followed by ammonia annealing. Journal of Solid State Chemistry. 2010, 183: 2456.

[3] Guo, Shu Yin, Lijun Huang, Lu Yang, and Tsugio Sato. Effects of Crystallization Atmospheres on the Near‐Infrared Absorbtion and Electroconductive Properties of Tungsten Bronze Type MxWO3 (M= Na, K). Chem. Commun. 2011, 47: 8853.

[4] Chongshen Guo, Shu Yin, Mei Yan, and Tsugio Sato. Facile synthesis of homogeneous CsxWO3 nanorods with excellent low-emissivity and NIR shielding property by a water controlled-release process. J. Mater. Chem. 2011, 21: 5099..

[5] Chongshen Guo, Shu Yin, Peilin Zhang, Mei Yan, Kenji Adachi, Takeshi Chonan, and Tsugio Sato. Novel synthesis of homogenous CsxWO3 nanorods with excellent NIR shielding properties by a water controlled-release solvothermal process. J. Mater. Chem. 2010, 20: 8227.

[6] Chongshen Guo, Shu Yin, Mei Yan, and Tsugio Sato. Facile synthesis of homogeneous CsxWO3 nanorods with excellent low-emissivity and NIR shielding property by a water controlled-release process. J. Mater. Chem. 2011, 21: 5099.

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