Photochromic Characteristics of Tungsten Trioxide Films Prepared by Electric Current Heating Using Tungsten Wire

Abstract:

Tungsten trioxide films were deposited on the glass substrate by electric current heating method using tungsten wire in air. Applied voltages to the tungsten wire were increased at rates of 0.05, 0.5, 5 and 50 V/min. The resultant films consisted of particles. A particle size of the crystals increased with increasing voltage applying rate. The effects of an ultraviolet (UV) irradiation on the reflectance of the films were investigated. The reflectance in the wavelength range of 500 - 2500 nm decreased by the irradiation. When the films were left in dark after the irradiation, the reflectance returned to the initial. With increasing voltage applying rate, absorption peak shifted toward longer wavelength in the region of 1250 - 1500 nm and the peak intensity decreased.

• Periodical: Key Engineering Materials (Volumes 421-422)
• Pages: 364-367
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.421-422.364
• Online since: December 2009
• Authors: Takumi Hagizawa, Takayuki Honma, Yuichiro Kuroki, Tomoichiro Okamoto, Masasuke Takata
• Keywords: Electric Current Heating, Near Infrared Absorption, Photochromism, Tungsten Trioxide

References

[11] and sublimation of WO3 occurred over 827 o C [11], resulting in the increase of the resistance. Figure 3 shows XRD patterns of the films on the substrate. The patterns revealed the existence of the WO3 crystalline with monoclinic α- and ε- phases. Fig. 2. Current-voltage characteristics of electric current heating using tungsten wire with various voltage applying rates. 0. 0 1. 0 2. 0 3. 0 4. 0 5. 0.

[5] [10] [15] [20] [25] [30] [35] 0. 05 0. 5.

[5] [50] V/min Current / A Voltage / V Intensity / a. u.

[22] 24 26 28 30 32 34 36 V/min (0.

[2] 0).

[50] [5] 0. 5 0. 05 (2.

[1] 2)(22.

) (1.

[1] 2) (0.

[1] [2] ) (11.

[2] ) (1.

[1] [0] ) ε-WO3 α-WO3 (1.

[2] 2) (1.

[22] ) (2.

[2] ) (2.

2) (0.

[2] [2] ) (1.

[1] 2) (12.

)(20.

) (0.

[2] 0)(0.

[2] ).

[2] θ / degree (Cu Kα) Fig. 3. XRD patterns of the WO3 films on the glass substrate with various voltage applying rates. Fig. 1. A schematic diagram of experiment system. Tungsten wire Glass substrate.

[1] cm.

[2] cm V A Figure 4 shows SEM images of the films on the substrate. The films consisted of particles. With increasing voltage applying rate, shape of the particles were changed as nanoparticle (a), sphere (b), octahedron (c) and, sphere and octahedron (d). In this experiment, the maximum temperature of the wire is considered to increase with increasing voltage applying rate because the electric power applied to the wire increased with increasing voltage applying rate as shown in Fig. 2. The obtained result agrees with Kaito's report that the spherical particles were prepared when the source temperature was lower, while in case of higher source temperature, octahedral particles were prepared by coalescence growth [12]. Figure 5 shows the relative diffuse reflectance spectra of the film prepared at the voltage applying rate of 5 V/min. In the wavelength shorter than 480 nm, the reflectance decreased dramatically because of the band edge absorption for the WO3.

DOI: 10.7554/elife.28034.008

[13] A moderate decrease of the reflectance was observed in the wavelength from 500 to 2500 nm. This is caused by the dispersion of refractive index. The dents at 1900 and 2250 nm were due to the absorptions of water [14] and -SiOH groups.

[15] After the films were irradiated with the UV light, the reflectance in the wavelength range of 500 - 2500 nm decreased (Fig. 5. (a) Coloration). On the other hand, when the films were left in dark after UV irradiation for 60 min, the reflectance returned gradually to the virgin state (Fig. 5. (b) Bleaching). Other films also showed the coloration and bleaching process in the wavelength range from 500 to 2500 nm. Figure 6 shows the difference in the reflectance (∆R) between the films before and after UV irradiation for 60 min. The ∆R means absorption caused by the UV irradiation. The increase of the peak intensity with decreasing voltage applying rate is supposed to be related to the decrease of particle size. The absorption on the surface of the particle is assumed to be higher than that in the particle because a lot of defects exist on the surface of the particle. For the small particle, the specific surface area should be large, resulting in the high absorption Fig. 4. SEM images of the films on the glass substrate with various voltage applying rates. (a) 0. 05 V/min (b) 0. 5 V/min (d) 50 V/min.

[1] µm Fig. 5. Relative diffuse reflectance of the WO3 film on the glass substrate in coloration (a) and bleaching (b) processes. The film was prepared with 5 V/min. 500 1000 1500 2000 2500 3000.

[20] [40] [60] [80] 100 -SiOH Water (b) Bleaching.

[10] Virgin state After irradiation for 60 min.

min.

[60] Relative diffuse reflectance / % Wavelength / nm Visible 500 1000 1500 2000 2500 3000.

[20] [40] [60] [80] 100 -SiOH Water (a) Coloration.

[1] min Virgin state.

[60] [3] Relative diffuse reflectance / % Wavelength / nm (c) 5 V/min peak intensity. The peak shifted toward longer wavelength with increasing voltage applying rate. Salje and Güttler reported that the absorption peak of WO3-x shifted toward longer wavelength with decreasing oxygen deficiency [16]. An oxidation of the metallic tungsten is thought to be accelerated in higher temperature. The maximum temperature of the wire is considered to increase with increasing voltage applying rate. Therefore, the oxygen deficiency of the WO3 decrease with increasing voltage applying rate. All absorption peaks were in near-infrared region (1250 - 1500 nm). The peak intensity and wave- length could be controlled by voltage applying rate for the film deposition. The obtained WO3 films may be applied to optical devices such as filter for near-infrared region. CO%CLUSIO% The tungsten trioxide films were prepared by electric current heating method using tungsten wire with various voltage conditions. The size of particle in the film was controlled by changing the rate of the voltage increase. The reflectance in the wavelength range of 500 - 2500 nm of the films decreased by the UV irradiation. When the films were left in dark after the irradiation, the reflectance returned to the initial. All absorption peaks were in near-infrared region (1250 - 1500 nm). The absorptions intensity and wavelength could be controlled by the voltage applying rate. ACK%OWLEDGEME%T This work was supported partially by Grant-in-Aid for Scientific Research (B) (20310057) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and partially by the New Energy and Industrial Technology Development Organization (NEDO) as part of the Development of Multiceramic Film for New Thermal Insulators Project promoted by the Ministry of Economy, Trade and Industry (METI), Japan. REFERE%CES.

[1] H. Takeda and K. Adachi, J. Am. Ceram. Soc., 90 (2007), p.4059.

[2] C. Bechinger, E. Wirth and P. Leiderer, Appl. Phys. Lett., 68 (1996), p.2834.

[3] Robert G. Palgrave and Ivan P. Parkin, J. Mater. Chem., 14 (2004), p.2864.

[4] C. O. Avellaneda and L. O. S. Bulhoes, Solid State Ionics, 165 (2003), p.117.

[5] J. Hao, S. A. Studenikin and M. Cocivera, J. Appl. Phys., 90 (2001), p.5064.

[6] N. E. Stankova, P. A. Atanasov and T. J. Stanimirova, Appl. Surf. Sci., 247 (2005), p.401.

[7] D. Nezaki, S. Takano, Y. Kuroki, Y. Kurihara, T. Okamoto and M. Takata, Trans Mater. Res. Soc. Jpn., 25 (2000), p.205.

[8] D. Nezaki, T. Okamoto, M. Takata and K. Ogawa, J. Ceram. Soc. Jpn., 112 (2004), p.958.

[9] D. Nezaki, M. Yasuda, T. Yasui and M. Takata, Solid State Ionics, 172 (2004), p.353.

DOI: 10.1016/j.ssi.2004.02.069

[10] H. Yamasaki, K. Minato, D. Nezaki, T. Okamoto, A. Kawamoto and M. Takata, Solid State Ionics, 172 (2004), p.349.

DOI: 10.1016/j.ssi.2004.02.050

[11] H. Yamamoto, Chemical properties of tungsten, Funtai Fummatsu Yakin Yogo Jiten, 1st edition, Edited by Japan Society of Powder and Powder Metallurgy, (The Nikkan Kogyo Shimbun, Ltd., Japan, 2001), p.313. (In Japanese).

[12] C. Kaito, Jpn. J. Appl. Phys., 17 (1978), p.601.

[13] K. -H. Hellwege and O. Madelung, Tungsten oxides, LANDOLT-BORNSTEIN Numerical Data and Functional Relationships in Science and Technology, Group 3, Vol. 17, Subvolume g, New series, Edited by O. Madelung, (Springer-Verlag, 1984), p.287.

[14] G. M. Hale and M. R. Querry, Appl Opt, 12 (1973), p.555.

[15] D. L. Wood, E. M. Rabinovich, D. W. Johnson, Jr., J. B. MacChesney and E. M. Vogel, J. Am. Ceram. Soc., 66 (1983), p.693.

[16] E. Salje and B. Güttler, Philos. Mag. B, 50 (1984).

[10] [20] [30] [40] [50] [60] Coloration 0. 5 V/min0. 5 V/min0. 5 V/min0. 5 V/min 5555 50505050 ∆R / % Wavelength / nm.