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Small Nanosized Oxygen-Deficient Tungsten Oxide Particles: Mechanistic Investigation with Controlled Plasma Generation in Water for Their Preparation Yohei Ishida,† Shingo Motono,† Wataru Doshin,† Tomoharu Tokunaga,‡ Hiroki Tsukamoto,† and Tetsu Yonezawa*,† †
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ‡ Department of Quantum Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *
ABSTRACT: Production of oxygen-deficient tungsten oxide nanoparticles with a diameter of around 10 nm have been successfully developed using a microwave-induced plasma in liquid technique. The prepared blue-green nanoparticles exhibit strong absorption in the visible region; thus, these could be efficient visible-light photocatalysts. The high-angle annular dark-field images revealed the dislocation of tungsten, which causes oxygen deficiencies.
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physical vapor deposition.7−12 However, the preparation of very small nanoparticles with diameters in the range of 1−10 nm, which are important for realizing efficient photocatalysis due to their relatively large specific areas, has still not been achieved. The purpose of the current work was to find a de novo simple strategy for the preparation of small WO3 or oxygendeficient WOx nanoparticles showing a strong absorption in the visible region, which are suitable for the use as efficient photocatalysts. Recently, the synthesis of metal nanocomposites using plasma in liquid has been developed by the authors’ group and others. Radio frequency (usually 13.56 MHz) equipment have been used to produce plasma in the last decade,13 but recently, microwave equipment (2.45 GHz) have become popular as an easy power source.14 Plasma can easily be produced with a small microwave equipment due to the high frequency of the microwave radiation. During the plasma production process, bubbles are continuously formed in the liquid phase, and plasma is generated inside the bubbles or around the bubble boundaries as reported in previous studies.14,15 We have previously reported the syntheses of Au,16,17 Ag,18 Pt,18 alloy NPs,19 ZnO,14 and black TiOx20 nanoparticles using microwave-induced plasma in aqueous solution containing
INTRODUCTION Considerable efforts have been made for developing clean, green, and renewable energy-related technologies, such as those involving photocatalysis, artificial photosynthesis, and the use of lithium-ion batteries, solar cells, and fuel cells.1 Various materials, including metals, metal chalcogenides, oxides, carbides, nitrides, and phosphides, have been developed, and tungsten oxides (WOx, x ≤ 3) have received considerable attention due to their abundance, highly tunable composition, and a high chemical stability at appropriate pH values.2 WO3 is made up of perovskite units and is one of the most attractive candidates for photocatalysis, as it exhibits approximately 12% absorption of the solar spectrum (Eg = 2.5−2.8 eV). WO3 is also well known for its nonstoichiometric properties, as the lattice can withstand a considerable number of oxygen vacancies. Many oxygen-deficient oxides (WOx, x < 3), such as WO2.98, WO2.96, WO2.9, WO2.83, and WO2.72, have been investigated and are found to have a stable crystal phase that is different from the surface-reduced WO3 and also exhibit a strong light absorption up to the near-IR region.3−6 Unlike WO3, which is yellow, WOx (x < 3) are either green or blue, with an additional broad absorption peak above 480 nm. This additional absorption is attributed to the new discrete energy bands below the conduction band, which are created by the oxygen vacancies. Nanostructured WO3 has been prepared by various methods, including thermal evaporation deposition, sputtering, and other © 2017 American Chemical Society
Received: July 13, 2017 Accepted: August 15, 2017 Published: August 29, 2017 5104
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(TEM) (Hitachi, H-9500 with acceleration voltage of 300 kV), FE-SEM (JEOL, JSM-6701F), HAADF−STEM (JEOL, JEMARM200, acceleration voltage of 200 kV), and UV−Vis diffuse reflectance spectroscopy (JASCO, V-670). The plasma generation was observed by a high-speed camera (Vision Research, PHANTOM Miro eX4, 8113 flames s−1). The emission spectrum of the generated plasma was observed by AvaSpec-2048 (Avantes). Brunauer−Emmett−Teller (BET) specific surface area was evaluated based on the nitrogen adsorption at 77 K using a Quantachrome NOVA 1200e surface area and a pore size analyzer. For the photocatalysis performance test, a 400 W high-pressure mercury arc lamp (SEN LIGHTS Co. Ltd, HL-400BL) was used.
metal ions or precursor metal complex. Moreover, we have recently succeeded in the preparation of W-doped TiOx nanoparticles that show a strong absorption in the visible region by modifying the commercially available TiO2 nanoparticles through the plasma process.15 Herein, we report a de novo and simple methodology to prepare WO3 nanoparticles 1−10 nm in diameter with oxygen vacancies by a plasma-inliquid technique using a bulk W wire as the metal source. In this method, the bulk W can be melted or evaporated by very high temperature of the plasma (several thousands °C) and is immediately cooled by the surrounding water (480 nm), which can be attributed to the new discrete energy bands that are created below the conduction band due to oxygen vacancies, thus changing its colors to blue or green. Thus, it is expected that our products may consists of deficient WOx (x < 3). The synthetic yields are summarized in Table 1. The synthetic yield is defined as eq 1
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EXPERIMENTAL SECTION Sample Preparation by Microwave-Induced Plasma in Liquid. Synthesis of O-deficient WO3 nanoparticles by microwave-induced plasma was carried using the procedure described below. A homemade microwave-induced plasma-inliquid system was used for the synthesis (Figure 1). In a typical
yield (%) =
amount of obtained powder × 100 amount of electrode loss × 1.261
(1)
where the constant value of 1.261 is the mass ratio between W metal and WO3, hypothesizing that all of electrode loss resulted in the formation of WO3. The synthetic yield at 100 Hz was found to be much smaller than that for 200−400 Hz. In fact, large W fragments were obtained as precipitates at 100 Hz due to the cavitation of the electrode. The synthetic yield was therefore decreased at 100 Hz. The differences in the synthetic yields will be discussed later, along with the mechanism of the nanoparticle formations. Figure 3 shows the transmission electron microscopic (TEM) images and the size distribution histograms of the obtained blue-green WO3 samples. In all of the conditions, spherical particles 10 nm in diameters were observed. Investigation of the Plasma Formation Process. To understand the mechanism that causes the differences in the synthetic yield as summarize in Table 1, we used a high-speed camera to record the plasma formation process (Figure 4). The detection interval was set at 8113 frames s−1. As shown in Figure 1, we observed an orange flame inside the bubble, which is due to the blackbody radiation of W metal,22 indicating plasma generation. Most importantly, the generated plasma completely disappeared before the introduction of the next pulse at 100 Hz (the pulse injection occurs every 10 ms). On the other hand, we could continuously observe the stable plasma at 300 Hz condition at any period during our observation. The stability of the plasma (i.e., the disappearance/continuity of plasma) should be significantly important for the formation of nanoparticles. At 100 Hz, the tip of W electrode was immediately cooled down by the surrounding water; thus, the reaction temperature should be lower than the other conditions. The W electrode was, on the other hand,
Figure 1. Experimental setup of the homemade microwave-induced plasma equipment. The microwaves were generated by UW-1500 (Micro-Denshi, Japan) at 2.45 GHz. Microwaves emitted from the magnetron pass through a WRJ-2 rectangular waveguide (109.22 × 54.61 mm2), a power meter, a tuner, and a waveguide to the coaxial adapter. A function generator (WF1973, NF corporation) was used to change the pulse frequency. The electrodes were made of tungsten (3.0 mmϕ, purity 99.96%).
process, 2 dm3 of distilled water was added to the reactor and stirred at 250 rpm. Using a functional generator, the pulse frequency was controlled from 100 to 400 Hz and the plasma reaction was continued for 1 h. A 3 mmϕ tungsten rod (purity 99.96%, Nippon Tungsten, Japan) was selected as the metal source. Plasma ignition was easiest using the rod of this diameter. The obtained solution was concentrated to ca. 50 cm3 under a reduced pressure and was then filtered (using a hydrophilic poly(tetrafluoroethylene) filter with 0.2 μm pore size, Advantec Toyo) and dried overnight under vacuum. Characterization. The obtained WO3 samples or electrodes after the reaction were characterized by X-ray diffraction (XRD) (Rigaku, MiniFlexII), transmission electron microscopy 5105
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Figure 2. Electric pictures of synthetic WO3 nanoparticles (prepared at 100−400 Hz pulse frequency) and commercially available WO3 powder.
electrode. On the other hand, rough surfaces with lots of small bumps were observed in the case of the samples obtained at 300 Hz. These rough surfaces and bumps strongly propose that the evaporation of electrode is the dominant process for the ejection of W from the electrode. Thus, the evaporation of W electrode resulted in the formation of spherical WO x nanoparticles (around 10 nm) observed in the TEM measurements (Figure 2). The formed W clusters or nanoparticles are readily combined with O radicals generated by the plasma from H2O molecules, resulting in the formation of WOx nanoparticles. A lower synthetic yield at 100 Hz condition compared with that in other conditions can be attributed to a low reaction temperature, which results in the melting of the electrode as a main process for the ejection of W. The formation of WOx nanoparticles, which are derived from the evaporation of W electrode, is thus suppressed. A large bulk of W precipitates, which should be formed via melting of the electrode at a lower reaction temperature at 100 Hz condition, therefore decreases the synthetic yield.
Table 1. Synthetic Yields of WO3 Nanoparticles under Different Plasma Frequencies frequency (Hz) yield (%)
100 2.2
200 14.4
300 15.3
400 12.9
continuously exposed to plasma at 300 Hz condition and, therefore, was subjected heating (as evidenced by a stable blackbody radiation of W), increasing the reaction temperature to as high as ∼9000 °C (see the estimation of plasma temperature by the emission spectrum of Hα and Hβ as shown in Figure S1 and detailed explanations). Because the nanoparticle formation of the current system is based on the evaporation or melting of W, the temperature of the electrode should affect the mechanism of the nanoparticle formation. To verify this assumption, we observed the field emission scanning electron microscopic (FE-SEM) images of the W electrodes after the plasma reaction (Figure S2). The FE-SEM image of the samples obtained at 100 Hz of microwave pulse frequency showed flat surfaces, suggesting that the melting of the electrode is a crucial process in ejecting W from the
Figure 3. TEM images and size distribution histograms of the synthetic WO3 nanoparticles under various plasma pulse frequencies. For the histograms, more than 100 particles from several TEM images were counted. 5106
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Figure 4. Pictures of plasma generations captured by a high-speed camera (8113 flames s−1).
Detailed Characterizations of the Obtained WO3 Nanoparticles. The obtained powders were characterized by XRD (Figure 5). The characteristic peaks of WO3, around 23.2, 24.3, 26.5, 29, 34, and 42°, were observed for all of the samples (broader peaks are observed around 29, 34, and 42°). Among several crystal structures of WO3-related materials, our results are in good agreement with the ε-WO3 crystal structure. The broad XRD peaks indicate that the obtained WO3 nanoparticles consist of a mixture of crystal grains, such as ε-, γ-, δ-, H0.1WO3, and WO2.98 (note that the crystal structure of WO2.98 is very similar to that of WO3, as determined by a previous study3). Because these tungsten oxides show similar XRD patterns and our patterns exhibited broad peaks, we were not able to estimate the exact ratios of the amounts of these components in our sample. However, it should be noted that this result might suggest the existence of low-valence state W atoms (as WO2.98 or other WOx with more oxygen vacancies, and H0.1-WO3) in our product. It is well known that ε-WO3 cannot be obtained as a main product by conventional WO3 synthesis and is a stable crystal phase below −43 °C.23,24 It has been reported that ε-WO3 was obtained when the synthesis was carried out at a very high temperature followed by an immediate cooling of the samples to room temperature (as reported by the flame spray pyrolysis
Figure 5. XRD patterns of synthetic WO3 nanoparticles. References were obtained from JCPDS data: γ-WO3 (00-043-1035), ε-WO3 (01087-2402), δ-WO3 (01-083-0947), WO3·H2O (01-084-0886), H0.1WO3 (01-006-0210), and α-W (00-004-0806).
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method).25 Our liquid plasma method also has a high reaction temperature (several thousands °C) and the nanoparticles were immediately cooled (several tens °C) by the surrounding water in the reaction bath. Thus, ε-WO3 could be the main product. WO3·H2O was observed only in the samples prepared at 100 Hz of microwave pulse frequency, as is evident from the peaks at 16.4 and 25.5°. This could be attributed to the introduction of water molecules due to the low reaction temperature. The pattern of metal W (α-W observed at 40.3°) indicates contamination of the electrode by cavitation. The photophysical property of the synthesized WO x nanoparticles was investigated by recording the diffuse reflectance spectra in the range of 200−800 nm with a baseline of BaSO4 (Figure 6). As expected from the sample colors
partially consist of low-valence state W atoms, which causes an improved absorption in the visible region. The decrease in the visible-region absorption with the pulse frequencies is probably due to the reaction temperature. The high microwave pulse frequency maintains the W electrode at a higher temperature during the synthesis. Decomposition of water into O or other radical proceeds efficiently at a high plasma temperature; thus, the concentration of O radical tends to be higher with the microwave pulse frequencies. This may lead to a slightly effective interaction between the formed W nanoparticles and the O radicals. Thus, the O deficiency of the obtained WOx nanoparticles is slightly decreased as the microwave pulse frequency is increased (Figure 5), which could cause a difference in their visible absorption. STEM Investigation of the Atomic Arrangement of Oxygen-Deficient WOx Nanoparticles and Their Properties. High-angle annular dark-field scanning transmission electron microscopic (HAADF−STEM) images of the obtained samples are shown in Figure 7. HAADF−STEM method enables us to observe the chemical contrast according to the atomic number (Z). Because the atomic number of 74W is very large, W atoms are observed as clear white spots in the HAADF images. We then could observe the crystalline atom arrangements using the HAADF images. Two kinds of lattice spacing values of 3.82 and 3.64 Å were observed, corresponding to {002} and {200} lattices of δ-WO3 and γ-WO3, respectively. Interestingly, we observed a few deformation sites in our HAADF−STEM image, marked by yellow circles, and a magnified image of one of these regions can be seen in Figure 7b. In the marked region in Figure 7b, the crystal structure is found to be nonuniform and distorted. Moreover, we observed an excess of W atoms in the region. This observation strongly suggests that the low-valence state W atoms are incorporated into the obtained WOx nanoparticles, resulting in their strong visible absorption. A preliminary investigation of the photocatalysis performance of (a) representative WOx product synthesized by 100 Hz plasma frequency and (b) the commercially available γ-WO3 nanoparticles (Beijing DK nano technology) was carried out by the rhodamine 6G (R6G) degradation test under the irradiation by 400 W high-pressure mercury arc lamp. Briefly, 2.4 mg of photocatalysis powder was dispersed in 12 mL of 10 mg L−1 R6G aqueous solution. The mixture was stirred for 30 min under dark. 1 mL of suspension was collected and centrifuged at 12 000 rpm for 10 min to precipitate the photocatalysis powders. The absorption spectrum of the supernatant was
Figure 6. UV−Vis diffuse reflectance spectra of the commercially available WO3 (black) and the synthetic WOx nanoparticles under various pulse frequencies.
(Figure 2), visible-light absorption above 480 nm was drastically improved on comparison with the commercially available WO3 particles. Ohtani et al. have reported similar absorption characteristics of WO3 films containing low-valence state W atoms that were produced by applying a negative potential.26 This electrochromism behavior can be attributed to the reduction of W6+ to W5+ by the potential and the intervalence charge transfer between these W ions, which resulted in the absorption in the visible region.27,28 On the basis of this assumption, we concluded that our WOx nanoparticles
Figure 7. (a) HAADF−STEM image of the obtained WO3 nanoparticles prepared at 300 Hz and (b) magnified image in red-square region in (a). Yellow circles indicate the deformation sites. 5108
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Institute for Materials Science and the High Voltage Electron Microscopy Laboratory of Nagoya University as a program of the microstructural characterization platform of Nanotechnology Platform of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Fruitful discussions with Prof. T. Yamamoto, Prof. S. Arai (Nagoya Univ.), Dr. M. T. Nguyen, and Y. Motokane (Faculty of Eng., Hokkaido Univ.) are gratefully acknowledged.
measured to determine the initial concentration of R6G. The sample was then irradiated and the absorption spectra of the supernatant were observed every 10 min. Figure S3 in the Supporting Information shows the raw absorption spectral change during irradiation. Figure S4 shows the concentration change of R6G ln(C0/C) as a function of irradiation time. The degradation of R6G was able to be analyzed by first-order kinetics, with the classical equation of ln(C0/C) = kt, where C0 and C denote the absorbance at 526 nm before and after the irradiation, respectively, and k and t denote the reaction rate constant and the irradiation time, respectively. The k values obtained were 6.2 × 10−1 and 3.3 × 10−1 h−1 for the synthetic WOx and the commercially available WO3, respectively. Because the used nanoparticles have different specific surface area (41 and 30 m2 g−1, determined by BET method), the obtained k values were normalized to determine the reaction activity at the same surface area after the subtraction of the degradation rate of R6G under the absence of photocatalysis (see Figure S4 in the Supporting Information). As a result, the synthetic WOx showed approximately 1.6 times higher photocatalytic activity than the commercially available WO3.
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(1) Huang, Z.-F.; Song, J.; Pan, L.; Zhang, X.; Wang, L.; Zou, J.-J. Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy. Adv. Mater. 2015, 27, 5309−5327. (2) Zheng, H.; Ou, J. Z.; Strano, M. S.; Kaner, R. B.; Mitchell, A.; Kalantar-zadeh, K. Nanostructured Tungsten Oxide − Properties, Synthesis, and Applications. Adv. Funct. Mater. 2011, 21, 2175−2196. (3) Gebert, E.; Ackermann, R. J. Substoichiometry of Tungsten Trioxide; the Crystal Systems of WO3.00, WO2.98, and WO2.96. Inorg. Chem. 1966, 5, 136−142. (4) Chen, Z.; Wang, Q.; Wang, H.; Zhang, L.; Song, G.; Song, L.; Hu, J.; Wang, H.; Liu, J.; Zhu, M.; Zhao, D. Ultrathin PEGylated W18O49 Nanowires as a New 980 nm-Laser-Driven Photothermal Agent for Efficient Ablation of Cancer Cells In Vivo. Adv. Mater. 2013, 25, 2095−2100. (5) Yan, J.; Wang, T.; Wu, G.; Dai, W.; Guan, N.; Li, L.; Gong, J. Tungsten Oxide Single Crystal Nanosheets for Enhanced Multichannel Solar Light Harvesting. Adv. Mater. 2015, 27, 1580−1586. (6) Wu, W.-T.; Liao, W.-P.; Chen, L.-Y.; Chen, J.-S.; Wu, J.-J. Outperformed electrochromic behavior of poly(ethylene glycol)template nanostructured tungsten oxide films with enhanced charge transfer/transport characteristics. Phys. Chem. Chem. Phys. 2009, 11, 9751−9758. (7) Thangala, J.; Vaddiraju, S.; Bogale, R.; Thurman, R.; Powers, T.; Deb, B.; Sunkara, M. K. Large-Scale, Hot-Filament-Assisted Synthesis of Tungsten Oxide and Related Transition Metal Oxide Nanowires. Small 2007, 3, 890−896. (8) Deniz, D.; Frankel, D. J.; Lad, R. J. Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition. Thin Solid Films 2010, 518, 4095−4099. (9) Di Fonzo, F.; Bailini, A.; Russo, V.; Baserga, A.; Cattaneo, D.; Beghi, M. G.; Ossi, P. M.; Casari, C. S.; Li Bassi, A.; Bottani, C. E. Synthesis and characterization of tungsten and tungsten oxide nanostructured films. Catal. Today 2006, 116, 69−73. (10) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. Crystallographically Oriented Mesoporous WO3 Films: Synthesis, Characterization, and Applications. J. Am. Chem. Soc. 2001, 123, 10639−10649. (11) Zhou, L.; Zou, J.; Yu, M.; Lu, P.; Wei, J.; Qian, Y.; Wang, Y.; Yu, C. Green Synthesis of Hexagonal-Shaped WO3·0.33H2O Nanodiscs Composed of Nanosheets. Cryst. Growth Des. 2008, 8, 3993−3998. (12) Sadek, A. Z.; Zheng, H.; Breedon, M.; Bansal, V.; Bhargava, S. K.; Latham, K.; Zhu, J.; Yu, L.; Hu, Z.; Spizzirri, P. G.; Wlodarski, W.; Kalantar-zadeh, K. Langmuir 2009, 25, 9545−9551. (13) Ihara, T.; Miyoshi, M.; Ando, M.; Sugihara, S.; Iriyama, Y. Preparation of a visible-light-active TiO2 photocatalyst by RF plasma treatment. J. Mater. Sci. 2001, 36, 4201−4207. (14) Yonezawa, T.; Hyono, A.; Sato, S.; Ariyada, O. Preparation of Zinc Oxide Nanoparticles by Using Microwave-induced Plasma in Liquid. Chem. Lett. 2010, 39, 783−785. (15) Ishida, Y.; Motokane, Y.; Tokunaga, T.; Yonezawa, T. Plasma induced tungsten doping of TiO2 particles for enhancement of photocatalysis under visible light. Phys. Chem. Chem. Phys. 2015, 17, 24556−24559. (16) Č empel, D.; Nguyen, M. T.; Ishida, Y.; Tsukamoto, H.; Shirai, H.; Wang, Y.; Wu, K. C.-W.; Yonezawa, T. Au Nanoparticles Prepared Using a Coated Electrode in Plasma-in-Liquid Process: Effect of the Solution pH. J. Nanosci. Nanotechnol. 2016, 16, 9257−9262.
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CONCLUSIONS In conclusions, we have developed a de novo method to produce oxygen-deficient tungsten oxide nanoparticles using a plasma-in-liquid technique. We have successfully obtained bluegreen spherical and well-crystalline nanoparticles with diameters of around 10 nm, and the product showed better photocatalytic R6G degradation performance than the commercially available WO3 nanoparticles. We believe that this method can be beneficial in the future for the development of efficient photocatalyst that utilizes the visible light.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00986. Table S1 and Figure S1 reporting details of plasma spectrum; Figure S2 collecting SEM images of the surface of the W electrode after plasma irradiation; Figures S3 and S4 reporting the photocatalytic activity results (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +81-11-706-7881. ORCID
Tetsu Yonezawa: 0000-0001-7371-204X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is partially supported from KAKENHI (A) (JP24241041 to TY) from JSPS, Japan, and Hokkaido University (Y.I. and T.Y.). A part of this work performed under the Research Program of “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “Network Joint Research Center for Materials and Devices.” A part of this work was also supported by the Advanced Characterization Nanotechnology Platform of the National 5109
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(17) Nishimoto, M.; Yonezawa, T.; Č empel, D.; Nguyen, M. T.; Ishida, Y.; Tsukamoto, H. Effect of H2O2 on Au Nanoparticle Preparation Using Microwave-Induced Plasma in Liquid. Mater. Chem. Phys. 2017, 193, 7−12. (18) Sato, S.; Mori, K.; Ariyada, O.; Atsushi, H.; Yonezawa, T. Synthesis of nanoparticles of silver and platinum by microwaveinduced plasma in liquid. Surf. Coat. Technol. 2011, 206, 955−958. (19) Shirai, H.; Nguyen, M. T.; Č empel, D.; Tsukamoto, H.; Tokunaga, T.; Liao, Y.-C.; Yonezawa, T. Preparation of Au/Pd Bimetallic Nanoparticles by a Microwave-Induced Plasma in Liquid Process. Bull. Chem. Soc. Jpn. 2017, 90, 279−285. (20) Ishida, Y.; Doshin, W.; Tsukamoto, H.; Yonezawa, T. Black TiO2 Nanoparticles by a Microwave-induced Plasma over Titanium Complex Aqueous Solution. Chem. Lett. 2015, 44, 1327−1329. (21) Hattori, Y.; Nomura, S.; Mukasa, S.; Toyota, H.; Inoue, T.; Kasahara, T. Synthesis of tungsten trioxide nanoparticles by microwave plasma in liquid and analysis of physical properties. J. Alloys Compd. 2013, 560, 105−110. (22) Judd, D. B. Changes in color temperature of tungsten-filament lamps at constant voltage. J. Res. Natl. Bur. Stand. 1936, 17, 679−694. (23) Yamaguchi, O.; Tomihisa, D.; Kawabata, H.; Shimizu, K. Formation and Transformation of WO3 Prepared from Alkoxide. J. Am. Ceram. Soc. 1987, 70, C-94−C-96. (24) Salje, E. K. H.; Rehmann, S.; Pobell, F.; Morris, D.; Knight, K. S.; Herrmannsdö rfer, T.; Dove, M. T. Crystal structure and paramagnetic behaviour of ε-WO3−x. J. Phys.: Condens. Matter 1997, 9, 6563−6577. (25) Wang, L.; Teleki, A.; Pratsinis, S. E.; Gouma, P. I. Ferroelectric WO3 Nanoparticles for Acetone Selective Detection. Chem. Mater. 2008, 20, 4794−4796. (26) Ohtani, B.; Atsumi, T.; Nishimoto, S.; Kagiya, T. Multiple-mode Responsive Device. Photo- and Electro-chromic Composit Thin Film of Tungsten Oxide with Titanium Oxide. Chem. Lett. 1988, 295−298. (27) Turyan, I.; Orel, B.; Reisfeld, R.; Mandler, D. Studying electron transfer at electrochromic tungsten oxide sol−gel films with scanning electrochemical microscopy (SECM). Phys. Chem. Chem. Phys. 2003, 5, 3212−3218. (28) Wang, D.; Li, J.; Cao, X.; Pang, G.; Feng, S. Hexagonal mesocrystals formed by ultra-thin tungsten oxide nanowires and their electrochemical behaviour. Chem. Commun. 2010, 46, 7718−7720.
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