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Synthesis of tungsten oxide nanoparticles ..
* Has a photocatalytic activity under visible light that is eight times higher than that of nitrogen-doped titanium oxide and three times higher than that of particulate tungsten oxide
* Compatible with large-scale synthesis at low cost
* Expected to be used to decompose indoor harmful volatile organic compounds
Masahiro Miyauchi (Senior Research Scientist) and Zhao Zhigang (Post-Doctoral Research Scientist), the Nano-Structured Material Group, the Nanotechnology Research Institute (Director: Nobutsugu Minami) of the National Institute of Advanced Industrial Science and Technology (AIST) (President: Hiroyuki Yoshikawa) have successfully synthesized WO3 nanotubes by a simple hydrothermal method. These nanotubes are composed of aggregates of crystallites and have a nanoporous structure with fine, nanometer-scale pores on their walls. This structure provides the nanotubes with a large specific surface area, enabling high photocatalytic activity.
When used to decompose gas-phase acetaldehyde, these nanotubes with platinum as a promoter exhibit visible-light-induced photocatalytic activities eight times higher than that of conventional N-doped TiO2, and three times higher than that of conventional particulate WO3 with a platinum promoter.
Since the hydrothermal synthesis method can produce large quantities at low cost, it is expected that this novel catalyst will be used commercially in products such as "safe and healthy interior construction materials" that can decompose harmful volatile organic compounds (VOCs) to purify indoor environments, where little ultraviolet (UV) radiation exists.
The results of this research have been published in Angewandte Chemie-International Edition, a German academic journal published by Wiley-VCH.
Images of tungsten oxide nanotubes observed usinga scanning electron microscope.
Social Background for Research
A photocatalyst, when exposed to light, decomposes harmful substances or exhibits antibacterial properties. Photocatalysts can also be coated onto the outer walls to make it difficult for dirt to attach to them, thereby producing "self-cleaning" surfaces. Titanium oxide (TiO2) is known as a typical photocatalyst, however it functions only when exposed to UV light. It is impractical in indoor environments, where little UV light exists. But the demand for photocatalysts that can work under indoor visible light is increasing. The photocatalysts can decompose harmful indoor VOCs, and can be a countermeasure against the sick house syndrome, for example. Although some visible light responsive photocatalysts such as N-doped TiO2 have been reported recently, none achieve practical levels of performance. However, recent research efforts have resulted in the discovery that nanoparticles of WO3, a simple oxide, together with promoters such as platinum or palladium particles or copper compounds embedded on their surfaces, exhibit high levels of activity when exposed to visible light. However, since commercially available WO3 particles are large in size and, therefore, low in specific surface area, they are not necessarily appropriate as a photocatalyst base material. Moreover, only few cases of WO3 nanoparticle synthesis have been reported, and the development of nanostructure-controlled WO3 nanoparticles is necessary for WO3-based photocatalysts with improved activities.
History of Research
The Energy Technology Research Institute of AIST previously discovered that the activity of WO3-based photocatalysts can be greatly improved by simply mixing them with promoter particles of palladium or copper compounds (press release on July 9, 2008). At the same time, the Nanotechnology Research Institute of AIST focused its attention on the base WO3 particles themselves, and has been making efforts aimed at creating a high-activity photocatalyst by controlling the nanostructure.
This development is the fruition of research conducted in collaboration with the University of Tokyo as part of the "Project to Create Photocatalyst Industry for Recycling-oriented Society" initiated by the New Energy and Industrial Technology Development Organization (NEDO).
Details of Research
Figure 1 shows scanning electron micrographs of the synthesized WO3 nanotubes. Each nanotube is composed of aggregates of crystallites, each 100 nm in size or smaller, and has a nanoporous structure with many fine pores, several tens of nanometers in size, on the tube wall, producing a high specific surface area. The nanotubes have outer diameters of 300-1000 nm and lengths of 2-20 µm. They can be synthesized in high yield by simply heating the starting materials and the solvent in a sealed container. In this study, a high-yield synthesis process was developed by discovering that the introduction of urea to the hydrothermal reaction solution enabled the formation of nanotubes. Since an expensive templating agent is not necessary in the hydrothermal method used, a low-cost, large scale manufacturing process can be achieved.
A gas-phase acetaldehyde decomposition test was conducted with the nanotubes obtained from this process, with fine particles of platinum applied to the surface of the nanotubes as promoters. Irradiation with visible light at wavelengths of 400 nm or longer initiated a reduction in the concentration of acetaldehyde and the simultaneous generation of carbon dioxide (CO2) as a decomposition product, showing the visible-light-induced photocatalytic activity. Figure 2 shows the rate of CO2 generation induced by visible light. The newly developed WO3 nanotubes (Fig. 2-4) showed approximately eight times higher activity than conventional N-doped TiO2 (Fig. 2-2). When compared with a platinum-supported commercial particulate WO3 (Fig. 2-3), the platinum-supported WO3 nanotubes showed more than three times higher visible-light-induced catalytic activity.
It was also confirmed that the newly developed WO3 nanotubes can completely decompose acetaldehyde to CO2 in the presence of visible light.
The high visible-light-induced activity of WO3 nanotubes is mainly due to their nanoporous structure, which provides them with a large specific surface area. In the future, we intend to further improve the activity by selectively introducing a promoter onto either the inner or outer walls of the nanotubes. We also intend to develop a process to manufacture nanotube thin-films for possible applications as coating materials.
IF-WS2 nanoparticles synthesized starting from tungsten oxide nanourchins have been investigated by using aberration corrected scanning transmission electron microscopy (Cs-STEM). The synthesis process produced IF-WS2 nanoparticles of two different and well differentiated ranges of size. High resolution HAADF-STEM images and their comparison with simulated STEM micrographs reveal the predominance of stacking of the type 1T close to the border of the structure; the observation of this kind of stacking, observed previously in IF-MoS2 but never reported before in the case of the IF-WS2 nanostructures, adds a new dimension to the existing understanding of structure and stacking in the case of the nanostructures of transition metal chalcogenides.
Novel Approach: Tungsten Oxide Nanoparticle as a …
Metal salts in which the metal center is in an oxidation state of less than 3+ do not form gels with epoxide-assisted gelation easily, especially the late (right-side) third-row transition metals.. Only recently have nickel oxide and copper oxide aerogels been prepared, and only very recently (2007) have zinc oxide aerogels been prepared by Prof. Louisa Hope-Weeks at Texas Tech University. Additionally, no synthetic route has been established for preparing aerogels of alkali oxides (Li2O, Na2O, K2O, Rb2O, Cs2O) or alkaline earth oxides (MgO, CaO, SrO, BaO, RaO). In all of these cases, it’s not hard to imagine that the reason no aerogels have been prepared is because a continuous expanse of metal-oxygen-metal network bonding is hard to form, since there are only one or two bonds to be made, and a three-dimensional structure cannot result. Furthermore, there are no reports of aerogels of noble metal oxides (the noble metals being Pd, Rh, Ir, and Pt), largely due to difficulties arising from their square-planar configurations and because they don’t readily form oxides.
Unfortunately, not all metal oxides can be made into aerogels easily, and for some metals, it may not be possible at all. Molybdenum oxide aerogels were elusive for quite some time, since molybdenum tends to form terminal Mo=O oxo bonds as opposed to Mo-O-Mo bridges. Without a macroscopic network of Mo-O-Mo bridges, a molybdenum oxide gel cannot form, and only a sol of unconnected nanoparticles or a precipitate will result. Molybdenum oxide aerogels were fiinally prepared in 1998 by Drs. Winny Dong and Bruce Dunn at the University of California–Los Angeles from molybdenum isopropoxide by performing a ligand exchange with acetonitrile (CH3CN) to replace some of the isopropoxide ligands with acetonitrile ligands. Hydrolysis of this new complex results in the formation of both Mo=O and Mo-O-Mo bonds, with Mo-O-Mo bonds favored enough that a gel can form. With a synthetic route to molybdenum oxide gels available, molybdenum oxide aerogels could be prepared. Since epoxide-assisted gelation of molybdenum(VI) chloride does not result in a gel (or at least is not straightforward), this remains as the only synthetic route to molybdenum oxide aerogels. As of now, vanadia and manganese oxide aerogels also need to be prepared from alkoxide precursors as opposed to epoxide-assisted gelation of their metal salts as well.
Hydrothermal Synthesis of Tungsten Oxide Nanoparticles
In general, metal oxide aerogels can be prepared by carefully hydrolyzing a respective metal alkoxide in a solvent. Because of the increased sensitivity of most metal alkoxides over silicon alkoxides, often times very dry solvents and tightly controlled amounts of water must be used, and often times also cryogenic temperatures to slow the reaction rates down to a productive speed. Schematically the process proceeds similar to silica alkoxide sol-gel chemistry, in that alkoxide groups are hydrolyzed to hydroxyl groups, and then hydroxyl groups from two molecules condense, resulting in a metal-oxygen-metal bridge. This proceeds, resulting in the formation of a metal oxide sol which can then crosslink into a metal oxide gel. The gels can then purified, aged, and supercritically dried similar to other gels. Generally speaking, though, the quality of metal oxide gels made through alkoxide chemistry is generally poor (mechanically and optically speaking).
Researchers are using photocatalytic copper tungsten oxide nanoparticles to into biodegradable compounds. The nanoparticles are in a grid that provides high surface area for the reaction, is activated by sunlight and can work in water, making them useful for cleaning up oil spills.
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Synthesis of Tungsten Oxide Particles by Chemical ..
By incorporating small tungsten oxide particles synthesized by microwave hydrothermal synthesis and then heat-treated with forming gas, in silica based coating, the IR shielding performance of this coating has been shown to be better than that of commercially available tungsten oxide nanoparticles.
Zinc oxide is an inorganic compound with the formula Zn O
Due to the high potential of photoconductive behavior used in electrochromic and sensor devices, tungsten oxide (WO3) is the most promising candidate applied for electrochromic materials, gas sensors, and dye sensitized solar cells. Adsorbing the visible blue light by tungsten oxide nanoparticles would thus be used efficiently to enhance the sun-light harvest efficiency. Recent investigations have been promoted by new methods of ultrafine powder production and their applications. Nanocrystalline tungsten oxide can be prepared by sputtering methods, gas condensation methods, and chemical deposition method. In this study, we would like to prepare of tungsten oxide nanoparticles by chemical deposition method and investigate the experimental parameters of various organic solutions and discuss the relationship between absorbance spectra and which by chemical deposition method. Purged O2 into the various solvent ( DMF and acetylacetone solution), WCl6 was added into the solvent slowly and stirred vigorously. During the filtration process solutions were characterized and analyzed by particles size distribution, UV-visible absorption, and TEM observation. The size of tungsten oxide particles synthesized in DMF is smaller than that in acetylacetone solutions from TEM observation. These results may be related to particle sizes or agglomerated behavior of tungsten oxide. In the photochromic data, it exhibits absorbance result due to valence change of tungsten ion and photochromic effect of tungsten oxide nanoparticles. Since some papers also have been reported that the highest intensity of the solar spectrum is in the blue region, the modification of the oxide semiconductor for blue-light absorption will effectively increase the photon harvest from the incident sunlight and thus generate more electron-hole pairs in addition to those from dyes. It would be appropriately applied in the design of solar cell devices.
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