2017年7月13日星期四

The Reaction Process from Violet Tungsten Oxide to Tungsten Dioxide

Violet tungsten oxide is kind of short rod fine crystal and between these short rod fine crystals there have many voids, particle size distribution is relatively loose. Which are conducive to discharge water and help hydrogen into the crystal particles, therefore, not only the restore process can take place inside also can restore on the surface, violet tungsten oxide advantageous for the producing ultrafine tungsten powder. Besides the reduction process conditions during the reaction should able to better control, such as low dew point of hydrogen, flow rate, amount of loading the boat.



After studies showed with the development of reduction reaction, the oxygen atom reduce and  oxygen vacancies increase of violet tungsten oxide, tungsten dioxide particles becomes more dense than γ- tungsten oxide’s. This is bad for hydrogen gas get into interior particles and the water which producing by reaction particles are not easily discharged. During the restore process, the reaction only happens on violet tungsten oxide surface but hardly occurs at particle interior. According to tungsten oxide reduction - precipitation growth mechanism, since violet tungsten oxide reduced to tungsten dioxide will lead to the production of tungsten powder particles grow up, so to produce ultra-fine tungsten powder or nano tungsten powder, so in the reduction process is best not restore to tungsten dioxide.

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Structure and Optical Property of Tungsten Oxide Film in Different Conditions

As we all know, annealing, or thermal, treatment is one of the most effective ways to influence the structure and the properties of tungsten oxide films. To investigate the effects of the annealing temperatures on the structure and optical property of tungsten oxide thin film that were deposited by magnetron sputtering of WO3 bulk in a vacuum, the deposited films were annealed at 200℃ and 300℃ for 60 min and at 400℃ for 60 min and 180 min in air, respectively. 

tungsten oxide thin film 

We find that the annealing temperature of 400℃ can effectively influence the structure and optical property of the deposited films. The films annealed at 200℃ and 300℃ display violetred under the sunlight as well as the as-deposited ones. Nevertheless, the film annealed at 400℃ exhibits transparency and appeared to be blue colored.

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Special Form Tungsten Oxide Producing Method

Nano tungsten oxide has strong organizational ability to grow and in a stable reaction conditions the interactions between molecular are fairly obvious. Molecular can epitaxial growth which is accordance with lattice arrangement strictly and to form the formation ratio complete and single component structure. With the improvement of nano technology, now there have been some special form of tungsten oxide, including arborescent, clitheriform, nanorods, nanowires and other special forms. 


Producing method of special form tungsten oxide which can be divided into three types: high-temperature synthesis, hydrothermal synthesis method, and solution low temperature synthesis method. Although high-temperature synthesis method is most common, but it has high requirements in equipment so this method is difficult wildly be used. The hydrothermal synthesis method is acidification W (CO) 6 and ammonium tungstate, the reaction is from 2 to 100 hours at 160-200 ℃ range in autoclave for produce different shapes of nanorods. This method is low cost so it can be widely used than high-temperature synthesis method. Solution low temperature synthesis method is a new invention so in produce special morphology tungsten oxide is not mature, but the method has attracted much attention and the prospects are very broad.

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Sodium Tungstate Preparing Pyrochlore Type Tungsten Oxide

The chemical formula of pyrochlore type tungsten oxide is  H2W2O7. Pyrochlore type tungsten oxide has recently found its wide applications in many different areas such as photoelectron-chemistry, functional material and sensing. In addition, pyrochlore type tungsten oxide can be used as intermediate product for the industrial production of WO3. There are many kinds of synthesis methods of the pyrochlore type tungsten oxide. And there are two main types: Aurivillius type compound as precursor, pyrochlore type tungsten oxide is obtained by acid treatment. The second method is hydrothermal synthesis method, namely sodium tungstate solution acidification to a certain pH range and it can be obtained by hydrothermal reaction for a certain time. The hydrothermal method that has overwhelming superiorities than others also has its own sets of flaws such as high cost, long reaction time and low quality product. Therefore, how to intensify the preparation process of H2W2O7 to obtain high quality product with low cost has become an urgent problem.

sodium tungstate photo


In this article, XRD, TGA-DSC, and SEM were used to characterize the products. The sodium content in the H2W2Ois
determined by chemical analysis and most sodium could be removed by ion exchange method. 

The main research results are listed as follows.

(I)The seeded precipitation and salting-out precipitation methods are not appropriate to prepare H2W2O7 but salting-out precipitation method suggests a new way to prepare Na2WO4 and Na2W2O7.
(2)Under hydrothermal conditions, the addition of oxalic acid has its optimum value. When C (Na2WO4) =150g/L, T=126℃, t=11h, adding oxalic acid 65g/L, the reaction ratio reaches its peak value 91%. Additive C plays a part in reaction ratio and seeds of H2W2O7 could accelerate reaction rate. The W7O246- ions may be the precursor in precipitating H2W2O7, but the Fe2+ ions seem to change the precipitation mechanism.            
(3)The TGA-DSC curve shows a continuous water loss from ambient to 450℃, and the H2W2O7 remains stable under 340℃. The SEM pictures of theH2W2O7 exhibit cubic morphology which can be changed by organic surfactant.
(4)Sodium is an inevitable impurity with its content varying from 3.3% to 4.9%, and ion exchange is a valid way to remove sodium content to about 0.3%.

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Structure and Optical Property of Tungsten Oxide Thin Films

The films annealed at 200C and 300 C display violet–red under the sunlight as well as the as-deposited ones. Nevertheless, the film annealed at 400C exhibits transparency and appeared to be blue colored. The film annealed at 200C is amorphous, i.e. does not exhibit diffraction peaks while those annealed at 400C exhibit several sharp diffraction peaks, indicating the crystallization of this film happened. The peaks shown in Figs. 1(b) and 1(c) can be attributed to a mixture of polycrystalline phases, which includes hexagonal h-WO3 phase (JCPDS PDF-33-1387) and triclinic t-WO3 phase (JCPDS PDF-30-1387). It is also seen that the films annealed at 400C were dominated by hexagonal h-WO3 phases. Acosta et al. had prepared the tungsten oxide thin films by sputtering WO3 bulk. Their results indicated that the films mainly contained hexagonal h-WO3 phase and monoclinic α-WO3 phase. Usually, the polycrystalline monoclinic and triclinic phases can be observed in the films deposited by reactive magnetron sputtering from W target, but few reports have also shown the presence of hexagonal phases. we can see that the growths of the film along (002) and (200) orientations of the triclinic phases are enhanced when thermal treatment time at 400C is prolonged from 60 min to 180 min.

YTO and BTO photo YTO photo

Under our experimental conditions, the thickness of the as-deposited film has been determined to be about 220 nm by an observation of its section SEM. SEM images of the tungsten oxide thin films annealed at different temperatures. All the sample films are very compact. The sizes of the sputtered tungsten oxide grains look very uniform and are a little less than 100 nm. The grain boundaries become more and more discernable as the annealing temperature increases from room temperature to 400C, whereas they become very indistinct again owing to the growth of the grains when the annealing time is prolonged from 60 min to 180 min at 400C. This process indicates the structure transformation of the sample films from the complete amorphous nature to crystallization and is in complete agreement with the results confirmed by XRD. The surfaces of the deposited films become rougher as the annealing temperatures rise. When the annealing time at 400C increases from 60 min to 180 min, the nanorod-like structures have grown from the surface of the deposited film. This kind of surface morphology of the samples is very different from those of the films prepared by sol–gel methods and by reactively sputtering tungsten metal target.

It is well known that during the sputtering, the interaction between Ar plasma and WO3 bulk can highly distort and tilt the form works of the WO6 octahedra and thus results in an amorphous film. At the same time, a large number of oxygen vacancies and defects, which can reduce the tungsten ions from W6+ to W5+ or W4+,would be produced in the deposited films. This is responsible for the violet–red color and the amorphous XRD data of the films annealed below 300C. The obvious crystallinity of the deposited films at 400C for 60 min implies that the defects and disordered structures in the films have been effectively activated by this temperature. Consequently, increasing the annealing time at 400C is obviously favorable to the crystallinity and the growth of the films.

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2017年7月12日星期三

Tungsten Oxide Nanorods

Tungsten trioxide has shown good sensing properties towards various gases. Recently thin nanostructured WO3 films have been tested. Due to their large surface area to volume ratio they exhibit good sensitivity depending on the grain size. However in conventional WO3 thin films the average grain size exceeds the thickness of the surface space charge layer, so the electrical conduction is mainly controlled by the carriers transport across the grain boundaries. An alternative way seems to be in a monocrystalline material with nanometric dimensions. 

tungsten oxide nanorods photo


Our objective is to fabricate nanosized tungsten oxide rods and to test their sensing properties under gas adsorption. We focus on the growth, the structure and the electrical properties of tungsten nanorods. The tungsten oxide nanorods were grown by vapour transport from a WO3 layer onto a substrate (Mica). The nanorods growth was controlled by the temperature gradient between the WO3 layer and the substrate.

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Synergistic Effect Between Ceria and Tungsten Oxide

Synergistic effect between ceria and tungsten oxide can make them to form WO3–CeO2–TiO2 catalysts for NO (nitrogen monoxide) reduction by ammonia were prepared by a sol–gel method. The catalysts were characterized by BET, XRD, Raman, NH3/NO adsorption and H2-TPR to investigate the relationships among the catalyst composition, structure, redox property, acidity and deNOx activity. WO3–CeO2–TiO2 catalysts show a high activity in a broad temperature range of 200–480 1C. 

tungsten oxide photo

The low-temperature activity of catalysts is sensitive to the catalyst composition especially under low-O2-content atmospheres. It may be related to the synergistic effect between CeOx and WOx in the catalysts. On one hand, the interaction betweenceria and tungsten oxide promotes the activation of gaseous oxygen to compensate the lattice oxygen consumed in NH3-SCR (selective catalytic reduction) reaction at low temperatures. Meanwhile, the Br nsted acid sites mainly arise from tungsten oxides, Lewis acid sites mainly arise from ceria. Both of the Br nsted and Lewis acid sites facilitate the adsorption of NH3 on catalysts and improve the stability of the adsorbed ammonia species, which are beneficial to the NH3-SCR reaction.

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Synthesis of Nanostructured Tungsten Oxide Crystalline Films

Over the last decade, nanostructured tungsten oxide materials have attracted much interest due to their potential for catalyst, gas sensors, crystalline film and electrochromic material applications. Several studies have been conducted by using various techniques, including the oxygen plasma processing, plasma sputtering, chemical solution, sol-gel techniques, electron beam evaporation deposition, electrochemical etching, and the chemical vapor deposition techniques. Most work concerned the synthesis of nanoparticles for catalytic applications based on chemical solution methods.The nanostructured tungsten oxide materials were synthesized using a simple hot filament CVD technique. The details of the process were described elsewhere in our previous publications. 

crystalline film photo


The tungsten filament acted as a precursor for tungsten oxide, and no catalyst or other tungsten-containing compound precursor was used. Both AlN and A ceramic substrates were used. Prior to the experiments, the substrates were ultrasonically washed in the methanol solution for 5 minutes, and dried with helium. After placing the substrate, the chamber was pumped down to Torr and then fed with the Ar gas (purity: 75%) to ambient pressure. During deposition, the gas inside the chamber was in a static state. The distance between the hot filament and substrate remained unchanged. The substrate temperature was controlled by adjusting electrical current on the hot filament, which was different from our previous experiments where the substrate temperature was controlled by simply changing the distance between the substrate and the hot filament.

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