2016年8月26日星期五

Fuel Cell Cathode Composite Catalyst -- Pyrochlore Tungsten Trioxide

The study found that adding the transition metal oxide into the precious metals will help improve the electrocatalytic activity. Tungsten oxide can form the synergistic catalytic effect with the noble metal platinum, and perform quite stable in an acidic environment, the composite catalyst uses tungsten trioxide as the carrier has been extensively studied. Pyrochlore tungsten trioxide has a layered structure of multi-dimensional channel, also has a three-dimensional channel, and this structure is conducive to rapid migration and ion exchange to improve the ionic conductivity; at the same time, it enhances its adsorption property, certain amount of ions embedded in the layered structure to form intercalation compound catalytic material.
yellow tungsten oxide photo

Synthesis of pyrochlore tungsten trioxide by hydrothermal synthesis method, take pyrochlore tungsten trioxide and carbon as the carrier to prepare Pt / WO3-C composite catalyst which is the proton exchange film of fuel cell catalytic material. Research showed:
1. The initial pH value of hydrothermal synthesis solution in the range of 1.0~4.0, with the pH value decreasing, the synthesized pyrochlore tungsten trioxide particle size becomes smaller;
2. Compared to Pt / C, Pt / WO3 has the poorer catalytic performance, due to tungsten trioxide performs semiconducting, it is poor in electronic conductivity which affects its electrocatalytic properties;
3. The catalytic activity of Pt/ WO3-C significantly enhanced indicates that a simple mechanical mixing can not make the catalyst loaded uniformly on the carriers, while directly loading through the liquid phase can obtain a higher dispersion of the composite catalyst, thereby enhance the catalytic performance;
4. The pyrochlore tungsten trioxide which uniformly dispersed in the toner has catalytically active sites on the surface after loading platinum, the layered pore structure of pyrochlore tungsten trioxide will be benefit to deintercalation of ions, and has the ability of accepting and donating proton in the catalytic reaction process, thereby increasing the reaction rate can be caused, and form the cooperative catalysis with platinum, thereby improving the catalytic activity, and ultimately improve the energy conversion efficiency of the fuel cell.


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Tungsten Trioxide Nanowire Gas Sensor

Tungsten trioxide is one transition metal oxide among a lot of metal oxide, belonging to the n-type semiconductor with its very widespread application. As a semiconductor gas sensor, tungsten trioxide has been considered one of the most promising new oxide sensitive materials to detect NO x, SOx, NH3, H2S, etc.. Since carbon nanotubes have been discovered in 1991, the more innovative electrical, magnetic, optical, thermal and other physical and chemical properties of one-dimensional nano-materials rendered have caused more and more attention, and showing its value and broad application prospects. While, compared to the traditional tungsten oxide material, tungsten trioxide nanowire because of its greater surface area has broad application prospects on the fields of gas sensors, electroluminescence, photoluminescence, conductivity electrode and photocatalysis. In addition, the tungsten oxide nanowires also have a higher surface activity and stronger adsorption capacity, thus to speed up the reaction with gas, and finally greatly improve the sensitivity and further reduce the operating temperature of the gas sensor.

nanowire gas sensor photo
Preparation steps of tungsten trioxide nanowire gas sensor are as follows:
1. Sodium tungstate as raw material to prepare tungsten trioxide nanowire by hydrothermal method;
2. Tungsten oxide nanowire as the main material, coordinating with adhesive-ethyl cellulose and terpineol, and add the appropriate amount of glass material to enhance the adhesion between the alumina substrate and the sensitive material, mixing and stirring uniformly of the above materials at appropriate proportion to obtain a gas-sensitive material slurry;
3. Gas sensor sintering process: print the above slurry on the alumina substrate which is covered with silver electrode and lead, then placed at 80°C for sufficiently drying in the air, and then transferred to a box furnace at 300~450°C for sintering for 1 to 2 hours to obtain the tungsten trioxide nanowire gas sensor element;
4. Component aging process: the gas sensor is aged at 300°C for 120 hours to the final tungsten oxide nanowire gas sensor.

Experiments showed that this tungsten oxide nanowires gas sensor has a high sensitivity, excellent repeatability and good stability for low concentrations (l~100ppm) hydrogen, CO and ammonia.

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Two-Dimensional Tungsten Disulfide/Tungsten Trioxide Monohydrate Lateral Heterojunction Preparation

Heterojunction two-dimensional material is the cornerstones of constructing nanoelectronics and optoelectronics "buildings", and to be tie basic elements of modern semiconductor industry, which plays an important role in the high-speed electronic devices and optoelectronic devices. Two-dimensional layered materials, including graphene, transition metal disulfide (tungsten, molybdenum, etc.), because of their unique electrical and optical properties, they can act as a constituent unit of the heterostructure. Lateral heterojunction two materials are connected by a covalent bond is formed, because of its simple method for constructing, it has a greater potential in terms of applying in band-gap engineering. With the advent of nano-science and nano technology, the development of plasmon nanostructure is really rapid, and scientists have prepared hydrogen doped of MoO3 and WO3 which are namely hydrogen bronze (hydrogen molybdenum bronze and hydrogen tungsten bronze) by simple H-overflow method, and exhibit strong localized surface plasmon resonance in the visible region. The results provide direct evidence on the hydrogen-doped metal oxide semiconductor implement plasmon resonance, and may allow large-scale application of low-cost and abundant earth element.

graphene WO3 lateral heterojunction delocalized rendering photo

This article provides a method for preparing two-dimensional tungsten disulfide / tungsten trioxide monohydrate lateral heterojunction, and its specific steps are as follows:
1. Disperse the 50-100 parts by mass of tungsten disulfide in 5-10 parts by volume of water or ethanol to form a dispersion, wherein the concentration of ethanol can be any volume;
2. Place the dispersion in the reactor of supercritical carbon dioxide at the conditions of 40-45°C, 6-20MPa to stir for 0.5~7h, decrease the atmospheric pressure to the normal value after the reaction is completed, and remove the unpeeled tungsten disulfide;
3. Remove the solvent after completing air oxidation (the solution appears in dark green before the oxidization, and turns pale yellow or bright yellow after it is oxidized), then obtain the two-dimensional tungsten disulfide / tungsten trioxide monohydrate lateral heterojunction.

Supercritical carbon dioxide with its high diffusion and low surface tension and other properties makes it can be the penetrant and expander to open gap between tungsten disulfide layers; stirring finally separate the sheet, so that the single layer of tungsten disulfide in the supernatant after centrifugation will be oxidized to monohydrate tungsten trioxide in the air, and to form two-dimensional / tungsten trioxide monohydrate lateral heterojunction eventually. This heterogeneity structure separates the electron-hole pairs, and has good prospects for development on the photocatalytic degradation of organic compounds and photocatalytic hydrogen production and other fields.

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Tungsten Trioxide Nanowire CO Gas Sensor Preparation

CO, the product of incomplete combustion of carbon fuels and cracking reactions occur at high temperatures, oxidation reaction ect. has become one of the major sources of air pollution, which greatly threats human life and health and environmental protection. The body of CO maximum allowable limit is 10-4, while in Europe the provisions of the environment CO must not exceed 10-5. So, the detection and controlling of CO is becoming urgent, tungsten trioxide based gas sensor is considered to be the most promising sensor for detecting NOx, O2 and NH3 and other poison gases, due to its simple structure, low cost, high sensitivity and gas sensing. 

CO gas sensor photo

This paper presents preparation method of  tungsten trioxide nanowire CO gas sensor which shows as follow:

1. Dissolve the weighed analytically pure sodium tungstate dihydrate (Na2WO4.2H2O) in an appropriate amount of deionized water, and taken out ice water bath, magnetically stirred 0.5 to 1 hour;
2. Slowly add, dropwise, 3mol/L hydrochloric acid to the sodium tungstate solution until complete the reaction to generate a pale yellow precipitate tungstate micelle, then stir for 1 hour and centrifuge for 20 minutes;
3. Add an appropriate amount of deionized water and chemically pure potassium sulfate (K2SO4), then turns into a volume of 50 ml reaction vessel after stirring with a glass rod, remarks: the reaction vessel is suggested to be filled with a volume of 80%;
4. Sealed, and carried out hydrothermal reaction for 6 to 72 hours in the oven under 180~270°C, remove the reactor and cooled to room temperature until the end of the reaction; then repeatedly wash and filter the product with deionized water and absolute ethanol, finally get tungsten trioxide nanowire after dried;
5. Weighing and mixing sensitive material of trioxide nanowires, solvents of terpineol and ethanol and binder of ethyl cellulose by a certain proportion, and also an appropriate amount of a low temperature glass frit slurry is appreciate be added to enhance the adhesion with the substrate, magnetic stirring for 2 hour. Ultrasonic treatment for 1.5 hours and then stirred sufficiently to get sensitive magnetic slurry;
6. Using a screen printing method to print sensitive slurry on the silver electrode interdigital alumina substrate, after thermal heat treatment under 250 °C~ 450°C in the air for 1 hour, we finally obtain WO3 nanowire CO gas sensor .

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Hexagonal Tungsten Trioxide

Tungsten trioxide is a unique n-type semiconductor material, that makes it one of the few oxide semiconductors which are easy-to-realize quantum size effect, and performances the excellent properties in photocatalysis, electrochromic, photochromic, gasochromic and other aspects, thus widely used in fields of chemical sensor, fuel cell, light catalyst and so on. Tungsten trioxide has orthogonal, monoclinic, cubic, hexagonal and other crystal structure. Among them, hexagonal tungsten oxide causes many concern because of its special hexagonal passage; many metal ions can be embedded in this hexagonal channel, thereby forming a hexagonal tungsten bronze which exhibits potential application in anode material and rechargeable lithium-ion battery.

hexagonal tungsten oxide crystal structure diagram

The crystal structure of the tungsten trioxide is ReO3 type, the result of A-site cation Absence of ABO3 perovskite structure, six oxygen atoms constituting the octahedron, W located therein and the neighbor WO6 octahedron links to form crystals by oxygen atom on the apical. Hexagonal tungsten trioxide has layered structure, and each layer octahedral connected by vertex to form a six-membered ring, in which the crystal axis direction will form a one-dimensional hexagonal channel. Further, the neighbor six-membered ring will form triangles which will also form a tripartite dimensional channel. Some scholars believe that the hexagonal and tripartite channels in hexagonal structure can accommodate cation, and has the chemical or adsorption interaction between each other, and each different cation can be substituted.

Hexagonal tungsten trioxide is a metastable crystalline phase, the preparation process is generally requiring moderate; it can generate by hydrothermal method from the raw materials of hydrochloric acid and sodium tungstate, with additives of potassium oxalate and potassium, respectively, hydrothermal synthesis method. Studies have shown that, at different temperatures, the various forms of tungsten trioxide crystal can be converted to each other, when the calcination temperature is 200°C, the product is orthorhombic tungsten trioxide; when the temperature is raised to 300°C, it begins to come out the peak of hexagonal phase tungsten trioxide; when the temperature reaches 450°C, hexagonal tungsten trioxide characteristic peaks disappears completely. Therefore, it can be concluded that hexagonal tungsten trioxide exists stably in interval of 200°C~450°C.


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Factors of Hydrothermal Method Preparing Hexagonal Tungsten Trioxide



Hydrothermal process is carried in a special closed reaction vessel (reactor), the aqueous solution as medium, by heating the reaction vessel to create a high-temperature, high-pressure reaction environment to make the generally insoluble or poorly soluble material to dissolve and recrystallization, and then by filtering, washing, drying and other separation means to get the ultra-fine, high-purity particles. Hydrothermal method for preparing hexagonal tungsten trioxide, its crystalline form is affected by many factors, such as pH value, hydrothermal temperature and additives and others, the following will specifically analyze the impact of various factors.

hexagonal WO3
In the hydrothermal environment to prepare tungsten trioxide, the existence of hydrogen ions (H+) is one of the important factors which affect crystal form of product, especially when the reaction condition is acidic environment. The influence mechanism of pH on tungsten trioxide crystal form is really complex, it is generally believed that when in a different pH environment, the solution ion balance is changed, so that the crystal growth environment changes thus to achieve the purpose of crystal form control, however the specific impact mechanism is still unknown. Studies have shown that, hexagonal tungsten trioxide will be generate in the pH value range of among 1.5~2.0; In addition, tungstate ions will be partially polymerized in the acidic environment to generate paratungstate or metatungstate ions, which is not conducive to the generating of nanoribbons, experiment results also showed that, pH in the range of 11~12, the resulting product was hexagonal tungsten trioxide nanoribbons.

Study found out that when the hydrothermal temperature is among 200 ~ 300 ° C, hexagonal tungsten trioxide will be prepared by the hydrothermal method; and when the temperature reaches to 350°C, the resulting product will be orthorhombic tungsten trioxide. The polymorph regulation of hydrothermal temperature on the tungsten trioxide depends on the energy of system changing. Moreover, a study that taking sodium tungstate and HCl as raw materials, citric acid as an additive, the result shows that hexagonal WO3.2H2O will generate at hydrothermal temperature of 150°C, however, the 180°C will get hexagonal WO3.

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