| Tungsten is a kind of rare metal that possessing high melting point and high hardness. From the18th century, large attention has been paid on the exploration and application of tungsten. Tungsten and tungsten alloy show wide application in the aerospace, marine, atomic energy, electronics, electrical industry, military equipment, automobile industry, chemical industry, and other fields, ascribed to their good comprehensive performances including high strength, high density, low thermal expansion coefficient, corrosion resistance and good machinability. In addition, tungsten oxide has also draw much attention in a long time because of its great utilization value in corrosion inhibitor, catalyst and many other fields. As a consequence, it can be conclude that the tungsten resources are of important strategy significance for a country.Tungsten resources are extremely rich and of highly centralized distribution in our country, the reserves, production and exports of which are all ranked first in the world. Although there are ten kinds of tungsten ores in the world, our country mainly owns two of them, that is, wolframite ((Mn, Fe) WO4) and scheelite (Ca (WO4)). Although reserve of scheelite is larger than wolframite in our country, mining object of tungsten ores major is wolframite for a long time due to the much rich ore and facile mining and beneficiation. However, as high-end products of tungsten ores, the application value and research feasibility of scheelite are much higher than wolframite. Therefore, it is very significant to efficiently control the supply of tungsten products and establish an industrial development pattern. Scheelite has stimulated great interest due to the large number of valuable and special performances.Scheelite (CaWO4) is an island of tungstate mine, and the tungsten can be replacement as isomorphism by molybdenum. As a result, scheelite crystal is composed of tungstate crystals (MeWO4, Me:Ba、Sr、Ca) and molybdate crystals (MeMoO4, Me:Ba、Sr、Ca), which belongs to tetragonal system. The crystalline nature of scheelite is very stable, which can resistance to high temperature and high pressure. Indeed, this advantage provides vital scientific evidence for scheelite as a preferred material in some application. Scheelite is also a self-activation fluorescent material and bears excellent luminescence property, which shows great potential application in scintillation counter, laser, detection, luminous and display, and other fields. Moreover, most tungstate crystals and molybdate crystals from scheelite are important scintillator or X ray illuminant, which can give out many spectrum from shallow blue to yellow under the UV light excitation. Thereby, scheelite plays an important role in fluorescence and laser photoelectric material, and has been widely applied in large screen display and light emitting diode. Besides, scheelite has also broad application space in gas sensitive element, color materials and off-color material, etcIt should be pointed out that almost all scheelite is artificially synthesized which is really used in high value-added functional materials, so that there are many researches have been focus on the synthesis of scheelite at present. While natural scheelite is seldom applied in high functional materials in fact, result from difficile beneficiation and low purity. For this reason, in order to effectively broaden the application in high-end functional materials field of scheelite resources and promote the utilization of resources of our country, the in-depth study of natural scheelite is going to be a focus. Many gangue minerals such as fine grained fluorite and calcite still exist in the natural scheelite after rough concentration. Accordingly, at present a lot of researches have focused on concentrate purification, such as Petrov’s method and Plants.the flotation. Despite of the certain improvement on the traditional method made by a lot of researchers, the purity of scheelite still is not meet the needs to be used high value-added functional materials. Continue to purify natural scheelite requires some special chemical methods and instruments. The existing further purification methods can further improve tungsten content from the natural scheelite, but these methods would desdroy chemical composition and crystal structure of scheelite. Meanwhile, it is noted that although scheelite has many advantages mentioned above, the related performance activities are still relatively low under the light/electric excitation, because the crystal structure symmetry of scheelite is good, which results in great limitations on the use of the high additional functional materials.Based on the present situation, a research strategy about further purifying scheelite and then preparing another kind of high-end product of tungsten resources, tungsten oxide, with scheelite as precursor gradually come into the researchers view. At the same time, the research of this aspect has certain progress at present. For example, Zhang et al. using scheelite for tungsten source successfully prepared the high purity tungsten trioxide by activated carbon adsorption and ethylene glycol purification method. This study injected new hope to expand future of development direction and application of the scheelite.WO3is an n-type semiconductor material, band gap is about3.25eV. The structure of WO3can be regarded as [WO6] octahedral linked together by corner-sharing, and results in a three-dimensional network structure. As a typical wide bandgap semiconductor, WO3has attracted much attention due to the excellent photoelectric properties, which makes it for wide applications in color, catalysis, sensitization, etc. At present, it is possible to prepare WO3by acidifying the scheelite to achieve improvement of tungsten content. Accordingly, further purifying scheelite and then turning it to WO3, not only can effectively solve the limited application of natural scheelite in high value-added functional materials, but also can improve the utilization efficiency of scheelite resources of our country.WO3has many excellent performances. Among them, photochromic property recently is the new research direction of this material. WO3has been regarded as promising applications in the smart window, information storage media, automobile reflective mirror, large area information display and so on, which makes it become the research focus. The photochromism refers such a phenomenon that materials can change color in a reversible way by electromagnetic radiation. The reverse process can take place by the effect of heat and light in the dark. The photochromism was first phenomenologically observed in organisms, which dates back to more than one hundred years. In the1940s, photochromic phenomenon has already been found in the organic compounds. As research continues, photochromic materials are fast developed towards the practical direction and industrialization because of their unique advantages. In recent years, it has become the focus of attention that photochromic materials are applied in light regulation, optical information storage, optical device materials, optical switch, modification gene chip materials, optical information genetic material and many other fields around the world.WO3has six kinds of crystal type including triclinic, monoclinic, orthogonal, tetragonal, hexagonal and cubic phase. While only those WO3productions that possess some metastable crystal types and special morphologies may have photochromic properties. Since it is still a challenge to prepare WO3materials with good photochromic properties, pepole seldom do research in the photochromic property of WO3. Thus, how to effectively improve photochromic properties of WO3materials has attracted increasing interest. WO3materials contain powder materials and thin film material. Up till now, a large number of preparation methods have been explored to obtain WO3materials with improved properties. Among them, hydrothermal method is extremely popular because of its economic, operation facile, morphology controllable etc. features. Just as the photochromic properties of WO3materials depend not only on WO3units’shape and size, but also on their morphologies and arrangement. Therefore, it has drawn much attention about the preparation and properties of WO3with special hierarchical structure and morphologies.As for the investigation of photo-sensitive WO3powders materials, hollow ball structures have been favored. Because hollow ball structures usually have high surface area and low density, and the internal space of hollow ball can hold molecules with certain size, which conducive to the utilizing efficiency of luminous energy due to the effect of photons localization and improve the photochromic properties of WO3. These advantages make WO3hollow structure show a broad application prospect as a new type of optic material. However so far, most synthetic for obtaining hollow ball often need to remove template including hard template and soft template by dissolving or calcining. Meanwhile, the shell of hollow structure will be instable when the templates are removed due the multiple valence states of tungsten base oxide, which often results in the collapse of hollow ball structure with the synthesis proceeding. Therefore, it is still difficult to prepare WO3micro/nanospheres, and there is seldom report about the WO3hollow sphere.In this project, WO3hollow spheres were successfully obtain by molybdenum ion regulating method together with a simple acidification purification and hydrothermal synthesis process, which uses scheelite for tungsten source, sodium molybdate for molybdenum sources. Indeed, inorganic ions could construct an organized reaction environment before crystallization, which can play an important role on the nucleation position and the formation of crystal. Hence, it may be an effective strategy that using inorganic ions regulating method to synthesize WO3powder with hollow spherical structure. From the characterization results, it is revealed that as-prepared powder is urchin-like and the crystal belongs to hexagonal phase structure. The surface area of the hollow sphere sample is101.8m2g-1(3.5times as high as the standard sample) and the band gap is decreased.In addition, soft template synthesis method to synthesize hollow structure is simple and mild. In general, soft template synthesis method firstly needs to introduce surfactant molecules into reaction system, and then remove the core through organic solvent or oxidation, ultimately obtain micro/nano hollow sphere structures. Moreover, bubble not only possessing advantages of ordinary soft template synthesis method, but also avoided introducing and removing the surfactant. Therefore, we provide another effective route to manufacture hollow sphere structure. Namely, the approach uses scheelite for tungsten source, hydrogen peroxide and oxalic acid for annexing agents, oxygen generated by hydrogen peroxide decomposition for template. In the consequence, we successfully obtained WO3hollow sphere with hydrothermal route. The analysis of XRD revealed that product is hexagonal structure of WO3as well, and crystallinity of samples synthetized with bubble as template is superior to standard sample. It is guessed that the nucleation process is slowed down due to the introducing of bubble, and it is beneficial to the growth of crystal. SEM patterns indicating that the synthetic powder is composed with a mass of microballoons whose diameter is0.7μm and sizes are uniform after the addition of H2O2. And from the further enlarged photograph, it could be found that particles with hollow structure are accumulated with nanorods whose length is near200nm. Specific surface area of WO3with hollow structures is twice larger than standard samples, and its absorption band occurred to an obvious red shift.The WO3hollow sphere structures prepared though this two approach bear improved photochromic properties, which may due to photons localization and large exposed area of the porous films together with the decreased band gap that is in favor of the production of electron-hole pairs.Using scheelite as precursor to synthesize photochromic WO3film material is another application approach. As for WO3film, it is attractive to prepare WO3film with various hierarchical structures, and as a result to improve the property. In this paper, scheelite was used as tungsten source and was acidified to purification. The WO3films were generated on the glass substrates, and we realized the controllable synthesis of morphologies of WO3films by modulating the hydrothermal reaction conditions. Method one:Three kinds of WO3hierarchical structure films assembled by tetragonal prisms were synthesized by using scheelite as tungsten source together with hydrochloric acid, sulfuric acid and nitric acid as inducer. The XRD and SAED results show that products are hexagonal WO3. Oxyacid introduced oxygen groups, which may result in generation of porous structure on surface of WO3films prepared with sulfuric acid and nitric acid. Because oxygen groups were absorbed on the surface of WO3films, which would makes tetragonal prisms to possess relative high polarity and be mutually exclusive assembled. As a result, surface area of WO3films was evidently increased due to bridge effect. Especially, WO3films prepared with nitric acid had higher nucleation barrier so that the pores on the surface of film were bigger, structure units were arranged more loosely. Thus WO3films prepared with nitric acid preformed better photochromic properties. Method two:The same preparation method was done using small molecule organics instead of inorganic acid to control the films’morphologies. Consequently, photochromic properties of WO3films were improved. In this route, ethanol and oxalic acid were using as inducer to in the preparation process of samples. Films prepared without capping agents were ordered nanorod arrays, while the ones obtained with ethanol and oxalic acid revealed peeled-orange-like and cauliflower-like hierarchical structure arrays, respectively. Both of the two hierarchical structures were composed of much thinner nanorods compared with the one obtained without capping agents. All the WO3films exhibited good photochromic properties and the two with inducers performed even better, which could be due to the loose porous structure and big exposed area. |
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