Synthesis Of Photochromic Tungsten Trioxide Particles By Self-induced Method

The material with hierarchical structure is one of the most challenging areas of research in nanotechnology today. The hierarchical structure means:some substances in nanometer scale are the units of the assembly, and then these assembled unit structures build into a novel three-dimensional super structure of the new structure or system along some regularity. These novel assembly hierarchical structures can provide more active sites for the material matrix itself to react with the reaction medium, which can effectively improve the absorption capacity of the material to the reaction medium (e.g., light, gas, etc.), at the same time due to the complex hierarchical structure assembly, the hierarchical structure materials have a strong trapping and locking effect on the reaction medium, and thus their absorption rate of the reaction medium is also high, which may results in the corresponding significantly improved performance of the obtained materials. In addition, the hierarchical structure owns another prominent advantage:their ordered assembling patterns usually make the material having a large number of effective channels and pore structure. Based on this character, the the transmission of charge carriers (e.g., electron, proton, and so on) in the material is more convenient, and accordingly, hierarchical structure materials can often exhibit more active electrical, optical and magnetic properties. Therefore, three-dimensional super-structure has an important research value in the reinforcement performance of materials. So far, there are two main directions to prepare the hierarchical structure materials; one is the "top-down" approach:the large-scale substances are carved into desired hierarchical structure morphologies, that is, the traditional etching technology. The other is the opposite, the small-scale assembly units are accumulated into three-dimensional complex structures from the "bottom-up" approach. Especially the self-assembly technology, which has become the most popular method of preparing the material of the hierarchical structure because of its efficient, flexible, controllable, etc advantages. Such as:the sol-gel method, chemical vapor deposition method, chemical vapor deposition method, hydrothermal or solvothermal method.Photochromism refers to the change of a material’s color, i.e. coloring under the radiation of excitation light, and the inverse process, i.e. the bleaching process can also occur under heating in the dark. This process is reversible under some certain external conditions. The photochromic materials have broad application prospect in many areas, including the reusable information storage, data display, an optical signal processor, smart windows, mirrors, and military camouflage, due to its flexible color change and the photo-sensitivity. Photochromic materials contain organic materials and inorganic materials. For a long time, a lot of research is mainly focused on organic photochromic materials, and then inorganic materials followed. Inorganic photochromic material has good thermal stability, fatigue resistance, and easy to control the morphology etc., which quickly attracted the attention of researchers, become research focus in the field of photochromic materials. After the pioneering work of Deb et al, the inorganic transition metal oxides and inorganic polyoxometallates, especially MoO3and WO3have drawn large attention of researchers.WO3belongs to the inorganic transition metal oxides photochromic materials, and is also a typical n-type semiconductor material. Its crystal structure is substantially connected with [WO6] octahedron by the angle of oxygen, thereby extending the three-dimensional network structure. The outer electron configuration of element W is complex, which results in that the compound containing W, the W has+2,+3,+4,+5,+6five valence state. In addition, except for single valence state, there are often several valence states coexist. It is well known that the color of substance is closely related to its micro-electronic structure. Accordingly, it is the complex and diverse electronic structure of WO3that the WO3can display a variety of color.WO3powder is typical inorganic photochromic material. Owning to its excellent photochromic properties, WO3powder shows potential applications in many types of storage, display, signal processing and other fields. Thereby, in recent years, WO3powder materials leap into the view of researchers, and some contributions and efforts have focused on the photochromic WO3powder. However, up to now, there is rarely study about the application of the photochromic WO3powder. Because, it is still a challenge that to prepare WO3powder materials with good photochromic properties. In order to broaden the WO3materials’ application in the reality, some certain preparation methods and performance optimization approaches should be adopted to synthesize WO3powder material with excellent photochromic property.As we all kown, many fundamental properties of materials often subject to the relative microscopic structure and morphology. Therefore, the controlled synthesis of WO3powder particles with good morphology will play an important role in the improvement of the corresponding photochromic properties. The hierarchical assembled structure of material can effectively optimize the optical, magnetic, electrical and other aspects of performances. As a result, synthesizing the ordered hierarchical WO3powder material is expected to significantly improve the photochromic performance, and increasing the range of applications of WO3photochromic device.In this paper, WO3powder particles were firstly synthesized by introducing ethanol as dispersants via typical hydrothermal method. From the results of XRD, SEM and TEM, it shows that the as-prepared products are hexagonal phase WO3, and the particles reveal spherical structure. It is obviously that the addition of ethanol significantly improved the dispersion of the WO3particles. When the volume ratio of ethanol and water was40%, the dispersion of the WO3microsphere is optimization. Ethanol is adsorbed on the WO3particle surface, which greatly reduces the polarity and the electrostatic attraction of particle surface. As a result, the dispersion of the particles is enhanced. On this basis, the pre-prepared well-dispersed WO3powder was added to the precursor solution as seeds, and the optimal volume of ethanol was used as dispersing agent. Ultimately the six-fold hierarchical WO3powder particles were synthesized by self-induced method combining with the same hydrothermal technique. XRD, SEM, TEM, HRTEM and SAED results show that the six-fold WO3hierarchical particles are the hexagonal phase, and assembled by single crystal nanorods. The analysis of BET nitrogen adsorption and desorption curve suggested that the hexagonal hierarchical structure of the WO3particles contain a large number of slit-pore structure, the color difference test results of the samples prove that compared to spherical WO3particles obtained without seed, the hexagonal symmetric WO3hierarchical structure exhibit a better photochromic activity, and the higher the regularity of hexagonal WO3particles, the better the corresponding photochromic properties. There are a large number of active positions on the micro-powder seeds surface, and the new generated WO3crystal belongs to the hexagonal system, which having inherent hexagonal symmetry. It is proposed that the two effects resulted in the formation of six-fold symmetrical WO3particles. The six-fold WO3hierarchical particles are regularly assembled by single crystal nanorods and contain a number of slit pore structure, which is conducive to the transmission of photo-generated charge carriers, and thus results in a better photochromic activity.