Hydrothermal Synthesis And Properties Of Niobate Micro/Nanomaterials

In recent years, a series of niobates semiconductor materials have attracted widespread attention for their high photocatalytic activities and good luminescence properties, as well as the tremendous application prospect in the field of microwave dielectric ceramics. However, most of niobates are synthesized by traditional solid-state reaction (SSR) method. Their practical applications are limited due to the cumbersome synthesis conditions and the lower active surface areas. In this thesis, after improving the activity of Nb₂O₅, we investigated the synthesis of columbite niobates in the mild hydrothermal conditions, systematically. The major research work is as follows:1. Hydrothermal synthesis and photocatalytic properties of CaNb₂O₆ hierarchical micro/nanostructure.Hierarchical micro/nanostructures, which are assembled from zero-dimensional (0D), 1D, and 2D micro/nanoscale building blocks, have attracted significant attention due to their unique physical and chemical properties. Research on hierarchical structures provides potential applications in the bottom-up fabrication of advanced functional devices, including sensors, fuel cells, and photocatalysts. The photocatalytic superiority of the hierarchical micro/nanostructures can be attributed to their special structural features. The special structural features can prevent aggregation to maintain a large active surface area and enhance the charge-transfer rates in materials. In this article, CaNb₂O₆ hierarchical micro/nanostructure was successfully fabricated under mild hydrothermal conditions in the absence of templates or organic additives. In this hierarchical micro/nanostructure, the nanosheets grow vertically on the surface of the one-dimensional microneedles. At the same time, selective synthesis of CaNb₂O₆ and Ca₂Nb₂O₇ could also be facilely achieved only by adjusting pH value. We investigated the effect of the reaction temperature, time and concentration on the growth of hierarchical structure, in order to explore the mechanism of its formation. We believe that the hierarchical structure is formed in a two step nucleation–growth process, at appropriate supersaturation and temperature conditions. Our result may provide a new route for the construction of a new hierarchical structure from a multistep nucleation–growth process dependent on the degree of supersaturation. The hierarchical structured CaNb₂O₆ exhibits a stronger enhancement of photocatalytic activity in the degradation of rhodamine B (RhB) than other CaNb₂O₆ samples with different morphologies, for the unique structural features of the CaNb₂O₆ hierarchical micro/nanostructure could efficiently promote the separation of photo-generated electron-hole pairs.2. Hydrothermal synthesis of transition-metal niobates.Based on the work of previous chapter, we synthesized the transition metal ions columbite niobates using low-cost hydrothermal method. After changing a variety of reaction conditions, such as reaction time, temperature and pH value, we got the suitable reaction conditions required for the synthesis of niobates. Ultimately, MnNb₂O₆, CdNb₂O₆, ZnNb₂O₆ and Pb₂Nb₂O₇ were synthesized under the low-temperature hydrothermal system. Compared with the traditional high temperature solid state reaction method, hydrothermal synthesis reactions are performed at lower temperature and samples with good purity and crystallinity could be obtained. The samples synthesized in the hydrothermal reaction usually showed regular morphologies with smaller size and good dispersion property. We also studied the properties of these niobates products, and found that the MnNb₂O₆ product presents paramagnetism and the Pb₂Nb₂O₇ product has good photocatalytic activity. Additionally, we have synthesized the InVO4 self-assembled microspheres in the hydrothermal conditions without templates.3. The controllable synthesis of binary and ternary sulfide nanocrystals.We synthesized the binary and ternary sulfide nanocrystals by hot injection and solvothermal method. Monodispersed PbS and CuInS2 nanocrystals with different morphologies and controllable sizes could be obtained by changing the reaction conditions, such as the concentration of ligand, reaction time and temperature.