Synthesis And Characterization Of Micro/nanomaterials Of Oxide And Oxysalt Of Chromium, Molybdenum And Tungsten

The synthesis and design for the nanomaterials, as forerunner of the material science, are the base of the application and the future development of nanoscience and nanotechnology. Up to now, it is still important task in the field of materials that how to develop a new method for preparing nanomaterials. Although there are many methods reported for preparing nanomaterials, it is still difficult to obtain materials with controllable morphologies and sizes. Therefore, it is attracting a great deal of attention of the chemists and materials researchers to explore a new method for obtaining low-cost mild reaction.In this dissertation, we focused on the synthesis and characterization of micro/nanomaterials of oxide and oxysalt of chromium, molybdenum and tungsten. We used precursor-calcined technique, catanionic reverse micelles method, soft-model technique and microemulsion method to get structure, morphology and size-controlled materials. Main points are summarized as follows:1. Precursor-calcined technique was developed to synthesize inorganic nanoscale materials. Organic alkali hydrolyzed slowly in H₂O and C₂H₅OH or the other mixed solvent. We got the precursor Cr(OH)₃ from the reaction of chromium salt and organic alkali in the above system. Then the precursor was calcined in the muffle at 600℃for 3 hours, and the final products Cr₂O₃ was obtained. Using different chromium salt, we got sphere-like precursor. The morphology of the precursor was almost the same in different solvent system. The kinds of organic alkali had the most important influence on the morphology and size of the precursor and product.2. We selectively synthesized two kinds of chromate. (a) Single-crystalline Pb₂CrO₅ with nanorod-shape has been synthesized by adjusting the pH value of the catanionic reverse micelles formed by a cationic surfactant CTAB and an anionic surfactant SDS, followed by a hydrothermal process. Discussion and analysis indicate that the kinds of the surfactants, the molar ratio (r) between the mixed cationic and anionic surfactants, reaction time and temperature play important roles in the crystallization and morphologies of Pb₂CrO₅ nanocrystals. Such catanionic reverse micelles systems may present promising media for the solution synthesis of one-dimensional inorganic nanostructures. (b) On the basis of relative references, we tried a facile, effective and new micelle system to synthesize micro/nanomaterials. In the new micelle system of PVP/H₂O/ cyclohexanol, BaCrO₄ micro/nanomaterials were synthesized practically in the low temperature. In comparison with the traditional microemulsion in which alkyl hydrocarbon was the oil phase, the new micelle system exhibited some particularity. Cyclohexanol as the oil phase can afford a stable and homogeneous reaction environments. And we could get products with uniform size and well-proportioned distribution. There was no need of extreme temperature and press in the system, and the equipment was simple and easy-controlled.3. We synthesized BaMoO₄ micro/nanomaterials through two kinds of different method, (a) Single-crystalline BaMoO₄ microcrystals with 3D bipyramidal-like architectures were synthesised by adjusting the pH value of the catanionic reverse micelles formed by a cationic surfactant CTAB (hexadecyltrimethylammonium bromide) and an anionic surfactant SDS (sodium dodecyl sulfonate), followed by a hydrothermal process. It was found that the external conditions such as the molar ratio of the two kinds of surfactants, reaction time, and temperature had no crucial influence on the crystallisation and the shape evolution of the products. The internal properties of the crystals were the real reason for the typical morphology. The coexistence of the dual surfactants and the cooperation of oriented attachment and Ostwald ripening played important roles in the formation of the crystals. (b) Large-scale high-quality BaMoO₄ nanocrystals have been synthesized in aqueous solutions under mild conditions with citrate as a simple additive. The crystals have dumbbell-like, spindle-like and wheatear-like morphologies assembled from nanoparticles, nanofibers. The results showed experimental parameters had great influences on the shape evolution of products. The adjustment of these parameters such as room temperature stirring time, reaction temperature and reaction time of hydrothermal reaction, can lead to obvious morphology changes of products, and the growth mechanism has been proposed. In addition, the optical properties of the BaMoO₄ products had been examined by PL spectrum. Furthermore, the feasibility and ease of this synthesis route are worth exploring for other inorganic systems.4. An additive-assisted solution-phase approach was developed to prepare SrWO₄ micro/nanomaterials. (a) We report a novel shape evolution of SrWO₄ crystals from 0-D microspheres to 1-D rods or belts and to 0-D shuttle-like particles with prolonged reactiontime and special reaction conditions. In contrast to the traditional crystal growth from 0-D to 1-D to 3-D, in this case, 0-D microparticles were easily obtained within a short reaction time, but it was different with the final products. Detailed experimental results revealed that the addition of the citrate and the hydrothermal reaction conditions and reaction time controlled the shape evolution of the SrWO₄ crystals. (b) We used CTAB as a model to synthesize SrWO₄ microspheres in the solution. In addition, the optical properties of the SrWO₄ products have been examined by PL spectrum. This method can be extended to other materials to purposefully prepare 0-D and 1-D structures.