Precursor-based Synthesis Of Colloidal Porous Materials And Their Applications

The porous materials, as a new class of structural materials, have received considerable attention since1990s, due to their large specific surface area, and good uniformity through strength, light weight, good sound and heat insulation, permeability, etc. These porous materials possess a lot of promising properties, which are different from their solid counterparts. The emergence of nanomaterials as well as the development of nano techno logy provides a new opportunity for the development and application of porous materials. Compared with the conventional bulk porous materials synthesized by templates, nanoscaled porous materials have better dispersibility. They are emerging as promising materials for broad applications in enzyme immobilization, adsorption, energy storage, drug delievery, CO2capture and catalysis, and can also serve as fundamental building blocks for complex structures. As supports for catalysts, monodisperse porous spheres inherit the merits of the large specific surface area of their large bulk counterparts and high dispersibility of tiny nanoscale particles in a liquid medium. Therefore, they will be able to well mediate the conflicts between short diffusion path, high catalytic activity, and facile recyclable separation after use. Furthermore, the specific pore structures in porous spheres have been used to immobilize functional molecules and nanoparticles to tailor the material properties or as templates for the synthesis of namomaterials replicas. In this regards, extensive research efforts have focused on the synthesis of diverse porous spheres during the past few years. In particular, research in the synthesis of porous nonsiliceous spheres has seen tremendous growth, which extensively broadened the scope of porous sphere materials. However, development of a low-cost, simple synthesis process for porous nanomaterials is still a great challenge in present study. In this dissertation, we explore sveral precursor-based synthetic routes for the controllable preparation of porous nanomaterials prepared with different morphologies. More details ae summarized below:(1) In chapter2, we have devised in this work a general synthetic strategy for preparation of single-and multicomponent rare-earth coordination polymer colloidal spheres (RE-CPCSs). This strategy is based on an integration of coordination chemistry and antisolvent effect for synchronized precipitation. Highly monodisperse RE-CPCSs with homogeneous mixing of RE elements, which are not readily attainable by any existing methods, have been successfully prepared for the first time. In addition, the type and molar ratio of these colloidal spheres can be adjusted easily in accordance to the variety and dosage of precursor salts. The molar ratio of RE elements in as-prepared colloidal spheres shows a linear relationship to that of starting reactants. Furthermore, the RE based core/shell colloidal spheres can be facilely prepared by introducing other metal salts (beyond lanthanide elements) owing to their different coordination chemistry and precipitation behavior. By adjusting concentrations of the ionic activators, luminescent properties can be tuned accordingly. Furthermore, the RE-CPCSs can be transformed to monodisperse lanthanide oxide spheres via simple heat treatment. We believe that the present synthetic strategy provides a viable route to prepare other lanthanide containing colloidal spheres that have enormous potential for future applications as optoelectronic devices, catalysts, gas sensors and solar cells.(2) In chapter3, we demonstrate a novel indirect synthetic route to prepare monodisperse porous multicomponent rare earth (RE) based colloidal sphere with uniform size and tunable compositions using RE-CPCSs as precursors. For the multicomponent system, the essential differences of lanthanides in solubility product constant (Ksp), saturated concentration and growth rate, make it almost impossible to synthesize multicomponent RE-based colloidal sphere with controlled morphology and porosity. Generally, the Ksp value of a lanthanide precipitation decreases with decreased ionic radius of the RE ions, following the lanthanide contraction law. Since the colloidal spheres are formed via nucle at ion/growth processes, nucleation can only takes place when the degree of supersaturation (S, given below) reaches the critical supersaturation. It thus can be inferred from that stable nuclei of RE ions with small ionic radii are easier to be formed compared to those with larger ionic radii. This conclusion has also been demonstrated by Matijevic’s experimental results, in which the light lanthanides tend to precipitate in a way different from those heavier ones in terms of morphology of the resultant particles during the direct precipitation process. However, the indirect synthetic strategy is based on the integration of coordination chemistry precipitation of RE ions and subsequent ion-exchange process so as to steer clear of obstacle from differences in solubility product constant.(3) In chapter4, we develop a low-cost reaction protocol to synthesize highly effective mesoporous Nb2O5-based solid acid catalysts with additional morphological control. In the synthesis, the monodisperse glycolated niobium oxide spheres (GNOS) were prepared by a simple antisolvent precipitation approach, and subsequently converted to mesoporous niobium oxide spheres (MNOS) with large surface area of312m2·g-1by hydrothermal treatment. The obtained mesoporous MNOS were further functionalized with sulfate anions at different temperatures or incorporated with tungstophosphoric acid (TPA) to obtain recyclable solid acid catalysts. The antisolvent acetone used in obtaining GNOS can be completely recovered at a high purity. Our MNOS-based catalysts have shown remarkable performance in a wide range of acid-catalyzed reactions such as Friedel-Crafts alkylation, esterification and hydrolysis of acetates. Because they are monodisperse spheres with diameters in submicrometer regime, the catalysts can be easily separated and reused.(4) In chapter5, we report a polyo1process for controlled growth of cobalt carbonate (COCO3). A preparative investigation on morphogenesis of COCO3crystals has been carried out, and three types of highly uniform COCO3products (i.e., peanut-like, capsule-like, and rhombus crystals) in the submicrometer region have been synthesized at200℃under batch conditions. Uniform particles of tricobalt tetroxide (CO3O4) have also been obtained at300℃from the COCO3submicrometer crystals after thermal transformation in laboratory air. The CO3O4powder products possess mesoporosity and essentially preserve the pristine morphologies of their respective solid precursors. Due to high crystal uniformity and specific surface areas in the range of140-149m2/g, the mesoporous CO3O4exhibits enhanced performances for ethanol and carbon monoxide sensing.(5) In chapter6, transition metal-based coordination polymer nanowires were synthesized using nitrilotriacetic acid (NA) as a chelating agent by a one-step hydrothermal approach. In the synthesis, transition metal ions were bonded with amino or carboxyl groups of NA to form one-dimension polymer nanowires, which can be confirmed by FTIR and TGA results. Our experimental results show that the morphologies of polymer nanowires greatly depend on the precursor salts, ratios between deionized water and isopropyl acohol. The probable molecular formula and growth mechanism have been proposed. After heat treatment, the cobalt ion-based coordination polymer nanowires can be converted into porous transition metal oxide nanowires, which completely preserved the nanowire-like morphology. When used as anodes in lithium-ion batteries, the obtained porous nanowires exhibited a high reversible capacity of810mAhg-1and stable cyclic retention at30th cycle. The good electrochemical performance could be attributed to the porous nanostructure, which provides pathways for easy accessibility of electrolytes and fast transportation of lithium ions.