On Synthesis And Properties Of Oxides With Biogenic Hierarchy Using Plant Leaves As Templates And Biogenic Nitrogen-doped Ceria Using Biomass As Nitrogen Source-Toward Superior Materials And Greener Chemistry

Nature is a mega-lab producing immense amounts of materials with intriguing structures. Especially, hierarchical structures combine merits of different size-scales and enable the organisms to be equipped with multi-functions in highly integrated manners. To human, hierarchical structures carry great technological as well as industrial significances because they can promote material performances in applications, like catalysis, fuel cells, gas sensors, light-harvesting, separation and filtration, etc. Though hierarchical structures are daily synthesized with ease by natural organisms under mild conditions, the fabrication of artificial structures with comparable hierarchy and intricacy is a great challenge. Moreover, many functional materials that we go after, like oxides, nitrides, carbides, noble metals, and most inorganic composites, are not synthesized naturally by the organisms.Biomorphic mineralization rises as a bio-inspired strategy for the facile fabrication of materials with biogenic morph-structures by mineralizing natural biological structures. This approach is environment-benign for dispensing with artificial templates or additives that are generally employed for producing artificial structures. Biomorphic mineralization processes are simple and low-cost, because many natural materials readily exist in abundance and are easy to harvest. Biomorphic mineralization has the chemical flexibility as well as morph-replication efficiency for producing diverse materials with complex biogenic structures. This is particularly attractive because it facilitates the study of structural-property relationship in materials and inspires future materials design.Besides morph-inheritance, biomorphic mineralization also implicates the utilization of biogenic elements. A bunch of studies have shown that biogenic elements can be preserved to contribute to the final products. For example, carbonaceous biomass, like wood tissue, cellulose fibers, bamboo, can be precursors to biomorphic carbon, carbides and carbide-based composites; siliciferous biomass like diatoms, diatomaceous earth, and plant tissues (arvense equisetum leaves and stems) can be silicon sources for growing zeolites or forming biomorphic silicates and silicon. Nitrogen is an effective dopant for modifying metal oxides, but conventional nitrogen-doping methods involve the use of N-containing chemicals, which are toxic to human and pose hazard to the environment, therefore a green nitrogen-doping route dispensing with those toxic chemicals is desired. Although nitrogen is a universal element in living things, the utilization of biogenic nitrogen in biomass for doping oxides is seldom investigated.This work contains two parts. Part I is on the synthesis and property of biomorphic oxides with structural hierarchy inherited from plant leaves. Plant leaves were chosen as the bio-template based on their natural abundance, morphological diversity, as well as low-cost. Biomorphic mineralization of plant leaves was carried out with oxides of distinctly different properties, so as to provide information about the effect of biogenic hierarchical structures on different material performances and to evaluate the potential of hierarchically-structured materials for respective applications. For ceria, a well-known material for catalysis, fuel cells, gas sensing, etc, emphasis is placed on its oxygen activity and oxygen storage capability examined with temperature-programmed techniques. For Eu(III)-doped CaO-SiO2, a phosphor system, light-harvesting and photoluminescence were examined with optical absorbance and photoluminescence measurements. For characterization of morphology and texture, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption-desorption measurements, were adopted. Characterizations of phases and composition were carried out with X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), etc.Part II reported the attempt to develop a green chemical route for producing nitrogen-doped oxides with biomass as the biogenic nitrogen source. Two types of protein-rich biomass -- rice (fruit of Oryza sativa) and soybeans (fruit of Giycine max(L)Merrill) were investigated. Ceria was chosen as the pilot oxide based on following considerations. Ceria is a versatile material of great importance to environment- and energy-related applications, however efforts to improve ceria concentrate on metal-doping whereas nitrogen-doped ceria is seldom investigated. On the other hand, oxygen defects, which strongly influence ceria performances, can be modified with the introduction of exotic elements, so the effect of N-doping on oxygen defects in ceria is of special interest. In this study, XPS was employed for detecting nitrogen levels in rice- and soybean-derived cerias, and possible chemical environments of the nitrogen in biomass-derived cerias were discussed. Oxygen defects were studied through Raman measurements as well as analysis of the high-resolution O1s peak. Oxygen activity and oxygen storage capability of biomass-derived cerias were examined with temperature-programmed techniques. Basic characterizations, including XRD and TEM imaging, were also carried out.