Study Of The Biological Template Preparation Methods For Metal Oxide Fibers

Due to its remarkable merits of a simple procedure, mild templating conditions, etc., template method is widely used to prepare nano materials having special microstructures. Inspired by the miscellaneous forms of biologic species in nature, a novel process of biotemplating is developed to synthesize metal oxides with sophisticated morphological and structural ordering analogous to biologic species. Thus, a strategy of surface sol-gel and its derivative methods is extensively studied as a potentially simple but efficient way to replicate the biotemplate with oxides in multi-scale levels. However, as a result of the high reactivity of the metal oxide precursors, i.e., metal alkoxides in general, and the double role of water as a ligand and a solvent, the replication of the biotemplate is very sensitive to the slight change in experimental conditions. Moreover, besides the utilization of the expensive and commercially limited alkoxides, the commonly negatively charged biological materials under moderate conditions are not ideal for coating the negatively charged sol solutions. Thus, facile and precise replication of the biotemplates with metal oxides is still a challenge although the nano-precision replication of cellulosic substances by the titania and the zirconia via a complex surface sol-gel procedure has been achieved.To overcome the shortcomings of the surface sol-gel and its derivative methods, a facile, versatile, and easily controlled two-step surface precipitation approach using an inorganic metal salt as a precursor is developed to faithfully replicate micro-fibrous oxides from biologic fibers composed of keratin in a nano scale. By taking the advantage of the wealthy functional groups of C=O and N-H in the keratin protein chain, metal cations can be selectively adsorbed via weak intermolecular forces, and coordinated and/or chelated when the fibrous keratin was immersed in the aqueous solution of metal cations. Moreover, the hydrogen bonding inside the bundles of the protein chains prevents the penetrating of metal cations to the interior of the keratin fiber, leading to the copy of the fiber surface with metal cations in a molecular level. To preserve or fix the surface copied with metal cations, a solvent having lower solvation ability to the metal cation than water (e.g., ethyl acetate, EA) is used as a medium to on-site precipitate the metal cations with an alkali. After burning the template, hollow fibrous metal oxide precisely recording the morphologic surface of the original template is obtained. The developed method can be easily extended to other biotemplates with different functional groups such cellulose having hydroxyl groups. The main works and conclusion of this thesis are as follows. (1) Alumina replicas were obtained by the two-step surface precipitation approach using keratin fiber (chicken feather) as template and Al(NO3)3 as precursor, respectively. The effect of solvents, precipitation cycles, and calcination temperatures on the stucture and morphology of the replicated Al2O3 was extensively investigated with SEM/EDS, FESEM, HRTEM, FT-IR, XRD, and N2 adsorption/desorption techniques. The templating mechanism of the developed method was revealed based on the characterization results. Under optimized experimental parameters, the nano-precision replicated alumina fibers with chicken feather were obtained. The procedure is readily applicable to prepare other metal oxides by simply changing the cationic precursors, and the fibrous oxides of CeO2, CO3O4, NiO, and SnO2 were precisely copied from the chicken feather.(2) Alumina replicas with a special scaly texture of the hair surface, were obtained by the two-step surface precipitation approach using hair as a template. The versatility of the two-step surface precipitation approach was demonstrated as a method for the production of various metal oxide fibers with special biological morphologies.(3) To extend the applicability of the developed methods, cotton fibers having abundant hydroxyl groups were used as a template. Under optimazed conditions, fibrous Al2O3, SnO2, ZrO2 were successfully replicated in a nano scale.(4) Compared with the sol-gel and its derived methods, the thus developed approach is simple, practical, low cost, and green. In addition, enhanced properties in catalysis, sensor, and other electronic devices are expectable based o the special fibrous structure of the replicated metal oxides.