Functional manganese oxide, silicon, and carbon materials: Synthesis, characterization, and applications

Date of Completion

January 2007

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

This research consists of two parts. The objective of the first part is to develop multifunctional manganese oxide nanomaterials with controlled shapes and crystalline structures. The second part is to investigate the growth of silicon nanostructures and carbon films on different substrates via a chemical vapor deposition (CVD) method. ^ Manganese oxide materials are a class of microporous transition metal oxides. They can form mixed-valent octahedral molecular sieves (OMS), which have porous structures, open tunnels, and high surface areas. The OMS materials can be applied extensively in catalysis, battery electrodes, and ion-sieves due to their distinctive properties. Because different shapes and crystallographic forms are generally believed to be responsible for their widely varying electrical and optical properties, fabricating transition metal oxide architectures with controlled structures and morphologies is highly desirable. Therefore, the objective of Part I is to synthesize manganese oxides with controlled shapes and crystal structures. Various types of manganese oxide nanostructures with tunable shapes, Na-OMS-2 (2x2 tunnel), K-OMS-2, Rb-OMS-2 (2x2 tunnel), pyrolusite (1x1 tunnel) and γ-MnO2 (an intergrowth of 1x1 and 1x2 tunnel), have been synthesized using alkaline dichromates as mild oxidants. ^ Polymorphs of Mn2O3 are known to be cheap, environmentally friendly catalysts for removing carbon monoxide and nitrogen oxide from waste gases. In section II of Part I of this research, a solvothermal technique is designed to selectively synthesize Mn2O3 nanomaterials with tunable nanostructures with controlled morphologies. By varying solvents, novel octahedral, truncated-octahedral and cubotahedral Mn2O 3 have been formed. In cyclohexane and benzene, gamma MnO2 has been obtained. Using this solvothermal technique, other transition metal oxides and metals, such as zinc oxides, iron oxides and Ag, can be prepared from the nitrate precursors as well. ^ Part II of this research is to study the formation of silicon nanomaterials and carbon films on different substrates for lithium ion batteries. Via a chemical vapor deposition (CVD) process, carbon films are grown on metal substrates including iron, nickel, and copper foils, while silicon nanostructures are prepared on graphite substrates including nanospheres on graphite rods and nanowires (SiNWs) on mesophase carbon microbean (MCMB) powder. The thickness of the carbon films can be controlled by changing the experimental factors, such as temperature, reaction time and reaction substrates. The SiNWs have a crystalline core with a very thin amorphous SiOx sheath (2-3 nm). The SiNWs obtained are homogeneous with average diameters below 50 run and lengths up to micrometers. Temperature, reaction time, and substrate effects on the growth of SiNWs are systematically studied. Higher reaction temperatures and longer reaction times result in larger diameters and higher yields of SiNWs. SiNWs with a better crystallinity can be obtained at higher temperatures and longer reaction times. Electrochemical performance of the SiNW-coated MCMB powder exhibits a potential to improve that of pure MCMB anodes. ^

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