Date of Completion

9-23-2013

Embargo Period

3-22-2014

Keywords

Nanomaterials, Energy

Major Advisor

Puxian Gao

Associate Advisor

S.Pamir Alpay

Associate Advisor

Mei Wei

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Because our limited fossil fuel reserves are being depleted while worldwide energy demands are increasing, an energy crisis has emerged. To mitigate the problem, one major approach is to design and discover new energy materials with superior cost-effectiveness and functionality. Along this line, this dissertation work presents a high-throughput, low temperature and solution-based, green synthesis and processing strategy to enable various nanostructured, metal and bi-metal stannates for future energy storage and harvesting applications.

Specifically, a generic green synthesis route was developed by applying a room temperature, solution-based process for nanostructured copper and copper-zinc hydroxystannate [CuSn(OH)6(CuHS) and CuxZn1-xSn(OH)6(CZHS)]. The hydroxystannate growth started from poorly crystalline, Sn-based aggregates as the intermediate growth building blocks in both metal and bi-metal cases. A complex, multi-stage self-assembly process was identified for the formation of hydroxystannate nanostructures in aqueous solution. The Cu2+: Zn2+ precursor ratio was found to be an effective parameter to tune the final nanocrystal composition and growth kinetics. The Cu: Zn substitution ratio was modified in the final products by tuning Cu2+: Zn2+ precursor ratios. A much faster growth rate was found with increasing Zn2+ content. These influences likely originated from the ionic activity and character difference of Cu2+ and Zn2+ in the solution. On the other hand, a cationic surfactant, CTAB, was successfully used toconstruct the final hydroxystannate nanocrystal assemblies with different morphology and porosity. Due to the amphiphilic nature of cationic head groups, CTAB-organized micelles in aqueous solution guided the unique self-assembly of mesoporous spherical CZHS nanocrystals by consuming the Sn-based intermediate aggregates.

A simple and mild post-thermal treatment of the hydroxystannates led to formation of a new class of amorphous and mesoporous CuSnO3 (a-CS) and CuxZn1-xSnO3 (CZS). These stannates displayed similar electrochemical reaction characteristics to those of crystalline SnO2-based Li ion battery anodes. Lithiation starting with reduction of stannates into freshly formed metal and Li2O, with SEI layer formation, would explain the irreversible capacity loss seen during the first couple of cycles. Cyclic voltammograms revealed an extra side reaction in the Cu-rich stannates at a higher potential than that of Li/Li+, which was likely due to the nanosized copper oxide. The extra reduction peak corresponding to reduction of ZnO was found in the Zn-rich stannates, suggesting extra electrochemical reactions after the phase segregation of the Zn-rich stannates.

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