Nanostructured Arrays of Semiconducting Octahedral Molecular Sieves by Pulsed Laser Deposition

Date of Completion

January 2011

Keywords

Chemistry, Molecular|Engineering, Materials Science

Degree

Ph.D.

Abstract

This research work focused on developing new synthetic methods to produce thin films of mixed-valent octahedral molecular sieves with better control of their crystallographic orientation. This study paved the way to diverge from the lengthy liquid approaches that are currently being employed. The as-produced thin films show promising use in catalysis, sensors, and photovoltaics applications. Three different parts are included in this work. ^ The first part presents the novel formation of three-dimensional (3D) nanostructured arrays of cryptomelane-type manganese oxide (OMS-2) by heteroepitaxial growth on (001) single-crystal strontium titanate (STO) substrates. This study was done to prove the feasibility of producing thin films of microporous materials by pulsed laser deposition by means of self-assembly while preserving the crystallinity, porosity, and mixed-valency from target to film. The use of seeds or templates to direct the crystallization was avoided which increased scale-up and cost-effectiveness. ^ The second part of this research demonstrates a proof-of-concept of heteroepitaxial mechanisms of OMS-2 on other single-crystal strontium titanate substrates, such as (110)STO and (111)STO. This study proved that the crystal domains of OMS-2 can be directed in nanostructured arrays by selecting different orientations of single-crystal STO. Moreover, the electrical resistivity of the oriented OMS-2 thin films is well correlated with the direction of the tunnel and angle of the grain boundaries. These results promise applications in sensors and transistors. ^ The third part of this thesis involves the comprehensive characterization of OMS-2 materials. Characterization includes elemental chemical analysis in measurement of AOS and argon sorption studies in measurement of micropore distribution. The obtained information demonstrates that the K+ balances the mixed-valent octahedral framework (Mn4+ and Mn 3+) described by the cryptomelane-exchange. The sorption measurements with argon revealed a good correlation of the microporosity and surface area with potassium content. The understanding of the AOS and micro-structure of OMS-2 will facilitate the selection of specific OMS-2 materials for required applications. ^

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