Date

2019

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

McIntosh, Steven

Abstract

Green synthesis methods of inorganic materials have gained lots of attention due to lower pollution and toxic precursors during the synthesis. One of the modern green methods of synthesis is biosynthesis in which a DNA, small peptide molecule, or amino acids plays the main role of synthesis. Biomineralization, the biosynthesis process happens in the nature by elements, has been considered as a green, low cost, and scalable method of synthesis. In comparison with conventional synthesis methods, biomineralization happens under ambient conditions in aqueous phase that can be applied for synthesis of inorganic nanostructure metal sulfide semiconductors gained interest due to their remarkable physical and chemical properties such as electronic, magnetic, and optical1. Their conventional synthesis methods of semiconductors are cost effective nowadays; however, Environmental-friendly biosynthesis is going to be a potential replacement in near future. Biomineralization of semiconductor nanocrystals (Quantum Dots) offers a low cost and greener approach of semiconductors under ambient conditions in aqueous phase in which a single enzyme plays the role of catalyzing the reaction and templating the nanostructure of quantum dots.To begin, we showed the templating role of a single protein in biosynthesis of CaCO3 crystallization. In nature, various crystal structure of CaCO3 formation happen by living organisms such as sea shells, snails, and corals. In this study, a single well-known protein, Silicatein α, has been utilized to be responsible for templating the crystal structure of CaCO3 mineralization at ambient conditions in aqueous phase. Templating vaterite and aragonite crystal structure, confirmed by XRD and SEM, in the presence of Silicatein showed a change in state level of CaCO3 crystal structure from calcite to vaterite and aragonite. Also, the influence of protein concentration on aragonite crystallization has been studied in this work that confirms the role of Silicatein in CaCO3 crystallization.Furthermore, we have focused on both roles of single enzyme in catalyzing and templating in biomineralization process. In this study, an engineered enzyme called cystathionine γ-lyase (CSE) has been playing two roles in biomineralization of various metal sulfides. First, CSE can turn over L-cysteine to generate reactive source of sulfur, H2S, in solution continuously. Second, CSE plays the role of templating the nanocrystal growth of metal sulfides.Single enzyme direct biomineralization of nontoxic metal chalcogenide such as ZnS by utilizing CSE enzyme was reported in this work. Synthesized ZnS, Zn1-xCdxS alloy, and ZnS-Zn1-xCdxS core-shell showed considerable optical properties such as great absorption and fluorescence lead to 97.6 ns increase in decay time confirmed by life time decay photoluminescent. Also, narrow size distribution, well crystallinity, nanomaterial composition and shape of QDs were characterized by HRTEM, HAADF, and EDAX. Biosynthesized method lead to achieve 7% enhancement in quantum yield.Also, we have synthesized SnS and Cu2ZnSnS4 alloy by utilizing CSE single enzyme in the presence of L-cysteine as a source of sulfur. Biosynthesized SnS nanocrystals and its relative compound, confirmed by XRD and HAADF, showed great photocatalytic property regard to the potential application in quantum dots-sensitized solar cell (QDSCs). SnS and CZTS have been applied as an absorber layer by in-situ growth on TiO2-paste as a working electrode in the presence of polysulfide electrolyte and Cu2S cathode. We have presented the uniform penetration of QDs into TiO2-paste by in-situ growth, confirmed by SEM-EDAX line scanning, that improved solar cell characterization parameters such as Voc, Jsc, and FF to 5.5 V, 3.1 mA/cm^2 , and 61%, respectively.

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