Access Type

Open Access Dissertation

Date of Award

January 2022

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Long Luo

Abstract

Quantum dots (QDs) are attractive because of their unique size-dependent optical and electronic properties and high surface area. They are tested in research for diverse applications, including energy conversion, catalysis, and sensing. Assembling QDs into functional solid-state devices while preserving their attractive properties is a challenge. Methods currently under the research are not effective in directly fabricating QDs onto devices, making large area assemblies, maintaining the high surface area by forming 3D porous structures, and conducting electricity for applications such as sensing. QD gels are an example of QD assemblies that consist of a 3D porous interconnected QD network. They are ideal candidates for gas sensing due to two main reasons. First, their extremely high surface area and the accessibility through porous openings provide a large number of interactions per unit volume of the material. Second, the partial removal of surface ligands during the gelation increases the active sites and, therefore, the number of signals generated compared to QDs covered by ligands. Preparation of QD gels was conventionally carried out by directly adding oxidizing agents to a stable QD dispersion.9 Dimensions and shapes of chemical gels are defined by a mold, so it does not allow the flexibility to introduce fine detail to the QD gel form. Using chemical gels in applications such as sensing or catalysis often requires a gel deposition on an electrode by following a technique such as drop-casting. This dissertation aims to develop and understand the electrochemical techniques to assemble QDs into porous networks. QD assemblies are prepared using two new methods: oxidative electrochemical gelation (OE-gelation) and metal-mediated electrogelation (ME-gelation). Material properties, mechanisms, and applications of the two gelation techniques are studied in detail. OE-gelation is the first use of electrochemical techniques for QD gel synthesis. This technique offers the ability to produce QD gels within minutes and tunable gelation by selecting different electrochemical parameters. The kinetics and thermodynamics studies have revealed that the electrogelation of metal chalcogenide QDs proceeds via a two-step mechanism: the electrochemical removal of the organic capping agents followed by the oxidative dichalcogenide bonds formation. More interestingly, we have found that the gelation process is reversible by applying a negative potential to reduce the dichalcogenide bonds that connect the QD in the gel network. The facile electrogelation of QDs significantly simplifies the preparation of gel-based sensors. We demonstrated the one-step fabrication of CdS xerogel sensors for NO2 gas sensing. The resulting CdS xerogel sensors exhibit an outstanding performance toward NO2 gas sensing at room temperature. While the OE-gelation technique offers unique advantages for applications such as gas sensing, OE-gels are unstable in reducing conditions as their disulfide bonds can go back to sulfide form. Therefore, electrochemistry was used to control the in-situ generation of metal ions, making QD ME-gels via metal-ligand bonding. ME-gelation was demonstrated using Co, Zn, and Cu electrodes. TEM images and inductively coupled plasma mass spectrometer (ICP-MS) analysis revealed that QDs in a ME-gel are connected via QD-metal ion-QD bonding. Gel growth was controlled by the charge employed in the oxidation of metal electrodes. The optical and physical properties of the formed gels were characterized, which confirmed the formation of quantum confined macroscale 3-D architectures of QDs. The use of the ME-gelation method in fast QD pattering was demonstrated using printed circuit board electrodes. Finally, a competition between ME- and OE-gelations was studied to understand the interplay of the two electrogelation methods. Elemental compositions and the microscopic connectivity in produced gels suggested the OE-gelation dominates the QD assembly. A byproduct formed during the OE-gelation blocked the electrode surface from further producing ME-gels.

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