Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemistry

Major Professor

Shengqian Ma, Ph.D.

Committee Member

Jing Wang, Ph.D.

Committee Member

Xiaodong Shi, Ph.D.

Committee Member

Julie Harmon, Ph.D.

Keywords

Metal-Organic Frameworks, Direct Conversion, Sensor, Optical Fiber

Abstract

Chemical sensor is working as a widely used device which can be applied to the detection of specific chemicals that are existing in the environment especially in gas phase. The detection of combustible and toxic chemicals can be extremely important in the field of both industrial and civil activities. The chemical sensor is commonly operating by utilizing a chemical or physical interaction between the specific chemical compound and the sensing functional unit, to obtain an electronic signal caused by the property change and realize the chemical detection. Traditional chemical gas sensors such as catalytic gas sensor, thermal conductivity gas sensor, optical gas sensor and others all share different principles together with distinct advantages and disadvantages. To optimize the functionality of chemical sensors in aspects of both selectivity and sensitivity, which decide the performance of the sensors, Metal-Organic Frameworks (MOFs) are introduced and widely applied to the research in the field of chemical sensors. MOFs is known as porous materials which possess the crystalline structure consists of metal ions and organic ligand linkers. The nature of this structure has endowed the material with high surface area, tailorable pore size and the ability to coordinate with specific chemical compounds, which makes MOFs as good candidates to improve the selectivity and sensitivity of sensor systems.

Here in this work, a novel method of synthesizing MOF thin films has been introduced, to provide a general way to grow different MOF thin films with Zn, Al, and Cu as coordinate centers. The MOF thin films with great adhesion and uniformity are achieved in a simple and convenient way, unlike conventional methods. It is shown that the MOF thin films can be synthesized by using metal oxide sputtered thin film as a template to provide metal source, and ligand solution as reaction environment. The morphology and thickness of certain MOFs can be adjusted by changing the parameters of the reaction condition, which can be possibly applied to the sensing system such as QCM and impedance sensor.

Additionally, the method to convert metal oxide thin layers into MOF thin films can be applied with little limitation on the substrates, which could be flexibly used for glass, silicon wafer, metal surface as well as polymers. This is due to the fact that a metal oxide thin layer still remaining between the substrate and MOF thin film and acting as a binding buffer to offer a strong connection between them.

Furthermore, using this method, MOF thin films has been grown on the polymer protective layer of optical fibers to fabricate optical sensing device which achieves a detection towards to ethanol vapor with the changes of partial pressures. The MOF thin film and polymer layer constitute a Fabry-Perot cavity, which creates an interference spectrum when light pass through. The interference spectrum shifts when dielectric properties of the transmission media changes, caused by an adsorption behavior of MOF layers. In other words, when the Fabry-Perot device exposed into an environment with target chemical gas, the interference spectrum peak shifts along with the changes of chemical concentration. A specific MOF thin film, SIFSIX-2-Cu-I, which is using CuSiF6 and dipyridilacetylene as its building units, has been uniformly grown on the surface of polymer layer, showing a significant interference peak change with the alternation of atmosphere with ethanol vapor.

It is proved in this dissertation that this simple, convenient, and easy-to-control method to fabricate Zn, Cu and Al based MOFs can be realized, and there is a huge potential to put the as-synthesized MOF thin films into designation of different types of chemical sensors, such as optical fiber or other fields.

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