Synthesis and Electrochemical Applications of Nanostructural and Nanoporous Thin Films
Date Issued
2015
Date
2015
Author(s)
Kung, Chung-Wei
Abstract
In this dissertation, various nanostructural and nanoporous electrode materials were synthesized directly onto the conducting substrates, and the obtained modified electrodes were applied for various electrochemical applications, including electrochemical sensors, supercapacitors, counter electrodes (CEs) of dye-sensitized solar cells (DSSCs), and electrochromic thin films. Since one-dimensional (1D) nanostructural arrays are expected to show a faster electronic conduction along the axial direction through the arrays to their substrate, 1D nanostructural arrays of electroactive materials were focused in the first part of this dissertation. The 1D bending acicular nanorod arrays (ANRAs) of cobalt oxide (Co3O4) were grown on conducting substrates by using chemical bath deposition (CBD) followed by calcination. The bending Co3O4 ANRAs were successfully applied for non-enzymatic electrochemical glucose sensors and achieved a promising performance. Thereafter, three different morphologies of Co3O4 nanostructural thin films, including the 1D straight ANRAs, 2D nanosheets, and 2D net-shaped nanosheets, were also grown on conducting substrates by similar approaches and compared with the original bending ANRAs. The electrocatalytic activities of these Co3O4 thin films for the oxidation of acetaminophen (AP) were investigated, and the result shows that the 1D straight ANRAs of Co3O4 exhibit the best electrocatalytic activity among them. Thereafter, the straight Co3O4 ANRAs were solvothermally converted into the 1D ANRAs of cobalt sulfide (CoS), which is very difficult to be grown as 1D nanostructure due to its two-dimensional (2D) nature. The obtained CoS ANRAs were used as the CEs of DSSCs to replace the conventional CE using platinum, and the obtained DSSC shows comparable cell efficiency compared to the DSSC using platinum. In the end of the first part of this dissertation, a solvothermal anion-exchange process was used to convert the straight ANRAs of layered cobalt carbonate hydroxide (LCCH), which were grown on conducting substrates by CBD, into the Co3O4 acicular nanotube arrays (ANTAs). Taking advantages of the higher surface area provided by nanotubes compared to that provided by nanorods, the Co3O4 ANTAs show a much higher specific capacitance compare to the original Co3O4 ANRAs obtained by calcining the LCCH ANRAs. On the other hand, 2D nanostructures are expected to exhibit a higher surface area compared to the 1D nanostructures. In the second part of this dissertation, 2D nanostructural thin films of electrochemically active materials were explored. Thin films of Co3O4 2D nanosheets were electrodeposited on flexible titanium substrates by a potentiostatic method, followed by an UV-ozone treatment. The obtained thin film of Co3O4 nanosheets was applied for supercapacitors; a high specific capacitance of 1,033.3 F/g was obtained at a charge-discharge current density of 2.5 A/g. Also, poly(3,4-ethylenedioxythiophene) (PEDOT) hollow microflower arrays (HMFAs), with several 2D hollow nanopetals on each of the microflowers were fabricated on a conducting substrate, by using ZnO microflower arrays (MFAs) as the template. The 2D nanostructures of PEDOT are expected to provide numerous electrocatalytic sites, which result in the promising performance when they were utilized as the CEs of DSSCs. Beyond the nanostructural materials, the nanoporous materials which possess even higher surface area started to be considered. Metal-organic frameworks (MOFs), which show the highest surface area among all categories of porous materials, were chosen as electrode materials in the last part of this dissertation. Thin films composed of 1D acicular nanorods of a MOF constructed from zirconium-based nodes and pyrene-based linkers were grown on conducting glass substrates. The pyrene-based linkers are electrochemically active and show electrochromic behavior. Due to the fast charge-transport rate provided by the 1D acicular nanorods and the facile ion diffusion in regular 1D pores of the MOF, the obtained MOF thin film can exhibit facile and reversible electrochromism. In order to make a MOF thin film that is electrochemically addressable and stable in aqueous solutions, uniform thin films of another MOF, which is constructed from the electrochemically active porphyrin linkers and zirconium-based nodes, were solvothermally grown on conducting substrates. The MOF thin films were found to be electrochemically addressable and stable in aqueous solution, which opens up several opportunities for utilizing such MOF thin films in various electrochemical applications operated in aqueous systems.
Subjects
Acicular nanorods
Dye-sensitized solar cells
Electrochemical sensors
Electrochromism
Metal-organic frameworks
Supercapacitors
Type
thesis