Molecular Design of Electrode Surfaces and Interfaces: For Optimized Charge Transfer at Transparent Conducting Oxide Electrodes and Spectroelectrochemical Sensing
AuthorMarikkar, Fathima Saneeha
KeywordsElectrode Surface Modification & Sub-Micron Patterning
Optimizing Indium Tin Oxide Electrodes
Diffraction Based Sensors
Electron Transfer Enhancement at Partially Blocked Electrodes
AdvisorArmstrong, Neal R.
Committee ChairArmstrong, Neal R.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThis dissertation has focused on i) optimizing charge transfer rates at indium-tinoxide (ITO) electrodes, and ii) characterization of the supramolecular structure and properties of ultra thin surface modifier films on modified electrodes for various device applications. Commercial ITO surfaces were modified using conducting polymer thin film architectures with and without various chemical activation procedures. Ferrocene derivatives were used as redox probes, which showed dramatic changes in electron transfer rate as the SA-PANI/PAA layers were added to the ITO surface. Highest rates of electron transfer were observed for DMFc, whose oxidation potential coincides with the potential region where these SA-PANI/PAA films reach their optimal electroactivity. Apparent heterogeneous electron transfer rate constants, kS, measured voltammetrically, were ca.10 x higher for SA-PANI/PAA films on ITO, versus clean ITO substrates. These films also showed linear potentiometric responses with retention of the ITO transparency with the capability to create smoothest films using an aqueous deposition protocol, which proved important in other applications. ITO electrodes were also modified via chemisorption of carboxy functionalized EDOTCA and electropolymerization of PEDOTCA/PEDOT copolymers, when properly optimized for thickness and structure, enhance voltammetrically determined electron transfer rates (kS) to solution probe molecules, such as dimethylferrocene (DMFc). Values of kS ≥ 0.4 cm•sec⁻¹, were determined, approaching rates seen on clean gold surfaces. ITO activation combined with formation of these co-polymer films has the effect of enhancing the electroactive fraction of electrode surface, versus a non-activated, unmodified ITO electrode, which acts as a “blocked” electrode. The electroactivity and spectroelectrochemistry of these films helped to resolve the electron transfer rate mechanism and enabled the construction of models in combination with AFM, XPS, UPS and RAIRS studies. The surface topography, structure, composition, work function and contact angle, also revealed other desirable properties for molecular electronic devices. The carboxylic functionality of the EDOTCA molecule adds more desirable properties compared to normal PEDOT films, such as favoring the deposition of smooth films, increasing the optical contrast, participating in hydrogen-bonding, chemisorption to oxide surface, self-doping and providing a linker for incorporation of different functional groups, new molecules, or nanoparticles. Periodic sub-micron electrode arrays can be created using micro-contact printing and electropolymerization. The sinusoidal modulation of the refractive index of such confined conducting polymer nanostructures or nanoparticle stripes allows efficient visible light diffraction. The modulation of the diffraction efficiency at PANI and PEDOT gratings in the presence of an analytical stimulus such as pH or potential demonstrate the sensing capability at these surfaces. The template stripped gold surfaces that are being developed in our lab demonstrate several advantages over commercially available evaporated gold films especially for nanoscale surface modification.
Degree GrantorUniversity of Arizona
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Nanoscale Characterization of the Electrical Properties of Oxide Electrodes at the Organic Semiconductor-Oxide Electrode Interface in Organic Solar CellsMacDonald, Gordon Alex (The University of Arizona., 2015)This dissertation focuses on characterizing the nanoscale and surface averaged electrical properties of transparent conducting oxide (TCO) electrodes such as indium tin oxide (ITO) and transparent metal-oxide (MO) electron selective interlayers (ESLs), such as zinc oxide (ZnO), the ability of these materials to rapidly extract photogenerated charges from organic semiconductors (OSCs) used in organic photovoltaic (OPV) cells, and evaluating their impact on the power conversion efficiency (PCE) of OPV devices. In Chapter 1, we will introduce the fundamental principles regarding the need for low cost power generation, the benefits of OPV technologies, as well as the key principles that govern the operation of OPV devices and the key innovations that have advanced this technology. In Chapter 2 of this dissertation, we demonstrate an innovative application of conductive probe atomic force microscopy (CAFM) to map the nanoscale electrical heterogeneity at the interface between an electrode, such as ITO, and an OSC such as the p-type OSC copper phthalocyanine (CuPc).(MacDonald et al. (2012) ACS Nano, 6, p. 9623) In this work we collected arrays of J-V curves, using a CAFM probe as the top contact of CuPc/ITO systems, to map the local J-V responses. By comparing J-V responses to known models for charge transport, we were able to determine if the local rate-limiting step for charge transport is through the OSC (ohmic) or the CuPc/ITO interface (nonohmic). These results strongly correlate with device PCE, as demonstrated through the controlled addition of insulating alkylphosphonic acid self-assembled monolayers (SAMs) at the ITO/CuPc interface. Subsequent chapters focus on the electrical property characterization of RF-magnetron sputtered ZnO (sp-ZnO) ESL films on ITO substrates. We have shown that the energetic alignment of ESLs and the organic semiconducting (OSC) active materials plays a critical role in determining the PCE of OPV devices and the appearance of, or lack thereof, UV light soaking sensitivity. For ZnO and fullerene interfaces, we have shown that either minimizing the oxygen partial pressure during ZnO deposition or exposure of ZnO to UV light minimizes the energetic offset at this interface and maximizes device PCE. We have used a combination of device testing, device modeling, and impedance spectroscopy to fully characterize the effects that energetic alignment has on the charge carrier transport and charge carrier distribution within the OPV device. This work can be found in Chapter 3 of this dissertation and is in preparation for publication. We have also shown that the local properties of sp-ZnO films varies as a function of the underlying ITO crystal face. We show that the local ITO crystal face determines the local nucleation and growth of the sp-ZnO films. We demonstrate that this effects the morphology, the chemical resistance to etching as well as the surface electrical properties of the sp-ZnO films. This is likely due to differences in the surface mobility of sputtered Zn and O atoms on these crystal faces during film nucleation. This affects the nanoscale distribution of electrical and chemical properties. As a result we demonstrate that the PCE, and UV sensitivity of the J-V response of OPVs using sp-ZnO ESLs are strongly impacted by the distribution of ITO crystal faces at the surface of the substrate. This work can be found in Chapter 4 of this dissertation and is in preparation for publication. These studies have contributed to a detailed understanding of the role that electrical heterogeneity, insulating barriers and energetic alignment at the MO/OSC interface play in OPV PCE.
CHARACTERIZATION OF CHARGE INJECTION PROCESSES OF THIN FILMS ON INDIUM TIN OXIDE ELECTRODES USING A NOVEL SPECTROELECTROCHEMICAL TECHNIQUE: POTENTIAL-MODULATED ATTENUATED TOTAL REFLECTANCE SPECTROSCOPYAraci, Zeynep (The University of Arizona., 2010)Understanding interfacial charge injection processes is one of the key factors needed for development of efficient organic electronic devices, such as biosensors and energy conversion systems, since these processes control the electrical characteristics of these devices. Spectroelectrochemical characterization of electron transfer processes occurring at the electrode - electroactive thin film interface has been evaluated to improve our understanding of charge transfer kinetics using a novel form of electroreflectance spectroscopy, potential-modulated attenuated total reflectance (PM-ATR), which makes it possible to sensitively monitor spectroscopic changes in thin films as a function of applied potential.PM-ATR was used to evaluate three different redox-active films deposited on indium tin oxide (ITO) electrodes to investigate: i) the orientation dependence of charge transfer rates of thin films of biomolecules, ii) surface treatment and modification effects on charge transfer kinetics of conducting polymers and, iii) estimation of rates of electron injection and conduction band edge of semiconductor nanocrystalline materials.First, Prussian blue film as a model system was used successfully to examine the PM-ATR technique for determination of the charge transfer rate constant between ITO and a molecular film.Second, an anisotropic and redox active protein film, cytochrome c, was used to probe charge transfer rates with respect to molecular orientation. The electron transfer rate measured using TM polarized light was four-fold greater than that measured using TE polarized light. These data are the first to correlate a distribution of molecular orientations with a distribution of electron transfer rates in a redox-active molecular film.Third, the effects of ITO surface treatment and modification on charge transfer kinetics on a conducting polymer, poly(3,4-ethylenedioxythiophene/)/poly(styrenesulfonate) (PEDOT/PSS), were studied. The apparent interfacial charge transfer rate constant for PEDOT/PSS on ITO has been reported for the first time which cannot be measured otherwise with conventional electrochemistry due to high non-Faradaic background of PEDOT/PSS films.Fourth, PM-ATR enabled characterization of reversible redox processes between submonolayer coverages of surface-tethered, CdSe nanocrystals and ITO for the first time. Optically determined onset potentials for electron injection were used for estimation for the conduction band and valance band energies (ECB and EVB, respectively).
Electrochemical Probing of Causes for Variation in Lifetime of Iridium-Tantalum Oxide Electrode Used in Copper ElectrowinningMa, Dongni (The University of Arizona., 2017)In hydrometallurgical copper production plants, titanium-based electrodes coated with a conductive layer of IrO2-Ta2O5 are widely used as anodes in acidic copper electrowinning baths because of their long service life and low overpotential for oxygen evolution. The presence of trace amounts of ions such as fluoride, aluminum, and iron in sulfate-based electrowinning baths is believed to affect the stability of IrO2-Ta2O5 coated anodes. Hence, in this study, the effect of fluoride and metallic cations on the lifetime of IrO2-Ta2O5 coated Ti electrodes in sulfuric acid solutions has been investigated, and a degradation mechanism for IrO2-Ta2O5 coatings in the presence of fluoride has been proposed. Typical lifetime of the conductive ceramic coated anodes is 1 to 2 years. In order to predict electrode performance over this long period, an accelerated laboratory test that can be carried out in a few weeks is often used. This test, known as accelerated lifetime test (ALT), is conducted by electrically stressing the anodes at a current density that is much higher than the current density used for electrowinning while monitoring the change in the cell potential. The time required for the cell voltage to increase by 5 V is taken as the accelerated lifetime of the oxide electrode. In this research, titanium mesh samples coated with mixed iridium oxide-tantalum oxide layers were tested as anodes in 2 M sulfuric solution a constant current density of 0.54 A/cm2. A two-electrode cell with a bare titanium mesh serving as the cathode was used for experiments. In addition to ALTs, anodic polarization measurements were also carried out to study the changes in overpotential for oxygen evolution on electrodes before and after ALTs. Additionally, morphology and chemical composition analyses were performed on electrodes before and after ALTs using various techniques such as scanning electron microscopy (SEM) analysis, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Chemical species that are likely to be present in the electrowinning bath were predicted using the computer software STABCAL and presented in distribution-pH and Pourbaix diagrams. The results of multiple ALTs in the absence and presence of various levels of fluoride indicate that the anode lifetime was greatly reduced by the presence of fluoride in sulfuric acid solutions. The greater the amount of fluoride added, the shorter the anode lifetime. Additionally, both in the absence and presence of fluoride, the molar ratio of IrO2 to Ta2O5 in the coating did not change during dissolution. In studying strategies to prolong the lifetime of the electrode in a fluoride-containing solution, a method of complexing fluoride ions using metallic cations such as Al3+ and Fe3+ was developed and demonstrated. The anode lifetime was successfully prolonged from 200 to over 500 hours with the addition of aluminum ions to a fluoride-containing solution. Compared with ferric ions, aluminum ions are more efficient in complexing with fluoride to extend the lifetime of electrodes.