Graduation Semester and Year

2020

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry and Biochemistry

First Advisor

Krishnan Rajeshwar

Abstract

Photoelectrochemical (PEC) water splitting via inorganic semiconductors has shown interest in technical applications such as harvesting sunlight as sustainable fuels. Chalcogen-based (S, Se, Te) semiconductors are important in numerous technology applications especially related to photovoltaic solar conversion, fuel cells, hydrogen generation etc. On the other hand, oxide semiconductors are great candidates due to their unique properties namely electrolyte stability, a wide range of bandgaps and easy access. In this vein, we investigated the quaternary metal chalcogenide, Ca(La1−xCex)2S4 (0 ≤ x ≤ 1) photoelectrochemical behavior in an aqueous redox electrolyte. These solid solution series were synthesized in Prof. Macaluso’s laboratory by a sealed ampule method. In this work, we mainly focused on the effect of electrons in f-orbitals in the PEC behavior of these solid solution series, where the presence of f electron density is zero in CaLa2S4 and is maximized in CaCe2S4. All the samples were found to be n-type semiconductors as synthesized. The second part of this research addressed the use of photocurrent polarity (i.e., whether anodic or cathodic) in a PEC situation to assess whether a Cu2O semiconductor electrode sample behaved as an n- or p-type semiconductor. Using electrodeposited copper(I) oxide film as a sample platform, complications arising from the presence of Cu as an unwanted impurity phase and/or PEC corrosion of the oxide film in the photocurrent polarity data are discussed. Such artefacts are shown to be a possible contributory factor in many previous studies that have (erroneously) identified n-type semiconductor behavior in electrodeposited copper oxide films. Among all the requisite properties of an inorganic semiconductor, the dynamics of interfacial charge transfer is an important common denominator for technological applications. Most attention in this regard has been paid to the dynamics of minority carrier transfer; only sporadic studies have focused on majority carrier transfer across the solid/electrolyte interface. However, recent studies have underlined the growing realization that the charge transfer kinetics of both minority and majority carriers are often coupled. Performance optimization of the interfaces must address both these charge transfer pathways such that deleterious carrier recombination can be minimized. Therefore, this portion of the dissertation study addresses the experimental understanding of interfacial charge transfer dynamics at semiconductor/electrolyte interfaces in the dark. For these experiments, binary n-type TiO2 and WO3 semiconductors were chosen because of their excellent PEC stability, high PEC activity, and overall robustness over a wide pH range. Similarly, binary and ternary p-type semiconductors such as copper (II) oxide (CuO), copper bismuth oxide (CuBi2O4), and silver vanadate (AgVO3) were selected to compare their charge transfer kinetics in the dark. Such comparative studies are conspicuously absent in the literature, and in particular, dark current-forward bias potential measurements are largely lacking for these compounds. Tafel analyses were used to evaluate the standard rate constant, k0, the exchange current density, j0, and the transfer coefficient, α in all these cases. Based on these kinetics data, a general conclusion could be made that p-type oxide semiconductors exhibited faster kinetics than their n-type counterparts. For comparison, Pt electrode was included as benchmark in these experiments. The data also indicated that heterogeneous charge transfer at metal/electrolyte interfaces had faster kinetics compared to semiconductor/electrolyte interfaces. This contrasting behavior stems from surface state density variations in the two sets of cases. The most marked feature of the experimental data was the evidence for localized states within the bandgap. The results were discussed based on a model involving charge transfer mediated by surface states. The practical implications of these data are finally discussed.

Keywords

Electrochemical, Photoelectrochemical, Charge transfer kinetics, Cathodic dark current, Anodic dark current, Exchange current

Disciplines

Chemistry | Physical Sciences and Mathematics

Comments

Degree granted by The University of Texas at Arlington

Included in

Chemistry Commons

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