THE CHARACTERIZATION OF SUB-BAND GAP PHOTOCURRENTS AT SINGLE CRYSTAL N-TYPE TITANIUM-DIOXIDE SEMICONDUCTOR ELECTRODES
Abstract
This thesis describes the application of laser-flash electrochemical studies to the study of sub-band gap photocurrents at single crystal n-type TiO(,2) semiconductor electrodes. The work involved the design and construction of a laser-flash coulostatic instrument capable of sub-microsecond electrochemical response as well as the experimental design and data analysis necessary for the characterization of the dynamic photochemical processes. The objectives of the study, presented in Part I, centered around the possible application of semiconductor indicator electrodes under flash irradiation to the study of chromophore excited state electrochemistry. Because of the presence of significant residual photocurrents, the study involved the characterization of the background response in the absence of any absorbing species. Included in this section is a brief summary of semiconductor electrochemistry and a discussion of instrumental electroanalytical limitations for studying fast photochemical reactions. Part II describes the general theory of semiconductor electrochemistry with special emphasis placed on the electrical characteristics of an ideal semiconductor electrode/solution interface. Important topics in this area include the interfacial differential capacitance, the potential and charge distribution, and the effect of surface states on the behavior of a semiconductor electrode. Part III describes the design and construction of the coulostatic apparatus and the experimental procedures followed during data acquisition. Part IV describes the experimental design and characterization of the sub-band gap photocurrent response to both continuous and pulsed irradiation. Experimental variables included the wavelength and intensity of light used, the solution composition, the electrode dopant level, and the electrode surface condition. The highlights of this work centered around the discovery of two separate pathways for photo-stimulated charge separation. The first pathway, involving surface states from crystal imperfections, resulted in faradaic oxidation of the electrolyte within the pulse width of the laser. The second pathway was characterized by internal electrode electron-hole generation in chemically modified (solution dependent) surface states. In this case the surface states did not give rise to any faradaic charge transfer but nonetheless yielded large transient coulostatic photovoltage signals over a narrow ((TURN)200 mV) potential region that obscured the observation of any coincident faradaic signal. The potential region over which the chemically modified surface state response occurred is discussed in terms of the solution composition. Finally, Part V presents a brief conclusion to the study with particular emphasis on the potential regions and electrode properties best suited for pulsed studies involving semiconductor electrode/solution chromophore excited state electrochemistry.
Degree
Ph.D.
Subject Area
Analytical chemistry
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