Rotating Ring‐Disk Electrode Study of Competitive Photo‐oxidation at α ‐ Fe2 O 3 Photoanodes in Aqueous Solution

Abstract

BENZOATE ESTER RADICAL ANIONS 1565The double potential step method is particularly attractive for measuring the homogeneous rate constant for EC systems because the number of variables used for fitting can be reduced to one. By stepping to a sufficiently forcing potential, the electrochemical reaction becomes diffusion controlled, and thus the charge transfer rate parameters have no influence on the relaxation currents. The dependence on electrode area as well as the dependence on the concentration and diffusion coefficient of the electroactive species is removed by using the ratio of the forward and reverse currents.In conclusion, we have demonstrated by several methods (voltammetry, double potential step experiments, coulometry, and product isolation) that the reduction of benzoate esters in dilute (1 mM) nonaqueous medium involves an EC mechanism. The system becomes more complex at higher concentrations. The initially formed radical anion cleaves to form benzoate ion and alkyl radical. This is one of a very few examples of the reduction of organic compounds by an EC mechanism.ABSTRACT Competition reactions at a-Fe203 photoanodes were studied using the RRDE technique. It was foundthat iodide, bromide, and chloride ions were photo-oxidized in preference to water in acidic solutions. The competition ratio was measured as a function of halide-ion concentration, halide-ion flux, light intensity, electrode potential, and solution pH. A direct correlation between the disk photocurrent and the competition ratio at a given halide flux was demonstrated. A lack in selectivity for the dark current was also observed.

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

“…Plots of steady-state photocurrent density, J , vs applied voltage, V , curves in response to varying light intensities, 10, 33, and 100 mW cm –2 , are shown in Figure a for pH 6.9 and Figure b for pH 13.3. Because the water oxidation potential and the hematite bands both shift at the Nernstian rate of 59 mV/pH, the potentials were normalized to the real hydrogen electrode reference (RHE). − The J ( V ) curves under 100 mW cm –2 illumination (1 sun) are plotted vs RHE in Figure c. The curves show remarkable overlap; however, the performance of the electrodes at pH 13.3 does show a somewhat improved current onset potential of about 100 mV.…”

Photoelectrochemical Investigation of Ultrathin Film Iron Oxide Solar Cells Prepared by Atomic Layer Deposition

“…Steady state current density, J , versus applied potential, E , measurements were performed on three series of thin film hematite electrodes ranging from 7 to 56 nm in contact with an aqueous ferricyanide/ferrocyanide electrolyte. , Figure a shows representative J − E curves measured in the dark and in response to simulated AM 1.5 illumination (100 mW cm −2 ). Figure b shows the average short circuit current densities, J SC , as a function of thickness.…”

Semiconductor Photooxidation of Pollutants Dissolved in Water: A Kinetic Model for Distinguishing between Direct and Indirect Interfacial Hole Transfer. I. Photoelectrochemical Experiments with Polycrystalline Anatase Electrodes under Current Doubling and Absence of Recombination

“…The notion of competitive oxidation has been used a number of times in previous reports. [39][40][41][42][43][44][45][46][47][48][49][50][51][52] This idea, which constitutes the key of the model presented here, has not been taken into account in most of the kinetic studies concerning heterogeneous photocatalytic systems, with a few remarkable exceptions. 22,53,54 The model here presented takes into account photooxidation processes showing the so-called "photocurrent multiplication" effect.…”

“…The model is based on the fact that the photooxidation of dissolved pollutant competes with the photooxidation of the solvent (water molecules). The notion of competitive oxidation has been used a number of times in previous reports. − This idea, which constitutes the key of the model presented here, has not been taken into account in most of the kinetic studies concerning heterogeneous photocatalytic systems, with a few remarkable exceptions. ,, …”