Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co–Pi”-Coated Hematite Electrodes

Uniform thin films of hematite (α-Fe2O3) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, “Co–Pi,” were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co–Pi thickness were performed in order to clarify the mechanism by which Co–Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co–Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co–Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co–Pi catalyst coupled with semiconductor electrodes for water-splitting applications

Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes

Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe2O3) on transparent conductive substrates for photocatalytic water oxidation studies. Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results

Water oxidation at hematite photoelectrodes: the role of surface states

Hematite (α-Fe2O3) constitutes one of the most promising semiconductor materials for the conversion of sunlight into chemical fuels by water splitting. Its inherent drawbacks related to the long penetration depth of light and poor charge carrier conductivity are being progressively overcome by employing nanostructuring strategies and improved catalysts. However, the physical–chemical mechanisms responsible for the photoelectrochemical performance of this material (J(V) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by atomic layer deposition to study the photoelectrochemical properties of this material under water-splitting conditions. We employed impedance spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity, and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat-band potential. These findings demonstrate the utility of impedance spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement

Water Oxidation on Hematite Photoelectrodes: Insight into the Nature of Surface States through In Situ Spectroelectrochemistry

Uniform planar films of hematite (α-Fe₂O₃), deposited by atomic layer deposition, were examined using in situ spectroelectrochemistry during photoinduced water oxidation. A change in the absorption spectrum of hematite electrodes during water oxidation was measured under illumination and applied potentials. The absorption was correlated to a charge measured by cyclic voltammetry and with a capacitance measured by impedance spectroscopy. Modification of the hematite surface with alumina reduced the absorption feature and the associated capacitance, suggesting that these features are associated with the surface. Comparing the spectral change of hematite to absorption features of molecular analogues allowed us to tentatively assign the absorbance and capacitive features to the oxidation of a low valent iron-aqua or iron-hydroxyl species to a high valent iron-oxo chemical species at the surface

Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co–Pi”-Coated Hematite Electrodes

Uniform thin films of hematite (α-Fe₂O₃) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, “Co–Pi,” were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co–Pi thickness were performed in order to clarify the mechanism by which Co–Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co–Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co–Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co–Pi catalyst coupled with semiconductor electrodes for water-splitting applications

Competitive Photoelectrochemical Methanol and Water Oxidation with Hematite Electrodes

Photocatalytic water and methanol oxidation were studied at thin film hematite electrodes synthesized by atomic layer deposition (ALD). Systematic photoelectrochemical characterization along with O₂ evolution measurements were carried out in order to better understand the mechanisms of both water and methanol oxidation at hematite electrodes. When both water and methanol are present in the solution, they are oxidized competitively with each other, allowing the detection and assignment of distinct surface states characteristic to each process. The measurement of different surface states for methanol and water oxidation, along with the absence of measurable surface states in an inert acetonitrile electrolyte, clearly shows that the detected surface states are chemically distinct reaction intermediates of water or methanol oxidation