Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Abstract

Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe 2 O 3 ) 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.

Dynamics of photogenerated holes in surface modified α-Fe 2 O 3 photoanodes for solar water splitting

“…Our transient kinetic studies also suggest that the lifetime required for water oxidation by hematite photoholes is on the order of 1 s, a conclusion supported by recent frequency domain photoelectrochemical analyses (18,32,41). In particular, we observe here a close correlation between the bias dependence of the appearance of the long-lived photohole transient signal and the bias requirement for the observation of anodic photocurrents, providing further confirmation of requirement of longlived holes to enable water oxidation.…”

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

“…39 Films were prepared by 1,000 ALD cycles and measured to be ~55 nm by absorption measurements (Perkin Elmer, Lambda 35 with a Labsphere integrating sphere) corrected for reflection as described previously, as well as ellipsometric measurements (Horiba Jobin Yvon, Smart-SE). 36 Films were characterized by Raman Spectroscopy and XRD of these films previously. 36 Hematite electrodes were masked with a 60 m Surlyn film (Solaronix) with a 0.28 cm 2 hole to define the active area and to prevent scratching of the thin films.…”

“…The thin film hematite was prepared by atomic layer deposition (ALD), [34][35][36] and were subsequently coated with Co-Pi films by photoelectrodeposition. These thin hematite films have been shown to be a good model system for studying the limitations of water oxidation at the hematite surface.…”

Interpretation of Cyclic Voltammetry Measurements of Thin Semiconductor Films for Solar Fuel Applications

“…It has been suggested that these surface traps are FeO intermediates and the formation of these species by proton-coupled oxidation of surface hydroxide species constitutes the first step of water oxidation on hematite electrodes. 16,25 In other materials, like GaN for instance, 26 surface traps are mainly due to the morphology of the material and are present in the dark and under illumination. Consequently, in the latter situation, the voltammetry plots display a comparable capacitive peak for both pretreatments.…”