Published asKlahrHowever, 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. 3 I ntroductionAs part of the quest to develop better and cleaner energy conversion and storage systems, the direct conversion of sunlight into chemical fuels has become a subject of renewed interest.One attractive example is the use of semiconductors to harness solar photons to split water, thereby producing hydrogen as a chemical fuel. In order to achieve this, a given material must satisfy a number of stringent requirements including visible light absorption, efficient charge carrier separation and transport, facile interfacial charge-transfer kinetics, appropriate positions of the conduction and valence band energy levels with respect to required reaction potentials and good stability in contact with aqueous solutions. 1 While such systems were heavily investigated several decades ago, no material so far has fulfilled all the required conditions. 2,3 Recent advances in nanotechnology and catalysis, however, greatly increase the prospects of developing a combination of materials capable of efficient conversion of sunlight to chemical fuels. 4 Hematite ( -Fe 2 O 3 ) is a very promising material for photoelectrochemical (PEC) water splitting due to its combination of sufficiently broad visible light absorption, up to 590 nm, and excellent stability under caustic operating conditions. 5,6 However, hematite electrodes are adversely affected by a number of factors including a long penetration depth of visible light due to its indirect band gap transition and a very short minority carrier lifetime and mobility; this combination hinders efficient collection of the minority carriers via the required interfacial charge-transfer reactions. Considerable effort has been devoted to improving the actual efficiency by employing nanostructuring strategies, which disconnects the ligh...

Uniform thin films of hematite ( -Fe

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.

A series of one-electron outersphere cobalt bipyridyl redox couples were used as redox shuttles in dyesensitized solar cells (DSSCs). Atomic layer deposition was used to deposit an ultrathin coating of alumina on nanoparticle-based TiO 2 DSSC photoanodes, which results in significantly improved quantum yields for all of the DSSCs containing outersphere redox systems. However, a significant discrepancy in performance remains between DSSCs containing the different cobalt redox shuttles. Variation of the driving force for regeneration by ∼500 mV, by employing [Ru(bpy)2(4,4′-dicarboxy-bpy)](PF6)2 as a dye, combined with concentration dependence studies indicates that the cobalt redox couples are not limited by dye regeneration; however, in certain cases the iodide electrolyte was, one of the very few systems where alternate redox couples perform significantly better than triiodide/iodide. Electron lifetimes were measured with the open circuit voltage decay technique. The differences in the lifetimes (recombination kinetics) of DSSCs employing cobalt redox couples correlate well with the differences in the incident photon-to-current efficiencies (IPCEs), providing strong evidence that the external quantum efficiencies of DSSCs with cobalt polypyridyl redox couples are limited by recombination. We further found that, contrary to previous reports, the cobalt(III/II) tris(4,4′-dimethyl-2,2′-bipyridine) couple can produce comparable external quantum yields to cobalt(III/II) tris(4,4′-di-tert-butyl-2,2′-bipyridine) when employed as redox shuttles in DSSCs. However, the photovoltaic performances of both are constrained by mass transport of the oxidized species through the nanoparticle photoelectrode.

Uniform thin films of hematite and Ti-doped hematite (a-Fe 2 O 3 ) were deposited on transparent conductive substrates using atomic layer deposition (ALD). ALD's epitaxial growth mechanism allowed the control of the morphology and thickness of the hematite films as well as the concentration and distribution of Ti atoms. The photoelectrochemical performances of Ti-doped and undoped hematite electrodes were examined and compared under water oxidation conditions. The incorporation of Ti atoms into hematite electrodes was found to dramatically enhance the water oxidation performance, with much greater enhancement found for the thinnest films. An optimum concentration $3 atomic% of Ti atoms was also determined. A series of electrochemical, photoelectrochemical and impedance spectroscopy measurements were employed to elucidate the cause of the improved photoactivity of the Ti-doped hematite thin films. This performance enhancement was a combination of improved bulk properties (hole collection length) and surface properties (water oxidation efficiency). The improvement in both bulk and surface properties is attributed to the resurrection of a dead layer by the Ti dopant atoms. Broader contextSolar energy to fuel conversion through photoelectrochemical (PEC) water oxidation at semiconductors is a promising approach to supply renewable and clean energy. Hematite (a-Fe 2 O 3 ) is one of the most studied semiconductor materials for solar hydrogen production via PEC water splitting due to its suitable combination of visible light absorption, favorable band gap positions, stability in aqueous solutions and abundance. A short charge collection length, however, has thus far prevented efficient water oxidation with hematite. Nanostructuring the electrode to minimize the charge collection distance has produced promising results, but just changing the dimensions has proven insufficient. The incorporation of a large concentration of impurity atoms, doping, has also led to improved performance, however the specic cause has not been unambiguously determined. It is vital to understand the fundamental physical effect of such dopant atoms in order to fully exploit this promising strategy. In this work, uniform thin lm Ti-doped hematite electrodes were made by atomic layer deposition (ALD) and examined by employing photoelectrochemical and impedance spectroscopic measurements under PEC water oxidation conditions; the cause of the improved performance is discussed.