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Hematite electrodes with variable morphologies were prepared via a simple electrodeposition (ED) method. The photoelectrochemical (PEC) properties of planar and nanostructured electrodes were examined under PEC water oxidation and compared to that of planar analogue prepared by atomic layer deposition (ALD). The water oxidation performance of electrodeposited planar thin films was comparable to the nanostructured films; surprisingly, both ED films significantly outperformed the ALD-made planar films. The superior performance is attributed to variations in the crystallographic properties which results in enhanced hole transport and collection, as confirmed by photoelectrochemical and electrochemical impedance spectroscopy measurements and structural analysis. These results indicate a nonzero hole diffusion length for the electrodeposited hematite thin films in contrast to the ALD counterparts. Electrodeposited hematite thin films modified with Co-Pi exhibit near-unity hole collection efficiencies and produce the highest photocurrent among reported planar electrodes. This approach thus provides a simple and scalable approach to prepare high-performance thin film absorber hematite electrodes for solar water splitting.

In this work, Ni1–x Fe x O y coated hematite electrodes are investigated as a model system of different semiconductor/catalyst interfaces. We found that the photoelectrochemical (PEC) performance of the electrodes strongly depends on both the way the hematite electrode is prepared and the composition of the catalyst. Two extreme behaviors are observed for electrodeposited hematite electrodes coated with different compositions of catalyst. In the case of Fe-rich catalyst (Ni0.25Fe0.75O y ), the performance is substantially enhanced compared to the bare electrode; however, the Ni-rich (Ni0.75Fe0.25O y ) catalyst inhibits the PEC performance. A combination of photoelectrochemical, intensity modulated photocurrent spectroscopy, and electrochemical impedance spectroscopy measurements collectively reveal the critical role that the interface states of the semiconductor and catalyst plays in controlling the key interfacial charge transfer and recombination reactions. The photogenerated holes are efficiently collected and stored into the catalyst layer for the Ni0.25Fe0.75O y coated hematite electrodes. An unusually large improvement in performance is attributed to this hole collection circumventing recombination at the hematite surface. For the Ni0.75Fe0.25O y coated hematite electrodes, however, there is a presence of interface trap states that act as recombination centers and pin the catalyst potential. These combined results provide important new understanding of the role of the interfaces at semiconductor/electrocatalyst junctions.

Electrocatalysts improve the efficiency of light-absorbing semiconductor photoanodes driving the oxygen evolution reaction, but the precise function(s) of the electrocatalysts remains unclear. We directly measure, for the first time, the interface carrier transport properties of a prototypical visible-light-absorbing semiconductor, α-Fe2O3, in contact with one of the fastest known water oxidation catalysts, Ni0.8Fe0.2Ox, by directly measuring/controlling the current and/or voltage at the Ni0.8Fe0.2Ox catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-Fe2O3 directly transfer to the catalyst film over a wide range of conditions and that the Ni0.8Fe0.2Ox is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-Fe2O3. The Ni0.8Fe0.2Ox therefore acts as both a hole-collecting contact and a catalyst for the photoelectrochemical water oxidation process. Separate measurements show that the illuminated junction photovoltage across the α-Fe2O3|Ni0.8Fe0.2Ox interface is significantly decreased by the oxidation of Ni2+ to Ni3+ and the associated increase in the Ni0.8Fe0.2Ox electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement.

The first example of tantalum nitride electrodes on transparent conductive oxide substrates, which enables solar water splitting, is presented.