The Interface Effect on Solar Water Oxidation for Ni_1-xFe_xO_y Modified Hematite Photoanodes

Hamed Hajibabaei, Thomas W. Hamann*

Abstract. In order to minimize the large overpotential associated with the sluggish kinetics of water oxidation, the surface of semiconductor photoanodes (PA) are commonly modified with electrocatalysts (EC). Despite the central role of the PA|EC interface, little attention has been paid to the fundamental charge transfer dynamics across the interface. In this work, we employed thin films of hematite which were prepared by atomic layer deposition (ALD) and electrodeposition (ED) as PA’s, which were coated with different compositions of Ni_1-xFe_xO_y EC’s. Regardless of the composition of catalyst, the performance of modified ALD hematite PA’s were improved. The photoelectrochemical (PEC) performance of the modified ED hematite PA’s, however, were found to strongly depend on the composition of the EC. In case of Fe-rich (e.g. Ni_0.25Fe_0.75O_y) coated ED electrodes, the PEC performance was substantially enhanced. In a peculiar observation, the PEC performance of Ni-rich (e.g. Ni_0.75Fe_0.25O_y) coated ED electrodes is inhibited. This is especially interesting in that this EC was identified as the most active EC for water oxidation and the ED electrode the most active PA. Systematic studies as a function of the thickness of the catalysts via steady state and transient photoelectrochemical as well as intensity modulated photocurrent spectroscopy and impedance spectroscopy measurements revealed the essential role of the PA|EC interface on PEC performance of the catalyst modified electrodes. It was found that for Fe-rich catalyst coated ED hematite electrodes and all (both Fe- and Ni-rich) catalyst modified ALD electrodes, the photogenerated holes were efficiently collected and stored in the catalyst layer which further facilitated the multi-hole transfer water oxidation reaction. This type of behavior is consistent with the “adaptive junction” model established for ion permeable EC’s. For the Ni-rich coated ED electrodes, however, the interface trap states emerge at the PA|EC junction which modulate the charge transfer between the underlying semiconductor to the electrocatalyst. As a result, the photogenerated holes were initially collected in the interface states which subsequently were injected into the electrocatalyst layer through a slower process. These combined results provide an important new understanding of the role of the interface at the semiconductor/electrocatalyst junction.

• Nanomaterial for Application in Energy Technology, Ventura Beach Marriott, Ventura, CA, 2017.