Oxygen deficient α-Fe₂O₃ photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Abstract

Intrinsic doping of hematite through the inclusion of oxygen vacancies (V_O) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe₂O_3-<i>x</i> photoelectrodes. Whilst it is widely accepted that the introduction of V_O leads to improved conductivities, little else is verified regarding the actual mechanism of enhancement. Here we employ transient absorption (TA) spectroscopy to build a comprehensive kinetic model for water oxidation on α-Fe₂O_3-<i>x</i> . In contrast to previous suggestions, the primary effect of introducing V_O is to block very slow (ms) surface hole - bulk electron recombination pathways. In light of our mechanistic research we are also able to identify and address a cause of the high photocurrent onset potential, a common issue with this class of electrodes. Atomic layer deposition (ALD) of Al₂O₃ is found to be particularly effective with α-Fe₂O_3-<i>x</i> , leading to the photocurrent onset potential shifting by <i>ca.</i> 200 mV. Significantly TA measurements on these ALD passivated electrodes also provide important insights into the role of passivating layers, that are relevant to the wider development of α-Fe₂O₃ photoelectrodes.