Abstract
Hematite (α-Fe2O3) is an earth-abundant indirect n-type semiconductor displaying a band gap of about 2.2 eV, useful for collecting a large fraction of visible photons, with frontier energy levels suitably aligned for carrying out the photoelectrochemical water oxidation reaction under basic conditions. The modification of hematite mesoporous thin-film photoanodes with Ti(IV), as well as their functionalization with an oxygen-evolving catalyst, leads to a 6-fold increase in photocurrent density with respect to the unmodified electrode. In order to provide a detailed understanding of this behavior, we report a study of Ti-containing phases within the mesoporous film structure. Using X-ray absorption fine structure and high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy, we find that Ti(IV) ions are incorporated within ilmenite (FeTiO3) near-surface layers, thus modifying the semiconductor–electrolyte interface. To the best of our knowledge, this is the first time that an FeTiO3/α-Fe2O3 composite is used in a photoelectrochemical setup for water oxidation. In fact, previous studies of Ti(IV)-modified hematite photoanodes reported the formation of pseudobrookite (Fe2TiO5) at the surface. By means of transient absorption spectroscopy, transient photocurrent experiments, and electrochemical impedance spectroscopy, we show that the formation of the Fe2O3/FeTiO3 interface passivates deep traps at the surface and induces a large density of donor levels, resulting in a strong depletion field that separates electron and holes, favoring hole injection in the electrolyte. Our results provide the identification of a phase coexistence with enhanced photoelectrochemical performance, allowing for the rational design of new photoanodes with improved kinetics.
Highlights
The need to find solutions to the global energy crisis has prompted the scientific community to propose various approaches aimed at the exploitation of renewable energy sources
In order to provide an in-depth understanding of the origin of the enhanced photoelectrochemical cells (PECs) performance of these modified photoanodes, the local structure and oxidation state of Fe and Ti have been studied by X-ray absorption fine structure (XAFS) and TEM−energy loss spectroscopy (EELS)
We demonstrate that the inclusion of Ti(IV) does not influence the local structure of Fe, which remains that of hematite, and that the local electronic and atomic structures around Ti are similar to those of ilmenite (FeTiO3)
Summary
The need to find solutions to the global energy crisis has prompted the scientific community to propose various approaches aimed at the exploitation of renewable energy sources. Solar energy conversion in the form of chemical energy (so-called artificial photosynthesis) can be effectively realized in photoelectrochemical cells (PECs) In such devices, the illumination of semiconductor-based materials triggers several complex charge separation processes, leading to the formation of energy-rich molecules, known as solar fuels.[1,2] One of the most investigated alternative fuels, hydrogen, can be obtained at the cathodic side of a PEC system for photoinduced water splitting, while at the anodic side oxygen evolves.[3,4] In order to realize optimized composite photoelectrodes it is of paramount importance to gain a thorough understanding of the physical properties of commonly used materials as well as to investigate new ones.
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