Better Together: Ilmenite/Hematite Junctions for Photoelectrochemical Water Oxidation

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.

Hematite Photoanodes for Water Oxidation: Electronic Transitions, Carrier Dynamics, and Surface Energetics

“…Instead, the broad photoinduced absorption signals peaking at % 650-700 nm were assigned to photogenerated holes. [20,53,54] In this case,t he study of photohole dynamics could be conducted [d] 3ps(Fe 4+ lifetime) [47] [a] Thin film, TF. [b] TASa fter 20 ps is comparable to steady-state thermal differencespectrum.…”

“…We note that later investigations updated the assignment of the 580 nm absorption feature to hole trapping processes in the intraband states (located a few hundred millivolts below the conduction band). Instead, the broad photoinduced absorption signals peaking at ≈650–700 nm were assigned to photogenerated holes [20, 53, 54] . In this case, the study of photohole dynamics could be conducted without interference of electron/hole trapping processes in these intraband states.…”

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

“…102 Recently, Berardi et al also reported the fabrication of the FeTiO 3 overlayer passivated hematite photoanode (FeTiO 3 / Fe 2 O 3 ) through a simple electrophoretic method. 103 In that study, the FeTiO 3 phase was confirmed by X-ray absorption fine structure (XAFS) and HRTEM with electron energy loss spectroscopy (EELS). It existed in the near-surface region and could passivate the deep surface traps and induce a large density of donor levels, leading to a strong depletion field for the facilitated charge separation and injection into the electrolyte.…”

Hematite Photoanodes for Water Oxidation: Electronic Transitions, Carrier Dynamics, and Surface Energetics

“…Instead, the broad photoinduced absorption signals peaking at % 650-700 nm were assigned to photogenerated holes. [20,53,54] In this case,t he study of photohole dynamics could be conducted Fe L-edge EELS [d] 3ps(Fe 4+ lifetime) [47] [a] Thin film, TF. [b] TASa fter 20 ps is comparable to steady-state thermal differencespectrum.…”