Enhanced Photoelectrochemical Oxidation of Water over Ti-Doped α-Fe₂O₃ Electrodes by Surface Electrodeposition InOOH

Abstract

Ti-doped α-Fe 2 O 3 (Ti-Fe 2 O 3 ) is regarded as one of the most promising hematite-based photoanodes for photoelectrochemical oxidation of water. However, the sluggish interfacial transfer and rapid recombination of photogenerated holes still limit its efficiency. In this article, we report that an approach of surface modification for this material by electrodeposition in In(NO 3 ) 3 containing aqueous solutions was attempted to overcome these disadvantages. The results show that a thin layer of InOOH was formed on the photoelectrodes, confirmed by X-ray photoelectron spectroscopy. This modification could enhance the photoelectrochemical oxidation rate of water. The reason for the improvement was investigated quantitatively by a combination of multiple (photo-)electrochemical techniques and transient absorption spectroscopy. It is demonstrated that the kinetics of photogenerated holes can be well described by a model of involving photogenerated surface-state mediated charge transfer and recombination. The improvement resulted predominantly from the enhanced interfacial transfer rate constant (k ct ) of the photoholes, despite that the surface recombination rate constant (k sr ) slightly increased. Energetically, the increase in k ct was attributed to more potential drop in the Helmholtz layer. The higher potential drop resulted from more accumulation of photoholes on the surface due to a larger band bending as the flatband potential negatively shifted. Physically, the increase in k ct can be explained by that InOOH acts as a so-called cocatalyst to promote the hole transfer. In addition, the implication of these findings is addressed for further designing efficient Ti-Fe 2 O 3 photoanodes.

Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes

“…In addition, as illustrated in Figure , for less positive applied potentials, optical signals can be observed in three of our studied photoanodes, indicative of charge accumulation in intra-bandgap states. Charge accumulation in such intra-bandgap states has previously been reported for hematite in several studies and proposed to drive water oxidation. − In contrast, the data herein shows that charge accumulating in these states does not contribute significantly to water oxidation, most likely due to their modest oxidation potentials relative to hematite’s valence band. These intra-bandgap states have been suggested to be catalytically inactive, instead, they are likely involved in competing trapping processes that can be mitigated with increasing applied potentials.…”

“…One of the key considerations for water oxidation on metal oxides, including hematite, is the potential role of intra-bandgap surface states. − Such surface states have often been related to oxygen vacancies and structural imperfections/defect sites. − Surface holes on hematite have been assigned to Fe IV O species, with these states being proposed as the first intermediate species of the OER . Some studies, including electrochemical impedance analyses, have provided evidence that midgap surface states participate as intermediates in the OER catalysis on such photoanodes. − Other studies, including transient absorption analyses, have suggested OER catalysis is driven by valence band holes localized at the metal oxide surface, with intra-bandgap (e.g., oxygen vacancy) states primarily functioning as electron/hole trapping sites, impacting upon bulk and back electron–hole recombination. ,− The impact of surface states on the water oxidation catalysis on metal-oxide photoanodes such as α-Fe 2 O 3 therefore remains controversial.…”

Impact Of Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes

“…Charge accumulation in such intra-bandgap states has previously been reported for hematite in several studies, and proposed to drive water oxidation. [13][14][15][16] In contrast, the data herein shows that charge accumulating in these states does not contribute significantly to water oxidation, most likely due to their modest oxidation potentials relative to hematite's valence band. These intra-bandgap states have been suggested to be catalytically inactive, 38 instead, they are likely involved in competing trapping processes that can be mitigated with increasing applied potentials.…”

“…12 Some studies, including electrochemical impedance analyses, have provided evidence that mid-gap surface states participate as intermediates in the OER catalysis on such photoanodes. [13][14][15][16] Other studies, including transient absorption analyses, have suggested OER catalysis is driven by valence band holes localized at the metal oxide surface, with intra-bandgap (e.g. : oxygen vacancy) states primarily function as electron / hole trapping sites, impacting upon bulk and back electron-hole recombination.…”

Surface-oxidized titanium diboride as cocatalyst on hematite photoanode for solar water splitting

“…[8][9][10] Considering the above, the targeted modification of α-Fe 2 O 3 is necessary to increase its PEC activity by: (i) nanoengineering α-Fe 2 O 3 to compensate for the short minority carrier diffusion length by maximizing the semiconductor-electrolyte interface; 11,12 (ii) increasing the electroconductivity by elemental doping (e.g., F, Ti, Ta); 7,13,14 (iii) passivating the surface states by the decoration of a metal oxide transition layer; 15,16 (iv) heterostructure engineering by utilizing the synergistic effects of different semiconductors; 17,18 and (v) loading cocatalyst to improve the water oxidation kinetics. 19 The state-of-the-art cocatalysts mainly include transition metal oxides (CoO X ), 20 hydroxides (Ni(OH) 2 ), 21 oxyhydroxides (FeOOH, NiOOH, InOOH), [21][22][23] and phosphates (CoPi, FeP). 24,25 Metal borides exhibit extremely rich diversity in crystal structure and chemical bond formation; thus they have potential applications in the aspects of superconductivity, electronics, and catalysis.…”