Rate-Limiting O–O Bond Formation Pathways for Water Oxidation on Hematite Photoanode

Abstract

Photoelectrochemical (PEC) water oxidation has attracted heightened interest in solar fuel production. It is well accepted that water oxidation on hematite is mediated by surface trapped holes, characterized to be the high valent -Fe═O species. However, the mechanism of the subsequent rate-limiting O-O bond formation step is still a missing piece. Herein we investigate the reaction order of interfacial hole transfer by rate law analysis based on electrochemical impedance spectroscopy (EIS) measurement and probe the reaction intermediates by operando Fourier-transform infrared (FT-IR) spectroscopy. Distinct reaction orders of ∼1 and ∼2 were observed in near-neutral and highly alkaline environments, respectively. The unity rate law in near-neutral pH regions suggests a mechanism of water nucleophilic attack (WNA) to -Fe═O to form the O-O bond. Operando observation of a surface superoxide species that hydrogen bonded to the adjacent hydroxyl group by FT-IR further confirmed this pathway. In highly alkaline regions, coupling of adjacent surface trapped holes (I2M) becomes the dominant mechanism. While both are operable at intermediate pHs, mechanism switch from I2M to WNA induced by local pH decrease was observed at high photocurrent level. Our results highlight the significant impact of surface protonation on O-O bond formation pathways and oxygen evolution kinetics on hematite surfaces.

Interrogating the Surface Intermediates and Water Oxidation Products of Boron‐Doped Diamond Electrodes with Scanning Electrochemical Microscopy

“…The greater resistance to charge transfer and lower electrochemically active surface area in low‐doped domains would produce lower feedback current. This has indeed been shown with SECM, where lower boron content leads to reduced electron transfer kinetics . Furthermore, lower doping and sp 2 content would provide fewer viable binding sites for surface intermediates, leading to lower effective surface charge and greater product desorption .…”

“…This has indeed been shown with SECM, where lower boron content leads to reduced electron transfer kinetics. [16] Furthermore, lower doping and sp 2 content would provide fewer viable binding sites for surface intermediates, leading to lower effective surface charge and greater product desorption. [4] Recent work with SECCM has shown that surface termination, in addition to doping levels, greatly influences water splitting characteristics, [14] both of which may also play a factor here.…”

“…[2,3] Most work on identifying surface-confined ROS, however, has focused on the ubiquitous photo(electro)catalyst TiO 2 , which can form a variety of surface complexes, including hydroxyl (À OH), hydroperoxo (À OOH), end-on (À OÀ O) or bridged (À OÀ OÀ ) peroxo, and oxo (=O) species. [15,16] Comparatively, however, very little about the surface ROS on borondoped diamond is known.…”

Visible‐Light‐Driven Water Oxidation with a Polyoxometalate‐Complexed Hematite Core of 275 Iron Atoms

“…[36] At the same time,the first-and third-order dependence of hole formation reported by Durrant, [7] and the first-and second-order kinetics and operando observations of electrochemical water oxidation by Chen and Song, [6] support O À Oformation both at as ingle Fe atom, and at larger hole densities and/or highly basic pH values via coupling between adjacent surfacetrapped holes.F or solutions of 1 at pH 8, OÀOf ormation is more likely to occur at asingle Fe atom, giving the iron(III)- Computational results argue that the distances between adjacent Fe atoms at the hematite surface are too large to give stable peroxidebridged di-iron intermediates,Fe III ÀOOÀFe III .…”

“…[2] Ag ood example is visible-light-driven water oxidation with hematite (a-Fe 2 O 3 ). Recent investigations with hematite photoanodes have quantified rates of carrier recombination, [3] identified intermediates such as Fe IV = O, [4] determined orders of reaction from one to three as afunction of the "hole" density [5] and pH value, [6] and noted the ratelimiting formation of OÀOb onds. [6,7] Forv isible-light-driven water oxidation with colloidal a-Fe 2 O 3 , [8] however,a lthough ashort hole-diffusion length of 2-4 nm [9] has been identified as the basis for increased activity at smaller particle size,very little mechanistic information is available.…”