Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy

Abstract

Semiconductor electrodes capable of using solar photons to drive water-splitting reactions, such as haematite (α-Fe2O3), have been the subject of tremendous interest over recent decades. The surface has been found to play a significant role in determining the efficiency of water oxidation with haematite; however, previous works have only allowed hypotheses to be formulated regarding the identity of relevant surface species. Here we investigate the water-oxidation reaction on haematite using infrared spectroscopy under photoelectrochemical (PEC) water-oxidation conditions. A potential- and light-dependent absorption peak at 898 cm(-1) is assigned to a Fe(IV)=O group, which is an intermediate in the PEC water-oxidation reaction. These results provide direct evidence of high-valent iron-oxo intermediates as the product of the first hole-transfer reaction on the haematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.

Rate Law Analysis of Water Oxidation and Hole Scavenging on a BiVO4 Photoanode

“…This contrasts with the four hole overall stoichiometry required to oxidize two molecules of water to one molecule of oxygen. However it is unclear from this study if this third order behavior is specific to the particular chemistry of water oxidation on hematite (which has been proposed to be associated with the formation of surface iron-oxo groups 24 ), or is more generally observed on other metal oxide surfaces. [25][26] Herein, we report the kinetics of water oxidation on undoped, dense BiVO4 photoanodes under quasi steady-state conditions, as determined by spectroelectrochemical photoinduced absorption (PIA) measurements employing pulsed LED excitation.…”

“…36 The third order reaction kinetics under high surface hole densities is consistent with, for example, a mechanism involving the formation of an O-O bridging bond between two reaction sites, which consumes three surface accumulated holes. Such mechanisms have been suggested for water oxidation on some metal oxide materials, including Co3O4 25 and α-Fe2O3 11,24 , from either detecting the time-resolved IR response during the formation of rate-limiting intermediates on Co3O4, or employing a rate law analysis to determine the reaction order. We are aware that studies of the mechanism of water oxidation are still limited due to the difficulties in probing the intermediates involved.…”

Photocatalysis: Basic Principles, Diverse Forms of Implementations and Emerging Scientific Opportunities

“…Recently, Hamann group reported a detailed study by operando PEC infrared measurements. [102] Their results showed direct evidence of the formation of Fe IV = O intermediates produced from the first holetransfer reaction on hematite surface.…”

The Principle of Photoelectrochemical Water Splitting

“…Zandi and Hamann [76] recently investigated the water-oxidation reaction on hematite using operando infrared spectroscopy under PEC water-oxidation conditions. They observed an absorption peak at 898 cm −1 , which was assigned to a Fe IV =O group (Figure 29).…”