Chapter 4: Unravelling the Charge Transfer Mechanism in Water Splitting Hematite Photoanodes

In this chapter, the mechanism of water oxidation on the surface of hematite and other relevant (photo)catalysts are discussed. Various (photo)electrochemical and spectroscopic methods with the aim to uncover the mechanisms of charge transfer at the surface of hematite are reviewed. It is shown that an elaborate combination of (photo)electrochemical methods can be utilised to detect and study the surface states and their role(s) on the photoelectrochemical water oxidation performance of hematite electrodes. We also show that the exclusive formation of these surface states under photoelectrochemical water oxidation conditions, strongly indicates that they are in fact, the intermediate(s) of water oxidation. We further discuss the spectroscopic measurements coupled with electrochemical methods to directly identify the chemical nature of these intermediates under operando conditions of water oxidation at the surface of hematite and analogous (photo)catalysts. Particularly, electrochemical cells coupled with attenuated total reflectance-infrared spectroscopy and ultraviolet-visible spectroscopy (spectroelectrochemistry) are discussed. Furthermore, the resemblance of water oxidation mechanism on the surface of hematite to the iron-based homogeneous catalysts are reviewed. Investigations of the surface states of other metal oxide photoanodes for water oxidation were also introduced in this chapter to demonstrate the general application of these methods for such interfacial studies.