# Characterizing two surface states and their role in the photoinduced oxygen evolution reaction on hematite via photocurrent kinetics

## TL;DR

This study identifies two surface states on hematite that influence the efficiency of the oxygen evolution reaction in solar water splitting.

## Contribution

The paper introduces a method using bias-dependent, wavelength-resolved photocurrent kinetics to distinguish surface states on hematite.

## Key findings

- Two surface states on hematite are identified as monodentate and bidentate coordinated surface oxygens.
- Bidentate coordinated surface O(H) are the active sites for the oxygen evolution reaction.
- Monodentate surface OH populations change with applied bias and control surface charge.

## Abstract

Hematite (α-Fe2O3) is a promising photoanode for solar water splitting, whose efficiency is limited by rapid charge recombination, sluggish hole transport and slow oxygen evolution reaction kinetics. Understanding which of these factors actually leads to inefficiency, i.e. non-unitary photon conversion, is challenging. Here we show, for a model hematite photoanode, that analysis of wavelength-dependent (405–645 nm) photocurrent kinetics as a function of bias (0.9–1.65 V vs. RHE) reveals two surface states. The observed bias dependence and relative size of the charge transfer resistances and capacitances associated with each state are most easily rationalized if our α-Fe2O3(0001) anode is characterized by a mixed Fe/O termination that results in populations of monodentate and bidentate coordinated surface oxygens. Bidentate coordinated surface O(H) are the active site for the photoinduced OER but populations of monodentate surface OH change in response to applied bias/illumination in a manner that controls surface charge. At potentials where OER occurs in the dark, both sites are catalytically active.

Bias-dependent, λ-resolved photocurrent kinetics and interfacial capacitor analysis separate bulk and surface processes on hematite, revealing monodentate and bidentate OH with distinct kinetics and showing the OER proceeds via bidentate OH.