# Observation of Transition from Rate Law to Butler–Volmer Controlled Water Oxidation Kinetics on Hematite Photoanodes

## TL;DR

This study shows how water oxidation on hematite photoanodes changes from being controlled by reaction rates to being governed by potential as hole density increases.

## Contribution

The paper unifies population-based and Butler–Volmer models by observing a mechanistic transition in water oxidation on hematite.

## Key findings

- At low hole densities, water oxidation follows a rate law mechanism.
- At high hole densities, the reaction shifts to a Butler–Volmer-like, potential-driven regime.
- The transition is triggered by band edge unpinning after surface M–OH species are fully oxidized.

## Abstract

Despite its central
role in photoelectrochemical (PEC) water splitting,
the mechanistic pathway of water oxidation on metal oxides remains
unresolved, with population-based and Butler–Volmer (BV) models
offering distinct views on how surface valence band holes drive the
reaction. Here, we bring together these two perspectives by combining
operando photoinduced absorption (PIA) spectroscopy with photocurrent
analyses on α-Fe2O3 (hematite) photoanodes
as a function of light intensity. We find a crossover from population-controlled,
rate law water oxidation at low hole densities to a BV-like, potential
driven regime at high densities, triggered by band edge unpinning
once surface M–OH species are fully oxidized, and excess holes
accumulate without compensation. This mechanistic transition unifies
competing models of interfacial charge transfer and reveals design
principles for optimizing water oxidation in metal oxide photoelectrodes.