Charge carrier accumulation and dynamics in metal oxide photoanodes
File(s)
Author(s)
Oldham, Louise Ilena
Type
Thesis
Abstract
In this thesis, time-resolved spectroscopy is employed to investigate charge carrier accumulation and dynamics in Fe2O3 and BiVO4-based photoanodes for photoelectrochemical (PEC) water splitting. The background and motivations for this research are presented in Chapter 1 and the experimental techniques employed within this thesis are presented in Chapter 2.
In Chapter 3, photoinduced absorption spectroscopy and step potential spectroelectrochemistry are employed to directly investigate the quasi-Fermi level splitting (QFLS) in a model Fe2O3 photoanode under operando conditions. This internal QFLS is observed to match the external photovoltage obtained from current-voltage curves, providing experimental evidence for the electrochemical origins of photovoltage in this system under applied bias. This differs from the electrostatic understanding of photovoltage in solar cells. Pinning of the hole quasi-Fermi level to midgap states ~150-200 mV above the valence band correlates with the observation of first order water oxidation kinetics, while accumulation of valence band holes is necessary for the third order oxygen evolution reaction.
In Chapter 4, a comparative study of BiVO4 performance in borate buffer (pH 9-9.5) and phosphate buffer (pH 6.9), demonstrates that the enhanced PEC performance observed in borate buffer is a result of reduced recombination, while the water oxidation kinetics remain unchanged.
In Chapter 5, transient absorption spectroscopy (TAS) measurements of a Fe2O3-based heterojunction demonstrate the role of a carbon nitride interlayer in facilitating hole transport from the Fe2O3 to co-catalyst particles. The improved charge separation of the system results in larger hole populations surviving on the timescales relevant to water oxidation.
Finally, in Chapter 6, a BiVO4-based heterojunction system is studied, which incorporates a ferromagnetic overlayer. After exposure to an external magnetic field, this heterojunction demonstrates a PEC performance enhancement, which TAS measurements correlate with improved charge separation.
In Chapter 3, photoinduced absorption spectroscopy and step potential spectroelectrochemistry are employed to directly investigate the quasi-Fermi level splitting (QFLS) in a model Fe2O3 photoanode under operando conditions. This internal QFLS is observed to match the external photovoltage obtained from current-voltage curves, providing experimental evidence for the electrochemical origins of photovoltage in this system under applied bias. This differs from the electrostatic understanding of photovoltage in solar cells. Pinning of the hole quasi-Fermi level to midgap states ~150-200 mV above the valence band correlates with the observation of first order water oxidation kinetics, while accumulation of valence band holes is necessary for the third order oxygen evolution reaction.
In Chapter 4, a comparative study of BiVO4 performance in borate buffer (pH 9-9.5) and phosphate buffer (pH 6.9), demonstrates that the enhanced PEC performance observed in borate buffer is a result of reduced recombination, while the water oxidation kinetics remain unchanged.
In Chapter 5, transient absorption spectroscopy (TAS) measurements of a Fe2O3-based heterojunction demonstrate the role of a carbon nitride interlayer in facilitating hole transport from the Fe2O3 to co-catalyst particles. The improved charge separation of the system results in larger hole populations surviving on the timescales relevant to water oxidation.
Finally, in Chapter 6, a BiVO4-based heterojunction system is studied, which incorporates a ferromagnetic overlayer. After exposure to an external magnetic field, this heterojunction demonstrates a PEC performance enhancement, which TAS measurements correlate with improved charge separation.
Version
Open Access
Date Issued
2025-09-26
Date Awarded
2026-03-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Durrant, James R
Kafizas, Andreas
Sponsor
Imperial College London
Publisher Department
Department of Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)