Charge Carrier Dynamics in Solar Water Oxidation

The search for sustainable energy sources is one of the greatest problems facing mankind in the 21st century. Most renewable sources do not have adequate potential to cover the growing need for energy in order to sustain economic and population growth. Solar power is a plausible way to fully cover mankind’s continuously growing need for energy. However, sunlight is diurnal, and the amount of sunlight received at different latitudes of the Earth varies drastically. Harnessing solar energy into chemical bonds is an attractive approach to enable the storage of energy for transportation and later use. Direct photoelectrochemical water splitting produces only oxygen and hydrogen, of which hydrogen can be used to sustain a possible hydrogen based economy. The materials used in this Thesis are metal oxide semiconductors that act as photoanodes, performing the water oxidation reaction on their surface and supplying electrons for the water reduction reaction.Hematite is an n-type metal oxide semiconductor that has a band gap suitable for the absorption of a noticeable fraction of solar radiation. The absorption of light leads to the generation of electron-hole pairs that are separated due to a built-in electric field. However, the conduction band level of hematite is not suitable for unassisted water splitting and it suffers from poor intrinsic charge transport properties. For this reason the photoanodes studied in this Thesis have been modified with doping and by forming heterojunctions with other metal oxide semiconductors, namely titanium dioxide.This Thesis studies the evolution of the primary charge carriers responsible for water splitting in modified hematite photoanodes. The method selected to probe the charge carrier dynamics is transient absorption spectroscopy that enables the monitoring of charge carriers from the subpicosecond timescale up to seconds. The measurements were performed in a three electrode photoelectrochemical cell to see the effects of additional bias voltage on the charge carrier dynamics and how the recombination and oxygen evolution reaction are changed when a photocurrent is generated.The results of this Thesis indicate that the modification of hematite has a profound effect on the charge carrier behaviour. The observed effects range from changes in recombination on the picosecond timescale, to nanosecond timescale trapping of electrons into intraband or surface states, and all the way to changes in the reaction rates of long-lived holes in the hundreds of milliseconds timescale.