CHAPTER 2: Kinetics and Mechanisms of Light-Driven Reactions at Semiconductor Electrodes: Principles and Techniques

Some basic principles of semiconductor electrochemistry are reviewed, with particular emphasis on the deviations from ideality that are expected when the reaction of minority carriers (holes in n-type semiconductors) is slow, as is normally the case in light-driven water splitting (photoelectrolysis). A simple kinetic approach is used to relate the rate constant for interfacial electron (hole) transfer to the build-up of minority carriers at the semiconductor/electrolyte interface during slow reactions such as light-driven oxygen evolution. The kinetic model is extended to consider transient and periodic techniques. It is shown that the rate constants for charge transfer and surface recombination can be obtained by analysing the transient photocurrent response. Intensity modulated photocurrent spectroscopy (IMPS) and photoelectrochemical impedance spectroscopy (PEIS) are discussed, and the application of the methods is illustrated with results for light-driven oxygen evolution on hematite electrodes. The principles and application of light-modulated microwave reflectance (LMMR) spectroscopy are presented and illustrated with the example of light-driven hydrogen evolution on p-Si electrodes. The chapter concludes with a section dealing with light-modulated absorption spectroscopy, a powerful tool for the detection of intermediates in light-driven reactions at semiconductor electrodes such as hematite.