Post-Excitation Transient IR Phenomena in α-Fe2O3 Films

Abstract

Hematite (α-Fe2O3) is one of the most studied materials for electrochemical water splitting and photovoltaic applications. A plethora of experimental techniques have been applied in order to unveil the mechanisms of charge migration in hematite and to understand the kinetics of the multistep processes responsible for its performance. The common concept is based on the formation of small electron polarons within a few picoseconds, having a lifetime of up to a few nanoseconds. In this work, step-scan transient IR spectroscopy was used to follow IR spectral changes in the semiconductor following pulsed UV excitation. The transient spectrum resembled the spectrum of maghemite, suggesting a similar local distortion following excitation. The most pronounced change was the appearance of an absorption peak at 640 cm–1, whose intensity was the highest at 40–50 ns after excitation, and its lifetime was found to be in the order of a few hundreds of nanoseconds that is considerably longer than what is usually considered as carriers’ lifetime in hematite. The intensity of the 640 cm–1 peak was found to change with the film thickness in a manner that correlated with the photoinduced current measured by linear sweep voltammetry. This correlation demonstrates that transient IR spectroscopy in the nanosecond range may be useful as a tool for studying photoinduced phenomena in photoactive materials.