Post-Excitation Transient IR Phenomena in α-Fe2O3 Films

Andraž Šuligoj,Dorit Grinberg,Yaron Paz

doi:10.1021/acs.jpcc.1c09118

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.

Highlights

Hematite (α-Fe2O3) is considered to be one of the most promising materials for photoelectrochemical (PEC) water splitting owing to the positions of its valence and conductance bands, stability in basic solutions, abundance, and low cost.[1−4] Despite its potential adequacy, its actual performance is rather meager: a solar to hydrogen conversion of no more than a few percent[5] compared to a theoretical value of 15%.6 The discrepancy between the expectation and the performance made hematite one of the most studied photocatalysts
Analysis of the crystallite sizes imaged by Transmission electron microscopy (TEM) (Figure S3,A) gave a distribution of sizes, peaked at 30−32 nm (Figure S3,B), similar to the values obtained from the XRD measurements
Three pronounced changes in the difference spectra are observed (Figure 4B). (1) A positive peak at ∼640 cm−1. This band is sharp at its highest intensity, ∼30−40 ns from excitation, after which the intensity reduces and the width increases and the difference peak eventually disappears

Summary

Introduction

Hematite (α-Fe2O3) is considered to be one of the most promising materials for photoelectrochemical (PEC) water splitting owing to the positions of its valence and conductance bands, stability in basic solutions, abundance, and low cost.[1−4] Despite its potential adequacy, its actual performance is rather meager: a solar to hydrogen conversion of no more than a few percent[5] compared to a theoretical value of 15%.6 The discrepancy between the expectation and the performance made hematite one of the most studied photocatalysts. 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 multistep processes responsible for its performance. This led to a multiplicity of claims, to some extent contradictory, showing rough correlation with the type of technique that was used. At large, this controversy relates to the types of species that are involved (free carriers, small electron polarons, hole polarons, and excitons) and the characteristic lifetime of the electronically excited state. While it is broadly accepted that a free carrier model is irrelevant for hematite,[10] considering the low mobilities that were measured, the nature of the alternative trapped-carrier mechanism is still a matter of dispute

Results

Discussion

Conclusion