Correlating Raman and X-Ray Absorption Spectroscopy to Analyze Defects in Hematite Photoandoes

Abstract

Over two decades of research on hematite photoanodes have generated significant information on the electronic structure, photophysics and electron transfer mechanism for the oxygen evolution reaction. A surprisingly high degree of variability in photoelectrocatalytic performance continues across the literature despite these advances, however, suggesting missing information and uncontrolled variables. This encouraged us to pursue spectroscopic methods capable of identification, and possibly quantification of specific defects within the hematite lattice, and to map their presence to behaviors observed in photoelectrochemistry. We approach this issue by applying structure-property analysis to hematite samples treated under either O2 or N2 environments with variable performance in photoelectrocatalytic oxygen evolution. X-ray absorption fine-structure spectroscopy and Raman spectroscopy can provide short-range coordination shells and longer-range order feature respectively to describe the structure of samples across the series. Different correlations between these structural parameters and photoelectrochemical performance reveal distinct defects for sample sets annealed in O2 or N2. These distortions can be observed by processed Raman spectrum data, suggesting that it may be possible to calibrate the width, energy, and intensity of peaks in Raman spectra to enable direct analysis of defects in hematite photoanodes.