Identifying Structural Defects in Hematite Photoanodes through Structure-Property Analysis

Abstract

Hematite photoanodes continue to exhibit highly variable photoelectrocatalytic water oxidation performance across the literature despite over two decades of intensive research. Studies have advanced the understanding of fundamental photophysical behavior of key electron transfer and enriched material design in hematite photoanodes, but poor photoelectrocatalytic performance and high variability in reported behavior indicate missing information. We believe that a lack of the chemical nature of structural defects in hematite and their specific impacts on material properties and photoelectrocatalytic water oxidation is a key problem. We target this issue with a structure-property analysis using photoelectrochemical, X-ray diffraction, Raman and UV-visible spectroscopic data on a series of hematite photoanodes. The analysis reveals a formally Raman inactive vibrational mode in hematite films prepared by annealing lepidocrocite films whose intensity varies with annealing protocols. Correlations between the intensity of this feature in the Raman spectrum with photocurrent density, semiconductor band structure, and the onset of photoelectrocatalysis signifies systematic changes in the magnitude of a crystal lattice distortion. Analysis of the nature of these key Raman vibrations and the synthetic conditions lead us to conclude that the observed defects are iron vacancies induced by trapped protons within the crystal lattice. This finding provides a means to rapidly diagnose a specific structural defect and will aid in the optimization of fabrication protocols for hematite photoanodes. Figure 1