Identifying protons trapped in hematite photoanodes through structure–property analysis

Abstract

Features in Raman spectra are found to correlate to the band structure and photoelectrochemical behavior across a series of hematite photoanodes. Analysis suggests the presence of iron vacancies caused by the trapping of interstitial protons.

Identification of Three Coexistent Defect Types in Hematite Photoanodes through Structure–Property Analysis

“…Growth in the intensity of this E u mode was previously correlated with improved performance in heteroatom‐doped α‐Fe 2 O 3 , [ 38 ] but degraded performance in pure α‐Fe 2 O 3 . [ 30 ] A positive linear correlation of I 499 / I 660 with both j PEC and E fb in undoped α‐Fe 2 O 3 films prepared by sintering lepidocrocite (γ‐FeOOH) previously demonstrated the utility of the Raman forbidden E u vibration at 660 cm −1 for diagnosis of trapped protons and associated iron vacancies. [ 30 ] Similar handling of the data acquired here reveals separation of the samples into two clear branches for j PEC , with humidified sintering environments yielding one branch and dry environments a second (Figure 2B).…”

“…[ 30 ] A positive linear correlation of I 499 / I 660 with both j PEC and E fb in undoped α‐Fe 2 O 3 films prepared by sintering lepidocrocite (γ‐FeOOH) previously demonstrated the utility of the Raman forbidden E u vibration at 660 cm −1 for diagnosis of trapped protons and associated iron vacancies. [ 30 ] Similar handling of the data acquired here reveals separation of the samples into two clear branches for j PEC , with humidified sintering environments yielding one branch and dry environments a second (Figure 2B). Correlation plots for E fb show similar separation into two apparent branches, but seemingly scattered distribution of fabrication parameters across both branches indicates that the underlying reasons are non‐trivial (Figure 2C).…”

“…It has previously been defined in a number of ways, including the potential where a given current density is attained, the potential where first derivative plots, d j /d E , reach 0.2 mA cm −2 V −1 , [ 36 ] and the potential where the first positive peak occurs in second derivative plots, d 2 j /d E 2 . [ 30 ] The two former approaches fail to capture any value for poorly performing samples; the need to analyze samples across the performance spectrum in this work therefore leads to adoption of the second derivative definition (Figure 1A). This enables a systematic mathematical definition to be applied across all samples, even when significant resistive effects distort the j–E response, or samples have very poor performance.…”

“…[ 28,29 ] In the context of α‐Fe 2 O 3 photoanodes, Raman spectroscopy‐based structure–property analysis of a series of α‐Fe 2 O 3 samples prepared by thermal treatment of lepidocrocite (γ‐FeOOH) revealed that the intensity of select peaks in the Raman spectra were highly correlated to band structure and overall PEC performance. [ 30 ] Variable temperature Raman spectroscopy supported the proposal that the correlations arose due to iron vacancies caused by the trapping of protons within the lattice. [ 31 ] The ability to correlate specific defects within α‐Fe 2 O 3 to properties related to PEC reactions may enable Raman spectroscopy to serve as a rapid and accessible technique to communicate complex structural information.…”

“…Further evidence of differences between the dry and humid segments is in the vibrational modes involved in correlations: the dry segment of the data relies entirely upon combinations involving I 660 , whereas the humid segment of the data rely upon combinations of vibrational modes at 412, 499, 613, and 660 cm À1 . Each of these vibrational modes that are relevant to humid conditions involves the movement of iron ions in the lattice, [30,41,42] revealing a third defect that is critically dependent on iron sites in the lattice. We tentatively assign this third defect as iron ions residing in interstitial sites, based on this being the second major iron-based defect known to exist natively in α-Fe 2 O 3 .…”