The consistent fabrication of high performance α-Fe2O3 photoanodes for the oxygen evolution reaction remains a challenge. We work towards resolving this issue by developing in situ variable temperature Raman spectroscopy as a means to better understand the formation of α-Fe2O3, using the conversion of γ-FeOOH to α-Fe2O3 under varied gaseous environments as a model case. The sensitivity of Raman spectroscopy to structural changes provides mechanistic insights that are not readily available in more conventional approaches, such as thermal gravimetric analysis and differential scanning calorimetry. The Raman spectra are combined with conventional thermal analyses to interpret the photoelectrocatalytic performance of a series of α-Fe2O3 photoanodes prepared by systematic variation of a three-stage annealing protocol. The combined results suggest that protohematite, a form of α-Fe2O3 where trapped hydroxyl ligands are balanced by Fe(III) vacancies, forms between 200 °C and 400 °C in a reaction environment-dependent fashion. This protohematite is shown to be remarkably persistent once formed, degrading photoelectrocatalytic performance. This research advances understanding of the γ-FeOOH to α-Fe2O3 structural transformation, illustrates a powerful method to study solid state phase transitions, and provides guidance for the synthesis of high quality α-Fe2O3 from a convenient precursor.
Structural defects present in hematite photoanodes are analyzed by correlating parameters derived from Raman spectra, band structure measurements and photoelectrochemical (PEC) oxygen evolution reaction performance. A series of photoanodes with varying quality of hematite are prepared by sintering akaganeite films with a systematically varied protocol. Data acquired through Raman microscopy, linear sweep voltammetry, and electrochemical impedance spectroscopy is parametrized and systematically compared to detect relationships amongst the data. Correlations between the structural information contained within the Raman spectra and PEC parameters enable the identification of three types of structural defects within the material. Each defect type is found to exert unique influence on PEC behavior and each became dominant under specific fabrication conditions. A straightforward approach to detecting defects in photoelectrodes and identifying their influence on photoelectrode properties and behavior is provided.
Hot-stamped ultrahigh strength steel components are pivotal to automotive light-weighting. Steel blanks, often coated with an aluminum-silicon (Al-Si) layer to protect them from oxidation and decarburization, are austenitized within a furnace and then simultaneously quenched and formed into shape. The Al-Si coating melts within the furnace and reacts with iron from the steel to yield an intermetallic phase that provides some long-term corrosion protection. During the intermediate liquid phase, some of the coating may transfer to the furnace components, leading to maintenance costs and operational downtime. A detailed understanding of the coating transformation mechanism is needed to avoid such production issues while ensuring that final intermetallic coatings conform to specifications. We introduce cross-sectional Raman microscopic mapping as a method to rapidly elucidate the coating transformation mechanism. Raman spectroscopic fingerprints for relevant intermetallic compounds were determined using synthesized Al-Fe-Si ternary and Al-Fe binary compounds. These fingerprints were used to map the spatial distribution of intermetallic compounds through cross sections of Al-Si-coated 22MnB5 specimens that were heated at temperatures between 570 and 900 °C. These chemical maps show that the intermetallic fraction of the coating does not grow significantly until formation of η (Al5Fe2) at the steel interface, suggesting that η facilitates extraction of iron from the steel and subsequent diffusion through the coating. Under the heating conditions used here, a series of reactions ultimately lead to a silicon-rich τ2 (Al3FeSi) phase on top of the binary η phase. The technique presented here simplifies structural analysis of intermetallic compounds, which will facilitate prototyping of strategies to optimize hot stamping.
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