In situ surface monitoring of energy materials during processing: impact of defect disorder on surface versus bulk semiconducting properties of photocatalytic hematite (Fe 2 O 3 )

• Published in: Energy advances 2024-03, Vol.3 (3), p.614-624
• Main Authors: Bak, T., StC. Black, D., Gannon, P., Gür, T. M., Jasiuk, I., Sherif, S. A., Sigmund, W. M., Nowotny, J.
• Format: Article
• Language: English
• ISSN: 2753-1457; 2753-1457
• DOI: 10.1039/D3YA00482A

Rational design of surface properties of oxide semiconductors for energy conversion requires in situ surface characterization. This work considers in situ monitoring of surface semiconducting properties of hematite (Fe 2 O 3 ) during isothermal oxidation and reduction at 1140 K, and the related charge transfer at the Fe 2 O 3 –O 2 interface using the measurements of work function (WF) changes during successive oxidation and reduction experiments under two extreme oxygen activities of 44 kPa and 220 Pa. The obtained results indicate that prolonged annealing of Fe 2 O 3 leads to the formation of an n-type surface layer on top of the bulk phase of hematite that exhibits intrinsic semiconducting properties. The derived theoretical model shows that the reactivity of Fe 2 O 3 with oxygen involves two parallel processes: (i) fast bulk diffusion of defects associated with penetration of oxygen activity into the oxide lattice, and (ii) slow surface segregation of defects, derived from the bulk phase, and resulting in the formation of a quasi-isolated surface structure (QISS) that plays a critical role in the performance of energy materials. The obtained results led to derivation of theoretical models that explain the role of the QISS in the performance of energy materials.