In situ surface monitoring of energy materials during processing: impact of defect disorder on surface versus bulk semiconducting properties of photocatalytic hematite (Fe2O3)

T. Bak; D. StC. Black; P. Gannon; T. M. Gür; I. Jasiuk; S. A. Sherif; W. M. Sigmund; J. Nowotny

doi:10.1039/D3YA00482A

In situ surface monitoring of energy materials during processing: impact of defect disorder on surface versus bulk semiconducting properties of photocatalytic hematite (Fe₂O₃)

T. Bak,

†^a D. StC. Black,

†^bc P. Gannon,

†^d T. M. Gür,

†^e I. Jasiuk,

†^f S. A. Sherif,

†^g W. M. Sigmund

†^h and J. Nowotny

†*^ci

Author affiliations

* Corresponding authors

^a Western Sydney University, School of Computer Data and Mathematical Sciences, Penrith, NSW 2751, Australia
E-mail: T.Bak@westernsydney.edu.au

^b School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
E-mail: D.Black@unsw.edu.au

^c Sustainable Energy Initiative Pty Ltd, 2 Hebron Ave, Mt Pleasant, NSW 2519, Australia

^d Chemical and Biological Engineering, Montana State University, Bozeman, MT 59717, USA
E-mail: Pgannon@montana.edu

^e Department of Materials Science and Engineering, Stanford333 University, Stanford, CA 94305-4034, USA
E-mail: Turgut@stanford.edu

^f Department of Mechanical Science and Engineering, University of Illinois at Urbana, Champaign, IL 61801, USA
E-mail: Ijasiuk@illinois.edu

^g Department of Mechanical and Aerospace Engineering, University of Florida, 1064 Center Drive, Room 181 NEB Building, Gainesville, FL 32611, USA
E-mail: sasherif@ufl.edu

^h Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400, USA
E-mail: Sigmund@ufl.edu

ⁱ Western Sydney University, School of Computer Data and Mathematical Sciences, Penrith, NSW 2751, Australia
E-mail: J.Nowotny@westernsydney.edu.au

Abstract

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 (Fe₂O₃) during isothermal oxidation and reduction at 1140 K, and the related charge transfer at the Fe₂O₃–O₂ 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 Fe₂O₃ 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 Fe₂O₃ 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.

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In situ surface monitoring of energy materials during processing: impact of defect disorder on surface versus bulk semiconducting properties of photocatalytic hematite (Fe₂O₃)

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In situ surface monitoring of energy materials during processing: impact of defect disorder on surface versus bulk semiconducting properties of photocatalytic hematite (Fe₂O₃)

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