TY - JOUR

T1 - Nucleation of Hematite: A Nonclassical Mechanism

AU - Scheck, Johanna

AU - Fuhrer, Lisa M.

AU - Wu, Baohu

AU - Drechsler, Markus

AU - Gebauer, Denis

N1 - Funding Information: D.G. was a Research Fellow of the Zukunftskolleg of the University of Konstanz during this work. We acknowledge support by the Fond der Chemischen Industrie and the German Research Foundation (DFG) within project GE 2278/6-1, which was part of the NSF-DFG ?Materials World Network for Particle-mediated Control over Crystallization: From the Pre-nucleation Stage to the Final Crystal.? M.D. thanks the Bavarian Polymer Institute (BPI) and the collaborative research center SFB840 of the German Research Foundation (DFG) for financial support. SAXS experiments were performed on beamline ID02 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France (proposal IHSC-1379). We thank Dr. Peter Boesecke for the assistance in using beamline ID02.

PY - 2019/10/8

Y1 - 2019/10/8

N2 - Hematite (α-Fe 2O 3) is thermodynamically stable under ambient conditions, of vast geological importance, and widely used in applications, for example, as corrosion protection and as a pigment. It forms at elevated temperatures, whereas room-temperature reactions typically yield metastable akaganéite or ferrihydrite. The mechanistic key changes underlying this observation were explored in the present study. The entropic contribution to the prenucleation hydrolysis reaction categorically implies the presence of prenucleation clusters (PNCs) as fundamental precursors. The formation of hematite is then due to a change in the reaction mechanism above approximately 50 °C, whereby the reaction limitation towards oxolation in phase-separated clusters is overcome. A model that rationalizes the occurrence of hematite, akaganéite, and ferrihydrite based on the chemistry of olation PNCs is proposed. Supersaturation and the temperature dependence of olation and oxolation rates from monomeric precursors are irrelevant in this nonclassical mechanism.

AB - Hematite (α-Fe 2O 3) is thermodynamically stable under ambient conditions, of vast geological importance, and widely used in applications, for example, as corrosion protection and as a pigment. It forms at elevated temperatures, whereas room-temperature reactions typically yield metastable akaganéite or ferrihydrite. The mechanistic key changes underlying this observation were explored in the present study. The entropic contribution to the prenucleation hydrolysis reaction categorically implies the presence of prenucleation clusters (PNCs) as fundamental precursors. The formation of hematite is then due to a change in the reaction mechanism above approximately 50 °C, whereby the reaction limitation towards oxolation in phase-separated clusters is overcome. A model that rationalizes the occurrence of hematite, akaganéite, and ferrihydrite based on the chemistry of olation PNCs is proposed. Supersaturation and the temperature dependence of olation and oxolation rates from monomeric precursors are irrelevant in this nonclassical mechanism.

KW - aggregation

KW - crystal growth

KW - hematite

KW - iron oxides

KW - prenucleation clusters

UR - http://www.scopus.com/inward/record.url?scp=85071920989&partnerID=8YFLogxK

U2 - 10.1002/chem.201902528

DO - 10.1002/chem.201902528

M3 - Article

VL - 25

SP - 13002

EP - 13007

JO - Chemistry – A European Journal

JF - Chemistry – A European Journal

SN - 0947-6539

IS - 56

ER -