Understanding the electrochemical double layer at the hematite/water interface: A first principles molecular dynamics study

Kanchan Ulman; Emiliano Poli; Nicola Seriani; Simone Piccinin; Ralph Gebauer

doi:10.1063/1.5047930

Understanding the electrochemical double layer at the hematite/water interface: A first principles molecular dynamics study

J Chem Phys. 2019 Jan 28;150(4):041707. doi: 10.1063/1.5047930.

Authors

Kanchan Ulman¹, Emiliano Poli¹, Nicola Seriani¹, Simone Piccinin², Ralph Gebauer¹

Affiliations

¹ The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34151 Trieste, Italy.
² CNR-IOM Democritos, c/o SISSA, Via Bonomea 265, 34136 Trieste, Italy.

PMID: 30709242
DOI: 10.1063/1.5047930

Abstract

Using first principles molecular dynamics simulations, we probe the electrochemical double layer formed at the interface between the hematite surface and water. We consider two terminations of the (001) surface, viz., the fully hydroxylated (OH) and the stoichiometric (FeO₃Fe) termination. We explicitly incorporate the counterions (Na⁺ and F^-) in the solution, and model both specific and nonspecific adsorption of F^- ions. We find that F^- ions prefer to bind directly to the Fe ions (specific adsorption), with a substantial energy gain (0.75 eV/ion). We investigate the effect of the interface and the counterions on the dipole of individual water molecules. We find significant deviations of +0.2/-0.15 D for dipoles of the first solvation shell water molecules of F^-/Na⁺ ions, respectively. Additionally, the hydration layers at the interface show an enhancement in the dipole moment resulting from stronger hydrogen bonding interactions between the water molecules and surface charged species. Furthermore, we analyze the electrostatic potential profile at the solid/liquid interface as a function of the kind of counterion present in the double layer and compute the capacitance of the compact (Helmholtz) layer. We find that our results (40.3 ± 3.5 μF/cm² for the OH termination and 51 ± 5 μF/cm² for the FeO₃Fe termination) compare favorably with values reported by potentiometric titration based experimental studies (10-100 μF/cm²).