IrO2 Surface and Nanostructure Stability from First Principles and Variable Charge Force Field Calculations

Sunday, 24 May 2015: 17:10
Conference Room 4D (Hilton Chicago)
F. G. Sen, A. Kinaci, B. Narayanan, M. J. Davis, S. K. Gray, S. K. R. S. Sankaranarayanan, and M. K. Y. Chan (Argonne National Laboratory)
IrO2 is one of the most efficient water-oxidation electrocatalysts and can also act as a photocatalyst for the water splitting reaction when used in the form of nano-sized clusters. The structure and surface properties of nanoclusters greatly influence the photocatalytic properties of IrO2, and atomistic scale understanding of these properties is essential to elucidate the photocatalytic mechanisms of IrO2. Here, we carried out first principles calculations based on spin polarized density functional theory (DFT) including spin-orbit coupling and the Hubbard U correction on the bulk and surface structures of IrO2. The stability and electronic structure of low index rutile (100), (001), (110) and (101) surfaces of IrO2, were studied. The relative surface energies were obtained as (110) < (101) < (100) < (001). The equilibrium shape of IrO2 nanoparticles was deduced using a Wulff construction. In order to study the structural stability of IrO2 nanoclusters and the long time-scale dynamics of IrO2 polymorphs larger than about 2 nm in diameter, we developed the first empirical interatomic potential (force field) for IrO2 based on Morse functional form coupled with a variable charge method (QEq). The Morse+QEq parameters were optimized using an evolutionary algorithm with respect to a DFT training set, and was shown to be successful in predicting bulk and surface properties of rutile IrO2 and various polymorphs including anatase, brookite, columbite and pyrite derived from DFT calculations. Pressure induced phase transformations of bulk IrO2 polymorphs were reported and thermodynamically stabile phases of IrO2 at nano-scale were obtained using the surface energies of stable polymorphs. Our results will shed light on the development of stable nanoscale IrO2 electrocatalysts and photocatalysts that can efficiently utilize solar energy for water splitting reaction.

Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.