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

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.