In iridium oxide catalysts, the electronic states whose energies are in the range of energetics and charge transfer kinetics of the oxygen evolution reaction (OER) originate from the Ir 5d orbital states. However, the understanding of the atomic structures and orbital states underlying catalytic reactivity in amorphous iridium oxide oxygen evolving catalysts (Ir-OECs) is incomplete compared to that of crystalline oxides, owing to a lack of direct experimental verification. Here, we present experimental approaches using resonant inelastic X-ray scattering (RIXS) to directly access Ir 5d orbital excitations at the Ir L3 edge and atomic pair distribution function (PDF) measurements to characterize electronic and coordination structures at the atomic scale. The so-called iridium blue layer (IrBL) and IrOx were formed from the organometallic precursor complex [Cp*Ir(H2O)3]SO4 and the inorganic precursor IrCl3, respectively. Ex situ IrBL and IrOx films for RIXS and PDF measurements were prepared by conditioning electrodeposited films at a low voltage. The incident energy RIXS profile of IrOx exhibited extra weak resonantly enhanced excitation below 2 eV energy loss. The feature was clearly different from a single high-energy excitation above 3 eV of IrBL related to the interband transition between π- and σ-antibonding states. The atomic structure refinement based on PDF measurements revealed the atomic structure domains to have edge- and corner-shared IrO6 octahedra with trigonal-type distortion. Density functional theory calculations guided by the refined atomic structures shed light on the electronic structure corresponding to experimental results, including insulating and metallic phases in ex situ IrBL and IrOx films, respectively. Our study establishes different Ir 5d orbital states and atomic structures in two amorphous Ir oxide OER catalysts in their reduction states.
|Number of pages||11|
|Publication status||Published - 2021 Aug 6|
Bibliographical noteFunding Information:
Research at beamline 28-ID-1 and 28-ID-2 of the National Synchrotron Light Source II was supported by U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. G.K. also gratefully acknowledges support of 11-ID-B and 6-ID-D to collect high-energy X-ray scattering data and also thanks Dr. Stafford Sheehan (Catalytic Innovations) for electrochemical deposition of IrBL. Work at Yale University (Ir precursor complex synthesis) and Argonne National Laboratory (PDF analysis) was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Contract Nos. DE-FG02-07ER15909 and DE-AC02-06CH11357, respectively. B.J.K. was supported by IBS-R014-A2. Experiments at 3A beamline of PLS-II were supported in part by MIST. S.H.C., J.E.H., and K.J.L. were supported by Basic Science Research Programs through the National Research Foundation of Korea (NRF-2019K1A3A7A09033393 and 2020R1C1C1012424). R.J.K. was supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DOE-BES) under Contract No. DE-SC0012704.
© 2021 American Chemical Society.
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