(Invited) Resolving Sites for Catalysis in Amorphous Metal Oxide and Molecular Water-Oxidation Catalysts Using in-Situ X-Ray Techniques

Tuesday, 3 October 2017: 14:40
National Harbor 6 (Gaylord National Resort and Convention Center)
D. M. Tiede (Argonne National Laboratory), G. Kwon (Northwestern University), I. S. Kim, A. B. F. Martinson (Argonne National Laboratory), J. Hoyoung, J. S. Lee (Stanford University), L. Kohlar, and K. Mulfort (Argonne National Laboratory)
A key challenge for the development of materials and technologies for solar-to-fuels energy conversion lies in resolving structure, structural dynamics, and mechanisms underlying solar fuels catalysis, and measuring these structural parameters during functional conditions. This presentation will discuss the resolution of sites for solar-driven catalysis in amorphous cobalt oxide and molecular water-oxidation catalysts using a combination of in-situ high energy X-ray (60 keV) scattering with atomic pair distribution function (PDF) analysis, cobalt L3 edge X-ray absorption spectroscopy, and anomalous wide angle X-ray scattering (AWAXS), with a goal of resolving structure, "one electron at a time", following successive, single-electron, photo-initiated electron transfers. We have extended the in-situ PDF technique by developing 3-D porous electrode architectures that enable structural characterization of interfacial thin films during photo-electrochemistry with 0.2 Å spatial resolution. PDF-electrochemistry measurements for the cobalt borate OEC film, CoBi, poised across the potentials from 0.5 V and 1.4 V vs NHE show lattice contraction of the domains as a result of bond shortening by the accumulation of metal centers in high valence oxidation states. The structural change is associated with increased metal-oxo covalency and charge delocalization. Comparable in-situ PDF measurements for the cobalt phosphate, CoPi, OEC show that charge accumulation leads to similar domain lattice contraction. In addition, for CoPi the CoIICoIII to CoIIICoIII transition is found to be coupled to Co-O peak broadening and amplitude decreases for pairs involving edge O atoms, identifying these as the sites for redox activity. Corresponding changes are seen in the P-O peak. These results provide experimental evidence for a Pi edge-associated model with bond length disorder. The PDF data shows that Pi remains edge-bound at the onset of water oxidation, suggesting the possible positioning of Pi for function in proton-coupled electron transfer. The cobalt OEC are also distinguished by X-ray absorption at the L3 edge, with CoPi showing both octahedral and tetrahedral Co(II) assigned to lattice and edge or extra-domain connecting sites, respectively. Finally, we demonstrate new AWAXS measurements for structure resolution in molecular and mixed metal OEC.