(Invited) Tracking Structures in Solar Fuels Catalysis: In-Situ X-Ray Structure Characterization of Interfacial Water-Splitting Molecular and Thin-Film Catalysts
Authors: Tiede David M , Kim Tae Wu , Sprague-Klein Emily , Kwon Gihan , Martinson Alex B. F. , Mulfort Karen L. 
Published: February 2020

A key challenge for the design of efficient photoelectrochemical devices that employ nanoscaled interfacial designs for solar energy-to-fuels conversion lies in resolving atomic structures and dynamics at the active interfaces, and relating these to the complex cascade of events which includes excited-state charge separation, charge-accumulation, and multi-step energy conversion and catalysis. Our research programs have been developing in-situ, time-resolved X-ray techniques aimed at the resolution of atomic scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function for both interfacial thin-films and molecular photosensitizers and catalysts bound to semiconductor surfaces.

This presentation will discuss our development of X-ray techniques aimed at the resolution of atomic scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function. We have developed high surface area TiO2 semiconductor and ITO and IZO conducting oxide porous assemblies that duplicate oxide-supported dye and catalyst architectures widely used in dye sensitized solar cells (DSSC) and photoelectrochemial (PEC) electrodes, but have been tailored to allow for interfacial X-ray structure characterization.

Thin-Film Metal-Oxide Water Oxidation Catalysts. Non-noble-metal, thin film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L-edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, while CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4p-3d hybridization through Co-O-Co bonding was detected by RXES and the inter-site dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure while CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance OEC performance.The combined electronic and structural analyses demonstrate that hole transfer to catalytic sites in amorphous cobalt oxide thin films is the rate-limiting step for electrochemical water-splitting rather than the multi-step catalytic events themselves. The macroscopic catalytic properties of the thin films were found to be correlated to the electronic structures measured at the atomic scale for the metal-oxo cluster domains. These results show the interplay between intrinsic catalytic activity and charge transport properties of semiconductor thin-film catalysts.

In-situ Structure-Function Analysis of Molecular Water Oxidation Catalysts.
Building from the microporous electrodes for in-situ X-ray analyses of amorphous oxide thin-film OECs, we have extended this approach by the development of nano-porous electrode architectures which enable the use of combined PDF and X-ray spectroscopy analyses of coordination structures for molecular photosensitizers and catalysts in solution and when bound to semiconductor oxide surfaces. PDF measurements require PEC oxide supports with sufficiently low background scattering to permit detection of the surface bound complexes. We have found that atomic layer deposition of semiconductor oxides on anodic aluminium oxide templates, AAO, provides a suitable layered, high-surface area architecture. PDF patterns measured for the N3 dye, cis-bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II), bound to an amorphous TiO2 surface reveal details on outer sphere ligand structures. Each of the PDF peaks can be assigned to the atom pair distances calculated from crystal and DFT structures. Measurements on ruthenium photosensitizer complexes bound to the TiO2 surface resolve distortions in the surface-coordinating ligand structures. On-going work is investigating the correlation between ligand structural distortion and the metal-to-ligand charge-transfer (MLCT) states with interfacial charge injection function.