Methods

XAS

XAS is a widely used technique for determining the local geometric and/or electronic structure of matter. In XAS, the absorption of photons excites electrons from deep core levels, such as 1s, of a selected atom in a molecule to unoccupied states, leaving behind a core hole. The decay of core holes may induce to an avalanche of electrons or photons; thus we can measure the total electron yield (TEY) or the fluorescence yield (FY) to obtain the x-ray absorption spectrum. XAS is an element specific technique. Pre-edge and Edge (XANES) are dominated by core transitions to quasi bound states (multiple scattering resonances) for photoelectrons and provides electronic information, eg. oxidation state. EXAFS (extended X-ray absorption fine structure) provides geometric information, eg. metal-oxygen bonding length.   

Our group recently developed in situ cell for XAS study which can be operated under fluoresce and transmission mode. We are using it to study the interface layer of OER catalysts/electrolyte under visible light illumination and potential. In detail, we aim to study the structure and valence change of transition metals eg. Ni, Co, Fe and Mn of OER catalysts on hematite photoanodes under visible light. Understanding the chemical structure and valence change at the catalyst/electrolyte is an essential step toward gaining a mechanistic understanding of water oxidation.

XES/RIXS

Like XAS, XES probe the energy distribution of electronic states in atoms, molecules, and solid state materials. In-situ and operando measurements. In XES, the core hole is filled by a valence electron causing the emission of an x-ray photon. XES gives information about chemical bonding in the molecule. By combining XAS and XES, one can obtain information about unoccupied states (conduction band for inorganic materials) and occupied states (valence band).

Resonant inelastic X-ray scattering (RIXS) spectroscopy is a powerful tool to reveal the electronic structure of strongly correlated electronic systems. With the capability of tuning incident photon energy, the specific intermediate state can be selected and the direct information of electronic structure can be obtained. It also has the advantages of bulk sensitivity, element specificity and symmetry selectivity.

Reference:

  1. Kanan, M. W.; Nocera, D.G. Science, 2008, 321, 1072-1075.
  2. McCrory, C.; Jung, S.; Ferrer, I.; Chatman, S.; Peters, J. C.; Jaramillo, T.; J. Am. Chem. Soc., 2015, 137, 4347-4357.