Fixed Energy X‑ray Absorption Voltammetry

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV represents a novel in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials and consists of recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the absorption coefficient. Although the information given by FEXRAV obviously does not supply the detailed information of X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS), it allows to quickly map the oxidation states of the element under consideration within the selected potential windows. This leads to the rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window) and is preliminary to more deep X-ray absorption spectroscopy (XAS) characterizations, like XANES or EXAFS. In addition, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis

Role of Interfacial Energy and Crystallographic Orientation on the Mechanism of the ZnO + Al₂O₃ → ZnAl₂O₄ Solid-State Reaction: I. Reactivity of Films Deposited onto the Sapphire (110) and (012) Faces

The initial steps of the reaction between ZnO and Al₂O₃ have been investigated with X-ray diffraction, atomic force microscopy, and X-ray absorption spectroscopy at the Zn–K edge starting from 45 nm thick zincite films deposited onto (110)- and (102)-oriented sapphire single crystals. The formation of nonequilibrium phase(s) has been detected for both orientations. For the (001)_zincite ∥ (110)_sapphire interface, the rate-determining step is the motion of the interface(s); the growth of the spinel layer is linear with time, with a rate constant <i>k</i> = 1.1(2) × 10^–9 cms^–1 at 1000 °C. At the (110)_zincite ∥ (012)_sapphire interface, the reaction shows dumped oscillations. The results are discussed along with a comparison with previous results on thinner films to clarify the role of interfacial free energy and crystallographic orientation