X-ray absorption fine structure(XAFS) spectroscopy has been widely used for decades in a wide range of scientific fields, including physics, chemistry, biology, materials sciences, environmental sciences, etc. This review article is devoted to the applications of XAFS in nanomaterials. The basic principles of XAFS are briefly described from the view point of practical application, including its theory, data analysis and experiments. Using selected examples from recent literatures, the power of XAFS in determination of local atomic/electronic structures is illustrated for various nanomaterials, covering metal and semiconductor nanoparticles, catalysts, core/shell structures, ultrathin nanosheets, and so on. The utilization of time-resolved XAFS technique is also briefly introduced, for in-situ probing the nucleation/growth processes of nanomaterials and identifying reaction intermediates of nanostructured catalysts under operando conditions.
Copper ion is the essential microelement to many organisms. In this paper, the local structure of Cu2+ in CuBr2 aqueous solutions with different concentrations are investigated by using X-ray absorption fine structure (XAFS) technique. XANES (X-ray Absorption Near Edge Structure) spectra indicate that charge transfer from Br- to Cu2+ decreases with the solution concentration, which lead to a shift of the absorption edge. The shoulder appearing at the rising edge proves to be characteristic of a tetragonal distortion. The Fourier transform magnitudes of EXAFS (Extended X-ray absorption fine structure) data of Cu species suggest that more Cu-Br bonds may exist in high concentrations. A fivefold coordination configuration like a pyramid is used as the fitting parameters. From the analysis of the coordination numbers, the proportion of Cu-O and Cu-Br is 4:1 in the saturated solution. The Br atom is on the equatorial plane of the model. The fitting results agree well with the experiment data.
DAI Binbin WANG Qian MA Jingyuan LI Jiong ZHANG Shuo HUANG Yuying HUANG Wei WU Guozhong ZOU Yang JIANG Zheng XU Hongjie
In situ quick X-ray absorption spectroscopy(QXAFS) at the Cu and Zn K-edge under operando conditions has been used to unravel the Cu/Zn interaction and identify possible active site of CuO/ZnO/Al_2O_3 catalyst for methanol synthesis. In this work, the catalyst, whose activity increases with the reaction temperature and pressure, was studied at calcined, reduced, and reacted conditions. TEM and EDX images for the calcined and reduced catalysts showed that copper was distributed uniformly at both conditions. TPR profile revealed two reduction peaks at 165 and 195 °C for copper species in the calcined catalyst. QXAFS results demonstrated that the calcined form consisted mainly of a mixed Cu O and Zn O, and it was progressively transformed into Cu metal particles and dispersed Zn O species as the reduction treatment. It was demonstrated that activation of the catalyst precursor occurred via a Cu^+intermediate, and the active catalyst predominantly consisted of metallic Cu and Zn O evenunder higher pressures. Structure of the active catalyst did not change with the temperature or pressure, indicating that the role of the Zn was mainly to improve Cu dispersion.This indicates the potential of QXAFS method in studying the structure evolutions of catalysts in methanol synthesis.
