Data acquisition

A detailed methodology for nano-XANES data collection was reported previously.¹⁰ XRF maps are collected across the Fe–K absorption edge at 7.112 keV (from 7.08 to 7.20 keV with 73 energy points). The total acquisition time for the model sample was about 24 h. The individual XRF maps were processed by fitting the XRF peaks and producing 3D image stacks, where each voxel contains the integrated Fe XRF signal. The individual Fe maps were spatially aligned by performing an image registration correction,²⁶ which is necessary because the focused beam position with respect to the sample could be shifted while changing the incidence X-ray beam energy. Chemical or oxidation-state images are produced from the aligned XRF image stacks using component analysis and XANES fitting analysis.