Supporting Information

Scanning transmission X-ray microscopy studies of chromium hydroxide hollow spheres and nanoparticles formed by gamma-radiation

Zhiqiang Wang,1 L. M. Alrehaily,1 Jiju Joseph,1 Jungsook Clara Wren,*,1 Jian Wang2 and Tsun-Kong Sham*,1

1 Department of Chemistry, the University of Western Ontario, London, Ontario N6A 5B7, Canada

2 Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada S7N 0X4

* Author to whom correspondence should be addressed. Email: and

Thickness modelling of Cr(OH)3 hollow spheres and solid nanoparticles

To obtain the thickness distribution of Cr(OH)3 hollow spheres and solid nanoparticles, firstly reference spectrum of Cr(OH)3 was acquired through stack scan at O K-edge. In the absence of absorption saturation, XANES spectrum is obtained through the transmitted light intensity and the incident beam intensity (recorded in the absence of sample region of the same stack) using the Beer-Lambert law:

(1)

where OD is the optical density, also called absorbance (A), T is the transmittance, I0 and I are the incident and transmitted X-ray photon flux (photons) respectively, μ is the energy dependent mass absorption coefficient (cm2/g), ρ is the density (g/cm3) of the material, and l is the sample thickness (nm). Then the spectrum was converted to absolute linear absorbance scales (i.e. optical density per nm thickness sample). This was manipulated using the software package aXis2000 by scaling the XANES spectrum of the pure material to match that of the linear X-ray absorption for the elemental composition of the material. For a specific compound, the elemental linear X-ray absorption, which neglects interactions such as bonding among the atoms, is calculated by aXis2000 using equation 1 and 2 (which computes sum of the weighted atomic photoabsorption cross sections) with a known density,

(2)

where NA is the Avogadro’s number, M is the molecular weight of a compound containing xi atoms of type i, σai is the atomic photoabsorption cross section (cm2/atom) for type i atom.

Specifically, O reference spectrum was obtained by fitting the original O K-edge XANES spectrum to match its calculated elemental linear X-ray absorption profile (Cr(OH)3: density = 3.11 g/cm3, and thickness = 1 nm) in the pre-edge and continuum. The reference spectrum of Cr(OH)3 and its elemental linear X-ray absorption profiles are shown in Figure S1. Then, the thickness was obtained by comparing the OD of each pixel in a STXM image and reference spectra.

Figure S1 Reference spectrum and elemental linear X-ray absorption profiles of Cr(OH)3 at O K-edge (black line: optical density spectrum of 1 nm thickness, red line: elemental linear X-ray absorption profile based on formula Cr(OH)3, density = 3.11 g/cm3, and thickness = 1 nm).

1