1156 Elliot Padgett et al.


Figure 6. a: Comparison of Au/STO tomogram cross-sections and intensity histograms for ±75° tilt range and full tilt range. Red lines show the result of automatic threshold segmentation to separate the Au/STO sample from the back- ground void and carbon fiber support. The threshold, chosen using Otsu’s method, is indicated with the red arrow in each intensity histogram. Histograms cover the same intensity range for each tomogram. b: Three-dimensional (3D) surface rendering of full-range tomogram segmented by threshold and morphological filtering, showing Au particles, STO, and internal voids. c: 3D rendering of STO surface in full-range tomogram with color indicating local mean cur- vature, separating curved convex (blue) and concave (yellow) regions from flat facets (green).


the segmented STOcomponent, and then down-sampled and smoothed to remove artifacts due to the discrete voxels of the segmented volume. For each vertex, the local curvature was calculated using a least-squares fit to the neighboring vertices, up to third-nearest neighbors (Kroon, 2014). Figure 6c shows a rendering of the isosurface mesh where the color of each triangular face is determined by the average of the mean curvature values at its three vertices. This allows flat facets, convex edges, and concave surfaces at cube junctions to be easily recognized and quantified, providing a route to connect the surface geometry to the chemical properties of the specimen.


CONCLUSIONS


Full-range electron tomography provides superior 3D recon- structions for quantitative analysis when compared to con- ventional electron tomography, which suffers distortions introduced by themissing wedge. Sample preparationmethods that have been previously demonstrated for full-range tomo- graphy are poorly suited to nanostructured powder specimens, which are commonly studied with electron tomography. In this work, we present a new sample preparation technique for full-range tomography,where particles are collected on carbon nanofibers,which act asweakly scattering supports that extend well beyond the end of a tungsten needle. The demonstrated procedure is simple and produces samples compatible with commercial on-axis tomography holders using inexpensive


consumables and equipment commonly available in sample preparation labs. This approach allows the creation of high- quality tomograms, as we have demonstrated for Pt/CNF fuel cell catalyst and Au/STO photocatalyst specimens using the simple, well-understood WBP algorithm. Full-range tomo- graphy enables accurate automatic segmentation and quanti- tative analysis. The sample preparation approach presented in this work could enable full-range electron tomography across a variety of samples that can be dispersed in liquid to provide accurate, quantitative analysis of their 3D nanostructure.


ACKNOWLEDGMENTS


This work was funded by Department of Energy SBIR grant DE-SC0011385. E.P. acknowledges support from a NSF Graduate Research Fellowship (DGE-1650441). J.C.D. acknowledges support from CAPES, Brazil (13159/13-5). Electron microscopy facility support from the NSF MRSEC program (DMR 1120296). The authors thank Dr. Ryo Wakabayashi, Dr. Megan Holtz, and Yi Jiang for assistance and useful discussions, and Dr. Zhongyi Liu and General Motors for providing the Pt/CNF sample.


REFERENCES


ANDRZEJCZUK, M., ROGUSKA, A., PISAREK, M., HOŁDYŃSKI, M., LEWANDOWSKA,M. & KURZYDŁOWSKI, K.J. (2017). Morphology of TiO2 nanotubes revealed through electron tomography. Micron 95,35–41.


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