Preparation of Particles for Full-Range Tomography 1153
needle was found to be relatively weak, causing the fibers to commonly fall from the needle tip within minutes of pre- paration. This problem was corrected by suspending the nanofibers at 0.5mg/mL in a solution containing a small concentration of Nafion polymer, made by diluting Nafion 1100EW (Sigma-Aldrich, St. Louis, MO, USA) to 0.02wt% Nafion in ethanol. This bound the fibers to the needle for the remaining preparation steps and tomography experiment, with no contamination observed during tomography. In our experience, with the added Nafion binder, the fibers remain on the needle for at least several weeks after successful sample preparation. In some cases, solvated binder alone may be insufficient to adhere fibers to the tungsten needle. In these cases, priming the needle surface by dipping in M-Bond and then in a dry carbon nanofiber powder before curing can promote adhesion. Needle tips were then dipped in a nanoparticle suspension at 0.5mg/mL concentration for 5–10 s to collect nanoparticles on the fibers.
Figure 3. Images of Pt/CNF (a–c) and Au/STO titanate (d–f) specimens as prepared by needle-dip method. Large-scale sample structure is shown in scanning transmission electron
microscope (STEM) stitched composite images (a,d) and in dark field visible light microscope images with extended depth of field (b,e). Areas selected for tomography are shown in STEM images (c,f) from regions highlighted in red boxes in (a,d). Composites made with Fiji Pairwise Stitching Plugin (Preibisch et al., 2009).
Tomography data for the Pt/CNF sample is available in a separate open data publication (Levin et al., 2016).
Generalized Preparation of a Particle Sample: Au/STO
This preparation procedure can be extended to many types of nanoparticles and nanostructures by collecting the sample on carbon nanofibers which are adhered to the needle tip. To demonstrate the more general procedure, we prepared a nanostructured water-splitting photocatalyst consisting of gold nanoparticles chemically deposited on cubic STO particles, referred to here as Au/STO. 3D tomographic images of this material are useful for measuring properties such as the faceting and pore structure of the STO and the
STEM Imaging and Tomography
For STEM experiments, samples were loaded in a Fischione Instruments Model 2050 on-axis tomography holder (E.A. Fischione Instruments, Inc., Export, PA, USA) with a custom-designed collet (made by Fischione) for compati- bility with 0.508-mm diameter Omniprobe Tungsten Probe Tips. Samples were cleaned with 5% oxygen, 95% argon plasma for 45 s in a Fischione Model 1020 plasma cleaner with a shielded specimen holder port. STEM imaging and tomography was performed in an FEI Tecnai F20 (FEI Company, Hillsboro, OR, USA) with a Schottky field emis- sion gun and a 200kV accelerating voltage. A convergence angle of ~6.9 mrad was used to provide sub-nanometer resolution over a large depth of field. The image signal was collected on an annular-dark field (ADF) detector with a camera length of 300mm for the Pt/CNF sample, and 200mm for the Au/STO sample. To reduce distortions in STEM images introduced by
sample drift and scan noise, at each tilt in the tomography experiments we acquired a series of fast-scan images with 1–2 s/frame and 16 s total acquisition. After data acquisition each image series was aligned by cross-correlation and summed to produce a high signal-to-noise ratio image at
sizes and spatial distribution of the Au particles. In this case, carbon nanofibers purchased from US Research Nanoma- terials (Research Grade Large Inner Diameter Thin Multi-Wall Carbon Nanotubes, id: US4440) were used as a specimen support. These fibers have a nominal outer dia- meter of 30–60nm and inner diameter of 20–50nm. Several other types of carbon nanotubes and nanofibers were investigated, but did not meet all the criteria needed for this application, including sufficiently small size for acceptable suspension, tubes/fibers which are straight enough to encourage fibers to extend well beyond the needle tip, and thin enough fibers or tube walls to maintain a low back- ground signal during tomography data acquisition. The interaction between the carbon fibers and tungsten
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