1152 Elliot Padgett et al.


a variety of materials such as nanoparticles, nanostructured microparticles, and nanofibers. This simple, bench-top procedure requires no special equipment beyond a full rotation tomography holder and allows for high-quality tomography reconstructions without the missing wedge. The method can be adapted to any type of on-axis or micro- pillar holder. We demonstrate this technique for two tech- nologically relevant nanoparticle specimens: (1) a Pt/CNF fuel cell catalyst in which the CNF structure and spatial relationship to Pt is of interest and (2) a gold-decorated strontium titanate (SrTiO3 or STO) nanoparticle photo- catalyst in which the carbon fiber is used as an ignorable specimen support.


MATERIALS AND EXPERIMENTAL METHODS


tip of the needle and other nanofibers that have been collected. It is convenient for the fiber to be oriented roughly parallel to the needle axis, although angles over 45° can be acceptable. The general arrangement of nanofibers on the needle tip can be observed in a visible light microscope (VLM), most easily in dark-fieldmode (Figs. 3b–3e).AVLMis useful for preliminary screening and guidance during the sample preparation, as the needle can be re-dipped until a suitable quantity and arrangement of fibers is achieved. Each sample takes only a few minutestoprepare,soit isconvenienttoprepare 5–10 samples that appear acceptable in VLM screening before proceeding to screen samples in the (S)TEM, so that at least one sample that is suitable fortomographyisproduced. In this investigation, we used Omniprobe Autoprobe-


General Sample Preparation Approach Samples for this investigation were supported on carbon nanofibers that adhere to the tip of a tungsten needle, which was loaded into an on-axis TEM holder (Fig. 2). Carbon nanofibers were collected on the needle by dipping the needle tip in a suspension of nanofibers in alcohol, which may also contain a small concentration of binder. Subsequently, particle samples may be loaded onto the nanofibers by dipping the needle tip again in a suspen- sion containing the sample.Abrief, step-by-step summary of the procedure is provided in the Appendix for practical reference. In an ideal sample, a single nanofiber extends beyond the


250 tungsten needles (Oxford Instruments, Abingdon, UK) which we cleaned in batches by cathodic etching in an argon plasma using a Bal-Tec SCD 050 sputter coater (Bal-Tec A.G., Balzers, Liechtenstein). We have observed that without cleaning, the needle tips may bend or curl upon contacting a liquid surface. Cleaning either with a 25% oxygen 75% argon plasma or with cathodic etching in an argon plasma was found to be effective. Nanoparticle and nanofiber suspensions were dispersed using a bath ultrasonicator, and pipetted into a shallow ceramic multiwelled plate for the dipping procedure. Samples were screened using an Olympus BH-2 VLM (Olym- pus Corporation, Tokyo, Japan) with an Infinity 2 camera at 500× magnification. Visible light images shown in Figures 3b


Figure 2. Schematic cartoon of sample preparation method. First, a tungsten needle is dipped in a suspension of nanofibers in order to collect fibers on the needle tip. To prepare a general nanoparticle sample, the needle is then dipped in a nanoparticle suspension to collect particles on the nanofiber support. Sample is loaded into an on-axis tomography holder for the tomography experiment in the scanning transmission electron microscope (STEM).


and3eweregiven asimpleextended-depth-of-field enhance- ment by summing three images at different specimen heights.


Preparation of a Simple Nanofiber Sample: Pt/CNF


Our first example of this preparation is a commercial fuel cell catalyst consisting of platinum nanoparticles chemically synthesized on carbon nanofibers, Pt/CNF. 3D tomographic images of this material are useful for measuring properties such as the distribution of Pt nanoparticle sizes, shapes, and locations on the carbon. The platinum nanoparticles are supported on the carbon fibers in the as-synthesized mate- rial, rather than being loaded on during sample preparation. This simplifies the sample preparation procedure. The Pt/CNF powder was dispersed in methanol at a concentra- tion of ~0.5mg/mL, and collected onto the needle tip in a single dipping step. No binding agent was added, as the sample interaction with the tungsten needle was sufficiently strong for fibers to adhere and remain on the needle.


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