Quietly Building Capabilities
allows in situ imaging of spatially resolved reaction kinetics and dynamics at specific catalytic reaction sites. In general, catalytic reactions are activated and have to be carried out at elevated temperatures. Using LT SPM in combination with collimated beams of reagents with well-defined hyperthermal kinetic energies allows probing reactions at low temperatures. Te instrument also enables detailed measurements of nucleation and growth processes important for materials synthesis, offers true cryo (5 K) scanning tunneling and non-contact atomic force microscopes, and can perform single-molecule vibrational spectroscopy. It combines in situ operation with traditional, ensemble-averaging surface analytical tools, providing a truly unique, state-of-the art capability. Ultra-High Vacuum, Variable-Temperature Scanning
Probe Microscope (UHV VT SPM). Tis existing EMSL instrument will be moved into the Quiet Wing to optimize its performance. Manufactured by Omicron NanoTechnology, it is primarily used for studies of model catalytic systems and associated surface thermal and photochemistry under ultrahigh vacuum conditions. It includes a pair of interlocked ultrahigh vacuum chambers: a scanning probe chamber and a sample preparation/characterization chamber with ensemble- averaged surface analysis capabilities. Both the scanning tunneling microscope (STM) and non-contact atomic force microscope provide atomic resolution in a full temperature range of 50–500 K.
Science Made Possible Te singular motivation for deploying these instruments
within the Quiet Wing is to enhance observation of scientific processes, with a specific focus on pushing the boundaries of in situ imaging for science areas of national and global importance. As scientists at EMSL, we are eager to collaborate with users to attain better images and spectra that lead to new discoveries. Although the Quiet Wing has yet to open, several of the tools described above have already yielded promising results for catalysis, energy storage, and health-related biology. Catalysis. A major challenge in TEM is to enable a deeper
understanding of catalytic reactions and interfacial/surface properties. Addressing this need calls for 3-D structural and chemical analyses of nanoparticles and nanostructures and, ideally, for the position and type of each atom involved. Te difficulty in obtaining this information using standard tomographic techniques is that a large number of images (~150) is required for 3-D reconstruction. Te total exposure time can be several hours, which results in an electron dose that causes severe radiation damage in many systems of scientific interest, such as nanoparticles with catalytic properties. Much of today’s catalysis-focused TEM is indeed hampered by low time-resolution, strong radiation damage, and poor stability. However, positive steps are taking place through atomic-level characterizations of transition metal catalytic clusters on oxide substrates in order to determine the structure-property relationships of small clusters. For example, Figure 4 shows iridium nanoscale clusters supported by MgAl2O4 in an image collected with EMSL’s aberration-corrected S/TEM. With high-angle annular dark field (HAADF) imaging, contrast is highly sensitive to the atomic number (Z1.7) [2]. A very high signal-to-noise ratio in Figure 4 enables correlation between the electron count rate at the position of atomic columns and
2011 September •
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Figure 4: Iridium clusters supported by MgAl2O4.
the number of atoms in these columns, which subsequently enables approximation of the full three-dimensional shape of the catalytic nanoclusters. Although this approach requires an acquisition of only a single image, it can lead to several possible structural models. Tus, ab initio calculations using supercomputing resources should be further used to identify the most likely model and achieve higher accuracy for atomic positions. Additional integration of the results with those from other tools can provide unprecedented analysis of how the catalyst interacts with its support. For example, high-resolution TEM phase contrast reveals the epitaxial relationship between the Ir clusters and the MgAl2O4 substrate, allowing quantitative analysis of the clusters. Te in situ challenge is to reveal how the shapes of catalytic nanoparticles change in real time, in realistic, gaseous environments. In addition to its microscopy efforts, EMSL is making strides in this area within the ultrahigh-field nuclear magnetic resonance capability. Energy Storage. Rechargeable lithium-ion batteries
are ubiquitous across today’s technologies, from laptops to cars, and they play a key role in the overarching goal of improving energy storage applications.
In exploring why
these batteries succeed or fail under operating conditions, in situ TEM has played a vital role. In a recent collaboration with the Center for Integrated Nanotechnologies, a concept was developed at EMSL to create a working lithium-ion nanobattery that used a single nanowire as an electrode and an ionic liquid-based electrolyte, designed specifically for placement inside a high-vacuum TEM to allow observation of the electrode’s structural evolution during charging of the nanobattery [3]. One outcome of the effort was a video [4] that captured the microstructural evolution of a tin oxide (SnO2) nanowire anode, which measured 100 nm in diameter and 10 µm in length. With the progression of the lithium injection into the SnO2, the nanowire exhibited swelling,
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