STEM with Atomic Resolution
Å), compared to the particle status in Figure 4a. In the corre- sponding enlarged panel, Figure 4e, it is evident that an extra atomic layer (layer #3) had grown on the (100) surface. Te lat- tice spacing between layer #2 and layer #3 is found to be 0.195 nm (1.95 Å), which is exactly half of Δd. Tis indicates another extra atomic layer has also started growing on the opposite (100) facet at the lower-right corner of Figure 4b. Similarly, by comparing Figures 4a and 4b, an extra layer can also be iden- tified on the other two (100) surfaces (the one at the top right and the one at lower leſt). Furthermore, at an elapsed time of 128 s, Figure 4c, the measured diameter, d128s
= 13.57 nm, indi-
cating a further increase of 0.39 nm (3.90 Å), demonstrating that the particle continued to undergo layer-by-layer growth at (100) surfaces. Te atomic resolution image of Figure 4f shows the newly grown layer #4. To determine the composition of the newly grown lay-
ers, HAADF-STEM intensity analysis was performed by using Gatan Microscopy Suite Soſtware. Intensity profiles were obtained from the same (020) plane indicated by blue boxes in Figures 4d–4f at t = 0, 64, and 128 s, respectively. Ten, as summarized in Figure 5, the new layers #3 and #4 can be quan- titatively represented in the sequential intensity profiles. Com- parison of profiles (ii) and (iii) indicates that the intensity of layer #3 is increasing during the interval of 64 s–128 s. At 128 s, layer #3 may be considered completely grown since the new layer #4 has started growing. Te columns in layer #3 exhibit
an intensity similar to that of the segregated Pt layers #1 and #2, which is higher than that of the inner Co columns. For con- firmation, a HAADF-STEM image simulation was performed based on a segregated intermetallic Pt3
Co model with a three-
layer pure Pt shell, by using the QSTEM simulation package [14]. According to the simulation results, the intensity profile of the three-layer Pt shell shows the same trend as the corre- sponding layers #1–#3 in the experimental profile (iii), sup- porting the experimental result that the newly grown layer is, indeed, a Pt-rich shell.
Discussion Using a windowed gas cell specimen holder for in situ
STEM, we observed surface phenomena that are difficult or impossible to observe by other methods. Direct observation of the reconstruction of metallic surfaces at realistic tempera- tures and pressures has been sought for decades. In the first experiment, the truncated octahedron shape
adopted by bare Pt nanoparticles was observed to reconstruct as a result of CO adsorption. In this facet-specific process, flat (100) facets roughen into vicinal stepped high Miller index fac- ets, while it is assumed that flat (111) facets remained intact. Te relatively high partial pressure of CO (> 20 Torr), allowed in the windowed gas cell, ensured saturation CO coverage, which triggered the observed surface reconstruction of Pt nanoparticles.
Figure 5: Intensity profiles taken along the blue boxes in Figure 4 (0 s, 64 s, and 128 s) and (iv) the result of an HAADF image simulation. Inset shows a simulated image of the three-layer Pt shell on the (100) surface.
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