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STEM with Atomic Resolution


{100} surface, indicated by the yellow arrow in the experimen- tal image (Figure 3b). A density functional theory (DFT) calculation based on


Wulff construction models was performed by using the real space grid-based projector-augmented wave method (GPAW) open source code [11]. Te surface energies of bare Pt facets and CO-covered Pt facets were calculated. Aſter CO adsorp- tion, the surface free energy of the {100} facet was comparable to that of stepped vicinal surfaces, for example, {210} and {310}, which are higher index surfaces likely to be more reactive. Tus, as shown in Figure 3c, the newly formed Layer 0 is created through the migration of Pt atoms from lower {100} layers (for example, Layer 1 and Layer 2). More under-coordinated Pt sur- face atoms (with metal−metal coordination numbers of 6 or 7), as indicated by blue atoms in Figure 3c, come into appearance next to the (100) surface. Both the STEM image and the DFT simulations suggest that under saturation CO coverage, the terminating Pt {100} facets will reconstruct to vicinal steps. Upon further CO annealing at a higher temperature of 300 °C for 10 minutes, the layer 0 was observed to disappear and the Pt particle reverted to its original shape [8]. Te reversibility of the Pt surface reconstruction reflects the desorption of CO molecules, shown by in situ infrared spectroscopy to take place above ∼230 °C [12]. Ordering and Pt surface enrichment in Pt3


Co nanopar-


ticles Te second example we present is the in situ STEM obser- vation of surface evolution in ordered Pt3


Co nanoparticles [13],


which is important for the oxygen reduction reaction (ORR) in fuel cell catalysts. To better understand the formation process of a Pt shell, which is crucial for enhancing ORR activity, we performed an in situ experiment on carbon-supported Pt3


Co


nanoparticles under static pure oxygen at 760 Torr. During the in situ experiment, the temperature was first


elevated to 720 °C, which is close but below to the order- disorder transition temperature of a 75 at%Pt-25 at%Co alloy, at a high rate of 5 °C/s. Upon oxygen annealing at 720 °C for 30 min, the initially disordered Pt3


formed to an ordered intermetallic L12


Co nanoparticles were trans- phase where Pt surface


segregation had also taken place to form a two-layer Pt-rich shell. Away from the particle surface, the ordered (100) planes alternate as pure Pt and pure Co planes. In the atomic-level HAADF-STEM imaging of the (100) planes, the last Co layer at the particle edge (low intensity) provided a marker and was labeled the #0 layer. Additional Pt layers caused by the process- ing in oxygen then could be identified and counted as layers #1 through #4. Aſter the processing above, the temperature was rap-


idly cooled to 300 °C. Te surface configuration of the Pt3Co nanoparticle was


Co nanoparticle was found to be


larger at an elapsed time of 64 s with the measured diameter, d64s


stable as shown in Figures 4a and 4d.


Sequential atomic-scale HAADF-STEM images were then taken at a scanning speed at one frame every 16 s at 300 °C. As shown in Figure 4b, the Pt3


, of 13.18 nm, showing an increase of Δd = 0.39 nm (3.90


Figure 4: In situ STEM observation of layer-by-layer growth of a (100) Pt shell on ordered Pt3


Co nanoparticles during oxygen annealing. (a–c) Sequential HAADF-


STEM images taken at 0 s, 64 s, and 128 s during oxygen annealing at 300 °C, respectively. Scale bar = 2 nm. (d–f) Corresponding enlarged false-color images of the (100) cross section of the surface from region indicated by red boxes in (a–c), respectively. Beam direction along [001]. Yellow and blue spheres represent Pt and Co atoms, respectively. Scale bar = 0.5 nm (5 Å).


2019 May • www.microscopy-today.com 19


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