Electrochemical Atomic-level Controlled Syntheses of Electrocatalysts for the Oxygen Reduction Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 153

Pd L
Pt L

150

d n 9 r 4 n g | 4

100

50

0

0 2 4 6 8 10

Distance (nm)

(a) (b)

Figure 6.3 Microscopic and compositional observations of a single Pt ML on Pd/C
nanoparticles. (a) HAADF/STEM Z-contrast image of carbon-supported
Pt ML /Pd nanoparticle. (b) Distribution of the elements in the Pt ML /Pd
nanoparticle, obtained by the line profile analysis using STEM/EDS.
Reprinted from ref. 28 (copyright 2010 with permission from Elsevier).

The HAADF technique can give the observations of interfaces/locations
between different elements since the image intensity from an element
(atomic number Z) follows approximately Z 2 dependence. The Z-contrast
image in Figure 6.3a shows bright shells on a relatively darker nanoparticle
core, which suggests the formation of a core-shell structure, i.e.,aPt(Z ¼ 78)
monolayer or bilayer (shell) on a Pd (Z ¼ 46) nanoparticle (core). Individual
bright dots are observed around the nanoparticle. They are presumably due
to Pt atoms bombarded by the high voltage electron beam during the ob-
servation. It should be noted that some HAADF images showed only partial
coverage of shells on core nanoparticles, or gave no clear contrast of core-
shell structures. Some HAADF images observed under zone axis conditions
showed contrast patterns due to strain fields but no contrasts due to the
difference in atomic weight. When the samples are tilted from the zone axis,
the image contrast becomes one predicted by atomic number variation
(Z-contrast). In STEM analysis the nanoparticles lie in completely random
orientations with respect to the electron beam and every single particle is
thus not amenable to the analysis. This is likely to be one of the reasons why
some pictures show incomplete Pt monolayers.
Figure 6.3b illustrates the STEM/EDS line profile analysis of a single
nanoparticle and shows the distribution of Pt and Pd components. It is seen
that the width of the Pd core is 8 nm. The Pt profile has the following three
marked features: (i) Pt atoms fully cover the Pd nanoparticle surface, (ii) the
Pt intensity is fairly constant along the center of Pd nanoparticle, and (iii) at
the both edges of Pd nanoparticle the Pt intensity is approximately doubled
in comparison with that at center. These features demonstrate the formation
of a core (Pd)-shell (Pt) structure. From the atomic ratio analysis of Pt and
Pd, and assuming that the height of Pd nanoparticle (normal to the picture)

.