Controllable Synthesis of Metal Nanoparticles for Electrocatalytic Activity Enhancement - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 237

Cu in FePt system and the significance of 1D nanostructures toward ORR
enhancement.

9.4.1.2 Pt-based Core-Shell NPs
Pt-based core-shell NPs are another class of electrocatalysts studied for ORR.
In particular, a thin shell of Pt or PtM alloy is deposited on the non-Pt NP
core and thus the usage of Pt can be greatly reduced. Furthermore, the ra-
tional design of core-shell nanostructures with Pt or PtM shell could
introduce (i) a 'ligand effect', which is caused by the atomic vicinity of two
dissimilar surface metal atoms that induces electronic charge transfer be-
tween the atoms, and thus affects their electronic band structure, and (ii) a
'strain effect', which originates from the lattice mismatch at the core-shell
interface that may include compressed or expanded arrangements of surface
atoms (surface strain). 43 These two effects are of great importance for ORR
activity enhancement. Therefore, design and synthesis of core-shell struc-
tures allows the tuning of the Pt electronic structure and surface strain for an
optimal ORR activity. So far several approaches have been applied to syn-
thesize Pt-based core-shell NPs for electrochemical applications and there
are already quite a few excellent reviews in this area. 52-54
Multi-metallic core-shell NPs with a layer of uniform FePt over Au, 55 Pd, 56
and even core-shell Pd/Au NPs 57 can be prepared by seed-mediated growth
method in an organic solution at a controlled temperature. For instance, Pd/
FePt NPs with 5 nm Pd core and uniform FePt shell were synthesized by the
nucleation and growth of Fe(CO) 5 in the presence of Pt(acac) 2 over the 5 nm
Pd NPs at 180 1C (Figure 9.9a-c). 56 The FePt thickness (1-3 nm) was con-
trolled by the amount of Fe(CO) 5 , Pt(acac) 2 , and the Pd seeds used in the

d n 9 r 4 n g | 5

.

Figure 9.9 (a) High-angle annular dark-field scanning TEM (HAADF-STEM), (b)

high-resolution HAADF-STEM, and (c) elemental mapping images of the
5 nm/1 nm Pd/FePt NPs. (d) ORR polarization curves for three types of
Pd/FePt NPs and the commercial Pt/C catalyst in 0.1 M HClO 4 . The
current was normalized against the total mass of NPs used, and the
electrode rotation rate was kept at 1600 rpm.
Reprinted from ref. 56 with permission by American Chemical Society.