Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 281

10.3.3.1.1 Electrophoresis Method. The concept of this method 113 is to
use a pre-sharpened needle-shaped substrate consisting of a Pt (or Pt-Rh)
atom probe samples produced by electropolishing in a molten salt mix-
ture. On this substrate, nanoparticles are deposited by electrophoresis. To
perform this deposition, a liquid droplet containing a suspension of nano-
particles is placed in a gold loop and a voltage bias is applied between the
loop and the substrate tip. On dipping the tip into the loop, nanoparticles
are drawn through the potential gradient by electrophoresis towards the
apex region of the specimen where they are subsequently immobilized.
A schematic representation can be found in Figure 10.17a, which is simi-
lar to the electropolishing procedure described in Section 10.2.1.1. Sam-
ples are then inspected by transmission electron microscopy (TEM) to
confirm the deposition of a suitable layer of nanoparticles. Figure 10.17b
shows a clean Pt-22at%Rh preformed tip, rather blunt, before deposition
and Figure 10.17c shows the same tip after deposition of Ag@Pd nano-
particles 113 (the notation 'Ag@Pd' denotes a core made of silver with a
shell made of palladium).
Factors affecting the deposition process include the particle type (colloidal
solution of isolated particles 113,114 or a suspension of crushed, catalyst
support material containing nanoparticles 14 ), concentration, size and charge
polarity, along with the specimen sharpness, loop voltage and duration of
immersion. An important factor in the success of APT study is the number of
deposited nanoparticles: agglomeration of nanoparticles makes the atom
probe analysis dicult and reconstructed atom maps susceptible to particle
overlapping from trajectory aberrations. In the case of supported samples,
the nanoparticles are relatively well separated, which is not the case for a
colloidal solution. A good dispersion of supported nanoparticles is required
and can be obtained via dissolution and immersion in an ultrasonic bath.
Typical deposition conditions are 5-20 V DC during

d n 9 r 4 n g | 8

.

B 10-20 s.

Figure 10.17 (a) Schematic representation of the electrophoresis method where a

Pt-22at%Rh pre-formed tip is dipped into a liquid droplet containing a
suspension of nanoparticles. (b) TEM of a Pt-22at%Rh specimen
before deposition. (c) TEM of the same specimen after deposition of
Ag@Pd nanoparticles.
(Panel (a) adapted with permission from ref. 14. Copyright (2014)
American Chemical Society; panels (b) and (c) adapted by permission
from Macmillan Publishers Ltd: Nature Nanotechnology, ref. 113,
copyright 2011).