Controllable Synthesis of Metal Nanoparticles for Electrocatalytic Activity Enhancement - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 232

molecules are well-organized and addition of FePt in this direction is more
dicult owing to the presence of the hydrophobic barrier. Area (2) has less
densely packed OAm and facilitates the growth of FePt along this direction
and the formation of NWs. Area (3) is the most readily accessible place for
the addition of FePt, leading to the fast growth of FePt and the rounded end
of the NWs.

d n 9 r 4 n g | 5

9.3 NP Activation for Catalysis
Metal NPs prepared from solution phase reactions are coated with a layer
of surfactant, which creates an insulating barrier around each NP, making
them inactive for electrocatalysis. Therefore, to function as electro-
catalysts, these metal NPs must first be activated by removing the surface
coating. To activate the NP catalysts, the NPs are first deposited onto a
high-surface-area and conductive support (e.g., carbon black) to ensure
the NP dispersion over the surface of the support and stabilization against
aggregation upon the surfactant removal. Such deposition is often done
by mixing the NP dispersion with the support under a sonication
condition.
The most straightforward way of removing surfactant is to anneal NPs at
high temperature in air to burn off or vaporize the organic surfactants. 13 For
example, OAm coated Au NPs deposited on a solid support can be treated at
300 1Cin8%O 2 /He mixture for 1 h to remove the OAm surfactant, after
which the supported Au NPs are highly active for CO oxidation, showing
100% CO conversion at 45 1C. An alternative approach is to neutralize the
surfactants so that their binding to the metal NP surface is weakened and
the NPs can be washed clean. 25 For example, on activating the OAm-coated
Pd NPs, the NPs deposited on the carbon support are immersed in acetic
acid. This acid neutralizes -NH 2 into -NH 3 1 , facilitating OAm removal from
the NP surface by simple washing. After this treatment, the Pd NPs are
catalytically active for electrochemical oxidation of formic acid.
If NPs, for example, FePt, CoPt, NiPt NPs, are subject to changes (NP
oxidation, NP aggregation or NP sintering) in air annealing or acid washing
conditions, surfactant exchange reaction may be applied to replace the long
chain surfactant with small molecules that bind weakly on the NP surface.
For instance, to activate OAm-coated NiPt 3 NPs, the NPs are dissolved in
butylamine at room temperature. Large amount of butylamine facilitates its
replacement of OAm, giving butylamine-coated NPs that can be further
cleaned by washing with other common solvents. The process preserves both
NP size and shape for electrocatalytic studies. 26 This strategy has been
extended successfully to activate Co and Co/CoO NPs for oxygen reduction
reaction in alkaline media. 27 Similarly, tetramethylammonium hydroxide
(TMAOH) 28 and tetrabutylammonium hydroxide (TBAOH) 29 can be used to
replace long-chain surfactant to facilitate NP cleaning. Recently, it is re-
ported that the organic surfactant present around NP can be even replaced
by small inorganic metal chalcogenide complexes. 30

.