Controllable Synthesis of Metal Nanoparticles for Electrocatalytic Activity Enhancement - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 229

9.2.3 NP Shape Control
The shape of metal NPs has a significant effect on their catalytic behaviors.
In an electrocatalytic reaction, the reactants are usually adsorbed on the NP
catalyst surface to facilitate electron transfer and product formation. The
energy barriers of reactant adsorption and product desorption play the key
role in determining the reaction activity and reaction selectivity. Catalytic
NPs with a uniform size and shape are often formed under a thermodynamic
growth condition, which means that the NP growth is relatively slow and
adatoms migrate into low energy sites following each step of deposition,
resulting in NPs covered by low energy facets. In order to obtain a desired NP
shape, surfactant-facet interaction or NP growth environment need to be
well-controlled so that the adatoms can be added on a specific crystal dir-
ection. Currently, studies on reactions in organic phase have led to a variety
of methods to control NP shapes, including: (i) kinetically controlled
growth, 14 (ii) growth in reverse micelle, 15 (iii) aggregation directed growth, 16
(iv) seed-mediated growth, 17 and (v) oxidative etching. 18 These techniques
induce the NP growth away from thermodynamic equilibrium state, leading
to the formation of high energy surfaces that contain a large amount of low-
coordinated atoms for active electrocatalysis. In this section, we focus on the
shape control of metal NPs by kinetic control and by growth in reverse
micelle.

d n 9 r 4 n g | 5

9.2.3.1 Kinetic Control
Growing NPs kinetically is an effective way to achieve shape control. Such a
control growth is realized via either surface energy differentiation or se-
lectively bonding of surfactants on NP surface. In the first case, the diffusion
time of an adatom from solution to the NP surface is typically shorter than
the time it takes for an adatom to migrate to an energetically preferable site.
Therefore, adatoms become kinetically locked into high energy positions,
resulting in diffusion controlled growth. This can lead to a number of high
energy surface including steps, kinks, terraces, islands and vacancies. 19 In
the presence of surfactants, the bonding strength of the surfactant on dif-
ferent crystallographic planes dominates the relative growth rates with the
weakly bound surface having a preferred growth.
Pt NPs with tetrahedral, cubic, and truncated octahedral shapes have been
prepared by controlling the concentration of surfactant. 20,21 One example is
in the synthesis of Pt NPs with shapes controlled to be polyhedral, truncated
cubic and cubic. 22 In the reaction, Pt(acac) 2 (acac ¼ acetylacetonate) was
mixed with OAm and oleic acid (OA) in octadecene (ODE) and the mixture
was heated to 200 1C while a small amount (less than 10%) of iron penta-
carbonyl (Fe(CO) 5 ) was injected to initiate nucleation of Pt. Injection of
Fe(CO) 5 at 180 1C led to fast nucleation/growth process, producing 3 nm Pt
NPs with thermodynamically more stable polyhedral shape (Figure 9.3a).
Injecting Fe(CO) 5 at lower temperatures (for example at 160 1C or 120 1C)

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