Electrochemical Atomic-level Controlled Syntheses of Electrocatalysts for the Oxygen Reduction Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 150

is energetically favored relative to the bulk elements whit a stoichiometry
defined by the surface chemistry of the substrate. With metals, a two-
dimensional bimetallic compound is formed. This process avoids formation
of three-dimensional crystallites and Stranski-Krastanov growth. Cu UPD,
Pb UPD and H UPD are three reactions used widely for deposition of Pt ML
on different supports by the SLRR strategy. 11,18,19 The SLRR methodology is
realized in two steps. In the first step a monolayer of Cu (Pb or H) is de-
posited at under-potentials on a substrate metal of interest, and in the
second step the UPD monolayer is displaced by Pt ML (Figure 6.1a). Tech-
nically, the SLRR protocol has been initially implemented either by shuttling
of the electrode between separate cells (multiple immersion) with different
solutions 10,20 or by subjecting it to an alternating flux of different solutions
in a flow cell. 12 Thus, the potentially controlled UPD layer formation and the
spontaneous redox replacement steps take place in different solution.
Most recently an alternative method for SLRR, known as 'one-cell con-
figuration' approach has been proposed. 11 It has been initially introduced as
a 'one-pot' configuration and first implemented in the growth of Cu on a
Au(111) single crystal surface. 21 In this approach the electrode is held all the
time in one cell, in contact with a solution containing both ions/complexes
of UPD and growing metals in a concentration ratio that would result in only
marginal (no more than 1 at.%) electro-deposition of the growing metal
during the sacrificial layer formation step.

d n 9 r 4 n g | 4

6.2.2.1 Surface-limited Redox Replacement of Cu UPD
Monolayer: a Highly Ecient Method for Synthesis of Pt
ML Core-Shell Type Electrocatalysts
The electrochemical atomic layer deposition strategy coincided with dem-
onstration of the Pt monolayer type electrocatalysts, which is considered as
the most advanced class of electrocatalysts for the ORR. The strategy has
been demonstrated for first time by deposition of a Pt ML on single crystal
Au(111) electrode. 10,18 The SLRR protocol has been performed in multiple
immersion configuration, where a Cu UPD ML is first deposited under po-
tential control from CuSO 4 plus H 2 SO 4 solution, and then is displaced in
[PtCl 6 ] 2 plus H 2 SO 4 solution. It is observed that the Au single crystal is
uniformly covered by one ML high Pt nanoclusters that does not form a
continuous layer. 10 The amount of Pt deposited by the displacement of a full
Cu UPD ML is limited to coverage of a 1/2 monolayer because two Cu 0 atoms
are oxidized in order to reduce one Pt 41 ion. Thus, the stochiometry of the
displacement is 2 : 1, and a maximum coverage of 50% could be achieved
when Pt 41 salt solution is used in the displacement step of the SLRR
protocol. Subsequent studies used Pt 21 and it has been confirmed 22 that if a
Pt 21 solution is used in the displacement step, the stochiometry of the
displacement is 1 : 1 and a uniform and pinhole-free Pt ML on Au surface is
formed. The stoichiometry of Pt deposition via surface-limited redox

.