Electrochemical Atomic-level Controlled Syntheses of Electrocatalysts for the Oxygen Reduction Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 149

perturbation of the double-layer structure that accompanies H ads saturation
of the Pt surface. Thus, the H-covered Pt surface impacts the adjacent water
structure, which leads to minimum in coupling between the electrode and
electrolyte. In addition, the quenching of the metal deposition reaction oc-
curs at potentials negative to the potential of zero charge of Pt, where all
anions are desorbed. The combination of these two phenomena exerts re-
markable effect whereby [PtCl 4 ] 2 reduction is completely quenched while
diffusion-limited proton reduction continues unabated.
Alternatively, an idea for Pt ML deposition on polycrystalline Au, based on
adsorption driven surface specific reaction, has been most recently pro-
posed. 13 In this work the authors studied the electro-deposition of Pt on Au
from CO saturated K 2 PtCl 4 plus H 2 SO 4 solution into thin layer flow cell
under potential control. The potential chosen is far below the Nernst redox
potential for the Pt 21 /Pt couple, and bulk Pt deposition would be expected.
However, a distinct reduction peak in the current transient curve corres-
ponding to reduction of Pt ions to Pt metal on Au surface is observed, fol-
lowed by fast decay of current to zero as soon as saturation Pt coverage is
reached. Thus, according to the authors, immediately after Pt ML deposition
the layer is covered by strongly adsorbed CO molecules that inhibit further Pt
ions reduction and prevent bulk Pt formation. The monolayer geometry is
confirmed by electrochemical and in situ IR spectroscopic data. The IR
spectra indicate that adsorbed CO revealed distinct electronic ligand and
strain effects, characteristic for an electronic modification of the Pt ML as
compared with bulk Pt(111). Also, it is reported that oxidative removal of the
CO adlayer results in a distinct restructuring of the Pt films, yielding three-
dimensional (3D) Pt nanostructures. 13 This finding puts into question the
applicability of this strategy for synthesis of highly ecient and durable Pt
ML type electrocatalysts for the ORR.
We note that these both strategies, self-termination of Pt ML by hydrogen
adsorption at high redox over-potentials and self-limited Pt ML deposition
by CO adsorption, have been demonstrated for Pt ML deposition on Au
substrates and have not been explored for synthesis of Pt ML core-shell
catalysts.

d n 9 r 4 n g | 4

.

6.2.2 Displacement-driven Surface-limited Reactions for
Atomic Monolayer Deposition
Under-potential deposition is a type of electrochemical SLR in which up to a
monolayer of metal is deposited on another metal at a potential prior to
(under) that needed to deposit the first metal on itself. 15 UPD is a thermo-
dynamic phenomenon, where the interaction energy between the two
elements is larger than the interaction of an element with itself, thus re-
sulting in the formation of a surface compound or alloy. 15-17
Generally, UPD involves deposition of less noble metal on a more noble
metal surface. The driving force for UPD is formation of a compound, which