Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

entirely quenched, presumably due to reaction of the highly active Ptn with species in the air. For Pt7, air
exposure leads to quenching of ORR activity, replaced by reversible redox of adventitious organic
species.
The instrument was modified to allow in
situ electrochemical studies to be made in the
lower chamber. A retractable electrochemical
cell was constructed of PEEK, consisting of a 4
mm diameter working section that contains a Pt
counter electrode (working cell volume ≈ 15
μL). One end of the working section is open,
with a lip holding a 3 mm diameter O-ring that
is used to seal the cell against the surface of the
GCE. The other end of the working section is
sealed to a reference electrode section,
communicating via a fritted Vycor disk. The
reference electrode was Ag/AgCl. Both
working and reference sections of the cell are
equipped with Teflon tubes used for injecting
electrolyte, and ports to allow electrolyte
solution to flow out of the cell. Electrolyte flows
are controlled by a syringe pump located outside
the vacuum system.
To allow in situ electrochemical
measurements, an electrode is first prepared by
depositing Ptn on a clean, degassed GCE in
UHV, which is then lowered through the triple
seal into the lower chamber while the
electrochemical cell is retracted. The
electrochemical cell is then pushed against the
surface of the GCE, sealing via the O-ring at the
open end of the working section. For in situ
experiments, the lower chamber was vented
with high purity Ar, then the working and
reference sections were filled with 0.1 M
HClO4, and 3 M NaCl, respectively. Cyclic
voltammograms (CVs) were acquired using a
CH Instruments model 600D potentiostat.
The ORR results for Pt7/GCE are show
in Fig. 1, in the form of CVs taken in both N2-
and O2-saturated HClO4. The structure of the
CV is similar to that seen both in our in situ cell
and in the literature for Pt nanoparticles on
GCE, and the activity per Pt atom is roughly
twice that of Pt nanoparticles, probably because
the small clusters have essentially all the Pt on
the surface, and are highly dispersed on the
GCE. Optical microscopy of the electrode after
several hundred CV cycles showed no signs of
any degradation.
Fig. 2 shows CVs for Pt4/GCE, and also
a post-electrochemistry optical micrograph of
Fig. 2: Top. CV of Pt4/GCE in N2- and O2-saturated
0.1M HClO4 at a scan rate of 1 Vs -1 from 0.4 to -0.1 V
vs. Ag/AgCl and back. Bottom. Optical micrograph of
the electrode after electrochemical cycling.
Fig. 3 XPS of as-deposited Ptn/GCE in UHV
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