posted on 2012-04-11, 00:00authored byJoaquín Rodríguez-López, Nicole
L. Ritzert, Jason A. Mann, Cen Tan, William R. Dichtel, Héctor D. Abruña
The surface diffusion of a cobalt bis-terpyridine, Co(tpy)2-containing tripodal compound (1·2PF6), designed to noncovalently adsorb to graphene
through three pyrene moieties, has been studied by scanning electrochemical
microscopy (SECM) on single-layer graphene (SLG). An initial boundary
approach was designed in which picoliter droplets (radii ∼15–50
μm) of the tripodal compound were deposited on an SLG electrode,
yielding microspots in which a monolayer of the tripodal molecules
is initially confined. The time evolution of the electrochemical activity
of these spots was detected at the aqueous phosphate buffer/SLG interface
by SECM, in both generation/collection (G/C) and feedback modes. The
tripodal compound microspots exhibit differential reactivity with
respect to the underlying graphene substrate in two different electrochemical
processes. For example, during the oxygen reduction reaction, adsorbed 1·2PF6 tripodal molecules generate
more H2O2 than the bare graphene surface. This
product was detected with spatial and temporal resolution using the
SECM tip. The tripodal compound also mediates the oxidation of a Fe(II)
species, generated at the SECM tip, under conditions in which SLG
shows slow interfacial charge transfer. In each case, SECM images,
obtained at increasing times, show a gradual decrease in the electrochemical
response due to radial diffusion of the adsorbed molecules outward
from the microspots onto the unfunctionalized areas of the SLG surface.
This response was fit to a simple surface diffusion model, which yielded
excellent agreement between the two experiments for the effective
diffusion coefficients: Deff = 1.6 (±0.9)
× 10–9 cm2/s and Deff = 1.5 (±0.6) × 10–9 cm2/s for G/C and feedback modes, respectively. Control experiments
ruled out alternative explanations for the observed behavior, such
as deactivation of the Co(II/III) species or of the SLG, and verified
that the molecules do not diffuse when confined to obstructed areas.
The noncovalent nature of the surface functionalization, together
with the surface reactivity and mobility of these molecules, provides
a means to couple the superior electronic properties of graphene to
compounds with enhanced electrochemical performance, a key step toward
developing dynamic electrode surfaces for sensing, electrocatalysis,
and electronic applications.