Page 61 - Analytical Electrochemistry 2d Ed - Jospeh Wang

46 STUDY OF ELECTRODE REACTIONS

2-3 SCANNING PROBE MICROSCOPY

The recently developed scanning probe microscopies (SPM) promise to revolu-
tionize the understanding of electrode processes. The purpose of this family of
microscopes is to acquire high-resolution data of surface properties. The various
scanning probe microscopies have similar subcomponents but different sensing
probes. These high-resolution microscopies rely on sensing the interactions between
a probe tip and the target surface while scanning the tip across the surface. Different
types of interactions can be sensed by the tip to yield different imaging signals. Such
signals are displayed as gray-scale portraits, re¯ecting the extent of the tip±surface
interaction. With microcomputers, the image processing becomes possible in very
short times. Among the various scanning probe microscopies, scanning tunneling
microscopy, atomic force microscopy, and scanning electrochemical microscopy
have been useful for imaging electrode surfaces directly (under potential control),
and have thus dramatically improved the understanding of electrode reactions.

2-3.1 Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) has revolutionized the ®eld of surface
science by allowing the direct imaging of surfaces on the atomic scale. The scanning
tunneling microscope consists of a very sharp metallic tip that is moved over the
surface of interest with a ceramic piezoelectric translator. The basis for its operation
is the electron tunneling between the metal tip and the sample surface. The tunneling
current (i ) that ¯ows when a voltage is applied between the sample and the tip is
t
extremely sensitive to the sample±tip separation. In the simplest one-dimensional
treatment i is given by
t
1=2
i a exp‰ 4Sp=h† 2mf† Š 2-18†
t
where S is the barrier width (equivalent to the shortest distance between the sample
surface and the end of the tip), h is the Planck constant, m is the electron mass, and f
is the barrier height (equivalent to the local work function). In practice, i can change
t
by a factor of 2 or more with a change of the tip-sample separation of only 1 A Ê .
Accordingly, i tends to vary with the sample topography.
t
Although much of the early STM work has focused on investigating surfaces in
vacuum, recent work has demonstrated that surface images can also be obtained in
liquid and air. In particular, the STM probing of electrode±electrolyte interfaces has
attracted considerable attention (43±45). The ability of STM to offer structural
information at the atomic level makes it highly suitable for in-situ studies of time-
dependent electrochemical processes, such as corrosion, electrodeposition, adsorp-
tion, as well as surface modi®cation, passivation, and activation. For example, Figure
2-13 shows a representative three-dimensional STM view of an electrochemically
pretreated glassy carbon electrode, while Figure 2-14 illustrates a STM image of a
spontaneously adsorbed alkanethiol monolayer on a gold surface. Useful insights
into the structural±preparation relationships of conducting polymers can also be