Chemistry Reference
In-Depth Information
The Helmholtz capacity of a metal electrode is typically on the order of
44
which is much greater than the value for a silicon electrode.
848
It has been
reported that the Helmholtz layer capacitance of silicon in
KC1 aqueous solution is about
94
In 0.1 M tetrabutylammonium perchlorate
486
A
(TBAP) in acetonitrile it is found to be about larger value,
has been found for deep accumulation of a silicon in acetonitrile and the
value varies with the accumulated charge.
941
1.3.5. Surface States
Surface states are the electronic energy levels that are located at the surface and
are capable of transferring charges with the solid and solutions. The surface states of a
clean surface, the dangling bonds resulting from the termination of the bulk structure
at the surface, are called intrinsic states. For silicon the dangling bonds on the surface
in solutions are generally terminated by hydrogen and other species so that there are
very few intrinsic surface states.
In addition to the intrinsic states, there are also Lewis sites and adsorbed elec-
troactive species. Lewis sites arise from the acid/base properties of the surface. A Lewis
acid site is a site attractive to electron pairs from an adsorbing molecule and a Lewis
basic site is able to donate electron pairs to an adsorbing molecule. For example,
a Lewis acid site M or a basic site N may adsorb a
ion or a
from water,
respectively:
Adsorption at acidic sites M causes the solution to become acidic and adsorption of
on Lewis basic sites causes the solution to become basic. Lewis sites are important in
two ways: they contribute to the Helmholtz double layer, and they result in chemical
adsorption and passivation of the intrinsic active surface sites. The surface of silicon is
dominated by basic Lewis sites as manifested by the strong hydrogen adsorption. But
the associated surface states are not active because they are located energetically in the
valence band.
99
Adsorbed species other than hydrogen and hydroxyl ions that are able to give
up or accept electrons are also surface states. The reaction intermediates that are
able to act as donors or acceptors through charge transfer reactions can be viewed
as surface states. As will be described in more detail in the section on anodic beha-
vior, partially oxidized silicon atoms, i.e., the reaction intermediates,
act as transient surface states and play an important role in a range of electrode
processes.
Surface inhomogeneity such as vacancies, steps, kink sites, emergent disloca-
tions, and foreign elements in the lattice has a significant effect on adsorption, bonding
energy, and redox reactions. Such surface heterogeneity causes the energy levels of a
specific type of surface state to appear as bands because the exact chemical nature of
