Interfacial
Chemistry in High Electric Fields
The electrochemical interface---a complex
environment consisting of electrode,
interfacial layer, solvent molecules, ions,
diffuse layer, and bulk solution---has
received much attention. Little is known
about the influence of electric fields at
the electrochemical interface, however, due
to difficulties in modeling them
theoretically and generating or detecting
them experimentally.
To illustrate the nature of electric fields
at the electrode/electrolyte interface,
Figures 1 and 2 depict, respectively,
classical and quantum mechanical models of
the double layer. The classical model shows
an anion specifically adsorbed at the metal
electrode surface. The specifically adsorbed
layer is the Helmholtz layer and has a width
approximately equal to one water monolayer
(about 3 Å). Beyond the specifically
adsorbed layer is the diffuse layer,
consisting of anions, cations, and other
water molecules. The potential changes
sharply at the surface, due to the change in
charge density, and decays more smoothly
through the diffuse layer.
The density functional theory (DFT) model of
Fig. 2, for three water layers and one
hydronium ion (dark blue/orange) at a
Pt(111) surface, focuses on the adsorbed or
near-surface layer The water molecules alter
their orientation in response to the
hydronium ion. The red curve shows the
potential profile, and the blue curve is the
field profile obtained by differentiating
the potential profile. The field profile
varies from zero in the metal, to a maximum
of 1 V/Å near the surface, and to zero
into the solution phase. There is an
appreciably wide region where the field
exceeds 0.3~V/Å, at which water
ionization is possible, according to work in
our laboratory. Comparison of this
experimental result with the computational
result shows that ionization would be
expected to occur over the range where the
field exceeds the ionization threshold, from
1.5 to 3.5 V/Å from the surface.
Experimental Studies of High
Interfacial Electric Fields
The magnitude of electric fields
necessary to influence surface chemistry
can be estimated by two simple
observations: first, most chemical bonds
are on the order of a few (1--5) eV;
second, most chemical bonds are on the
order of a few (1--3) Å. The ratio
of these two estimates is of the order of
1 eV/Å, or simply 1 V/Å, as an
electron has one unit of charge. This
corresponds to a field of 100 MV/cm, a
very large quantity!
Obviously, traditional electrode
arrangements cannot be used to generate
such high fields. A more practical means
is to take advantage of field
amplification at sharp metal tips. A field
emitter tip with a tip radius of 300
Å can produce fields of 1 V/Å
or greater with tip potentials of about
5,000 V. Surface chemical reactions can be
studied on the tip in an ultrahigh vacuum
chamber as a function of tip temperature,
field, and partial pressure of reactant.
Figure 3 shows a schematic of the field
ionization system used in our laboratory.
The system has the following capabilities
(the letters correspond the letters in the
figure):
- FEM: Field emission microscopy
Image of field-emitted electrons, useful
for mapping work function changes or
certain regions of the tip surface; does
not provide atomic resolution; shown is
an image of a Pt(111) oriented tip.
- FIM: Field ionization microscopy
Atomic resolution of tip surface
achieved by ionization of Ne+
or other probe species; shown is an
image of a Pt (111) oriented tip.
- MR-FIM: Mass-resolved field ionization
microscopy
Ionization images obtained by
time-of-flight (ToF) gating of the
imaging screen; useful for identifying
spatio-temporal ionization
characteristics of specific species;
shown is an image of hydronium ions (H3O+)
desorbing from a Pt(111) tip at 180 K
under a constant supply of water to the
tip.
- IVS: Ion velocity selection
Continuous ion beams are separated by a
Wien (ExB) filter; permits reaction
studies under continuous conditions,
rather than the pulsing conditions of
ToF mass spectrometer; shown are images
of di-hydrated [(H2O)2H+]
and tri-hydrated [(H2O)3H+]
protons.
- PFDMS: Pulsed-field desorption mass
spectrometry
A sharp field pulse (from 0 to about
6,000 V/Å in 30 ns) desorbs all
species as ions, which are detected by a
ToF mass spectrometer; useful for
studies of surface reactions; shown is a
PFD spectrum of methanol from a Pt tip.
RFD: Ramped field desorption
Field desorption during a ramped
electric field at constant temperature
and other conditions; used to measure
field effects in adsorption and
interaction with temperature; analogous
to thermal desorption spectroscopy
(TDS); shown is RFD of water from a Pt
tip at temperature below 170 K.
SFD: Stepped field desorption
Similar to RFD, except that the field is
stepped and held at a constant value;
shown is SFD of water from a Pt tip at
temperature below 170 K.
Examples of Field
Ionization of Water
Dielectric properties of adsorbed water were
measured by RFD for temperatures of 100--170
K, shown in the lower portion of Fig.
4. The measurements were
analyzed according to the charge exchange
model, in which the ionization rate is given
where E is the electric field and Q(E)
the field-dependent activation energy for
ionization, given by
with Ha the field-free
activation energy for desorption, μ the
dipole moment, and α the polarizability.
These last two quantities can be determined
by a plot of Q(E) vs.
E, which is shown in the upper
portion of Fig. 4. From a curve fit the
values of μ = -1.16 D and α = -0.8 Å3
are obtained. (The equation in Fig. 4 is in
atomic units.) The negative value of the
dipole moment means that the water dipole is
aligned opposite to the field. (The
field is positive, so water is aligned with
its negative end toward the surface.)
It's value is less than the gas phase dipole
moment of water (1.8 D), due to a
distribution of orientations in the water
layer. The negative value of the
polarizability is non-physical and can be
ascribed to error in estimating the very
slight curvature of the Q(E)
vs. E plot in Fig. 4.
Figure 5 shows a series of mass-resolved,
FIM images of protonated water clusters from
dynamic water layers at 180 and 230 K.
A 10 s continuous exposure was used for each
micrograph so that the bright spots
represent continuous emission for the entire
exposure period, and the black regions the
absence of emission. The bright spots
are termed ``ion flares.'' They shine
continuously and sway around a home
position. The ion flares repel each
other; when a new flare appears the others
rearrange so as to maximize distance between
them. If an ion flare extinguishes,
the other flares again move to equalize
distances.
The effect of surface structure can be seen
in Fig. 6, which compares the field
ionization micrograph of the clean surface
(a) with the water ionization image for H3O+(b).
There is a ring of spots in the lower
portion of (b) that passes through the
center of the stereographic triangle.
Figure 7 illustrates the significant spots
on the ring and their relation to surface
structure. Ion emission occurs
preferentially at the (210) and (311)
surfaces. These have the highest
density of step defects, which create field
enhancements that favor ionization.
Ion beams also occur within the
stereographic triangle at the locations
shown in the figure. During the
experiment, the ion beams fluctuate in
position and may appear or extinguish during
the experiment, so a ionization does not
necessarily occur at a certain
location. Rather, higher surface
defect density, which occurs at stepped and
kinked surfaces, promote ionization due to
their higher fields.
Acknowledgement
This work was supported by the Office of
Naval Research.
Last revised: January 9, 2013
|
Figure 1. Classical model of the
electrochemical double layer.
Figure 2. A DFT/molecular dynamics
simulation of a charged Pt(111) slab with
3 water layers and one solvated hydronium
ion. Adapted from J. Rossmeisl, E.
Skúlason, M. E. Björketun, V. Tripkovic
́, and J. K. Nørskov, “Modeling the
electrified solid-liquid interface,” Chem.
Phys. Lett., 466 (2008), 68--71.
Figure 3. Field ioinization system with
capabilities for FEM (a), FIM (b), MR-FIM
(c), IVS (d) and PFDMS, RFD, SFD (e).
Figure 4. RFD curves (lower) as a
function of temperature for water layers
of approximately 900 Å thickness on
a Pt tip of 300 Å radius. The upper
portion shows the value of Q taken
from these data and the corresponding fit.
Figure 5. Mass-resolved, field ion
micrographs of protonated water clusters
(H2O)mH+
for water ionization at 180 K and 230
K. A 10 second exposure was used for
all water ionization micrographs.
The partial pressure of water was 5 x 10-6
Torr for (a--h). Image (i) is a FIM
image of the clean Pt tip. Table
\ref{ion_flare_tab} lists cluster sizes,
fields, and temperatures for each image.
Figure 6. Comparison of field ionization of
the clean Pt tip (a) with (H2O)H+
ionization (b).
|
