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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):

  1. 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.

  2. 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.

  3. 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.

  4. 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.


  5. 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
Ionization
                                rate equation

where E is the electric field and Q(E) the field-dependent activation energy for ionization, given by

Field-dependent activation energy

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


Classical double layer

Figure 1. Classical model of the electrochemical double layer.

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Quantum 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.

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Field ionization system

Figure 3. Field ioinization system with capabilities for FEM (a), FIM (b), MR-FIM (c), IVS (d) and PFDMS, RFD, SFD (e).

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Ramped field desorption of
                                water

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.
 
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Mass resolved field ionization
                                of water

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.

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H3O field ionization

Figure 6. Comparison of field ionization of the clean Pt tip (a) with (H2O)H+ ionization (b).
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H3O ionization - composite

Figure 7. Superposition of H3O+ ionization beams (ring only) onto the stereographic triangle of the tip.  The panels show: (a) the full stereographic triangle with ring of ion beams; (b) detail of the (311) and (210) stepped planes and kinked surfaces between them; and (c) close-up of ion beams from the center of the triangle, which has a high step/kink density.  The tip atoms are shown to scale with the tip curvature (350 Å radius). The tip surface was generated numerically and contains over 32,000 Pt atoms in 3-4 layers. The white atoms are a simulated field ionization image. Gray atoms are the remaining surface atoms.  The dark spots in (c), for example, show subsurface atoms (colored purple) that are visible due to the open structure of the defected surface. The hydronium ions are enlarged to represent the ion beam.

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