Change of the work function of platinum electrodes induced by halide adsorption

Abstract

The properties of a halogen-covered platinum(111) surface have been studied by using density functional theory (DFT), because halides are often present at electrochemical electrode/electrolyte interfaces. We focused in particular on the halogen-induced work function change as a function of the coverage of fluorine, chlorine, bromine and iodine. For electronegative adsorbates, an adsorption-induced increase of the work function is usually expected, yet we find a decrease of the work function for Cl, Br and I, which is most prominent at a coverage of approximately 0.25 ML. This coverage-dependent behavior can be explained by assuming a combination of charge transfer and polarization effects on the adsorbate layer. The results are contrasted to the adsorption of fluorine on calcium, a system in which a decrease in the work function is also observed despite a large charge transfer to the halogen adatom.

Keywords: density functional theory; ionicity; polarizability; surface dipole; work function.

Figure 1

The figures show the relaxed structures of different coverages of chlorine on a Pt(111) surface.

Figure 2

Calculated change of the work function vs coverage for the adsorption of fluorine, chlorine, bromine and iodine on Pt(111). The high value for the 0.5 ML calculation of iodine is due to a double layer structure of the adsorbates, caused by the larger size of iodine atoms.

Figure 3

Charge density difference Δλ(z) for the adsorption of fluorine, chlorine, bromine, and iodine on Pt(111) at the fcc hollow position for a coverage of 1/16 ML. The subsurface region corresponds to the gray-shaded area at z < 0.

Figure 4

Calculated work function versus dipole moment. The solid line corresponds to the expectation according to Equation 1.

Figure 5

Calculated normalized dipole moment as a function of the coverage of fluorine, chlorine, bromine and iodine on Pt(111).

Figure 6

Contributions to the total dipole moment change Δμ according to Equation 6 and Equation 7 as a function of halogen coverage. The term Δμ_q describes the purely charge transfer induced dipole moment and Δμ_pol the polarization induced dipole moment; Δμ_A shows the effect of the adsorbate layer on the total dipole moment and Δμ_S indicates substrate effects. The color code denoting the different halogen atoms is the same as used in the previous figures.

Figure 7

Cross sections of electron density difference ρ_diff(r) at the surface. Solid-blue (dashed-red) contours denote regions of electron buildup (depletion). The interval between contours of constant electron density is 0.01 electrons/ų. The region of the metal slab is shaded gray as a visual aid.