Effect of surface crystallographic orientation upon specific adsorption of azide at silver electrodes as examined by potential-difference infrared spectroscopy

Abstract

The specific adsorption of azide anions on the three low-index faces of silver, having (111), (100), and (110) orientations, has been studied by means of potential-difference infrared spectroscopy using an FTIR spectrometer in order to examine the influence of surface crystallography upon the adsorbate binding. Bipolar infrared spectra were obtained in the 2050–2100 cm −1 region, with a negative-going (i.e., decreased transmittance) feature, v a (sur), at 2075–2100 cm −1 due to the asymmetric N-N-N vibration for adsorbed azide, and its positive-going band partner, v a (sol), at 2048 cm −1 arising from the corresponding loss of an equal amount of solution azide in the thin-layer cavity brought about by the positive alteration in electrode potential. Information on the adsorbate structure was obtained from the relative intensities, I sur and I sol, as well as frequencies, of v a(sur) and v a(sol). For the Ag (111) and (100) faces, I sur⪡ I sol small azide surface concentrations (Γ ⩽ 4×10 −10 mol cm −2), whereas I sur≈ I sol at higher coverages. These findings are interpreted in terms of the surface infrared selection rule as indicting a prominence of “flat” and “vertical” orientations, respectively, of adsorbed azide under these conditions; the presence of the latter, end-on bonded, form is also consistent with the observed v a(sur) frequencies. In contrast, for the Ag (110) face, I sur⪡ I sol over the entire accessible coverage range (up to Γ ≈ 1×10 −9 mol cm −2, indicating that a flat azide orientation is especially favored at this surface. This is attributed to the adsorption of the linear N 3 − ions along the atomic-scale “furrows” which characterize the (110) face. Polycrystalline silver exhibited spectral behavior that is in some respects intermediate between that observed for the low-index single-crystal faces. Comparisons are also made between the surface infrared results and corresponding capacitance-potential data.