In-situ infrared study of the adsorption and oxidation of oxalic acid at single-crystal and thin-film gold electrodes: a combined external reflection infrared and ATR-SEIRAS approach

Layered Sn-Au Thin Films for Increased Electrochemical ATR-SEIRAS Enhancement

Operando electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (EC ATR-SEIRAS) is a valuable method for a fundamental understanding of electrochemical interfaces under real operating conditions. The applicability of this method depends on the ability to tune the optical and catalytic properties of an electrode film, and it thus requires unique optimization for any given material. Motivated by the growing interest in Sn-based electrocatalysts for selective reduction of CO2 to formate species, we investigate several Sn thin-film synthesis routes for the resulting SEIRA signal response. We compare the SEIRA performance of thermally evaporated metallic Sn to a series of Sn-based films on top of a SEIRA-active Au substrate (metallic Sn, oxide-derived metallic Sn, and metal oxide SnOx). Using alkanethiol self-assembled monolayers as a probe, we find that electrodepositing metallic catalyst films on top of SEIRA-active Au substrates yield higher signal relative to thermal evaporation as well as higher signal than the independent SEIRA-active Au underlayer. These observations come despite the fact that thermally evaporated Sn has a significantly higher surface roughness (and thus higher adsorbate population), suggesting specific SEIRA-magnifying effects for the stacked films. Finally, we applied these films to observe the electrochemical conversion of CO2. Differences are observed in spectral features based on the composition of the electrode being either metallic or oxide-derived metallic Sn, implying differences in their respective reaction pathways.

Adsorption of Acetate on Au(111): An in-situ Scanning Tunnelling Microscopy Study and Implications on Formic Acid Electrooxidation

The adsorption of acetate on an Au(111) electrode surface in contact with acetic acid at pH 2.7 was imaged in-situ using scanning tunnelling microscopy (STM). Two different ordered structures were imaged for acetate adsorbed in the bidentate configuration on the unreconstructed 1 × 1 surface at 0.95 V (vs. the saturated calomel electrode, SCE). The first structure, ( 19 × 19 ) R 23 . 45 ∘ , is metastable and transforms at constant potential within 20 minutes to a ( 2 × 2 ) structure, which is thermodynamically more favourable. The ( 2 × 2 ) acetate adlayer starts to form at step edges and propagates via nucleation and growth onto terraces. The findings from in-situ STM are in agreement with the electrochemical behaviour of acetate on Au(111) characterized by voltammetry. A comparison is made with formate adsorption on Au(111). While acetate is not reactive, in contrast to formate, it can act as a spectator species in formic acid electrooxidation.

Electrochemical modification of surface morphology of Au/Ti bilayer films deposited on a Si prism for in situ surface-enhanced infrared absorption (SEIRA) spectroscopy

Surface-enhanced infrared absorption (SEIRA)-active Au/Ti bilayer films sputter deposited on Si substrates have been prepared by an electrochemical annealing (ECA) treatment for the first time. The application of Au/Ti bilayer films on Si substrates to the spectroscopic technique is a promising alternative to the conventional technique using directly deposited Au films on Si substrates, offering excellent adhesive durability of the deposited metal films. However, Au/Ti bilayer films have never been selected for the spectroscopy technique because the films in the as-prepared state exhibit relatively smooth surface morphology: the excitation of the localized surface plasmon is vital to achieving SEIRA enhancements but could hardly be observed on the smooth morphology. It is shown by ex situ scanning tunneling microscopy measurements that the unfavorable smooth morphology of the as-prepared Au/Ti bilayer films can be modified by the ECA treatment to a reasonably rough, island-structure morphology similar to that of the conventional SEIRA-active Au films. In situ infrared absorption spectroscopy of adsorbed sulfate anions has been conducted on the Au/Ti bilayer film both before and after ECA treatment. The spectroscopy measurements demonstrate that the SEIRA activity of the film after being subjected to the treatment is significantly improved so that the technique could detect adsorbates on the film electrodes even with the submonolayer coverage. As an additional benefit, the ECA treatment has brought about a substantial increase in the fraction of Au(111) domains on the polycrystalline Au film surfaces. Accordingly, this approach enables us to prepare SEIRA-active Au films having sufficient adhesion to the Si substrates as well as the highly preferred (111) orientation.

Roughening of gold atomic steps induced by interaction with tetrahydrofuran

Exposure of a clean gold surface to tetrahydrofuran (THF) under ambient conditions was observed to cause roughening of atomic step edges. This change was followed in situ using a scanning tunneling microscope during the exposure of a gold surface to a controlled stream of THF vapor. THF is a common solvent used in depositing molecules, self-assembled monolayers, and polymer films on surfaces, in electrochemistry, and in chemical reactions. Unlike other solvents, such as methanol, ethanol and diethyl ether, however, we found that THF itself has a profound effect on the surface morphology that needs to be taken into account when reporting on the interactions of solutes with a gold surface. At the same time, this finding may present new opportunities in catalysis or nanostructuring of surfaces.

Biochemical applications of surface-enhanced infrared absorption spectroscopy

An overview is presented on the application of surface-enhanced infrared absorption (SEIRA) spectroscopy to biochemical problems. Use of SEIRA results in high surface sensitivity by enhancing the signal of the adsorbed molecule by approximately two orders of magnitude and has the potential to enable new studies, from fundamental aspects to applied sciences. This report surveys studies of DNA and nucleic acid adsorption to gold surfaces, development of immunoassays, electron transfer between metal electrodes and proteins, and protein–protein interactions. Because signal enhancement in SEIRA uses surface properties of the nano-structured metal, the biomaterial must be tethered to the metal without hampering its functionality. Because many biochemical reactions proceed vectorially, their functionality depends on proper orientation of the biomaterial. Thus, surface-modification techniques are addressed that enable control of the proper orientation of proteins on the metal surface.

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