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

Abstract

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 CO 2 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 SnO x ). 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 CO 2 . 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.

Insights into Electrochemical CO2 Reduction on Metallic and Oxidized Tin Using Grand-Canonical DFT and In Situ ATR-SEIRA Spectroscopy

“…With some degree of understanding of what might be present on the surface during CO 2 R from the GC-DFT analysis described above, we performed ATR-SEIRAS experiments to validate and refine this understanding. We previously reported a method to prepare both fully metallic and oxidized SEIRAS-active Sn films, which we used in the present study to discern differences in adsorbed species during CO 2 R . Before we begin the discussion of the SEIRAS results, we would like to point out that ex situ characterization of the two Sn films after the reductive treatment shows that they have similar degrees of oxidation.…”

“…With some degree of understanding of what might be present on the surface during CO 2 R from the GC-DFT analysis described above, we performed ATR-SEIRAS experiments to validate and refine this understanding. We previously reported a method to prepare both fully metallic and oxidized SEIRAS-active Sn films, which we used in the present study to discern differences in adsorbed species during CO 2 R. 32 Before we begin the discussion of the SEIRAS results, we would like to point out that ex situ characterization of the two Sn films after the reductive treatment shows that they have similar degrees of oxidation. Both Sn films undergo spontaneous oxidation in air, which complicates the assignment of the exact oxidation state of the two films during CO 2 R. Dutta et al performed operando EXAFS experiments on SnO 2 -based catalysts and found that metallic Sn coordination was not observed until −0.88 V RHE , and SnO 2 and SnO features were still observed even at −1 V RHE .…”

“…While the CHE is elegantly simple and has led to valuable insights into electrocatalytic mechanisms, it also suffers from key limitations, namely that all molecular geometries, including the catalyst adsorbate structures, are relaxed at the potential of zero charge (PZC), that all electron transfers must be charge neutral (e.g., proton-coupled electron transfer, PCET, rather than sequential proton/electron transfer), and that it neglects the potential dependence on energetics of chemical steps. − GC-DFT accurately describes electrochemical mechanisms due to its self-consistent treatment of the electrified interface under an applied potential and its ability to capture decoupled charge transfer, widely believed to be a relevant elementary step in CO 2 R (CO 2 + * + e – → CO 2 – *, where * represents a surface active site). ,, We also use attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to investigate metallic and oxidized Sn surfaces during electrocatalysis. We recently developed a method of preparing Sn-based films that exhibit excellent surface enhancement in ATR-SEIRAS . Much of the ongoing debate in the community regarding CO 2 R on Sn originates from the lack of reliability in preparing and characterizing model Sn surfaces for spectroscopic investigation, as well as ambiguous frequency assignment. , By carefully comparing our observations from the GC-DFT and ATR-SEIRAS studies, we show that CO 2 R is feasible on both metallic and oxidized Sn through a combination of pathways that lead to adsorbed formate.…”

“…The spectro-electrochemical experiments were performed in a J1W Jackfish spectro-electrochemical cell (PIKE Technologies) with a PTFE/PEEK base. The synthesis and characterization of ATR-SEIRAS-active Sn-based films have been described in detail elsewhere . Other reports have also detailed the synthesis of electrodeposited metal films for SEIRAS, but not films of metal oxides. , Briefly, a polycrystalline Au underlayer was chemically deposited onto a Si(100) specialized 1 ATR element (single-bounce ATR crystal, IRUBIS) according to the procedure reported by Osawa .…”

“…We recently developed a method of preparing Sn-based films that exhibit excellent surface enhancement in ATR-SEIRAS. 32 Much of the ongoing debate in the community regarding CO 2 R on Sn originates from the lack of reliability in preparing and characterizing model Sn surfaces for spectroscopic investigation, as well as ambiguous frequency assignment. 10 , 21 By carefully comparing our observations from the GC-DFT and ATR-SEIRAS studies, we show that CO 2 R is feasible on both metallic and oxidized Sn through a combination of pathways that lead to adsorbed formate.…”