In Situ Wide-Frequency Surface-Enhanced Infrared Absorption Spectroscopy Enables One to Decipher the Interfacial Structure of a Cu Plating Additive

Uncovering the Dissociative Adsorption of the Leveler Janus Green B on Cu Electrodes at the Molecular Level

The interfacial adsorption structure of an organic leveler decides its functionality in Cu interconnect electroplating and is yet far from clear. In this work, in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and electrochemical quartz crystal microbalance (EQCM) in conjunction with density functional theory (DFT) calculations are applied to unravel the interfacial adsorption of the classic dye leveler Janus Green B (JGB) at a Cu electrode and understand its polarization property against Cu electrodeposition from an adsorption structure perspective. ATR-SEIRAS measurements and DFT calculations reveal that the N=N bond of the JGB molecule splits via reductive hydrogenation, forming two fragments of contrasting adsorption configurations. JGB exhibits the strongest inhibition effect on Cu deposition among all the tested additives including individual and mixed fragments, due to the highest coverage of organic adsorbates from JGB dissociation, as measured by EQCM. This work highlights the advantage of surface sensitive analytical tools in understanding the structure-performance of levelers.

Atomic Layer Deposition of TiO2 on Si Window Enables In Situ ATR-SEIRAS Measurements in Strong Alkaline Electrolytes

The Si window is the most widely used internal reflection element (IRE) for electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), yet local chemical etching on Si by concentrated OH- anions bottlenecks the reliable application of this method in strong alkaline electrolytes. In this report, atomic layer deposition of a 25 nm nonconductive TiO2 barrier layer on the reflecting plane of a Si prism is demonstrated to address this challenge. In situ ATR-SEIRAS measurement on a Au film electrode with the Si/TiO2 composite IRE in 1 M NaOH reveals reversible global spectral features without spectral distortion at 1000-1300 cm-1, in stark contrast to those obtained with a bare Si window. By applying this structured ATR-SEIRAS, ethanol electrooxidation on a Pt/C catalyst in 1 and 5 M NaOH is explored, manifesting that such high pH values prevent the adsorption of as-formed acetate in the C2 pathway but not that of CO intermediate in the C1 pathway.

Nano-Plasticity of an Electrified Ionic Liquid/Electrode Interface: Uncovering Hard-Soft Structuring via Controlled Metal Fill Factor

Ionic liquids (ILs) nanostructuring at electrified interfaces is of both fundamental and practical interest as these materials are increasingly gaining prominence in energy storage and conversion processes. However, much remains unresolved about IL potential-controlled (re)organization under highly polarized interfaces, mostly due to the difficulty of selectively probing both the distal and proximal surface layers of adsorbed ions. In this work, the structural dynamics of the innermost layer (<10 nm from the surface) were independently interrogated from that of the ionic layers in the sub-surface region (>100 nm from the surface), using an infrared (IR) spectroscopy approach. By tuning the metal fill factor of gold films deposited on conductive metal oxide-modified IR internal reflection elements, the charge-driven (re)structuring of the inner and distal layers of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate is unveiled. Within a relatively wide potential region (∼±1 V) bounding the potential of zero charges, the ionic liquid is shown to undergo a reversible (i.e., soft) reorganization whereby the innermost layer of anions (cations) is exchanged by a layer of cations (anions). Kinetically unhindered changes in the number density of constituent cations and anions largely follow electrostatic expectations in the subsurface region, whereas the innermost layer exhibits a pronounced hysteresis and very slow relaxation. Under larger negative potential bias, IL restructuring is characterized by a highly irreversible (i.e., hard) and intense interfacial densification of the BMPy+ cations, consistent with the formation of nanoscale segregated liquids. The outcomes of this work reveal a plastic IL nanostructuring under a strong electric field.

Time-resolved in situ vibrational spectroscopy for electrocatalysis: challenge and opportunity

Understanding the structure-activity relationship of catalysts and the reaction pathway is crucial for designing efficient, selective, and stable electrocatalytic systems. In situ vibrational spectroscopy provides a unique tool for decoding molecular-level factors involved in electrocatalytic reactions. Typically, spectra are recorded when the system reaches steady states under set potentials, known as steady-state measurements, providing static pictures of electrode properties at specific potentials. However, transient information that is crucial for understanding the dynamic of electrocatalytic reactions remains elusive. Thus, time-resolved in situ vibrational spectroscopies are developed. This mini review summarizes time-resolved in situ infrared and Raman techniques and discusses their application in electrocatalytic research. With different time resolutions, these time-resolved techniques can capture unique dynamic processes of electrocatalytic reactions, short-lived intermediates, and the surface structure revolution that would be missed in steady-state measurements alone. Therefore, they are essential for understanding complex reaction mechanisms and can help unravel important molecular-level information hidden in steady states. Additionally, improving spectral time resolution, exploring low/ultralow frequency detection, and developing operando time-resolved devices are proposed as areas for advancing time-resolved techniques and their further applications in electrocatalytic research.

Electrolyte effect for carbon dioxide reduction reaction on copper electrode interface: A DFT prediction

An insightful understanding of the interaction between the electrolyte and reaction intermediate and how promotion reaction occurs of electrolyte is challenging in the electrocatalysis reaction. Herein, theoretical calculations are used to investigate the reaction mechanism of CO₂ reduction reaction to CO with different electrolytes at the Cu(111) surface. By analyzing the charge distribution of the chemisorbed CO₂ (CO₂ ^δ-) formation process, we find that the charge transfer is from metal electrode transfer to CO₂ and the hydrogen bond interaction between electrolytes and CO₂ ^δ- not only plays a key role in the stabilization of CO₂ ^δ- structure but also reduces the formation energy of *COOH. In addition, the characteristic vibration frequency of intermediates in different electrolyte solutions shows that H₂O is a component of HCO₃ ^-, promoting CO₂ adsorption and reduction. Our results provide essential insights into the role of electrolyte solutions in interface electrochemistry reactions and help understand the catalysis process at the molecular level.