Why Is Surface-Enhanced Raman Scattering Insensitive to Liquid Water?

Surface-enhanced Raman scattering (SERS) is widely recognized as a remarkably powerful analytical technique that enables trace-level detection of organic molecules on a metal surface in aqueous systems with negligible spectral interference of water. This insensitivity of SERS to liquid water is violated in a restrictive manner under specific electrochemical conditions. However, the origin of such different SERS sensitivities to liquid water remains unclear.

Probing collective terahertz vibrations of a hydrogen-bonded water network at buried electrochemical interfaces.

The exceptional properties of liquid water such as thermodynamic, physical, and dielectric anomalies originate mostly from the hydrogen-bond networks of water molecules. The structural and dynamic properties of the hydrogen-bond networks have a significant impact on many biological and chemical processes in aqueous systems. In particular, the properties of interfacial water molecules with termination of the network at a solid surface are crucial to understanding the role of water in heterogeneous reactions.

Revisit of the plasmon-mediated chemical transformation of -aminothiophenol.

Fingerprint Raman features of -aminothiophenol (pATP) in surface-enhanced Raman scattering (SERS) spectra have been widely used to measure plasmon-driven catalytic activities because the appearance of characteristic spectral features is purported to be due to plasmon-induced chemical transformation of pATP to ,'-dimercaptoazobenzene (-DMAB). Here, we present a thorough comparison of SERS spectra for pATP and -DMAB in the extended range of frequencies covering group vibrations, skeletal vibrations, and external vibrations under various conditions. Although the fingerprint vibration modes of pATP could be almost mistaken with those of -DMAB, the low-frequency vibrations revealed distinct differences between pATP and DMAB.

In-situ Surface-Enhanced Raman Spectroscopy Reveals a Mars-van Krevelen-Type Gas Sensing Mechanism in Au@SnO Nanoparticle-Based Chemiresistors.

Molecular-level understandings of gas sensing mechanisms of oxide-based chemiresistors are significant for designing high-performance gas sensors; however, the mechanisms are still controversial due to the lack of direct experimental evidence. This work demonstrates efficient in situ surface-enhanced Raman spectroscopy (SERS) tracing of the highly representative SnO-ethanol gas sensing using Au@SnO nanoparticles (NPs), where the Au core and SnO shell provide SERS activity and a gas sensing response, respectively. The in situ SERS evidence suggests that the sensing follows a Mars-van Krevelen mechanism rather than the prevailing adsorbed oxygen (AO) model.

Engineered Au@CuO Nanoparticles for Wide-Range Quantitation of Sulfur Ions by Surface-Enhanced Raman Spectroscopy.

Efficient detection of sulfide ions (S), especially in a wide quantitative range, is of significance but faces challenges. This work strategizes and fabricates Au@CuO nanoparticles for quantitative surface-enhanced Raman spectroscopy (SERS) detection of the S ions based on the S concentration-dependent ion-solid interactions. We have achieved fast and quantitative S detection in a wide range from 5 ppb to 64,000 ppm (saturation concentration of the S source).

A single spectroscopic probe for in situ analysis of electronic and vibrational information at both sides of electrode/electrolyte interfaces using surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) at electrode/electrolyte interfaces includes inelastic light scattering not only by molecular vibrations in the electrolyte phase but also by conduction electrons in the metal electrode phase. While the former, i.e.

surface-enhanced electronic and vibrational Raman scattering spectroscopy at metal/molecule interfaces.

SERS signals from nanostructured surfaces of Au, covered with thiol monolayers, were monitored under application of various electrochemical potentials over a wide Raman-shift range of both the Stokes and anti-Stokes branches. The background continuum in the SERS spectra varied in intensity with apparent correlations with breaking of Au-S bond or evolution of Au-O. This clearly indicates that the origin of the background can be ascribed to non-resonant electronic Raman scattering, which is sensitive to the electronic density at the surface.

Origin of a High Overpotential of Co Electrodeposition in a Room-Temperature Ionic Liquid.

Metal electrodeposition in room-temperature ionic liquids (RTILs) often shows high overpotentials. Although this is often explained by the formation of a negatively charged metal complex due to the coordination of RTIL anions and the hindrance of its close approach onto the negatively charged electrode, we propose an alternative model based upon surface-enhanced infrared absorption spectroscopy measurements under Co electrodeposition. We found that the anionic first layer exists on the negatively charged electrode, and its replacement with a cationic one and Co electrodeposition both begin at an identical onset potential.

Electrochemical THz-SERS Observation of Thiol Monolayers on Au(111) and (100) Using Nanoparticle-assisted Gap-Mode Plasmon Excitation.

Surface-enhanced Raman scattering (SERS) microscopy using nanoparticle-assisted gap-mode plasmon excitation, which enables us to observe an atomically defined planar metal surface, was combined with THz-Raman spectroscopy to observe ultra-low-frequency vibration modes under electrochemical conditions. This combination helps us to gain deeper insights into electrode/electrolyte interfaces via direct observation of extramolecular vibrations including information on intermolecular and substrate/molecule interactions. Electrochemical reductive desorption of benzenethiol derivatives from Au(111) and (100) was monitored to demonstrate the power of this spectroscopy.

Tracking a Common Surface-Bound Intermediate during CO2-to-Fuels Catalysis.

Rational design of selective CO2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO2-to-fuels catalyst, capable of generating a wide array of value-added products, including methane, ethylene, and ethanol. All of these products are postulated to form via a common surface-bound CO intermediate.

Speciation of Adsorbed Phosphate at Gold Electrodes: A Combined Surface-Enhanced Infrared Absorption Spectroscopy and DFT Study.

Despite the significance of phosphate buffer solutions in (bio)electrochemistry, detailed adsorption properties of phosphate anions at metal surfaces remain poorly understood. Herein, phosphate adsorption at quasi-Au(111) surfaces prepared by a chemical deposition technique has been systematically investigated over a wide range of pH by surface-enhanced infrared absorption spectroscopy in the ATR configuration (ATR-SEIRAS). Two different pH-dependent states of adsorbed phosphate are spectroscopically detected.

Inhibited proton transfer enhances Au-catalyzed CO2-to-fuels selectivity.

CO2 reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H2 We combine in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical kinetic studies to probe the mechanistic basis for kinetic bifurcation between H2 and CO production on polycrystalline Au electrodes. Under the conditions of CO2 reduction catalysis, electrogenerated CO species are irreversibly bound to Au in a bridging mode at a surface coverage of ∼0.2 and act as kinetically inert spectators.

Adsorption of water dimer on platinum(111): identification of the -OH···Pt hydrogen bond.

The fundamental structure of an isolated water dimer on Pt(111) was determined by means of a spectroscopic method using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Two water molecules on adjacent atop sites form a dimer through a hydrogen bond, and they rotate even at a substrate temperature of 5 K. Action spectroscopy using STM (STM-AS) for water dimer hopping allows us to obtain the vibrational spectrum of a single water dimer on Pt(111).

Dissociation pathways of a single dimethyl disulfide on Cu(111): reaction induced by simultaneous excitation of two vibrational modes.

We present a novel reaction mechanism for a single adsorbed molecule that proceeds via simultaneous excitation of two different vibrational modes excited by inelastic tunneling electrons from a scanning tunneling microscope. Specifically, we analyze the dissociation of a single dimethyl disulfide (DMDS, (CH3S)2) molecule on Cu(111) by using a versatile theoretical method, which permits us to simulate reaction rates as a function of sample bias voltage. The reaction is induced by the excitation of C-H stretch and S-S stretch modes by a two-electron process at low positive bias voltages.

Insight into action spectroscopy for single molecule motion and reactions through inelastic electron tunneling.

We propose a versatile formula that describes action spectra for vibrationally mediated reactions of single molecules with a scanning tunneling microscope. Spectral fitting of the formula to CO hopping and the configurational change of the cis-2-butene molecule on Pd(110) enables us to determine the vibrational energy, reaction order, and transition rate associated with anharmonic coupling between the modes excited by tunneling electrons and the reaction-coordinate modes. The formula proposed here is general and easy to apply to any vibrationally mediated motion and reaction of single molecules.

State-selective dissociation of a single water molecule on an ultrathin MgO film.

The interaction of water with oxide surfaces has drawn considerable interest, owing to its application to problems in diverse scientific fields. Atomic-scale insights into water molecules on the oxide surface have long been recognized as essential for a fundamental understanding of the molecular processes occurring there. Here, we report the dissociation of a single water molecule on an ultrathin MgO film using low-temperature scanning tunnelling microscopy.