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Understanding the structure and dynamic process of water at the solid–liquid interface is an extremely important topic in surface science, energy science and catalysis1–3. As model catalysts, atomically flat single-crystal electrodes exhibit well-defined surface and electric field properties, and therefore may be used to elucidate the relationship between structure and electrocatalytic activity at the atomic level4,5. Hence, studying interfacial water behaviour on single-crystal surfaces provides a framework for understanding electrocatalysis6,7. However, interfacial water is notoriously difficult to probe owing to interference from bulk water and the complexity of interfacial environments8. Here, we use electrochemical, in situ Raman spectroscopic and computational techniques to investigate the interfacial water on atomically flat Pd single-crystal surfaces. Direct spectral evidence reveals that interfacial water consists of hydrogen-bonded and hydrated Na+ ion water. At hydrogen evolution reaction (HER) potentials, dynamic changes in the structure of interfacial water were observed from a random distribution to an ordered structure due to bias potential and Na+ ion cooperation. Structurally ordered interfacial water facilitated high-efficiency electron transfer across the interface, resulting in higher HER rates. The electrolytes and electrode surface effects on interfacial water were also probed and found to affect water structure. Therefore, through local cation tuning strategies, we anticipate that these results may be generalized to enable ordered interfacial water to improve electrocatalytic reaction rates. Interfacial water consists of hydrogen-bonded water and Na·H2O, its structure changes at hydrogen evolution reaction (HER) potentials, and when structurally ordered it aids interfacial electron transfer, resulting in higher HER rates.

If hydrogen can be stored and carried safely at a high density, hydrogen-fuel cells offer effective solutions for vehicles. The stable chemisorption of atomic hydrogen on single layer graphene (SLG) seems a perfect solution in this regard, with a theoretical maximum storage capacity of 7.7 wt %. However, generating hydrogenated graphene from H2 requires extreme temperatures and pressures. Alternatively, hydrogen adatoms can easily be produced under mild conditions by the electroreduction of protons in solid/liquid systems. Graphene is electrochemically inert for this reaction, but H-chemisorption on SLG can be carried out under mild conditions via a novel Pt-electrocatalyzed "spillover-surface diffusion-chemisorption" mechanism, as we demonstrate using dynamic electrochemistry and isotopic Raman spectroscopy. The apparent surface diffusion coefficient (∼10-5 cm2 s-1), capacity (∼6.6 wt %, ∼85.7% surface coverage), and stability of hydrogen adatoms on SLG at room temperature and atmospheric pressure are significant, and they are perfectly suited for applications involving stored hydrogen atoms on graphene.

Understanding the fundamental insights of oxygen activation and reaction at metal-oxide interfaces is of significant importance yet remains a major challenge due to the difficulty in in situ characterization of active oxygen species. Herein, the activation and reaction of molecular oxygen during CO oxidation at platinum-ceria interfaces has been in situ explored using surface-enhanced Raman spectroscopy (SERS) via a borrowing strategy, and different active oxygen species and their evolution during CO oxidation at platinum-ceria interfaces have been directly observed. In situ Raman spectroscopic evidence with isotopic exchange experiments demonstrate that oxygen is efficiently dissociated to chemisorbed O on Pt and lattice Ce-O species simultaneously at interfacial Ce3+ defect sites under CO oxidation, leading to a much higher activity at platinum-ceria interfaces compared to that at Pt alone. Further in situ time-resolved SERS studies and density functional theory simulations reveal a more efficient molecular pathway through the reaction between adsorbed CO and chemisorbed Pt-O species transferred from the interfaces. This work deepens the fundamental understandings on oxygen activation and CO oxidation at metal-oxide interfaces and offers a sensitive technique for the in situ characterization of oxygen species under working conditions.

RuO2 is one of the most promising catalysts for oxygen evolution reaction (OER), a key reaction in water electrolysis. However, the fundamental insights about the structural evolution of RuO2 under different electrochemical environments remain unclear. Herein, the molecular reaction pathways of the electroreduction of RuOx in alkaline and neutral conditions are identified using in-situ electrochemical surface-enhanced Raman spectroscopy. Au@RuO2 core–shell nanostructures are fabricated to amplify the Raman signals of species adsorbed on RuOx surfaces. Using such core–shell nanostructures, direct spectroscopic evidences of OH intermediate during the RuO2 electroreduction process are obtained, which is further confirmed by D2O isotopic substitution experiments. A molecular pathway that RuO2 is reduced to Ru through a two-step reduction process has been proposed. This work provides insightful information on the structural evolution of RuOx with electrochemical environments.

As energy demands increase, electrocatalysis serves as a vital tool in energy conversion. Elucidating electrocatalytic mechanisms using in situ spectroscopic characterization techniques can provide experimental guidance for preparing high-efficiency electrocatalysts. Surface-enhanced Raman spectroscopy (SERS) can provide rich spectral information for ultratrace surface species and is extremely well suited to studying their activity. To improve the material and morphological universalities, researchers have employed different kinds of nanostructures that have played important roles in the development of SERS technologies. Different strategies, such as so-called borrowing enhancement from shell-isolated modes and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)-satellite structures, have been proposed to obtain highly effective Raman enhancement, and these methods make it possible to apply SERS to various electrocatalytic systems. Here, we discuss the development of SERS technology, focusing on its applications in different electrocatalytic reactions (such as oxygen reduction reactions) and at different nanostructure surfaces, and give a brief outlook on its development.

Interfacial host–guest complexation offers a versatile way to functionalize nanomaterials. However, the complicated interfacial environment and trace amounts of components present at the interface make the study of interfacial complexation very difficult. Herein, taking the advantages of near-single-molecule level sensitivity and molecular fingerprint of surface-enhanced Raman spectroscopy (SERS), we reveal that a cooperative effect between cucurbit[7]uril (CB[7]) and methyl viologen (MV2+2I−) in aggregating Au NPs originates from the cooperative adsorption of halide counter anions I−, MV2+, and CB[7] on Au NPs surface. Moreover, similar SERS peak shifts in the control experiments using CB[n]s but with smaller cavity sizes suggested the occurrence of the same guest complexations among CB[5], CB[6], and CB[7] with MV2+. Hence, an unconventional exclusive complexation model is proposed between CB[7] and MV2+ on the surface of Au NPs, distinct from the well-known 1:1 inclusion complexation model in aqueous solutions. In summary, new insights into the fundamental understanding of host–guest interactions at nanostructured interfaces were obtained by SERS, which might be useful for applications related to host–guest chemistry in engineered nanomaterials.

Elucidating hydrogen oxidation reaction (HOR) mechanisms in alkaline conditions is vital for understanding and improving the efficiency of anion-exchange-membrane fuel cells. However, uncertainty remains around the alkaline HOR mechanism owing to a lack of direct in situ evidence of intermediates. In this study, in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and DFT were used to study HOR processes on PtNi alloy and Pt surfaces, respectively. Spectroscopic evidence indicates that adsorbed hydroxy species (OHad ) were directly involved in HOR processes in alkaline conditions on the PtNi alloy surface. However, OHad species were not observed on the Pt surface during the HOR. We show that Ni doping promoted hydroxy adsorption on the platinum-alloy catalytic surface, improving the HOR activity. DFT calculations also suggest that the free energy was decreased by hydroxy adsorption. Consequently, tuning OH adsorption by designing bifunctional catalysts is an efficient method for promoting HOR activity.

Molecular face-rotating polyhedra (FRP) exhibit complex stereochemistry, rendering it challenging to manipulate their assembly in a stereoselective manner. In our previous work, stereocontrolled FRP were gained at the cost of losing the confined inner space, which hampers their host–guest interactions and potential applications. Through a rational design approach, herein we demonstrate the successful construction of hollow FRP with high diastereoselectivity. Whereas the [4 + 4] imine condensation of meta-formyl substituted C3h-symmetric TAT-m and C3-symmetric Tri-NH2 led to the formation of all feasible FRP-12 diastereoisomers; the para-substituted constitutional isomer, TAT-p, exclusively assembled into a pair of homo-directional enantiomeric FRP-13-CCCC/AAAA with a cavity size larger than 600 Å3. Detailed structural characterizations and theoretical investigations revealed the thermodynamic landscape of FRP assembly can be effectively shaped by modulating the van der Waals repulsive forces among the facial building blocks. Our work provided a novel strategy towards stereospecific assembly of pure organic cages, opening up new opportunities for further applications of these chiral materials., The rationally engineered facial units, TAT-m and TAT-p, resulted in distinct diastereoselectivity of face-rotating polyhedra (FRP).

The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres1–6. Plasmon-enhanced applications include advanced spectroscopies7–10, optomechanics11, optomagnetics12 and biosensing13–17. However, precise determination of plasmon field intensity distribution within a nanogap remains challenging. Here, we demonstrate a molecular ruler made from a set of viologen-based, self-assembly monolayers with which we precisely measures field distribution within a plasmon nanocavity with ~2-A spatial resolution. We observed an unusually large plasmon field intensity inhomogeneity that we attribute to the formation of a plasmonic comb in the nanocavity. As a consequence, we posit that the generally adopted continuous media approximation for molecular monolayers should be used carefully. The strength of the plasmonic field between a plasmonic particle and a Au surface can be measured at ~2-A resolution by following the Raman peaks of a suitably labelled self-assembly monolayer.

The electrical double layer (EDL) is the extremely important interfacial region involved in many electrochemical reactions, and it is the subject of significant study in electrochemistry and surface science. However, the direct measurement of interfacial electric fields in the EDL is challenging. In this work, both electrochemical resonant Raman spectroscopy and theoretical calculations were used to study electric field distributions in the EDL of an atomically flat single-crystal Au(111) electrode with self-assembled monolayer molecular films. This was achieved using a series of redox-active molecules containing the 4,4'-bipyridinium moiety as a Raman marker that were located at different precisely controlled distances away from the electrode surface. It was found that the electric field and the dipole moment of the probe molecule both directly affected its Raman signal intensity, which in turn could be used to map the electric field distribution at the interface. Also, by variation of the electrolyte anion concentration, the Raman intensity was found to decrease when the electric field strength increased. Moreover, the distance between adjacent Raman markers was ∼2.1 Å. Thus, angstrom-level spatial resolution in the mapping of electric field distributions at the electrode-electrolyte interface was realized. These results directly evidence the EDL structure, bridging the gap between the theoretical and experimental understandings of the interface.

The spillover of hydrogen species and its role in tuning the activity and selectivity in catalytic hydrogenation have been investigated in situ using surface-enhanced Raman spectroscopy (SERS) with 10 nm spatial resolution through the precise fabrication of Au/TiO2 /Pt sandwich nanostructures. In situ SERS study reveals that hydrogen species can efficiently spillover at Pt-TiO2 -Au interfaces, and the ultimate spillover distance on TiO2 is about 50 nm. Combining kinetic isotope experiments and density functional theory calculations, it is found that the hydrogen spillover proceeds via the water-assisted cleavage and formation of surface hydrogen-oxygen bond. More importantly, the selectivity in the hydrogenation of the nitro or isocyanide group is manipulated by controlling the hydrogen spillover. This work provides molecular insights to deepen the understanding of hydrogen activation and boosts the design of active and selective catalysts for hydrogenation.

The dependence of plasmon resonance on the size, shape, and interparticle spacing of single, isolated nanostructures inherently limits their light-harvesting capability to a narrow spectral band. Here, we report a facile overcurrent electrodeposition strategy to prepare fractal plasmonic black gold (B-Au) with broad-band absorption properties (over 80% throughout the range of 300–1800 nm). The broad-band absorption properties are attributed to the excitation of multiple plasmons in the B-Au, which results in strong light–matter interaction over a broad-band spectral window. Consequently, the B-Au can produce strong broad-band surface-enhanced Raman scattering (SERS) regardless of the excitation light used. These findings demonstrate that the fractal B-Au allows efficient utilization of broad spectral photons and opens up exciting opportunities for highly sensitive SERS detection, photocatalysis, and photovoltaic devices.

Summary Plasmon-mediated photocatalysis via hot-electron transfer attracts increasing interest due to its capability to improve energy utilization efficiency. However, more insightful information is still needed to reveal the mechanism of interfacial hot-electron transfer. Herein, the plasmon-induced hot-electron transfer at different plasmonic interfaces, including Au-insulator (SiO2), Au-semiconductor (TiO2 and Cu2O), and Au-metal (Pd and Pt), is directly investigated using surface-enhanced Raman spectroscopy (SERS) and density functional theory calculation with a (sub)nanometer spatial resolution, through the fabrication of well-defined plasmonic nanostructures. (Sub)nanometer-distance dependence of interfacial hot-electron transfer has been identified for the first time. Hot electrons can migrate across the Au-semiconductor or Au-metal interfaces and transfer more than 10 nm in semiconductors but decay to thermally equilibrated states rapidly in metals in less than 1 nm. Such a transfer process is blocked at the Au-insulator interface. This work promotes the fundamental understanding of plasmon-induced hot-electron transmission and photocatalysis at plasmonic interfaces.

It is widely accepted that the sensitivity of surface-enhanced Raman spectroscopy (SERS) is mainly manipulated by the electromagnetic enhancement mechanism (EM). Herein, we determined that the direct adsorption of the target on the SERS active surface is vital as well, through the systematic investigation of the SERS behavior of three positively charged molecules on negatively charged gold (Au) or silver nanoparticles (Ag NPs). Facilitated by the synergistic effect among the molecule, the surface, and the specific adsorbed halide ions (Cl-, Br-, and I-), high SERS sensitivity for trace target was realized, which was mainly from the directly adsorbed molecules. Noteworthy, little contribution from the nondirectly adsorbed molecules was discernible, although the EM enhancement was at the same level for these two surface species dwelling within a distance significantly less than 1 nm from the surface. Further, the related strategy for trace detection sheds light on how to realize sensitive SERS detection of new targets.

In dye-sensitized solar cells (DSSCs), the TiO2/dye interface significantly affects photovoltaic performance. However, the adsorption and photoinduced behavior of dye molecules on the TiO2 substrate remains unclear. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to study the adsorption and photoinduced behavior of dye (N719) molecules on different TiO2(hkl) surfaces. On TiO2(001) and TiO2(110) surfaces, the in situ SHINERS and mass spectrometry results indicate S Created by potrace 1.16, written by Peter Selinger 2001-2019 C bond cleavage in the anchoring groups of adsorbed N719, whereas negligible bond cleavage occurs on the TiO2(111) surface. Furthermore, DFT calculations show the stability of the SC anchoring group on three TiO2(hkl) surfaces in the order TiO2(001) < TiO2(110) < TiO2(111), which correlated well with the observed photocatalytic activities. This work reveals the photoactivity of different TiO2(hkl) surface structures and can help with the rational design of DSSCs. Thus, this strategy can be applied to real-time probing of photoinduced processes on semiconductor single crystal surfaces., In dye-sensitized solar cells (DSSCs), the TiO2/dye interface significantly affects photovoltaic performance.

Plasmonic metals under photoexcitation can generate energetic hot electrons to directly induce chemical reactions. However, the capability and fundamental insights of the transportation of these hot electrons at plasmonic metal-2D material interfaces remain unclear. Herein, hot-electron transfer at Au-graphene interfaces has been in situ studied using surface-enhanced Raman spectroscopy (SERS) with atomic layer accuracy. Combining in situ SERS studies with density functional theory calculations, it is proved that hot electrons can be injected from plasmonic Au nanoparticles to graphene and directly penetrate graphene to trigger photocatalytic reactions. With increasing graphene layers, the transportation of hot electrons decays rapidly and would be completely blocked after five layers of graphene. Moreover, the transfer of hot electrons can be modulated by applying an external electric field, and the hot-electron transfer efficiency under electrochemical conditions is improved by over three times in the presence of a monolayer of graphene. These fundamental understandings about hot-electron transfer provide insightful information to promote the design of more efficient plasmonic materials and devices.

Surface enhanced Raman spectroscopy (SERS) is an ultrasensitive label-free analytical technique that can provide unique chemical and structural fingerprint information. However, gaining reliable quantitative analysis with SERS remains a huge challenge because of poor reproducibility and the instability of nanostructured SERS active surfaces. Herein, an effective strategy of coating Au nanoparticles (NPs) with ultrathin and uniform Prussian blue (PB) shell (Au@PB NPs) was developed for quantitative detection of dopamine (DA) concentrations in blood serum and crystal violet (CV) contaminants in lake water. The only intense PB Raman signal at 2155 cm-1 served as an ideal and interference-free internal standard (IS) for correcting fluctuations in the Raman intensities of analytes. Also, the stability of Au@PB NPs was investigated, exhibiting good functionality in strong acid solutions and thermal stability at 100 °C. This work demonstrates a convenient and fast quantitative SERS technique for detecting analyte concentrations in complex systems and has a great number of potential applications for use in analytical chemistry.

It is vital to understand the oxygen reduction reaction (ORR) mechanism at the molecular level for the rational design and synthesis of high activity fuel-cell catalysts. Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of detecting the bond vibrations of surface species in the low wavenumber range, however, using it to probe practical nanocatalysts remains extremely challenging. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to investigate ORR processes on the surface of bimetallic Pt3 Co nanocatalyst structures. Direct spectroscopic evidence of *OOH suggests that ORR undergoes an associative mechanism on Pt3 Co in both acidic and basic environments. Density functional theory (DFT) calculations show that the weak *O adsorption arise from electronic effect on the Pt3 Co surface accounts for enhanced ORR activity. This work shows SHINERS is a promising technique for the real-time observation of catalytic processes.

Sulfadiazine, as a class of antibiotics, has been widely used in the world for decades; however, its surface-enhanced Raman spectra (SERS) on gold colloids are obviously different from ordinary Raman spectra in the solid powder and liquid solution. To explore the reasons for such significant differences, we used density functional theory calculations and normal-mode analysis to investigate the effects of the configuration, conformation, protonation, hydrogen-bonding interaction, and adsorption configurations of sulfadiazine on gold clusters to check these different effects on the vibrational assignments. Our calculated results can be summarized as two points. First, the Raman spectra strongly depend on the configuration, conformation, protonation, and hydrogen bonding of sulfadiazine. Second, the wagging vibration displays a significant vibrational frequency shift and a very strong SERS peak responsible for the observed SERS signal when sulfadiazine is adsorbed on gold clusters through the terminal amino group. This is different from another adsorption configuration through two oxygen atoms of the -SO2NH- group on gold clusters. Finally, we further investigate the potential energy surfaces along the wagging vibration and the binding interaction of -NH2 adsorbed on different sites of gold surfaces.

Recently, surface-enhanced Raman Spectroscopy (SERS) has been widely applied for rapid detection of trace targets in various fields. However, two key issues are still being explored: (1) how to for...

Improving time resolution of Raman imaging is essential for the observation of dynamic processes involved in interfacial catalysis and biological systems. The crucial step is how to recognize and extract weak Raman signals overwhelmed in the strong noise under the low signal-to-noise ratio (SNR) condition. Here, by exploring the relationship between the SNR of a single Raman spectrum and the structural similarity (SSIM; the key parameter evaluating the image quality) of the whole image, we determined a semiempirical threshold with SNR = 0 dB for clear imaging for the first time. Therefore, we proposed one signal processing algorithm for fast Raman imaging by reconstructing the Raman spectrum with the aid of weak signal processing: extracting the reliable Raman signal of the target under the low SNR and then determining the suitable scanning time to obtain the Raman image with a trustworthy image quality. In the first step, fast Fourier transform (FFT), least squares, and 2-D median filter are sequentially applied to improve the SNR of each raw Raman spectrum. In the second step, a local SNR evaluation strategy is developed to predict image quality as well as the determination of clear imaging. The proposed method was successfully applied to the fast imaging of the cell under the low SNR condition.

Adsorption and chemical enhancement of p-methylbenzenethiol (PMBT) on silver and gold nanoparticle surfaces have been studied using surface enhanced Raman spectroscopy (SERS) and density functional...

Adsorption plays a critical role in surface and interface processes. Fractional surface coverage and adsorption free energy are two essential parameters of molecular adsorption. However, although adsorption at the solid-gas interface has been well-studied, and some adsorption models were proposed more than a century ago, challenges remain for the experimental investigation of molecular adsorption at the solid-liquid interface. Herein, we report the statistical and quantitative single-molecule measurement of adsorption at the solid-liquid interface by using the single-molecule break junction technique. The fractional surface coverage was extracted from the analysis of junction formation probability so that the adsorption free energy could be calculated by referring to the Langmuir isotherm. In the case of three prototypical molecules with terminal methylthio, pyridyl, and amino groups, the adsorption free energies were found to be 32.5, 33.9, and 28.3 kJ mol-1 , respectively, which are consistent with DFT calculations.

The fundamental understanding of the subtle interactions between molecules and plasmons is of great significance for the development of plasmon-enhanced spectroscopy (PES) techniques with ultrahigh sensitivity. However, this information has been elusive due to the complex mechanisms and difficulty in reliably constructing and precisely controlling interactions in well-defined plasmonic systems. Herein, the interactions in plasmonic nanocavities of film-coupled metallic nanocubes (NCs) are investigated. Through engineering the spacer layer, molecule-plasmon interactions were precisely controlled and resolved within 2 nm. Efficient energy exchange interactions between the NCs and the surface within the 1-2 nm range are demonstrated. Additionally, optical dressed molecular excited states with a huge Lamb shift of ≈7 meV at the single-molecule (SM) level were observed. This work provides a basis for understanding the underlying molecule-plasmon interaction, paving the way for fully manipulating light-matter interactions at the nanoscale.

Coordination behavior between theophylline (THP) molecule and Au(III) ion as well as electrochemical reduction of THP-Au(III) complexing ion were studied in detail. UV–vis absorption spectroscopy clarifies that Au(III) is coordinated with THP in a 1:2 M ratio. Raman spectroscopy indicates that it is N in THP chemically bonding to Au(III), and density functional theory (DFT) together with time-dependent density functional theory (TD-DFT) further reveal that the specific N atom is N9 in the imidazole ring (N7H form). Based on the invented formula of cyanide-free gold electroplating with THP as a complexant, the electrode behavior of THP-Au(III) ion on the gold electrode was studied. The results show that the stability constant Kf of THP-Au(III) ion is 2.7 × 1035, and the reduction of THP-Au(III) ion is in irreversible with diffusion control.

Plasmonic "hot spots" play a key role in surface-enhanced Raman scattering (SERS) enabling its ultrahigh surface sensitivity. Thus, precise prediction and control of the location of hot spots in surface nanostructures is of great importance. However, it is difficult to predict the exact location of hot spots due to complex plasmon competition and synergistic effects in three-dimensional (3D) multiparticle surface configurations. In this work, three types of Au@probe@SiO2 core-shell nanoparticles were prepared and a 3D hot spots matrix was assembled via a consecutive layer on layer deposition method. Combined with SERS, distinct probe molecules were integrated into different layers of the 3D multiparticle nanostructure allowing for the hot spots to be precisely located. Importantly, the hot spots could be controlled and relocated by applying different excitation wavelengths, which was verified by simulations and experimental results. This work proposes a new insight and provides a platform for precisely probing and controlling chemical reactions, which has profound implications in both surface analysis and surface plasmonics.

Understanding the role of the morphology and particle–particle interactions on the plasmonic properties is of significant importance for the development of nanomaterials with excellent optical properties. However, the preparation of precisely defined nanomaterials with sizes that span a large range and their controllable self-assembly still remain a great challenge. Here, a multistep seed-mediated method has been established for preparing uniform Ag nanocubes over a broad size range from nanoscale (50 nm) to microscale (1400 nm) and with different hierarchical nanostructures range from “zero-dimension” (“0D”) to “three-dimension” (“3D”). The influence of the size and the interactions between the Ag nanocubes on their surface-enhanced Raman scattering (SERS) properties have been systematically and quantitatively investigated. It is demonstrated through experiments and finite-difference time-domain (FDTD) calculations that the SERS activity is dependent on the matching of the nanocube size to the excitation wavelength. The optimal combinations are 80, 110 and 130 nm nanocubes with respect to 532, 638 and 785 nm excitation wavelength, respectively. Furthermore, the Raman enhancement of the Ag nanocube hierarchical nanostructures increases rapidly from “0D” to “3D”, due to the extra increase of the hot spots that is attributed to the out-of-plane plasmonic coupling realized in the “3D” hierarchical nanostructures. This work clearly illustrates the quantitative role of the size and dimension of Ag nanocubes on their SERS properties and provides fundamental information for the design of advanced nanomaterials with higher SERS sensitivity.

In recent years, ensuring the rational use and effective control of antibiotics has been a major focus in the eco-environment, which requires an effective monitoring method. However, on-site rapid detection of antibiotics in water environments remains a challenging issue. In this study, surface-enhanced Raman spectroscopy (SERS) was used to systematically achieve selective, rapid, and highly sensitive detection of sulfonamides, based on their fingerprint characteristics. The results show that the trade-off between the competitive and coadsorption behaviors of target molecules and agglomerates (inorganic salts) on the surface of the SERS substrate determines whether the molecules can be detected with high sensitivity. Based on this, the qualitative differentiation and quantitative detection of three structurally similar antibiotics, sulfadiazine, sulfamerazine, and sulfamethazine, were achieved, with the lowest detectable concentration being 1 μg/L for sulfadiazine and 50 μg/L for sulfamerazine and sulfamethazine.

In spectroscopic analysis, push-to-the-limit sensitivity is one of the important topics, particularly when facing the qualitative and quantitative analyses of the trace target. Normally, the effective recognition and extraction of weak signals are the first key steps, for which there has been considerable effort in developing various denoising algorithms for decades. Nevertheless, the lower the signal-to-noise ratio (SNR), the greater the deviation of the peak height and shape during the denoising process. Therefore, we propose a denoising algorithm along with peak extraction and retention (PEER). First, both the first and second derivatives of the Raman spectrum are used to determine Raman peaks with a high SNR whose peak information is kept away from the denoising process. Second, an optimized window smoothing algorithm is applied to the left part of the Raman spectrum, which is combined with the untreated Raman peaks to obtain the denoised Raman spectrum. The PEER algorithm is demonstrated with much better signal extraction and retention and successfully improves the temporal resolution of Raman imaging of a living cell by at least 1 order of magnitude higher than those by traditional algorithms.

Over the past decade, many efforts have been devoted to designing and fabricating substrates for surface-enhanced Raman spectroscopy (SERS) with abundant hot spots to improve the sensitivity of detection. However, there have been many difficulties involved in causing molecules to enter hot spots actively or effectively. Here, we report a general SERS method for actively capturing target molecules in small gaps (hot spots) by constructing a nanocapillary pumping model. The ubiquity of hot spots and the inevitability of molecules entering them lights up all the hot spots and makes them effective. This general method can realize the highly sensitive detection of different types of molecules, including organic pollutants, drugs, poisons, toxins, pesticide residues, dyes, antibiotics, amino acids, antitumor drugs, explosives, and plasticizers. Additionally, in the dynamic detection process, an efficient and stable signal can be maintained for 1-2 min, which increases the practicality and operability of this method. Moreover, a dynamic detection process like this corresponds to the processes of material transformation in some organisms, so the method can be used to monitor transformation processes such as the death of a single cell caused by photothermal stimulation. Our method provides a novel pathway for generating hot spots that actively attract target molecules, and it can achieve general ultratrace detection of diverse substances and be applied to the study of cell behaviors in biological systems.

Surface-enhanced Raman spectroscopy (SERS) is a promising ultrasensitive analysis technology due to outstanding molecular fingerprint identification. However, the measured molecules generally need to be adsorbed on a SERS substrate, which makes it difficult to detect weakly adsorbed molecules, for example, the volatile organic compound (VOC) molecules. Herein, we developed a kind of a SERS detection method for weak adsorption molecules with Au@ZIF-8 core-shell nanoparticles (NPs). The well-uniformed single- and multicore-shell NPs can be synthesized controllably, and the shell thickness of the ZIF-8 was able to be precisely controlled (from 3 to 50 nm) to adjust the distance and electromagnetic fields between metal nanoparticles. After analyzing the chemical and physical characterization, Au@ZIF-8 core-shell NPs were employed to detect VOC gas by SERS. In contrast with multicore or thicker-shell nanoparticles, Au@ZIF-8 with a shell thickness of 3 nm could efficiently probe various VOC gas molecules, such as toluene, ethylbenzene, and chlorobenzene. Besides, we were capable of observing the process of toluene gas adsorption and desorption using real-time SERS technology. As observed from the experimental results, this core-shell nanostructure has a promising prospect in diverse gas detection and is expected to be applied to the specific identification of intermediates in catalytic reactions.

Raman and infrared (IR) spectroscopy are powerful analytical techniques, but have intrinsically low detection sensitivity. There have been three major steps (i) to advance the optical system of the light excitation, collection, and detection since 1920s, (ii) to utilize nanostructure-based surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) since 1990s, and (iii) to rationally couple (i) and (ii) for maximizing the total detection sensitivity since 2010s. After surveying the history of SERS and SEIRA, we outline the principle of plasmonics and the different mechanisms of SERS and SEIRA. We describe various interactions of light with nano/microstructures, localized surface plasmon, surface plasmon polariton, and lightning-rod effect. Their coupling effects can significantly increase the surface sensitivity by designing nanoparticle–nanoparticle and nanoparticle–substrate configuration. As the nano/microstructures have specific optical near-field and far-field behaviors, we focus on how to systematically design the macro-optical systems to maximize the excitation efficiency and detection sensitivity. We enumerate the key optical designs in particular ATR-based operation modes of directional excitation and emission from visible to IR spectral region. We also present some latest advancements on scanning-probe microscopy-based nanoscale spectroscopy. Finally, prospects and further developments of this field are given with emphasis on emerging techniques and methodologies., This review focuses on the advance of optical design from nano/micro to macro in SERS and SEIRA, especially on the optical coupling between nano/micro and macro scales.

Developing advanced characterization techniques for single-atom catalysts (SACs) is of great significance to identify their structural and catalytic properties. Raman spectroscopy can provide molecular structure information, and thus, the technique is a promising tool for catalysis. However, its application in SACs remains a great challenge because of its low sensitivity. We develop a highly sensitive strategy that achieves the characterization of the structure of SACs and in situ monitoring of the catalytic reaction processes on them by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) for the first time. Using the strategy, Pd SACs on different supports were identified by Raman spectroscopy and the nucleation process of Pd species from single atoms to nanoparticles was revealed. Moreover, the catalytic reaction processes of the hydrogenation of nitro compounds on Pd SACs were monitored in situ, and molecular insights were obtained to uncover the unique catalytic properties of SACs. This work provides a new spectroscopic tool for the in situ study of SACs, especially at solid-liquid interfaces.

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article., Funding is acknowledged from the European Research Council (ERC Advanced Grant No. 787510-4DBIOSERS to L.M.L.-M., ERC Advanced Grant No. 789104-eNANO to F.J.G.A., ERC Starting Grant No. 259432-MULTIBIOPHOT to J.K., ERC Consolidator Grant No. 772108-DarkSERS); the Department of Education of the Basque Government (Grant No. IT1164-19 to J.A.); the Spanish MINECO (CTQ2017-88648-R to R.A.-P., MAT2016-77809-R to I.P.-S. and J.P.-J.); the EPSRC (EP/P034063/1 to S.B., EP/L027151/1 to J.B., EP/L014165/1 to D.G. and K.F.); IDUN-Danish National Research Foundation (DNRF122) and Villum Fonden (Grant No. 9301) to A.B.; the National Research Foundation of Korea (Grant No. 2019R1A2C3004375 to J.B.); the German Science Foundation, DFG (SFB 1278 Polytarget (Project B4) to V.D., Grant No. SCHL 594/13-1 to S.S., Germany’s Excellence Strategy (EXC 2089/1-390776260) to S.M.); the Federal Ministry of Education and Research, Germany (BMBF) (Grant InfectoGnostics 13GW0096F to D.C.-M. and J.P.); DARPA-16-35-INTERCEPT-FP-018 to L.F.; the UK BBSRC (Grant No. BB/L014823/1 to R.G.); the Department of Science and Technology (DST Nanomission Project SR/NM/NS-23/2016 to K.G.T.); the U.S. National Science Foundation (Grant No. CHE-1707859 to A.J.H., Center for Sustainable Nanotechnology CHE-1503408 (Centers for Chemical Innovation Program) to C.L.H., Center for Chemical Innovation Chemistry at the Space-Time Limit (CaSTL) CHE-1414466 to G.C.S. and R.P.V.D., Grant No. CHE-1807269 to K.A.W.); the Knut and Alice Wallenberg Foundation to M.K.; the Office of Naval Research (Grant No. N00014-18-1-2876 to N.A.K.); Royal Society of New Zealand Te Apa̅rangi to E.L.R. and B.A.; Singapore Ministry of Education, Tier 1 (RG11/18) to X.Y.L.; the Photoexcitonix Project in Hokkaido Univ., Japan, to K.M.; BioNano Health-Guard Research Center funded by the Ministry of Science and ICT (MSIT) of Korea as Global Frontier Project (Grant No. H-GUARD_2013M3A6B2078947) to J.-M.N.; NSFC of P. R. China (Grant No. 21705015 to Y.O., Grant No. 21633005 to B.R.); National Key R&D Program (2017YFA0206902) to C.X. This work was coordinated under the Maria de Maeztu Units of Excellence Program from the Spanish State Research Agency—Grant No. MDM-2017-0720.

The adsorption and electrooxidation of CO molecules at well-defined Pt(hkl) single-crystal electrode surfaces is a key step towards addressing catalyst poisoning mechanisms in fuel cells. Herein, we employed in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) coupled with theoretical calculation to investigate CO electrooxidation on Pt(hkl) surfaces in acidic solution. We obtained the Raman signal of top- and bridge-site adsorbed CO* molecules on Pt(111) and Pt(100). In contrast, on Pt(110) surfaces only top-site adsorbed CO* was detected during the entire electrooxidation process. Direct spectroscopic evidence for OH* and COOH* species forming on Pt(100) and Pt(111) surfaces was afforded and confirmed subsequently via isotope substitution experiments and DFT calculations. In summary, the formation and adsorption of OH* and COOH* species plays a vital role in expediting the electrooxidation process, which relates with the pre-oxidation peak of CO electrooxidation. This work deepens knowledge of the CO electrooxidation process and provides new perspectives for the design of anti-poisoning and highly effective catalysts.

Real-time detection of photo-induced bond cleavage of a single molecule was enabled by plasmon-enhanced spectroscopy., Determining structural transformations of single molecules (SMs) is an important fundamental scientific endeavor. Optical spectroscopies are the dominant tools used to unravel the physical and chemical features of individual molecules and have substantially contributed to surface science and biotechnology. In particular, Raman spectroscopy can identify reaction intermediates and reveal underlying reaction mechanisms; however, SM Raman experiments are subject to intrinsically weak signal intensities and considerable signal attenuation within the spectral dispersion systems of the spectrometer. Here, to monitor the structural transformation of an SM on the millisecond time scale, a plasmonic nanocavity substrate has been used to enable Raman vibrational and fluorescence spectral signals to be simultaneously collected and correlated, which thus allows a detection of photo-induced bond cleavage between the xanthene and phenyl group of a single rhodamine B isothiocyanate molecule in real time. This technique provides a novel method for investigating light-matter interactions and chemical reactions at the SM level.

Knowledge of atomistic structures at solid/liquid interfaces is essential to elucidate interfacial processes in chemistry, physics, and materials sciences. The (√3 × √7) structure associated with a pair of sharp reversible current spikes in the cyclic voltammogram on a Au(111) electrode in sulfuric acid solution represents one of the most classical ordered structures at electrode/electrolyte interfaces. Although more than 10 adsorption configurations have been proposed in the past four decades, the atomistic structure remains ambiguous and is consequently an open problem in electrochemistry and surface science. Herein, by combining high-resolution electrochemical scanning tuning microscopy, electrochemical infrared and Raman spectroscopies, and, in particular, the newly developed quantitative computational method for electrochemical infrared and Raman spectra, we unambiguously reveal that the adstructure is Au(111)(√3 × √7)-(SO4···w2) with a sulfate anion (SO4*) and two structured water molecules (w2*) in a unit cell, and the crisscrossed [w···SO4···w]n and [w···w···]n hydrogen-bonding network comprises the symmetric adstructure. We further elucidate that the electrostatic potential energy dictates the proton affinity of sulfate anions, leading to the potential-tuned structural transformations. Our work enlightens the structural details of the inner Helmholtz plane and thus advances our fundamental understanding of the processes at electrochemical interfaces. The authors acknowledge funding by the National Natural Science Foundation of China (91950121, 21727807, 21403179, 21872115, and 21533006).

A surface-enhanced Raman scattering-chiral anisotropy (SERS-ChA) effect is reported that combines chiral discrimination and surface Raman scattering enhancement on chiral nanostructured Au films (CNAFs) equipped in the normal Raman scattering Spectrometer. The CNAFs provided remarkably higher enhancement factors of Raman scattering (EFs) for particular enantiomers, and the SERS intensity was proportional to the enantiomeric excesses (ee) values. Except for molecules with mesomeric species, all of the tested enantiomers exhibited high SERS-ChA asymmetry factors (g), ranging between 1.34 and 1.99 regardless of polarities, sizes, chromophores, concentrations and ee. The effect might be attributed to selective resonance coupling between the induced electric and magnetic dipoles associated with enantiomers and chiral plasmonic modes of CNAFs.

In situ monitoring of electrocatalytic processes at solid-liquid interfaces is essential for the fundamental understanding of reaction mechanisms, yet quite challenging. Herein, Pt-on-Au nanocatalysts with a Au-core Pt-satellite superstructure have been fabricated. In such Pt-on-Au nanocatalysts, the Au cores can greatly amplify the Raman signals of the species adsorbed on Pt, allowing the in situ surface-enhanced Raman spectroscopy (SERS) study of the electrocatalytic reactions on Pt. Using the combination of an electrochemical method and in situ SERS, size effects of Pt on the catalytic performance of the core-satellite nanocomposites towards CO and methanol electrooxidation are revealed. It is found that such Pt-on-Au nanocomposites show improved activity and long-term stability for the electrooxidation of CO and methanol with a decrease in the Pt size. This work demonstrates an effective strategy to achieve the in situ monitoring of electrocatalytic processes and to simultaneously boost their catalytic performance towards electrooxidation.

Plasmonic optical antennas (POAs), often constructed from gold or silver nanostructures, can enhance the radiation efficiency of emitters coupled to POAs and are applied in surface-enhanced Raman spectroscopy (SERS) and light-emitting devices. Over the past four decades, radiation enhancement factors (REFs) of POA-emitter systems were considered to be difficult to calculate directly and have been predicted indirectly and approximately, assuming POAs are illuminated by electromagnetic plane waves without emitters. The validity of this approximation remains a significant open problem in SERS theory. Herein, we develop a method based on the rigorous optical reciprocity theorem for accurately calculating the REFs of emitters in nanoparticle-substrate nanogaps for single-molecule SERS and scanning probe-substrate nanogaps for tip-enhanced Raman spectroscopy. We show that the validity of the plane wave approximation breaks down if high-order plasmonic modes are excited. The as-developed method paves the way toward designing high-REF POA nanostructures for luminescence-related devices.

ConspectusThe rational design of highly efficient catalysts relies on understanding their structure-activity relationships and reaction mechanisms at a molecular level. Such an understanding can be obtained by in situ monitoring of dynamic reaction processes using surface-sensitive techniques. Surface-enhanced Raman spectroscopy (SERS) can provide rich structural information with ultrahigh surface sensitivity, even down to the single-molecule level, which makes it a promising tool for the in situ study of catalysis. However, only a few metals (like Au, Ag, and Cu) with particular nanostructures can generate strong SERS effects. Thus, it is almost impossible to employ SERS to study transition metals (like Pt, Pd, Ru, etc.) and other nonmetal materials that are usually used in catalysis (material limitation). Furthermore, SERS is also unable to study model single crystals with atomically flat surface structures or practical nanocatalysts (morphology limitation). These limitations have significantly hindered the applications of SERS in catalysis over the past four decades since its discovery, preventing SERS from becoming a widely used technique in catalysis. In this Account, we summarize the extensive efforts done by our group since the 1980s, particularly in the past decade, to overcome the material and morphology limitations in SERS. Particular attention has been paid to the work using core-shell nanostructures as SERS substrates, because they provide high Raman enhancement and are highly versatile for application on different catalytic materials. Different SERS methodologies for catalysis developed by our group, including the "borrowing" strategy, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and SHINERS-satellite strategy, are discussed in this account, with an emphasis on their principles and applications. These methodologies have successfully overcome the long-standing limitations of traditional SERS, enabling in situ tracking of catalysis at model single-crystal surfaces and practical nanocatalysts that can hardly be studied by SERS. Using these methodologies, we systematically studied a series of fundamentally important reactions, such as oxygen reduction reaction, hydrogen evolution reaction, electrooxidation, CO oxidation, and selective hydrogenation. As such, direct spectroscopic evidence of key intermediates that can hardly be detected by other traditional techniques was obtained. Combined with density functional theory and other in situ techniques, the reaction mechanisms and structure-activity relationships of these catalytic reactions were revealed at a molecular level. Furthermore, the future of SERS in catalysis has also been proposed in this work, which we believe should be focused on the in situ dynamic studies at the single-molecule, or even single-atom, level using techniques with ultrahigh sensitivity or spatial resolution, for example, single-molecule SERS or tip-enhanced Raman spectroscopy. In summary, core-shell nanostructure-enhanced Raman spectroscopies are shown to greatly boost the application of SERS in catalysis, from model systems like single-crystal surfaces to practical nanocatalysts, liquid-solid interfaces to gas-solid interfaces, and electrocatalysis to heterogeneous catalysis to photocatalysis. Thus, we believe this Account would attract increasing attention to SERS in catalysis and opens new avenues for catalytic studies.

In situ electrochemical infrared spectroscopy and Raman spectroscopy are powerful tools for probing potential-dependent adstructures at solid/liquid electrochemical interfaces. However, it is very difficult to quantitatively interpret the observed spectral features including potential-dependent vibrational frequency and spectral intensity, even from model systems such as single-crystal electrode/liquid interfaces. The clear understanding of electrochemical vibrational spectra has remained as a fundamental issue for four decades. Here, we have developed a method to combine computational vibrational spectroscopy tools with interfacial electrochemical models to accurately calculate the infrared and Raman spectra. We found that the solvation model and high precision level in the self-consistent-field convergence are critical elements to realize quantitative spectral predictions. This method's predictive power is verified by analysis of a classic spectroelectrochemical system, saturated CO molecules electro-adsorbed on a Pt(111) electrode. We expect that this method will pave the way to precisely reveal the physicochemical mechanism in some electrochemical processes such as electrocatalytic reactions., An integrated approach for quantitatively predicting the electrochemical-infrared and electrochemical-Raman spectra and STM images of Pt(111)(2 × 2)-3CO adstructures has been developed.

Understanding the reaction mechanism for the catalytic process is essential to the rational design and synthesis of highly efficient catalysts. MoS2 has been reported to be an efficient catalyst toward the electrochemical hydrogen evolution reaction (HER), but it still lacks direct experimental evidence to reveal the mechanism for MoS2-catalyzed electrochemical HER process at the atomic level. In this work, we develop a wet-chemical synthetic method to prepare the single-layer MoS2-coated polyhedral Ag core-shell heterostructure (Ag@MoS2) with tunable sizes as efficient catalysts for the electrochemical HER. The Ag@MoS2 core-shell heterostructures are used as ideal platforms for the real-time surface-enhanced Raman spectroscopy (SERS) study owing to the strong electromagnetic field generated in the plasmonic Ag core. The in situ SERS results provide solid Raman spectroscopic evidence proving the S-H bonding formation on the MoS2 surface during the HER process, suggesting that the S atom of MoS2 is the catalytic active site for the electrochemical HER. It paves the way on the design and synthesis of heterostructures for exploring their catalytic mechanism at atomic level based on the in situ SERS measurement. Ministry of Education (MOE) Accepted version This research was financially supported by MOE under AcRF Tier 1 (Project No. 2017-T1-002-119) and AcRF Tier 2 (Project No. MOE2017-T2-1-162; MOE2016-T2-2-103) in Singapore, NTU’s Start-Up Grant (Project No. M4081296.070.500000), and NSFC (21775127 and 21522508). H.Z. acknowledges the financial sup-port from ITC via the Hong Kong Branch of National Precious Metals Material Engineering Research Center (NPMM), and the grants (Project No. 9380100, 9610478 and 1886921) in the City University of Hong Kong.

Developing an understanding of structure–activity relationships and reaction mechanisms of catalytic processes is critical to the successful design of highly efficient catalysts. As a fundamental reaction in fuel cells, elucidation of the oxygen reduction reaction (ORR) mechanism at Pt(hkl) surfaces has remained a significant challenge for researchers. Here, we employ in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculation techniques to examine the ORR process at Pt(hkl) surfaces. Direct spectroscopic evidence for ORR intermediates indicates that, under acidic conditions, the pathway of ORR at Pt(111) occurs through the formation of HO2*, whereas at Pt(110) and Pt(100) it occurs via the generation of OH*. However, we propose that the pathway of the ORR under alkaline conditions at Pt(hkl) surfaces mainly occurs through the formation of O2−. Notably, these results demonstrate that the SERS technique offers an effective and reliable way for real-time investigation of catalytic processes at atomically flat surfaces not normally amenable to study with Raman spectroscopy. This work was supported by the NSFC (21522508, 21427813, 21521004, 21533006, 21621091 and 21775127), “111” Project (B17027), Natural Science Foundation of Guangdong Province (2016A030308012), the Fundamental Research Funds for the Central Universities (20720180037), and the Thousand Youth Talents Plan of China. Support from MINECO and Generalitat Valenciana (Spain), through projects CTQ2016–76221-P (AEI/FEDER, UE) and PROMETEOII/2014/013, respectively, is greatly acknowledged. V.B.-M. acknowledges MINECO for the award of a pre-doctoral grant (BES-2014–068176, project CTQ2013–44803-P).

Heterogeneous metal interfaces play a key role in determining the mechanism and performance of catalysts. However, in situ characterization of such interfaces at the molecular level is challenging. Herein, two model interfaces, Pd and Pt overlayers on Au single crystals, were constructed. The electronic structures of these interfaces as well as effects of crystallographic orientation on them were analyzed by shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) using phenyl isocyanide (PIC) as a probe molecule. A clear red shift in the frequency of the C≡N stretch (νNC ) was observed, which is consistent with X-ray photoelectron spectroscopy (XPS) data and indicates that the ultrathin Pt and Pd layers donate their free electrons to the Au substrates. Furthermore, in situ electrochemical SHINERS studies showed that the electronic effects weaken Pt-C/Pd-C bonds, leading to improved surface activity towards CO electrooxidation.

Currently, lithium-ion batteries play a key role in energy storage; however, their applications are limited by their low energy density. Here, we design a facile method to prepare mesoporous ZnMn2O4 microspheres with ultrahigh rate performance and ultralong cycling properties by finely tuning the solution viscosity during synthesis. When the current density is raised to 2 A·g−1, the discharge capacity is maintained at 879 mA·h·g−1 after 500 cycles. The electrochemical properties of mesoporous ZnMn2O4 microspheres are better than that for most reported ZnMn2O4. To understand the electrochemical processes on the mesoporous ZnMn2O4 microspheres, in situ Raman spectroscopy is used to investigate the electrode surface. The results show that mesoporous ZnMn2O4 microspheres have a great potential as an alternative to commercial carbon anode materials.

Ionic liquids (ILs) are novel type of electrolytes and have found various applications in electrochemistry. Understanding their electrochemical stability has been one of fundamental and important subject of researches, which would promote further application of ionic liquids in electrochemistry. Electrochemical surface enhanced Raman spectroscopy (EC-SERS) can provide abundant finger print information of surface-bound species for elucidating reaction mechanism, while normal Raman (NR) spectroscopy of thin layer region near electrode surface can provide more information about products without interference of signals from surface adsorption. Here we report combined EC-SERS and NR studies with DFT calculations on the cathodic process of 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMITFSI) on Ag electrodes, aiming to identify reaction products and to reveal reaction mechanisms. It has been found that during the reduction of EMI cation, N-heterocyclic carbenes (NHCs) as an intermediate product were formed first and then underwent dimerization to form double-bond dimer. Single-bond dimer were also generated through single-electron radicals whose lifetime was too short to be detected directly though. In addition, dealkylation may happen, leading to formation of methylimidazole and ethylimidazole. This work not only deepens our insight into the cathodic process of EMI + , but also provides a guideline for electrochemical Raman study to be employed for tracing and understanding the process of electrode reaction.