English

• 1. INTRODUCTION

  Many energy-related processes are strongly dependent on the structure and performance of electrochemical interfaces^[1-4]. The study of electrochemical interfacial reactions has generated considerable research interest. The discovery of surface-enhanced Raman scattering (SERS) effect was a historic breakthrough for the development and of application Raman spectroscopy in surface science^{[5, 6]}. SERS overcame many of the shortcomings of Raman spectroscopy, such as its low signal strength and surface sensitivity, allowing it to be applied to the study of numerous fields, like electrochemistry, biomedicine, catalysis, environmental science, material science, etc^[7-12]. More importantly, it can give signals in the low-wavenumber spectral region, and avoid interference from water signals, so it is widely used in research on the electrode interface. However, only a few noble metals such as Ag, Au, Cu, and so on with rough or nanoscale structures have SERS activity, which for a long time limited the scope of SERS applications in the research of interfacial reactions on atomically flat single crystal surfaces^[13].

  In 2010, our group developed the shell-isolated nanoparticles enhanced Raman spectroscopy (SHINERS) technology, which overcomes the material and morphological limitation of SERS^{[7, 14-16]}. As shown in Fig. 1, Au nanoparticles with high SERS activity are coated by an ultrathin and chemically inert SiO₂ layer. The Au core acts as a signal amplifier, while the SiO₂ shell completely separates it from the surrounding environment. Enhanced Raman signals can be readily obtained by simply assembling SHINERS nanoparticles on a surface. And the enhancement is high enough for most adsorbed molecules. Overall, SHINERS has the following advantages compared to conventional Raman and SERS techniques: 1) Au nanoparticles can be physically separated from contacting the substrate to ensure that the signal comes from molecules at the substrate being studied; 2) Charge transfer processes between Au nanoparticle cores and the substrate are eliminated; 3) Interference signal from contact between external impurities and Au nanoparticles can be effectively avoided; 4) The shell isolated nanoparticles (SHINs) exhibit long-term stability due to the inert SiO₂ protective shells. SHINERS technology has been applied to in situ monitoring of single-crystal electrode reactions, such as Au(hkl), Pt(hkl) and Cu(hkl), and its specific applications for studying single-crystal interfaces are described below.

  Figure 1

  

  Figure 1.  Working mode of Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)

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  2. INVESTIGATION OF INTERFACE ELECTRONIC STRUCTURE

  In surface studies, the electronic structure of the electrode materials can strongly influence interactions between the metal surface and reacting species^[17-20]. For example, the "volcano" relationship between oxygen reduction reaction (ORR) activity and activation energy of materials indicates that the electronic structures of different materials will influence the performance of interfacial catalytic reactions^[21]. Therefore, it is of great significance to study the electronic structure of the interface of different catalysts. Our group investigated the electronic structure of heterogeneous single-crystal interfaces by carefully constructing monolayers of Pd, or Pt on the Au single-crystal surface and using phenyl isocyanide (PIC) as a Raman probe molecule. In this work, 1~5 monolayers of Pd were constructed on different Au(hkl) single crystal surfaces by underpotential deposition (UPD), and then the changes on the electronic structure of the interface were investigated in detail (Fig. 2a). Fig. 2b shows the SHINERS results of PIC molecules adsorbed on Au(111) covered by Pd overlayers. 2044 and 2155 cm^-1 are stretching vibrations of bridge and top adsorbed PIC on the Pd monolayers, respectively. With increasing Pd overlayers, the bridge adsorption peaks (νNC_Pd(bridge)) shift to 2002 cm^-1, indicating that the electronic structure changes depending on the number of Pd monolayers on Au(111). In order to verify this phenomenon, the same experiment was carried out using X-ray photoelectron spectroscopy (XPS). The corresponding XPS results showed that the binding energy of Pd_3d decreased with the increase of Pd overlayers, indicating that the electrons will transfer from Pd to Au, especially in the case of a single Pd monolayer. From the relationship between Raman frequency and the binding energy of Pd_3d/2 (Fig. 2c), we found that as the number of Pd overlayers increases, the electron density on Pd surface increases, thus leading to a lower binding energy. Therefore, the interaction is enhanced and leads to a redshift in νNC bands. In this work, SHINERS technique was used to probe the electronic structure of heterogeneous metal interfaces. Combined with DFT calculations, we found that Pd ovelayers will donate the free electrons to the more electronegative Au substrate. This indicates that the SHINERS technology can be effectively applied to the analysis of electronic structure and catalytic performance of heterogeneous metal monocrystalline interfaces, which can provide experimental and theoretical guidance for the design of catalysts with better performance.

  Figure 2

  

  Figure 2.  (a) Schematic of in situ studies of the interfacial structure by SHIENRS. (b) SHINERS spectra of PIC molecule adsorbed on Au(111) covered by different layers of Pd. (c) Correlation of Pd overlayer thickness against the frequency of νNC_Pd(bridge) and the binding energies of Pd_3d3/2

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  3. INVESTIGATION OF INTERFACE ADSORBED MOLECULES

  SHINERS technology can not only be used to study the electronic structure of heterogeneous metal electrodes interface, but also be widely used to detect the adsorption configuration of molecules on the electrode surface, such as pyridine^[22], DNA base^[23], dye molecules^[24] and so on. In electrochemistry, the detailed structure of the interfacial water, such as its orientation in the electric double layers (EDL) and the complex network of hydrogen bonds, has a significant influence on the electrochemical performance of electrode materials^{[25, 26]}. Research on the atomically flat well-defined single-crystal surfaces will help to reveal the interfacial reaction mechanism and configuration-related activities, which is of great significance for the study of the electrochemical interface. The Raman signal of interfacial water was firstly obtained on Au(111) and Au(100) surfaces using SHINERS without interference from bulk water signals, and the transformation of different configurations of interfacial water was observed during hydrogen evolution reaction (HER)^[27]. It is found that the interfacial water changes from a "parallel" structure to "one-H-down", and then to a "two-H-down" water configuration with potential shifting in the negative direction (Fig. 3a). Combined with the theoretical studies, three configurations of the interfacial water and the corresponding number of hydrogen bonds in the EDL at different potentials were simulated, which is in good agreement with the experimental data and further reveals the structure of EDL. This work is the first time to correlate the configuration transformation and hydrogen bond of the interface water with the precise electrode potential experimentally and theoretically, which is instructive for the exploration of the three-dimensional structure of the electric double layers.

  Figure 3

  

  Figure 3.  (a) In situ SHIENRS research of the interfacial water structure on Au(111) single crystal surface. (b) In situ SHINERS research of ORR on Pt(111) and Pt(100) single crystal surface

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  SHINERS has also been used to monitor the dynamic electrocatalytic processes occurring at single-crystal interfaces in situ^[28-31]. As the most important cathode reaction of fuel cells, the ORR reaction on Pt-based surfaces has been widely studied for a long time. However, since direct spectroscopic information of the intermediate species of ORR reactions has not been obtained on the single-crystals with well-defined surface structures, the specific ORR path has not been determined. Recently, our group used in situ SHINERS to explore ORR processes at the Pt(hkl) single-crystal interface^{[29, 32]}. At the Pt(111) surface, a peak at 732 cm^-1 was observed during the ORR (Fig. 3a). Combined with DFT calculations, we assigned this peak as a HO₂^* species. While at Pt(100), and Pt(110) surfaces, a peak at 1080 cm^-1 was visible, assigned to OH^* species (Fig. 3b). Finally, based on in situ spectroscopic experiments and the results of previous studies, we proposed a reasonable ORR mechanism, which provides direct experimental evidence and an in-depth understanding of ORR process at the Pt(hkl) single crystal interface. It also shows that the SHINERS technique can be effectively extended to in situ studies of single-crystal catalytic reactions at other transition metals.

  4. SUMMARY AND PERSPECTIVE

  In summary, SHINERS is an advanced technique that has been widely employed in the research of interfacial structures, including the electronic structure of heterogeneous metal surfaces, and the molecules absorbed on the surface. These studies illuminate our understanding of molecular adsorption, bonding, and orientational transformations at single-crystal interfaces on the molecular level. Some future development directions for SHINERS studies of interfacial structures can be addressed as follows. (1) Due to the SHINERS technique showing unique advantages for studying interfacial reactions, its applications can be extended from studying electrocatalytic reactions on well-defined single-crystals model surfaces to those on practical electro-catalyst surfaces. (2) Compared with Pt(hkl) crystal surfaces, alloy single-crystal electrode surfaces, such as Pt₃Ni(hkl), PtRu(hkl), and other single-crystal electrode surfaces, usually exhibit better catalytic activity. It is believed that in situ studies of alloy single-crystal catalytic reactions at the interface using SHINERS have more significance for tailoring actual electrocatalytic systems. (3) High-index single-crystal surfaces usually have better catalytic activity due to the more accessible surface structures and lower atomic coordination numbers, and they also exhibit higher stability because the surface atoms are short-range ordered, and not easily disturbed. Therefore, the SHINERS technique can be further extended to the study of high index single-crystal interfacial reactions. (4) With the rapid development of nano synthesis technologies, SHINERS will be further developed with higher enhancement, and it can be employed in the research of interfacial reaction processes such as single-atom catalysis, cluster catalysis, 2D material catalysis, biomimetic material interfacial catalysis, etc.

  
  
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  Figure 1  Working mode of Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)

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  Figure 2  (a) Schematic of in situ studies of the interfacial structure by SHIENRS. (b) SHINERS spectra of PIC molecule adsorbed on Au(111) covered by different layers of Pd. (c) Correlation of Pd overlayer thickness against the frequency of νNC_Pd(bridge) and the binding energies of Pd_3d3/2

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  Figure 3  (a) In situ SHIENRS research of the interfacial water structure on Au(111) single crystal surface. (b) In situ SHINERS research of ORR on Pt(111) and Pt(100) single crystal surface

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