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Perspective
In situ analysis of surface catalytic reactions using SHINERS Yao-Hui Wang, Jie Wei, Petar Radjenovic, Zhongqun Tian, and Jian-Feng Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05499 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019
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Analytical Chemistry
In Situ Analysis of Surface Catalytic Reactions Using SHINERS Yao-Hui Wang†,#, Jie Wei†,, Petar Radjenovic†, Zhong-Qun Tian†, and Jian-Feng Li*,†,‡ †MOE
Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. ‡Shenzhen Research Institute of Xiamen University, Shenzhen 518000, China. *E-mail:
[email protected]. Phone: +86-592-2186192
ABSTRACT: Electrochemistry and heterogeneous catalysis continue to attract enormous interest. In situ surface analysis is a dynamic research field capable of elucidating the catalytic mechanism of reaction processes. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a nondestructive technique that has been cumulatively used to probe and analyze catalytic reaction processes, providing important spectral evidence about reaction intermediates produced on catalyst surfaces. In this perspective, we review recent electrochemical and heterogenous catalytic studies using SHINERS and highlight its advantages; summarize the flaws and prospects for improving the SHINERS technique; and give insight into its future research directions.
Due to surface sensitivity and selectivity, innovations in surface analysis techniques have presented considerable opportunities for a large number of fields,1-5 including: catalysis,6-8 energy,9 biomedicine,10-17 environmental science1821 etc. Surface analysis is an indispensable part of analytical chemistry. At present, the field of surface analysis has received increasing attention and become one of the most active subjects in the world. As a surface science, surface analysis can provide information about the composition, structure and energy state2224 of the outermost several nanometers (l-10 atomic layer(s)) of a surface and can be used to study physical and chemical reactions that take place at the interface of two phases, such as: solid-liquid,25,26 solid-gas,27,28 solid-vacuum29 and liquid-gas interfaces.30 Moreover, surface analysis can also monitor catalytic reaction processes in situ and provide information about reaction intermediates and changes in surface atoms. In surface science research, the study of surface catalytic reactions has received special attention. Catalytic reactions usually occur at the surface of catalysts and are complex processes with a variety of reaction trajectories. Different reaction trajectories result from the adsorption and desorption processes of intermediary species in the catalytic process. Thus, the ability to completely characterize and understand catalytic reactions is a great challenge. In situ studies capable of determining chemical and physical changes on the catalyst; i.e. the adsorption of reactants and desorption of products, especially capturing and identifying the intermediate species, are essential.31 Analyzing the intermediate species of catalytic reactions and the physicochemical changes of the relevant catalysts using surface analysis techniques, preferably with time-resolved in situ methods and under realistic conditions, is key to developing a clear understanding of practical catalytic mechanisms. Additionally, improving the understanding of surface structural requirements will facilitate the rational design of more efficient catalysts in electrochemistry and heterogeneous catalysis.32,33 Considering these obvious needs, some in situ techniques
including X-ray absorption spectroscopy (XAS),34 X-ray photoelectron spectroscopy (XPS),35 scanning tunneling microscope (STM)36 and synchrotron-based X-ray diffraction (XRD)33,37 have been used to characterize catalytic reactions and related phenomena (e.g., surface composition, charge transfer mechanisms, surface restructuring, chemical states, morphologies and absorption/desorption speciation.).38 These results have improved our understanding of many catalytic processes. The direct observation of intermediate species in catalytic reactions is a daunting challenge for surface analysis techniques due to the lower concentration of intermediate in the dynamic reaction process. Vibrational spectroscopic surface analysis techniques can be advantageous for observing trace amounts of intermediate species with short lifetimes. Interface-specific vibrational spectroscopies, including: infrared (IR) spectroscopy,39,40 Raman spectroscopy,41,42 and second order non-linear optical spectroscopy,43,44 are the most widely used in situ techniques to study surface catalytic reactions. However, the main difficulty for in situ IR is studying the adsorption of low concentrations of intermediates from the aqueous phase on the catalyst surface. In situ IR also suffers from strong signal interference by water and CO2 in the air under ambient conditions and lacks resolution in the low wavenumber region. Unlike IR spectroscopy, Raman spectra only have weak water bands and can detect signals in the low wavenumber region.45 Consequently, Raman spectroscopy has been adopted to analyze surface oxidation processes, to identify hydroxyl groups, active oxygen species, metal-C bonds and metal oxide complexes by their specific Raman peaks.27,41,46 However, as a two photon process, Raman scattering is intrinsically of lowprobability (only one per million photons is scattered).47 Thus, the sensitivity of ordinary Raman spectroscopy does not meet the requirements for monitoring trace amounts of surface species in catalytic processes. In 1977, Van Duyne et al. discovered the surface-enhanced Raman scattering (SERS) effect on roughened Ag electrode
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surface with ~105-106 Raman enhancement, which made a significant breakthrough in Raman spectroscopy.48-50 SERS produced by plasmonic resonances on nanostructured metal surfaces, can tremendously enhance the Raman signal of bond vibrations close to the surface. Surface-enhanced Raman scattering (SERS), produced by plasmonic resonances on nanostructured metal surfaces, can tremendously enhance the Raman signal of bond vibrations close to the surface.51 Due to the enormous signal enhancement, SERS allows rapid and surface-selective detection down to the single-molecule level;52 thus, enabling direct in situ analysis of catalytic reactions. However, chemical and thermal stability is an essential requirement for the SERS substrate and only rare nanostructured materials, such as: Au, Ag, and Cu, possess adequate SERS activity.53 Additionally, the main flaws of SERS are that the SERS-active nanostructure must be in close proximity or direct contact with the catalyst, distorting real catalytic activity. For example, when monitoring the electrocatalysis of Pt with SERS, the electrocatalytic effect is not only related to the catalyst Pt but also has energetic contributions from the SERS substrate. Consequently, to date, in situ SERS studies for catalytic reactions have rarely been reported. In 2010, our group established a new technique called ‘shellisolated nanoparticle-enhanced Raman spectroscopy’ or ‘SHINERS’, to overcome the drawbacks of SERS.54 SHINERS has since drawn a significant amount of attention due to its great advantage in surface analysis. For the SHINERS technique, the pivotal issue is the synthesis of shell-isolated nanoparticles (SHINs) that are special core-shell heterostructures with a SERS active core and an inert, compact and ultra-thin shell that inhibits any catalytic effect from the core by preventing its contact with the external chemical environment. The SHIN core can be synthesized as spheres, cubes, or rods with diameters from 40 up to 200 nm currently possible.55 The shell material can be adapted to various pH environments, with: SiO2, Al2O3, MnO2 and TiO2 all being used effectively.54,56 The sensitivity of SHINERS is a key factor in detecting trace amounts of surface and interface species. Increasing the enhancement effect of SHINs is pivotal to improving the sensitivity of SHINERS. It can be achieved by changing the material (Au, Ag, and Cu) and size of the nanoparticle core (to match the correct wavelength of excitation light) or by decreasing the shell thickness. In principle, the shell should be preferably less than 3 nm and pinhole-free. These features of SHINERS eliminate the possibility of distorting real catalytic effects and meet the in situ and time-resolved research requirements for the study of electrocatalysis and heterogeneous catalysis in different environments. This Perspective summarizes the field of in situ analysis of catalytic reaction processes using SHINERS. We cover most of the applications of SHINERS in monitoring catalytic reactions, but we are unable to provide a comprehensive overview of this topic here. Our goal is to give readers an insight into the benefits the current exciting field of SHINERS and reaction monitoring processes offer as well as insight into problems they face. Also, the future prospects of SHINERS are described in the future directions and conclusions section.
SHINERS ANALYSIS CRYSTAL SURFACES
OF
MODEL
SINGLE-
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Monitoring electrochemical and catalytic reaction processes with SHINERS is a relatively new research area that holds great potential and has been regarded as a ‘next generation advanced spectroscopy’.57 Indeed, detection of the intermediate species of electrocatalysis reaction processes has already been obtained by the in situ SHINERS technique.58,59 As is well known, singlecrystal electrodes maintain flat surfaces at the atomic scale and, as a model catalyst, can provide a far significant understanding about the mechanism of electrocatalytic reaction at a molecular level, which cannot be provided by a polycrystalline electrode. Thus, studying the electrocatalytic reaction on single-crystals is excellent for guiding the synthesis of catalysts. However, traditional SERS is unable to obtain the Raman signal on the single-crystal surface and the sensitivity of TERS cannot reach the extent of in situ directly detecting the intermediate species of electrocatalytic reaction. As a highly selective surface analysis technology that is not affected by matrix molecules in the surrounding medium, in situ electrochemical SHINERS has shown great applications in identifying the electrocatalytic reaction species on single-crystal surfaces.41,59-61 In the analysis of electrocatalytic reaction processes, the SHINs are usually assembled on the metal electrode surface to amplify surface Raman signals via surface plasmon (SP) coupling in the region between the SHINs and the metal electrode, known as hotspot regions. Figure 1 shows the working principle of SHINs on a single-crystal electrode and provides the Raman information of surface species in hotspots. In general, SHINERS can usually probe the region within 20 nm around SHINs. The areas with the strongest enhancements, i.e. the hotspot, locate at the nanogaps between two SHINs or between SHINs and the substrate. However, the probe molecules tend not to absorb on the shell of SHINs due to the weak interactions with SiO2 shell. Thus, the molecules are not in the hotspots. Therefore, we normally cannot obtain SERS signals of the probe molecules on SHINs itself, but we can enhance the Raman signals of the probe molecules on target substrate using SHINs as a signal amplifier. According to the 3D-FDTD simulations, the average enhancement ability of coupling is ~106 magnitude between SHINs and singlecrystal.41,62 The enormous enhancement effect ensures that the trace intermediates on single-crystal surfaces can be detected by SHINERS.
Figure 1. Elementary diagram of the operating principle of SHINs on single-crystal electrode surface.
The analysis of the interfacial electronic structure of catalysts at a molecular level is significant for the elucidation of surface reaction processes and understanding its catalytic properties. However, conventional XPS techniques are often complicated to operate and require high vacuum conditions, which are not suitable for in situ analysis. Based on this, our group developed a rapid method to analyze the electronic structure of two model catalysts (Pd and Pt overlayers on Au single-crystal electrodes) by SHINERS using phenyl isocyanide (PIC) as a probe molecule, schematically illustrated in Figure 2a.23 We combined structurally defined single-crystal facets and
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SHINERS with sensitiveness.
Analytical Chemistry morphological
generality
and
ultra-
As shown in Figure 2c and d, there was a 42 cm-1 and 23 cm-1 redshift in the C≡N (nNC) stretching frequency of the phenyl isocyanide (PIC) probe molecule adsorbed on Au(111) substrate with 1-5 monolayers (MLs) of Pd (bridge configuration) and Pt (top configuration), respectively. These large redshifts indicate that the C≡N bond was much more strongly adsorbed on 1 ML Pd and Pt surfaces than on 5 ML surfaces due to their distinct surface electronic structures. In recent papers, the 5s and 2p* molecular frontier orbitals of nNC interact with the metal and generate s-d donation and d-p* back donation, respectively.63 Weakening the s-d interaction or increasing the d-p* backdonation will enhance the C≡N stretching vibration, resulting in a blue shift in the frequency of nNC. The d-p* interaction presented in Figure 2b correlates the frequency of the C≡N stretching band against the number of Pd or Pt overlayers. For 1 ML overlayer on the Au substrate, Pd or Pt donated free electrons to the Au, verified by XPS measurements. Therefore, there was less free electron availability at the surface to feedback into the empty 2p* orbitals of the C≡N bond. Consequently, the frequencies of nNC Pd(bridge) and nNC Pt(top) blue shifted to higher wavenumbers due to weakening d-p* interactions. These SHINERS and XPS results coincided and demonstrate that SHINERS can be used as an efficient and time-saving method for investigating the electronic structure of heterogeneous metal surfaces at room temperature and pressure.
Figure 2. (a) Schematic illustration of SHINERS for the analysis of electronic structures and catalytic properties. (b) Diagram depicting the d-p* PIC to surface interaction and the dependence of the Raman shift of the nNC bond vibration on Pd/Pt surfaces. SHINER spectra of PIC adsorbed on Au(111) substrate with 1-5 ML (c) Pd and (d) Pt, respectively.
found a new peak at 790 cm-1 during electro-oxidation. According to the D2O experiments, DFT calculations and recent research, this peak was attributed to a gold-hydroxide bending mode (AuOH) of the top sites of OH. The different pH, anion, and crystallographic orientation were also systematically investigated for electrochemical oxidation. OH species were selectively adsorbed on three Au(hkl) surfaces with an intensity order Au(100) ≪ Au(110)
