Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy at Single-Crystal Electrode Surfaces
Jin-Chao Dong , Rajapandiyan Panneerselvam , Ying Lin , Xiang-Dong Tian , * and Jian-Feng Li *
DOI: 10.1002/adom.201600223
additional insights about various reactions at the electrochemical interfaces, e.g., qualitatively detect the surface bonding, adsorption confi guration and orientation of probe molecules. [ 6–9 ] However, as a two-photon process, the Raman cross section of probe molecules usually (even below) 10 −29 cm −2 sr −1 , which limited the detec-tion sensitivity of Raman spectroscopy. [ 10 ] Therefore, it was very diffi cult to perform Raman experiments in electrochemical interfaces.
In 1974, through electrochemical roughening method, Fleischmann et al. obtained unusually intense Raman spectra of pyridine adsorbed on a roughened silver surface. [ 6 ] At fi rst, they thought that the
unexpected high quality Raman spectrum was obtained because of the high electrode surface area after oxidation-reduction cycling. In 1977, after carefully checking the experimental and theoretical calculations, Van Duyne and Creighton explained that the enhanced Raman signals was due to an enhancement (10 5 –10 6 ) in Raman cross-section of pyridine molecules. [ 7,11 ] Later, the effect was called as surface-enhanced Raman scat-tering (SERS) effect. [ 12 ] It is widely accepted that there are two mechanisms contribute to the SERS enhancement namely: an electromagnetic enhancement mechanism (EM) and a chem-ical enhancement mechanism (CM). However, electromagnetic enhancement plays a major role when compared to chemical enhancement. The chemical enhancement is due to the chem-isorption interaction and the photon-driven charge transfer between adsorbed molecules and metal nanostructures. [ 13 ]
The electromagnetic fi eld enhancement is originated from surface plasmon resonance (SPR) of metal nanostructures. Sur-face plasmon resonance can be excited in free-electron metals with nanostructured surfaces if the wavelength of excitation line satisfi es the requirements of the resonance frequency of conduction band electrons in metals. As a result, a strong local electrical fi eld around the metal surfaces arises from the resonance interaction, characterized by the wavelength and distance dependent fi eld magnitude. [ 14,15 ] The enhanced near fi eld, existing in a tiny volume around the metallic surfaces, is the physical origin of the Raman signal surface-amplifi ca-tion. Molecules located in the vicinity of the nanostructured surfaces can exhibit huge increase in Raman cross section, making SERS a powerful spectroscopic technique with down to single-molecule sensitivity. [ 16 ]
After SERS discovery, surface and material scientists have carried out numerous experimental and theoretical studies, which have greatly promoted SERS applications in the fi eld of
As an innovative technique, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) eliminates the material and morphology generality problems of surface-enhanced Raman spectroscopy (SERS). For the past few years, SHINERS has been extensively employed in many fi elds, especially in the electrochemistry fi eld. This article renders a brief overview of the develop-ments of SHINERS technique and its applications in electrochemistry. First, we clearly explain the basic principles of the SHINERS technique, such as design principles, materials synthesis, characterization methods, and related theo-retical calculation methods. We then describe about the signifi cant applications of electrochemical SHINERS (EC-SHINERS) with a focus of study on various single-crystal electrode surfaces. Finally, we summarize the recent develop-ments and give an outlook for future developments in the SHINERS fi eld.
1. Introduction
In 1928, the Raman scattering effect was discovered by the Indian physicist C. V. Raman, since then Raman spectroscopy obtained a great attention from the entire scientifi c commu-nity. [ 1,2 ] As a molecular specifi c technique, Raman spectroscopy can offer much more information about the structure and prop-erties of the surface species or interface elements from elec-trodes while maintaining the capability of offering real-time information in a non-invasive manner. [ 3–5 ] Moreover, it can offer
J. C. Dong, Dr. R. Panneerselvam, Y. Lin, Prof. J. F. Li MOE Key Laboratory of Spectrochemical Analysis and Instrumentation Xiamen University Xiamen 361005 , China E-mail: [email protected] J. C. Dong, Dr. R. Panneerselvam, Y. Lin, Prof. J. F. Li State Key Laboratory of Physical Chemistry of Solid Surfaces Xiamen University Xiamen 361005 , China J. C. Dong, Dr. R. Panneerselvam, Y. Lin, Prof. J. F. Li College of Chemistry and Chemical Engineering Xiamen University Xiamen 361005 , China Prof. J. F. Li Department of Physics Xiamen University Xiamen 361005 , China Dr. X. D. Tian Xiamen Institute of Rare-earth Materials Chinese Academy of Sciences Xiamen 361005 , China E-mail: [email protected]
surface science. [ 17–23 ] Additionally, SERS have been used for in situ investigations at solid-solid or solid-liquid interfaces in elec-trochemistry fi eld. [ 4,9,24–26 ] Generally, SERS effect is associated with the following features: 1) in the most of the cases, only a few metals such as Ag, Au, Cu, and Al can be used as a tradi-tional SERS substrate to obtain strong/huge enhancement; [ 27 ] 2) the strength of SERS effect not only depends on the metal itself, but also depends on the size, shape, and nanogap dimen-sion of the nanostructures. The shape and size of metallic nanoparticles prescribe the spectral signature of its plasmon resonance, and the ability to change these two parameters and evaluate the effect on the localized surface plasmon resonance (LSPR) is an important experimental parameter. Similarly, the LSPR effect is also dictated by the nano gap dimensions and surroundings; [ 14 ] 3) SERS technique provides ultra-sensitive detection by measuring the fi rst layer molecules on the metal surface; [ 28 ] 4) SERS enhancement factors vary among different probe molecules due to the distinct properties of molecules or ions adsorption on the metal surfaces. [ 29–34 ]
To obtain huge SERS enhancement, several researchers employ noble metals such as Au, Ag, or Cu metal nanostruc-tures for SERS studies. Though, several materials such as transi-tion metals, [ 27 ] metal oxide, [ 35 ] and graphene [ 36 ] can enhance the Raman signals, the enhancement obtained from these materials are signifi cantly lower than the noble metals. Thus the lack of SERS substrate material has greatly restricted the number of specifi c applications of SERS. The limitation becomes promi-nent when SERS is extended to study the spectroelectrochem-istry of single crystals surfaces which are usually employed in electrochemical catalysis and surface science. [ 16,37–42 ] A signifi -cant research impetus has emerged to overcome the material and morphology limitation of SERS. [ 43–49 ] In order to expand the SERS effect to other materials or surfaces, a “borrowing” strategy was proposed during the 1980s. The “borrowing” strategy mainly include two diferent pathways: either by coating SERS-active metals on non-SERS-active substrates or by coating non-SERS-active materials over SERS-active substrates surface. [ 50–54 ] This “borrowing” strategy employ the long-range effect of the electromagnetic fi eld which generated by SERS-active mate-rials, including Ag or Au, to increase the Raman signal of probe molecules adsorbed on the non-SERS-active substrates. Several researchers have obtained SERS signals from some transition metals (e.g., Pt, Pd, Rh, etc.) by coating ultrathin shells of these transition metals on Au or Ag NPs or other relevant nanostruc-tured surfaces. [ 27,55–58 ] However, for oxides, insulators or bio-logical membranes and many other materials, it is impractical or almost impossible to deposit them on Au or Ag NPs surface as a uniform ultrathin shell. Moreover, it is noteworthy to men-tion that the enhanced electromagnetic fi eld will decrease sig-nifi cantly with the thickness of the shell material increasing, so the shell thickness should be compact and ultra-thin (<4 nm).
To apply SERS in other fi elds, it is necessary to analyse the working modes of SERS in all kinds of SERS measurements. Generally, SERS working modes can be classifi ed into two types, namely contact mode, and non-contact mode ( Figure 1 ). In contact mode, the noble metal nanoparticles not only act as signal amplifi ers, but also act as a host to accomodate probe molecules (Figure 1 A,B). In non-contact mode, the metal nanoparticles are separated from the surface of interest/probe
Xiang-Dong Tian obtained his B.Sc., M.Sc., and Ph.D. from Xiamen University. After postdoctoral research at the University of Connecticut, he joined the Xiamen Institute of Rare-earth Materials, Chinese Academy of Sciences. His research focuses on nano-particle assemblies with sub-wavelength dimension and their applications in surface-
enhanced Raman spectroscopy.
Jian-Feng Li is a Professor of Chemistry at Xiamen University. He received his Doctor Degree in chemistry from Xiamen University. Professor Li is the principal inventor of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). His research interests mainly include surface-enhanced Raman
spectroscopy, core–shell plasmonic nanostructures, electrochemistry, and surface catalysis.
molecules. Thus, non-contact mode is a key step forward to extend the applications of SERS in other fi elds where molecules will be adsorbed on any substrate/any morphology.
In 2000, the invention of tip-enhanced Raman spectroscopy (TERS) [ 59–65 ] is a major breakthrough in Raman spectroscopy fi eld because it solved a few problems associated with contact mode SERS ( Figure 1 C). Interestingly, TERS technique merges Raman spectroscopy with scanning probe microscopy (SPM) to provide chemical information with high spatial information. [ 66–68 ] In TERS, a sharp plasmonic gold or silver tip generates a well-defi ned hot spot on the target surface under a suitable excitation wavelength. [ 69 ] When the tip is positioned at the centre of a laser focus, the electromagnetic fi eld is signifi cantly enhanced due to a combination of localized surface plasmon resonance (LSPR) and lightning rod effect. [ 70 ] As a result, the Raman signals of a target surface can be improved by this enhanced EM fi eld, no matter of material and morphology. Though the obtained Raman signals from probe molecules around a single tip is quite weak, TERS technique offers sub-nm spatial resolution with the sensitivity of single molecule detection. [ 71 ] However, TERS tip is prone to contamination and sensitivity problems under electro-chemical conditions. [ 72 ] The adsorption of solution species on the tip surface rather than the electrode surface may yield mis-leading information. Particularly when electrodes are used for EC-TERS measurements, one has to work in a side-illumination mode, and the tip has to be immersed in solution, in which it is impractical to use a high numerical aperture objective with a
short working distance. Because of the refraction at the air-water interface, it is quite diffi cult to form a good focus. [ 73 ] Therefore, EC-TERS needs a special set up to obtain Raman signals under electrochemical conditions.
In 2010, our team introduced a novel “shell-isolated mode” to overcome the issues associated with material and morphology problems of SERS (Figure 1 D). [ 74 ] In SHINERS, the TERS tip concept was substituted by a layer of Au-core SiO 2 -shell nano-particles (Au@SiO 2 NPs or SHINs). In this case, each gold nanoparticle core of SHINs can act as a single TERS tip, which means many SHINs are excited in one micron-sized laser spot simultaneously. The SERS-active Au NPs core of SHINs act as “signal amplifi ers” which can increase the Raman signals of probe molecules around or adsorbed on the target surface. The outer ultrathin yet pinhole-free silica shells (2–5 nm) can effectively protect the core Au NPs from the outer chemical environment, and preclude the Au NPs contact with the sur-face species/probe molecules. [ 75 ] As a result, this technique can provide the original information about the target systems. We named this technique as “shell-isolated nanoparticle-enhanced Raman spectroscopy” or “SHINERS.” Importantly, SHINERS does not suffers from material or morphology generality prob-lems, because SHINs can be employed to examine any target surface with diverse compositions and morphologies, and the SHINERS technique has already been used to investigate a number of challenging systems. [ 75–94 ]
Notably, SHINERS technique employs shell-isolated mode which is different from contact mode and non-contact mode. Thus, the technique overcomes the long-standing limitations of SERS by characterizing various materials and morphologies. Gen-erally, several research groups utilize bare Ag or Au NPs for SERS measurements because bare plasmonic nanoparticles exhibit strong SERS effect. [ 95–98 ] However, certain problems may arise
when we use bare nanoparticles for SERS measurements. First, metal nanoparticles (NPs) may interact with the chemical spe-cies, especially in a solution environment ( Figure 2 A), and many other species in the reaction system may adsorb on the nanopar-ticles surface, which can interfere with the Raman signals. [ 2,99,100 ] Second, due to the Fermi level difference between the NPs and the surface material, charge transfer may happen between the NPs and the different metal or semiconductor surfaces. If we just directly spread bare metallic NPs on a target surface (Figure 2 B), in some cases, the interaction between the metallic NPs and the target surface may signifi cantly disturb the electronic structure of the target system. Eventually, it will affect the Raman signals of the target system. [ 101,102 ] Last, probe molecules may interact with the bare NPs, and the direct contact also would signifi cantly affect the Raman spectral features because the electron density distribution of probe molecules and its adsorption behavior will be changed (Figure 2 C). [ 103–108 ] However, the shell-isolated mode of SHINERS overcomes the above-mentioned problems by pro-tecting the NPs with a thin inert shell, which protect the bare NPs from the chemical environment such as solution, gas or probe molecules (Figure 2 D). For instance, a chemically inert shell can avoid inadvertent adsorption, charge transfer, and photocatalytic reactions. In addition, the shell can signifi cantly improve the shelf-life/stability of metallic nanoparticles. [ 109 ]
2. 3D-FDTD Simulations
As an effi cient computational electrodynamic method, 3D-fi nite-difference time-domain (3D-FDTD) can be employed to examine the local electromagnetic fi elds by solving Max-well’s equations. [ 42,43,110–112 ] Chen et al. [ 113 ] used 3D-FDTD method to simulate the EM infl uence of SHINs on a smooth gold surface, and compared with bare gold nanoparticles under
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Figure 1. Schematic illustration of different working modes of Raman techniques. A) Probe molecules adsorbed on bare Au NPs: SERS contact mode; B) Probe molecules adsorbed on Au NP cores-transition metal shells : contact mode; C) TERS: non-contact mode; D) SHINERS: shell-isolated mode. A–D) Reproduced with permission. [ 74 ] Copyright 2010, Macmillan Publishers.
different conditions ( Figure 3 ). To evaluate the SPR properties of SHINs, the scattering property of a single SHIN of 80 nm Au@ 1 nm SiO 2 was examined (Figure 3 A). The green spec-trum in Figure 3 A clearly shows there are three LSPR peaks at 535, 570 and 633 nm. The peak at 535 nm is attributed to the Au NP normal plasmon band, and peaks at 570 and 633 nm can be attributed to the quadrupole and dipole coupling mode of the SHINs and the smooth Au fi lm, respectively. [ 15,114 ] How-ever, the bare nanoparticle without visible quadrupole coupling mode, which plays a dominating role to the Raman scattering in the far fi eld, and this behaviour arises mainly due to the
charge transfer effect between the gold fi lm and nanoparticle (Figure 3 A–C).
Meanwhile, when the shell thickness was increased, the quadrupole coupling mode shifted to a lower wavenumber and quickly disappeared. From Figure 3 D the authors also found that the suitable SiO 2 thickness is 1–2 nm for the most strong plasmon coupling between the SHINs and the smooth gold fi lm. Moreover, the optimal enhancement position of SHINs are not accurately located at the interaction point between SHINs and the smooth gold fi lm. Importantly, the diameter of the SERS-active nanoparticles play a critical role in the EM
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Figure 3. A) Scattering spectra of single bare Au NP (80 nm) ( black curve) and Au@1 nm SiO 2 NP (green curve) on the smooth gold fi lm, respectively. Vector distributions and electric fi eld of steady state correspond to cross-sectional view under 633 nm B) and 570 nm C) excitation; D) Electromagnetic fi eld (EM) enhancement with different silica shell thicknesses under 633 nm laser excitation. A–D) Reproduced with permission. [ 113 ] Copyright 2015, American Chemical Society.
Figure 2. Schematic illustration of three different interferences in contact mode SERS. A) Contact with matrix molecules/impurities; B) Electrical contact with the metal surface; C) Chemical/photochemical interaction with the probe molecules; D) No interferences in shell-isolated mode. A–D) Reproduced with permission. [ 74 ] Copyright 2010, Macmillan Publishers.
enhancement, for instance, the electric fi eld intensity gradually increased when the NPs diameter was increased from 40 to 120 nm. But if the nanoparticle diameter exceeded 200 nm, the electric fi eld intensity was signifi cantly changed.
Recently, Chen et al. employed 3D-FDTD method to system-atically analyze the plasmonic modes and their contributions to the formation of hot spots in multiparticle−fi lm confi gura-tions. [ 115 ] There were four distinct plasmonic modes at 525, 580, 645, and 850 nm in the scattering spectrum for the dimer of Au@SiO 2 SHINs on a smooth gold fi lm under the charger-conjugate image and near-fi eld coupling effect ( Figure 4 A,B).The mode IV at 525 nm is an uncoupled mode results from the dipole resonance of an isolated Au@SiO 2 SHIN; mode I is the dipole coupling mode between the gold fi lm and the SHINs dimer; mode II was considered as the dipole coupling mode of the single SHIN and gold fi lm and the mode III was resulted from the quadruple coupling mode of the single particle and gold fi lm. [ 15,116–119 ] Besides the maximum EFs of mode IV being in the particle-particle gap, the maximum EFs of the other three modes are all located at the particle-fi lm junctions (Figure 4 C). This study opens the door to acquire strong Raman signals from molecules situated on single crystal surfaces and simul-taneously avoid signal interference from particle-particle gaps.
3. Synthesis of Various Types of SHINs
Figure 5 shows an overview of the experimental procedure for the preparation of SHINs. [ 120 ] In order to meet different system requirements, different types of SHINs have been prepared. To employ SHINERS technique, it is essential to understand the following parameters: fi rst, the metallic nanoparticle core should be SERS-active with an appropriate size. In our case, the suitable diameter of gold cores is 55 nm [ 121 ] for SHINERS measurements, and bigger diameter gold NPs, such as 120 nm, can be synthesized for enormous enhancements ( Figure 6 A,B). Shell-isolated gold nanocubes and nanorods (Figure 6 C,D) were
synthesised as their maximum SPR peak can be simply tuned by adjusting the aspect ratio. [ 15,120,122–125 ] The SPR property of nanoparticles will be affected by its shape and size. For Au nanorods, following its aspect ratio increase, its SPR absorption maximum can be tuned from the visible to the near infrared range. Additionally, Ag@SiO 2 NPs were synthesized because Ag NPs exhibit higher enhancement compared to the Au NPs under the same size. And the SPR peak position of the cor-responding Ag NPs can be shifted to any wavenumber at a lager scale by varying the particle size. [ 126,127 ] Second, the inert shell should be very thin as the SHINERS signal intensity will be signifi cantly decreased with the increasing shell thickness. For most of the experiment systems, the shell thickness of the SHINs should be in the range of 1–5 nm to exhibit signifi cant electromagnetic fi eld enhancement. The shell thickness can be varied by carefully changing the synthesis conditions such as the seed concentration, reaction time, reaction temperature, pH values and so on. Third, the shell material of SHINs should be inert to avoid any chemical reaction and electrical interfer-ences. For example, the shell material of SHINs can be TiO 2 (Figure 6 G), Al 2 O 3 (Figure 6 H), MnO 2 (Figure 6 I), and Ag 2 S
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Figure 4. A) Scattering spectra of a dimer of SHINs sited on a gold fi lm. The scattering effi ciency is normalized in the range 0–1; B) Surface charge distributions of an Au@SiO 2 SHIN dimer over a gold fi lm with the different excitation wavelengths corresponding to four scattering peaks; C) Calculated SERS enhancement distributions at the xz -plane under various wavelengths. The EF is shown as the scale ruler. A–C) Reproduced with permission. [ 115 ] Copyright 2016, American Chemical Society.
Figure 5. General overview of the SHINs synthesis.
(Figure 6 J), which can protect SHINs from various chemical and physical environments. [ 120,128,129 ]
Here, we describe the detailed experimental procedure of SHINs using 55 nm Au@SiO 2 as an example. The Au NPs (55 nm) were synthesized by classical Frens procedure. [ 121 ] In detail, 200 mL 0.01% HAuCl 4 solution was stirred in a round-bottom fl ask under heating conditions, after boiling, 1.4 mL 1% sodium citrate solution was added and continued the reaction for 1 h, then cooled down at room temperature. The SHINs were prepared as following [ 120 ] : added 30 mL Au NPs solution into a 100 mL round-bottom fl ask for stirring at room temper-ature and then droped 400 µL 1 mM APTMS solution. After 15 min stirring, 3 mL 0.54 % Na 2 SiO 3 solution (pH ≈ 10.3) was added into the above solution. Three minutes later, the mixed solution was transferred to a 90 °C bath and stirred for some time. Through controlling the reaction time, pH values, temperature and gold nanoparticles concentration, the SiO 2 shell could be tuned with different thicknesses. [ 130 ] After certain heating time, the hot solution should be quickly cooled down and then centrifuged for several times. At last, the concentrated SHINs was diluted with pure water to certain volume for fur-ther electrochemical or Raman measurements. It is worthy to mention that the SHINs can easily form a submonolayer on a target surface almost without obvious aggregates. Importantly, to get reproducible Raman signals and high enhancement, one should make sure that the target surface should be covered with an appropriate amount of SHINs.
Last, the shell of SHINs should be pinhole free. One of the most important and challenging requirements of a SHINERS experiment is to fabricate a uniform ultrathin shell. However, if the silica shell is too thin (<2 nm), pinholes can be easily formed and the probe/matrix molecules can easily contact the SERS-active Au core. This contact can yield false informar-tion from the target system. [ 120 ] If silica shells are fi lled with pinholes, its isolating function will be affected and the probe molecules or impurity from the outer environment can con-tact gold or silver nanoparticles core, which could yield false
information. Therefore, it is very important to notice that the SHINs should be pinhole-free so that the obtained Raman sig-nals only originate from the probe molecules which adsorbed on the target surface. Overall, SHINs should have ultrathin, uniform, and pinhole-free shells to fi nd applications in a wide range of chemical/electrochemical systems. [ 74 ]
4. Characterization of SHINs
Before SHINERS measurements, it is essential to characterize the pinhole-free property of SHINs. [ 74,120 ] To characterize its property, high-resolution transmission electron microscopy (HR-TEM) images, electrochemical cyclic voltammetry (CV) and SERS spectra from pyridine molecules are primarily used. The HR-TEM images of SHINs are shown in Figure 6 . How-ever, it is almost impossible to directly fi nd/detect the pinholes from the TEM images. So the electrochemical CV method was employed to obtain cyclic voltamograms of SHINs. [ 74 ] As shown in Figure 7 A, the blue curve (Figure 7 A–a) shows the typical electrochemical feature of bare Au NPs with a typical reduction peak around 0.9 V (vs. SCE), while the pinhole free SHINs are represented by the black curve (Figure 7 A–c), without the gold characteristic reduction peak, which indicates the Au cores are isolated by the inert silica shells. Furthermore, Raman spectros-copy was employed to investigate the pinhole effect of SHINs. As a normal Raman probe molecule, pyridine molecules exhibit strong affi nity toward metal NPs, but it does not adsorb on the inert shell surface. By this way, the presence of pinholes on SHINs can be characterized using cyclic voltametry and Raman measurements.
The procedure is as follows: fi rst, 10µL SHINs solution was deposited (with pinhole and centrifuged) on a Si wafer and dried for a certain time; then 20 µL of 10 mM pyridine solution was deposited on the SHINs and covered with a thin quartz window. Finally, the sample was subjected to Raman measure-ments. Undoubtedly, strong Raman signals from pyridine were
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Figure 6. HR-TEM pictures of different kinds of SHINs. A) 55 nm Au@SiO 2 NPs, B) 120 nm Au@SiO 2 NPs, C) a nanocube core of Au@SiO 2 NPs, D) a nanorod core of Au@SiO 2 NPs, E) Ag@SiO 2 NPs, F)55 nm Au@20 nm SiO 2 NP, G)Ag@TiO 2 , H)Ag@Al 2 O 3 , I)MnO 2 , J) and Ag 2 S. A,F) Repro-duced with permission. [ 74 ] Copyright 2010, Macmillan Publishers. B–D) Reproduced with permission. [ 120 ] Copyright 2013, Macmillan Publishers. G,H) Reproduced with permission. [ 129 ] Copyright 2015, Wiley.
seen at 1011 and 1035 cm −1 because pyridine can interact with the gold cores through pinholes (Figure 7 B–a). When the pro-cedure was repeated with pinhole-free SHINs, the Raman sig-nals of pyridine was not observed for a long time (Figure 7 B–b). When the Au(111) electrode surface was covered with pinhole-free SHINs, the typical Raman signals of pyridine at 1011 and 1034 cm −1 were observed (Figure 7 B–c), which originated from the adsorbed pyridine molcules on the Au(111) single crystal surface. In order to confi rm this result, the pinhole-free SHINs were placed on a smooth Ag fi lm and the typical Raman peaks of pyridine at 1005 cm −1 and 1034 cm −1 were observed (Figure 7 B–d), which indicates that the Raman signals just arise from the adsorbed pyridine molecules on the Ag fi lm.
5. SHINERS Studies on Single-Crystal Surfaces
As we discussed earlier, an unequalled application of SHINERS over other spectroscopic techniques is its applicability on single-crystal surfaces in electrochemistry fi eld. Single-crystal sur-faces possess well-known metal electronic levels and surface structure, which are basic parameters to unravel the chemical enhancement mechanism of SERS and other important reac-tion mechanism. Several analytical techniques have been used to investigate electrochemical processes on single crystal surfaces. For instance, surface-enhanced infrared refl ection absorption spectroscopy (SEIRAS) can be employed for spectro-electrochemical studies, but IR spectroscopy suffers from the interference of H 2 O absorption, and cannot characterize the low wavenumber region. [ 131,132 ] SERS technique has been used to acquire the direct molecule-metal bond information at low wave-numbers without any interference from CO 2 and H 2 O. [ 32,133–138 ] However, SERS enhancement has been obtained from a limited number of noble metals and other materials. Additionally, the atomically fl at single-crystal surfaces cannot support strong SPR effectively, [ 139,140 ] so it is diffi cult to obtain Raman signals from a single crystal surface using SERS method.
Tip-enhanced Raman spectroscopy (TERS) and attenuated total refl ection (ATR) methods can be used to obtain molecular information at single crystal surfaces. [ 65,71,141 ] Unfortunately,
TERS exhibit low SERS enhancement because the metallic tip provide a single hotspot to measure a limited number of molecules. Due to this reason, TERS is used to detect large cross-section Raman molecules with an excellent spatial resolu-tion. ATR is not so easy to control and the enhancement factor (EF) is only 1 to 2 orders of magnitude.
With the new working mode, SHINs can create a large electromagnetic fi eld enhancement, and SHINERS can be employed to characterize the adsorption and orientation of probe molecules on a variety of single-crystal surfaces. As a new technique, SHINERS method has been successfully applied to investigate a wide range of probe molecules and to detect the mechanism of various electrocatalytic processes on single crystal surfaces.
In the following sections, fi rst, we highlight some of the applications of SHINERS on single crystal surfaces with dif-ferent materials such as Au, Pt, Rh, and Cu, and from (111) facets to (100) and (110) facets electrodes. Next, we expanded the application of SHINERS from a aqueous solution to a non-aqueous (ionic liquid) system. The above-mentioned SHINERS works are mainly focusing on studying the molecular adsorp-tion at the surface. Finally, we present an example for in situ investigation of chemical reactions at a single crystal surface. By obtaining the directly spectral proof of the intermediates during the surface reaction, the mechanism of the surface electro-oxidation are better understood. Overall, SHINERS can be employed on a variety of single crystal surfaces to investi-gate the molecular orientations or chemical reactions in both aqueous and non-aqueous systems.
Generally, the surfactants originating from the synthesis of shell-isolated nanoparticles (SHINs) can adsorb on the electrode surface which may generate misleading spectroscopic features. To effi ciently overcome this problem, we introduced a hydrogen evolution reaction (HER) [ 92 ] treatment to get high-quality Raman spectra on single crystal electrodes surfaces. Under the hydrogen evolution potential, the impurities or the surfactants at the elec-trode and the NPs surface will be departed off and diffused into the solution which can be removed by exchanging the solution. In this study, we used a Au( hkl ) or Pt( hkl ) electrode assembled with a submonolayer of SHINs. [ 142 ] The Au( hkl ) electrode was
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Figure 7. A) CVs of 55 nm Au NPs ( a , red line), Au@SiO 2 NPs without pinhole ( c -black line) and Au@SiO 2 NPs with pinhole ( b -red line) on glassy carbon (GC) electrodes in 0.5 M H 2 SO 4 solution; B) To investigate the pinhole effect, SHINERS spectra were obtained from adsorbed pyridine molecules; a) SHINs with and b) without pinholes on a Si surface; c) SHINs without pinholes on a Au(111) electrode surface; d) SHINs without pinholes on a smooth Ag fi lm surface. A,B) Reproduced with permission. [ 74 ] Copyright 2010, Macmillan Publishers.
placed in a vertically confi gured three-electrode thin-layer Raman cell fi lled with deoxygenated neutral 0.1 M NaClO 4 solution. Then, the potential was kept at −2.0 V (vs. Ag/AgCl) for about 100 s. [ 118 ] The hydrogen evolution reaction proceeded vigorously, but the submonolayer of SHINs was stable due to the thin liquid layer between the single crystal surface and the quartz window surface, which effi ciently prevents the big H 2 bubbles formation.
After HER treatment, the electrochemical experiments were carried out in 0.1 M H 2 SO 4 and 0.1 M HClO 4 solution ( Figure 8 ) respectively. As shown in Figure 8 , the surface reconstruction peak (P1/P1’), sulfate phase transition peak (P3/P3’), surface oxida-tion (P4, P5) and reduction peaks (P5’) of Au(111) electrode con-sistent well with the ideally Au(111) electrode surface which had not covered with SHINs before (Figure 8 , black lines). Moreover, we should notice that the SHINs on the electrode surface could occupy some surface sites, which may partially affect the single crystal electrode surface structure and the electrochemical proper-ties of the reaction system. Overall, researchers can obtain high-quality Raman spectra on well-defi ned single-crystal surfaces.
5.1. SHINERS Investigation of Hydrogen Adsorption on Pt (111) and Rh (111) Electrodes Surfaces
Investigation of hydrogen adsorption on metal single crystal surfaces is an imperative task in surface science, electrochem-istry, including fuel cell applications. [ 143,144 ] However, it is very diffi cult to obtain the Raman spectra of hydrogen from single crystal surfaces using the conventional methods due its very small Raman cross-section. With the advent of SHINERS, we were able to detect the hydrogen adsorption on single crystal surfaces. We examined the hydrogen absorption on Pt(111) and Rh(111) single crystal surfaces using electrochemical SHINERS (EC-SHINERS) method. [ 74,76,78 ] In this work, SHINs were assembled on a Pt(111) single crystal surface and then controlled the potential at the hydrogen evolution reaction zone to collect Raman signals in 0.1 M NaClO 4 solution. [ 74 ] As can be seen from Figure 9 A, we found that there was a broad Raman peak around 2023 cm −1 , this band is assigned to the Pt-H stretching vibration mode. [ 49 ] If there is no SHINs on the Pt(111) surface, no Raman peak frequency was observed around 2030 cm −1 . It clearly shows that SHINs are indispensable to obtain the Raman information from a single crystal surface.
Similarly, we used EC-SHINERS method to examine the hydrogen absorption behaviour on Rh(111) single crystal sur-face in the hydrogen evolution potential range in 0.1 M NaClO 4 solution. With the help of SHINERS, we observed an obvious Raman peak at 1907 cm −1 when the potential was stepped in the negative direction at about −1.0 V (Figure 9 B), that peak is assigned to the Rh-H stretching mode. The spectral peaks of Rh-H in SHINERS are much sharper than SERS spectra of Au@Rh NPs as the single crystal electrode surfaces posses well-defi ned surface structure. [ 78 ] This study exemplifi ed tran-sition metal single crystal surfaces like Pt and Rh electrodes could be examined by SHINERS method, whereas only rough-ened surfaces could be studied by SERS technique.
5.2. SHINERS Study of Pyridine Adsorption on Au ( hkl ) Electrodes Surfaces
Pyridine (Py) is a standard probe molecule that can be used to understand the coordination and orientation behaviour of chemical molecules at metal interfaces. Our group utilized
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Figure 8. Cyclic voltammograms (CVs) of Au(111) electrodes unmodifi ed with (solid blue lines) and without (solid black lines) SHINs. CV of “as-prepared” NPs (dotted blue curves) and the CV of HER-SHINs (solid blue lines). Solution: 0.1 M A) H 2 SO 4 and B) HClO 4 . A,B) Reproduced with permission. [ 142 ] Copyright 2013, American Chemical Society.
Figure 9. SHINERS spectra of adsorbed hydrogen (H) on Pt(111) and Rh(111) electrode surfaces. A) SHINER spectra of adsorbed H on a Pt(111) surface: a) without Au/SiO 2 (curve a); b) Au/SiO 2 with a thicker shell at −1.9 V; B) SHINER spectra of adsorbed H on a Rh(111) surface. A) Repro-duced with permission. [ 74 ] Copyright 2010, Macmillan Publishers. B) Repro-duced with permission. [ 76,78 ] Copyright 2011, Royal Society of Chemistry.
EC-SHINERS method to examine the electrochemical adsorp-tion behavior of Py on Au( hkl ) single crystal surfaces in 0.1 M NaClO 4 solution. [ 77,92,142 ] Different factors, such as the crystal-lographic orientation, Py concentration and applied potential were examined systematically. Figure 10 A shows the CVs of 1 mM Py on three basic low-index Au( hkl ) single crystal elec-trodes in 0.1 M NaClO 4 solution. Figure 10 B shows the typical EC-SHINERS spectra of Py on the Au(111) surface from −0.8 V to 0.4 V potential range. The peak at 1011 cm −1 was attributed to the Py ν 1 ring breathing mode, and the peak at 1035 cm −1 was attributed to the Py ν 12 symmetric triangular ring deforma-tion mode. [ 32,57,145–147 ]
Generally, the Raman peak of a probe molecule is associ-ated with adsorption confi guration and surface coverage rate. By using ν 1 mode of Py on Au(111) as an example, we found that the Raman frequency and intensity of ν 1 mode around 1011 cm −1 increases with the increasing potential (Figure 10 B), but the enhancement factor is not same in dif-ferent potential regions as shown in Figure 10 C,D. At low potential region (E < 0.1 V), the Raman frequency of ν 1 mode increased little, but when E > 0.1V, the frequency of ν 1 mode undergoes obvious change and the Stark tuning rate is about 5.6 cm −1 V –1 , which indicates the adsorption confi guration or orientation of Py is different compared to the negative poten-tial region. When Py molecules were fl at-adsorbed on the elec-trode surface, there was a weak binding interaction between π-orbital electrons and the electrode surface, thus, the rel-evant Raman signal was weak; When Py molecules were ver-tically adsorbed on the electrode surface, there was a strong interaction between the nitrogen atom lone pair of electrons
and the electrode surface, so the relevant Raman signal was strong. From the potential-Raman frequency relationship and potential-Raman intensity relationship results we can con-clude that Py fl at-adsorbed on the Au(111) electrode surface when E < 0.1 V, and vertically adsorbed on the Au(111) elec-trode surface when E ≥ 0.1 V, which was in good agreement with the reported articles that Py forms a full monolayer on Au(111) at higher potentials. [ 148–150 ]
Additionally, we found that the orientation of Py on Au(100) and Au(110) surface would be changed from fl at-adsorbed to vertically adsorbed following the potential increase too. [ 151,152 ] The potentials of Py monolayer completion decreasing in the order of Au(111) > Au(100) > Au(110) on three basic-index gold single crystal surfaces, this trend is same as the poten-tial of zero charge (E PZC ) order obtained without Py. It means that the electrode surface charge plays an important role during the adsorption process. But the Raman peak intensity of ν 1 mode for adsorbed Py molecules on three basic-index Au( hkl ) electrode surfaces increasing as the order of Au(111) < Au(100) << Au(110) (Figure 10 D). Thus, EC-SHINERS can be used to investigate the adsorption behaviour of probe molecules on Au single crystal electrodes at electrochemical interfaces.
5.3. EC-SHINERS In Situ Study of Benzotriazole Film Formation on Cu( hkl ) and Cu(poly) Electrodes Surfaces
To effi ciently prevent the copper and relevant alloys cor-rosion, benzotriazole (BTAH) is widely applied in the pol-ishing and plating fi elds. [ 153 ] Interestingly, Gewirth’s group
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Figure 10. A) CVs of Au( hkl ) electrodes in 0.1 M NaClO 4 solution containing 1 mM Py. B) SHINER spectra of Py molecules on Au(111). The relationship of C) Raman frequencyand D) normalized Raman intensity with potentials for the ν 1 mode, and accompanied with surface concentration isotherms (bold curves). A–D) Reproduced with permission. [ 92 ] Copyright 2015, American Chemical Society.
used SHINERS to directly investigate the fi lm formation mechanism on both single-crystal and unroughened poly-crystalline Cu electrodes. [ 81 ] As shown in Figure 11 A, the peaks intensity of 1020 and 1190 cm −1 increased during the anodic direction scanning on Cu( hkl ) single crystal electrode surface relative to the adjacent peaks, but this phenomenon was not found on polycrystalline electrodes. Another Raman peak around 1140 cm −1 is typically corresponds to the NH bending mode of adsorbed BTAH.
As shown in Figure 11 B, the 1190 cm −1 /1140 cm −1 peak intensities ratio increased following the potential increased during the anodic scanning direction on both Cu(111) and Cu(100) single crystal electrode surfaces. However, the 1190 cm −1 /1140 cm −1 peak intensities ratio increasing untill −0.2 V for Cu(100) surface and −0.3 V for Cu(111) surface during the cathodic swept direction. This behavior indicated that the fi lm growth behavior is different at three different electrode surfaces. According to the SHINERS experiment results, compared to the reversible process of fi lm forma-tion on Cu(poly) electrode surface in H 2 SO 4 solution, the BTA− fi lm always exist during the cathodic sweep direction. This systematic report unveils that the crystallographic orienta-tion of Cu( hkl ) signal crystal surface can obviously affect the BTA− fi lm formation, and the reversible growth of BTA-fi lm on Cu(poly) surface maybe because of the grain boundaries pres-ence. Overall, EC-SHINERS has been employed to investigate various probe molecules on different single crystal surfaces including Au, Cu, Pt, and Rh electrodes. Thus, the invention of SHINERS rules out the need for the target surface to be SERS active and allows the direct investigation of single crystal electrodes.
5.4. SHINERS Study of Ionic Liquids on Au( hkl ) Electrode Surfaces
Ionic liquids got signifi cant attention in electrochemistry and energy devices because of its wide electrochemical window, electrochemical stability, high viscosity and conductivity. [ 154,155 ] Generally, in ionic liquid systems, the strong interactions between the solvent ions and the metal electrode surfaces can complicate the Raman measurements in electrochemical interfaces. It is well-known that multilayered structures can be formed in IL systems, which is potential dependent and much thicker than the double layer of aqueous systems. Therefore, it is necessary to research the detailed mechanism on well-defi ned single crystal surfaces. [ 156 ]
Signifi cantly, Tian’s group combined SHINERS method with density functional theory (DFT) calculations to investigate 1-butyl-3-methylimidazolium hexafl uorophosphate (BMIPF 6 ) and 1-octyl-3-methylimidazolium hexafl uorophosphate (OMIPF 6 ) on Au(111) surface in a wide potential range. [ 157 ] Figure 12 A reveals that BMI + through HC 4 C 5 H side of the ring interacts with the Au(111) electrode surface. [ 158,159 ] The intensity of imidazolium ring ( related to 1340 and 1390 cm −1 peaks) and the butyl group (related with 1115, 2880, 2919 and 2944 cm −1 peaks) is stronger following the potentials decreased, which indicates that the BMI + is located closely to the nega-tively charged electrode surface. The orientation change of BMI + was deducted by the intensity vs. potential curve, which clearly shows the difference in intensities at various potentials. Usually, the cations or anions do not adsorb strongly on the electrode surface when the potential close to PZC. [ 160 ] when the electorde surface is negatively charged, the cations prefer a nearly fl at confi guration and eventually vertical confi guration
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Figure 11. A) SHINER spectra of Cu( hkl ) and Cu(poly) in 0.75 mM BTA + 0.1 M H 2 SO 4 for the positive scan direction (a,c,e,g) and the negative scan direction (b,d,f,h). B) The peak intensities ratio of 1190 cm −1 /1140 cm −1 bands for Cu electrodes under different potentials. A,B) Reproduced with permission. [ 81 ] Copyright 2012, Wiley.
at decreasing potential (Figure 12 B). Thus, SHINERS has the ability to detect analytes in both aqueous and non-aqueous systems. This combined SHINERS-DFT approach rendered important information about the ionic liquid interfacial processes at single crystal electrodes.
5.5. In Situ Investigation of Electrooxidation Processes on Au( hkl ) Single Crystal Surfaces
In electrocatalytic processes, the formation of oxide greatly affects the performance of a catalyst and also the reaction mech-anism. [ 161,162 ] Therefore, it is imperative to understand the elec-trocatalytic process to fabricate or design effi cient new catalysts. However, the conventional analytical techniques are not able to detect the key intermediates because of the complexity in in situ studies. Most of the reaction mechanisms were deduced based on the electrochemical techniques and combined with the theoretical calculations, but it was still a challenge to prove these results by direct in situ Raman spectra. [ 163,164 ]
To solve this issue, in situ EC-SHINERS technique pro-vides a novel way to real-time investigate the electrocatalytic reaction pathways at well-defi ned noble metal single crystal surfaces. Our group employed in situ EC-SHINERS method to investigate the electrooxidation processes at Au( hkl ) single crystal surfaces, and provided spectral evidence for the elec-trooxidation intermediates. [ 91 ] As shown in Figure 13 , there is no obvious Raman peak in the range of 300–800 cm −1 during the positive scan until 0.4 V on Au(111) single crystal electrode surface. There was a Raman peak around 790 cm −1 following the potential increased to 0.4 V, and its intensity reached max-imum at 0.5 V, then decreased and almost disappeared at 0.9 V.
According to the references and the DFT calculation results, we considered the peak at 790 cm −1 belonged to the gold-hydroxide bending mode δ AuOH of the adsorption on top sites. [ 133,165,166 ]
To confi rm our conclusion, a deuterium isotopic experi-ment was also carried out, and the results shown that the bending mode of AuOH at 790 cm −1 shifts toward the lower wavenumber 694 cm −1 in deuterated water, which clearly con-fi rms that the band around 790 cm −1 was assigned to the gold-hydroxide bending mode δ AuOH . We also found that there was a broad band at 360–420 cm −1 , when the potential reached to 0.3 V, the band blue-shifted about 20 cm −1 in heavy water exper-iment, which is also in agreement with the previous references. When the potential was at 0.7 V, there was another broad band at 567 cm −1 , it shifted to 593 cm –1 following the potential increased, which is attributed to Au–O stretching.
We then investigated the effect of crystallographic orienta-tion, anion and pH values for the electrooxidation process on Au( hkl ) single electrode surface. As shown in Figure 13 , the intensities of the bending mode δ AuOH at three single-crystal facets increased in the order of Au(111) > Au(110) >> Au(100), which revealed the OH species are selectively adsorbed on three low-index gold single surfaces. However, this order is opposite to the order of activity in the oxygen reduction reac-tion, [ 135,167–170 ] which may be because of OH formation during the electrooxidation of Au surface. Along with the crystallo-graphic orientation, pH, and anion effects, EC-SHINERS was employed to systematically characterize and monitor the elec-trooxidation process on gold single crystal electrodes. In situ electrochemical SHINERS offers a unique opportunity for a real-time investigation of electrocatalytic reaction processes at noble metal single crystal electrode surfaces, to pave a way for technological innovations in surface catalysis.
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Figure 12. A) EC-SHINERS of BMIBF 6 on a Au(111) surface. B) The suggested potential-dependent structure of the imidazolium/Au( hkl ) electrode-based ionic liquid. Reproduced with permission. [ 157 ] Copyright 2014, Royal Society of Chemistry.
In the previous sections, we have elaborately discussed the developments of three different modes/techniques such as SERS, TERS and SHINERS. Signifi cantly, the SHINERS tech-nique has overcome the material and morphology limitations problems of SERS. Thus, SHINERS have been successfully used to study various analytes and intermediates on various single crystal electrodes (e.g. Au, Pt, Rh, and Cu). Moreover, now it is possible to examine non-aqueous solvents like ionic liquid at electrochemical interfaces. The combination of SHINERS technique and theoretical methods can provide addi-tional details about electrochemical systems, which will enrich our knowledge in electrochemical interfaces.
Strikingly, the shell-isolated nanoparticles strategy has been utilized for infrared absorption spectroscopy (IR) and sum fre-quency generation (SFG) and other spectroscopic techniques. For instance, Aroca and Li et al. have implemented the concept to develop shell-isolated nanoparticle-enhanced fl uorescence (SHINEF). [ 171,172 ] Yang and Li et al. have extended the strategy to Second-Harmonic Generation (SHG). [ 173 ] New scanning probe microscopy tip structures inspired by SHINERS, such
as shell-isolated tip-enhanced spectroscopy (SITERS), are now under development in Zenobi and Li groups. [ 72 ]
As an analytical technique, SHINERS technique is simple, rapid, and economic with signifi cant advantages. The shell-isolated mode is also extremely simple and employed in several fi elds such as electrochemistry, surface science and food science. However, it is necessary to point out that the fabrication of a pinhole-free ultrathin shell is indeed very dif-fi cult. For instance, our group took several years to achieve this goal and many groups have failed to reproduce it. Thus, it needs more experience and effort to prepare pinhole-free SHINs for SHINERS studies. Though the chemically stable and inert shell can improve the stability of nanoparti-cles and the target systems, the silica shell can be dissolved in basic solution, which limits the applications of SHINs (Au@SiO 2 ) in basic solution systems. In addition, SiO 2 is typically porous and not compact enough, which restricts the massive preparation of pinhole-free shell-isolated NPs with shell thickness down to 2 nm. Therefore, it is necessary to develop other shell materials, such as TiO 2 , MnO 2 , Al 2 O 3 and Ag 2 S, that can be easily prepared for different pH or requirements. [ 174–182 ]
Figure 13. A) The SHINERS spectra of electrooxidation at Au(111), Au(100) and Au(110) electrode surfaces in 0.1 M NaClO 4 (pH is ≈9); B) Normal-ized SHINERS intensities of the stretching mode of AuO and the bending mode of AuOH at different potentials. CV of Au(111) electrode in 0.1 M NaClO 4 is presented (pH is ≈9, scan rate is 2 mV s –1 ); C) Schematic diagram of OH species on three low index Au( hkl ) surfaces. A–C) Reproduced with permission. [ 91 ] Copyright 2015, American Chemical Society.
We believe that the preliminary achievements made by SHINERS in the past six years are promising, but has not been fully exploited. With the development of nanotechnology, there will be a growing number of applications on SHINERS for the in-situ investigation of electrochemical processes on atomically smooth single crystal surfaces. The developments in SHINERS technique indicate that SHINERS will become a powerful tool for fundamental studies and practical applications in electro-chemical science and widespread acceptance of this technique can be achieved in future.
Acknowledgements The authors thank Z. L. Yang and Z. Q. Tian for helpful discussions. This work was supported by NSFC (21522508, 21503231 and 21427813), Open Research Fund of Top Key Discipline of Chemistry in Zhejiang Provincial Colleges (Zhejiang Normal University ZJHX201501), and Thousand Youth Talents Plan of China.
Received: March 30, 2016 Revised: May 19, 2016
Published online: July 21, 2016
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