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Shell-Isolated Nanoparticles-Enhanced Raman Spectroscopy
Shell-Isolated Nanoparticles-Enhanced Raman Spectroscopy J-F Li and J-C Dong, Xiamen University, Xiamen, China © 2018 Elsevier Inc. All rights reserved.
Introduction Basic Principles of SHINERS Technique Preparation of SHINs Characterization of SHINs Application of SHINERS in Electrochemical Surface Science In Situ Monitoring of Pyridine Behaviors at Au(hkl) Electrode Surfaces by SHINERS Technique Real-Time Research of the Growth Process of Benzotriazole Film at Single Crystal Cu(hkl) and Polycrystalline Cu(poly) Electrodes Surfaces by SHINERS Technique Real-Time Monitoring of the Electrooxidation Processes of Au(hkl) Electrodes In Situ Research of the Adsorption and Hydrogenation Behavior of Ethyl Pyruvate (EP) at Pt(hkl) Electrode Surfaces by SHINERS The Quantitative Analysis of Temporal Changes by SHINERS Technique Summary and Prospective References Further Reading
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Introduction Surface reactions occurring at different solid/liquid or solid/gas interfaces have attracted significant attention within the basic research scientific community for decades. In particular, the surface reactions are gaining increasing attention, and they are of paramount importance for electrochemical interfaces studies which contribute immeasurably to the mechanistic understanding of catalytic processes at the molecular scale.1,2 The interfacial reaction involves some key factors that correlate with mass transfer, molecular adsorption, and charge transfer. Though conventional electrochemical techniques, such as cyclic voltammetry (CV) and rotating ring disk electrode technique, can provide important information about the interfacial reactions, it is just an average message from a macroscopic view. In essence, the key point to understand interfacial reaction is from molecular even atomic level to reveal its mechanism. Therefore, improved techniques must be introduced for investigating the interfacial electrochemical systems, in particular, the use of real-time spectroscopy methods offering information in a noninvasive manner. As a powerful “fingerprint” technique, Raman spectroscopy can not only provide information in a nondestructive manner about structures and properties of the interfacial elements but also adsorption configurations, orientations, and surface bonding of probe molecules at reaction surfaces.3–12 However, Raman spectroscopy lacks detection sensitivity due to the small Raman scattering cross section of analyte molecules, thus the normal Raman signals are usually very weak, which seriously limit the development and application of the Raman technique.13 In 1970s, the emergence of surface-enhanced Raman spectroscopy (SERS) technique aided in realization of an ultrahigh sensitivity down to the single-molecule level at noble metal substrate surfaces (Fig. 1A).5–9,14,15 Since the introduction of SERS effect, numerous experimental studies and theoretical researches have been carried out; which largely promoted the development of SERS technique.16–21 With the further understanding of SERS effect, it was found that SERS enhancement is mainly attributed to electromagnetic enhancement mechanism (EM).22,23 As a key content of SERS effect, electromagnetic field enhancement originates from surface plasmon resonance (SPR) of metal nanostructures.24,25 However, the shortcomings of SERS technique, as clear as its advantages, include the lack of materials and morphological generality, which has significantly restricted the practical application of SERS in variety of fields. The limitation becomes prominent when SERS is extended to study the spectroelectrochemistry of single crystal surfaces which are usually employed in electrochemical catalysis and surface science. Long-time significant research efforts devoted to SERS indicated that only a few noble metal materials, such as gold (Au), silver (Ag), copper (Cu), and few alkali metals yield a strong SERS effect. However, unfortunately, most of normal metals, in particular transition metals, which are extensively used in catalytic field, do not exhibit obvious SERS effect. Moreover, as the SERS substrates must be rough at the nanoscale; therefore, SERS is not suitable to research the reactions at atomically flat single crystal surfaces, which commonly serve as research model in theoretical simulation systems.26 To overcome the long-standing limitation of SERS technique, significant research impetus has emerged to systematically explore the pathways to expand SERS technique at transition metal surfaces, which partly solved the substrate generality problem of SERS (Fig. 1B).27–32 In 2000, the invention of tip-enhanced Raman spectroscopy (TERS) technique converted the direct contact mode of SERS to a noncontact mode (Fig. 1C), which resulted in directly obtaining the Raman signals from different metal single crystal surfaces.15,33–35 Though TERS technique offers high spatial resolution, it is limited to large Raman cross-section molecules and special experimental equipment.36–38
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Fig. 1 Different work modes of Raman methods: (A) contact mode of SERS; (B) target molecules adsorbed on the Au core-transition metal shell NPs surfaces; (C) noncontact mode of TERS; and (D) shell-isolated work mode of SHINERS. Reproduced with permission Li, J. F.; Huang, Y. F.; Ding, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464(7287), 392–395. Copyright 2010, Macmillan Publishers Ltd.
In 2010, the invention of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)39 technique by our group helped in significantly overcoming the materials and morphological generality problems of SERS (Fig. 1D). In SHINERS method, the electromagnetic field of inner Au nanoparticles (Au NPs) of Au@SiO2 shell-isolated nanoparticles (SHINs) can greatly enhance the Raman signals of nearby molecules, and the outer inert SiO2 shells can efficiently isolate the signal amplifiers (core Au NPs) from the outer environment and preclude the Au NPs contact with the surface species/probe molecules.40,41 Simply speaking, we can directly obtain the Raman signals just by spreading SHINs on any surfaces with diverse compositions and morphologies. The SHINERS method, with higher sensitivity and lower technical threshold, has been widely applied in different fields, in particular, in electrochemical interfaces and surface science research fields.41–58 Therefore, herein we mainly focused on the basic principles and recent development in SHINERS method with the objective of introducing this novel technique in surface science fields.
Basic Principles of SHINERS Technique The first step in performing SHINERS experiment includes the preparation of high-quality SHINs for different requirements. Further, an appropriate characterization method and theoretical simulation can also provide important supporting information for the research system. Therefore, the basic principles and relevant contents of SHINERS are introduced in the following sections.
Preparation of SHINs For different experimental conditions, different types of SHINs are required. Noteworthy, the core NPs of SHINs should be SERS-active and the outer shell materials of SHINs should be stable in the research system. The most common core NP for SHINs is 55 nm Au NPs, and greater diameter of Au NPs ( 130 nm) can provide higher enhancement under 633 nm laser.59 As the SPR peaks of SHINs can be tuned by adjusting the size and shape of the core; therefore, different shapes and sizes of core NPs could also be prepared to tune their maximum SPR peak position.25,40,45,60–62 The core material of SHINs is another important factor for the enhancement. For example, Ag NPs usually exhibit stronger enhancement compared to the Au NPs with the same size, thus the Ag core SHINs is also a considerable choice for some weak interfacial researches. As EM of SHINs is a long range effect, it obviously increases with the decrease in the shell thickness; therefore, the inert shell of SHINs should be very thin (1–2 nm) for higher enhancement.59 The shell materials of SHINs should be stable and inert during the Raman test to forbid any electrochemical reactions or physical interferences. The reported shell materials of SHINs are silica (SiO2), alumina (Al2O3), titania (TiO2), and manganese dioxide (MnO2) (Fig. 2).40,54,63,64 We then introduced the detailed preparation process for normal Au@SiO2 SHINs. Au NPs (55 nm) solution was prepared by Frens method,65 and the SHINs were synthesized as follows40: Au NPs solution (30 mL, 55 nm) was added into a 100-mL round-bottom flask and stirred for 15 min at room temperature, following which 3-(aminopropyl)triethoxysilane solution (APTES, 400 mL, 1 mM) was added to it. Then Na2SiO3 solution (3.2 mL, pH 10.3, 0.54%) was added into the above-mentioned solution. Subsequently, after 3 min, the mixed solution was transferred to a 90 C bath and stirred for some time. The shell thickness of SHINs can be precisely controlled by changing the Au NPs seed concentration, heating temperature, reaction time, and pH of Na2SiO3 solution. After certain heating time, the hot SHINs solution was rapidly cooled down in an ice bath and centrifuged for two or three times, and then it was finally diluted to certain volume for characterization or Raman experiment.
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Fig. 2 The TEM images of different types of SHINs: (A) Au@SiO2 NP with a 55 nm Au NP core; (B) Au@SiO2 NP with a 120 nm Au NP core; (C) Au@SiO2 NP with Au nanocube core; (D) Au@SiO2 NP with Au nanorod core; (E) Ag@SiO2 NP with Ag NP core; (F) 55 nm Au@20 nm SiO2 NP; (G) Ag@TiO2 NP; (H) Ag@Al2O3 NP; (I) MnO2 shell; and (J) Ag2S shell NPs. Reproduced with permission from Li, J. F.; Huang, Y. F.; Ding, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464(7287), 392–395. Copyright 2010, Macmillan Publishers Ltd. Reproduced with permission from Li, J. F.; Tian, X. D.; Li, S. B.; et al. Surface Analysis Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nat. Protocols 2013, 8(1), 52–65. Copyright 2013, Macmillan Publishers Ltd. Reproduced with permission from Zhang, W.; Dong, J. C.; Li, C. Y.; et al. Large Scale Synthesis of Pinhole-Free Shell-Isolated Nanoparticles (SHINs) Using Improved Atomic Layer Deposition (ALD) Method for Practical Applications. J. Raman Spectrosc. 2015, 46(12), 1200–1204. Copyright 2015, John Wiley & Sons, Ltd. Reproduced with permission Lin, X. D.; Uzayisenga, V.; Li, J. F.; et al. Synthesis of Ultrathin and Compact Au@MnO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS). J. Raman Spectrosc. 2012, 43(1), 40–45. Copyright 2012, John Wiley & Sons, Ltd. Reproduced with permission from Lin, X. D.; Li, J. F.; Huang, Y. F.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Nanoparticle Synthesis, Characterization and Applications in Electrochemistry. J. Electroanal. Chem. 2013, 688, 5–11. Copyright 2013, Elsevier Inc.
Characterization of SHINs Obviously, the outer ultrathin yet pinhole-free shells of SHINs plays a key role during the experimental process. With the effective protection and isolation provided by outer SiO2 shell, some problems can be efficiently avoided. First, if outer inert SiO2 shell protection is not provided, some chemical species present in environment may interact with bare Au core or other metallic NPs, which directly interfere with the Raman signals (Fig. 3A).11,66 Second, attributed to different Fermi levels between substrate material and bare metallic NPs, charge transfer may occur between them, which then further disturbs the electronic structure of the research system.67,68 Last but more importantly, the bare NPs may interact with some target molecules, and this direct contact significantly affects the Raman signal because of the change in the molecule adsorption behaviors and electron density distribution during the
Fig. 3 The models of different interferences in bare Au NPs: (A) contact with adsorbed molecules or impurities; (B) charge transfer between Au NPs and the metal surface; (C) the interaction between bare Au NPs and probe molecules; and (D) No interferences due to SHINs. Reproduced with permission from Li, J. F.; Huang, Y. F.; Ding, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464(7287), 392–395. Copyright 2010, Macmillan Publishers Ltd.
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contact process (Fig. 3C).69–71 The above-mentioned analysis clearly indicates that the shell-isolated mode can easily resolve the aforesiad problems just by coating an ultrathin inert SiO2 shell on the bare metallic NPs surface (Fig. 3D), and the chemical inert shell can also efficiently prevent the charge transfer, inadvertent adsorption, or photocatalytic reactions. SHINs can be characterized by transmission electron microscopy (TEM), CV, and surface-enhanced Raman test. TEM is an efficient method to characterize the size and the shell thickness of SHINs (Fig. 2); however, detection of pinholes present in the shell by TEM is extremely difficult, thus the electrochemical CV and Raman test methods can be employed. For the shell of SHINs with pinholes, a characteristic Au reduction peak is observed around 0.9 V (vs. SCE) in CV curves.39 However, detection of very small or tiny pinholes by CV method is still a challenge, thus a more sensitive-enhanced Raman spectral method using pyridine as a probe molecule has been introduced to examine the pinhole effect.40 Presence of pinholes in the SiO2 shell leads to the occurrence of two Raman peaks around 1011 and 1035 cm 1 because pyridine molecules get adsorbed on the surface of Au and interact with the Au core through pinholes. The above-mentioned peaks are assigned to n1 ring breathing mode and n12 symmetric triangular ring deformation mode of adsorbed pyridine molecules on Au NPs surfaces, respectively. Other than experimental approach, theoretical simulations of SHINs can also provide theoretical supports for SHINERS researches. The most common theoretical simulation method for SHINERS systems is three-dimensional finite-difference time-domain (3D-FDTD) modeling, which can be efficiently used to test the local electromagnetic fields of SHINs on smooth metal substrate surfaces.59,72–76
Application of SHINERS in Electrochemical Surface Science The significant advantages of SHINERS technique compared to SERS and TERS are as follows: SHINERS can not only directly provide the Raman signals from any substrates and any materials surfaces but also with high sensitivity and under simple operating conditions. Moreover, owing to the protection provided by inert SiO2 shell, the SHINs do not participate in or disturb the outer reactions, thus maintaining the reliability of Raman signals. The SHINERS technique can be effectively applied in different fields, in particular, in electrochemical surface science research. In this section, the recent development in SHINERS technique in surface science field based on the concrete application examples is illustrated. Compared to other spectroscopic methods, the most successful application of SHINERS is its applicability at single crystal surface.39,43,50,53,77,78 Single crystals with atomically flat surfaces, well-defined structures, and well-known electronic levels have attracted special attention of surface scientists. In early time, several spectroscopic methods were employed to study the reactions at single crystal surfaces, such as surface-enhanced infrared reflection absorption spectroscopy (SEIRAS),79,80 X-ray photoelectron spectroscopy (XPS),81–83 and so forth; however, these methods were restricted by water or requirement of special experiment conditions or equipment. Though SERS is a powerful technique, it is difficult to directly obtain the Raman signals from single crystal surfaces.84,85 TERS with high spatial resolution and high sensitivity of single molecules detection can be employed to study the reactions at single crystal surfaces; nonetheless, the total Raman signals of TERS are too weak and the tip gets easily contaminated by outer air or solution environment, which may yield misleading information. Owing to strong electromagnetic field enhancement through “shell-isolated” working mode, SHINERS technique has been employed to research the adsorption and orientation of probe molecules and catalytic processes at different types of single crystal surfaces. Furthermore, the SHINERS technique can also be explored for researches in some challenging fields, such as corrosion resistance research and surface quantity analysis.
In Situ Monitoring of Pyridine Behaviors at Au(hkl) Electrode Surfaces by SHINERS Technique Pyridine molecule plays an important role in the Raman research fields, and the first SERS effect was discovered through obtaining high-quality Raman signals of pyridine molecules at Ag electrode surface.5 Our group employed electrochemical SHINERS (EC-SHINERS) technique to research the electrochemical adsorption behaviors of pyridine at Au(hkl) electrodes in NaClO4 solution. Furthermore, the crystallographic orientation of electrode surfaces under applied potential was investigated.78 In general, different surface coverage rates and adsorption behaviors of target molecule at electrode surface usually correlate with different Raman peaks. Considering n1 ring breathing mode of pyridine molecule at Au(111) surface as an example, the correlated Raman peak was at around 1011 cm 1. Though the Raman intensity and frequency of n1 ring breathing mode around 1011 cm 1 both increased following positive scanning direction, they exhibited different enhancement rates between low potential range and high potential range (Fig. 4). At high potential range, the Raman frequency increased rate was more obvious than that at the low potential and with a 5.6 cm 1/V Stark tuning rate, which clearly illustrated the variation in the adsorbed orientations or structures of pyridine molecules at electrode surface at high potential range compared to that at low potential range. If pyridine was flat adsorbed at Au electrode surface, there should be a weak binding interaction between the Au electrode and p-orbital electrons of pyridine molecules, thus leading to a weak Raman signal. If pyridine was vertically adsorbed at the Au electrode surface, there should be a strong interaction between the Au electrode and the nitrogen atom lone pair of electrons of pyridine, thus leading to a strong Raman signal. Based on this information and the potential Raman intensity/frequency relationship of SHINERS data, it could be concluded that pyridine molecules were flat-adsorbed at the Au(111) electrode surface at low potential range, and vertically adsorbed at the Au(111) electrode surface at high potential range, which correlated well with the previous related researches that a monolayer of pyridine molecules is formed at Au(111) electrode in higher potentials range.86,87 Besides Au(111), Au(110), and Au(100) also exhibited the occurrence of same phenomenon, and the potentials trend order of pyridine monolayer
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Fig. 4 (A) CVs of pyridine (1 mM) in NaClO4 (0.1 M) solution at Au(hkl) electrodes and (B) SHINERS results of pyridine at Au(111) electrode surface. The relationship of (C) Raman frequency and (D) normalized Raman intensity with potentials for n1 ring breathing mode around 1011 cm 1, and combined with surface concentration isotherms (bold curves). Reproduced with permission from Li, J. F.; Zhang, Y. J.; Rudnev, A. V.; et al. Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Correlating Structural Information and Adsorption Processes of Pyridine at the Au(hkl) Single Crystal/Solution Interface. J. Am. Chem. Soc. 2015, 137(6), 2400–2408. Copyright 2015, American Chemical Society.
completion at three low-index Au(hkl) surfaces was same as the potential of zero charge (PZC) order of three single crystal surfaces, that is, Au(111) > Au(100) > Au(110). From this perspective, it can be considered that the surface charge of electrode surfaces plays a key role in the electrochemical adsorption process. The n1 ring breathing mode intensity decreased in the following order: Au(110) [ Au(100) > Au(111). In this study, the SHINERS results and the electrochemical behaviors correlated well. Thus, the SHINERS technique is an efficient method to investigate the behaviors of adsorbed molecules at different electrochemical interfaces.
Real-Time Research of the Growth Process of Benzotriazole Film at Single Crystal Cu(hkl) and Polycrystalline Cu(poly) Electrodes Surfaces by SHINERS Technique Benzotriazole (BTAH) is widely used for polishing and plating purposes to prevent Cu and relevant alloys from corrosion because BTAH can form a coordination polymer film on the surface of Cu to prevent its oxidation.88 Gewirth’s group employed SHINERS technique for real-time investigation of processes involving formation of BTA film at Cu(hkl) and Cu(poly) electrode surfaces.50 Fig. 5 clearly exhibits the peak around 1020 cm 1 which is assigned to benzene skeletal and CeH bend, and the peak around 1190 cm 1 is attributed to triazole ring breathing mode (CeCeC in-plane bending). During the positive scanning process at Cu(hkl) electrode surfaces, the Raman intensities of both the above-mentioned peaks increased; however, the similar phenomenon at Cu(poly) electrode surface was not observed during the same scanning process. Furthermore, the peak intensities ratio of 1190/ 1140 increased with the increase in the potential at Cu(100) and Cu(111) electrodes surfaces. The peak at 1140 cm 1 is assigned to NeH bending mode of adsorbed BTAH at Cu electrode surfaces. However, SHINERS results indicated different interpretation for both single crystal surfaces. The intensities ratio corresponding to 1190/1140 stopped increasing later for Cu(111) electrode surface compared to Cu(100) at the negative scanning direction, and the correlated stop potentials were 0.3 and 0.2 V, respectively. During the negative scanning direction at Cu(poly) electrode surface, the SHINERS results again exhibited different phenomena compared to the single crystal electrode surface, thus indicating different growth behaviors for the BTA-Cu(I) film formation at different Cu electrodes surfaces. The film formation was found to be irreversible at single crystal Cu(hkl) electrodes, while it was reversible at Cu(poly) surfaces. The systematic research reveals that different crystallographic orientation of Cu(hkl) electrodes surfaces, with evident effect on BTA-Cu(I) film growth, and the presence of grain boundaries together lead to the cathodic degradation of BTA-polymeric films at Cu(poly) electrode.
Real-Time Monitoring of the Electrooxidation Processes of Au(hkl) Electrodes The formation of oxide during the electrocatalytic processes significantly affects the function of catalyst. Therefore, it is important to reveal the electrocatalytic mechanisms for the effective designing and preparation of efficient new catalysts.89,90 Although the conventional spectroscopy methods are used to study the catalytic reactions in electrochemical systems, difficulties are encountered
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Fig. 5 (A) SHINERS results of 0.75 mM BTA in 0.1 M H2SO4 solution during positive scan direction (left group images) and negative scan direction (right group images) at Cu(poly) and Cu(hkl) electrode surfaces and (B) the 1190/1140 bands intensities ratio for different Cu electrodes under different potentials. Reproduced with permission from Honesty, N. R.; Gewirth, A. A. Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) Investigation of Benzotriazole Film Formation on Cu(100), Cu(111), and Cu(poly). J. Raman Spectrosc. 2012, 43(1), 46–50. Copyright 2012, John Wiley & Sons, Ltd.
in observing the key intermediates species because of the complexity of in situ researches. Thus it is still a huge challenge to use an in situ spectroscopy method to study the reaction mechnisms in electrochemical systems.91,92 The SHINERS technique provides an efficient pathway for in situ monitoring of the electrocatalytic reactions processes at single crystal electrodes. Our team employed EC-SHINERS technique for in situ investigation of the electrooxidation processes of Au(hkl) electrodes and found direct in situ spectral evidences for reaction intermediates.77 Fig. 6 shows the absence of obvious Raman peaks at initial potential in the range of 300–800 cm 1. At positive scanning direction, appearance of an obvious Raman peak around 790 cm 1 is observed. The intensity of this peak continues to increase until 0.5 V, then decreases, and almost disappears at 0.9 V. The peaks around 790 cm 1 are assigned to the top site gold–hydroxide (Au OH) bending mode (dAuOH) at Au(hkl) electrode surfaces. Following the diappearance of Au OH bending mode, the Au O stretching mode (nAu–O) around 570–590 cm 1 becomes more and more obvious with the continuous increase in the potential. The different influencing factors, such as crystallographic orientation, anions, and pH of the electrooxidation system at Au(hkl) single electrode surfaces, were carefully investigated. The SHINERS results indicated that the Au OH bending mode intensities increased in the following order: Au(100) Au(110) < Au(111), which is opposite to the oxygen reduction reaction activity order of three low-index Au(hkl) electrodes under the same conditions. This should be attributed to the fact that OH formed at Au(hkl) electrode surfaces retards the oxygen reduction reaction.91,93–95 Different pH of the electrooxidation system also leads to different SHINERS results. When the pH of the system was too high (pH 11), the peak intensity of AueOH bending mode was weaker compared to that at the pH 9 condition. Furthermore, under the acidic condition, the AueOH bending mode almost disappeared, which indicated that the electrooxidation process of Au(hkl) electrode surfaces under conditions of different pH possibly followed different mechanistic pathways. Moreover, the presence of different anions in the the electrolye also leads to different phenomena, and the SHINERS results also correlate well with the previous studies. The present study clearly indicates that the SHINERS technique offers a unique opportunity for many researchers to directly study the catalytic reactions at well-defined metal electrode surfaces through a real-time pathway in surface catalysis fields, which can definitely open the door for technological innovations in surface catalysis.
In Situ Research of the Adsorption and Hydrogenation Behavior of Ethyl Pyruvate (EP) at Pt(hkl) Electrode Surfaces by SHINERS Weak coupling effect between SHINs and platinum hinders the enhancement of SHINs and correlated Raman intensities obtained from platinum surface is usually weakened by several magnitude orders compared to Au surfaces under the same conditions.43 Recently, Attard et al. employed EC-SHINERS technique to investigate the adsorption behavior and hydrogenation of ethyl pyruvate (EP) at Pt(hkl) electrode surfaces.96 They observed two types of reaction intermediates during the hydrogen evolution potential range, that is, the half-hydrogenation state (HHS) obtained by addition of a hydrogen atom to the keto carbonyl group and the intact chemisorbed EP bound in a m2(C,O) configuration. Based on the SHINERS results, they employed density functional theory (DFT) calculation method to analyze the adsorption of EP at Pt(hkl) surfaces. The m2(C,O) configuration of EP was observed at Pt(100) and Pt(111) electrodes surfaces; however, HHS was not observed until the Pt(hkl) electrodes surfaces were roughened by electrochemical methods.
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Fig. 6 (A) The SHINERS results of in situ electrooxidation processes at different Au(hkl) electrode surfaces in NaClO4 solution (0.1 M, pH 9); (B) normalized Raman peaks intensities of nAu–O and dAuOH at different potentials; and (C) the models of OH species adsorbed at three basic Au(hkl) electrodes. Reproduced with permission from Li, C. Y.; Dong, J. C.; Jin, X.; et al. In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2015, 137(24), 7648-7651. Copyright 2015, American Chemical Society.
The Quantitative Analysis of Temporal Changes by SHINERS Technique It is well known that quantitative analysis is a huge challenge encountered during plasmon-enhanced Raman spectroscopy studies. As a novel technique, SHINERS method has already been utilized in surface analysis fields. Gold-thiosulfate leaching process is inhibited by the formation of the passive layer at Au electrode surfaces which can prevent the dissolution of Au from ore samples. Lipkowski’s group employed SHINERS technique for quantitative analysis of temporal changes in the passive layer at Au material surfaces in thiosulfate solution.57 In this study, the APTES band of SHINs was used as internal standard to compensate the surface enhancement fluctuation. After subtracting the SHINERS spectra in NaF solution, the APTES band could be efficiently removed and the corrected Raman spectra correlated well with the relevant SERS results. The SHINERS spectra for long immersion times in thiosulfate electrolyte with several new bands compared to the spectra recorded at shorter immersion times (Fig. 7A), provided a good chance to correlate the quantitative changes in Raman bands intensity with the rate of Au leaching in thiosulfate solution. Clearly, during the first 50-min immersion time, the Au leaching rate rapidly decreases and then gradually becomes down to an almost linear decay following the time passed. Fig. 7B exhibits the relationship between the Raman peaks and the immersion time after correction of the backgrounds, and normalized Raman intensities of 382, 316, and 460 cm 1, correspond to [Au(S2O3)2]3 complex, adsorbed sulfide, and polymeric sulfur species at the electrode surfaces, respectively. The peak intensity of [Au(S2O3)2]3 complex around 382 cm 1 first decreases rapidly during the initial immersion time and then decreases gradually with longer immersion time, which correlated well with the leaching current. However, the adsorbed sulfide peak at 316 cm 1 and the polymeric sulfur species peak at 460 cm 1 exhibited different phenomena. The peaks intensities for both sulfide and polymeric sulfur species increased with the increase in the immersion time. The SHINERS result clearly provided the quantitative evidences that the formation of the elemental sulfur species could efficiently prevent Au leaching in thiosulfate solutions. This should be primarily attributed to the fact that the adsorbed sulfur species occupied the Au electrode surface and blocked the interaction
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Fig. 7 (A) Raman results of SHINs/Au electrode after being immersed in 0.1 M Na2S2O3 solution (pH 10.0) between 300 and 600 min; (B) the normalized integrated band intensity as a function of immersion time, IDt, of adsorbed sulfide (316 cm 1), [Au(S2O3)2]3 complex (382 cm 1), and polymeric sulfur (460 cm 1). Reproduced by permission from Smith, S. R.; Leitch, J. J.; Zhou, C.; et al. Quantitative SHINERS Analysis of Temporal Changes in the Passive Layer at a Gold Electrode Surface in a Thiosulfate Solution. Anal. Chem. 2015, 87(7), 3791–3799. Copyright 2015, American Chemical Society.
between Au surface and thiosulfate, eventually preventing the [Au(S2O3)2]3 complex formation. This methodological advancement allows us to monitor the temporal changes in the passive layer composition at Au electrode surface through correlating the Au dissolution rate with relevant Raman bands, and then to further identify the formation mechanism of passivating species. Overall, SHINERS technique can be easily employed in anticorrosion research at different substrates for qualitative and quantitative analysis. Noteworthy, the strategy of quantitative SHINERS analysis method at different electrode surfaces species undeniably provides a potential pathway for fundamental researches and practical applications.
Summary and Prospective As a novel-enhanced spectroscopy technique, SHINERS technique breakthrough the material and morphology generality limitations of SERS technique, thus making it possible to obtain the Raman signals from any substrates and materials surfaces. Moreover, the SHINERS experiment, with low technical restriction and easy operation, is very conducive to be applied in different surface science fields. In the above-mentioned contents, we tried to use the simplest and easily understandable language to introduce the SHINERS technique development history, basic principles, and relevant contents. Further, we introduced in detail several representative application examples of SHINERS in surface science fields. The applications not only include the probe molecules adsorption studies and the real-time catalytic reactions monitoring at single crystal electrodes but also the quantitative analysis in practical application systems. Moreover, the shell-isolated mode of SHINERS technique can be explored to other areas as well by integrating this technique with other research methods, such as infrared absorption spectroscopy (IR), fluorescence spectroscopy, sum frequency generation, and tip-enhanced spectroscopy.97–100 Though the normal SiO2 shell of SHINs can meet most of requirements, it does not remain stable in solution with high pH (pH > 12) for long immersion time. Furthermore, thiner shell of SHINs offers higher enhancement; therefore, the shell of SHINs should be very thin down to 1–2 nm; however, pinholes may appear when the shell becomes ultrathin. Importantly, a lot more systematic explorations are demanded to improve the SHINs preparation methods and explore new shell materials. Overall, the achievements of SHINERS technique in the past few years are encouraging. There will be numerous emerging new applications of SHINERS in different surface science fields following the rapid development in nanotechnology. The existing developments and huge potential of SHINERS technique indicate that the SHINERS technique and the shell-isolated mode must become a powerful tool for practical applications and fundamental researchs in surface science fields, and widespread acceptance of SHINERS can come true in the near future.
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Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) Investigation of Benzotriazole Film Formation on Cu(100), Cu(111), and Cu(Poly). J. Raman Spectrosc. 2012, 43 (1), 46–50. Kudelski, A.; Wojtysiak, S. Silica-Covered Silver and Gold Nanoresonators for Raman Analysis of Surfaces of Various Materials. J. Phys. Chem. C 2012, 116 (30), 16167–16174. Zhang, Z. M.; Liu, R.; Xu, D. M.; Liu, J. F. In Situ Detection of Acid Orange II in Food Based on Shell-Isolated Au@SiO2 Nanoparticle-Enhanced Raman Spectroscopy. Acta Chim. Sin. 2012, 70 (16), 1686–1689. Li, J. F.; Rudnev, A.; Fu, Y. C.; Bodappa, N.; Wandlowski, T. In Situ SHINERS at Electrochemical Single-Crystal Electrode/Electrolyte Interfaces: Tuning Preparation Strategies and Selected Applications. ACS Nano 2013, 7 (10), 8940–8952.
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Pursuing Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) for Concomitant Detection of Breast Lesions and Microcalcifications. Nanoscale 2015, 7 (40), 16960–16968. Chen, S.; Yang, Z.; Meng, L.; et al. Electromagnetic Enhancement in Shell-Isolated Nanoparticle-Enhanced Raman Scattering From Gold Flat Surfaces. J. Phys. Chem. C 2015, 119 (9), 5246–5251. Ye, X.; Jin, L.; Caglayan, H.; et al. Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods Through the Use of Aromatic Additives. ACS Nano 2012, 6 (3), 2804–2817. Pérez-Juste, J.; Correa-Duarte, M. A.; Liz-Marzán, L. M. Silica Gels With Tailored, Gold Nanorod-Driven Optical Functionalities. Appl. Surf. Sci. 2004, 226 (1), 137–143. Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15 (10), 1957–1962. Zhang, W.; Dong, J. C.; Li, C. Y.; et al. Large Scale Synthesis of Pinhole-Free Shell-Isolated Nanoparticles (SHINs) Using Improved Atomic Layer Deposition (ALD) Method for Practical Applications. J. Raman Spectrosc. 2015, 46 (12), 1200–1204. Lin, X. D.; Uzayisenga, V.; Li, J. F.; et al. Synthesis of Ultrathin and Compact Au@MnO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS). J. Raman Spectrosc. 2012, 43 (1), 40–45. Frens, G. Controlled Nucleation for Regulation of Particle-Size in Monodisperse Gold Suspensions. Nat. Phys. Sci. 1973, 241 (105), 20–22. Gersten, J.; Nitzan, A. Electromagnetic Theory of Enhanced Raman Scattering by Molecules Adsorbed on Rough Surfaces. J. Chem. Phys. 1980, 73 (7), 3023–3037. Rodriguez, J. A.; Goodman, D. W. The Nature of the Metal Metal Bond in Bimetallic Surfaces. Science 1992, 257 (5072), 897–903. Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters. 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Li, C. Y.; Meng, M.; Huang, S. C.; et al. “Smart” Ag Nanostructures for Plasmon-Enhanced Spectroscopies. J. Am. Chem. Soc. 2015, 137 (43), 13784–13787. Osorio-Roman, I. O.; Guerrero, A. R.; Albella, P.; Aroca, R. F. Plasmon Enhanced Fluorescence With Aggregated Shell-Isolated Nanoparticles. Anal. Chem. 2014, 86 (20), 10246–10251. Guerrero, A. R.; Aroca, R. F. Surface-Enhanced Fluorescence With Shell-Isolated Nanoparticles (SHINEF). Angew. Chem. Int. Ed. 2011, 50 (3), 665–668.
Further Reading Ding, S. Y.; Yi, J.; Li, J. F.; et al. Nanostructure-Based Plasmon-Enhanced Raman Spectroscopy for Surface Analysis of Materials. Nat. Rev. Mater. 2016, 1, 16021. Dong, J. C.; Panneerselvam, R.; Lin, Y.; Tian, X. D.; Li, J. F. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy at Single-Crystal Electrode Surfaces. Adv. Opt. Mater. 2016, 4 (8), 1144–1158. Ozaki, Y., Kneipp, K., Aroca, R., Eds. Frontiers of Surface-Enhanced Raman Scattering: Single Nanoparticles and Single Cells, Wiley: Chichester, 2014.
Introduction Basic Principles of SHINERS Technique Preparation of SHINs Characterization of SHINs Application of SHINERS in Electrochemical Surface Science In Situ Monitoring of Pyridine Behaviors at Au(hkl) Electrode Surfaces by SHINERS Technique Real-Time Research of the Growth Process of Benzotriazole Film at Single Crystal Cu(hkl) and Polycrystalline Cu(poly) Electrodes Surfaces by SHINERS Technique Real-Time Monitoring of the Electrooxidation Processes of Au(hkl) Electrodes In Situ Research of the Adsorption and Hydrogenation Behavior of Ethyl Pyruvate (EP) at Pt(hkl) Electrode Surfaces by SHINERS The Quantitative Analysis of Temporal Changes by SHINERS Technique Summary and Prospective References Further Reading
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Introduction Surface reactions occurring at different solid/liquid or solid/gas interfaces have attracted significant attention within the basic research scientific community for decades. In particular, the surface reactions are gaining increasing attention, and they are of paramount importance for electrochemical interfaces studies which contribute immeasurably to the mechanistic understanding of catalytic processes at the molecular scale.1,2 The interfacial reaction involves some key factors that correlate with mass transfer, molecular adsorption, and charge transfer. Though conventional electrochemical techniques, such as cyclic voltammetry (CV) and rotating ring disk electrode technique, can provide important information about the interfacial reactions, it is just an average message from a macroscopic view. In essence, the key point to understand interfacial reaction is from molecular even atomic level to reveal its mechanism. Therefore, improved techniques must be introduced for investigating the interfacial electrochemical systems, in particular, the use of real-time spectroscopy methods offering information in a noninvasive manner. As a powerful “fingerprint” technique, Raman spectroscopy can not only provide information in a nondestructive manner about structures and properties of the interfacial elements but also adsorption configurations, orientations, and surface bonding of probe molecules at reaction surfaces.3–12 However, Raman spectroscopy lacks detection sensitivity due to the small Raman scattering cross section of analyte molecules, thus the normal Raman signals are usually very weak, which seriously limit the development and application of the Raman technique.13 In 1970s, the emergence of surface-enhanced Raman spectroscopy (SERS) technique aided in realization of an ultrahigh sensitivity down to the single-molecule level at noble metal substrate surfaces (Fig. 1A).5–9,14,15 Since the introduction of SERS effect, numerous experimental studies and theoretical researches have been carried out; which largely promoted the development of SERS technique.16–21 With the further understanding of SERS effect, it was found that SERS enhancement is mainly attributed to electromagnetic enhancement mechanism (EM).22,23 As a key content of SERS effect, electromagnetic field enhancement originates from surface plasmon resonance (SPR) of metal nanostructures.24,25 However, the shortcomings of SERS technique, as clear as its advantages, include the lack of materials and morphological generality, which has significantly restricted the practical application of SERS in variety of fields. The limitation becomes prominent when SERS is extended to study the spectroelectrochemistry of single crystal surfaces which are usually employed in electrochemical catalysis and surface science. Long-time significant research efforts devoted to SERS indicated that only a few noble metal materials, such as gold (Au), silver (Ag), copper (Cu), and few alkali metals yield a strong SERS effect. However, unfortunately, most of normal metals, in particular transition metals, which are extensively used in catalytic field, do not exhibit obvious SERS effect. Moreover, as the SERS substrates must be rough at the nanoscale; therefore, SERS is not suitable to research the reactions at atomically flat single crystal surfaces, which commonly serve as research model in theoretical simulation systems.26 To overcome the long-standing limitation of SERS technique, significant research impetus has emerged to systematically explore the pathways to expand SERS technique at transition metal surfaces, which partly solved the substrate generality problem of SERS (Fig. 1B).27–32 In 2000, the invention of tip-enhanced Raman spectroscopy (TERS) technique converted the direct contact mode of SERS to a noncontact mode (Fig. 1C), which resulted in directly obtaining the Raman signals from different metal single crystal surfaces.15,33–35 Though TERS technique offers high spatial resolution, it is limited to large Raman cross-section molecules and special experimental equipment.36–38
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Fig. 1 Different work modes of Raman methods: (A) contact mode of SERS; (B) target molecules adsorbed on the Au core-transition metal shell NPs surfaces; (C) noncontact mode of TERS; and (D) shell-isolated work mode of SHINERS. Reproduced with permission Li, J. F.; Huang, Y. F.; Ding, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464(7287), 392–395. Copyright 2010, Macmillan Publishers Ltd.
In 2010, the invention of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)39 technique by our group helped in significantly overcoming the materials and morphological generality problems of SERS (Fig. 1D). In SHINERS method, the electromagnetic field of inner Au nanoparticles (Au NPs) of Au@SiO2 shell-isolated nanoparticles (SHINs) can greatly enhance the Raman signals of nearby molecules, and the outer inert SiO2 shells can efficiently isolate the signal amplifiers (core Au NPs) from the outer environment and preclude the Au NPs contact with the surface species/probe molecules.40,41 Simply speaking, we can directly obtain the Raman signals just by spreading SHINs on any surfaces with diverse compositions and morphologies. The SHINERS method, with higher sensitivity and lower technical threshold, has been widely applied in different fields, in particular, in electrochemical interfaces and surface science research fields.41–58 Therefore, herein we mainly focused on the basic principles and recent development in SHINERS method with the objective of introducing this novel technique in surface science fields.
Basic Principles of SHINERS Technique The first step in performing SHINERS experiment includes the preparation of high-quality SHINs for different requirements. Further, an appropriate characterization method and theoretical simulation can also provide important supporting information for the research system. Therefore, the basic principles and relevant contents of SHINERS are introduced in the following sections.
Preparation of SHINs For different experimental conditions, different types of SHINs are required. Noteworthy, the core NPs of SHINs should be SERS-active and the outer shell materials of SHINs should be stable in the research system. The most common core NP for SHINs is 55 nm Au NPs, and greater diameter of Au NPs ( 130 nm) can provide higher enhancement under 633 nm laser.59 As the SPR peaks of SHINs can be tuned by adjusting the size and shape of the core; therefore, different shapes and sizes of core NPs could also be prepared to tune their maximum SPR peak position.25,40,45,60–62 The core material of SHINs is another important factor for the enhancement. For example, Ag NPs usually exhibit stronger enhancement compared to the Au NPs with the same size, thus the Ag core SHINs is also a considerable choice for some weak interfacial researches. As EM of SHINs is a long range effect, it obviously increases with the decrease in the shell thickness; therefore, the inert shell of SHINs should be very thin (1–2 nm) for higher enhancement.59 The shell materials of SHINs should be stable and inert during the Raman test to forbid any electrochemical reactions or physical interferences. The reported shell materials of SHINs are silica (SiO2), alumina (Al2O3), titania (TiO2), and manganese dioxide (MnO2) (Fig. 2).40,54,63,64 We then introduced the detailed preparation process for normal Au@SiO2 SHINs. Au NPs (55 nm) solution was prepared by Frens method,65 and the SHINs were synthesized as follows40: Au NPs solution (30 mL, 55 nm) was added into a 100-mL round-bottom flask and stirred for 15 min at room temperature, following which 3-(aminopropyl)triethoxysilane solution (APTES, 400 mL, 1 mM) was added to it. Then Na2SiO3 solution (3.2 mL, pH 10.3, 0.54%) was added into the above-mentioned solution. Subsequently, after 3 min, the mixed solution was transferred to a 90 C bath and stirred for some time. The shell thickness of SHINs can be precisely controlled by changing the Au NPs seed concentration, heating temperature, reaction time, and pH of Na2SiO3 solution. After certain heating time, the hot SHINs solution was rapidly cooled down in an ice bath and centrifuged for two or three times, and then it was finally diluted to certain volume for characterization or Raman experiment.
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Fig. 2 The TEM images of different types of SHINs: (A) Au@SiO2 NP with a 55 nm Au NP core; (B) Au@SiO2 NP with a 120 nm Au NP core; (C) Au@SiO2 NP with Au nanocube core; (D) Au@SiO2 NP with Au nanorod core; (E) Ag@SiO2 NP with Ag NP core; (F) 55 nm Au@20 nm SiO2 NP; (G) Ag@TiO2 NP; (H) Ag@Al2O3 NP; (I) MnO2 shell; and (J) Ag2S shell NPs. Reproduced with permission from Li, J. F.; Huang, Y. F.; Ding, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464(7287), 392–395. Copyright 2010, Macmillan Publishers Ltd. Reproduced with permission from Li, J. F.; Tian, X. D.; Li, S. B.; et al. Surface Analysis Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nat. Protocols 2013, 8(1), 52–65. Copyright 2013, Macmillan Publishers Ltd. Reproduced with permission from Zhang, W.; Dong, J. C.; Li, C. Y.; et al. Large Scale Synthesis of Pinhole-Free Shell-Isolated Nanoparticles (SHINs) Using Improved Atomic Layer Deposition (ALD) Method for Practical Applications. J. Raman Spectrosc. 2015, 46(12), 1200–1204. Copyright 2015, John Wiley & Sons, Ltd. Reproduced with permission Lin, X. D.; Uzayisenga, V.; Li, J. F.; et al. Synthesis of Ultrathin and Compact Au@MnO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS). J. Raman Spectrosc. 2012, 43(1), 40–45. Copyright 2012, John Wiley & Sons, Ltd. Reproduced with permission from Lin, X. D.; Li, J. F.; Huang, Y. F.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Nanoparticle Synthesis, Characterization and Applications in Electrochemistry. J. Electroanal. Chem. 2013, 688, 5–11. Copyright 2013, Elsevier Inc.
Characterization of SHINs Obviously, the outer ultrathin yet pinhole-free shells of SHINs plays a key role during the experimental process. With the effective protection and isolation provided by outer SiO2 shell, some problems can be efficiently avoided. First, if outer inert SiO2 shell protection is not provided, some chemical species present in environment may interact with bare Au core or other metallic NPs, which directly interfere with the Raman signals (Fig. 3A).11,66 Second, attributed to different Fermi levels between substrate material and bare metallic NPs, charge transfer may occur between them, which then further disturbs the electronic structure of the research system.67,68 Last but more importantly, the bare NPs may interact with some target molecules, and this direct contact significantly affects the Raman signal because of the change in the molecule adsorption behaviors and electron density distribution during the
Fig. 3 The models of different interferences in bare Au NPs: (A) contact with adsorbed molecules or impurities; (B) charge transfer between Au NPs and the metal surface; (C) the interaction between bare Au NPs and probe molecules; and (D) No interferences due to SHINs. Reproduced with permission from Li, J. F.; Huang, Y. F.; Ding, Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464(7287), 392–395. Copyright 2010, Macmillan Publishers Ltd.
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contact process (Fig. 3C).69–71 The above-mentioned analysis clearly indicates that the shell-isolated mode can easily resolve the aforesiad problems just by coating an ultrathin inert SiO2 shell on the bare metallic NPs surface (Fig. 3D), and the chemical inert shell can also efficiently prevent the charge transfer, inadvertent adsorption, or photocatalytic reactions. SHINs can be characterized by transmission electron microscopy (TEM), CV, and surface-enhanced Raman test. TEM is an efficient method to characterize the size and the shell thickness of SHINs (Fig. 2); however, detection of pinholes present in the shell by TEM is extremely difficult, thus the electrochemical CV and Raman test methods can be employed. For the shell of SHINs with pinholes, a characteristic Au reduction peak is observed around 0.9 V (vs. SCE) in CV curves.39 However, detection of very small or tiny pinholes by CV method is still a challenge, thus a more sensitive-enhanced Raman spectral method using pyridine as a probe molecule has been introduced to examine the pinhole effect.40 Presence of pinholes in the SiO2 shell leads to the occurrence of two Raman peaks around 1011 and 1035 cm 1 because pyridine molecules get adsorbed on the surface of Au and interact with the Au core through pinholes. The above-mentioned peaks are assigned to n1 ring breathing mode and n12 symmetric triangular ring deformation mode of adsorbed pyridine molecules on Au NPs surfaces, respectively. Other than experimental approach, theoretical simulations of SHINs can also provide theoretical supports for SHINERS researches. The most common theoretical simulation method for SHINERS systems is three-dimensional finite-difference time-domain (3D-FDTD) modeling, which can be efficiently used to test the local electromagnetic fields of SHINs on smooth metal substrate surfaces.59,72–76
Application of SHINERS in Electrochemical Surface Science The significant advantages of SHINERS technique compared to SERS and TERS are as follows: SHINERS can not only directly provide the Raman signals from any substrates and any materials surfaces but also with high sensitivity and under simple operating conditions. Moreover, owing to the protection provided by inert SiO2 shell, the SHINs do not participate in or disturb the outer reactions, thus maintaining the reliability of Raman signals. The SHINERS technique can be effectively applied in different fields, in particular, in electrochemical surface science research. In this section, the recent development in SHINERS technique in surface science field based on the concrete application examples is illustrated. Compared to other spectroscopic methods, the most successful application of SHINERS is its applicability at single crystal surface.39,43,50,53,77,78 Single crystals with atomically flat surfaces, well-defined structures, and well-known electronic levels have attracted special attention of surface scientists. In early time, several spectroscopic methods were employed to study the reactions at single crystal surfaces, such as surface-enhanced infrared reflection absorption spectroscopy (SEIRAS),79,80 X-ray photoelectron spectroscopy (XPS),81–83 and so forth; however, these methods were restricted by water or requirement of special experiment conditions or equipment. Though SERS is a powerful technique, it is difficult to directly obtain the Raman signals from single crystal surfaces.84,85 TERS with high spatial resolution and high sensitivity of single molecules detection can be employed to study the reactions at single crystal surfaces; nonetheless, the total Raman signals of TERS are too weak and the tip gets easily contaminated by outer air or solution environment, which may yield misleading information. Owing to strong electromagnetic field enhancement through “shell-isolated” working mode, SHINERS technique has been employed to research the adsorption and orientation of probe molecules and catalytic processes at different types of single crystal surfaces. Furthermore, the SHINERS technique can also be explored for researches in some challenging fields, such as corrosion resistance research and surface quantity analysis.
In Situ Monitoring of Pyridine Behaviors at Au(hkl) Electrode Surfaces by SHINERS Technique Pyridine molecule plays an important role in the Raman research fields, and the first SERS effect was discovered through obtaining high-quality Raman signals of pyridine molecules at Ag electrode surface.5 Our group employed electrochemical SHINERS (EC-SHINERS) technique to research the electrochemical adsorption behaviors of pyridine at Au(hkl) electrodes in NaClO4 solution. Furthermore, the crystallographic orientation of electrode surfaces under applied potential was investigated.78 In general, different surface coverage rates and adsorption behaviors of target molecule at electrode surface usually correlate with different Raman peaks. Considering n1 ring breathing mode of pyridine molecule at Au(111) surface as an example, the correlated Raman peak was at around 1011 cm 1. Though the Raman intensity and frequency of n1 ring breathing mode around 1011 cm 1 both increased following positive scanning direction, they exhibited different enhancement rates between low potential range and high potential range (Fig. 4). At high potential range, the Raman frequency increased rate was more obvious than that at the low potential and with a 5.6 cm 1/V Stark tuning rate, which clearly illustrated the variation in the adsorbed orientations or structures of pyridine molecules at electrode surface at high potential range compared to that at low potential range. If pyridine was flat adsorbed at Au electrode surface, there should be a weak binding interaction between the Au electrode and p-orbital electrons of pyridine molecules, thus leading to a weak Raman signal. If pyridine was vertically adsorbed at the Au electrode surface, there should be a strong interaction between the Au electrode and the nitrogen atom lone pair of electrons of pyridine, thus leading to a strong Raman signal. Based on this information and the potential Raman intensity/frequency relationship of SHINERS data, it could be concluded that pyridine molecules were flat-adsorbed at the Au(111) electrode surface at low potential range, and vertically adsorbed at the Au(111) electrode surface at high potential range, which correlated well with the previous related researches that a monolayer of pyridine molecules is formed at Au(111) electrode in higher potentials range.86,87 Besides Au(111), Au(110), and Au(100) also exhibited the occurrence of same phenomenon, and the potentials trend order of pyridine monolayer
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Fig. 4 (A) CVs of pyridine (1 mM) in NaClO4 (0.1 M) solution at Au(hkl) electrodes and (B) SHINERS results of pyridine at Au(111) electrode surface. The relationship of (C) Raman frequency and (D) normalized Raman intensity with potentials for n1 ring breathing mode around 1011 cm 1, and combined with surface concentration isotherms (bold curves). Reproduced with permission from Li, J. F.; Zhang, Y. J.; Rudnev, A. V.; et al. Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy: Correlating Structural Information and Adsorption Processes of Pyridine at the Au(hkl) Single Crystal/Solution Interface. J. Am. Chem. Soc. 2015, 137(6), 2400–2408. Copyright 2015, American Chemical Society.
completion at three low-index Au(hkl) surfaces was same as the potential of zero charge (PZC) order of three single crystal surfaces, that is, Au(111) > Au(100) > Au(110). From this perspective, it can be considered that the surface charge of electrode surfaces plays a key role in the electrochemical adsorption process. The n1 ring breathing mode intensity decreased in the following order: Au(110) [ Au(100) > Au(111). In this study, the SHINERS results and the electrochemical behaviors correlated well. Thus, the SHINERS technique is an efficient method to investigate the behaviors of adsorbed molecules at different electrochemical interfaces.
Real-Time Research of the Growth Process of Benzotriazole Film at Single Crystal Cu(hkl) and Polycrystalline Cu(poly) Electrodes Surfaces by SHINERS Technique Benzotriazole (BTAH) is widely used for polishing and plating purposes to prevent Cu and relevant alloys from corrosion because BTAH can form a coordination polymer film on the surface of Cu to prevent its oxidation.88 Gewirth’s group employed SHINERS technique for real-time investigation of processes involving formation of BTA film at Cu(hkl) and Cu(poly) electrode surfaces.50 Fig. 5 clearly exhibits the peak around 1020 cm 1 which is assigned to benzene skeletal and CeH bend, and the peak around 1190 cm 1 is attributed to triazole ring breathing mode (CeCeC in-plane bending). During the positive scanning process at Cu(hkl) electrode surfaces, the Raman intensities of both the above-mentioned peaks increased; however, the similar phenomenon at Cu(poly) electrode surface was not observed during the same scanning process. Furthermore, the peak intensities ratio of 1190/ 1140 increased with the increase in the potential at Cu(100) and Cu(111) electrodes surfaces. The peak at 1140 cm 1 is assigned to NeH bending mode of adsorbed BTAH at Cu electrode surfaces. However, SHINERS results indicated different interpretation for both single crystal surfaces. The intensities ratio corresponding to 1190/1140 stopped increasing later for Cu(111) electrode surface compared to Cu(100) at the negative scanning direction, and the correlated stop potentials were 0.3 and 0.2 V, respectively. During the negative scanning direction at Cu(poly) electrode surface, the SHINERS results again exhibited different phenomena compared to the single crystal electrode surface, thus indicating different growth behaviors for the BTA-Cu(I) film formation at different Cu electrodes surfaces. The film formation was found to be irreversible at single crystal Cu(hkl) electrodes, while it was reversible at Cu(poly) surfaces. The systematic research reveals that different crystallographic orientation of Cu(hkl) electrodes surfaces, with evident effect on BTA-Cu(I) film growth, and the presence of grain boundaries together lead to the cathodic degradation of BTA-polymeric films at Cu(poly) electrode.
Real-Time Monitoring of the Electrooxidation Processes of Au(hkl) Electrodes The formation of oxide during the electrocatalytic processes significantly affects the function of catalyst. Therefore, it is important to reveal the electrocatalytic mechanisms for the effective designing and preparation of efficient new catalysts.89,90 Although the conventional spectroscopy methods are used to study the catalytic reactions in electrochemical systems, difficulties are encountered
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Fig. 5 (A) SHINERS results of 0.75 mM BTA in 0.1 M H2SO4 solution during positive scan direction (left group images) and negative scan direction (right group images) at Cu(poly) and Cu(hkl) electrode surfaces and (B) the 1190/1140 bands intensities ratio for different Cu electrodes under different potentials. Reproduced with permission from Honesty, N. R.; Gewirth, A. A. Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) Investigation of Benzotriazole Film Formation on Cu(100), Cu(111), and Cu(poly). J. Raman Spectrosc. 2012, 43(1), 46–50. Copyright 2012, John Wiley & Sons, Ltd.
in observing the key intermediates species because of the complexity of in situ researches. Thus it is still a huge challenge to use an in situ spectroscopy method to study the reaction mechnisms in electrochemical systems.91,92 The SHINERS technique provides an efficient pathway for in situ monitoring of the electrocatalytic reactions processes at single crystal electrodes. Our team employed EC-SHINERS technique for in situ investigation of the electrooxidation processes of Au(hkl) electrodes and found direct in situ spectral evidences for reaction intermediates.77 Fig. 6 shows the absence of obvious Raman peaks at initial potential in the range of 300–800 cm 1. At positive scanning direction, appearance of an obvious Raman peak around 790 cm 1 is observed. The intensity of this peak continues to increase until 0.5 V, then decreases, and almost disappears at 0.9 V. The peaks around 790 cm 1 are assigned to the top site gold–hydroxide (Au OH) bending mode (dAuOH) at Au(hkl) electrode surfaces. Following the diappearance of Au OH bending mode, the Au O stretching mode (nAu–O) around 570–590 cm 1 becomes more and more obvious with the continuous increase in the potential. The different influencing factors, such as crystallographic orientation, anions, and pH of the electrooxidation system at Au(hkl) single electrode surfaces, were carefully investigated. The SHINERS results indicated that the Au OH bending mode intensities increased in the following order: Au(100) Au(110) < Au(111), which is opposite to the oxygen reduction reaction activity order of three low-index Au(hkl) electrodes under the same conditions. This should be attributed to the fact that OH formed at Au(hkl) electrode surfaces retards the oxygen reduction reaction.91,93–95 Different pH of the electrooxidation system also leads to different SHINERS results. When the pH of the system was too high (pH 11), the peak intensity of AueOH bending mode was weaker compared to that at the pH 9 condition. Furthermore, under the acidic condition, the AueOH bending mode almost disappeared, which indicated that the electrooxidation process of Au(hkl) electrode surfaces under conditions of different pH possibly followed different mechanistic pathways. Moreover, the presence of different anions in the the electrolye also leads to different phenomena, and the SHINERS results also correlate well with the previous studies. The present study clearly indicates that the SHINERS technique offers a unique opportunity for many researchers to directly study the catalytic reactions at well-defined metal electrode surfaces through a real-time pathway in surface catalysis fields, which can definitely open the door for technological innovations in surface catalysis.
In Situ Research of the Adsorption and Hydrogenation Behavior of Ethyl Pyruvate (EP) at Pt(hkl) Electrode Surfaces by SHINERS Weak coupling effect between SHINs and platinum hinders the enhancement of SHINs and correlated Raman intensities obtained from platinum surface is usually weakened by several magnitude orders compared to Au surfaces under the same conditions.43 Recently, Attard et al. employed EC-SHINERS technique to investigate the adsorption behavior and hydrogenation of ethyl pyruvate (EP) at Pt(hkl) electrode surfaces.96 They observed two types of reaction intermediates during the hydrogen evolution potential range, that is, the half-hydrogenation state (HHS) obtained by addition of a hydrogen atom to the keto carbonyl group and the intact chemisorbed EP bound in a m2(C,O) configuration. Based on the SHINERS results, they employed density functional theory (DFT) calculation method to analyze the adsorption of EP at Pt(hkl) surfaces. The m2(C,O) configuration of EP was observed at Pt(100) and Pt(111) electrodes surfaces; however, HHS was not observed until the Pt(hkl) electrodes surfaces were roughened by electrochemical methods.
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Fig. 6 (A) The SHINERS results of in situ electrooxidation processes at different Au(hkl) electrode surfaces in NaClO4 solution (0.1 M, pH 9); (B) normalized Raman peaks intensities of nAu–O and dAuOH at different potentials; and (C) the models of OH species adsorbed at three basic Au(hkl) electrodes. Reproduced with permission from Li, C. Y.; Dong, J. C.; Jin, X.; et al. In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2015, 137(24), 7648-7651. Copyright 2015, American Chemical Society.
The Quantitative Analysis of Temporal Changes by SHINERS Technique It is well known that quantitative analysis is a huge challenge encountered during plasmon-enhanced Raman spectroscopy studies. As a novel technique, SHINERS method has already been utilized in surface analysis fields. Gold-thiosulfate leaching process is inhibited by the formation of the passive layer at Au electrode surfaces which can prevent the dissolution of Au from ore samples. Lipkowski’s group employed SHINERS technique for quantitative analysis of temporal changes in the passive layer at Au material surfaces in thiosulfate solution.57 In this study, the APTES band of SHINs was used as internal standard to compensate the surface enhancement fluctuation. After subtracting the SHINERS spectra in NaF solution, the APTES band could be efficiently removed and the corrected Raman spectra correlated well with the relevant SERS results. The SHINERS spectra for long immersion times in thiosulfate electrolyte with several new bands compared to the spectra recorded at shorter immersion times (Fig. 7A), provided a good chance to correlate the quantitative changes in Raman bands intensity with the rate of Au leaching in thiosulfate solution. Clearly, during the first 50-min immersion time, the Au leaching rate rapidly decreases and then gradually becomes down to an almost linear decay following the time passed. Fig. 7B exhibits the relationship between the Raman peaks and the immersion time after correction of the backgrounds, and normalized Raman intensities of 382, 316, and 460 cm 1, correspond to [Au(S2O3)2]3 complex, adsorbed sulfide, and polymeric sulfur species at the electrode surfaces, respectively. The peak intensity of [Au(S2O3)2]3 complex around 382 cm 1 first decreases rapidly during the initial immersion time and then decreases gradually with longer immersion time, which correlated well with the leaching current. However, the adsorbed sulfide peak at 316 cm 1 and the polymeric sulfur species peak at 460 cm 1 exhibited different phenomena. The peaks intensities for both sulfide and polymeric sulfur species increased with the increase in the immersion time. The SHINERS result clearly provided the quantitative evidences that the formation of the elemental sulfur species could efficiently prevent Au leaching in thiosulfate solutions. This should be primarily attributed to the fact that the adsorbed sulfur species occupied the Au electrode surface and blocked the interaction
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Fig. 7 (A) Raman results of SHINs/Au electrode after being immersed in 0.1 M Na2S2O3 solution (pH 10.0) between 300 and 600 min; (B) the normalized integrated band intensity as a function of immersion time, IDt, of adsorbed sulfide (316 cm 1), [Au(S2O3)2]3 complex (382 cm 1), and polymeric sulfur (460 cm 1). Reproduced by permission from Smith, S. R.; Leitch, J. J.; Zhou, C.; et al. Quantitative SHINERS Analysis of Temporal Changes in the Passive Layer at a Gold Electrode Surface in a Thiosulfate Solution. Anal. Chem. 2015, 87(7), 3791–3799. Copyright 2015, American Chemical Society.
between Au surface and thiosulfate, eventually preventing the [Au(S2O3)2]3 complex formation. This methodological advancement allows us to monitor the temporal changes in the passive layer composition at Au electrode surface through correlating the Au dissolution rate with relevant Raman bands, and then to further identify the formation mechanism of passivating species. Overall, SHINERS technique can be easily employed in anticorrosion research at different substrates for qualitative and quantitative analysis. Noteworthy, the strategy of quantitative SHINERS analysis method at different electrode surfaces species undeniably provides a potential pathway for fundamental researches and practical applications.
Summary and Prospective As a novel-enhanced spectroscopy technique, SHINERS technique breakthrough the material and morphology generality limitations of SERS technique, thus making it possible to obtain the Raman signals from any substrates and materials surfaces. Moreover, the SHINERS experiment, with low technical restriction and easy operation, is very conducive to be applied in different surface science fields. In the above-mentioned contents, we tried to use the simplest and easily understandable language to introduce the SHINERS technique development history, basic principles, and relevant contents. Further, we introduced in detail several representative application examples of SHINERS in surface science fields. The applications not only include the probe molecules adsorption studies and the real-time catalytic reactions monitoring at single crystal electrodes but also the quantitative analysis in practical application systems. Moreover, the shell-isolated mode of SHINERS technique can be explored to other areas as well by integrating this technique with other research methods, such as infrared absorption spectroscopy (IR), fluorescence spectroscopy, sum frequency generation, and tip-enhanced spectroscopy.97–100 Though the normal SiO2 shell of SHINs can meet most of requirements, it does not remain stable in solution with high pH (pH > 12) for long immersion time. Furthermore, thiner shell of SHINs offers higher enhancement; therefore, the shell of SHINs should be very thin down to 1–2 nm; however, pinholes may appear when the shell becomes ultrathin. Importantly, a lot more systematic explorations are demanded to improve the SHINs preparation methods and explore new shell materials. Overall, the achievements of SHINERS technique in the past few years are encouraging. There will be numerous emerging new applications of SHINERS in different surface science fields following the rapid development in nanotechnology. The existing developments and huge potential of SHINERS technique indicate that the SHINERS technique and the shell-isolated mode must become a powerful tool for practical applications and fundamental researchs in surface science fields, and widespread acceptance of SHINERS can come true in the near future.
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Further Reading Ding, S. Y.; Yi, J.; Li, J. F.; et al. Nanostructure-Based Plasmon-Enhanced Raman Spectroscopy for Surface Analysis of Materials. Nat. Rev. Mater. 2016, 1, 16021. Dong, J. C.; Panneerselvam, R.; Lin, Y.; Tian, X. D.; Li, J. F. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy at Single-Crystal Electrode Surfaces. Adv. Opt. Mater. 2016, 4 (8), 1144–1158. Ozaki, Y., Kneipp, K., Aroca, R., Eds. Frontiers of Surface-Enhanced Raman Scattering: Single Nanoparticles and Single Cells, Wiley: Chichester, 2014.