Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive analytical technique based on Raman scatter and utilizes the nanostructures of active metals, such as gold and silver, with roughened surfaces as a signal amplifier. With its enhancement effect and “fingerprint” ability, in situ SERS is able to capture the dynamics of microstructure evolution and trace surface species in real time, which provides direct information for the analysis of a reaction mechanism in various surface processes, including heterogeneous catalysis, electrochemical reactions, etc. SERS has been widely used in operando analysis of surface processes under ordinary temperatures. For application in high-temperature processes, the harsh environment puts forward additional requirements in addition to high sensitivity for the SERS nanostructures, especially concerning thermal stability, chemical inertness, and surface universality.
1. Introduction
SERS has drawn huge interest in trace analysis and chemical reaction mechanism analysis due to its high detection sensitivity, low demanding conditions, and abundant surface information, among others [
1,
2,
3,
4,
5,
6]. Despite unique advantages of SERS, materials exhibiting high SERS activity is only limited to a few nano-rough coinage metals, typically Au, Ag, and Cu, because of their unique plasmonic properties [
7]. As a result, studies on SERS-active material substrates progressed easily early on, but major challenges arose when scientists tried to extend SERS to other substrates. Therefore, numerous research efforts were devoted to improving enhancement capabilities of other substrates, generally by introducing SERS-active nanostructures, or Raman amplifiers, to the desired substrates, which is also referred to as the “borrowing strategy” [
8,
9]. In 1983, researchers had managed to deposit discrete Ag nanoparticles on the surface of n-GaAs, a SERS-inert material, and successfully observed Raman signals of probe molecules absorbing on the SERS-inert material [
10,
11,
12]. However, the shortcoming of the method relates to the miscellaneous peaks that stem from the interaction of Ag and the probe molecules, since Ag is a highly chemically active material. Secondly, charge transfers between the electrode materials and Raman amplifiers would also bring interferences to the Raman spectra if the coinage metal is directly deposited on the surface of the materials to be characterized.
So far, SERS has been widely used in ordinary temperature conditions, such as trace molecular detection, electrochemical adsorption, catalysis, etc. SERS technology can detect molecules in ultralow concentration, and even a single molecule [
29,
30,
31,
32]. In the field of electrochemical reactions, SERS technology can be used to study the adsorption and desorption behavior of molecules and detect the chemical reaction on the electrode surface in real time [
33,
34]. In the field of catalysis, SERS can be used to obtain direct information on the evolution of catalyst surface species and local structure during the reaction processes [
35,
36].
2. Design of Nanostructures for High-Temperature SERS
2.1. Substrate-Specific Nanostructures
2.1.1. Layer Deposition
The layer deposition technique suggests that the thicker the protective layer (generally SiO2 or Al2O3), the lower the SERS activity exhibited when compared to bare metal nanostructures, which means that thermal robustness is improved at the cost of SERS activity. On the other hand, the coating of protective layer on the SERS-active nanostructures and the substrate limits the studies to SiO2- and Al2O3-related systems [45,47]. Nevertheless, it is still a milestone improvement since more SERS activity is retained for the protected SERS-active nanostructures compared to their unprotected counterparts after the heating treatment.
2.1.2. Entrapped Nanorods
To stabilize Raman amplifiers without the presence of protective capping layer, Du et al. have managed to entrap Au nanorods (Au NRs) in anodized aluminum oxide (AAO) scaffolds, which make full use of the substrate to offer thermal protection and can be seen as a partial but ultrathick protective layer. The EF of the nanostructure is two to three orders of magnitude for Rhodamine 6G, which shows no significant change after 800 °C treatment for 1 h [
49]. It is also worth noting that Au exhibits great chemical-inertness even at high temperature, while Ag exhibits very limited resistance to oxidation [
50]. As a result, partial exposure of Au under high temperature could cause far less interference to the reaction system. The strength of the method is that it is capable of exhibiting relatively high SERS activity at ultrahigh temperature, compared to previously described layer deposition methods. However, for as-prepared samples that exhibit excellent performance, scaffold preparations cover or even deconstruct the interface of the samples so as to entrap Au NR, which unavoidably interferes with the analysis and limits the adoption of this approach to create the scaffold-related materials. Moreover, partial exposure of Au NR is likely to introduce a miscellaneous peak to the Raman spectra. Both of these could hinder the extension of the method to broad material systems.
2.2. Substrate-Universal Nanostructures
2.2.1. Core–Shell Nanostructures
To extend the substrate universality, researchers have managed to coat Raman amplifiers with a protective layer before it is deposited on the substrate. Weckhuysen et al. synthesized Au@SiO
2 and Au@TiO
2 shell-isolated nanoparticles, respectively, typically with the diameter of Au spheres being 76 nm and thickness of protective shell being 1–3 nm [
51]. Both Au@SiO
2 and Au@TiO
2 nanoparticles with a 2.2–2.7 nm protective layer exhibited an enhancement factor of four orders of magnitude for Rhodamine 6G after 400 °C treatment for 3 h, which was less than one order lower compared to their unheated counterparts. The stability of Au@SiO
2 and Au@TiO
2 nanoparticles with a 1.2–1.3 nm protective layer were found insufficient above 300 °C. When the temperature was raised to 500 °C, all the structures collapsed.
To extend SERS to even higher temperature, Liu et al. [
43] developed Ag@SiO
2 core–shell nanoparticles, with 60 nm Ag spheres being the core to offer SERS activity and SiO
2 shell being 10 nm thick to offer thermal stability, which maintained more than one third of its original enhanced capabilities after heating treatment at 500 °C for 30 min. However, the EF of the as-prepared Ag@SiO
2 nanoparticles was calculated to be around 150 for Rhodamine 6G due to the exponential decay of SERS activity with the increased contact distance from Ag core to probe molecules [
43].
2.2.2. Embedded Core–Shell Nanostructure
Recently, a group developed a novel hybrid nanostructure of embedded Ag nanoparticles confined by core–shell silica nanospheres, denoted as ESCNs [55]. The approach is to plant ~30 nm Ag nanoparticles on the surface of the ~500 nm SiO2 core first, and then deposit 3~5 nm SiO2 shell afterward. The SiO2 core–shell architecture functions as a thermally robust framework for embedded Ag nanoparticles due to the nanoconfinement effect. With introduction of SiO2 as thermally robust core and decreased thickness of outermost SiO2 shell, the EF of ESCNs for Rhodamine 6G reaches 4.45 × 104, 1 × 104, 69 after 400 °C, 450 °C, 500 °C heat treatment for 2 h, respectively. In addition, it is revealed by 3D-FDTD (three-dimensional finite-difference time-domain) calculation that the self-assembled Ag nanoparticles that embedded in the SiO2 core–shell consist of plasmonic “hot spots”, which contribute a strong enhancement on individual ECSNs. The theoretical calculation is found to be in good agreement with the experimental observation of ESCNs with different Ag embedding. Therefore, the SERS activity of the ECSNs does not depend on assembling dimers or trimers, as required in previously reported approaches.
3. Application of SERS Nanostructure in High-Temperature Processes
3.1. Structure Evolution
3.1.1. Thermal Crystallization
The in situ analysis of lattice structure evolution could provide direct information for understanding thermal crystallization. The capability of high-temperature Raman spectra to monitor microstructural changes in melt makes it an effective tool to study the structure of melt at high temperature and provide information to expand the understanding of crystal growth. Sobol et al. studied the microstructure of Li2O-B2O3-MoO3 using high-temperature in situ Raman spectroscopy [56].
In situ high-temperature SERS can monitor the phase development of crystals during heat treatment with enhanced sensitivity, which could provide abundant information for the study of the thermal crystallization. Tri (8-hydroxyquinoline) aluminum (Alq3) film is one of the most widely used electron transfer and emission materials for organic photodiodes (OLEDs) [58]. However, the crystallization of Alq3 film and the increase in surface roughness leads to the degradation of OLED [59].
3.1.2. Interfacial Interaction
The ability for high-temperature SERS to observe interfacial interactions is of general significance for catalysis, as interfacial bonds play a key role in catalytic reactions. Dai et al. utilized robust SERS nanostructures to study in situ the effects of heating a solid acid, phosphotungstic acid (PTA), on an alumina surface while operating at elevated temperatures in various atmospheres [
61]. The robust SERS nanostructures were generated by depositing an ultrathin protective coating (1.2 nm) of alumina on top of silver NWs via ALD. A thin layer of PTA was subsequently decorated on top of the nanostructures. The PTA powder on the silicon chip was tested to initialize the in situ Raman analysis. Raman bands at 1015 and 992 cm
−1 were observed, which were attributed to the ν
s (W=O) and ν
as (W=O), respectively, while bands at 935 and 890 cm
−1 were attributed to bridging ν
as (W-O-W) modes. When PTA was loaded onto SERS substrates covered with an Al
2O
3 protective layer, differences arose. Band intensities of ν
s (W-O) and ν
as (W-O) reversed, which corresponded to the analysis results of PTA adsorbed onto bare Ag colloids, which suggested that the orientation of PTA on the Al
2O
3 surface was likely similar to that on the Ag surface. When the analysis was performed in H
2 atmosphere at 400 °C, the intensity of the 890 cm
−1 band increased.
3.1.3. Lattice Dynamics
High-temperature SERS has been applied to monitor the lattice evolution of CeO
2 in reducing and oxidizing atmospheres. CeO
2 is an important functional material for various applications, such as heterogeneous catalysis, solid oxide fuel cells, and functional ceramics [
62]. The functionality of CeO
2 depends greatly on the oxygen storage and release capability, which is endowed by the generation and consumption of a key lattice defect, oxygen vacancies. The in situ observation of the lattice dynamics related with oxygen vacancies is critical to reveal the mechanism of CeO
2 functionality as either a structure stabilizer or electronic promotor in wide applications.
3.2. Heterogeneous Catalytic Reaction
3.2.1. Ethylene Epoxidation
Raising temperatures can significantly improve reaction kinetics. Although the reaction energy barrier can be lowered by using catalysts, high temperature is still necessary for a large number of catalytic reactions to proceed effectively. SERS can be used to study the heterogeneous reaction in catalytic systems, which can provide valuable insights for understanding the active sites and reaction mechanisms. Kondarides et al. utilized in situ SERS to follow the changes in adsorption state of oxygen and ethylene epoxidation on Ag catalyst surfaces at 1 atmosphere and temperature between 25 and 400 °C [
63]. Interactions and reaction mechanisms were carefully examined in three types of silver catalysts: (I) Ag supported on α-Al
2O
3, (II) Ag films deposited on SiO
2, and (III) Ag films deposited on the oxygen ion conducting solid electrolyte material, Y
2O
3-stabilized ZrO
2 (YSZ). For the Ag/YSZ catalyst, Raman spectra were obtained after the catalyst was heated to 300 °C and then cooled to room temperature in oxygen gas flow. Raman bands at 240, 345, 815, 870, and 980 cm
−1 were observed, for which 815, 870, and 980 cm
−1 were ascribed to the ν (O-O) vibrations of adsorbed molecular oxygen, while 240 and 345 cm
−1 were ascribed to Ag-O vibrations of Ag-O
2 interactions.
3.2.2. NO Reduction
Williams et al. utilized in situ SERS to monitor the exact reactive nature of surface species during reduction of NO by CO on Pt and Pd [
3]. The SERS architecture was prepared by electrodepositing ultrathin transition-metal films onto electrochemically roughened gold. When the Pt surface was exposed to an equivalent mixture of NO and CO at room temperature, SERS spectra showed bands at 240, 325, 470, 640, 2080, 2160, and 2230 cm
−1, for which the 240 and 470 cm
−1 bands were attributed to Pt-NO stretching and Pt-N-O bending modes, respectively; the 325 cm
−1 band was attributed to the Pt-NO stretch of NO adsorbed on bridge sites; the 470 and 2080 cm
−1 bands were assigned to the Pt-CO and C-O stretches of terminally adsorbed CO, and the 2160 and 2230 cm
−1 bands were attributed to reaction products of CO and NO.
3.2.3. CO Oxidation
The oxygen storage capacity of CeO2, which is the ability to accommodate a large number of oxygen vacancies in reducing atmospheres (becoming CeO2−x) without structural destruction of the fluorite ceria lattice and could be oxidized back to CeO2 in oxidizing atmospheres, is widely acknowledged as one of the reasons for improving metal/CeO2 catalyst activity. Catalytic tests showed that the reaction rate of CO oxidation on Pt/CeO2 was enhanced by about two orders of magnitude compared to Pt/SiO2. Li et al. designed Au@Pt-CeO2 satellite nanostructure to achieve in situ SERS investigations of oxygen activation and reaction at Pt-CeO2 interfaces [67]. The in situ SERS spectra of CO oxidation on Au@Pt-CeO2 showed a Raman band at ~2090 and ~2110 cm−1, which were ascribed to CO adsorbed on Pt surfaces and Pt-CeO2 interfaces, respectively.
3.3. High-Temperature Electrochemistry
3.3.1. Molten Carbonate Electrolysis
Hu et al. employed Raman spectroscopy to investigate the solubility of CO2 in the molten Li2O-LiCl system with 4–8 wt% Li2O at 873–973 K [68]. When CO2 was dissolved in LiCl-Li2O melts, CO2 interacted with O2− to produce CO32−, and the three species were at equilibrium when the temperature was held constant. For quantitative analysis of Raman spectroscopy, 13.33 wt% Li2SO4 was used as the external standard. Characteristic bands of SO42− and CO32− were obtained at 988 and 1074 cm−1, respectively. After baseline subtraction and bandfitting to the Lorentzian equation, the area of the two bands for each spectrum could be attained, which was further utilized to study the relationship between the conversion rate of Li2O to Li2O3 and the concentration of Li2O in the Li2O-LiCl system, along with the relationship of CO2 solubility and temperature in the Li2O-LiCl system. Experimental results indicated that the solubility of CO2 increases with Li2O concentration in the Li2O-LiCl system, and it reached a maximum when the Li2O concentration was 8 wt% at 923 K.
3.3.2. Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFCs) operating at around 600–1000 °C have the advantages of high energy conversion efficiency, low pollutant emission, and great fuel flexibility [72,73,74,75]. However, the practical application of SOFC has been limited by the degradation issue, in which the adoption of high-temperature SERS could provide important information by monitoring the evolution of the electrode surfaces related to the formation of undesirable phases and species. For example, nickel-based anodes are susceptible to carbon deposition when hydrocarbons are used as fuel. Liu et al. used Ag@SiO2 as Raman amplifiers to study the deposition and removal of carbon on nickel-based anodes through in situ SERS [76]. Ag@SiO2 nanoparticle effectively increased the sensitivity toward trace analysis of carbon species, therefore making the tracking of initial carbon deposition feasible. The initiation of coking was found at the exposure of the first 20 mL propane, and carbon deposition was accumulated gradually with continuous exposure. The in situ SERS analysis also revealed that the introduction of CeO2 increased the resistance of coking on Ni due to surface modification. The in situ SERS study of the kinetics of the initial carbon deposition provided insights into the intrinsic coking tolerance of the material surface. Additionally, fast degradation of cathodes for SOFC due to poisoning by contaminants usually existing in air, such as CO2, is one of the major challenges for the development of high-performing cathodes for SOFC.
4. Summary
For structure design of high-temperature SERS, two types of strategies have been reported and categorized, which are substrate-specific and substrate-universal nanostructures. The substrate-specific nanostructures, prepared by layer deposition or entrapped metal NR, generally exhibit great thermal robustness with considerable SERS activity. However, this type of nanostructure could only apply to a minority of substrates due to the interference on the sample surfaces. The substrate-universal nanostructures, including core–shell nanostructures and embedded core–shell nanostructure, could apply to a wide range of substrates due to their “first assemble, then deposit” strategy. It is noted that when the thickness of protective layer is held constant, the substrate-universal nanostructures do not exhibit as great a thermal robustness as substrate-specific ones due to the higher surface energy of the former. This challenge is largely resolved by the introduction of a thermal-robust core in embedded core–shell nanostructure. For high-temperature applications, the thermal crystallization properties and local structural changes in the materials during heating treatment have been studied by employing specialized SERS nanostructures. For high-temperature catalytic reactions, the strength of in situ SERS analysis has also been demonstrated to a great extent. For example, with the assembling of “satellite” nanocatalysts around the SERS nanostructure, the reaction mechanism of CO oxidation is investigated by in situ monitoring the characteristic peak during the reaction process. In the field of high-temperature electrochemical reactions, thermally robust SERS nanostructures have been employed to identify the key intermediates in molten carbonate electrolysis and solid oxide fuel cells.
This entry is adapted from the peer-reviewed paper 10.3390/chemosensors11010021
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