Practical Guidelines for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy of Heterogeneous Catalysts

Impact of Surface Enhanced Raman Spectroscopy in Catalysis

Catalysis stands as an indispensable cornerstone of modern society, underpinning the production of over 80% of manufactured goods and driving over 90% of industrial chemical processes. As the demand for more efficient and sustainable processes grows, better catalysts are needed. Understanding the working principles of catalysts is key, and over the last 50 years, surface-enhanced Raman Spectroscopy (SERS) has become essential. Discovered in 1974, SERS has evolved into a mature and powerful analytical tool, transforming the way in which we detect molecules across disciplines. In catalysis, SERS has enabled insights into dynamic surface phenomena, facilitating the monitoring of the catalyst structure, adsorbate interactions, and reaction kinetics at very high spatial and temporal resolutions. This review explores the achievements as well as the future potential of SERS in the field of catalysis and energy conversion, thereby highlighting its role in advancing these critical areas of research.

Alternative nano-lithographic tools for shell-isolated nanoparticle enhanced Raman spectroscopy substrates

Chemically synthesized metal nanoparticles (MNPs) have been widely used as surface-enhanced Raman spectroscopy (SERS) substrates for monitoring catalytic reactions. In some applications, however, the SERS MNPs, besides being plasmonically active, can also be catalytically active and result in Raman signals from undesired side products. The MNPs are typically insulated with a thin (∼3 nm), in principle pin-hole-free shell to prevent this. This approach, which is known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), offers many advantages, such as better thermal and chemical stability of the plasmonic nanoparticle. However, having both a high enhancement factor and ensuring that the shell is pin-hole-free is challenging because there is a trade-off between the two when considering the shell thickness. So far in the literature, shell insulation has been successfully applied only to chemically synthesized MNPs. In this work, we alternatively study different combinations of chemical synthesis (bottom-up) and lithographic (top-down) routes to obtain shell-isolated plasmonic nanostructures that offer chemical sensing capabilities. The three approaches we study in this work include (1) chemically synthesized MNPs + chemical shell, (2) lithographic substrate + chemical shell, and (3) lithographic substrate + atomic layer deposition (ALD) shell. We find that ALD allows us to fabricate controllable and reproducible pin-hole-free shells. We showcase the ability to fabricate lithographic SHINER substrates which report an enhancement factor of 7.5 × 103 ± 17% for our gold nanodot substrates coated with a 2.8 nm aluminium oxide shell. Lastly, by introducing a gold etchant solution to our fabricated SHINER substrate, we verified that the shells fabricated with ALD are truly pin-hole-free.

Operando Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy of the NO Reduction Reaction over Rhodium-Based Catalysts

Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) with on-line mass spectrometry (MS) has been used to investigate the surface species, such as NO, NOH, NO2 , N2 O, and reaction products of the NO reduction reaction with CO and H2 over supported Rh-based catalysts in the form of catalyst extrudates. By correlating surface intermediates and reaction products, new insights in the reaction mechanism could be obtained. Upon applying different reaction conditions (i. e., H2 or CO), the selectivity of the catalytic reaction could be tuned towards the formation of N2 . Furthermore, in the absence of Rh, no reaction products were detected. The importance of the operando SHINERS as a surface-sensitive characterization technique in the field of heterogeneous catalysis provides routes towards a better understanding of catalytic performance.

New advances in using Raman spectroscopy for the characterization of catalysts and catalytic reactions

Gaining insight into the mode of operation of heterogeneous catalysts is of great scientific and economic interest. Raman spectroscopy has proven its potential as a powerful vibrational spectroscopic technique for a fundamental and molecular-level characterization of catalysts and catalytic reactions. Raman spectra provide important insight into reaction mechanisms by revealing specific information on the catalysts' (defect) structure in the bulk and at the surface, as well as the presence of adsorbates and reaction intermediates. Modern Raman instrumentation based on single-stage spectrometers allows high throughput and versatility in design of in situ/operando cells to study working catalysts. This review highlights major advances in the use of Raman spectroscopy for the characterization of heterogeneous catalysts made during the past decade, including the development of new methods and potential directions of research for applying Raman spectroscopy to working catalysts. The main focus will be on gas-solid catalytic reactions, but (photo)catalytic reactions in the liquid phase will be touched on if it appears appropriate. The discussion begins with the main instrumentation now available for applying vibrational Raman spectroscopy to catalysis research, including in situ/operando cells for studying gas-solid catalytic processes. The focus then moves to the different types of information available from Raman spectra in the bulk and on the surface of solid catalysts, including adsorbates and surface depositions, as well as the use of theoretical calculations to facilitate band assignments and to describe (resonance) Raman effects. This is followed by a presentation of major developments in enhancing the Raman signal of heterogeneous catalysts by use of UV resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and shell-isolated nanoparticle surface-enhanced Raman spectroscopy (SHINERS). The application of time-resolved Raman studies to structural and kinetic characterization is then discussed. Finally, recent developments in spatially resolved Raman analysis of catalysts and catalytic processes are presented, including the use of coherent anti-Stokes Raman spectroscopy (CARS) and tip-enhanced Raman spectroscopy (TERS). The review concludes with an outlook on potential future developments and applications of Raman spectroscopy in heterogeneous catalysis.

Shell-isolated nanoparticle-enhanced Raman spectroscopy for characterization of living yeast cells

Studying the biochemistry of yeast cells has enabled scientists to understand many essential cellular processes in human cells. Further development of biotechnological and medical progress requires revealing surface chemistry in living cells by using a non-destructive and molecular structure sensitive technique. In this study shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was applied for probing the molecular structure of Metschnikowia pulcherrima yeast cells. Important function of studied cells is the ability to eliminate iron from growth media by precipitating the insoluble pigment pulcherrimin. Comparative SERS and SHINERS analysis of the yeast cells in combination with bare Au and shell-isolated Au@SiO₂ nanoparticles were performed. It was observed that additional bands, such as adenine ring-related vibrational modes appear due to interaction with bare Au nanoparticles; the registered spectra do not coincide with the spectra where Au@SiO₂ nanoparticles were used. SHINERS spectra of M. pulcherrima were significantly enhanced comparing to the Raman spectra. Based on first-principles calculations and 830-nm excited Raman analysis of pulcherrimin, the SHINERS signatures of iron pigment in yeast cells were revealed. Being protected from direct interaction of metal with adsorbate, Au@SiO₂ nanoparticles yield reproducible and reliable vibrational signatures of yeast cell wall constituents.

Core-Shell Nanostructure-Enhanced Raman Spectroscopy for Surface Catalysis

ConspectusThe rational design of highly efficient catalysts relies on understanding their structure-activity relationships and reaction mechanisms at a molecular level. Such an understanding can be obtained by in situ monitoring of dynamic reaction processes using surface-sensitive techniques. Surface-enhanced Raman spectroscopy (SERS) can provide rich structural information with ultrahigh surface sensitivity, even down to the single-molecule level, which makes it a promising tool for the in situ study of catalysis. However, only a few metals (like Au, Ag, and Cu) with particular nanostructures can generate strong SERS effects. Thus, it is almost impossible to employ SERS to study transition metals (like Pt, Pd, Ru, etc.) and other nonmetal materials that are usually used in catalysis (material limitation). Furthermore, SERS is also unable to study model single crystals with atomically flat surface structures or practical nanocatalysts (morphology limitation). These limitations have significantly hindered the applications of SERS in catalysis over the past four decades since its discovery, preventing SERS from becoming a widely used technique in catalysis. In this Account, we summarize the extensive efforts done by our group since the 1980s, particularly in the past decade, to overcome the material and morphology limitations in SERS. Particular attention has been paid to the work using core-shell nanostructures as SERS substrates, because they provide high Raman enhancement and are highly versatile for application on different catalytic materials. Different SERS methodologies for catalysis developed by our group, including the "borrowing" strategy, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and SHINERS-satellite strategy, are discussed in this account, with an emphasis on their principles and applications. These methodologies have successfully overcome the long-standing limitations of traditional SERS, enabling in situ tracking of catalysis at model single-crystal surfaces and practical nanocatalysts that can hardly be studied by SERS. Using these methodologies, we systematically studied a series of fundamentally important reactions, such as oxygen reduction reaction, hydrogen evolution reaction, electrooxidation, CO oxidation, and selective hydrogenation. As such, direct spectroscopic evidence of key intermediates that can hardly be detected by other traditional techniques was obtained. Combined with density functional theory and other in situ techniques, the reaction mechanisms and structure-activity relationships of these catalytic reactions were revealed at a molecular level. Furthermore, the future of SERS in catalysis has also been proposed in this work, which we believe should be focused on the in situ dynamic studies at the single-molecule, or even single-atom, level using techniques with ultrahigh sensitivity or spatial resolution, for example, single-molecule SERS or tip-enhanced Raman spectroscopy. In summary, core-shell nanostructure-enhanced Raman spectroscopies are shown to greatly boost the application of SERS in catalysis, from model systems like single-crystal surfaces to practical nanocatalysts, liquid-solid interfaces to gas-solid interfaces, and electrocatalysis to heterogeneous catalysis to photocatalysis. Thus, we believe this Account would attract increasing attention to SERS in catalysis and opens new avenues for catalytic studies.

In Situ Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy of Nickel-Catalyzed Hydrogenation Reactions

Synthesis methods to prepare lower transition metal catalysts and specifically Ni for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) are explored. Impregnation, colloidal deposition, and spark ablation have been investigated as suitable synthesis routes to prepare SHINERS-active Ni/Au@SiO2 catalyst/Shell-Isolated Nanoparticles (SHINs). Ni precursors are confirmed to be notoriously difficult to reduce and the temperatures required are generally harsh enough to destroy SHINs, rendering SHINERS experiments on Ni infeasible using this approach. For colloidally synthesized Ni nanoparticles deposited on Au@SiO2 SHINs, stabilizing ligands first need to be removed before application is possible in catalysis. The required procedure results in transformation of the metallic Ni core to a fully oxidized metal nanoparticle, again too challenging to reduce at temperatures still compatible with SHINs. Finally, by use of spark ablation we were able to prepare metallic Ni catalysts directly on Au@SiO2 SHINs deposited on a Si wafer. These Ni/Au@SiO2 catalyst/SHINs were subsequently successfully probed with several molecules (i. e. CO and acetylene) of interest for heterogeneous catalysis, and we show that they could be used to study the in situ hydrogenation of acetylene. We observe the interaction of acetylene with the Ni surface. This study further illustrates the true potential of SHINERS by opening the door to studying industrially relevant reactions under in situ or operando reaction conditions.

Controlled fabrication of gold nanobipyramids/polypyrrole for shell-isolated nanoparticle-enhanced Raman spectroscopy to detect γ-aminobutyric acid

Shell-isolated nanoparticle-enhanced Raman Spectroscopy (SHINERS) has been a non-destructive, highly sensitive, specific and powerful sensing method. Detection of γ-aminobutyric acid (GABA) and glutamate, main neurotransmitters in the human brain, is important to diagnosis the neurological disorder. The purpose of this study is preparing a simple, rapid and inexpensive fabrication of Au nanobipyramids/polymer core/shell as a SHINERS-based biosensor to detect different neurotransmitters such as GABA and glutamate with high sensitivity and specificity. Au nanobipyramids/polymer core/shell was fabricated by using two steps process. In the first Au nanobipyramids with longitude and latitude axial of about 100 nm and 10 nm, respectively, was prepared based on the chemical reduction of Au ions by using sodium borohydride as a reducing agent. Then a thin layer of polypyrrole was used for decorating the Au nanobipyramids by using direct polymerization in the presence of Au nanobipyramids. The sensor composed Au nanobipyramids with a thin layer of polypyrrole that could measure GABA within a wide range of concentrations in the presence of human serum. And this sensor was used for direct monitoring of GABA and glutamate. The proposed biosensor can be applied to monitor the level of neurotransmitters accurately for the diagnosis of various neurological disorders with optical signal enhancement.

Extending Surface-Enhanced Raman Spectroscopy to Liquids Using Shell-Isolated Plasmonic Superstructures

Plasmonic superstructures (PS) based on Au/SiO₂ were prepared for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) in liquid phase applications. These superstructures are composed of functionalized SiO₂ spheres with plasmonic Au nanoparticles (NPs) on their surface. Functionalization was performed with (3-aminopropyl)trimethoxysilane, (3-mercaptopropyl)trimethoxysilane and poly(ethylene-imine) (PEI). Of these three, PEI-functionalized spheres showed the highest adsorption density of Au NPs in TEM, UV/Vis and dynamic light scattering (DLS) experiments. Upon decreasing the Au NP/SiO₂ sphere size ratio, an increase in adsorption density was also observed. To optimize plasmonic activity, 61 nm Au NPs were adsorbed onto 900 nm SiO₂ -PEI spheres and these PS were coated with an ultrathin layer (1-2 nm) of SiO₂ to obtain Shell-Isolated Plasmonic Superstructures (SHIPS), preventing direct contact between Au NPs and the liquid medium. Zeta potential measurements, TEM and SHINERS showed that SiO₂ coating was successful. The detection limit for SHINERS using SHIPS and a 638 nm laser was around 10^-12  m of Rhodamine (10^-15  m for uncoated PS), all with acquisition settings suitable for catalysis applications.

Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

In 2010, Tian et al. reported the development of a new, relatively sensitive method of the chemical analysis of various surfaces, including buried interfaces (for example the surfaces of solid samples in a high-pressure gas or a liquid), which makes it possible to analyze various biological samples in situ. They called their method shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). SHINERS spectroscopy is a type of surface-enhanced Raman spectroscopy (SERS) in which an increase in the efficiency of the Raman scattering is induced by plasmonic nanoparticles acting as electromagnetic resonators that locally significantly enhance the electric field of the incident electromagnetic radiation. In the case of SHINERS measurements, the plasmonic nanoparticles are covered by a very thin transparent protective layer (formed, for example, from various oxides such as SiO2, MnO2, TiO2, or organic polymers) that does not significantly damp surface electromagnetic enhancement, but does separate the nanoparticles from direct contact with the probed material and keeps them from agglomerating. Preventing direct contact between the metal plasmonic structures and the analyzed samples is especially important when biological samples are investigated, because direct interaction between the metal nanoparticles and various biological molecules (e.g., peptides) may lead to a change in the structure of those biomolecules. In this mini-review, the state of the art of SHINERS spectroscopy is briefly described.

In Situ Analysis of Surface Catalytic Reactions Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Electrochemistry and heterogeneous catalysis continue to attract enormous interest. In situ surface analysis is a dynamic research field capable of elucidating the catalytic mechanisms of reaction processes. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a nondestructive technique that has been cumulatively used to probe and analyze catalytic-reaction processes, providing important spectral evidence about reaction intermediates produced on catalyst surfaces. In this perspective, we review recent electrochemical- and heterogeneous-catalysis studies using SHINERS, highlight its advantages, summarize the flaws and prospects for improving the SHINERS technique, and give insight into its future research directions.