Bifunctional Ag@Pd-Ag Nanocubes for Highly Sensitive Monitoring of Catalytic Reactions by Surface-Enhanced Raman Spectroscopy

Realization of red plasmon shifts by the selective etching of Ag nanorods

The red plasmon shifts is realized through selective deposition of Au atoms and etching of Ag atoms on the Ag nanorods.

Ascorbic Acid-Assisted Polyol Synthesis of Iron and Fe/GO, Fe/h-BN Composites for Pb2+ Removal from Wastewaters

Iron powders and Fe/graphene oxide and Fe/boron nitride composites were synthesized by means of a polyol synthesis method. The effect of NaOH/Fe and ascorbic acid/Fe ratios on the characteristics of synthesized products were evaluated. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, low-temperature nitrogen adsorption and Raman-spectroscopy. Ascorbic acid-assisted polyol synthesis resulted in the 10-fold decrease of the iron particles' size and almost 2-fold increase of lead removal efficiency. The deposition of iron on the surface of graphene oxide lead to the formation of small 20-30 nm sized particles as well as bigger 200-300 nm sized particles, while the reduction in presence of boron nitride resulted in the 100-200 nm sized particles. The difference is attributed to the surface state of graphene oxide and boron nitride. Adsorption properties of the obtained materials were studied in the process of Pb²⁺ ion removal from wastewater.

Quasi-Metal for Highly Sensitive and Stable Surface-Enhanced Raman Scattering

Compared with the noble-metal surface-enhanced Raman scattering (SERS) substrates activated by the surface plasmon resonance (SPR)-induced electromagnetic mechanism (EM), the relative low sensitivity and stability of the chemical mechanism (CM)-based substrates are the biggest obstacles to their applications. Herein, we report that quasi-metallic VO₂ nanosheet arrays can be used as a sensitive and stable SERS substrate. The lowest detectable limit of analyte adsorbed on the VO₂ nanosheets achieves 10⁻¹⁰ M and the maximum Raman enhancement factor (EF) reaches 6.7 × 10⁷, which is comparable with that of the noble metals. The experimental and theoretical results demonstrate that the SERS performance of the VO₂ nanosheets comes from the strong interfacial interactions based on charge transfer and the vigorous SPR effects. Our research results demonstrate that quasi-metals are very promising SERS detection platforms and reveal that CM, like EM, contributes significantly to the SERS activity of quasi-metals.

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Surface-enhanced Raman scattering (SERS) on quasi-metallic VO₂

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High SERS enhancement factor and low limit of detection have been achieved

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Synergistic effect of electromagnetic enhancement and chemical enhancement

A composite prepared from gold nanoparticles and a metal organic framework (type MOF-74) for determination of 4-nitrothiophenol by surface-enhanced Raman spectroscopy

Core-shell nanoparticles (NPs) consisting of a gold core and a metal-organic framework shell (type MOF-74) were synthesized via one-pot synthesis. The NPs exhibit highly sensitive and stable SERS activity for the detection of 4-nitrothiophenol, with a specific band at 1337 cm^-1. The method has a linear response in 0.10-10 μmol·L^-1 analyte concentration range and a lower detection limit of 69 nmol·L^-1. The potential application of this novel SERS substrate was evaluated by two model reactions involving 4-nitrothiophenol. The first involves in-situ SERS monitoring of the surface plasmon-induced nitration of aromatic rings without adding conventional acid catalyst. The second involves the photocatalytic reduction of 4-nitrothiophenol to 4-thioaminophenol in the presence of Au/MOF-74 under 785-nm laser irradiation. The plasmon-assisted dimerization of 4-nitrothiophenol to form 4,4'-dimercaptoazobenzene can also be monitored simultaneously. Graphical abstract Schematic presentation of a nanoparticle SERS substrate consisting of gold core and MOF-74 shell, which was applied to detection of 4-nitrothiophenol. The Au/MOF-74 was successfully used for in-situ monitoring of two model reactions involving 4-nitrothiophenol by SERS.

Fabrication of Ag-Pd concave nanocrystals through facet-selective oxidation of Ag atoms

We report the fabrication of Ag-Pd concave nanocrystals by introducing the Pd(ii) precursor into an aqueous suspension of Ag nanocubes in the presence of cetyltrimethylammonium chloride (CTAC) under ambient conditions. Different from the previously reported work that involved the oxidation of Ag and deposition of Pd at random sites on the surface for the generation of Ag-Pd hollow nanocrystals, we demonstrate that the Cl- ions from CTAC can confine the oxidation of Ag atoms to the side faces of a nanocube while the resultant Pd atoms are deposited on the edges in an orthogonal manner. By controlling the amount of the Pd(ii) precursor involved in a synthesis, we can transform Ag nanocubes into Ag-Pd nanocrystals with different degrees of concaveness for the side faces and controllable Pd contents. We characterize the outermost layer of concave surfaces for the as-obtained Ag-Pd nanocrystals by surface-enhanced Raman scattering (SERS) through the use of an isocyanide probe. This facile approach would enable the fabrication of Ag-based concave nanocrystals for applications in plasmonics and catalysis.

pH-Dependent growth of atomic Pd layers on trisoctahedral gold nanoparticles to realize enhanced performance in electrocatalysis and chemical catalysis

In this work, the controlled epitaxial growth of ultrathin Pd shells of a few atomic layers (denoted as nL) on the surfaces of gold nanoparticle (Au NP) cores of different morphologies (trisoctahedral, cubic, and spherical shapes) in the presence of cetyltrimethylammonium chloride (CTAC) was achieved by regulating the pH value of the aqueous CTAC solution and finely tuning the amount of the Pd precursor. It was found that the critical shell thickness for epitaxial Pd growth at the optimal pH value was 4 atomic layers, taking {331}-faceted trisoctahedral (TOH) Au@Pd_nL NPs as an example, on the basis of the results of atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. Moreover, the resulting TOH Au@Pd_1L NPs (100.9 m² g^-1, 13.2 A mg_Pd^-1 and 13.1 mA cm^-2) exhibited excellent electrocatalytic performance and long-term electrocatalytic activity for ethanol oxidation, around 4.8-fold, 66-fold, and 21.8-fold better than commercial Pd/C catalysts (31 m² g^-1, 0.2 A mg_Pd^-1, and 0.6 mA cm^-2). Furthermore, the resulting TOH Au@Pd_1L NPs not only markedly enhance the chemical catalytic activity for the reduction of 4-nitrophenol (4-NP), but also allow the in situ surface-enhanced Raman spectroscopy (SERS) monitoring of the reaction process of the Pd-catalyzed reduction of 4-NTP. Thus, our work may provide a new way to fabricate core-shell (CS) bimetallic NPs with the merits of both metal outer shells (excellent catalytic performance in electrocatalysis and chemical catalysis) and Au NP cores (reaction process by in situ SERS monitoring).

Stabilization of noble metal nanostructures for catalysis and sensing

Noble metal nanocrystals have been widely used as active components in catalysis and chemical/bio-sensing. The sizes, structures, and shapes of noble metal nanocrystals are crucial to their electronic, optical, and catalytic properties. However, metal nanocrystals tend to lose their structural and morphological properties when they are subjected to thermal and chemical treatment. Therefore, stabilization of noble metal nanostructures remains a challenge. In this feature article, we present our recent efforts on the stabilization of noble metal nanocrystals, i.e., using inorganic and non-metal solids as supports and physical barriers, protecting the nanocrystal surface by a metal coating, and forming alloys with other metals. At the end of this review, we provide our perspectives on the future development of effective methods for nanocrystal stabilization.

In Situ Atomic-Level Tracking of Heterogeneous Nucleation in Nanocrystal Growth with an Isocyanide Molecular Probe

We report the use of 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a spectroscopic probe to study the heterogeneous nucleation and deposition of Pd on Ag nanocubes under different conditions by surface-enhanced Raman scattering. As a major advantage, the spectroscopic analysis can be performed in situ and in real time with the nanoparticles still suspended in the reaction solution. The success of this method relies on the distinctive stretching frequencies (ν_NC) of the isocyanide group in 2,6-DMPI when it binds to Ag and Pd atoms through σ donation and π-back-donation, respectively. Significantly, we discovered that ν_NC was sensitive to the arrangement of Pd adatoms on the Ag surface. For example, when the isocyanide group bound to one, two, and three Pd atoms, we would observe the atop, bridge, and hollow configurations, respectively, at different ν_NC frequencies. As such, the ν_NC band could serve as a characteristic reporter for the Pd adatoms being deposited onto different types of facets on Ag nanocubes with atomic-level sensitivity. When 2,6-DMPI molecules were introduced into the reaction solution, we further demonstrated in situ tracking of heterogeneous nucleation and early stage deposition of Pd on Ag nanocubes by monitoring the evolution of ν_NC bands for both Ag and Pd surface atoms as a function of reaction time. This in situ technique opens up the opportunity to investigate the roles played by reaction temperature and the type of Pd(II) precursor in influencing the heterogeneous nucleation and growth of bimetallic nanocrystals. The sensitivity of isocyanide group to Pd atoms helps elucidate some of the details on the reduction, deposition, and diffusion processes involved in heterogeneous nucleation.

Immobilization of ultrafine Ag nanoparticles on well-designed hierarchically porous silica for high-performance catalysis

Catalyst immobilization is of much significance not only for maintaining the high activity of the ultrafine catalyst, but also for the separation of catalyst during the practical application. Herein, a novel support material, three-dimensional hierarchically porous silica (HPS) with interconnected micro-meso-macro pores and high specific surface area was successfully fabricated though a freeze-drying technique in the presence of poly(vinyl alcohol) (PVA) and subsequent calcination process. A series of characterizations revealed that the specific surface area of HPS can be well adjusted by changing the addition of PVA. The specific surface area of HPS was as high as 360 m² g^-1, which was 211-fold higher than HPS-0 (silica prepared without using PVA). To demonstrate the potential application of such novel support material, highly dispersed silver nanoparticles (AgNPs) were immobilized on the surfaces of HPS and HPS-0 through in-situ reduction. By contrast, the catalytic activity of AgNPs anchored on HPS (531 s^-1 g^-1) was about 42-fold higher than that of AgNPs anchored on HPS-0 (12.67 s^-1 g^-1). The significantly enhanced catalytic activity of AgNPs/HPS was believed to be related to their high specific surface area and interconnected macroporous scaffolds, which could provide numerous reactive sites and mass transfer routes for the reactants.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals by Replicating the Surface Atomic Structure on the Seed

Controlling the surface structure of metal nanocrystals while maximizing the utilization efficiency of the atoms is a subject of great importance. An emerging strategy that has captured the attention of many research groups involves the conformal deposition of one metal as an ultrathin shell (typically 1-6 atomic layers) onto the surface of a seed made of another metal and covered by a set of well-defined facets. This approach forces the deposited metal to faithfully replicate the surface atomic structure of the seed while at the same time serving to minimize the usage of the deposited metal. Here, the recent progress in this area is discussed and analyzed by focusing on the synthetic and mechanistic requisites necessary for achieving surface atomic replication of precious metals. Other related methods are discussed, including the one-pot synthesis, electrochemical deposition, and skin-layer formation through thermal annealing. To close, some of the synergies that arise when the thickness of the deposited shell is decreased controllably down to a few atomic layers are highlighted, along with how the control of thickness can be used to uncover the optimal physicochemical properties necessary for boosting the performance toward a range of catalytic reactions.

Facet-selective deposition of Au and Pt on Ag nanocubes for the fabrication of bifunctional Ag@Au-Pt nanocubes and trimetallic nanoboxes

We report a facile route to the synthesis of Ag@Au-Pt trimetallic nanocubes in which the Ag, Au, and Pt atoms are exposed at the corners, side faces, and edges, respectively. Our success relies on the use of Ag@Au nanocubes, with Ag2O patches at the corners and Au on the side faces and edges, as seeds for the site-selective deposition of Pt on the edges only in a reaction system containing ascorbic acid (H2Asc) and poly(vinylpyrrolidone). At an initial pH of 3.2, H2Asc can dissolve the Ag2O patches, exposing the Ag atoms at the corners of a nanocube. Upon the injection of the H2PtCl6 precursor, the Pt atoms derived from the reduction by both H2Asc and Ag are preferentially deposited on the edges, leading to the formation of Ag@Au-Pt trimetallic nanocubes. We demonstrate the use of 2,6-dimethylphenyl isocyanide as a molecular probe to confirm and monitor the deposition of Pt atoms on the edges of nanocubes through surface-enhanced Raman scattering (SERS). We further explore the use of these bifunctional trimetallic nanoparticles with integrated plasmonic and catalytic properties for in situ SERS monitoring the reduction of 4-nitrothiophenol by NaBH4. Upon the removal of Ag via H2O2 etching, the Ag@Au-Pt nanocubes evolve into trimetallic nanoboxes with a wall thickness of about 2 nm and well-defined openings at the corners. The trimetallic nanoboxes embrace plasmon resonance peaks in the near-infrared region with potential in biomedical applications.

Overgrowth Versus Galvanic Replacement: Mechanistic Roles of Pd Seeds during the Deposition of Pd-Pt

Here, a systematic study of the roles played by Pd seeds during seed-mediated coreduction of Pd-Pt is presented. Either nanoparticles with porous, hollow architectures or concave nanocubes were achieved, depending on whether the synthesis conditions favored galvanic replacement or overgrowth. Prior works have shown that the galvanic replacement reaction between seeds and a precursor can be suppressed by introducing a faster, parallel reaction that removes one of the reagents (e.g., adatom generation in solution rather than surface-catalyzed precursor reduction). Here, we show that the galvanic replacement reaction depends on the size and concentration of the Pd seeds; the former of which can be manipulated during the course of the reaction through the use of a secondary reducing agent. This insight will guide future syntheses of multimetallic nanostructures by seeded methods, allowing for a range of nanocrystals to be precisely engineered for a variety of applications.

Surface-enhanced Raman spectroscopic detection of molecular chemo- and plasmo-catalysis on noble metal nanoparticles

The in situ detection of reactions catalyzed by metal NPs is challenging because the underlying chemical transformations occur at interfaces. Surface-enhanced Raman scattering (SERS), a surface-selective, sensitive and label-free vibrational spectroscopic technique, is ideally suited for monitoring of heterogeneous catalysis with high chemical specificity. A major limitation in the past, however, was that small, catalytically active metal NPs do not exhibit the high plasmonic activity required for SERS. This feature article focuses on the design, synthesis and use of bifunctional NPs with both catalytic and plasmonic activity for in situ SERS detection of reactions catalyzed by metal NPs. We focus on model reactions induced by chemical reducing agents such as hydride or molecular hydrogen as well as on plasmon-induced photo-catalysis including both photo-oxidation and photo-reduction. Finally, we highlight the concept of photo-recycling on halide-containing silver surfaces for unprecedented multi-electron reduction chemistry.

Immobilized Seed-Mediated Growth of Two-Dimensional Array of Metallic Nanocrystals with Asymmetric Shapes

Bottom-up fabrication of such arrays with specific orientation of nanoparticles remains a challenge. In this paper, we report an immobilized seed-mediated growth strategy for the fabrication of two-dimensional (2D) arrays of mono- and bimetallic polyhedral nanocrystals with well-defined shapes and orientations on a substrate. This method relies on the controlled solution-phase deposition of metals (i.e., Au and Pd) on a selectively exposed surface of self-assembled seed nanoparticles that are immobilized on a substrate through collapsed polymer brushes. By using this approach, we demonstrated the preparation of various 2D arrays of shaped Au nanocrystals and Au core/Pd shell nanocrystals with asymmetric geometry of two halves and controlled orientations with respect to the substrate. The shape evolution of seeds to final nanocrystals was systematically monitored and evaluated by electron microscopic imaging. Our study suggests that the shape and orientation of nanocrystals within arrays is determined by the preferential orientation of assembled seed nanoparticles on the substrate and controllable deposition of metals on exposed crystal facets of immobilized seeds. The synthetic approach we developed presents an important addition to current tools for the fabrication of substrate-supported functional nanostructures.

Synthesis of ultrathin platinum nanoplates for enhanced oxygen reduction activity

Ultrathin Pt nanostructures exposing controlled crystal facets are highly desirable for their superior activity and cost-effectiveness in the electrocatalytic oxygen reduction reaction (ORR), and they are conventionally synthesized by epitaxial growth of Pt on a limited range of templates, such as Pd nanocrystals, resulting in a high cost and less structural diversity of the ultrathin Pt nanostructures. To solve this problem, we demonstrate that ultrathin Pt nanostructures can be synthesized by templating conveniently available Ag nanocrystals without involving galvanic replacement, which enables a much-reduced cost and controllable new morphologies, such as ultrathin Pt nanoplates that expose the {111} facets. The resulting ultrathin Pt nanoplates are ∼1-2 nm in thickness, which show an ∼22-fold increase in specific activity (5.3 mA cm-2), an ∼9.5-fold increase in mass activity (1.62 A mg-1) and significantly enhanced catalytic stability in the ORR, compared with the commercial Pt/C catalyst. We believe this strategy opens a door to a highly extendable family of ultrathin noble metal nanostructures, thus promising excellent activity and stability in a broad range of catalytic applications.

Plasmonic core–shell ionic microgels for photo-tuning catalytic applications

NIR laser acts as a motor to drive and control Au-based catalysts which exhibit highly catalytic activity.

Addressing Challenges and Scalability in the Synthesis of Thin Uniform Metal Shells on Large Metal Nanoparticle Cores: Case Study of Ag-Pt Core-Shell Nanocubes

Bimetallic nanoparticles in which a metal is coated with an ultrathin (∼1 nm) layer of a second metal are often desired for their unique chemical and physical properties. Current synthesis methods for producing such core-shell nanostructures often require incremental addition of a shell metal precursor which is rapidly reduced onto metal cores. A major shortcoming of this approach is that it necessitates precise concentrations of chemical reagents, making it difficult to perform at large scales. To address this issue, we considered an approach whereby the reduction of the shell metal precursor was controlled through in situ chemical modification of the precursor. We used this approach to develop a highly scalable synthesis for coating atomic layers of Pt onto Ag nanocubes. We show that Ag-Pt core-shell nanostructures are synthesized in high yields and that these structures effectively combine the optical properties of the plasmonic Ag nanocube core with the surface properties of the thin Pt shell. Additionally, we demonstrate the scalability of the synthesis by performing a 10 times scale-up.

Bimetallic Effect of Single Nanocatalysts Visualized by Super-Resolution Catalysis Imaging

Compared with their monometallic counterparts, bimetallic nanoparticles often show enhanced catalytic activity associated with the bimetallic interface. Direct quantitation of catalytic activity at the bimetallic interface is important for understanding the enhancement mechanism, but challenging experimentally. Here using single-molecule super-resolution catalysis imaging in correlation with electron microscopy, we report the first quantitative visualization of enhanced bimetallic activity within single bimetallic nanoparticles. We focus on heteronuclear bimetallic PdAu nanoparticles that present a well-defined Pd-Au bimetallic interface in catalyzing a photodriven fluorogenic disproportionation reaction. Our approach also enables a direct comparison between the bimetallic and monometallic regions within the same nanoparticle. Theoretical calculations further provide insights into the electronic nature of N-O bond activation of the reactant (resazurin) adsorbed on bimetallic sites. Subparticle activity correlation between bimetallic enhancement and monometallic activity suggests that the favorable locations to construct bimetallic sites are those monometallic sites with higher activity, leading to a strategy for making effective bimetallic nanocatalysts. The results highlight the power of super-resolution catalysis imaging in gaining insights that could help improve nanocatalysts.

Core-Shell Nanoparticle Clusters Enable Synergistic Integration of Plasmonic and Catalytic Functions in a Single Platform

Designing controlled hybrid nanoarchitectures between plasmonic and catalytic materials is of paramount importance to fully exploit each function of constituent materials. This study reports a new synthetic strategy for the realization of colloidal clusters of core-shell nanoparticles with plasmonic cores and catalytically active shells. The Au@M (M = Pd or Pt) nanoparticle clusters (NPCs) with a high density of sub-1 nm interparticle gaps are successfully prepared by the deposition of M shells onto thermally activated Au NPCs. NPCs with other metal, metal sulfide, and metal oxide shells can also be synthesized by using the present approach. The prepared Au@M NPCs show remarkably enhanced plasmonic performance compared to their Au@M nanoparticle counterparts due to the localization of a strong electromagnetic field at the interparticle gaps, while the inherent catalytic function of shells is intact. In situ real-time Raman spectroscopy and plasmon-enhanced electrocatalysis experiments demonstrate that the controlled assembly of core-shell nanoparticles is a very effective route for the synergistic integration of plasmonic and catalytic functions in a single platform.

Pt-Ag cubic nanocages with wall thickness less than 2 nm and their enhanced catalytic activity toward oxygen reduction

We report a facile synthesis of Pt-Ag nanocages with walls thinner than 2 nm by depositing a few atomic layers of Pt as conformal shells on Ag nanocubes and then selectively removing the Ag template via wet etching. In a typical process, we inject a specific volume of aqueous H₂PtCl₆ into a mixture of Ag nanocubes, ascorbic acid (H₂Asc), NaOH, and poly(vinylpyrrolidone) in water under ambient conditions. At an initial pH of 11.9, the Pt(iv) precursor is quickly reduced by an ascorbate monoanion, a strong reducing agent derived from the neutralization of H₂Asc with NaOH. The newly formed Pt atoms are deposited onto the edges and then corners and side faces of Ag nanocubes, leading to the generation of Ag@Pt core-shell nanocubes with a conformal Pt shell of approximately three atomic layers (or, about 0.6 nm in thickness) when 0.4 mL of 0.2 mM H₂PtCl₆ is involved. After the selective removal of Ag in the core using an etchant based on a mixture of Fe(NO₃)₃ and HNO₃, we transform the core-shell nanocubes into Pt-Ag alloy nanocages with an ultrathin wall thickness of less than 2 nm. We further demonstrate that the as-obtained nanocages with a composition of Pt₄₂Ag₅₈ exhibit an enhanced catalytic activity toward the oxygen reduction reaction, with a mass activity of 0.30 A mg^-1 and a specific activity of 0.93 mA cm^-2, which are 1.6 and 2.5 times, respectively, greater than those of a commercial Pt/C catalyst.

Ag Nanorods-Oxide Hybrid Array Substrates: Synthesis, Characterization, and Applications in Surface-Enhanced Raman Scattering

Over the last few decades, benefitting from the sufficient sensitivity, high specificity, nondestructive, and rapid detection capability of the surface-enhanced Raman scattering (SERS) technique, numerous nanostructures have been elaborately designed and successfully synthesized as high-performance SERS substrates, which have been extensively exploited for the identification of chemical and biological analytes. Among these, Ag nanorods coated with thin metal oxide layers (AgNRs-oxide hybrid array substrates) featuring many outstanding advantages have been proposed as fascinating SERS substrates, and are of particular research interest. The present review provides a systematic overview towards the representative achievements of AgNRs-oxide hybrid array substrates for SERS applications from diverse perspectives, so as to promote the realization of real-world SERS sensors. First, various fabrication approaches of AgNRs-oxide nanostructures are introduced, which are followed by a discussion on the novel merits of AgNRs-oxide arrays, such as superior SERS sensitivity and reproducibility, high thermal stability, long-term activity in air, corrosion resistivity, and intense chemisorption of target molecules. Next, we present recent advances of AgNRs-oxide substrates in terms of practical applications. Intriguingly, the recyclability, qualitative and quantitative analyses, as well as vapor-phase molecule sensing have been achieved on these nanocomposites. We further discuss the major challenges and prospects of AgNRs-oxide substrates for future SERS developments, aiming to expand the versatility of SERS technique.

Revealing the Role of Interfacial Properties on Catalytic Behaviors by in Situ Surface-Enhanced Raman Spectroscopy

Insightful understanding of how interfacial structures and properties affect catalytic processes is one of the most challenging issues in heterogeneous catalysis. Here, the essential roles of Pt-Au and Pt-oxide-Au interfaces on the activation of H₂ and the hydrogenation of para-nitrothiophenol (pNTP) were studied at molecular level by in situ surface-enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Pt-Au and Pt-oxide-Au interfaces were fabricated through the synthesis of Pt-on-Au and Pt-on-SHINs nanocomposites. Direct spectroscopic evidence demonstrates that the atomic hydrogen species generated on the Pt nanocatalysts can spill over from Pt to Au via the Pt-Au and Pt-TiO₂-Au interfaces, but would be blocked at the Pt-SiO₂-Au interfaces, leading to the different reaction pathways and product selectivity on Pt-on-Au and Pt-on-SHINs nanocomposites. Such findings have also been verified by the density functional theory calculation. In addition, it is found that nanocatalysts assembled on pinhole-free shell-isolated nanoparticles (Pt-on-pinhole-free-SHINs) can override the influence of the Au core on the reaction and can be applied as promising platforms for the in situ study of heterogeneous catalysis. This work offers a concrete example of how SERS/SHINERS elucidate details about in situ reaction and helps to dig out the fundamental role of interfaces in catalysis.

Enriching Silver Nanocrystals with a Second Noble Metal

Noble-metal nanocrystals have received considerable interests owing to their fascinating properties and promising applications in areas including plasmonics, catalysis, sensing, imaging, and medicine. As demonstrated by ample examples, the performance of nanocrystals in these and related applications can be augmented by switching from monometallic to bimetallic systems. The inclusion of a second metal can enhance the properties and greatly expand the application landscape by bringing in new capabilities. Seeded growth offers a powerful route to bimetallic nanocrystals. This approach is built upon the concept that preformed nanocrystals with uniform, well-controlled size, shape, and structure can serve as seeds to template and direct the deposition of metal atoms. Seeded growth is, however, limited by galvanic replacement when the deposited metal is less reactive than the seed. The involvement of galvanic replacement not only makes it difficult to control the outcome of seeded growth but also causes degradation to some properties. We have successfully addressed this issue by reducing the salt precursor(s) into atoms with essentially no galvanic replacement. In the absence of self-nucleation, the atoms are preferentially deposited onto the seeds to generate bimetallic nanocrystals with controlled structures. In this Account, we use Ag nanocubes as an example to demonstrate the fabrication of Ag@M and Ag@Ag-M (M = Au, Pd, or Pt) nanocubes with a core-frame or core-shell structure by controlling the deposition of M atoms. A typical synthesis involves the titration of Mⁿ⁺ (a precursor to M) ions into an aqueous suspension containing Ag nanocubes, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. In one approach, aqueous sodium hydroxide is introduced to increase the initial pH of the reaction system. At pH = 11.9, ascorbic acid is dominated by ascorbate monoanion, a much stronger reductant, to suppress the galvanic replacement between Mⁿ⁺ and Ag. In this case, the M atoms derived from the reduction by ascorbate monoanion are sequentially deposited on the edges, corners, and side faces to generate Ag@M core-frame and then core-shell nanocubes. The other approach involves the use of ascorbic acid as a relatively weak reductant while Mⁿ⁺ is cotitrated with Ag⁺ ions in the absence of sodium hydroxide. At pH = 3.2, when the molar ratio of Ag⁺ to Mⁿ⁺ is sufficiently high, the added Ag⁺ ions can effectively push the galvanic reaction backward and thus inhibit it. As a result, coreduction of the two precursors by ascorbic acid produces Ag and M atoms for the generation of Ag@Ag-M core-frame nanocubes with increasingly thicker ridges. The Ag@Ag-Pd core-frame nanocubes can serve as a dual catalyst to promote the stepwise reduction of nitroaromatics to aminoaromatics and then oxidation to azo compounds. The consecutive reactions can be monitored using surface-enhanced Raman scattering (SERS). The Ag@Au core-shell nanocubes with Au shells of three or six atomic layers exhibit plasmonic peaks almost identical to those of the Ag nanocubes while the chemical stability and SERS activity are substantially augmented. For both types of bimetallic nanocubes, the Ag cores can be selectively removed to generate nanoframes and nanoboxes.

Observing the Overgrowth of a Second Metal on Silver Cubic Seeds in Solution by Surface-Enhanced Raman Scattering

We report the development of an isocyanide-based molecular probe for in situ characterizing the overgrowth of a second metal on silver nanocrystal seeds in solution by surface-enhanced Raman scattering (SERS). As the first demonstration, we elucidate that the vibrational frequency of 2,6-dimethylphenyl isocyanide (2,6-DMPI) can serve as a distinctive reporter for capturing the nucleation of Pt on the edges of Ag nanocubes in the aqueous solution containing a Pt precursor, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. Our success relies on the difference in stretching frequency for the NC bond when the isocyanide group binds to the Ag and Pt atoms. Specifically, σ donation from the antibonding σ* orbital of the -NC group to the d-band of Ag would strengthen the NC bond, blue shifting the stretching frequency. In contrast, π-back-donation from the d-band of Pt to the π* antibonding orbital of the -NC group would weaken the NC bond, leading to a red shift of stretching frequency. Therefore, it is feasible to in situ characterize the outermost surface that consists of both newly deposited Pt atoms and remaining Ag atoms by following the stretching frequencies and intensities of 2,6-DMPI in real time. Because the SERS hot spots on the edges of Ag nanocubes coincide with the {110} facets preferred for the nucleation of Pt atoms, this technique is capable of resolving 27 Pt atoms being deposited on each edge of a 39 nm Ag nanocube in the original growth solution. Collectively, in situ SERS, with its consummate sensitivity to molecular structure and bonding of isocyanide-based molecular probe, could elucidate the mechanistic details involved in the seeded overgrowth of a catalytically significant metal, such as Pt, Pd, Ir, Rh, and Ru, on the surface of a Ag or Au nanocrystal seed.

A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy

Compared with noble metals, semiconductors with surface plasmon resonance effect are another type of SERS substrate materials. The main obstacles so far are that the semiconducting materials are often unstable and easy to be further oxidized or decomposed by laser irradiating or contacting with corrosive substances. Here, we report that metallic MoO₂ can be used as a SERS substrate to detect trace amounts of highly risk chemicals including bisphenol A (BPA), dichloropheno (DCP), pentachlorophenol (PCP) and so on. The minimum detectable concentration was 10⁻⁷ M and the maximum enhancement factor is up to 3.75 × 10⁶. To the best of our knowledge, it may be the best among the metal oxides and even reaches or approaches to Au/Ag. The MoO₂ shows an unexpected high oxidation resistance, which can even withstand 300 °C in air without further oxidation. The MoO₂ material also can resist long etching of strong acid and alkali.

Semiconducting materials are potential SERS substrates as alternatives to noble metals, but often suffer from poor stabilities and sensitivities. Here, the authors use molybdenum dioxide as a SERS material, showing high enhancement factors and stability to oxidation even at high temperatures.

Multifaceted Gold-Palladium Bimetallic Nanorods and Their Geometric, Compositional, and Catalytic Tunabilities

Kinetically controlled, seed-mediated co-reduction provides a robust and versatile synthetic approach to multimetallic nanoparticles with precisely controlled geometries and compositions. Here, we demonstrate that single-crystalline cylindrical Au nanorods selectively transform into a series of structurally distinct Au@Au-Pd alloy core-shell bimetallic nanorods with exotic multifaceted geometries enclosed by specific types of facets upon seed-mediated Au-Pd co-reduction under diffusion-controlled conditions. By adjusting several key synthetic parameters, such as the Pd/Au precursor ratio, the reducing agent concentration, the capping surfactant concentration, and foreign metal ion additives, we have been able to simultaneously fine-tailor the atomic-level surface structures and fine-tune the compositional stoichiometries of the multifaceted Au-Pd bimetallic nanorods. Using the catalytic hydrogenation of 4-nitrophenol by ammonia borane as a model reaction obeying the Langmuir-Hinshelwood kinetics, we further show that the relative surface binding affinities of the reactants and the rates of interfacial charge transfers, both of which play key roles in determining the overall reaction kinetics, strongly depend upon the surface atomic coordinations and the compositional stoichiometries of the colloidal Au-Pd alloy nanocatalysts. The insights gained from this work not only shed light on the underlying mechanisms dictating the intriguing geometric evolution of multimetallic nanocrystals during seed-mediated co-reduction but also provide an important knowledge framework that guides the rational design of architecturally sophisticated multimetallic nanostructures toward optimization of catalytic molecular transformations.

Core-Shell Nanoparticle-Enhanced Raman Spectroscopy

Core-shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core-shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core-shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core-shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core-shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core-shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core-shell nanomaterials with different functionalities for SERS and other surface-enhanced spectroscopies.

Green in Situ Synthesis of Clean 3D Chestnutlike Ag/WO3-x Nanostructures for Highly Efficient, Recyclable and Sensitive SERS Sensing

Surface-enhanced Raman scattering (SERS) has proven to be an effective technique for identifying and providing fingerprint structural information on various analytes in low concentration. However, this analytical technique has been plagued by the ubiquitous presence of organic contaminants on roughened SERS substrate surfaces, which not only often result in poorer detection sensitivity but also significantly affect the reproducibility and accuracy of SERS analysis. Herein, we developed a clean, stable, and recyclable three-dimensional (3D) chestnutlike Ag/WO_3-x (0 < x < 0.28) SERS substrate by simple hydrothermal reaction and subsequent green in situ decoration of silver nanoparticles. None of the organic additives were used in synthesis, which ensures the substrate surfaces are completely clean and free of interferences from impurities. The innovative design combines the SERS enhancement effect and self-cleaning property, making it a multifunctional and reusable SERS platform for highly sensitive SERS detection. Using malachite green as a model target, the as-prepared SERS substrates exhibited good reproducibility (relative standard deviation of 7.5%) and pushed the detection limit down to 0.29 pM. The enhancement factor was found to be as high as 1.4 × 10⁷ based on the analysis of 4-aminothiophenol. The excellent regeneration performance indicated that the 3D biomimetic SERS substrates can be reused many times. In addition, the fabricated substrate was successfully employed for detecting thiram in water with a detection limit of 0.32 nM, and a good linear relationship was obtained between the logarithmic intensities and the logarithmic concentrations of thiram ranging from 1 nM to 1 μM. More importantly, the resultant SERS-active colloid can be used for accurate and reliable determination of thiram in real fruit peels. These results predict that the proposed SERS system have great potential toward rapid, reliable, and on-site analysis, especially for food safety and environmental supervision.

Boosting catalytic activity of metal nanoparticles for 4-nitrophenol reduction: Modification of metal naoparticles with poly(diallyldimethylammonium chloride)

Most of the previously reported studies have focused on the change in the size, morphology, and composition of metal nanocatalysts for improving their catalytic activity. Herein, we report poly(diallyldimethylammonium chloride) [PDDA]-stabilized nanoparticles (NPs) of platinum (Pt) and palladium (Pd) as highly active and efficient catalysts for hydrogenation of 4-nitrophenol (4-NP) in the presence of NaBH4. PDDA-stabilized Pt and Pd NPs possessed similar particle size and same facet with citrate-capped Pt and Pd NPs, making this study to investigate the inter-relationship between catalytic activity and surface ligand without the consideration of the effects of particle size and facet. Compared to citrate-capped Pt and Pd NPs, PDDA-stabilized Pt and Pd NPs exhibited excellent pH and salt stability. PDDA could serve as an electron acceptor for metal NPs to produce the net positive charges on the metal surface, which provide strong electrostatic attraction with negatively charged nitrophenolate and borohydride ions. The activity parameter and rate constant of PDDA-stabilized metal NPs were higher than those of citrate-capped metal NPs. Compared to the previously reported Pd nanomaterials for the catalysis of NaBH4-mediated reduction of 4-NP, PDDA-stabilized Pd NPs exhibited the extremely high activity parameter (195s^-1g^-1) and provided excellent scalability and reusability.

Pd-P nanoparticles supported on PxOy-incorporated carbon nanotubes for enhanced methanol oxidation in an alkaline medium

In comparison to Pd-P/OCNTs, an enhanced electrochemical performance of Pd-P/PCNTs towards methanol oxidation is observed.

Core–shell and alloy integrating PdAu bimetallic nanoplates on reduced graphene oxide for efficient and stable hydrogen evolution catalysts

PdAu nanoplates with different core–shell structures on rGO were generated by manipulating the competition between galvanic replacement and chemical reduction with the alloy and core–shell integrating nanoplates exhibiting superior HER properties.

A general seed-mediated approach to the synthesis of AgM (M = Au, Pt, and Pd) core–shell nanoplates and their SERS properties

Graphical abstract describing a general pH-dependent synthetic steps for the preparation of AgM (Au, Pt and Pd) triangular core–shell nanoplates and hollow nanoframes.

Fabrication of Sesame Sticks-like Silver Nanoparticles/Polystyrene Hybridnanotubes and Their Catalytic Effects

A novel and efficient catalyst is one of the goals in the material field, and the involvement of nanoscience and technology has brought new vigor to the development of catalyst. This research aimed to develop a simple two-step route to fabricate Fe3O4@PS/PDA-Ag hybridnanotubes with size-controllable and highly dispersed silver nanoparticles (NPs). First, Fe3O4@PS nanotubes of a sound mechanical property were prepared using polystyrene (PS)/toluene solution containing highly dispersed oleic acid modified Fe3O4 particles in a commercial AAO template. Next, the facile technique was used to form in situ silver NPs on the surface of magnetic PS (Fe3O4@PS) nanotubes through dopamine coating. The catalytic effects of the prepared Fe3O4@PS/PDA-Ag hybridnanotubes with highly dispersed AgNPs were characterized using a range of analytical methods, including transmission electron microscopy, thermogravimetric analysis, UV-Visible spectroscopy, and X-ray diffraction. It was found that such prepared Fe3O4@PS/PDA-Ag hybridnanotubes had a large specific surface area. They possessed excellent activities in catalyzing the reduction of 4-nitrophenol (4-NP) by NaBH4 in the aqueous phase. Furthermore, they were readily separated from fluid and retrieved by an external magnet. Their catalyst activity and recyclability demonstrated that this approach we proposed had the potential to become a new idea and route for catalytic platform.