Bimetallic Core-shell Nanoparticles Research Articles

Optimization of gold-palladium core-shell nanowires towards H2O2 reduction by adjusting shell thickness.

Designable bimetallic core–shell nanoparticles exhibit superb performance in many fields including industrial catalysis, energy conversion and chemical sensing, due to their outstanding properties associated with their tunable electronic structure. Herein, Au–Pd core–shell (AurichPd@AuPdrich) nanowires (NWs) were synthesized through a one-pot facile chemical reduction method in the presence of cetyltrimethyl ammonium bromide (CTAB) surfactant. The thickness of the Pd shell could be adjusted by directly controlling the Au/Pd feeding ratio while maintaining the nanowire morphology. The as-obtained Au75Pd25 core–shell NWs with a thin Pdrich shell showed significantly enhanced activities towards the reduction of hydrogen peroxide with the sensitivity reaching 338 μA cm−2 mM−1 and a linear range up to 10 mM. In sum, Pd shell thickness could be used to adjust the electronic structure, thereby optimizing the catalytic activity.

How interface properties control the equilibrium shape of core-shell Fe-Au and Fe-Ag nanoparticles.

While combining two metals in the same nanoparticle can lead to remarkable novel applications, the resulting structure in terms of crystallinity and shape remains difficult to predict. It is thus essential to provide a detailed atomistic picture of the underlying growth processes. In the present work we address the case of core-shell Fe-Au and Fe-Ag nanoparticles. Interface properties between Fe and the noble metals Au and Ag, computed using DFT, were used to parameterize Fe-Au and Fe-Ag pairwise interactions in combination with available many-body potentials for the pure elements. The growth of Au or Ag shells on nanometric Fe cores with prescribed shapes was then modelled by means of Monte Carlo simulations. The shape of the obtained Fe-Au nanoparticles is found to strongly evolve with the amount of metal deposited on the Fe core, a transition from the polyhedral Wulff shape of bare iron to a cubic shape taking place as the amount of deposited gold exceeds two monolayers. In striking contrast, the growth of silver proceeds in a much more anisotropic, Janus-like way and with a lesser dependence on the iron core shape. In both cases, the predicted morphologies are found to be in good agreement with experimental observations in which the nanoparticles are grown by physical deposition methods. Understanding the origin of these differences, which can be traced back to subtle variations in the electronic structure of the Au/Fe and Ag/Fe interfaces, should further contribute to the better design of core-shell bimetallic nanoparticles.

Predicting the adsorption capacity of iron nanoparticles with metallic impurities (Cu, Ni and Pd) for arsenic removal: a DFT study

The potential capacities of bimetallic nanoclusters, constituted by Fe doped with metal atoms of Cu, Ni and Pd, for the H3AsO3 adsorption and reduction, were studied by density functional theory calculations. Both the pure Fe nanocluster and the one doped with a Ni atom on an edge, show greater adsorbent and reducing capacities than the others substrates. Then, the structural and electronic properties of bimetallic core–shell nanoparticles constituted by 80 atoms were also studied. The highest adsorption capacity was found on cFe/sNi core–shell nanoparticle, decreasing the activity in this order: cFe/sNi > cNi/sFe > cFe/sCu > cCu/sFe. The interaction found between the atom of As and the surface atom of Ni coincides with a significant hybridization between the s–p As states and the sp and d bands of the metal atom. The charge transfer from the core atoms to the surface generates a charge accumulation on the cFe/sNi surface, and a surface–subsurface dipole. We have also observed that higher adsorption energies correspond linearly with more pronounced displacement of the d band center from the Fermi level. Finally, we want to highlight the reductive capacity of this material (cFe/sNi) to adsorption Arsenious acid, which is certainly favorable for the immobilization of this pollutant.

Absorption and plasmon resonance of Bi-metallic core-shell nanoparticles on a dielectric substrate near an external tip

Absorption efficiency profiles and localized surface plasmon resonance (LSPR) wavelengths are reported for metallic core-shell nanoparticles (NPs) placed over a BK7 glass substrate. A numerical study is performed with the vectorized version of the discrete dipole approximation with surface interactions (DDA-SI-v). Gold (Au) and silver (Ag) metallic components are used for the simulations of two different core-shell structures. Absorption enhancement and the hybrid modes of plasmon resonances of the core-shell structures are compared by using a measure that defines a size configuration. It is observed that a small volume fraction of the core sizes results in shell domination over the plasmon response. An additional study is conducted to discern the sensitivity of the refractive index of nanoparticles in different surrounding environments. With a selected core-shell size configuration of Ag-Au pairs, a significant absorption enhancement with a redshift of LSPR wavelength is observed for both Ag core-Au shell and Au core-Ag shell NPs. The absorption behavior of the bare metallic NPs and selected core-shell pairs in proximity to an external probe's tip is also analyzed. The gallium phosphide (GaP) and silicon (Si) tip usage are investigated with transverse electric (TE) and transverse magnetic (TM) wave polarizations. It is observed that the dominance of light polarization on the absorption enhancement of the NPs switches at different wavelengths, where the dielectric transition for tip materials occurs. These findings show the possible targeted uses of metallic core-shell nanoparticles in several areas such as nanomanufacturing, localized heating, bio-sensing, and material detection applications.

Rapid analysis of herbicide diquat in apple juice with surface enhanced Raman spectroscopy: Effects of particle size and the ratio of gold to silver with gold and gold-silver core-shell bimetallic nanoparticles as substrates

Twelve different Au nanoparticles (NPs; diameter, 19–87 nm) and Au–Ag core-shell NPs (diameters of core and particles: 19 nm, 35–91 nm; 43 nm, 66–127 nm) varying in size and the ratio of gold to silver were employed as surface enhanced Raman scattering (SERS) substrates for rapid analysis of herbicide diquat cation in apple juice. The minimum detectible concentrations for diquat cation standards with three selected optimal SERS substrates were 0.005 mg/L (43 nm Au NPs), 0.05 mg/L (91 nm Au–Ag NPs, 19 nm core), and 0.025 mg/L (78 nm Au–Ag NPs, 43 nm core), respectively. However, only the 78 nm Au–Ag NPs led to acceptable SERS enhancement effect for diquat cation in apple juice with minimum sample preparation, allowing its detection at as low as 0.025 mg/L. In addition to providing a rapid analysis approach for diquat in fruit juices, this study indicates that the influences of particle size and composition of NPs on SERS enhancement effects for an analyte in standard solutions could be quite different from that in a real food sample, while the optimal ratio of gold to silver of the bimetallic NPs depended on the Au seeds and sample matrix.

Green Synthesis and Characterization of Ag@Au Core-shell Bimetallic Nanoparticles using the Extract of Hamelia patens Plant

Journal Article Green Synthesis and Characterization of Ag@Au Core-shell Bimetallic Nanoparticles using the Extract of Hamelia patens Plant Get access K Chavez, K Chavez Instituto de investigación en Metalurgia y Materiales, UMSNH, Morelia, México Corresponding author: kchavez@umich.mx Search for other works by this author on: Oxford Academic Google Scholar G Rosas G Rosas Instituto de investigación en Metalurgia y Materiales, UMSNH, Morelia, México Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1102–1103, https://doi.org/10.1017/S143192761900624X Published: 01 August 2019

Surface Plasmon Resonant Gold-Palladium Bimetallic Nanoparticles for Promoting Catalytic Oxidation

AbstractColloidal gold-palladium (Au-Pd) bimetallic nanoparticles were used as catalysts to study the ethanol (EtOH) photo-oxidation cycle, with an emphasis towards driving carbon-carbon (C-C) bond cleavage at low temperatures. Au-Pd bimetallic alloy and core-shell nanoparticles were prepared to synergistically couple a plasmonic absorber (Au) with a catalytic metal (Pd) with composite optical and catalytic properties tailored towards promoting photocatalytic oxidation. Catalysts utilizing metals that exhibit localized surface plasmon resonance (SPR) can be harnessed for light-driven enhancement for small molecule oxidation via augmented photocarrier generation/separation and photothermal conversion. The coupling of Au to Pd in an alloy or core-shell nanostructure maintains SPR-induced charge separation, mitigates the poisoning effects on Pd, and allows for improved EtOH oxidation. The Au-Pd nanoparticles were coupled to semiconducting titanium dioxide photocatalysts to probe their effects on plasmonically-assisted photocatalytic oxidation of EtOH. Complete oxidation of EtOH to CO2 under solar simulated-light irradiation was confirmed by monitoring the yield of gaseous products. Bimetallics provide a pathway for driving desired photocatalytic and photoelectrochemical reactions with superior catalytic activity and selectivity.

Synthesis of ˜10 nm size Cu/Ag core-shell nanoparticles stabilized by an ethoxylated carboxylic acid for conductive ink

Bimetallic Cu-Ag Core-shell nanoparticles were synthesized by the reduction of copper 2-[2-(2-methoxyethoxy)ethoxy]acetate with hydrazine hydrate in benzyl alcohol followed by the reduction of silver ions on the copper surface using a galvanic replacement reaction. The morphology and Core-shell structure of the nanoparticles so prepared were investigated by X-ray phase analysis, high-resolution transmission electron microscopy, X-ray energy dispersive spectrometry and optical spectroscopy. The effect of synthesis conditions such as temperature, synthesis time, silver-to-copper molar ratio, and the rate of addition of silver nitrate to the system on thickness, density, and uniformity of the silver shell was studied. The silver coated copper nanoparticles were shown to exhibit improved stability to oxidation.

Iron-nickel bimetallic nanoparticles: Surfactant assisted synthesis and their catalytic activities

Iron-Nickel core-shell bimetallic nanoparticles was synthesized by seed-growth co-reduction of their corresponding metal precursors (Fe(NO3)3 and Ni(NO3)2) for the first time by using chemical reduction method in absence and presence of shape-controlling cetyltrimethylammonuim bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS). UV–visible spectra revealed that the formation of core-shell NPs strongly depends on the reduction potential of Fe3+ and Ni2+ ions in an aqueous solution. Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and X-ray diffraction (XRD) were used to determine the surface morphology and elemental composition of the NPs. The Fe-Ni NPs was used as a catalyst to the hydrogen generation from hydrolysis of sodium borohydride under various experimental conditions. The various activation parameters such as Ea = 75.1 kJ mol−1, ΔH# = 69.9 kJ mol−1 and ΔS# = −104.5 JK−1 mol−1 were calculated using Arrhenius and Eyring equations and discussed. Fe/-Ni was also used as an effective adsorbent for the removal of golden yellow MR from dying wastewater. Langmuir adsorption parameters were evaluated by using linear-form of the isotherm. The maximum adsorption capacity (Q0max = 181.1 mg/g) was calculated for Fe25-Ni75.

Core-shell structure Ag@Pd nanoparticles supported on layered MnO2 substrate as toluene oxidation catalyst

The silver palladium bimetallic core-shell structure nanoparticles (Ag@Pd NPs) were synthesized by a thin Pd shell slowly generating on the outmost of Ag nanoparticle according to galvanic replacement mechanism. Then, the 2D layered structure manganese dioxide (MnO2) substrate was used to support the as-synthesize Ag@Pd NPs to prepare Ag@Pd/MnO2 catalyst for toluene purification in oxidation reaction. Physicochemical properties of the samples were characterized by a number of different analytical techniques. It is found that the Ag@Pd NPs are homogeneously spherical shape with an average particle diameter size of 7.1 nm. And the MnO2 substrate can not only uniformly disperse Ag@Pd active component on its surface for its high specific surface area and mesopore volume, but also provide the reactive lattice oxygen by its mixed oxidation states of Mn3+/Mn4+. Moreover, the core-shell configuration of Ag@Pd NPs can improve the state phase transformation from Pd0 to PdO2 with the aid of lattice oxygen of MnO2 substrate, because of the lattice oxygen migration controlled strong metal-support interaction (SMSI) effect between palladium active component and MnO2 substrate. So, there would be more exposed PdO2 active sites on the Ag@Pd/MnO2 catalyst relative to monometallic Pd/MnO2 catalyst. Hence, the as-prepared bimetallic core-shell Ag@Pd/MnO2 catalyst exhibits greatly higher toluene oxidation activity at lower reaction temperature than monometallic Pd/MnO2 catalyst, indicating it is a promising catalyst for use in volatile organic compounds (VOCs) purification.

X-ray powder diffraction to analyse bimetallic core-shell nanoparticles (gold and palladium; 7-8 nm).

A comparative X-ray powder diffraction study on poly(N-vinyl pyrrolidone) (PVP)-stabilized palladium and gold nanoparticles and bimetallic Pd–Au nanoparticles (both types of core–shell nanostructures) was performed. The average diameter of Au and Pd nanoparticles was 5 to 6 nm. The two types of core–shell particles had a core diameter of 5 to 6 nm and an overall diameter of 7 to 8 nm, i.e. a shell thickness of 1 to 2 nm. X-ray powder diffraction on a laboratory instrument was able to distinguish between a physical mixture of gold and palladium nanoparticles and bimetallic core–shell nanoparticles. It was also possible to separate the core from the shell in both kinds of bimetallic core–shell nanoparticles due to the different domain size and because it was known which metal was in the core and which was in the shell. The spherical particles were synthesized by reduction with glucose in aqueous media. After purification by multiple centrifugation steps, the particles were characterized with respect to their structural, colloid-chemical, and spectroscopic properties, i.e. particle size, morphology, and internal elemental distribution. Dynamic light scattering (DLS), differential centrifugal sedimentation (DCS), atomic absorption spectroscopy (AAS), ultraviolet-visible spectroscopy (UV-vis), high-angle annular dark field imaging (HAADF), and energy-dispersed X-ray spectroscopy (EDX) were applied for particle characterization.

Universal description of heating-induced reshaping preference of core-shell bimetallic nanoparticles.

To achieve universal description of the reshaping process of core-shell bimetallic nanoparticles, we combined the tight-binding Ising Hamiltonian model with molecular dynamic simulations to propose a general theoretical model at the atomic scale while considering the temperature, bond energy, atomic size, and surface energy effects. Based on this model, we can quantitatively analyze the tendency of core-shell structured bimetallic nanoparticles toward the reshaping phenomenon upon heating. By rapidly screening 196 types of bimetallic nanoparticles (containing transition metal elements from VIII to IIB groups in the fourth, fifth, and sixth rows of the periodic table), we identified forty-four kinds of bimetallic nanoparticles with reshaping tendency upon heating, which was validated by molecular dynamic simulations and available experimental results. With increasing temperature, the bimetallic nanoparticles with reshaping preference were transformed from an icosahedron to a star-like shape. In contrast, the structure of bimetallic nanoparticles without reshaping preference was transformed from an icosahedron to a sphere shape, which is usually considered to be the normal pre-melting phenomenon. Further structural analysis indicated that the reshaping of bimetallic nanoparticles could be ascribed to different diffusion mechanisms, where a dominant unidirectional mechanism leads to reshaped bimetallic nanoparticles and a bidirectional diffusion mechanism results in no-reshaped bimetallic nanoparticles. This study provides a deep insight into the origin of reshaping in bimetallic nanoparticles, and it may stimulate extensive studies on engineering bimetallic nanoparticles to switch on/off reshaping upon heating, for example, by modifying the structures, atomic arrangement or composites of bimetallic systems in future.

Structure, stability, electronic, magnetic, and catalytic properties of monometallic Pd, Au, and bimetallic Pd-Au core-shell nanoparticles.

Bimetallic core-shell nanoparticles (CSNPs) often exhibit excellent and tunable properties, depending on their composition, sizes, morphology, atomic arrangement, thickness, and sequence of both core and shell. In this study, the geometrical structure, thermodynamic stability, chemical activity, electronic and magnetic properties, and catalytic activity in the hydrogen evolution reaction (HER) of 13- and 55-atom Pd, Au NPs, and Pd-Au CSNPs were systematically investigated using density functional theory calculations. The results showed that Au atoms prefer to segregate to the surface-shell, while Pd atoms were inclined to aggregate in the core region for bimetallic Pd-Au CSNPs; therefore, Pd@Au CSNPs with an Au surface-shell were thermodynamically more favorable than both the monometallic Pd/Au NPs and the Au@Pd CSNPs with a Pd surface-shell. The Pd surface-shell of the Au@Pd CSNPs displayed a positive charge, while the Au surface-shell of the Au@Pd CSNPs exhibited a negative charge due to the charge transfer in the Pd-Au CSNPs, resulting in that the d-band center of Au@Pd with the Pd surface-shell showed larger shift toward the Fermi level and higher chemical activity. The Pd@Au CSNPs with the Au surface-shell showed similar d-band curves and d-band centers with monometallic Au NPs. All 13-atom Pd, Au NPs, and Pd-Au CSNPs were magnetic, while the 55-atom NPs were non-magnetic with symmetry partial density of states' curves except for Pd55. Changing the location of Pd and Au atoms in the Pd-Au CSNPs influenced their total magnetic moments. In addition, an opposite trend was found: small 13-atom NPs with a Pd surface-shell showed superior HER activity to the ones with an Au surface-shell, while large 55-atom NPs with an Au surface-shell possessed higher HER activity than the ones with a Pd surface-shell.

Computational design of bimetallic core-shell nanoparticles for hot-carrier photocatalysis

Computational design can accelerate the discovery of new materials with tailored properties, but applying this approach to plasmonic nanoparticles with diameters larger than a few nanometers is challenging as atomistic first-principles calculations are not feasible for such systems. In this paper, we employ a recently developed material-specific approach that combines effective mass theory for electrons with a quasistatic description of the localized surface plasmon to identify promising bimetallic core-shell nanoparticles for hot-electron photocatalysis. Specifically, we calculate hot-carrier generation rates of 100 different core-shell nanoparticles and find that systems with an alkali-metal core and a transition-metal shell exhibit high figures of merit for water splitting and are stable in aqueous environments. Our analysis reveals that the high efficiency of these systems is related to their electronic structure, which features a two-dimensional electron gas in the shell. Our calculations further demonstrate that hot-carrier properties are highly tunable and depend sensitively on core and shell sizes. The design rules resulting from our work can guide experimental progress towards improved solar energy conversion devices.

Synthesis and Optical Properties of Fe@Au, Ni@Au Bimetallic Core–Shell Nanoparticles

Fe@Au and Ni@Au core–shell nanoparticles (NPs) were synthesized by liquid-phase reduction of iron and nickel compounds by sodium borohydride in an aqueous medium. Transmission electron microscopy, X-ray powder diffraction, and spectrophotometry were used to confirm the structure of the NPs and to determine their shape and the average core and shell size.

Fluorometric and colorimetric sensor array for discrimination of glucose using enzymatic-triggered dual-signal system consisting of Au@Ag nanoparticles and carbon nanodots

In the present study, a dual-signal senor for glucose detection has been established based on Au@Ag bimetallic core-shell nanoparticles (Au@Ag NPs) and carbon nanodots (C-dots). As a donor of surface plasmon-enhanced energy transfer (SPEET) in this senor, the fluorescence of C-dots was quenched by silver shell (acceptor) of the Au@Ag NPs. Furthermore, Au@Ag NPs serve as colorimetric reporter for the etching effect of H2O2 produced from enzymatic-triggered oxidation of glucose on silver shell, the fluorescence of C-dots recovery. Therefore, the change of color and fluorescence of this senor are directly related to the concentration of glucose. The linear relationship between fluorescence (absorbance) intensity and glucose was in the range of 2.0–400 μM (0.50–300 μM) with a low limit of detection of 0.67 μM (0.20 μM). Besides, this dual-signal senor also exhibited excellent selectivity for distinguishing glucose from common substances. This sensor system was successfully applied to the analysis of glucose in pretreated blood sample with satisfactory results, demonstrating the application prospect for practical sample analysis.

Natural solar light-driven preparation of plasmonic resonance-based alloy and core-shell catalyst for sustainable enhanced hydrogen production: Green approach and characterization

Plasmonic Co and/or Ag monometal and their combinations as bimetal alloy and core-shell nanoparticles were prepared under natural sun light, using aqueous glycerol (in-situ reducing agent) and TNP (stabilizer). The formation of bimetallic alloy and core-shell NPs, and their uniform dispersion on the surface of TNP were evidenced by different characterization techniques. Ultraviolet-visible diffuse reflectance spectroscopy evidenced the two distinct characteristic surface plasmon resonance (SPR) absorption bands for the core-shell nanoparticles and a single distinct characteristic SPR band for the alloy. X-ray photoelectron spectroscopy revealed the presence of Ag and Co in the metallic form. The results of H2-temperature programmed reduction of fresh and used samples emphasized the characteristic reduction peaks for the reduction of monometal/bimetal oxide to metallic/bimetallic particles. The interaction of the loaded bimetallic particles with TNP resulted in a shift in the Raman bands. The photoluminescence spectra of the used samples revealed the formation of bimetallic alloy and core-shell structures, which resulted in a decrease in the recombination of charge carriers. X-ray diffraction, electron paramagnetic resonance spectroscopy, high-resolution transmission electron microscopy, and cyclic voltammetry analyses substantiated the same. The monometal-loaded photocatalysts and the bimetal alloy and core-shell nanoparticle-loaded photocatalysts were further examined for H2 production with pure water/aqueous glycerol/crude glycerol under natural solar light irradiation. The (Ag-Co)coloadedTNP catalyst yielded the maximum H2 evolution rate of 63 mmol g−1. For comparison, experiments were conducted under artificial solar light irradiation with similar experimental conditions. Based on the results, different mechanistic paths for insitu photoreduced Ag-Co bimetallic alloy and core-shell nanoparticles on TNP for H2 production are envisaged.

Tunable plasmon-enhanced broadband light harvesting for perovskite solar cells

In this work, we report a reliable method for synthesizing (Au, Au/Ag core)/(TiO2 shell) nanostructures with their plasmonic wavelengths covering the visible light region for perovskite solar cells. The mono- and bi-metallic core-shell nanoparticles exhibit tunable localized surface plasmon resonance wavelength and function as “light tentacle” to improve the photo-electricity conversion efficiency. Plasmonic nanoparticles with different sizes and shapes, different thicknesses of TiO2 shell and Ag interlayer are found to have a strong influence on the localized surface plasmon resonance enhancement effect. The experimental photovoltaic performance of perovskite solar cells is significantly enhanced when the plasmonic nanoparticles are embedded inmesoporous TiO2 scaffolds. A champion photo-electricity conversion efficiency of 17.85% is achieved with nanoparticles (Au/Ag, λLSPR = 650 nm), giving a 18.7% enhancement over that of the pristine device (15.04%). Finite-difference time-domain simulations show that nanorod Au in mesoporus TiO2 scaffold induces the most intense electromagnetic coupling, and provides a novel emitter for photon flux in mesoporous perovskite solar cells. These theoretical results are consistent with the corresponding experimental those. Thus, enhancing the incident light intensities around 650 nm will be most favorable to the improvement of the photo-electricity conversion efficiency of perovskite solar cells.

Colloidal Clusters of Bimetallic Core–Shell Nanoparticles for Enhanced Sensing of Hydrogen in Aqueous Solution

AbstractDeveloping sensitive and stable hydrogen sensors is of great importance for sustainable energy development. Here, a novel hydrogen sensing platform is described based on colloidal clusters of Au@Pd core–shell nanoparticles (Au@Pd NPCs) with characteristic localized surface plasmon resonance (LSPR) properties. Au@Pd NPCs with well‐controlled topologies exhibit highly pronounced sensitivity for LSPR‐based hydrogen sensing in aqueous solution in comparison to their nanoparticle counterparts and previously reported Au–Pd bimetallic nanostructures, which can be attributed to highly promoted plasmonic field resulting from the cluster formation. Furthermore, Au@Pd NPCs show stable sensing capability for repeated hydrogen sensing cycles. The present strategy for devising high‐performance LSPR‐based hydrogen sensors via the controlled assembly of bimetallic nanoparticles can be applied to the development of efficient plasmonic platforms for various sensing applications.

Core size matters! High Raman enhancing core tunable Au/Ag bimetallic core-shell nanoparticles

Bimetallic core-shell nanostructures have been attracted tremendous attention due to their ability to form novel materials with unique chemical, optical, and physical properties. Here, we have studied the influence of core size of Au/Ag bimetallic core-shell nanostructures on the Raman enhancement efficiency with the Raman-active probe methylene blue. The surface-enhanced Raman scattering intensity is increased with increase in the core size of Au/Ag bimetallic core-shell nanoparticles. Interestingly, the enhancement factor is found to be 6.58 × 107 for the Au100/Ag core-shell nanoparticles and allows easy detection of analyte methylene blue. Thus, surface-enhanced Raman scattering properties of the metal nanoparticles are significantly enhanced due to the Au/Ag core-shell structures and the enhancement factor is dependent on the size of the core of the bimetallic nanoparticles.