Highly sensitive SERS monitoring of catalytic reaction by bifunctional Ag-Pd triangular nanoplates

Surface-enhanced Raman spectroscopy (SERS) can attain the “fingerprint” information of molecules from their vibrational transitions for detecting chemical species and thus displays extraordinary application value in studying chemical reaction mechanism catalyzed by noble metal nanoparticles in recent years. Herein, we successfully fabricated bifunctional Ag-Pd triangular nanoplates with integration of catalytic and SERS activities, using Ag triangular nanoplates as templates and Na₂PdCl₄ as Pd precursor in the presence of ascorbic acid acting as reducing agent and polyvinylpyrrolidone serving as stabilizing agent. We found slowly titrating Na₂PdCl₄ solution, compared with the one-shot injection during reaction, can strongly restrain the galvanic replacement reaction and maintain the Ag content, therefore retaining the plasmonic and SERS properties of Ag-Pd triangular nanoplates. By easily adjusting the amount of Na₂PdCl₄, we can optimize the SERS and catalytic activities of Ag-Pd triangular nanoplates. The optimal Ag-Pd triangular nanoplates with dual functionalities are used to follow the catalytic reduction process of 4-nitrothiophenol in the presence of NaBH₄ by SERS. The results reveal 4-nitrothiophenol is directly transformed to 4-aminothiophenol through a one-step route. Thereby, the prepared Ag-Pd triangular nanoplates are effective and suitable for sensitively investigating the catalytic reaction process by in situ SERS.     

Fabrication and Formation Mechanism of Ag Nanoplate‐Decorated Nanofiber Mats and Their Application in SERS

We report a new simple method to fabricate a highly active SERS substrate consisting of poly‐m‐phenylenediamine/polyacrylonitrile (PmPD/PAN) decorated with Ag nanoplates. The formation mechanism of Ag nanoplates is investigated. The synthetic process of the Ag nanoplate‐decorated PmPD/PAN (Ag nanoplates@PmPD/PAN) nanofiber mats consists of the assembly of Ag nanoparticles on the surface of PmPD/PAN nanofibers as crystal nuclei followed by in situ growth of Ag nanoparticles exclusively into nanoplates. Both the reducibility of the polymer and the concentration of AgNO₃ are found to play important roles in the formation and the density of Ag nanoplates. The optimized Ag nanoplates@PmPD/PAN nanofiber mats exhibit excellent activity and reproducibility in surface‐enhanced Raman scattering (SERS) detection of 4‐mercaptobenzoic acid (4‐MBA) with a detection limit of 10^?10 m , making the Ag nanoplates@PmPD/PAN nanofiber mats a promising substrate for SERS detection of chemical molecules. In addition, this work also provides a design and fabrication process for a 3D SERS substrate made of a reducible polymer with noble metals.     

Visible-light-driven heterostructure Ag/Bi2WO6 nanocomposites synthesized by photodeposition method and used for photodegradation of rhodamine B dye

Visible-light-driven heterostructure Ag/Bi₂WO₆ nanocomposites were prepared by transforming Ag⁺ ions into metallic Ag⁰ nanoparticles loaded on top of Bi₂WO₆ nanoplates under visible light irradiation for 1 h. XRD, XPS, SEM and TEM analyses indicated that spherical metallic Ag nanoparticles were uniformly dispersed on top of orthorhombic Bi₂WO₆ thin nanoplates. Rhodamine B (RhB) was used as a dye model for investigation of photocatalytic performance of Bi₂WO₆ nanoplates with different weight contents of Ag nanoparticles illuminated by visible radiation. In this research, 10% Ag/Bi₂WO₆ nanocomposites have the highest photocatalytic activity in the degradation of RhB at 94.21% within 210 min because of the rapid diffusion of electronic charge through the Schottky barrier between metallic Ag nanoparticles and Bi₂WO₆ thin nanoplates, good electrical conductivity of metallic Ag nanoparticles, inhibited recombination of charge carriers and enhanced photocatalytic activity of Ag/Bi₂WO₆ nanocomposites. Main active species of the photocatalysis and stability of the photocatalyst were also evaluated.

     

Silver nanoplates formed at the air/water and solid/water interfaces

Silver nanoparticles and nanoplates were prepared at the air/AgNO₃ aqueous solution interfaces under poly(9-vinylcarbazole) (PVK) monolayers when illuminated by UV-light at room temperature and elevated temperatures, respectively. When the illuminated films at the air/water interfaces were covered by carbon-coated copper grids, nanoplates were formed even at room temperature, and the size of the nanoplates was much larger than those formed at the air/water interface under the same experimental conditions, indicating that copper took part in the formation of Ag nanoplates through the galvanic displacement reaction between Cu and Ag⁺ ions with the help of carbon layer to conduct electrons. It was found that the basal plane of these nanoplates is the (1 1 1) face of a face-centered cubic (fcc) Ag crystal. Although platelike structure can be formed at the carbon-coated copper grid/AgNO₃ aqueous solution interface without PVK film, it shows different features from those with PVK films, indicating that PVK film plays an important role in the formation of regular large nanoplates. Further observations indicate that special restrained microenvironment, adsorption of PVK molecules on a specific crystal face, anisotropic growth and attachment of the nanoparticles are responsible for the formation of the nanoplates.     

Liquid-liquid interface assisted synthesis of size- and thickness-controlled Ag nanoplates

Here we proposed a synthetic method of high-purity Ag nanoplates by the reduction of aqueous Ag⁺ ions at the aqueous-organic interface with the reductant ferrocene. We demonstrated that the as-prepared Ag nanoplates can be widely tunable from 600 nm to 7 μm in size and from 10 to 35 nm in thickness, simply by adjusting the component of organic phase. To our knowledge, there are few methods to tailor the size and the thickness of metal nanoplates in such a large range although many efforts have been made aiming to realize it. Our proposed synthetic strategy is rapid, template-free, seed-less, and high-yield, and could be applied to synthesize analogous two-dimensional nanostructures of other noble metals, such as Pt, Au, and Pd.     

Enhanced photocatalytic properties of Bi2MoO6 nanoplates deposited with intermetallic AgPd nanoparticles by photoreduction method

Bi₂MoO₆ nanoplates modified with intermetallic AgPd nanoparticles synthesized by photoreduction deposition method were used for visible-light-driven photodegradation of rhodamine B. The as-synthesized AgPd/Bi₂MoO₆ nanocomposites were identified by XRD. The 2θ diffraction angle of the (111) plane of pure metallic cubic Ag at 38.11° of heterostructure 10% Ag/Bi₂MoO₆ nanocomposites shifted to a higher diffraction angle at 38.17° of heterostructure 10% Ag_0.7Pd_0.3/Bi₂MoO₆ nanocomposites. TEM images of the as-synthesized nanocomposites showed good metallic Ag and intermetallic AgPd nanoparticles with particle size of 10–12 nm which were fully supported on top of Bi₂MoO₆ nanoplates. Bi₂MoO₆ nanoplates deposited with intermetallic AgPd nanoparticles show significant photocatalytic activity better than Ag/Bi₂MoO₆ and Bi₂MoO₆ due to the formation of AgPd/Bi₂MoO₆ Schottky barrier.

     

Kinetically controlled synthesis of triangular and hexagonal nanoplates of palladium and their SPR/SERS properties

The rapid reduction of Na(2)PdCl(4) by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) has recently been demonstrated as a convenient method of generating Pd cubooctahedra and twinned nanoparticles. Here we describe a new procedure where Pd triangular or hexagonal nanoplates could be selectively synthesized by manipulating the reduction kinetics of the polyol process. More specifically, the reduction rate was substantially reduced through the introduction of Fe(III) species and the O(2)/Cl(-) pair, two wet etchants for Pd(0). The etching power of the O(2)/Cl(-) pair could be further enhanced by adding an acid to lower the pH of the reaction solution. Unlike the previously reported synthesis of Ag and Au nanoplates, light was found to have no indispensable role in the formation of Pd nanoplates. Both triangular and hexagonal nanoplates of Pd exhibited surface plasmon resonance (SPR) peaks in the visible region, and their positions matched with the results of discrete dipole approximation (DDA) calculation. Thanks to their sharp corners and edges, these Pd nanoplates could serve as active substrates for surface-enhanced Raman scattering (SERS).     

Synthesis of Ultrathin Face‐Centered‐Cubic Au@Pt and Au@Pd Core–Shell Nanoplates from Hexagonal‐Close‐Packed Au Square Sheets

The synthesis of ultrathin face‐centered‐cubic (fcc) Au@Pt rhombic nanoplates is reported through the epitaxial growth of Pt on hexagonal‐close‐packed (hcp) Au square sheets (AuSSs). The Pt‐layer growth results in a hcp‐to‐fcc phase transformation of the AuSSs under ambient conditions. Interestingly, the obtained fcc Au@Pt rhombic nanoplates demonstrate a unique (101)_f orientation with the same atomic arrangement extending from the Au core to the Pt shell. Importantly, this method can be extended to the epitaxial growth of Pd on hcp AuSSs, resulting in the unprecedented formation of fcc Au@Pd rhombic nanoplates with (101)_f orientation. Additionally, a small amount of fcc (100)_f‐oriented Au@Pt and Au@Pd square nanoplates are obtained with the Au@Pt and Au@Pd rhombic nanoplates, respectively. We believe that these findings will shed new light on the synthesis of novel noble bimetallic nanostructures.     

Catalytic properties of nanostructured Pd–Ag catalysts in the liquid-phase hydrogenation of terminal and internal alkynes

A comparative catalytic study of Pd–Ag bimetallic catalysts and the commercial Lindlar catalyst (Pd–Pb/CaCO₃) has been carried out in the hydrogenation of phenylacetylene (PA) and diphenylacetylene (DPA). The Pd–Ag catalysts have been prepared using the heterobimetallic complex PdAg₂(OAc)₄(HOAc)₄ supported on MgAl₂O₄ and aluminas (α-Al₂O₃ and γ-Al₂O₃). Physicochemical studies have demonstrated that the reduction of supported Pd–Ag complex with hydrogen results in homogeneous Pd–Ag nanoparticles. Equal in selectivity to the Lindlar catalyst, the Pd–Ag catalysts are more active in DPA hydrogenation. The synthesized Pd–Ag catalysts are active and selective in PA hydrogenation as well, but the unfavorable ratio of the rates of the first and second stages of the process makes it difficult to kinetically control the reaction. The most promising results have been obtained for the Pd–Ag₂/α-Al₂O₃ catalyst. Although this catalyst is less active, it is very selective and allows efficient kinetic control of the process to be carried out owing to the fact that, with this catalyst, the rate of hydrogenation of the resulting styrene is much lower than the rate of hydrogenation of the initial PA.     

Pd nanoparticle-modified Bi2WO6 nanoplates used for visible-light-driven photocatalyst

Pd nanoparticles supported on Bi₂WO₆ nanoplates used for visible-light-driven photocatalyst were successfully synthesized by photoreduction deposition method under visible-light irradiation. Different analytical techniques including X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and photoluminescence spectroscopy revealed that face-centered cubic metallic Pd nanoparticles were uniformly loaded on top of orthorhombic Bi₂WO₆ nanoplates to form heterostructure Pd/Bi₂WO₆ nanocomposites. Photocatalytic activities of pure Bi₂WO₆ sample and heterostructure Pd/Bi₂WO₆ nanocomposites were studied through the photodegradation of rhodamine B (RhB) under visible-light irradiation. The photocatalytic efficiency of Bi₂WO₆ was increased to the highest by being loaded with 5 wt% Pd and then decreased by being loaded with 10 wt% Pd. The improved photocatalytic efficiency caused by high-efficiency diffusion and separation of photo-generated charge carriers was explained and can lead to superior photodegradation of RhB under visible-light irradiation.

     

Comparative study of electrolessly deposited Pd/Ag films onto p-silicon (100)-activated seed layers of Ag and Pd

Pd/Ag films were electrolessly deposited onto p-silicon (100)-activated seed layers of Ag and Pd, respectively, in the solution of 0.005 mol l⁻¹ AgNO₃ + 0.005 mol l⁻¹ PdCl₂ + 4.5 mol l⁻¹ NH₃ + 0.16 mol l⁻¹ Na₂EDTA+0.1 mol l⁻¹ NH₂NH₂ (pH 10.5) at room temperature. The morphology and composition of the films were studied comparatively by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Cathodic polarization curves for hydrogen evolution were recorded in 0.5-mol l⁻¹ H₂SO₄ without illumination, in which the obtained films served as working electrodes. The experimental results show that the film obtained on the Ag seed layer was rather a pure Ag film and not a Pd/Ag film, and the Ag deposition rate on Pd sites was much faster than that on Ag sites.     

Surface-enhanced Raman scattering characteristics of 4-nitrobenzenethiol adsorbed on palladium and silver thin films

Palladium is an important catalytic metal, and it is desirable to develop a surface-enhanced Raman scattering (SERS) technique to investigate the reagent and product species adsorbed on its surface. Unfortunately, Pt-group metals, e.g., Pt and Pd, have been commonly considered as non- or weak-SERS-active substrates. In this work, Ag and Pd thin films were deposited very efficiently and evenly onto the surface of glass substrates by using only corresponding metal nitrate salts (AgNO₃ and Pd(NO₃)₂) with butylamine in ethanolic solutions. In this process, pure ethanol was used for Ag deposition, while an ethanol–water (8:2) mixture was used for Pd deposition. The as-prepared Ag and Pd films exhibited SERS activity over a large area. The surface-induced photoconversion capabilities of these Ag and Pd films were then tested on 4-nitrobenzenethiol by means of SERS. It was found that at least under visible laser irradiation, the surface-catalyzed photoreaction occurs more readily on a Ag film than on a Pd film for the conversion of 4-nitrobenzenethiol to 4-aminobenzenethiol, even though Pd is known to be an important transition metal with high catalytic activity.     

Synthesis of rhodanine-bonded silica gel and its application in the preconcentration and separation of noble metals

A silica gel based sorbent containing rhodanine as functional group (RDSG) was prepared. Its adsorption and separation characteristics for Ag(I), Au(III) and Pd(II) were studied by flow-injection (FI) on-line preconcentration. Influence of different experimental parameters such as acidity, eluent, co-existing ions were investigated. Trace amounts of Ag, Au and Pd could be efficiently adsorbed by rhodanine-bonded silica gel from acidic solution and eluted with thiourea solution. Common co-existing ions exhibited virtually no interference to the preconcentration and determination. The adsorption capacity of RDSG was 0.0352, 0.107 and 0.122 mmol/g for Ag, Au and Pd, respectively. Detection limits of 0.004, 0.022 and 0.019 μg/mL for Ag, Au and Pd, respectively, were achieved with a sampling time of 60 s at a flow rate of 5.0 mL/min. The relative standard deviation were 0.5%, 0.9% and 1.7% for 0.040 μg/mL Ag, 0.200 μg/mL Au and 0.300 μg/¶mL Pd. The sorption property did not change after 1000 cycles of sorption-desorption. The contents of Ag and Au in three national certified ore samples and Pd in a secondary nickel alloy, an anode slime and a CoCl₂ electrolytic solution were determined. The results showed good agreement with the certified values.     

Density functional study of AgnPd and AgnPdH clusters

Small Ag_nPd (n = 5) clusters and their hydrides Ag_nPdH (n = 5) have been studied by density functional theory calculations. For bare clusters, the structures in which the Pd atom has a maximum number of neighboring Ag atoms tend to be energetically favorable. Hydrogen prefers binding to Ag? Pd bridge site of Ag_nPd clusters except for Ag₅Pd. The binding energy has a strong odd–even oscillation. The electron transfers are from Ag atoms to Pd in bare clusters and are from metal clusters to H in cluster hydrides. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Electroless Pd and Ag deposition kinetics of the composite Pd and Pd/Ag membranes synthesized from agitated plating baths

The atomic absorption spectroscopy (AAS) has been successfully utilized for the measurement of the Pd and Ag ion concentrations in the plating baths and to elucidate the effects of temperature, initial metal ion and reducing agent concentrations and agitation on the electroless plating kinetics of Pd and Ag metals. The initial metal ion concentrations for Pd and Ag were varied over a range of 8.2–24.5 mM and 3.1–12.5 mM, respectively. The plating reactions were conducted in a constant temperature electroless plating bath over a temperature range of 20–60 °C and an initial hydrazine concentration range of 1.8–5.4 mM. It was found that the electroless plating of both Pd and Ag were strongly affected by the external mass transfer in the absence of bath agitation. The external mass transfer limitations for both Pd and Ag deposition have been minimized at or above an agitation rate of 400 rpm, resulting in a maximum conversion of the plating reaction at 60 °C and dramatically shortened plating times with the added advantage of uniform deposition morphology. The derivation of the differential rate laws and the estimation of the reaction orders and the activation energies for the electroless Pd and Ag kinetics were conducted via non-linear regression analysis based on the method of initial rates. For a constant-volume batch reactor, the integrated rate law was solved to calculate the conversion and the reactant concentrations as a function of plating time. The model fits were in good agreement with the experimental data. Furthermore, the bath agitation and the plating conditions used in the kinetics study were adopted for the synthesis of 16–20 μm thick composite Pd/Ag membranes (10–12 wt% Ag) and a pure-Pd membrane with a hydrogen selective dense Pd layer as thin as 4.7 μm. While hydrogen permeance of the Pd/Ag membranes A and B at 450 °C were 28 and 32 m³/m²-h-atm^0.5, the H₂ permeance for the 4.7 μm thick pure-Pd membrane at 400 °C was as high as 63 m³/m²-h-atm^0.5_. The long-term permeance testing of all the membranes synthesized from agitated plating baths resulted in a relatively slow leak growth due primarily to the improved morphology obtained via the bath agitation and modified plating conditions.     

Preparation of carrier-free111mCd and105,106mAg and their chemical behavior

It was found that not only^111mCd but also¹⁰⁵Ag and^106mAg sublimate at 1050 °C under a reduced pressure from Pd foils irradiated with -particles. The sublimation method has enabled rapid separation of the isotopes and repeated use of valuable Pd foils. Adsorption onte -Fe₂O₃ and coprecipitation with ferric hydroxide were studied by using radioactive tracers.     

Formation of oxides and their role in the growth of Ag nanoplates on GaAs substrates

Simple galvanic reactions between highly doped n-type GaAs wafers and a pure aqueous solution of AgNO 3 at room temperature provide an easy and efficient protocol to directly deposit uniform Ag nanoplates with tunable dimensions on the GaAs substrates. The anisotropic growth of the Ag nanoplates in the absence of surfactant molecules might be partially ascribed to the codeposition of oxides of gallium and arsenic, which are revealed by extensive data from electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, during the growth of the Ag nanoplates. The electron microscopic characterization shows that each Ag nanoplate has a "necked" geometry, that is, it pins on the GaAs lattices through only a tiny neck (with sizes of <10 nm). In addition, the as-grown Ag nanoplates exhibit strong enhancement toward Raman scattering of materials on (or around) their surfaces.     

Improvement on Electrical Properties and Magnetoresistance Induced by Pd or Ag Addition in La0.67(Ca0.65Ba0.35)0.33MnO3 Manganites

Electrical properties and magnetoresistance have been studied in two series of xAg-La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃ and xPd-La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃ (abbreviated by xAg-LCBMO and xPd-LCBMO) composites. Both Pd and Ag addition induce a decrease in resistivity and an increase in temperature at which the resistivity reaches its maximum. This is mainly due to the improvement of grain boundaries caused by the segregation of good conductive metal grains on the grain boundaries/surfaces. In addition, both Pd and Ag addition induce a large enhancement of room temperature magnetoresistance (RTMR). Note that 27% molar ratio of Ag addition induces a large RTMR of about 70%, about ten times larger than pure LCBMO, whereas 27% molar ratio Pd addition brings a much larger RTMR of about 170%. The large enhancements of MR can be attributed to the decrease in resistivity of the samples caused by the good conductive metal. On the other hand, the polarization of Pd atoms near the Mn ions on the grain surfaces/boundaries plays a very im-portant role in the increase in MR, which induces a large number of spin clusters in Pd-added samples.     

Nucleation Phenomenon in Silica Xerogels and Pd/SiO2, Ag/SiO2, Cu/SiO2 Cogelled Catalysts

Pd/SiO₂, Ag/SiO₂ and Cu/SiO₂ xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Pd, Ag and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). It appears that, in cogelled samples, the metal complex acts as a nucleation agent in the formation of silica particles. The resulting catalysts are then composed of completely accessible metallic crystallites with a diameter of about 3 nm located inside silica porous particles with a monodisperse microporous distribution. Xerogels without metal synthesized with EDAS and TEOS (C. Alié, R. Pirard, A.J. Lecloux, and J.-P. Pirard, J. Non-Cryst. Solids 289, 88 (2001)) verify this hypothesis of nucleation by EDAS.     

Heteropolymetallic Architectures as Snapshots of Transmetallation Processes at Different Degrees of Transfer

Novel heteropolymetallic architectures have been built by integrating Pd, Au and Ag systems. The dinuclear [(CNC)(PPh₃)Pd-^G11M(PPh₃)](ClO₄) (^G11M=Au ( 3 ), Ag ( 4 ); CNC=2,6-diphenylpyridinate) and trinuclear [{(CNC)(PPh₃)Pd}₂^G11M](ClO₄) (^G11M=Au ( 6 ), Ag ( 5 )) complexes have been accessed or isolated. Structural and DFT characterization unveil striking interactions of one of the aryl groups of the CNC ligand(s) with the ^G11M center, suggesting these complexes constitute models of transmetallation processes. Further analyses allow to qualitatively order the degree of transfer, proving that Au promotes the highest one and also that Pd systems favor higher degrees than Pt. Consistently, Energy Decomposition Analysis calculations show that the interaction energies follow the order Pd−Au > Pt−Au > Pd−Ag > Pt−Ag. All these results offer potentially useful ideas for the design of bimetallic catalytic systems.