Galvanic replacement reactions of active-metal nanoparticles

We present a systemic investigation of a galvanic replacement technique in which active-metal nanoparticles are used as sacrificial seeds. We found that different nanostructures can be controllably synthesized by varying the type of more noble-metal ions and liquid medium. Specifically, nano-heterostructures of noble metal (Ag, Au) or Cu nanocrystals on active-metal (Mg, Zn) cores were obtained by the reaction of active-metal nanoparticles with more noble-metal ions in ethanol; Ag nanocrystal arrays were produced by the reaction of active-metal nanoparticles with Ag(+) ions in water; spongy Au nanospheres were generated by the reaction of active-metal nanoparticles with AuCl(4)(-) ions in water; and SnO(2) nanoparticles were prepared when Sn(2+) were used as the oxidant ions. The key factors determining the product morphology are shown to be the reactivity of the liquid medium and the nature of the oxidant-reductant couple, whereas Mg and Zn nanoparticles played similar roles in achieving various nanostructures. When microsized Mg and Zn particles were used as seeds in similar reactions, the products were mainly noble-metal dendrites. The new approach proposed in this study expands the capability of the conventional nanoscale galvanic replacement method and provides new avenues to various structures, which are expected to have many potential applications in catalysis, optoelectronics, and biomedicine.     

Advanced Catalytic Performance of Au–Pt Double‐Walled Nanotubes and Their Fabrication through Galvanic Replacement Reaction

Bimetallic tubular nanostructures have been the focus of intensive research as they have very interesting potential applications in various fields including catalysis and electronics. In this paper, we demonstrate a facile method for the fabrication of Au–Pt double‐walled nanotubes (Au–Pt DWNTs). The DWNTs are fabricated through the galvanic displacement reaction between Ag nanowires and various metal ions, and the Au–Pt DWNT catalysts exhibit high active catalytic performances toward both methanol electro‐oxidation and 4‐nitrophenol (4‐NP) reduction. First, they have a high electrochemically active surface area of 61.66 m² g^?1, which is close to the value of commercial Pt/C catalysts (64.76 m² g^?1), and the peak current density of Au–Pt DWNTs in methanol oxidation is recorded as 138.25 mA mg^?1, whereas those of Pt nanotubes, Au/Pt nanotubes (simple mixture), and commercial Pt/C are 24.12, 40.95, and120.65 mA mg^?1, respectively. The Au–Pt DWNTs show a markedly enhanced electrocatalytic activity for methanol oxidation compared with the other three catalysts. They also show an excellent catalytic performance in comparison with common Au nanotubes for 4‐nitrophenol (4‐NP) reduction. The attractive performance exhibited by these prepared Au–Pt DWNTs can be attributed to their unique structures, which make them promising candidates as high‐performance catalysts.     

Formation of Metal Selenide and Metal–Selenium Nanoparticles using Distinct Reactivity between Selenium and Noble Metals

Small Se nanoparticles with a diameter of ≈20 nm were generated by the reduction of selenium chloride with NaBH₄ at ?10 °C. The reaction with Ag at 60 °C yielded stable Ag₂Se nanoparticles, which subsequently were transformed into M–Se nanoparticles (M=Cd, Zn, Pb) through cation exchange reactions with corresponding ions. The reaction with Pt formed Pt layers that were evenly coated on the surface of the Se nanoparticles, and the dissolution of the Se cores with hydrazine generated uniform Pt hollow nanoparticles. The reaction with Au generated tiny Au clusters on the Se surface, and eventually formed acorn‐shaped Au–Se nanoparticles through heat treatment. These results indicate that small Se nanoparticles with diameters of ≈20 nm can be used as a versatile platform for the synthesis of metal selenide and metal–selenium hybrid nanoparticles with complex structures.     

Plasmon‐Mediated Syntheses of Metallic Nanostructures

The ability to prepare noble metal nanostructures of a desired composition, size, and shape enables their resulting properties to be exquisitely tailored, which has led to the use of these structures in numerous applications, ranging from medicine to electronics. The prospect of using light to guide nanoparticle reactions is extremely attractive since one can, in principle, regulate particle growth based on the ability of the nanostructures to absorb a specific excitation wavelength. Therefore, using the nature of light, one can generate a homogenous population of product nanoparticles from a heterogeneous starting population. The best example of this is afforded by plasmon‐mediated syntheses of metal nanoparticles, which use visible light irradiation and plasmon excitation to drive the chemical reduction of Ag⁺ by citrate. Since the initial discovery that Ag triangular prisms could be prepared by the photo‐induced conversion of Ag spherical nanoparticles, plasmon‐mediated synthesis has become a highly controllable technique for preparing a number of different Ag particles with tight control over shape, as well as a wide variety of Au‐Ag bimetallic nanostructures. We discuss the underlying physical and chemical factors that drive structural selection and conclude by outlining some of the important design considerations for controlling particle shape as learned through studies of plasmon‐mediated reactions, but applicable to all methods of noble metal nanocrystal synthesis.     

Surface Segregated AgAu Tadpole‐Shaped Nanoparticles Synthesized Via a Single Step Combined Galvanic and Citrate Reduction Reaction

New AgAu tadpole nanocrystals were synthesized in a one‐step reaction involving simultaneous galvanic replacement between Ag nanospheres and AuCl₄^?_(aq.) and AuCl₄^?_(aq.) reduction to Au in the presence of citrate. The AgAu tadpoles display nodular polycrystalline hollow heads, while their undulating tails are single crystals. The unusual morphology suggests an oriented attachment growth mechanism. Remarkably, a 1 nm thick Ag layer was found to segregate so as to cover the entire surface of the tadpoles. By varying the nature of the seeds (Au NPs), double‐headed Au tadpoles could also be obtained. The effect of a number of reaction parameters on product morphology were explored, leading to new insights into the growth mechanisms and surface segregation behavior involved in the synthesis of bimetallic and anisotropic nanomaterials.     

Discrete Silver(I)‐Palladium(II)‐Oxo Nanoclusters, {Ag4Pd13} and {Ag5Pd15}, and the Role of Metal–Metal Bonding Induced by Cation Confinement

We introduce the class of discrete silver(I)‐palladium(II)‐oxo nanoclusters with the preparation of {Ag₄Pd₁₃} and {Ag₅Pd₁₅}. Both polyanions represent the first examples of noble metal‐capped polyoxo‐noble‐metalates in a fully inorganic assembly, featuring an unprecedented host–guest mode containing hetero‐ and homometallic Ag–Pd and Ag–Ag bonding interactions. Comprehensive theoretical calculations suggest that the Ag–Pd metallic bonds originate partially from surface confinement of Ag^I guest ions onto the anionic polyoxopalladate host that is induced by strong electrostatic forces. This work opens the field of fully inorganic silver‐palladium‐oxo nanoclusters, which can be considered as discrete mixed noble metal precursors for the formation of monodisperse core–shell nanoparticles, with high relevance for catalysis.     

Rapid Synthesis of Hollow Ag–Au Nanodendrites in 15 Seconds by Combining Galvanic Replacement and Precursor Reduction Reactions

Metallic nanomaterials displaying hollow interiors as well as sharp tips/branches at their surface (such as hollow nanodendrites) are attractive, because these features enable higher surface‐to‐volume ratios than their solid and/or rounded counterparts. This paper describes a simple strategy for the synthesis of Ag–Au nanodendrites in 15 s using Ag nanospheres prepared in a previous synthetic step as seeds. Our approach was based on the utilization of Ag nanospheres as seeds for Au deposition by a combination of galvanic replacement reaction between Ag and AuCl₄^?(aq) and AuCl₄^?(aq) reduction using hydroquinone in the presence of polyvinylpyrrolidone (PVP) as a stabilizer and water as the solvent. The produced Ag–Au nanodendrites presented monodisperse sizes, and their surface morphologies could be tuned as a function of growth time. Owing to their hollow interiors and sharp tips, the Ag–Au nanodendrites performed as effective substrates for surface‐enhanced Raman scattering (SERS) detection of 4‐MPy (4‐mercaptopyridine) and R6G (rhodamine 6G) as probe molecules. We believe that the approach described herein can serve as a protocol for the fast and one‐step synthesis of Ag–Au hollow nanondendrites with a wide range of sizes, compositions, and surface morphologies for applications in SERS and catalysis.     

Delivery of Highly Active Noble‐Metal Nanoparticles into Microspherical Supports by an Aerosol‐Spray Method

Noble metal nanoparticles (NPs) with 1–5 nm diameter obtained from NaHB₄ reduction possess high catalytic activity. However, they are rarely used directly. This work presents a facile, versatile, and efficient aerosol‐spray approach to deliver noble‐metal NPs into metal oxide supports, while maintaining the size of the NPs and the ability to easily adjust the loading amount. In comparison with the conventional spray approach, the size of the loaded noble‐metal nanoparticles can be significantly decreased. An investigation of the 4‐nitrophenol hydrogenation reaction catalyzed by these materials suggests that the NPs/oxides catalysts have high activity and good endurance. For 1 % Au/CeO₂ and Pd/Al₂O₃ catalysts, the rate constants reach 2.03 and 1.46 min^?1, which is much higher than many other reports with the same noble‐metal loading scale. Besides, the thermal stability of catalysts can be significantly enhanced by modifying the supports. Therefore, this work contributes an efficient method as well as some guidance on how to produce highly active and stable supported noble‐metal catalysts.     

In situ Deposition of ‘Naked’ Gold Nanoparticles Supported on Silica Spheres as Recyclable Catalysts in Styrene Epoxidation

Tlie rational designs of particle size, morphology and surface states of the Au nanoparticles(AuNPs) are crucial for Au nanocatalyst. We herein report a method to synthesize the silica microspheres supported AuNPs(ca.1 nm) and their application in controlling the reaction conversion and selectivity in styrene epoxidation. Surfactant-ftee AuNPs deposited on silica microspheres were in situ fabricated with aid of the Ag nanoparticles (AgNPs) as sacrificial template by galvanic replacement reaction, leading to AuNPs/SiO2 catalyst directly without any post-treatment to expose crystal facets.A high conversion of 46.7% and selectivity of 91.7% to styrene oxide was achieved with H2O2 as oxidant in ethanol. The solid catalyst could be reused at least 10 reaction cycles without significant decrease in activity and selectivity. This study not only supplies an active, recoverable catalyst for styrene oxidation with green oxidant and solvent, but also demonstrates that the silica microspheres functionalized with thiol groups have a superior ability in stabilizing noble metal nanoparticles even without any surfactant.     

A General and High‐Yield Galvanic Displacement Approach to Au?M (M=Au,Pd, and Pt) Core–Shell Nanostructures with Porous Shells and Enhanced Electrocatalytic Performances

In this work, we utilize the galvanic displacement synthesis and make it a general and efficient method for the preparation of Au? M (M=Au, Pd, and Pt) core–shell nanostructures with porous shells, which consist of multilayer nanoparticles. The method is generally applicable to the preparation of Au? Au, Au? Pd, and Au? Pt core–shell nanostructures with typical porous shells. Moreover, the Au? Au isomeric core–shell nanostructure is reported for the first time. The lower oxidation states of Au^I, Pd^II, and Pt^II are supposed to contribute to the formation of porous core–shell nanostructures instead of yolk‐shell nanostructures. The electrocatalytic ethanol oxidation and oxygen reduction reaction (ORR) performance of porous Au? Pd core–shell nanostructures are assessed as a typical example for the investigation of the advantages of the obtained core–shell nanostructures. As expected, the Au? Pd core–shell nanostructure indeed exhibits a significantly reduced overpotential (the peak potential is shifted in the positive direction by 44 mV and 32 mV), a much improved CO tolerance (I_f/I_b is 3.6 and 1.63 times higher), and an enhanced catalytic stability in comparison with Pd nanoparticles and Pt/C catalysts. Thus, porous Au? M (M=Au, Pd, and Pt) core–shell nanostructures may provide many opportunities in the fields of organic catalysis, direct alcohol fuel cells, surface‐enhanced Raman scattering, and so forth.     

Au–Ag Nanoflower Catalysts with Clean Surfaces for Alcohol Oxidation

Shape‐controlled metal nanocrystals, such as nanowires and nanoflowers, are attractive owing to their potentially novel catalytic properties and bimetallic nanocrystals composed of two distinct metals are expected to act as highly active catalysts. However, their catalytic activities are limited because of the capping agents adsorbed on the metal surfaces, which are necessary for the preparation and dispersion of these nanocrystals in solvents. Therefore, the preparation of bimetallic shape‐controlled noble metal nanocrystals with clean surfaces, devoid of almost all capping agents, are expected to have high catalytic activity. Herein, we report the preparation of bimetallic Au–Ag nanoflowers using melamine as the capping agent. The bimetallic Au–Ag nanoflowers with a clean surface were subsequently obtained by a support and extraction method. The bimetallic nanoflowers with a clean surface were then used for the aerobic oxidation of 1‐phenylethyl alcohol and they exhibited high rates for the formation of acetophenone compared to Au nanoflowers and spherical nanoparticles with almost the same size and Au/Ag ratio. We also show that Au–Ag nanoflowers containing only 1 % Ag (Au₉₉–Ag₁NFs) exhibit the highest rate of acetophenone formation among Au–Ag nanoflowers with different Au/Ag ratios owing to an increase in the electron density of the Au atoms that act as active sites for the oxidation of 1‐phenylethyl alcohol.     

In situ facile loading of noble metal nanoparticles on polydopamine nanospheres via galvanic replacement reaction for multifunctional catalysis

Noble metal nanoparticles(Pd,Ag,Pt,Au) with small and relatively uniform sizes were loaded on polydopamine nanospheres through in situ galvanic replacement reaction in aqueous solution.No additional reductant,surfactant or organic solvent was needed.X-ray photoelectron spectroscopy results revealed that the amount of quinone increased,while the amount of phenolic hydroxyl decreased on PDA nanospheres,indicating that the galvanic displacement reaction occurred between catechol groups and noble metal ions.The as-prepared PDA/Pd exhibited high catalytic activity and excellent stability in styrene hydrogenation.Moreover,PDA spheres retains the photo-thermal effect to serve as a nano-sized heater to accelerate the catalytic reactions under near-infrared illumination.     

Formation of gold and silver nanostructures within polyvinylpyrollidone (PVP) gel

Study on reduction of Au(III) and Ag(I) and the formation of Au and Ag nanostructures was performed on the gels of metal precursor and PVP polymer mixture. Some comparing samples were prepared for better understanding the role of reactants on the reduction of metal ions and further growth of nanocrystals. The results suggest that, in addition to its function of generating stable colloids, PVP not only has a reducing effect on metal ions, but also acts as a crystal growth modifier. At low temperatures, the reducing effect of PVP is strong on Ag(I) ions in AgNO₃, while the reduction of complex Au(III) ions in HAuCl₄ is slow, involving two steps of Au(III)→Au(I)→Au. In the study of temperature disturbance on crystal growth, Au nanoplates of new and well-defined star shape were observed. The differences in the size and shape of nanoparticles are discussed from the colloid chemistry.     

The Fourth Alloying Mode by Way of Anti‐Galvanic Reaction

Anti‐galvanic reaction (AGR) not only defies classic galvanic theory but is a promising method for tuning the compositions, structures, and properties of noble‐metal nanoparticles. Employing AGR for the preparation of alloy nanoparticles has recently received great interest. Herein, we report an unprecedented alloying mode by way of AGR, in which foreign atoms induce structural transformation of the mother nanoparticles and enter the nanoparticles in a non‐replacement fashion. A novel, active‐metal‐doped, gold nanoparticle was synthesized by this alloying mode, and its structure resolved. A CdSH motif was found in the protecting staples of the bimetal nanoparticle. DFT calculations revealed that the Au₂₀Cd₄(SH)(SR)₁₉ nanoparticle is a 8e superatom cluster. Furthermore, although the Cd‐doping does not essentially alter the absorption spectrum of the mother nanocluster, it distinctly enhances the stability and catalytic selectivity of the mother nanoclusters.     

Facile Synthesis of Gold Icosahedra in an Aqueous Solution by Reacting HAuCl4 with N‐Vinyl Pyrrolidone

Herein we describe a protocol that generates Au icosahedra in high yields by simply mixing aqueous solutions of HAuCl₄ and N‐vinyl pyrrolidone. Our mechanistic study reveals that water plays an important role in this synthesis: as a nucleophile, it attacks the gold–vinyl complex, leading to the production of an alcohol‐based Au^I intermediate. This intermediate then undergoes a redox reaction in which Au^I is reduced to Au⁰, leading to the formation of Au atoms and then Au icosahedra of about 18 nm in size at a yield of 94 %, together with a carboxylic acid in the final product. This new protocol has also been employed to prepare multiply twinned nanoparticles of Ag (15–20 nm in size), spherical aggregates (25–30 nm in size) of Pd nanoparticles, and very small nanoparticles of Pt (2 nm in size). Since no organic solvent, surfactant, or polymer stabilizer is needed for all these syntheses, this protocol may provide a simple, versatile, and environmentally benign route to noble‐metal nanoparticles having various compositions and morphologies.     

Green preparation of hollow mesoporous silica nanosphere inside-loaded gold nanoparticles and the catalytic activity

In this work, an active nano-catalyst with gold nanoparticles loaded in hollow mesoporous silica nanospheres (HMSNs/Au) was prepared by a one-pot sol-gel method, in which gold ions were loaded in hollow mesoporous silica spheres followed by sodium alginate reduction. The characterization of the HMSNs/Au were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption–desorption isotherms (BET). The high catalytic activity of HMSNs/Au, denoted as apparent turn-over frequency (TOF), was detected by UV-Vis spectrophotometer for the catalytic reduction of 4-nitrophenol (74.5 h^?1) and 2-nitrophenol (108.7 h^?1) in the presence of sodium borohydride solution due to the small gold nanoparticles size and overall exposure of active sites. It is expected that this ecofriendly approach to prepare inorganic composited nanoparticles as high active catalysts based on hollow mesoporous materials was a promising platform for loading noble metal nanoparticles.     

聚苯胺/贵金属复合纳米材料的可控合成及催化应用

导电高分子/贵金属复合纳米材料因其在催化、传感、表面增强拉曼、光热治疗等诸多领域的应用前景而受到广泛关注.本文主要介绍我们课题组近年来利用可控合成策略制备的负载型和包埋型两种结构聚苯胺/贵金属复合纳米材料,以及利用复合纳米材料的结构和功能特性,对其在多相催化领域的应用、结构与催化性能之间构效关系的探索.     

Evaluation of the plasmonic effect of Au and Ag on Ti-based photocatalysts in the reduction of CO_2 to CH_4

Crystalline TiO_2(P25) and isolated titanate species in a ZSM-5 structure(TS-1) were modified with Au and Ag, respectively, and tested in the gas-phase photocatalytic CO_2 reduction under high purity conditions. The noble metal modification was performed by photodeposition. Light absorbance properties of the catalysts are examined with UV–Vis spectroscopy before and after the activity test. In the gas-phase photocatalytic CO_2 reduction, it was observed that the catalysts with Ag nanostructures are more active than those with Au nanostructures. It is thus found that the energetic difference between the band gap energy of the semiconductor and the position of the plasmon is influencing the photocatalytic activity.Potentially, plasmon excitation due to visible light absorption results in plasmon resonance energy, which affects the excitation of the semiconductor positively. Therefore, an overlap between band gap energy of the semiconductor and metal plasmon is needed.     

Seeded growth core-shell (Ag–Au–Pd) ternary nanostructure at room temperature for potential water treatment

Seed-mediated growth is a promising technique for preparation of multi-metallic nanostructures, in which reduction of metal ions takes a place over the surface of another one. Herein, a seed growth mechanism was investigated for synthesis of core-shell Ag–Au–Pd ternary nanostructures through a facile method at room temperature. Ascorbic acid and sodium alginate were used as nano-generator and stabilizing agent, respectively. Spherical shaped monocular Ag nanostructure with size of 13.6 nm grew to 24.4 nm of Ag–Au binary and to 58.8 nm of Ag–Au–Pd ternary core-shell nanostructures. The crystalline shape of nanostructures was approved by X-Ray diffraction analyses. While, FT-IR data approved the redox mechanism for synthesis the as-required nanostructures. The catalytic reactivity of the prepared nanostructures in reductive degradation of methylene blue dye was studied. The results approved the role of Pd in perfection of catalytic degradation of the as-tested dye. The rate constant of dye degradation was considerably enlarged from 62.1 × 10⁻³ m⁻¹ for Ag monocular nanostructures to 403.3 × 10⁻³ m⁻¹ for Ag–Pd binary and to 852.4 × 10⁻³ m⁻¹ for Ag–Au–Pd ternary core-shell nanostructures. The obtained results offer an energy saving method to fabricate core-shell catalytically active ternary nanostructures with promising applicability in water treatment.     

Dealloying to Nanoporous Silver and Its Implementation as a Template Material for Construction of Nanotubular Mesoporous Bimetallic Nanostructures

Nanoporous silver (NPS) is fabricated by selectively dissolving Al from AgAl alloys in corrosive electrolytes at room temperature. Electron spectroscopy characterizations demonstrate that the NaOH electrolyte is beneficial to the formation of a three‐dimensional bicontinuous porous nanostructure with uniform and tunable pore and ligament dimensions of a few tens of nanometers, while processing in HCl electrolyte easily lead to coarsened porous nanostructures. The high‐surface‐area Ag nanostructures are demonstrated as novel effective template materials to the construction of nanotubular mesoporous Pt/Ag and Pd/Ag alloy structures, which are realized via room temperature galvanic replacement reactions with H₂PtCl₆ and K₂PdCl₄ solutions by adding a high concentration of Cl^? ions as a coordinating agent. Electrochemical measurements indicate that the resulting hollow and porous bimetallic nanostructures show enhanced electrocatalytic activities and CO‐tolerance with better durability toward methanol and formic acid oxidation due to alloying with Ag.