Synthesis and Characterization of Silver–Platinum Bimetallic Nanowires and Platinum Nanotubes

A galvanic replacement reaction was used to prepare silver–platinum bimetallic nanowires and platinum nanotubes. Silver nanowires, prepared by boiling aqueous silver nitrate with sodium citrate in the presence of small amount of sodium hydroxide, were used as the sacrificial template in the galvanic reaction to prepare silver–platinum bimetallic nanowires and ultimately hollow platinum nanotubes. The resulting nanomaterials are stable and can be isolated without core aggregation or decomposition. These new materials have been characterized by transmission electron microscopy, energy dispersive X-ray analysis, and inductively coupled plasma atomic emission spectroscopy.     

Mechanistic studies on the galvanic replacement reaction between multiply twinned particles of Ag and HAuCl4 in an organic medium

This article presents a mechanistic study on the galvanic replacement reaction between 11- and 14-nm multiply twinned particles (MTPs) of Ag and HAuCl4 in chloroform. We monitored both morphological and spectral changes as the molar ratio of HAuCl4 to Ag was increased. The details of reaction were different from previous observations on single-crystal Ag nanocubes and cuboctahedrons. Because Au and Ag form alloys rapidly within small MTPs rich in vacancy and grain boundary defects, a complete Au shell did not form on the surface of each individual Ag template. Instead, the replacement reaction resulted in the formation of alloy nanorings and nanocages from Ag MTPs of decahedral or icosahedral shape. For the nanorings and nanocages derived from 11-nm Ag MTPs, the surface plasmon resonance (SPR) peak can be continuously shifted from 400 to 616 nm. When the size of Ag MTPs was increased to 14 nm, the SPR peak can be further shifted to 740 nm, a wavelength sought by biomedical applications. We have also investigated the effects of capping ligands and AgCl precipitate on the replacement reaction. While hollow structures were routinely generated from oleylamine-capped Ag MTPs, we obtained very few hollow structures by using a stronger capping ligand such as oleic acid or tri-n-octylphosphine oxide (TOPO). Addition of extra oleylamine was found to be critical to the formation of well-controlled, uniform hollow structures free of AgCl contamination thanks to the formation of a soluble complex between AgCl and oleylamine.     

Formation of Ag2Se nanotubes and dendrite-like structures from UV irradiation of a CSe2/Ag colloidal solution

Ag2Se nanotubes have been successfully synthesized by UV photodissociation of adsorbed CSe2 on the surface of Ag nanowires under ambient conditions. Transmission electron microscopy was used to trace the growth of hollow interiors, allowing a detail study of the Kirkendall effect in 1-dimensional nanosystems. Voids were observed to grow from both ends of the nanowires along the longitudinal axis and ultimately merged to form hollow nanotubes. This phenomenon is attributed to the crystallographic selective adsorption of poly(vinylpyrolidone) on Ag nanowire templates. In addition, we observed the formation of dendrites on aging of the reaction mixture and explained its growth based on a diffusion-limited aggregation model.     

In Situ Single‐Nanoparticle Spectroscopy Study of Bimetallic Nanostructure Formation

Bimetallic nanostructures (NSs), with utility in catalysis, are typically prepared using galvanic exchange (GE), but the final catalyst morphology is dictated by the dynamics of the process. In situ single nanoparticle (NP) optical scattering spectroscopy, coupled with ex situ electron microscopy, is used to capture the dynamic structural evolution of a bimetallic NS formed in a GE reaction between Ag and [PtCl₆]^2?. We identify an early stage involving anisotropic oxidation of Ag to AgCl concomitant with reductive deposition of small Pt clusters on the NS surface. At later stages of GE, unreacted Ag inclusions phase segregate from the overcoated AgCl as a result of lattice strain between Ag and AgCl. The nature of the structural evolution elucidates why multi‐domain Ag/AgCl/Pt NSs result from the GE process. The complex structural dynamics, determined from single‐NP trajectories, would be masked in ensemble studies due to heterogeneity in the response of different NPs.     

Controllable Galvanic Synthesis of Triangular Ag–Pd Alloy Nanoframes for Efficient Electrocatalytic Methanol Oxidation

Triangular Ag–Pd alloy nanoframes were successfully synthesized through galvanic replacement by using Ag nanoprisms as sacrificial templates. The ridge thickness of the Ag‐Pd alloy nanoframes could be readily tuned by adjusting the amount of the Pd source during the reaction. These obtained triangular Ag–Pd alloy nanoframes exhibit superior electrocatalytic activity for the methanol oxidation reaction as compared with the commercial Pd/C catalyst due to the alloyed Ag–Pd composition as well as the hollow‐framed structures. This work would be highly impactful in the rational design of future bimetallic alloy nanostructures with high catalytic activity for fuel cell systems.     

Morphological Evolution of Single-Crystal Ag Nanospheres during the Galvanic Replacement Reaction with HAuCl(4)

This paper presents a systematic study of the galvanic replacement reaction between 23.5 nm single-crystal Ag nanospheres and HAuCl(4) in an aqueous medium. We have monitored both morphological and spectral changes as the molar ratio of HAuCl(4) to Ag is increased. The replacement reaction on single-crystal Ag nanospheres results in the formation of a series of hollow and porous nanostructures composed of Au-Ag alloys. By varying the molar ratio of HAuCl(4) to Ag, we are able to control the size and density of the pores. In addition, the localized surface plasmon resonance peaks of these nanostructures can be readily tuned from 408 to 791 nm as the product becomes increasingly more hollow and porous.     

Synthesis and optical properties of nanorattles and multiple-walled nanoshells/nanotubes made of metal alloys

The galvanic replacement reaction between silver and chloroauric acid has been exploited as a powerful means for preparing metal nanostructures with hollow interiors. Here, the utility of this approach is further extended to produce complex core/shell nanostructures made of metals by combining the replacement reaction with electroless deposition of silver. We have fabricated nanorattles consisting of Au/Ag alloy cores and Au/Ag alloy shells by starting with Au/Ag alloy colloids as the initial template. We have also prepared multiple-walled nanoshells/nanotubes (or nanoscale Matrioshka) with a variety of shapes, compositions, and structures by controlling the morphology of the template and the precursor salt used in each step of the replacement reaction. There are a number of interesting optical features associated with these new core/shell metal nanostructures. For example, nanorattles made of Au/Ag alloys displayed two well-separated extinction peaks, a feature similar to that of gold or silver nanorods. The peak at approximately 510 nm could be attributed to the Au/Ag alloy cores, while the other peak was associated with the Au/Ag alloy shells and could be continuously tuned in the spectral range from red to near-infrared.     

Synthesis and alignment of silver nanorods and nanowires and the formation of Pt, Pd, and core/shell structures by galvanic exchange directly on surfaces

Here we describe the synthesis of Ag nanorods (NRs) (aspect ratio <20) and nanowires (NWs) (aspect ratio > or =20) directly on surfaces by seed-mediated growth. The procedure involves attaching gold seed nanoparticles (Au NPs) to 3-mercaptopropyltrimethoxysilane (MPTMS)-functionalized silicon or glass surfaces and growing them into NRs/NWs by placing the substrates into a solution containing cetyltrimethylammonium bromide (CTAB), silver nitrate, and ascorbic acid with the pH ranging from 7 to 12. Under our conditions, Ag NRs/NWs grow optimally at pH 10.6 with a 3% yield, where spherical, triangular, and hexagonal nanostructures represent the other byproducts. The length of Ag NRs/NWs ranges from 50 nm to more than 10 microm, the aspect ratio (AR) ranges from 1.4 to >300, and the average diameter is approximately 35 nm. Approximately 40% of the 1D structures are NRs, and 60% are NWs as defined by their ARs. We also report the alignment of Ag NRs/NWs directly on surfaces by growing the structures on amine-functionalized Si(100) surfaces after an amidation reaction with acetic acid and a method to improve the percentage of Ag NRs/NWs on the surface by removing structures of other shapes with adhesive tape. Surface-grown Ag NRs/NWs also react with salts of palladium, platinum, and gold via galvanic exchange reactions to form high-surface-area 1D structures of the corresponding metal. The combination of the seed-mediated growth of Ag on Au NRs followed by the galvanic exchange of Ag with Pd leads to interesting core/shell NRs grown directly on surfaces. We used scanning electron microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy to characterize the surface-grown nanostructures.     

Identification of a Critical Intermediate in Galvanic Exchange Reactions by Single‐Nanoparticle‐Resolved Kinetics

The realization of common materials transformations in nanocrystalline systems is fostering the development of novel nanostructures and allowing a deep look into the atomistic mechanisms involved. Galvanic corrosion is one such transformation. We studied galvanic replacement within individual metal nanoparticles by using a combination of plasmonic spectroscopy and scanning transmission electron microscopy. Single‐nanoparticle reaction trajectories showed that a Ag nanoparticle exposed to Au³⁺ makes an abrupt transition into a nanocage structure. The transition is limited by a critical structural event, which we identified by electron microscopy to comprise the formation of a nanosized void. Trajectories also revealed a surprisingly strong nonlinearity of the reaction kinetics, which we explain by a model involving the critical coalescence of vacancies into a growing void. The critical void size for galvanic exchange to spontaneously proceed was found to be 20 atomic vacancies.     

Controlled synthesis of Ag/TiO2 core-shell nanowires with smooth and bristled surfaces via a one-step solution route

Ag/TiO2 core-shell nanowires were synthesized via a one-step solution method without using a template. Interestingly, the shell morphologies can be controlled to be smooth or bristled by altering the reaction temperature. Moreover, the TiO2 shell thickness and bristle length can be tuned by changing the AgNO3 concentration. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), energy-dispersive X-ray analysis (EDS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the resultant Ag/TiO2 core-shell nanowires. Moreover, the absorption peaks of our samples are significantly red-shifted compared with those of the uncoated pure silver nanowires, indicating that interaction between the core and shell occurred. On the basis of the experimental results, we proposed a template-induced Oswald ripening mechanism to explain the formation of the Ag/TiO2 core-shell nanowires.     

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.     

Synthesis and characterization of Ag nanoshells by a facile sacrificial template route through in situ replacement reaction

A facile in situ replacement reaction route was successfully introduced for synthesizing Ag nanoshells with outer diameters of 40-50 nm and inner diameters of 20-30 nm using Co nanoparticles as sacrificial templates. The products were characterized by XRD, TEM, SAED, and UV-vis absorption spectra. The formation mechanism was also discussed. The reaction driving force comes from the large reduction potential gap between the Ag+/Ag and Co2+/Co redox couples, which results in the consumption of Co cores and the formation of a hollow cavity of Ag nanoshells. The UV-vis spectrum of this nanostructure exhibits a distinct difference from that of solid nanoparticles, which makes it a good candidate for application in photothermal materials.     

Tubular carbon nano-/microstructures synthesized from graphite powders by an in situ template process

Through the use of commercial graphite powders as the carbon sources, a variety of interesting tubular carbon nano- and microstructures, such as networked carbon nanotubes, aligned carbon microtubes with hexagonal cross-sections, aligned tapered carbon tubes, and hollow carbon microhorns, have been successfully synthesized. As-grown tubular carbon structures were characterized using scanning electron microscopy, transmission electron microscopy, and X-ray energy-dispersive spectroscopy. An in situ template mechanism was proposed to explain the possible growth process. The vibrational properties of the synthesized tubular carbon structures were also studied by Raman spectroscopy.     

Preparation and characterization of copper/silver bimetallic nanowires with core‐shell structure

Core‐shell copper/silver bimetallic nanowires were prepared by replacement reaction with citric acid and polyvinylpyrollidone at room temperature. A uniform silver coating was obtained by strictly controlling the molar ratio of Ag/Cu. The copper/silver composite was characterized by X‐ray diffraction, scanning electron microscopy, electron probe microanalysis and X‐ray photoelectron spectroscopy. Microscopic analysis shows that a well‐copper/silver core‐shell structure was formed. Thermo‐gravimetry and differential thermal analysis to the composite nanowires show that the silver coatings efficiently inhibit the oxidation of Cu. Copyright © 2015 John Wiley & Sons, Ltd.     

Self-supported particle-track-etched polycarbonate membranes as templates for cylindrical polypyrrole nanotubes and nanowires: an X-ray scattering and scanning force microscopy investigation

Self-supported particle-track-etched polycarbonate membranes with nearly perfect cylindrical pores are used for the preparation of similarly perfect cylindrical polypyrrole nanowires and nanotubes. A complete investigation of the structural properties that result at different stages of the preparation route of polypyrrole nanowires and nanotubes is based on a combination of real and reciprocal space techniques. Nanoporous membranes with nominal pore size ranging from 5 to 150 nm and pore density up to 10(9) pores/cm(2) made from 21-microm-thick polycarbonate films are used. Polypyrrole nanotubes or nanowires are synthesized inside the pores. A real-space picture of the nanomaterial results from scanning force microscopy (SFM) on ultrathin sections made in two directions to obtain structures in the sample surface as well as perpendicular to the surface. From a model-based fit to the small-angle and ultra-small-angle X-ray scattering (SAXS/USAXS) data, the geometric pore structure is obtained and compared to values determined with scanning electron microscopy (SEM). Nanopores, nanowires, and nanotubes are described by uniform solid cylinders or hollow tubes, which are oriented highly parallel to each other and exhibit a small size distribution. Below a critical pore diameter, solid nanowires are produced whereas above this limit hollow nanotubes result.     

Preparation of carbon supported Pd–Pb hollow nanospheres and their electrocatalytic activities for formic acid oxidation

Pd–Pb hollow nanospheres dispersed on carbon black were developed by a galvanic replacement reaction between sacrificial cobalt nanoparticles and Pd²⁺, Pb²⁺ ions. The as-prepared catalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The electrochemical measurements show that the as-prepared catalysts have excellent catalytic activity for formic acid electrooxidation, which is attributed to the large surface area caused by the hollow structure and the lead doping effect which might modify the electronic structure of the catalysts.     

Preparation and characterization of hollow carambola-shaped silver sulfide microspheres using a microwave-assisted template-free method

A simple and convenient method, free of template, has been proposed to synthesize hollow carambolashaped Ag_2 S microspheres with Ag NO3, thiourea(TU), Na Cl and diethanolamine as reagents using a microwave-assisted method, at low temperatures of below 100 8C. Powder X-ray diffraction(XRD),scanning electron microscopy(FESEM) and high resolution transmission electron microscopy(HRTEM)were employed to characterize the morphology and composition of those microspheres. The results indicated that the hollow carambola-shaped silver sulfide microspheres(with high purity and homogeneous morphology) were prepared by an Ostwald ripening process. A possible formation mechanism of hollow carambola-shaped Ag2 S microspheres was proposed.     

Nitrate ion promoted formation of Ag nanowires in polyol processes: a new nanowire growth mechanism

In polyol processes, it was widely accepted that Ag nanowires (NWs) were formed via uniaxial growth of multiple twinned decahedral particles (MTPs) along the {111} facets. Herein, we show that the above MTP uniaxial growth mechanism for growth of nanorods (NRs) and short nanowires (NWs) is different from that for the growth of long Ag NWs. We provide experimental evidence to show that polycrystalline long Ag NWs (up to ~100 μm) could be formed in high yield (~90%) by a completely different growth mechanism via self-assembly of Ag NPs/NRs. Transmission electron microscope (TEM) measurements show that long Ag NWs are composed of crystalline Ag NPs and NRs with multiple crystal orientations, and many NRs have crystalline structures with pentagonal cross section. Solution phase in situ X-ray diffraction (XRD) measurements show that a strained face-centered tetragonal (fct) phase was gradually formed during the formation and growth of long Ag NWs, in addition to the normal face-centered cubic (fcc) phase. The strained fct phase disappears after partial etching by HAuCl(4) and Fe(NO(3))(3). The working conditions for the MTP uniaxial growth mechanism and the current nitrate-promoted self-assembly growth mechanism will be compared and discussed.     

石墨烯负载的空心银钯纳米粒子的制备及其在过氧化氢检测中的应用

制备了在石墨烯上修饰的AgPd合金纳米粒子,并将其应用于对过氧化氢(H2O2)的电化学检测。通过两步合成得到此纳米复合物:利用柠檬酸钠作为保护剂和还原剂,合成石墨烯负载的Ag纳米粒子(Ag/GO);在100℃下,通过Ag与Pd2+之间的置换反应得到石墨烯负载的具有空心结构的AgPd合金纳米粒子(AgPd/GO)。用紫外-可见光谱(UV-vis),电子显微镜(TEM)等对此纳米复合物的结构及其吸收光谱和形貌进行了表征。电化学循环伏安法(CV)和计时安培法(i-t)的研究结果表明,修饰在石墨烯上的空心AgPd合金纳米粒子对H2O2还原具有显著的催化活性,对H2O2检测的线性范围为0.01～1.4 mmol/L(R=0.990);检出限为1.4μmol/L。     

Tetraethylenepentamine-directed controllable synthesis of wurtzite ZnSe nanostructures with tunable morphology

A novel tetraethylenepentamine (TEPA)-directed method has been successfully developed for the controlled synthesis of ZnSe particles with distinctive morphologies, including nanobelts, nanowires, and hierarchically solid/hollow spheres. These structures, self-assembled from nanobelts and nanorods, have been synthesized by adjusting the reaction parameters, such as the solvent composition, reaction temperature, and the aging time. Results reveal that the volume ratio of H2O and TEPA plays a crucial role in the final morphology of ZnSe products. The mechanisms of phase formation and morphology control of ZnSe particles are proposed and discussed in detail. The products have been characterized by means of X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy (TEM), selected area electron diffraction, high-resolution TEM, Raman spectra and luminescence spectroscopy. The as-prepared ZnSe nanoparticles display shape- and size-dependent photoluminescent optical properties. This is the first time to report preparation of complex hollow structures of ZnSe crystals with hierarchy through a simple solution-based route. This synthetic route is designed to exploit a new H2O/TEPA/N2H4H2O system possibly for the preparation of other semiconductor nanomaterials.