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.     

Core-shell Ag–Pt nanoparticles: A versatile platform for the synthesis of heterogeneous nanostructures towards catalyzing electrochemical reactions

Heterogeneous nanostructures that are defined as a hybrid structure consisting of two or more nanoscale domains with distinct chemical compositions or physical characteristics have attracted intense efforts in recent years. In this review, we focus on the introduction of a number of heterogeneous nanostructures derived using core-shell Ag–Pt nanoparticles as starting materials, including hollow, dimeric and composite structures and also highlight their application in catalyzing electrochemical reactions, e.g., methanol oxidation reaction and oxygen reduction reaction. This review not only shows the capability of core-shell Ag–Pt nanoparticles in producing various heterogeneous nanostructures as starting templates, but also highlights the structural design or electronic interaction that endows the heterogeneous nanostructures with enhanced catalytic properties either in methanol oxidation or in oxygen reduction. Further, we also make some perspectives for more heterogeneous nanostructures that may be prepared by using core-shell Ag–Pt particles or their derivatives so as to offer the readers the opportunities and challenges in this field.     

Preparation of carbon-supported core-shell Au-Pt nanoparticles for methanol oxidation reaction: The promotional effect of the Au core

Core-shell Au-Pt nanoparticles with intimate contact of Pt and Au were prepared by a displacement reaction without formation of monometallic Au nanoparticles. The Au-Pt nanoparticles were dispersed on carbon (Au@Pt/C) and were used to catalyze methanol electrooxidation in acidic solutions at room temperature. The core-shell nanostructure was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy, and specific catalytic activities were evaluated by CO anodic stripping voltammetry in 0.5 M H(2)SO(4) and by cyclic voltammetry in 1 M CH(3)OH + 0.5 M H(2)SO(4). The Au@Pt/C catalyst demonstrated enhanced specific activity in methanol electrooxidation and showed multiple CO stripping peaks which were all negatively shifted with respect to a similarly prepared Ag@Pt/C catalyst. The activity enhancement is attributed to the presence of Au underneath a very thin Pt shell where electron exchange between Au and Pt had promoted the formation of active oxygen species on Pt, which facilitated the removal of inhibiting CO-like reaction intermediates.     

Trimetallic Au@PdPt core-shell nanoparticles with ultrathin PdPt skin as highly stable electrocatalysts for the oxygen reduction reaction in acid solution

To design efficient and low-cost core-shell electrocatalysts with an ultrathin platinum shell, the balance between platinum dosage and durability in acid solution is of great importance. In the present work, trimetallic Au@PdPt core-shell nanoparticles(NPs)with Pd/Pt molar ratios ranging from 0.31:1 to 4.20:1 were synthesized based on the Au catalytic reduction strategy and the subsequent metallic replacement reaction. When the Pd/Pt molar ratio is 1.19:1(designated as Au@Pd_(1.19) Pt_1 NPs), the superior electrochemical activity and stability were achieved for oxygen reduction reaction(ORR) in acid solution. Especially, the specific and mass activities of Au@Pd_(1.19) Pt_1 NPs are 1.31 and 6.09 times higher than those of commercial Pt/C catalyst. In addition, the Au@Pd_(1.19) Pt_1 NPs presented a good durability in acid solution. After 3000 potential cycles between 0.1 and 0.7 V(vs. Ag/AgCl), the oxygen reduction activity is almost unchanged. This study provides a simple strategy to synthesize highperformance trimetallic ORR electrocatalyst for fuel cells.     

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.     

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.     

Carbon-supported Pt^Ag nanostructures as cathode catalysts for oxygen reduction reaction

Pt(m)^Ag nanostructures (m being the atomic Pt/Ag ratio, m = 0.1-0.6) were prepared by reflux citrate reduction of PtCl(6)(2-) ions in aqueous solution containing colloidal Ag (6.3 ± 3.9 nm). A distinct alloying of Pt with Ag was detected due to an involvement of the galvanic replacement reaction between PtCl(6)(2-) and metallic Ag colloids. The nanostructure transformed from a structure with an Ag-core and an alloyed PtAg-shell to a hollow PtAg alloy structure with the increase in m. Compared to a commercial E-TEK Pt/C catalyst, the catalytic performance of Pt in the Pt(m)^Ag/C samples for the cathode oxygen reduction reaction (ORR) strongly correlated with the electronic structure of Pt, as a consequence of varied Pt dispersion and Pt-Ag interaction. With either H(2)SO(4) or KOH as an electrolyte, Pt in the Pt(m)^Ag nanostructures with a relatively high m (≥0.4) showed significantly enhanced intrinsic activity whereas Pt in those catalysts with low m (≤0.2) appeared less active than the Pt/C catalyst. These data are used to discuss the role of electronic structure and geometric effects of Pt toward ORR.     

Characterization of Ag/Pt core-shell nanoparticles by UV-vis absorption, resonance light-scattering techniques

The water-soluble Ag/Pt core-shell nanoparticles were prepared by deposition Pt over Ag colloidal seeds with the seed-growth method using K2PtCl4 with trisodium citrate as reduced agent. The Ag:Pt ratio is varied from 9:1 to 1:3 for synthesizing Pt shell layer of different thickness. A remarkable shift and broadening of Ag surface plasmon band around 410 nm was observed. The contrast of TEM images of Ag/Pt colloids has been obtained. Various techniques, such as transmission electron microscopy (TEM), UV-vis absorption and resonance light-scattering spectroscopy were used to characterize nanoparticles. The data of TEM, UV-vis and resonance light-scattering spectrum all confirm formation of Ag/Pt core-shell nanoparticles. Resonance light-scattering and emission spectrum show the Ag and Ag/Pt core-shell nanoparticles have a nonlinear light-scattering characteristic.     

Ultrafast spectroscopy and coherent acoustic phonons of Au-Ag core-shell nanorods

We performed the first investigations of coherent acoustic phonons in Au-Ag core-shell nanorods, which were compared with the results of parental Au nanorods. Both breathing and extensional modes were observed in Au-Ag core-shell nanorods with ~11 nm Ag shell while only extensional modes were detected in other core-shell nanorods with 4-7 nm Ag shell. Young's modulus estimated from the oscillation period of extensional modes was found to be larger for Au-Ag core-shell nanorods with ~4 nm Ag shell, as compared with that of Au nanorods. The value of Young's modulus decreases with the increase of the Ag shell thickness and finally becomes smaller than that of Au nanorods. This phenomenon is interpreted in terms of the surface effects and the existence of grain boundaries in the lattice structure of Ag shell.     

贵金属在Ag_2S纳米颗粒中由内向外的迁移现象

纳米颗粒具有明显区别于块体材料的新奇特性,本文利用透射电镜观察,描述并讨论一种发生在贵金属(Au、Ag、Pd和Pt)和硫化银(Ag_2S)构成的核壳结构纳米颗粒中的有趣现象,即贵金属在Ag_2S纳米颗粒中由内向外的迁移。迁移可在室温下进行,其最终结果使最初的核壳结构颗粒演变成由贵金属和Ag_2S构成的异质纳米二聚体结构,如Au-Ag_2S、Ag-Ag_2S、PdAg_2S和Pt-Ag_2S。电镜表征表面实验条件下贵金属在Ag_2S的迁移类似于一种整体迁移的模式且迁移过程中伴随着颗粒形貌结构的演变。贵金属在Ag_2S中的经空位互换的扩散机制或半导体纳米颗粒的自纯化机制可以用来解释这种迁移现象。     

Mechanistic study on the replacement reaction between silver nanostructures and chloroauric acid in aqueous medium

The replacement reaction between silver nanostructures and an aqueous HAuCl(4) solution has recently been demonstrated as a versatile method for generating metal nanostructures with hollow interiors. Here we describe the results of a systematic study detailing the morphological, structural, compositional, and spectral changes involved in such a heterogeneous reaction on the nanoscale. Two distinctive steps have been resolved through a combination of microscopic and spectroscopic methods. In the first step, silver nanostructure (i.e., the template) is dissolved to generate gold atoms that are deposited epitaxially on the surface of each template. Silver atoms also diffuse into the gold shell (or sheath) to form a seamless, hollow nanostructure with its wall made of Au-Ag alloys. The second step involves dealloying, a process that selectively removes silver atoms from the alloyed wall, induces morphological reconstruction, and finally leads to the formation of pinholes in the walls. Reaction temperature was found to play an important role in the replacement reaction because the solubility constant of AgCl and the diffusion coefficients of Ag and Au atoms were both strongly dependent on this parameter. This work has enabled us to prepare metal nanostructures with controllable geometric shapes and structures, and thus optical properties (for example, the surface plasmon resonance peaks could be readily shifted from 500 to 1200 nm by controlling the ratio between Ag and HAuCl(4)).     

Formation of multishell Au@Ag@Pt nanoparticles by coreduction method: a microscopic study

Metallic nanoparticles of Ag–Pt double-shell on Au-core (Au@Ag@Pt core@multishell NPs) with a hollow-granular shell structure were synthesized by coreduction method, a combination of galvanic replacement reaction with a coreducing agent. Their nanostructures were examined in detail at different reduction reaction periods to gain insights into the mechanism of Ag–Pt double-shell formation on Au-core, as well as the competitive role of each reduction reaction. The multishell NPs were found to contain a degree of hollows with the inner surface composed of both Ag and Pt, while the outer surface composed of granular Pt. The coreduction method contributed to the success of increasing Pt surface area that will further benefit catalytic applications of the NPs.     

Au/Ag核一壳结构复合纳米粒子形成机制的研究

在已制备好的Au纳米粒子表面,通过化学还原的方法沉积生长Ag包覆层,通过 控制Au, Ag的比列,制备了粒度均匀且粒径可控的Au/Ag核-壳结构纳米粒子。利用 UV-vis吸收光谱和透射电子显微镜（TEM）对SAu, Ag摩尔比为1：10的复合纳米粒 子的光学性质和形态进行随时监测,直接观察了核-壳结构纳米粒子的生长过程： 一部分Ag+在Au核表面还原生长,溶液中其余Ag+还原形成银的纳米团簇向粒子表面 的继续沉积生长,壳层增厚。     

Synergistic catalysis of Au@Ag core-shell nanoparticles stabilized on metal-organic framework

For the first time, this work presents Au@Ag core-shell nanoparticles (NPs) immobilized on a metal-organic framework (MOF) by a sequential deposition-reduction method. The small-size Au@Ag NPs reveal the restriction effects of the pore/surface structure in the MOF. The modulation of the Au/Ag ratio can tune the composition and a reversed Au/Ag deposition sequence changes the structure of Au-Ag NPs, while a posttreatment process transforms the core-shell NPs to a AuAg alloy. Catalytic studies show a strong bimetallic synergistic effect of core-shell structured Au@Ag NPs, which have much higher catalytic activities than alloy and monometallic NPs.     

Fabrication of nanorattles with passive shell

This investigation describes the formation of a metal nanorattle with a pure metal shell by varying experimental parameters. The galvanic replacement reaction between silver and chloroauric acid was adopted to prepare hollow metal nanoparticles. This approach is extended to produce nanorattles of Au cores and Au shells by starting with Au(core)Ag(shell) nanoparticles as templates. The effect of temperature on the nanostructure of the final product is also considered. The composition of the shell in nanorattles can be controlled by varying the reaction temperature (to form pure gold or gold-silver alloy, for example). X-ray absorption fine structure spectroscopy is conducted to elucidate the fine structure of these nanoparticles. Partial alloying between the Au core and the Ag shell is observed by extended X-ray absorption fine structure (EXAFS).     

A general method for the rapid synthesis of hollow metallic or bimetallic nanoelectrocatalysts with urchinlike morphology

We have reported a facile and general method for the rapid synthesis of hollow nanostructures with urchinlike morphology. In-situ produced Ag nanoparticles can be used as sacrificial templates to rapidly synthesize diverse hollow urchinlike metallic or bimetallic (such as Au/Pt) nanostructures. It has been found that heating the solution at 100 degrees C during the galvanic replacement is very necessary for obtaining urchinlike nanostructures. Through changing the molar ratios of Ag to Pt, the wall thickness of hollow nanospheres can be easily controlled; through changing the diameter of Ag nanoparticles, the size of cavity of hollow nanospheres can be facilely controlled; through changing the morphologies of Ag nanostructures from nanoparticle to nanowire, hollow Pt nanotubes can be easily designed. This one-pot approach can be extended to synthesize other hollow nanospheres such as Pd, Pd/Pt, Au/Pd, and Au/Pt. The features of this technique are that it is facile, quick, economical, and versatile. Most importantly, the hollow bimetallic nanospheres (Au/Pt and Pd/Pt) obtained here exhibit an area of greater electrochemical activity than other Pt hollow or solid nanospheres. In addition, the approximately 6 nm hollow urchinlike Pt nanospheres can achieve a potential of up to 0.57 V for oxygen reduction, which is about 200 mV more positive than that obtained by using a approximately 6 nm Pt nanoparticle modified glassy carbon (GC) electrode. Rotating ring-disk electrode (RRDE) voltammetry demonstrates that approximately 6 nm hollow Pt nanospheres can catalyze an almost four-electron reduction of O(2) to H(2)O in air-saturated H(2)SO(4) (0.5 M). Finally, compared to the approximately 6 nm Pt nanoparticle catalyst, the approximately 6 nm hollow urchinlike Pt nanosphere catalyst exhibits a superior electrocatalytic activity toward the methanol oxidation reaction at the same Pt loadings.     

Au-Ag合金纳米粒子制备及其表面增强拉曼光谱研究

首先采用柠檬酸钠法制得Au-Ag合金纳米种子, 然后采用盐酸羟胺生长法得到不同组成的Au-Ag合金纳米粒子. 在其UV-Vis光谱中只观察到一个位于单金属银和金之间的等离子体共振峰, 表明Au-Ag合金纳米粒子已经形成. TEM结果表明, 合金纳米粒子的粒径约为60 nm, 且颜色均一, 没有明显的核壳结构. 用苯硫酚(TP)作为探针分子研究了合金纳米粒子的表面增强拉曼光谱(SERS). 结果表明, SERS强度与合金纳米粒子的组成和尺寸有关. 当纳米粒子粒径一定时, 除Au25Ag75外, 随着金的增加SERS强度增强. Au25Ag75的粒径比Ag小, 导致SERS强度比Ag低. Au50Ag50和Au75Ag25加入TP分子后, 其聚集方式与Au相似, 等离子体共振峰逐渐靠近1064 nm, 金含量较高时, TP的SERS归于聚集体的等离子体共振增强的贡献.     

Bimetallic (Ag)Au nanoparticles prepared by the seed growth method: two-dimensional assembling, characterization by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and surface enhanced raman spectroscopy, and proposed mechanism of growth

Layered core-shell bimetallic silver-gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2'-bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.     

Interfacial deposition of Ag on Au seeds leading to AucoreAgshell in organic media

A seed mediated procedure for the synthesis of hydrophobic Au(core)Ag(shell) nanoparticles in toluene is demonstrated. The reaction proceeds by way of the interfacial reduction of silver ions by 3-pentadecylphenol followed by their deposition on hydrophobized Au nanoparticles. Such a hitherto unreported interfacial seeded growth reaction leads to the formation of phase pure Au(core)Ag(shell) nanoparticles that retain the hydrophobicity of the seed particles and remain stable in toluene. Such core-shell structures are however not formed in the aqueous phase. The core-shell architecture was verified using TEM analysis and the formation process was studied by recording the UV-vis spectra of the organic phase nanoparticles as a function of time. TEM kinetics also showed gradual increase in the silver layer thickness. Conclusive evidence was however obtained on examination of the HRTEM images of the products formed. Elemental analysis using X-ray photoelectron spectroscopy of the Au(core)Ag(shell) nanostructure revealed the presence of metallic silver. Moreover changing the surface capping of the Au seed does not affect the formation of the Au(core)Ag(shell) nanostructure.     

Pt@Au/Al2O3核壳纳米粒子的制备和表征及其在催化氧化甲苯中的应用

Customizing core-shell nanostructures is considered to be an efficient approach to improve the catalytic activity of metal nanoparticles. Various physiochemical and green methods have been developed for the synthesis of core-shell structures. In this study, a novel liquid-phase hydrogen reduction method was employed to form core-shell Pt@Au nanoparticles with intimate contact between the Pt and Au particles, without the use of any protective or structure-directing agents. The Pt@Au core-shell nanoparticles were prepared by depositing Au metal onto the Pt core; AuCl⁴⁻ was reduced to Au(0) by H₂ in the presence of Pt nanoparticles. The obtained Pt@Au core-shell structured nanoparticles were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution TEM, fast Fourier transform, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and H₂-temperature programmed reduction (H₂-TPR) analyses. The EDX mapping results for the nanoparticles, as obtained from their scanning transmission electron microscopy images in the high-angle annular dark-field mode, revealed a Pt core with Au particles grown on its surface. Fourier transform measurements were carried out on the high-resolution structure to characterize the Pt@Au nanoparticles. The lattice plane at the center of the nanoparticles corresponded to Pt, while the edge of the particles corresponded to Au. With an increase in the Au content, the intensity of the peak corresponding to Pt in the FTIR spectrum decreased slowly, indicating that the Pt nanoparticles were surrounded by Au nanoparticles, and thus confirming the core-shell structure of the nanoparticles. The XRD results showed that the peak corresponding to Pt shifted gradually toward the Au peak with an increase in the Au content, indicating that the Au particles grew on the Pt seeds; this trend was consistent with the FTIR results. Hence, it can be stated that the Pt@Au core-shell structure was successfully prepared using the liquid-phase hydrogen reduction method. The catalytic activity of the nanoparticles for the oxidation of toluene was evaluated using a fixed-bed reactor under atmospheric pressure. The XPS and H₂-TPR results showed that the Pt₁@Au₁/Al₂O₃ catalyst had the best toluene oxidation activity owing to its lowest reduction temperature, lowest Au 4d & 4f and Pt 4d & 4f binding energies, and highest Au⁰/Au^δ+ and Pt⁰/Pt²⁺ proportions. The Pt₁@Au₂Al₂O₃ catalyst showed high stability under dry and humid conditions. The good catalytic performance and high selectivity of Pt@Au/Al₂O₃ for toluene oxidation could be attributed to the high concentration of adsorbed oxygen species, good low-temperature reducibility, and strong interaction.