Au Nanoparticles Modified with Pt,Ru and SnO2 as Electrocatalysts for Ethanol Oxidation Reaction in Acids

The anodic reaction in direct ethanol fuel cells (DEFCs), ethanol oxidation reaction (EOR) faces challenges, such as incomplete electrooxidation of ethanol and high cost of the most efficient electrocatalyst, Pt in acidic media at low temperature. In this study, core‐shell electrocatalysts with an Au core and Pt‐based shell (Au@Pt) are developed. The Au core size and Pt shell thickness play an important role in the EOR activity. The Au size of 2.8 nm and one layer of Pt provide the most optimized performance, having 6 times higher peak current density in contrast to commercial Pt/C. SnO₂ as a support also enhances the EOR activity of Au@Pt by 1.73 times. Further modifying the Pt shell with Ru atoms achieve the highest EOR current density that is 15 and 2.5 times of Pt/C and Au@Pt. Our results suggest the importance of surface modification in rational design of advanced electrocatalysts.     

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.     

Plasmonic/Nonlinear Optical Material Core/Shell Nanorods as Nanoscale Plasmon Modulators and Optical Voltage Sensors

Herein, we report the design and synthesis of plasmonic/non‐linear optical (NLO) material core/shell nanostructures that can allow dynamic manipulation of light signals using an external electrical field and enable a new generation of nanoscale optical voltage sensors. We show that gold nanorods (Au NRs) can be synthesized with tunable plasmonic properties and function as the nucleation seeds for continued growth of a shell of NLO materials (such as polyaniline, PANI) with variable thickness. The formation of a PANI nanoshell allows dynamic modulation of the dielectric environment of the plasmonic Au NRs, and therefore the plasmonic resonance characteristics, by an external electrical field. The finite element simulation confirms that such modulation is originated from the field‐induced modulation of the dielectric constant of the NLO shell. This approach is general, and the coating of the Au NRs with other NLO materials (such as barium titanate, BTO) is found to produce a similar effect. These findings can not only open a new pathway to active modulation of plasmonic resonance at the sub‐wavelength scale but also enable the creation of a new generation of nanoscale optical voltage sensors (NOVS).     

Surface-enhanced Raman spectroscopy using gold-core platinum-shell nanoparticle film electrodes: toward a versatile vibrational strategy for electrochemical interfaces

The aim of this work is to further improve the molecular generality and substrate generality of SERS (i.e., to fully optimize the SERS activity of transition-metal electrodes). We utilized a strategy of borrowing high SERS activity from the Au core based on Au-core Pt-shell (Au@Pt) nanoparticle film electrodes, which can be simply and routinely prepared. The shell thickness from about one to five monolayers of Pt atoms can be well controlled by adjusting the ratio of the number of Au seeds to Pt(IV) ions in the solution. The SERS experimental results of carbon monoxide adsorption indicate that the enhancement factor for the Au@Pt nanoparticle film electrodes is more than 2 orders of magnitude larger than that of electrochemically roughened Pt electrodes. The practical virtues of the present film electrodes for obtaining rich and high-quality vibrational information for diverse adsorbates on transition metals are pointed out and briefly illustrated with systems of CO, hydrogen, and benzene adsorbed on Pt. We believe that the electrochemical applications of SERS will be broadened with this strategy, in particular, for extracting detailed vibrational information for adsorbates at transition-metal electrode interfaces.     

Adsorption of CO on various M@Pt core–shell nanoparticles: Surface-enhanced infrared absorption and DFT studies

Addition of some other metals to platinum causes significant increase of its catalytic activity towards ethanol electrochemical oxidation. This may be caused by different adsorption of CO molecules on the surface of the catalyst, and hence different resistance of the M@Pt nanostructures to poisoning by CO. In this work we attempt to verify this hypothesis analyzing vibrational spectra of CO adsorbed on various metal nanoparticles. Au@Pt nanoparticles revealing significantly higher catalytic activity towards ethanol oxidation than one-element Pt nanoparticles have been synthesized. Surface-enhanced infrared absorption (SEIRA) spectra of CO adsorbed on Au@Pt and Pt nanoparticles have been measured. Obtained spectra were very similar, which suggests that the higher catalytic activity of Au@Pt nanoparticles is rather not caused by different adsorption of CO molecules on Pt and Au@Pt nanoparticles. We suppose that better performance of core–shell M@Pt nanoparticles than one elements Pt nanoparticles towards ethanol electrochemical oxidation can be explained as follows: core–shell nanoparticles are probably much more defected than one-element nanoparticles, hence the M@Pt nanoparticles posses greater number of active sites (kinks, adatoms, and so on) for ethanol electrochemical oxidation. Analysis of the catalytic activity and CO adsorption have been also carried out for other nanoparticles including: Sn@Pt, Pb@Pt, Pd, Au@Pd, Sn@Pd and Pb@Pd. Density functional theory (DFT) calculations of CO modes for CO adsorbed on tetrahedral Pt₁₀ or Pd₁₀ clusters with different metal–metal distance have been also performed.     

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.     

Au@Pt纳米粒子催化O2还原反应的电化学研究

以100 nm的Au粒子为核,抗坏血酸为还原剂,H2PtCl6·6H2O为前驱体,合成了Pt包Au核壳结构纳米粒子( Au@ Pt)及其修饰的玻碳(GC)电极(Au@ Pt/GC).采用旋转圆盘电极等常规电化学方法,比较了Au@ Pt/GC和商用碳载铂(Pt/C)修饰的玻碳电极(Pt/C/GC)催化O2还原反应活性及耐甲醇性能,发现Au@ Pt纳米粒子在铂用量很低的情况下,其催化O2还原反应活性仍与商用Pt/C相当,而且还具有优良的耐甲醇性能;其催化O2还原反应机理按O2直接还原成H2O的四电子历程进行.     

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.     

Facile Synthesis of Quasi‐One‐Dimensional Au/PtAu Heterojunction Nanotubes and Their Application as Catalysts in an Oxygen‐Reduction Reaction

An intermediate‐template‐directed method has been developed for the synthesis of quasi‐one‐dimensional Au/PtAu heterojunction nanotubes by the heterogeneous nucleation and growth of Au on Te/Pt core–shell nanostructures in aqueous solution. The synthesized porous Au/PtAu bimetallic nanotubes (PABNTs) consist of porous tubular framework and attached Au nanoparticles (AuNPs). The reaction intermediates played an important role in the preparation, which fabricated the framework and provided a localized reducing agent for the reduction of the Au and Pt precursors. The Pt₇Au PABNTs showed higher electrocatalytic activity and durability in the oxygen‐reduction reaction (ORR) in 0.1 M HClO₄ than porous Pt nanotubes (PtNTs) and commercially available Pt/C. The mass activity of PABNTs was 218 % that of commercial Pt/C after an accelerated durability test. This study demonstrates the potential of PABNTs as highly efficient electrocatalysts. In addition, this method provides a facile strategy for the synthesis of desirable hetero‐nanostructures with controlled size and shape by utilizing an intermediate template.     

Attenuating surface plasmon resonance via core/alloy architectures

The layer-by-layer processing of Au/Au(x)Pd(1-x) core/alloy nanoparticles via microwave irradiation (MWI) based hydrothermal heating is described. Alloy shell growth was monitored by the attenuation of surface plasmon resonance (SPR) as a function of shell thickness and composition. Discrete dipole approximation (DDA) correlated the SPR to particle morphology.     

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.     

Electrocatalytic properties of Au@Pt nanoparticles: effects of Pt shell packing density and Au core size

A detailed study of electrocatalytic properties of Au@Pt nanoparticles (NPs) as functions of Pt shell packing density and Au core size in terms of CO/methanol oxidation and oxygen reduction reactions is reported here. While most samples studied showed inferior catalytic activities to those of the commercial Pt black that fall reasonably well in a d-band-center up-shift (i.e., stronger surface bonding) regime, the steepest activity recovery trend as manifested by the smallest Au-core samples suggests a plausible transition to a d-band-center down-shift (i.e., weaker surface bonding) regime as the Au core becomes smaller.     

Electrochemical Designing of Au/Pt Core Shell Nanoparticles as Nanostructured Catalyst with Tunable Activity for Oxygen Reduction

Au/Pt core shell nanoparticles (NPs) have been prepared via a layer‐by‐layer growth of Pt layers on Au NPs using underpotential deposition (UPD) redox replacement technique. A single UPD Cu monolayer replacement with Pt(II) yielded a uniform Pt film on Au NPs, and the shell thickness can be tuned by controlling the number of UPD redox replacement cycles. Oxygen reduction reaction (ORR) in air‐saturated 0.1 M H₂SO₄ was used to investigate the electrocatalytic behavior of the as‐prepared core shell NPs. Cyclic voltammograms of ORR show that the peak potentials shift positively from 0.32 V to 0.48 V with the number of Pt layers increasing from one to five, suggesting the electrocatalytic activity increases with increasing the thickness of Pt shell. The increase in electrocatalytic activity may originate mostly from the large decrease of electronic influence of Au cores on surface Pt atoms. Rotating ring‐disk electrode voltammetry and rotating disk electrode voltammetry demonstrate that ORR is mainly a four‐electron reduction on the as‐prepared modified electrode with 5 Pt layers and first charge transfer is the rate‐determining step.     

Heterogeneous Au-Pt nanostructures with enhanced catalytic activity toward oxygen reduction

Heterogeneous Au-Pt nanostructures have been synthesized using a sacrificial template-based approach. Typically, monodispersed Au nanoparticles are prepared first, followed by Ag coating to form core-shell Au-Ag nanoparticles. Next, the galvanic replacement reaction between Ag shells and an aqueous H(2)PtCl(6) solution, whose chemical reaction can be described as 4Ag + PtCl(6)(2-)→ Pt + 4AgCl + 2Cl(-), is carried out at room temperature. Pure Ag shell is transformed into a shell made of Ag/Pt alloy by galvanic replacement. The AgCl formed simultaneously roughens the surface of alloy Ag-Pt shells, which can be manipulated to create a porous Pt surface for oxygen reduction reaction. Finally, Ag and AgCl are removed from core-shell Au-Ag/Pt nanoparticles using bis(p-sulfonatophenyl)phenylphosphane dihydrate dipotassium salt to produce heterogeneous Au-Pt nanostructures. The heterogeneous Au-Pt nanostructures have displayed superior catalytic activity towards oxygen reduction in direct methanol fuel cells because of the electronic coupling effect between the inner-placed Au core and the Pt shell.     

A facile method for gold decoration of Te@CdTe nanorods in aqueous solution

Colloidal synthesis of metal-semiconductor hybrid nanostructures is mainly achieved in organic solution. In some applications of hybrid nanoparticles relevant in aqueous media, phase transfer of hydrophobic metal-semiconductor hybrid nanostructures is essential. In this work, we present a simple method for direct synthesis of water-soluble gold (Au) decorated Te@CdTe hybrid nanorods (NRs) at room temperature by using aqueous Te@CdTe NRs as templates, which were preformed by using CdTe nanocrystals (NCs) as precursor in the presence of hydrazine hydrate (N(2)H(4)). Our results showed that NRs were decorated with Au islands both on tips and along the surface of the NRs. The size and density of Au islands can be controlled by varying the amount of Au precursor (mixture of HAuCl(4) and thioglycolic acid (TGA)) and TGA/HAuCl(4) ratio. A possible growth mechanism for the Au decoration of Te@CdTe NRs is concluded as three steps: (1) the formation of AuTe(1.7) via the substitution reaction of Cd(2+) by Au(3+), (2) adsorption of Au-TGA complex onto the preformed AuTe(1.7) anchors and following reduction by CdTe and N(2)H(4), leading to the formation of small Au NCs, (3) Au NCs grow to bigger ones, followed by reduction of more Au precursor by N(2)H(4).     

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。     

Plasmon Coupling-Induced Hot Electrons for Photocatalytic Hydrogen Generation

We present the fabrication of core-shell-satellite Au@SiO₂-Pt nanostructures and demonstrate that LSPR excitation of the core Au nanoparticle can induce plasmon coupling effect to initiate photocatalytic hydrogen generation from decomposition of formic acid. Further studies suggest that the plasmon coupling effect induces a strong local electric field between the Au core and Pt nanoparticles on the SiO₂ shell, which enables creation of hot electrons on the non-plasmonic-active Pt nanoparticles to participate hydrogen evolution reaction on the Pt surface. In addition, small SiO₂ shell thickness is required in order to obtain a strong plamon coupling effect and achieve efficient photocatalytic activities for hydrogen generation.     

Ptshell–Aucore/C electrocatalyst with a controlled shell thickness and improved Pt utilization for fuel cell reactions

Nanoscale Pt_shell–Au_core/C with a controlled shell thickness was successfully synthesized based on a successive reduction strategy. With a Au core size of 4.8 nm, a complete Pt shell of thickness ∼0.6 nm was formed at Pt/Au mole ratio 1:1. The complete coverage of Au core with Pt shell was suggested by various techniques including TEM, UV–vis and cyclic voltammetry. A higher specific activity compared to conventional Pt/C was demonstrated using the probe reaction of methanol electro-oxidation, proving the improved Pt utilization with this core-shell structure.     

Directing the growth of highly aligned gold nanorods through a surface chemical amidation reaction

This paper describes the seed-mediated growth of highly aligned gold nanorods (Au NRs) over large areas directly on a Si(100) surface. The Si(100) surface is NH2-functionalized with (aminopropyl)triethoxysilane (APTES) followed by a DCC-catalyzed surface amidation reaction with acetic acid. After exposure to a gold nanoparticle (Au NP) "seed" solution, chemical seed-mediated growth of the surface-bound seeds via reduction of AuCl4- by ascorbic acid in the presence of cetyltrimethylammonium bromide leads to the growth of highly aligned Au NRs on the surface. About 80% of the NRs are aligned in the same direction within a +/-30 degrees range. Au NRs account for 19% of the nanostructures with average aspect ratio (AR) of approximately 20. The alignment direction did not correlate with the atomic arrangement of the Si(100) crystal since it varied over different regions of the sample, rotating by 90 degrees from top to bottom of an approximately 5 mm sample. Si crystallinity may still be important since alignment is not observed on amorphous glass. Surface functionalization is the key since alignment is only observed following the amidation reaction and not on NH2-functionalized, SH-functionalized, or bare Si(100) surfaces. Alignment also occurred for Au NRs grown on Si(100)/APTES reacted with succinic acid and on Ag NRs grown on Si(100)/APTES/acetic acid surfaces. This unique alignment of metal NRs promoted by a surface amidation chemical reaction may find use in nanoelectronics, chemical sensing, and plasmonics applications.     

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.