Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

In this study, we used size-exclusion chromatography (SEC) to evaluate the sizes of Au and Au/Pd core/shell nanoparticles (NPs) that had been subjected to thermal treatment, with the eluted NPs monitored through diode array detection (DAD) of the surface plasmon (SP) absorption of the NPs. In the absence of an adequate stabilizer, thermal treatment resulted in longer retention times for the Au NPs and shorter retention times for the Au/Pd core/shell NPs in the SEC chromatograms. Thus, thermal treatment influenced the sizes of these Au and Au/Pd core/shell NPs, through digestive ripening and Ostwald-type growth, respectively. In addition, the trends in the SP absorption phenomena of the NPs in the eluted samples, as measured using DAD, were consistent with the trends of their size variations, as measured from their elution profiles. In the presence of 3A-amino-3A-deoxy-(2AS,3AS)-??-cyclodextrin (H₂N-??-CD) as a stabilizer, the retention times and SP absorptions of the eluted Au and Au/Pd NP samples remained constant. Thus, H₂N-??-CD is a good stabilizer against size variation duration the thermal treatment of Au and Au/Pd core/shell NPs. A good correlation existed between the sizes obtained using SEC and those provided by transmission electron microscopy. Therefore, this SEC strategy is an effective means of further searching for suitable stabilizers for NPs, especially those exposed to harsh reaction conditions (e.g., in catalytic reactions).     

Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

In this study, we used size-exclusion chromatography (SEC) to evaluate the sizes of Au and Au/Pd core/shell nanoparticles (NPs) that had been subjected to thermal treatment, with the eluted NPs monitored through diode array detection (DAD) of the surface plasmon (SP) absorption of the NPs. In the absence of an adequate stabilizer, thermal treatment resulted in longer retention times for the Au NPs and shorter retention times for the Au/Pd core/shell NPs in the SEC chromatograms. Thus, thermal treatment influenced the sizes of these Au and Au/Pd core/shell NPs, through digestive ripening and Ostwald-type growth, respectively. In addition, the trends in the SP absorption phenomena of the NPs in the eluted samples, as measured using DAD, were consistent with the trends of their size variations, as measured from their elution profiles. In the presence of 3A-amino-3A-deoxy-(2AS,3AS)-β-cyclodextrin (H₂N-β-CD) as a stabilizer, the retention times and SP absorptions of the eluted Au and Au/Pd NP samples remained constant. Thus, H₂N-β-CD is a good stabilizer against size variation duration the thermal treatment of Au and Au/Pd core/shell NPs. A good correlation existed between the sizes obtained using SEC and those provided by transmission electron microscopy. Therefore, this SEC strategy is an effective means of further searching for suitable stabilizers for NPs, especially those exposed to harsh reaction conditions (e.g., in catalytic reactions).

     

Hyperbranched polyglycidol assisted green synthetic protocols for the preparation of multifunctional metal nanoparticles

Biocompatible hyperbranched polyglycidol (HBP) has been demonstrated to be an effective reducing and stabilizing agent for the synthesis of highly water-soluble monometallic (Au, Ag, Pt, Pd, and Ru) and bimetallic (Au/Pt, Au/Pd, and Au/Ru) nanoparticles (NPs), which provides a general and green protocol to fabricate metal NPs. The HBP-assisted reduction of metal ions follows an analogous polyol process. The reduction reaction rate increases sharply by increasing the temperature and the molecular weight of HBP. The size of NPs is controllable simply by changing the concentration of the metal precursor. High molecular weight HBP is favorable for the formation of NPs with uniform size and improved stability. By utilizing hydroxyl groups in the HBP-passivation layer of Au NPs, TiO(2)/Au, GeO(2)/Au, and SiO(2)/Au nanohybrids are also fabricated via sol-gel processes, which sets a typical example for the creation of versatile metal NPs/inorganic oxide hybrids based on the as-prepared multifunctional NPs.     

Monitoring Stability and Sizes of Au/Pd Core/Shell Nanoparticles by SEC

This paper describes how size exclusion chromatography (SEC) can be used to rapidly characterize Au/Pd core/shell nanoparticles (NPs). We monitored the sizes of Au/Pd core/shell NPs by effecting SEC separation using a mobile phase of 10 mM sodium dodecyl sulfate (SDS); the plot of retention time with respect to the standard size of the Au NPs was linear (R ² = 0.991) for diameters falling in the range from 12.1 to 59.9 nm; for five consecutive runs, the relative standard deviations of these retention times were less than 0.4%. Under the optimized separation conditions, we found that the addition of the surfactant SDS stabilized the Au/Pd core/shell NP samples. In addition, SEC analysis revealed that the sizes of the Au/Pd core/shell NPs could be controlled via modification of the rate of addition of the reducing agent and the use of adequate volumes of the seed and shell precursor metal ion solutions. When using these conditions to analyze the Au/Pd core/shell NPs produced through seed-assisted synthesis, a good correlation existed between the sizes determined through SEC and transmission electron microscopy. Our results suggest that SEC is a useful technique for monitoring the sizes of NPs and nanomaterials in general.     

纳米Pd和Au催化CO2还原粒径效应的密度泛函理论计算研究

以可再生能源为能量来源,在水溶液中进行的光(电)催化CO₂还原生成高附加值化学品和燃料是解决能源危机与环境污染的有效途径之一.CO是一种简单却很重要的CO₂还原产物,它可以作为水煤气变换反应与费托合成的重要原料.具有较高CO选择性的贵金属纳米颗粒催化剂(如Au和Pd)一直受到研究者的广泛关注.一般来说,金属颗粒催化剂的催化性能与粒径大小密切相关,即所谓的粒径效应.然而在实际的理论计算研究中,由于受到计算能力的限制,催化剂模型都仅局限于简单的周期性模型或小的金属团簇模型,无法准确描述真实颗粒上复杂的反应位点的性质,导致了对催化行为的误解.因此,建立更加真实的颗粒模型对探究纳米颗粒催化剂上活性位点的性质,解释其粒径效应至关重要.本文旨在阐述Au与Pd纳米颗粒催化剂不同活性位点上CO₂还原反应与产H2副反应的竞争机制,并解释Au与Pd纳米颗粒催化剂在CO₂电还原中表现出不同粒径效应的原因.本文基于密度泛函理论,采用VASP软件,BEEF-vdW泛函进行计算.分别建立了原子数为55,147,309和561的颗粒模型和高CO^*覆盖度模型,避免了传统周期性模型的局限性,探究了金属颗粒催化剂不同反应位点上的CO选择性.结果表明,对于颗粒模型来说,(100)位点对CO的选择性优于边缘位点;但对于周期性模型来说,Au(211)对CO的选择性则优于Au(100).产生这种反差的主要原因在于Au颗粒的边缘位点对H*的吸附过强.通过对比,我们直观地展现了颗粒模型上平面位点和Edge位点与相对应的周期性模型上CO选择性的区别,突出了模型选择对揭示活性位点性质的重要性.在此基础上,通过计算理论CO法拉第效率,发现Au颗粒随着粒径的减小,CO选择性降低,与实验的趋势一致.对于Pd催化剂来说,低覆盖度模型无法正确预测活性位点的性质;而高CO覆盖度的情况下,Pd颗粒的边缘位点对COOH^*吸附能更强,这是导致边缘位点上CO选择性更高的主要原因.同样通过计算理论CO法拉第效率,发现随着粒径的减小,Pd颗粒上CO选择性升高.本文不仅成功揭示了Au与Pd颗粒催化剂上活性位点的性质,对粒径效应做出了合理解释,也强调了合理的计算模型是理论研究的基础.     

Organosulfur-functionalized Au, Pd, and Au-Pd nanoparticles on 1D silicon nanowire substrates: preparation and XAFS studies

A hybrid preparative method was developed to prepare organosulfur-functionalized Au nanoparticles (NPs) on silicon nanowires (SiNWs) by reacting HAuCl(4) with SiNW in the presence of thiol. A number of organosulfur molecules-dodecanethiol, hexanethiol, 1,6-hexanedithiol, and tiopronin-were used to functionalize the Au surface. Size-selected NPs ranging from 1.6 to 7.5 nm were obtained by varying the S/Au ratio and the concentration of HAuCl(4). This method was further extended to the preparation Pd and Pd-Au bimetallic NPs on SiNWs. The morphology of the metal nanostructures was examined by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The local structure and bonding of the SiNW-supported metal nanostructures were studied using X-ray absorption fine structures (XAFS) [including both X-ray near-edge structures (XANES) and extended X-ray absorption fine structures (EXAFS)] at the Au L(3)-, Pd K-, S K-, and Si K-edges. It was also found that the annealing of the thiol-capped Au NPs up to 500 degrees C transforms the surface of the thiol-capped NPs to gold sulfide, as identified using Au L(3)- and S K-edge XANES. We also illustrate that this preparative approach can be used to form size-controllable Au NPs on carbon nanotubes.     

Highly catalytic spherical carbon nanocomposites allowing tunable activity via controllable Au-Pd doping

We report the synthesis of highly catalytic spherical carbon composite particles with Au-Pd bimetallic nanoparticle doping using a microwave-assisted technique that allows control over the location of the nanoparticles (NPs), putting them into stable interior, but still near-surface locations (within a 100 nm thick shell). First, composite particles with Pd NPs inside of nanoporous carbon spheres (CSs) were synthesized. Subsequent immersion of the composite particles in HAuCl(4) solutions containing PVP led to an addition of Au near the Pd. Au-Pd/CS composites with Au:Pd atomic ratios varying from 0.4 to 4.6 were prepared. The growth of Au and its location relative to the carbon's surface and the Pd are discussed. The catalytic activity towards the reduction of 4-nitrophenol is tunable via the Au:Pd atomic ratio. Optimizing the composition increases the activity a hundredfold over that of the corresponding monometallic Pd/CS. The catalytic activity arises from the synergy between different contributing mechanisms, here especially the interaction between the carbon matrix and metals, metal-metal interfaces, and the hydrogen absorption capabilities of Pd.     

Bimetallic palladium–gold nanoparticles synthesized in ionic liquid microemulsion

The palladium and gold precursors were dissolved in dispersive and continuous phase of ionic liquid microemulsion (H₂O/Triton X-100 (TX-100)/1-butyl-3-methylimidazolium hexafluorophosphate), respectively. [PdCl₆]^2? ions were reduced in situ by TX-100 in dispersive phase (H₂O) to prepare Pd nanoparticles (NPs) and then [AuCl₄]^? crossed through the interface film and reacted with the as-prepared Pd NPs to form Pd₄Au NPs. The as-prepared Pd₄Au NPs were characterized by transmission electronic microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and ultraviolet–visible spectroscopy. The as-prepared Pd₄Au NPs suspension and carbon nanotubes (CNTs) suspension were vigorously stirred to prepare the electrocatalyst supported on the CNTs with a total metal loading of 20?wt.% (denoted by Pd₄Au/CNTs). Cyclic voltammetry and chronoamperometry tests show that the Pd₄Au/CNTs are very promising for the oxidation of ethanol in alkaline medium. The result can be attributed to the synergistic effect between Pd and Au during the catalytic process.     

Observations of shape-dependent hydrogen uptake trajectories from single nanocrystals

In this work, H(2) absorption and desorption in faceted, crystalline Au/Pd core/shell nanocrystals and their interaction with a SiO(x)/Si support were studied at the single-particle level. Dark-field microscopy was used to monitor the changing optical properties of these Au/Pd nanoparticles (NPs) upon exposure to H(2) as reversible H(2) uptake from the Pd shell proceeded. Analysis of the heterogeneous ensemble of NPs revealed the H(2) uptake trajectory of each nanocrystal to be shape-dependent. Differences in particle uptake trajectories were observed for individual particles with different shapes, faceting, and Pd shell thickness. In addition to palladium hydride formation, the single-particle trajectories were able to decipher specific instances where palladium silicide formation and Au/Pd interdiffusion occurred and helped us determine that this was more frequently seen in those particles within an ensemble having thicker Pd shells. This noninvasive, plasmonic-based direct sensing technique shows the importance of single-particle experiments in catalytically active systems and provides a foundation for studying more complex catalytic processes in inhomogeneous NP systems.     

On the Hopping Efficiency of Nanoparticles in the Electron Transfer across Self‐Assembled Monolayers

Redox reactions of solvated molecular species at gold‐electrode surfaces modified by electrochemically inactive self‐assembled molecular monolayers (SAMs) are found to be activated by introducing Au nanoparticles (NPs) covalently bound to the SAM to form a reactive Au–alkanedithiol–NP–molecule hybrid entity. The NP appears to relay long‐range electron transfer (ET) so that the rate of the redox reaction may be as efficient as directly on a bare Au electrode, even though the ET distance is increased by several nanometers. In this study, we have employed a fast redox reaction of surface‐confined 6‐(ferrocenyl) hexanethiol molecules and NPs of Au, Pt and Pd to address the dependence of the rate of ET through the hybrid on the particular NP metal. Cyclic voltammograms show an increasing difference in the peak‐to‐peak separation for NPs in the order Au<Pt<Pd, especially when the length of the alkanedithiol increases from octanedithiol to decanedithiol. The corresponding apparent rate constants, k_app, for decanedithiol are 1170, 360 and 14 s^?1 for NPs of Au, Pt and Pd, respectively, indicating that the efficiency of NP mediation of the ET clearly depends on the nature of the NP. Based on a preliminary analysis rooted in interfacial electrochemical ET theory, combined with a simplified two‐step view of the NP coupling to the electrode and the molecule, this observation is referred to the density of electronic states of the NPs, reflected in a broadening of the molecular electron/NP bridge group levels and energy‐gap differences between the Fermi levels of the different metals.     

Novel radical tandem 1,6-enynes thioacylation/cyclization: Au–Pd nanoparticles catalysis versus thermal activation as a function of the substrate specificity

We studied the reactivity of 1,6-enynes with thioacetic acid (AcSH) under either thermal conditions or in the presence of catalytic amounts of supported Au or Au–Pd nanoparticles (NPs) under mild conditions. The 1,6-enynes undergo a tandem thioacylation/cyclization to original cyclic products featuring either a homoallylic thioester function or an enol thioester function depending on the substrate topology. Interestingly, the former process was found more efficient when performed in the presence of Au–Pd NPs while the latter process can be efficiently carried out under thermal conditions (100 °C). The reaction proceeds by a radical mechanism and the presence of precious metal NPs seems to stabilize the formation of free radical intermediates, as supported by experimental and theoretical results.     

低温静电自组装法制备贵金属修饰TiO_2纳米结构薄膜及其增强的光催化性能(英文)

以碱-水热法在金属Ti片上原位生长了TiO2纳米结构(纳米花和纳米线)薄膜,并采用低温静电自组装方法将超细贵金属(金、铂、钯)纳米颗粒均匀沉积于多孔TiO2薄膜上.负载于Ti片上的贵金属/TiO2纳米结构薄膜具有一体化结构、多孔架构和高光催化活性.超高分辨率场发射扫描电子显微镜(FESEM)直接观察表明贵金属纳米颗粒在TiO2表面分布均匀,且颗粒之间相互分离,金、铂、钯纳米颗粒的平均粒径分别约为4.0、2.0和10.0nm.俄歇电子能谱(AES)纵深成分分析表明贵金属不仅沉积于薄膜表面,且大量分布于TiO2纳米结构薄膜内部,其深度超过580 nm.X射线光电子能谱(XPS)分析表明,经300°C下在空气中热处理后,纳米金仍保持金属态,纳米铂部分被氧化成PtOabs,而钯粒子则完全被氧化成氧化钯(PdO).以低温静电自组装法沉积贵金属,贵金属负载量可通过调节组装时间与溶胶pH值来控制.光催化降解甲基橙的结果表明,沉积的纳米金和铂能显著增加TiO2纳米结构薄膜的光催化活性,说明金和铂粒子可促进光生载流子的分离;但负载的PdO对TiO2薄膜的光催化性能增强几乎无作用.     

In Situ Fabrication of Noble Metal Nanoparticles Modified Multiwalled Carbon Nanotubes and Related Electrocatalysis

Using multiwalled carbon nanotubes (MWNTs) as templates, noble metal (Au, Ag, Pt or Pd) nanoparticles (NPs) were fabricated in situ by electrochemistry with a diameter of 40–60 nm. Further, catalytic behaviors of these composite materials were investigated. Experiments showed that such carbon nanotubes decorated with Pd NPs modified glassy carbon electrodes exhibited higher electrocatalytic ability to some molecules such as evolution of hydrogen, reduction of oxygen and oxidation of ascorbic acid. Atomic force microscopy, X‐ray photoelectron spectroscopy and cyclic voltammetry were used to characterize the film formation and their properties.     

Reagentless electrochemical immunoassay using electrocatalytic nanoparticle-modified antibodies

We describe a new approach for reagentless electrochemical immunoassay sensing in which Au/Pd NPs can be "loaded" onto antibodies to create an electrocatalytic antibody that is sensitive to the oxygen reduction reaction.     

Colloidal gold nanoparticles as catalyst for carbon-carbon bond formation: application to aerobic homocoupling of phenylboronic acid in water

Gold nanoparticles (<2 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP NPs) were prepared by reduction of AuCl4- with NaBH4 in the presence of PVP and characterized via an array of methods including optical absorption spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray absorption near-edge structure, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopy. We demonstrate for the first time that the Au:PVP NPs act as catalyst toward homocoupling of phenylboronic acid in water under aerobic conditions. Suppression of biphenyl formation under anaerobic conditions indicates that molecular oxygen dissolved in water is intimately involved in the coupling reactions. A mechanism of the aerobic homocoupling catalyzed by the Au:PVP NPs is proposed on the basis of a crucial role of dissolved oxygen, steric effects on the product yields, and the well-established mechanism for the Pd(II)-based catalysts.     

Ampholyte polystyrene spheres self-regulated deposition of noble metal nanoparticles for catalyst applications

The polystyrene (PS) spheres having dual functional groups were synthesized using emulsifier-free emulsion polymerization based on St/potassium persulfate/water system in the presence of V-50 as co-initiator. Fourier transform infrared (FTIR) confirms the presence of the sulfate and the amino groups on the surfaces of PS spheres. Transmission electron microscopy and X-ray photoelectron spectra revealed that the PS spheres were successfully deposited with catalytic palladium (Pd) or gold (Au) nanoparticles (NPs). The reduction of 4-nitroaniline to 4-phenylenediamine used as a model reaction was performed for catalysis studies and examined by ultraviolet. It was found that both Au and Pd PS dispersions show high catalytic activity. The Pd PS dispersion of 200 μl with only Pd content of 1.09 wt.% exhibits an excellent catalytic effect superior to the commercial Pd/C catalyst, i.e., less than 35 s taken for the completion of the reduction of 4-nitroaniline.     

N-Functionalised Imidazoles as Stabilisers for Metal Nanoparticles in Catalysis and Anion Binding

Metal nanoparticles (NPs) have physicochemical properties which are distinct from both the bulk and molecular metal species, and provide opportunities in fields such as catalysis and sensing. NPs typically require protection of their surface to impede aggregation, but these coatings can also block access to the surface which would be required to take advantage of their unusual properties. Here, we show that alkyl imidazoles can stabilise Pd, Pt, Au, and Ag NPs, and delineate the limits of their synthesis. These ligands provide an intermediate level of surface protection, for which we demonstrate proof-of-principle in catalysis and anion binding.     

Synergistic catalysis of metal-organic framework-immobilized Au-Pd nanoparticles in dehydrogenation of formic acid for chemical hydrogen storage

Bimetallic Au-Pd nanoparticles (NPs) were successfully immobilized in the metal-organic frameworks (MOFs) MIL-101 and ethylenediamine (ED)-grafted MIL-101 (ED-MIL-101) using a simple liquid impregnation method. The resulting composites, Au-Pd/MIL-101 and Au-Pd/ED-MIL-101, represent the first highly active MOF-immobilized metal catalysts for the complete conversion of formic acid to high-quality hydrogen at a convenient temperature for chemical hydrogen storage. Au-Pd NPs with strong bimetallic synergistic effects have a much higher catalytic activity and a higher tolerance with respect to CO poisoning than monometallic Au and Pd counterparts.     

Relative stability of icosahedral and cuboctahedral metallic nanoparticles

The size and form of metallic nanoparticles (NPs) significantly affects their adsorptive, chemical, and catalytic activity. One of the most interesting nanoscale size effects is the transition from icosahedral to octahedral forms with growth in the NP size. We compared the stability of icosahedral, decahedral and cuboctahedral NPs made from eight metals Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au using the local optimization of total energy, which was computed from the tight-binding second moment approximation and quantum Sutton–Chen potentials. The obtained results predicted that the icosahedral form would be most stable for Ni, and least stable for Au. For Rh, and especially for Ir, a strong dependency of the stability of the different forms on the NP size was revealed.     

Bimetallic Au-Pd NPs Embedded in MOF Ultrathin Nanosheets with Tuned Surface Electronic Properties for High-performance Benzyl Alcohol Oxidation

Bimetallic Au-Pd nanoparticles(NPs) with synergistic effect between Au and Pd atom have shown excellent catalytic activity toward benzyl alcohol oxidation. The catalytic activities of metal NPs supported within metal-organic frameworks (MOFs) are affected by the electronic interactions between metal NPs and MOFs. Taking the advantages of ultrathin nanosheets, we confine the highly dispersed Au-Pd NPs within ultrathin nanosheets of MOF-Ni(NMOF-Ni) to fabricate Au_xPd_y@NMOF-Ni as catalysts. Under base-free and atmospheric pressure conditions, the as-prepared Au_xPd_y@NMOF-Ni catalysts exhibit superior activity and selectivity for benzyl alcohol oxidation. This work highlights the synergistic effects among different components in composite catalysts effectively improving the activity and offers a new way for designing efficient catalysts toward benzyl alcohol oxidation.