Growth of Ag and Au Nanoparticles on Reduced and Oxidized Rutile TiO2(110) Surfaces

The nucleation and growth of Au and Ag nanoparticles on rutile TiO₂(110)–(1 × 1) surfaces in different oxidation states is studied by means of photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM). Au and Ag nanoparticles were found to bind much more strongly to oxidized TiO₂(110) model supports than to reduced TiO₂(110) surfaces, as directly revealed by STM. Detailed PES studies addressing small Au and Ag particles complete this picture and show that the PES core level spectra acquired on Au/TiO₂(110) and Ag/TiO₂(110) can be best described by fitting with two binding energy (BE) components. Particularly for coverages in the sub-monolayer regime and for depositions at low temperatures (100 K) the PES core level spectra must be fitted with at least two BE components. The higher BE component is attributed to atoms at the interface between the metal clusters and the TiO₂(110) support. For Au/TiO₂(110), the two BE components were evident in the core level spectra for higher coverage than for Ag/TiO₂(110), consistent with different growth modes for Au (initially 2D) and Ag (3D) on TiO₂(110). Finally, strong evidence for charge transfer from Ag nanoparticles to the TiO₂(110) support is presented, whereas the charge transfer between Au nanoparticles and the TiO₂(110) support is very small.     

Trimetallic nanoparticles having a Au-core structure

The Au/Pt/Rh trimetallic nanoparticles were successfully synthesized by a combination of a co-reduction and a self-organization method using poly(N-vinyl-2-pyrrolidone) (PVP) as a protecting reagent. The triple core/shell structure was suggested by HR-TEM, EF-TEM and FT-IR-CO for the trimetallic nanoparticles. The present trimetallic nanoparticles had much higher catalytic activity for hydrogenation than the corresponding monometallic and bimetallic nanoparticles. This high catalytic activity can be due to the sequential electronic effect between different atoms of a particle, which is supported by XPS data.     

Theoretical study of CO adsorption on Au catalysts under environmental catalytic conditions

Density Functional Theory calculations with both standard GGA and hybrid functionals are performed on Au adatoms, steps, and low index surfaces with coordination numbers (CNs) varying from 3 to 9. The results are used to study adsorption thermodynamics and reactivity of CO on Au nanoparticles. We find that the hybrid functional improves calculated site preferences and predicts CO top site adsorption, regardless of the Au CN, in good agreement with experiments. The calculated adsorption energies vary monotonically with respect to Au CNs, and the results from the hybrid functional are around 20% smaller than the corresponding values from the GGA–PBE functional. A comparison with experimental adsorption energies suggests that these functionals may bound the true CO–Au interaction strength, and seven-coordinated Au atoms may be the active low-coordinated sites on many Au single crystal surfaces. However, thermodynamic analysis on Wulff-like Au particles at ambient temperatures shows that, even though the number of 6-coordinated corner Au atoms is much less than the number of 7-coordinated edge Au atoms and of higher-coordinated Au atoms, they are the dominant sites for CO adsorption on Au nanoparticles with sizes up to 10 nm. In addition, we find that CO adsorption is not influenced by the shape of Au nanoparticles, but the CO oxidation reaction may be.     

Control over the surface plasmon band of block copolymer/Ag/Au nanoparticles composites by the addition of single walled carbon nanotubes

In this study, we demonstrate control over the localized surface plasmon band (SPB) of a micellar poly(styrene-block-4vinylpyridine) (PS-b-P4VP) copolymer thin film composite that includes Ag and Au nanoparticles (NPs) in the presence of single walled carbon nanotubes (SWCNTs). Ag and Au NPs are preferentially located in the P4VP core and the PS corona of ordered spherical PS-b-P4VP copolymer micelles, respectively. This structure gave rise to a single SPB due to the coupling of Ag and Au SPBs. The non-covalent addition of SWCNTs in the block copolymer micelles shifts the coupled SPB to a lower wavelength. The maximum shift in the coupled SPB of approximately 30 nm was achieved in the PS-b-P4VP/Ag/Au NPs composite. The carbon nanotube induced modulation of the coupled SPB stems from the charge accumulation effect of the SWCNTs placed between the two types of nanoparticles.     

Clustered assembly of Au nanoparticles from spherical diblock copolymer micelles encapsulating Au nanoparticle

Superstructures composed of diblock copolymer micelles and inorganic nanoparticles are quite interesting because the specific arrangement of inorganic nanoparticles within the micellar structure can reveal interesting opportunities in many field of science. In this perspective, we report a simple method to produce clustered assembly of Au nanoparticles in the micelles in attempt to induce plasmonic coupling among multiple Au nanoparticles in the assembled structures. Here, we utilized polystyrene‐block‐poly(acrylic acid), PS‐PAA, micelles containing single Au nanoparticle in the core (Au@PS‐PAA micelles) as building materials to initiate next‐level assembling process. In particular, the addition of HCl to the solution of Au@PS‐PAA micelles affected the overall equilibrium condition as well as kinetic process in the micellar solution. As a result, individual Au@PS‐PAA micelles could be merged together to form more large micelles with inclusion of multiple nanoparticles in the core, the process of which was accompanied with plasmonic coupling of Au nanoparticles. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44693.     

Enhanced antibacterial activity of bimetallic gold-silver core-shell nanoparticles at low silver concentration

Herein we report the development of bimetallic Au@Ag core-shell nanoparticles (NPs) where gold nanoparticles (Au NPs) served as the seeds for continuous deposition of silver atoms on its surface. The core-shell structure and morphology were examined by UV-Vis spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD). The core-shell NPs showed antibacterial activity against both gram negative (Escherichia coli and Pseudomonas aeruginosa) and gram positive (Enterococcus faecalis and Pediococcus acidilactici) bacteria at low concentration of silver present in the shell, with more efficacy against gram negative bacteria. TEM and flow cytometric studies showed that the core-shell NPs attached to the bacterial surface and caused membrane damage leading to cell death. The enhanced antibacterial properties of Au@Ag core-shell NPs was possibly due to the more active silver atoms in the shell surrounding gold core due to high surface free energy of the surface Ag atoms owing to shell thinness in the bimetallic NP structure.     

Electrocatalytic activity of carbon-supported Pt-Au nanoparticles for methanol electro-oxidation

Pt-modified Au nanoparticles on carbon support were prepared and analyzed as electrocatalysts for methanol electro-oxidation. In this paper, a novel chemical strategy is described for the preparation and characterization of carbon-supported and Pt-modified Au nanoparticles, which were prepared by using a successive reduction process. After preparing Au colloid nanoparticles (∼3.5 nm diameter), Au nanoparticles were supported spontaneously on the surface of carbon black in the aqueous solution. Then a nanoscaled Pt layer was deposited on the surface of carbon-supported Au nanoparticles by the chemical reduction. The structural information and electrocatalytic activities of the Pt-modified Au nanoparticles were confirmed by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and cyclic voltammetry (CV). The results indicate that carbon-supported Au nanoparticles were modified with the reduced Pt atoms selectively. The Pt-modified Au nanoparticles showed the higher electrocatalytic activity for methanol electro-oxidation reaction than the commercial one (Johnson-Matthey). The increased electrocatalytic activity might be attributed to the effective surface structure of Pt-modified Au nanoparticles, which have a high utilization of Pt for surface reaction of methanol electro-oxidation.     

Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

The interaction of gold atoms with CeO₂ nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition?Cprecipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2?C6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O?CH bond cleavage appears to be rate limiting, while this shifts to C?CH bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria.     

Quantum sized gold nanoclusters with atomic precision

Gold nanoparticles typically have a metallic core, and the electronic conduction band consists of quasicontinuous energy levels (i.e. spacing δ ? k(B)T, where k(B)T is the thermal energy at temperature T (typically room temperature) and k(B) is the Boltzmann constant). Electrons in the conduction band roam throughout the metal core, and light can collectively excite these electrons to give rise to plasmonic responses. This plasmon resonance accounts for the beautiful ruby-red color of colloidal gold first observed by Faraday back in 1857. On the other hand, when gold nanoparticles become extremely small (<2 nm in diameter), significant quantization occurs to the conduction band. These quantum-sized nanoparticles constitute a new class of nanomaterial and have received much attention in recent years. To differentiate quantum-sized nanoparticles from conventional plasmonic gold nanoparticles, researchers often refer to the ultrasmall nanoparticles as nanoclusters. In this Account, we chose several typical sizes of gold nanoclusters, including Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60), to illustrate the novel properties of metal nanoclusters imparted by quantum size effects. In the nanocluster size regime, many of the physical and chemical properties of gold nanoparticles are fundamentally altered. Gold nanoclusters have discrete electronic energy levels as opposed to the continuous band in plasmonic nanoparticles. Quantum-sized nanoparticles also show multiple optical absorption peaks in the optical spectrum versus a single surface plasmon resonance (SPR) peak at 520 nm for spherical gold nanocrystals. Although larger nanocrystals show an fcc structure, nanoclusters often have non-fcc atomic packing structures. Nanoclusters also have unique fluorescent, chiral, and magnetic properties. Due to the strong quantum confinement effect, adding or removing one gold atom significantly changes the structure and the electronic and optical properties of the nanocluster. Therefore, precise atomic control of nanoclusters is critically important: the nanometer precision typical of conventional nanoparticles is not sufficient. Atomically precise nanoclusters are represented by molecular formulas (e.g. Au(n)(SR)(m) for thiolate-protected ones, where n and m denote the respective number of gold atoms and ligands). Recently, major advances in the synthesis and structural characterization of molecular purity gold nanoclusters have made in-depth investigations of the size evolution of metal nanoclusters possible. Metal nanoclusters lie in the intermediate regime between localized atomic states and delocalized band structure in terms of electronic properties. We anticipate that future research on quantum-sized nanoclusters will stimulate broad scientific and technological interests in this special type of metal nanomaterial.     

Preparation of Au/TiO2 catalysts from Au(I)–thiosulfate complex and study of their photocatalytic activity for the degradation of methyl orange

Gold loaded on TiO₂ (Au/TiO₂) catalysts were prepared using Au(I)–thiosulfate complex (Au(S₂O₃)₂³⁻) as the gold precursor for the first time. The samples were characterized by UV–vis diffuse reflectance spectra, X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), and X-ray photoelectron spectroscopy (XPS) methods. Using Au(S₂O₃)₂³⁻ as gold precursor, ultra-fine gold nanoparticles with a highly disperse state can be successfully formed on the surface of TiO₂. The diameter of Au nanoparticles increases from 1.8 to 3.0 nm with increasing the nominal Au loading from 1% to 8%. The photocatalytic activity of Au/TiO₂ catalysts was evaluated from the analysis of the photodegradation of methyl orange (MO). With the similar Au loading, the catalysts prepared with Au(S₂O₃)₂³⁻ precursor exhibit higher photocatalytic activity for methyl orange degradation when compared with the Au/TiO₂ catalysts prepared with the methods of deposition–precipitation (DP) and impregnation (IMP). The preparation method has decisive influences on the morphology, size and number of Au nanoparticles loaded on the surface of TiO₂ and further affects the photocatalytic activity of the obtained catalysts.     

Crystallinity dynamics of gold nanoparticles during sintering or coalescence

The crystallinity of gold nanoparticles during coalescence or sintering is investigated by molecular dynamics. The method is validated by the attainment of the Au melting temperature that increases with increasing particle size approaching the Au melting point. The morphology and crystal dynamics of nanoparticles of (un)equal size during sintering are elucidated. The characteristic sintering time of particle pairs is determined by tracing their surface area evolution during coalescence. The crystallinity is quantified by the disorder variable indicating the system's degree of disorder. The atoms at the grain boundaries are amorphous, especially during particle adhesion and during sintering when grains of different orientation are formed. Initial grain orientation affects final particle morphology leading to exposure of different crystal surfaces that can affect the performance of Au nanoparticles (e.g., catalytic efficiency). Coalescence between crystalline and amorphous nanoparticles of different size results in polycrystalline particles of increasing crystallinity with time and temperature. Crystallinity affects the sintering rate and mechanism. Such simulations of free‐standing Au nanoparticle coalescence are relevant also to Au nanoparticles on supports that do not exhibit strong affinity or strong metal support interactions. © 2015 American Institute of Chemical Engineers AIChE J, 62: 589–598, 2016     

Feasibility of methyl mercaptane as probe molecule for supported gold nanoparticle surface area determination

Abstract: Gold nanoparticles supported on TiO2 were probed by adsorption of methyl mercaptane (MM), and the process was quantified gravimetrically. This method allowed discrimination between weakly adsorbed (physisorbed) and strongly bound (chemisorbed) methyl mercaptane. Strong adsorption of MM occured on exposed Au faces, while low-temperature pre-treatment (30 degrees C) completely suppressed adsorption of MM on the TiO2 support. The thus obtained high selectivity of MM adsorption on Au enabled characterization of the gold surface area and the resulting values are comparable with other noble metal systems of similar average particle size. The estimated adsorption stoichiometry indicates that the entire Au surface is probed.     

微波合成(Au)_核·(Ag)_壳纳米粒子及其共振散射光谱研究

以柠檬酸化学还原法制备的金纳米粒子作晶种,采用微波高压液相合成技术,制备出分散性较好、规则球形的(Au)核·(Ag)壳复合纳米粒子。研究了(Au)核·(Ag)壳复合纳米粒子的紫外可见吸收光谱和共振散射光谱特性,在470nm处有最强共振散射峰,在404nm处产生一个吸收峰。结果表明,(Au)核·(Ag)壳复合纳米粒子的形成是导致470nm共振散射的根本原因。     

Photocatalytic Preparation of Encapsulated Gold Nanoparticles by Jingle-bell-shaped Cadmium Sulfide–silica Nanoparticles

Gold (Au) nanoparticles were deposited inside silica: (SiO₂) shells containing cadmium sulfide (CdS) nanoparticles through photocatalytic reduction of potassium dicyanogold(I) by CdS. Photocatalytic Au deposition occurred only when core-shell nanoparticles having a void space between the core and shell, i.e., a jingle-bell-shaped structure, were used. These core-shell nanoparticles were prepared by size-selective photoetching of SiO₂ -covered CdS nanoparticles. The size of Au nanoparticles could be controlled by adjustment of the void space in SiO₂-covered CdS. Dissolution of CdS by acid treatment from the Au-deposited jingle-bell nanoparticles did not have any effect on the surface-plasmon absorption by Au. These facts indicate that Au nanoparticles of adjustable size can be prepared in an SiO₂ shell that prevents mutual coalescence of Au nanoparticles but allows permeation of molecules and ions.     

Desulfurization Reactions on Surfaces of Metal Carbides: Photoemission and Density–Functional Studies

High-resolution photoemission and density functional (DF) calculations were used to study the interaction of atomic sulfur and S-containing molecules with metal carbides in which the carbon/metal ratio varies from 0.5 to 1 (M₂C and MC, M = Ti, V or Mo). In these compounds, the C sites cannot be considered as simple spectators. They moderate the reactivity of the metal centers and provide bonding sites for adsorbates. For example, the adsorption of S on TiC(001) induces a large positive shift (1.0–1.3 eV) in the C 1s core level. DF calculations give a CTiTi hollow as the most stable site for the S adatoms. There is a correlation between the adsorption energy of S or thiophene and shifts in the centroid of the metal d band induced by metal–carbon bonding in the metal carbides. The M₂C and MC carbides have difficulty obeying Sabatier’s principle for being good HDS catalysts because some of them interact too strongly with the products (M₂C stoichiometry) and the others have problems dissociating the reactants (MC stoichiometry). The addition of small Au nanoparticles is an efficient way for enhancing the HDS activity of MC catalysts. In spite of the very poor desulfurization performance of TiC and MoC, the Au/TiC and Au/MoC systems display an HDS activity comparable or higher than that of conventional Ni/MoS _x catalysts. The Au nanoparticles probably increase the HDS activity of the metal carbides by enhancing the adsorption energy of thiophene and by helping in the dissociation of H₂ to produce the hydrogen necessary for the hydrogenolysis of C–S bonds and the removal of sulfur.     

AuAg alloy nanomolecules with 38 metal atoms

Au(38-n)Ag(n)(SCH(2)CH(2)Ph)(24) alloy nanomolecules were synthesized, purified and characterized by MALDI TOF mass spectrometry. Similar to 25 and unlike 144 metal atom count AuAg alloy nanomolecules, incorporation of Ag atoms here results in loss or smearing out of distinct UV-vis features. We propose that the short and long staples contain Au atoms, while the inner core consists of both Au and Ag atoms.     

柠檬酸盐保护下光化学一步法合成Au核-Pt壳纳米粒子及其表征研究

柠檬酸钠做保护剂,光化学同时还原Au(Ⅲ)与Pt(Ⅳ)离子混合物合成Au核Pt壳纳米粒子。应用XPS、UV-V is、TEM等分析手段研究了Au-Pt纳米粒子的结构、形貌、化学状态等。结果表明:所合成的双金属纳米粒子为球形,具有Au核-Pt壳复合结构,平均粒径范围为8.1~5.1 nm。     

Characterization of Pt?CAu and Ni?CAu Clusters on TiO2(110)

The surface composition and properties of Pt?CAu and Ni?CAu clusters on TiO₂(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS) and soft X-ray photoelectron spectroscopy (sXPS). STM studies show that bimetallic clusters are formed during sequential deposition of the two metals, regardless of the order of deposition. At the 2?ML of Au/2?ML of Pt or Ni coverages studied here, the second metal contributes to the growth of existing clusters rather than forming new pure metal clusters. LEIS experiments demonstrate that the surfaces of the bimetallic clusters are almost 100% Au when 2?ML of Au is deposited on top of 2?ML of Pt or Ni. However, a much larger fraction of Pt or Ni (50 and 20%, respectively) remains at the surface when 2?ML of Pt or Ni is deposited on 2?ML of Au, most likely due to limited diffusion of atoms within the clusters at room temperature. According to sXPS investigations, the binding energies of the metals in the bimetallic clusters are shifted from those observed for pure metal clusters; the Pt(4f_7/2) and Ni(3p_3/2) peaks are shifted to lower binding energies while the position of the Au(4f_7/2) peak is dominated by surface core level shifts. Pure Pt clusters as well as 0.4?ML of Au on 2 ML of Pt clusters reduce the titania support upon encapsulation after annealing to 800?K, whereas 2?ML of Au on 2?ML of Pt clusters do not reduce titania, presumably because there is no Pt at the surface of the clusters. Pure Ni clusters are also known to become encapsulated upon heating, but the reduction of titania is much less extensive compared to that of pure Pt clusters.     

Au/Pd core-shell nanoparticles with varied hollow Au cores for enhanced formic acid oxidation

A facile method has been developed to synthesize Au/Pd core-shell nanoparticles via galvanic replacement of Cu by Pd on hollow Au nanospheres. The unique nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, and electrochemical measurements. When the concentration of the Au solution was decreased, grain size of the polycrystalline hollow Au nanospheres was reduced, and the structures became highly porous. After the Pd shell formed on these Au nanospheres, the morphology and structure of the Au/Pd nanoparticles varied and hence significantly affected the catalytic properties. The Au/Pd nanoparticles synthesized with reduced Au concentrations showed higher formic acid oxidation activity (0.93 mA cm^-2 at 0.3 V) than the commercial Pd black (0.85 mA cm^-2 at 0.3 V), suggesting a promising candidate as fuel cell catalysts. In addition, the Au/Pd nanoparticles displayed lower CO-stripping potential, improved stability, and higher durability compared to the Pd black due to their unique core-shell structures tuned by Au core morphologies.     

One-pot synthesis of PdAu bimetallic composite nanoparticles and their catalytic activities for hydrogen peroxide generation

We report a facile one-pot aqueous-phase synthesis of PdAu bimetallic nanoparticles with different Pd/Au ratio. The synthesis was conducted by co-reduction of Pd and Au precursor using ascorbic acid as a reducing agent and in the presence of polyallylamine hydrochloride (PAH). By high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectrometry (EDS) analyses, we found that the synthesized nanoparticles had an onion-like core/shell/shell/shell structure with Au-rich core, Pd-rich shell, Au-rich shell, and Pd shell, respectively. We also investigated the catalytic performance of the synthesized PdAu nanoparticles toward hydrogen peroxide generation reaction.