Electronic and geometric properties of Au nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG) studied using X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM)

Au nanoparticles grown on mildly sputtered Highly Ordered Pyrolytic Graphite (HOPG) surfaces were studied using Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). The results were compared with those of Ag nanoparticles on the same substrate. By varying the defect densities of HOPG and the Au coverages, one can create Au nanoparticles in various sizes. At high Au coverages, the structures of the Au films significantly deviate from the ideal truncated octahedral form: the existence of many steps between different Au atomic layers can be observed, most likely due to a high activation barrier of the diffusion of Au atoms across the step edges. This implies that the particle growth at room temperature is strongly limited by kinetic factors. Hexagonal shapes of Au structures could be identified, indicating preferential growth of Au nanostructures along the (111) direction normal to the surface. In the case of Au, XPS studies reveal a weaker core level shift with decreasing particle size compared to the 3d level in similarly sized Ag particles. Also taking into account the Auger analysis of the Ag particles, the core level shifts of the metal nanoparticles on HOPG can be understood in terms of the metal/substrate charge transfer. Ag is (partially) positively charged, whereas Au negatively charged on HOPG. It is demonstrated that XPS can be a useful tool to study metal-support interactions, which plays an important role for heterogeneous catalysis, for example.     

Preparation of uniform Au@SiO2 particles by direct silica coating on citrate-capped Au nanoparticles

Preparation of Au@SiO₂ particles by direct silica coating on three different sized citrate-capped Au nanoparticles (17, 25 and 33 nm) with Stöber method was investigated in this work. It was found that the uniformity of the resulting Au@SiO₂ particles was related to both the sizes of the Au nanoparticles and the concentration of citrate during the particle synthesis. When the citrate concentration during the particle synthesis was low, the 25 and 33 nm Au nanoparticles could be well dispersed in the Stöber system, thus resulting the formation of uniform Au@SiO₂ particles containing single core. However, small Au nanoparticles (17 nm) were identified to show poor stability in the Stöber system even under low citrate concentration, the silica coating must be performed in a pre-hydrolyzed Stöber system to get the uniform Au@SiO₂ particles. This approach was also applicable to citrate capped Ag nanoparticles. After removal of the excess citrate in the Ag prepared by citrate reduction, uniform Ag/SiO₂ particles containing single core colloids could also be prepared by the direct silica coating.     

Synthesis and characterization of stable organosols of silver nanoparticles by electrochemical dissolution of silver in DMSO

Stable organosols of silver nanoparticles (AgNPs) without any capping agents have been synthesized by an electrochemical dissolution of a sacrificing silver electrode in dimethyl sulfoxide (DMSO). The peak at 425 +/- 5 nm observed in the UV-vis spectra was attributed to the surface plasmon resonance for silver. The formation of nanoparticles of silver was confirmed by X-ray diffraction analysis (XRD). In TEM, three ranges of particle size were observed, namely, 3.75 +/- 0.8, 6.25 +/- 0.5, and 9.25 +/- 0.3 nm. The correlation among these sizes was explained by a new model based on a droplet coalition. Based on that, sizes correlation fits very well in the empirical formula 5d(1)(3)n' + (3 - n')d(2)(3) = d(3)(3) where d(1), d(2), and d(3) are three sizes of particles and n' is an integer having values 0, 1, 2, 3, .... The sols prepared in DMSO were stable against flocculation for months. An unusual solution stability without any capping agents was attributed to the formation of Ag(I)DMSO complex on particle surface, which was confirmed by FTIR, fluorescence spectroscopy, and thermal analysis.     

Thiosalicylic acid-functionalized silver nanoparticles synthesized in one-phase system

A series of silver colloidal dispersions were prepared by two protocols, i.e., addition of the reductant-NaBH(4) and the stabilizer-thiosalicylic acid (TSA) into Ag(+) solution simultaneously or successively. The products were compared and characterized by TEM, electrochemical measurements, XPS, UV-vis, and FT-IR spectra. The size distributions of the Ag nanoparticles prepared by the former and latter protocols are bimodal and monodisperse, respectively. The analytic results of UV-vis spectra coincide with the TEM observation. A tentative explanation was given to the relationship between particle sizes and different synthetic protocols. The changes of the reduction potential of the reductant invoked a variance in particle diameter and size distribution. Electrochemical measurements corroborated our assumption. The composition information of TSA-derived silver nanoparticles was obtained from XPS and FT-IR spectroscopic measurements.     

Synthesis of PVP stabilized Cu/Pd nanoparticles with citrate complexing agent and its application as an activator for electroless copper deposition

A simple method has been developed to synthesize Cu/Pd nanoparticles in aqueous solution in ambient condition with the addition of complexing agent, trisodium citrate. UV-vis spectra confirmed the complexing behavior of trisodium citrate and metal ions. The particles synthesized with trisodium citrate were well dispersed with particle size ranging between 3-4 nm while the particles without trisodium citrate were larger and aggregated, as demonstrated by transmission electron microscopy (TEM). X-ray diffraction patterns (XRD) indicated the formation of bimetallic nanoparticles without impurities in the complexing agent-supplemented system. In contrast, large amounts of PdO and Cu(OH)(2) were precipitated along with the formation of particles in the complexing agent-free system. X-ray photoelectron spectroscopy (XPS) revealed small amounts of oxidized Pd on the surface of particles and the existence of zerovalent Cu and oxidized Cu in particles with trisodium citrate. With a simpler process for electroless copper deposition, the Cu/Pd nanoparticle activator with less Pd metal used exhibited comparable catalytic activity to conventional Pd/Sn colloidal activator. In summary, application of Cu/Pd nanoparticles synthesized with the complexing agent as an activator suggested a novel, simpler and inexpensive process in PCB industry.     

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.     

Preparation of highly uniform Ag/TiO2 and Au/TiO2 supported nanoparticle catalysts by photodeposition

Photodeposition of Ag nanoparticles on commercial TiO2 particles and nanoparticles was performed in order to provide direct visualization of the spatial distribution of photoactive sites on sub-micrometer-scale and nanoscale TiO2 particle surfaces and to create materials for potential catalytic applications. HRTEM (high-resolution transmission electron microscopy) and HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) were used to characterize these materials. The size and spatial distributions of the Ag nanoparticles on the commercial TiO2 were not uniform; the concentration of Ag was higher on grain boundaries and at the edges of these submicrometer particles. In the case of TiO2 nanoparticles, the size distribution of the Ag nanoparticles deposited was relatively uniform and independent of irradiation time and photon energy. The amount of Ag deposited on TiO2 nanoparticles was at least 6 times higher than that on the commercial samples for comparable irradiation conditions. Compared to the case of Ag photodeposition, the difference in the amount of Au photodeposited on TiO2 particles and nanoparticles was even greater, especially at low precursor concentrations. Photodeposition on TiO2 nanoparticles is suggested as a potential method for the preparation of Au/TiO2 catalysts, as loadings in excess of 10 wt % of uniform 1 nm metal particles were achieved in this work.     

Studies on surface plasmon resonance and photoluminescence of silver nanoparticles

Silver nanoparticles of different sizes were prepared by citrate reduction and characterized by UV-vis absorbance spectra, TEM images and photoluminescence spectra. The morphology of the colloids obtained consists of a mixture of nanorods and spheres. The surface plasmon resonance (SPR) and photoemission properties of Ag nanoparticles are found to be sensitive to citrate concentration. A blue shift in SPR and an enhancement in photoluminescence intensity are observed with increase in citrate concentration. Effect of addition of KCl and variation of pH in photoluminescence was also studied.     

Silver colloid nanoparticles: synthesis, characterization, and their antibacterial activity

A one-step simple synthesis of silver colloid nanoparticles with controllable sizes is presented. In this synthesis, reduction of [Ag(NH(3))(2)](+) complex cation by four saccharides was performed. Four saccharides were used: two monosaccharides (glucose and galactose) and two disaccharides (maltose and lactose). The syntheses performed at various ammonia concentrations (0.005-0.20 mol L(-1)) and pH conditions (11.5-13.0) produced a wide range of particle sizes (25-450 nm) with narrow size distributions, especially at the lowest ammonia concentrations. The average size, size distribution, morphology, and structure of particles were determined by dynamic light scattering (DLS), transmission electron microscopy (TEM), and UV/Visible absorption spectrophotometry. The influence of the saccharide structure (monosacharides versus disaccharides) on the size of silver particles is briefly discussed. The reduction of [Ag(NH(3))(2)](+) by maltose produced silver particles with a narrow size distribution with an average size of 25 nm, which showed high antimicrobial and bactericidal activity against Gram-positive and Gram-negative bacteria, including highly multiresistant strains such as methicillin-resistant Staphylococcus aureus. Antibacterial activity of silver nanoparticles was found to be dependent on the size of silver particles. A very low concentration of silver (as low as 1.69 mug/mL Ag) gave antibacterial performance.     

Nonaqueous synthesis and characterization of capped α-Fe2O3 nanoparticles from iron(III) hydroxy-oleate precursor

The synthesis of capped α-Fe₂O₃ nanoparticles from thermal treatment of iron (III) hydroxy-oleate in boiling organic solvents around 250 °C with retention of the integrity of the oleate units during the reaction process is reported. The formation of capped iron oxide particles is accomplished under aerobic conditions while the solvents used in the synthesis have strong influence on the nature and morphology of nanoparticles. These nanoparticles are studied by means of X-ray powder diffraction, IR and XPS while the morphology and particle size of nanocrystals are evaluated using SEM and TEM analysis suggesting the formation of monocrystalline α-Fe₂O₃ particles having cubical and spherical morphologies with sizes ranging from 20 to 30 nm. This organophilic material with oleate capping around the surfaces can be readily dispersed in organic solvents thus forming organosols. These organosols exhibit band-edge emission photoluminescence band both in toluene as well as in solid state while FT-IR analysis reveals formation oleate capped nanoparticles The XPS data indicate ferric state having doublet from Fe 2p_3/2 and Fe 2p_1/2 core-level electrons.     

On the morphology and stability of Au nanoparticles on TiO2(110) prepared from micelle-stabilized precursors

The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.     

单分散性银纳米粒子的可控制备

以卤化银或氧化银作为前驱体,室温下以水为溶剂,在较高溶液浓度下,利用化学还原法制备了单分散性银纳米粒子,并通过改变前驱体的种类,实现了粒径可控制备。采取扫描电子显微镜（SEM）、紫外-可见光谱仪（UV-Vis）、X射线-粉末衍射仪（XRD）、X射线-光电子能谱仪（XPS）等对所制备的银纳米粒子的形貌及成分进行了表征。结果显示,所制备的银纳米粒子具有较高的单分散性,粒径在40~150 nm之间,具有面心立方多晶结构。该方法制备的银纳米粒子可用于喷墨打印RFID天线。     

Synthesis and surface structure of thymine-functionalized, self-assembled monolayer-protected gold nanoparticles

Thymine-functionalized SAM-protected gold nanoparticles with diameters of 2.2 +/- 0.3 nm and 7.0 +/- 1.0 nm were prepared via a modified two-phase transfer method. UV-vis spectra showed that particle size and solvent type, as well as surface charge, influenced the gold surface plasmon band absorption, along with the interaction between thymine terminal groups in the solution. Although the bulky thymine end groups interacted strongly on the particle surface, a well-ordered monolayer of thyminethiol derivatives with a long hydrocarbon chain was formed on the particle surface, exhibiting an ordered, all-trans conformation of the methylene backbone, similar to those of corresponding self-assembled monolayers (SAMs) generated from normal alkanethiols. A larger particle size and a longer reaction time facilitated the formation of more ordered thymine-terminated thiol SAMs. Thermal analysis indicated that reorientation of the SAMs during heat treatment occurred by two processes, caused possibly by the separate recrystallization of the hydrocarbon long chains and thymine units. More ordered SAMs with a higher thermal stability were formed on the larger particle surfaces when compared with those on the smaller ones. A greater density of molecular packing was found on the smaller particle surfaces. However, SAMs formed on the larger gold particles resembled 2D SAMs on the smooth, flat gold surfaces. XPS results confirmed the thymine structure as well as the chemical bond between gold and sulfur. One type of adsorbed sulfur species was observed for the smaller particles and two for the larger ones, but a slightly higher binding energy of thiolate was found for the smaller ones.     

Remarkable nanosize effect of zirconia in Au/ZrO2 catalyst for CO oxidation

Nanosize effect of ZrO2 in Au/ZrO2 catalyst was studied by deposition-precipitation of Au nanoparticles in similar sizes (4-5 nm) on ZrO2 nanoparticles of varying sizes. The catalysts were characterized with XRD, TEM, XPS, and nitrogen adsorption to understand the effect of ZrO2 particle size on the catalytic nanostructures. Nanocomposite Au/ZrO2 catalysts consisting of comparably sized Au-metal (4-5 nm) and ZrO2 (5-15 nm) nanoparticles are found advantageous over those containing similarly sized Au-metal but larger ZrO2 (40-200 nm) particles for CO oxidation. This finding may have important implications on the designed preparation of advanced nanostructured catalysts and other chemical materials.     

Size control synthesis of sulfur doped titanium dioxide （anatase） nanoparticles, its optical property and its photo catalytic reactivity for CO2 ＋ H2O conversion and phenol degradation

Sulfur doped anatase TiO2 nanoparticles (3 nm−12 nm) were synthesized by the reaction of titanium tetrachloride, water and sulfuric acid with addition of 3M NaOH at room temperature. The electro-optical and photocatalytic properties of the synthesized sulfur doped TiO2 nanoparticles were studied along with Degussa commercial TiO2 particles (24 nm). The results show that band gap of TiO2 particles decreases from 3.31 to 3.25 eV and for that of commercial TiO2 to 3.2 eV when the particle sizes increased from 3 nm to 12 nm with increase in sulfur doping. The results of the photocatalytic activity under UV and sun radiation show maximum phenol conversion at the particle size of 4 nm at 4.80% S-doping. Similar results are obtained using UV energy for both phenol conversion and conversion of CO2+H2O in which formation of methanol, ethanol and proponal is observed. Production of methanol is also achieved on samples with a particle size of 8 and 12 nm and sulfur doping of 4.80% and 5.26%. For TiO2 particle of 4 nm without S doping, the production of methanol, ethanol and proponal was lower as compared to the S-doped particles. This is attributed to the combined electronic effect and band gap change, S dopant, specific surface area and the light source used.     

Ubiquitous pentacene monolayer on metals deposited onto pentacene films

Photoelectron spectroscopy (XPS and UPS) was used to study the deposition of metal layers (Ag, Cu, and Au) onto pentacene films. Very low work functions were measured (PhiAg = 3.91 eV, PhiCu = 3.93 eV, and PhiAu = 4.3 eV) for all of the metals, in agreement with results from the literature. The intensities of the C 1s core-level signals from pentacene that were monitored during stepwise metal deposition leveled off at a value of about 30% of a thick pentacene film. This C 1s intensity is comparable to that of one monolayer of pentacene deposited onto the respective metal. The valence band spectra of metals deposited onto pentacene and spectra collected for pentacene deposited onto bare metal surfaces are very similar. These findings lead to the conclusion that approximately one monolayer of pentacene is always present on top of the freshly deposited metal film, which explains the very low work function of the metals when they are deposited onto organic films. We expect similar behavior with other nonreactive metals deposited onto stable organic layers.     

A comparative studyof heat‐treated Ag:SiO2 nanocomposites synthesized by cosputtering and sol‐gel methods

In this work, we compared formation and properties of heat‐treated Ag nanoparticles in silica matrix synthesized by RF‐reactive magnetron cosputtering and sol–gel methods separately. The sol–gel and sputtered films were annealed at different temperatures in air and in a reduced environment, respectively. The optical UV‐visible spectrophotometry have shown that the absorption peak appears at 456 and 400 nm wavelength indicating formation of silver nanoparticles in SiO₂ matrix for both the sol–gel and sputtering methods at 100 and 800 °C, respectively. XPS measurements showed that the metallic Ag⁰ nanoparticles can be obtained from both the techniques at these temperatures. According to XPS and AFM analysis, by increasing annealing temperature, the concentration of the Ag nanoparticles on the surface decreased and the nanoparticles diffused into the substrate for the sol–gel films, while for the films deposited by cosputtering method, the Ag surface concentration increased by increasing the temperature. Based on AFM observations, the size of nanoparticles on the surface were obtained at about 25 and 55 nm for sputtered and sol–gel films, respectively, supporting our optical data analysis. In comparison, the sputtering technique can produce Ag metallic nanoparticles with a narrower particle size distribution relative to the sol–gel method. Copyright © 2008 John Wiley & Sons, Ltd.     

Charge transfers at metal/oxide interfaces: a DFT study of formation of K delta+ and Au delta- species on MgO/Ag(100) ultra-thin films from deposition of neutral atoms

Ultra-thin oxide films grown on a metal substrate and of thickness smaller than 1 nm may exhibit unusual properties with respect to thicker films or single crystal oxide surfaces. In a previous study [G. Pacchioni, L. Giordano and M. Baistrocchi, Phys. Rev. Lett., 2005, 94, 226104] we have suggested that a Au atom adsorbed on a MgO/Mo(100) thin film becomes negatively charged by direct electron tunneling from the Mo metal and that this is related to the low MgO/Mo(100) work function. Here we show, based on periodic DFT supercell calculations, that charge transfer can occur also in the opposite direction by adsorption of electropositive K atoms on MgO/Ag(100) films. We predict the occurrence of a charge transfer also for Au on MgO/Ag(100) films despite the fact that here the work function is 1 eV larger than in MgO/Mo(100). The formation of a layer of adsorbed negative (Au delta-/MgO/Ag) or positive (K delta+/MgO/Ag) adsorbates results in an increase or decrease, respectively, of the MgO/Ag(100) work function as predicted by the classical Gurney model for ionic adsorbates on metal surfaces.     

Green synthesis and characterization of polymer-stabilized silver nanoparticles

Silver nanoparticles (Ag-NPs) were synthesized using a facile green chemistry synthetic route. The reaction occurred at ambient temperature with four reducing agents introduced to obtain nanoscale Ag-NPs. The variables of the green synthetic route, such as acidity, concentration of starting materials, and molar ratio of reactants were optimized. Dispersing agents were employed to prevent Ag-NPs from aggregating. Advanced instrumentation techniques, such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet–visible spectroscopy (UV–vis), and phase analysis light scattering technique (ZetaPALS) were applied to characterize the morphology, particle size distribution, elemental composition, and electrokinetic behavior of the Ag-NPs. UV–vis spectra detected the characteristic plasmon at approximately 395–410 nm; and XRD results were indicative of face-centered cubic phase structure of Ag. These particles were found to be monodispersed and highly crystalline, displaying near-spherical appearance, with average particle size of 10.2 nm using citrate or 13.7 nm using ascorbic acid as reductants from particle size analysis by ZetaPALS, respectively. The rapid electrokinetic behavior of the Ag was evaluated using zetapotential (from −40 to −42 mV), which was highly dependant on nanoparticle acidity and particle size. The current research opens a new avenue for the green fabrication of nanomaterials (including variables optimization and aggregation prevention), and functionalization in the field of nanocatalysis, disinfection, and electronics.     

Preparation of airborne Ag/CNT hybrid nanoparticles using an aerosol process and their application to antimicrobial air filtration

Carbon nanotubes (CNTs) have been widely used in a variety of applications because of their unique structure and excellent mechanical and electrical properties. Additionally, silver (Ag) nanoparticles exhibit broad-spectrum biocidal activity toward many different bacteria, fungi, and viruses. In this study, we prepared Ag-coated CNT hybrid nanoparticles (Ag/CNTs) using aerosol nebulization and thermal evaporation/condensation processes and tested their usefulness for antimicrobial air filtration. Droplets were generated from a CNT suspension using a six-jet collison nebulizer, passed through a diffusion dryer to remove moisture, and entered a thermal tube furnace where silver nanoparticles were generated by thermal evaporation/condensation at ～980 °C in a nitrogen atmosphere. The CNT and Ag nanoparticle aerosols mixed together and attached to each other, forming Ag/CNTs. For physicochemical characterization, the Ag/CNTs were introduced into a scanning mobility particle sizer (SMPS) for size distribution measurements and were sampled by the nanoparticle sampler for morphological and elemental analyses. For antimicrobial air filtration applications, the airborne Ag/CNT particles generated were deposited continuously onto an air filter medium. Physical characteristics (fiber morphology, pressure drop, and filtration efficiency) and biological characteristics (antimicrobial tests against Staphylococcus epidermidis and Escherichia coli bioaerosols) were evaluated. Real-time SMPS and transmission electron microscopy (TEM) data showed that Ag nanoparticles that were <20 nm in diameter were homogeneously dispersed and adhered strongly to the CNT surfaces. Because of the attachment of Ag nanoparticles onto the CNT surfaces, the total particle surface area concentration measured by a nanoparticle surface area monitor (NSAM) was lower than the summation of each Ag nanoparticle and CNT generated. When Ag/CNTs were deposited on the surface of an air filter medium, the antimicrobial activity against test bacterial bioaerosols was enhanced, compared with the deposition of CNTs or Ag nanoparticles alone, whereas the filter pressure drop and bioaerosol filtration efficiency were similar to those of CNT deposition only. At a residence time of 2 h, the relative microbial viabilities of gram-positive S. epidermidis were ～32, 13, 5, and 0.9% on the control, CNT-, Ag nanoparticle-, and Ag/CNT-deposited filters, respectively, and those of gram-negative E. coli were 13, 2.1, 0.4, and 0.1% on the control, CNTs, Ag nanoparticles, and Ag/CNTs, respectively. These Ag/CNT hybrid nanoparticles may be useful for applications in biomedical devices and antibacterial control systems.