The effect of preadsorbed K on the size distribution of Au nanoparticles on TiO2(1 1 0) surface

The effect of K on the morphology of Au nanoparticles deposited on TiO₂(1 1 0) surface is investigated by STM-STS and AES methods. For comparison, the enhanced concentration of oxygen defect sites generated by Ar⁺ bombardment was also studied. It was found that both the K additive and the oxygen defect sites induce a pronounced decrease in the average size of the Au nanoparticles evolved at 320 K. On the clean TiO₂(1 1 0) the average size of Au particles is 4.3 nm at approximately monolayer coverage of gold, while in the presence of K or oxygen vacancies this value decreased to 2.5 nm. In spite of the reduced average diameter detected at room temperature, the mean size of the Au nanoparticles increased significantly from 2.5 nm up to 7 nm on the effect of annealing at 500-700 K for K precoverages of 0.3-1 ML. For the clean and the Ar⁺ pretreated TiO₂(1 1 0) surfaces the mean size of the Au particles changed only slightly on the effect of the same thermal treatments.     

Oxygen adsorption and oxidation reactions on Au(2 1 1) surfaces: Exposures using O2 at high pressures and ozone (O3) in UHV

Nanosized gold particles supported on reducible metal oxides have been reported to show high catalytic activity toward CO oxidation at low temperature. This has generated great scientific and technological interest, and there have been many proposals to explain this unusual activity. One intriguing explanation that can be tested is that of Nørskov and coworkers [Catal. Lett. 64 (2000) 101] who suggested that the “unusually large catalytic activity of highly-dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au-support interface”. In particular, their calculations indicated that the Au(2 1 1) stepped surface would be much more reactive towards O₂ dissociative adsorption and CO adsorption than the Au(1 1 1) surface. We have now studied the adsorption of O₂ and O₃ (ozone) on an Au(2 1 1) stepped surface. We find that molecular oxygen (O₂) was not activated to dissociate and produce oxygen adatoms on the stepped Au(2 1 1) surface even under high-pressure (700 Torr) conditions with the sample at 300-450 K. Step sites do bind oxygen adatoms more tightly than do terrace sites, and this was probed by using temperature programmed desorption (TPD) of O₂ following ozone (O₃) exposures to produce oxygen adatoms up to a saturation coverage of θ_O = 0.90 ML. In the low-coverage regime (θ_O ? 0.15 ML), the O₂ TPD peak at 540 K, which does not shift with coverage, is attributed to oxygen adatoms that are bound at the steps on the Au(2 1 1) surface. At higher coverages, an additional lower temperature desorption peak that shifts from 515 to 530 K at saturation coverage is attributed to oxygen adsorbed on the (1 1 1) terrace sites of the Au(2 1 1) surface. Although the desorption kinetics are likely to be quite complex, a simple Redhead analysis gives an estimate of the desorption activation energy, E_d, for the step-adsorbed oxygen of 34 kcal/mol and that for oxygen at the terraces near saturation coverage of 33 kcal/mol, values that are similar to others reported on Au surfaces. Low Energy Electron Diffraction (LEED) indicates an oxygen-induced step doubling on the Au(2 1 1) surface at low-coverages (θ_O = 0.08-0.17 ML) and extensive disruption of the 2D ordering at the surface for saturation coverages of oxygen (θ_O ? 0.9 ML). Overall, our results indicate that unstrained step sites on Au(2 1 1) surfaces of dispersed Au nanoparticles do not account for the novel reactivity of supported Au catalysts for CO oxidation.     

亚纳米厚的氧化铝包裹层可以显著提高负载型金纳米颗粒催化剂的热稳定性

负载型金纳米颗粒催化剂在许多催化反应中展现出非常好的催化活性,但是金纳米颗粒在高温等反应条件下容易烧结团聚,极大地限制了金催化剂的应用。利用原子层沉积技术在Au/TiO₂催化剂表面分别精确沉积了一层超薄的二氧化钛和氧化铝包裹层,并对比研究了包裹层对金纳米颗粒的热稳定性影响。原位红外漫反射CO吸附和x-射线光电子能谱数据证实了氧化物包裹层的存在。发现亚纳米厚的氧化铝包裹层能够在600 C完全避免金纳米颗粒的团聚;相反,二氧化钛包裹层对金纳米颗粒稳定性的提高没有明显效果。通过CO氧化探针反应的活性测试,发现随着煅烧温度的升高氧化铝包裹的Au/TiO₂ 催化剂的活性逐渐提高,表明高温处理可以促进被包裹金原子的暴露并表现出催化活性。提供了提高金纳米颗粒稳定性的有效方法,为拓展金催化剂在条件苛刻的反应中的应用奠定了技术基础.     

Quantum spins in Mackay icosahedral gold nanoparticles

We report on the direct observation of the intrinsic magnetization behavior of Mackay icosahedral Au nanoparticles. The spin arrangements in 3.5-nm icosahedral Au particles are found to be ferrimagnetic-like, where the core and surface moments point in opposite directions, with a net spontaneous magnetic moment of 16 μ_B per particle. The unpaired spins behave as J = 1/2 quantum spins. The average level separation is found to be a factor of ~1.53 larger than that estimated according to the Kubo formula for spherical Au particles. This reflects the fact that there are considerably fewer atoms packed in a particle with an icosahedral geometry than with a spherical one.     

Au/TiO2/Ru(0 0 0 1) model catalysts and their interaction with CO

Au/TiO₂/Ru(0 0 0 1) model catalysts and their interaction with CO were investigated by scanning tunneling microscopy and different surface spectroscopies. Thin titanium oxide films were prepared by Ti deposition on Ru(0 0 0 1) in an O₂ atmosphere and subsequent annealing in O₂. By optimizing the conditions for deposition and post-treatment, smooth films were obtained either as fully oxidized TiO₂ or as partly reduced TiO_x, depending on the preparation conditions. CO adsorbed molecularly on both oxidized and reduced TiO₂, with slightly stronger bonding on the reduced films. Model catalyst surfaces were prepared by depositing submonolayer quantities of Au on the films and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. From X-ray photoelectron spectroscopy, a weak interaction between the Au and the TiO₂ substrate was found. At 100 K CO adsorption occurred on both the TiO₂ film and on the Au nanoparticles. CO desorbed from the Au particles with activation energies between 53 and 65 kJ/mol, depending on the Au coverage. If the Au deposit was annealed to 770 K prior to CO exposure, the CO adsorption energy decreased significantly. STM measurements revealed that the Au particles grow upon annealing, but are not encapsulated by TiO_x suboxides. The higher CO adsorption energy observed for smaller Au coverages and before annealing is attributed to a significantly stronger interaction of CO with mono- and bilayer Au islands, while for higher particles, the adsorption energy becomes more bulk-like. The implications of these effects on the known particle size effects in CO oxidation over supported Au/TiO₂ catalysts are discussed.     

Metal-on-oxide nanoparticles produced using laser ablation of microparticle aerosols

A continuous aerosol process has been studied for producing nanoparticles of oxides that were decorated with smaller metallic nanoparticles and are free of organic stabilizers. To produce the oxide carrier nanoparticles, an aerosol of 3–6 μm oxide particles was ablated using a pulsed excimer laser. The resulting oxide nanoparticle aerosol was then mixed with 1.5–2.0 μm metallic particles and this mixed aerosol was exposed to the laser for a second time. The metallic micron-sized particles were ablated during this second exposure, and the resulting nanoparticles deposited on the surface of the oxide nanoparticles producing an aerosol of 10–60 nm oxide nanoparticles that were decorated with smaller 1–5 nm metallic nanoparticles. The metal and oxide nanoparticle sizes were varied by changing the laser fluence and gas type in the aerosol. The flexibility of this approach was demonstrated by producing metal-decorated oxide nanoparticles using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.     

The investigation of oxidized silver nanoparticles prepared by thermal evaporation and radio-frequency sputtering of metallic silver under oxygen

The investigation of oxidized silver nanoparticles by the photoemission (XPS, UPS) and HRTEM methods was performed. The nanoparticles of oxidized silver were obtained in the vacuum chamber by two methods of synthesis: thermal evaporation of silver nanoparticles followed by transferring in convective gas flow and sputtering of oxidized clusters under the action of plasma. Both methods indicated that oxygen interaction with silver nanoparticles depends strongly on its size. It was shown that the chemical bonding of oxygen species stabilized on small particles differs from the oxygen species adsorbed on bulk silver surfaces (monocrystals, foils and large particles). The low charged oxygen with molecular type of bonding stabilizes on particles of size approximately 5 nm and smaller. Increasing particle size leads to the dissociation of molecular oxygen species and the formation of strongly charged oxygen composed of oxide nanoparticles like Ag₂O or AgO type. The presence of extended defects in the microdomain large nanoparticles facilitates the formation of Ag₂O or AgO layers covering metallic nanosilver.     

A comparative STM study of Ru nanoparticles deposited on HOPG by mass-selected gas aggregation versus thermal evaporation

Scanning tunneling microscopy was used to compare the morphologies of Ru nanoparticles deposited onto highly-oriented graphite surfaces using two different physical vapour deposition methods; (1) pre-formed mass-selected Ru nanoparticles with diameters between 2 nm and 15 nm were soft-landed onto HOPG surfaces using a gas-aggregation source and (2) nanoparticles were formed by e-beam evaporation of Ru films onto HOPG. The particles generated by the gas-aggregation source are round in shape with evidence of facets resolved on the larger particles. Annealing these nanoparticles when they are supported on unsputtered HOPG resulted in the sintering of smaller nanoparticles, while larger particles remained immobile. Nanoparticles deposited onto sputtered HOPG surfaces were found to be stable against sintering when annealed. The size and shape of nanoparticles deposited by e-beam evaporation depend to a large extent on the state of the graphite support and the temperature. Ru deposition onto unsputtered HOPG is characterised by bimodal growth with large flat particles formed on the substrate terraces and smaller diameter particles aligned along the substrate steps. Evaporation onto sputtered HOPG results in the formation of 2 nm round particles with a narrow size distribution. Finally, thermal deposition onto both sputtered and unsputtered HOPG at 660 °C results in larger particles showing a flat Ru(0 0 0 1) top facet.     

Characterization of Ag nanoparticles on Si wafer prepared using Tollen's reagent and acid-etching

Ag nanoparticles on SiO₂/Si surfaces synthesized using the Tollen's reagent and a subsequent acid-etching were characterized using X-ray photoelectron spectroscopy (XPS). Combining the reduction of the Tollen's reagent and the chemical etching, one can create naked Ag nanoparticles with various sizes in the size range below ∼10 nanometers (nm). The reduced particle size by the chemical etching was identified using positive core level shifts with increasing etching time. Ag nanoparticles smaller than ∼3 nm undergo a reversible oxidation and reduction cycle by reacting with H₂O₂/H₂O and a subsequent heating under vacuum to 150 °C, which was not found for the bulk counterparts and larger particles, demonstrating unique chemical properties of nanoparticles compared to the bulk counterparts.     

Size-selected Au clusters deposited on SiO2/Si: Stability of clusters under ambient pressure and elevated temperatures

This study examined the oxidation and reduction behavior of mass-selected Au clusters consisting of 2-13 atoms deposited on silica. An atomic oxygen environment was used for the oxidation of Au. X-ray photoelectron spectroscopy (XPS) was used to identify Au(III) and Au(O). Au₅, Au₇ and Au₁₃ clusters deposited on the as-prepared SiO₂/Si substrates were highly inert towards oxidation, whereas the other clusters could be oxidized, i.e. the chemical property drastically changed with the number of atoms in a cluster. The size-selectivity in chemical reactivity remained unchanged upon air-exposure. The chemical properties of the deposited Au clusters were unchanged after annealing at 250 °C. Annealing at higher temperatures caused structural changes to the surface, as determined by the oxidation behavior. XPS of the deposited Au clusters upon annealing indicated charge transfer from Au to silica.     

Surface oxidation of liquid Sn

We report the results of an X-ray scattering study that reveals oxidation kinetics and formation of a previously unreported crystalline phase of SnO at the liquid-vapour interface of Sn. Our experiments reveal that the pure liquid Sn surface does not react with molecular oxygen below an activation pressure of ∼5.0 × 10⁻⁶ Torr. Above that pressure a rough solid Sn oxide grows over the liquid metal surface. Once the activation pressure has been exceeded the oxidation proceeds at pressures below the oxidation pressure threshold. The observed diffraction pattern associated with the surface oxidation does not match any of the known Sn oxide phases. The data have an explicit signature of the face-centred cubic structure, however it requires lattice parameters that are about 9% smaller than those reported for cubic structures of high-pressure phases of Sn oxides.     

Interactions of oxygen and CO with AgAu bimetallic nanoparticles on sputtered highly ordered pyrolytic graphite (HOPG) surfaces

Oxidation of AgAu bimetallic nanoparticles on sputtered HOPG by atomic oxygen and reduction of the oxidized surface by CO at room temperature were studied using X-ray photoelectron spectroscopy (XPS). For 2 nm-sized nanoparticles, prepared by postdeposition of Ag on Au-core, atomic oxygen exposure mostly leads to the formation of chemically inert oxygen species. This result is analogous to that of pure Ag and Au nanoparticles of similar sizes on the same substrate. In contrast, “Au on Ag-core” nanoparticles form chemically active oxygen species, suggesting that depending on detailed structures of bimetallic nanoparticles, diverse chemical properties can be obtained.     

Oxidation of Au nanoparticles on HOPG using atomic oxygen

Various Au nanostructures prepared on highly ordered pyrolytic graphite (HOPG) were oxidized using atomic oxygen under ultrahigh-vacuum conditions, and the oxygen species formed in the Au nanostructures were characterized using X-ray photoelectron spectroscopy (XPS). For an Au thin film, only a single oxygen species could be identified in the O 1s spectrum, which can be attributed to Au-oxide. For Au nanoparticles smaller than ∼10 nm, in contrast, two different oxygen species were detected, which are suggested to be Au-oxide and subsurface oxygen (or dissolved oxygen), respectively. CO titration experiments confirm the formation of different oxygen species depending on the particle size.     

Size effect of gold nanoparticles on the electrocatalytic oxidation of carbon monoxide in alkaline solution

This paper describes preliminary findings of Au nanoparticle on glassy carbon (GC) electrodes for electro-oxidation of carbon monoxide in basic conditions. Electrochemical cyclic voltammetric results showed that CO oxidation simultaneously occurred in the anodic and cathodic sweeps during one cycle, and CO electro-oxidation activity was remarkably different in the anode for the different sized Au nanoparticles. The ultrafine catalyst metal particles (2 and 6 nm Au) were more active compared to the larger ones (12, 24 and 41 nm Au). The dependence of the activity on the particle size can be explained in terms of their effect on the number of active sites and the different surface gold oxide species.     

Sonochemical synthesis of porous gold nano- and microparticles in a Rosette cell

We report the synthesis of Au nano- and microparticles that relies on α-D-glucose (C₆H₁₂O₆) as the reducer and stabilizer in a Rosette cell under 20 kHz ultrasound irradiation. The chemical and physical effects of ultrasonic irradiation on the synthesis were investigated. The results showed that an optimum pH is required for the formation of insoluble Au(0) particles. Upon irradiation, low pH yielded Au nanoparticles while high pH resulted in microparticles. The Au surface capping by α-D-glucose hydroxyl and carbonyl groups was confirmed by Fourier transform infrared (FT-IR) spectroscopy. X-ray diffraction (XRD) analysis indicated that the Au particles crystallize within the face-centered-cubic (FCC) cell lattice. Moreover, continuous sonication reduced larger amounts of the Au precursor compared to the intermittent mode. Furthermore, tuning sonication time and mode influences the particle size and porosity as characterized by scanning and transmission electron microscopy. Our results shed a new light into the importance of the experimental and ultrasound parameters in obtaining Au particles of desired features through sonochemistry.     

Comparing the Contribution of Visible‐Light Irradiation,Gold Nanoparticles,and Titania Supports in Photocatalytic Nitroaromatic Coupling and Aromatic Alcohol Oxidation

Under visible‐light irradiation, gold nanoparticles (Au NPs) supported by titania (TiO₂) nanofibers show excellent activity and high selectivity for both reductive coupling of nitroaromatics to corresponding azobenzene or azoxylbenzene and selective oxidation of aromatic alcohols to corresponding aldehydes. The Au NPs act as active centers mainly due to their localized surface plasmon resonance (LSPR) effect. They can effectively couple the photonic energy and thermal energy to enhance reaction efficiency. Visible‐light irradiation has more influence on the reduction than on the oxidation, lowering the activation energy by 24.7 kJ mol^?1 and increasing the conversion rate by 88% for the reductive coupling, compared to merely 8.7 kJ mol^?1 and 46% for the oxidation. Furthermore, it is found that the conversion of nitroaromatics significantly depends on the particle size and specific surface area of supported Au NPs; and the catalyst on TiO₂(B) support outperforms that on anatase phase with preferable ability to activate oxygen. In contrast, for the selective oxidation, the effect of surface area is less prominent and Au NPs on anatase exhibit higher photo‐catalytic activity than other TiO₂ phases. The catalysts can be recovered efficiently because the Au NPs stably attach to TiO₂ supports by forming a well‐matched coherent interface observed via high‐resolution TEM.     

Low temperature and low pressure CO oxidation on gold clusters supported on MgO(100)

CO oxidation has been investigated on Au/MgO(100) model catalysts at RT and low pressure. The gold particles prepared by UHV evaporation on clean MgO surfaces are characterized by HRTEM. The gold particles are FCC single crystals or multiple twins with five-fold symmetry. Infrared spectroscopy indicates that two types of adsorption sites are present which correspond to loosely and strongly bound CO. The equilibrium CO coverage for the strongly bound CO is smaller than 0.1 ML. CO titration experiments show that oxygen does not dissociate on the gold nanoparticles. The CO oxidation reaction is studied at RT by molecular beam methods. A steady state CO reaction probability up to 0.50 is measured, for the first time at low pressure, for gold particles with a mean size of 1.5 nm. A reaction mechanism is proposed in which CO adsorbed on low coordinated gold atoms reacts with oxygen adsorbed molecularly, possibly at the Au/MgO interface.     

Self-organization of bimetallic PdAu nanoparticles on SiO<Subscript>2</Subscript> surface

Bimetallic PdAu nanoparticles on SiO₂ substrate were produced by a sequential room-temperature sputtering deposition method. By the atomic force microscopy technique we studied the nanoparticles self-organization mechanisms in various conditions. First, Pd nucleation and growth proceeds at the substrate defects and the Pd nanoparticles density increase rapidly. During the second sputtering deposition, Au atoms adsorb on the SiO₂ and diffuse toward Pd nanoparticles without forming new nuclei. The Au atoms are trapped by the preformed Pd nanoparticles, forming PdAu bimetallic nanoparticles which size increases. Furthermore, fixing the amount of deposited Pd and increasing the amount of deposited Au, we analyzed the evolution of the PdAu film surface morphology: we observe that the PdAu grows initially as three-dimensional islands; then the PdAu film morphology evolves from compact three-dimensional islands to partially coalesced worm-like structures, followed by a percolation morphology and finally to a continuous and rough film. The application of the interrupted coalescence model allowed us to evaluate the critical mean island diameter R _c ≈ 2.8 nm for the partial coalescence process. The application of the dynamic scaling theory of growing interfaces allowed us to evaluate the dynamic growth exponent β = 0.21 ± 0.01 from the evolution of the film surface roughness. Finally, fixing the amount of deposited Pd and Au we studied the self-organization mechanism of the PdAu nanoparticles induced by thermal processes performed in the 973–1173 K temperature range. The observed kinetic growth mechanism is consistent with a surface diffusion-limited ripening of the nanoparticles with a temperature-dependent growth exponent. The dependence of the growth exponent on the temperature is supposed to be linked to the variation with the temperature of the characteristics of the PdAu alloy. The activation energy for the surface diffusion process was evaluated in 0.54 ± 0.03 eV.     

Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe₃O₄-supported Pd model catalyst by molecular beam (MB) methods, time resolved IR reflection absorption spectroscopy (TR-IRAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The well-shaped Pd particles were prepared in situ by metal evaporation and growth under ultrahigh vacuum (UHV) conditions on a well-ordered Fe₃O₄ film on Pt(1 1 1).It is found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (∼10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface, stabilized by the iron-oxide support. Their formation and reduction is fully reversible. Upon decomposition, oxygen is released which migrates back onto the metallic part of the Pd surface. In consequence, the Pd interface oxide layer acts as an oxygen reservoir, the capacity of which by far exceeds the amount of chemisorbed oxygen on the metallic surface.Additionally, Pd surface oxides can also be formed at temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature. This effect is addressed to different onset temperatures for oxidation of the particle facets and sites. It is shown that the presence of Pd surface oxides sensitively modifies the adsorption and reaction properties of the model catalyst, i.e. by lowering the CO adsorption energy and CO oxidation probability. Still, a complete reduction of the Pd surface oxides can be obtained by extended CO exposure, fully reestablishing the metallic Pd surface.