Zoltán Pászti

Electronic structure of gold nanoparticles deposited on SiOx/Si(100)

Nanosize gold particles were prepared by Ar+ ion sputtering of island-like 10-nm-thick film deposited onto Si/SiOx substrate. The valence band of the gold particles was measured by means of photoelectron spectroscopy and infrared (IR) absorbance. The size of the particles was determined by transmission electron microscopy (TEM). The valence band of Au nanoparticles is strongly redistributed with decreasing size, involving mostly the lowest and the highest binding energy part of the Au 5d valence states. This effect can be attributed more to the redistribution than to the narrowing of the 5d states.

Electronic structure and catalytic properties of transition metal nanoparticles: The effect of size reduction

Size reduction of metal particles results in the formation of nanoparticles having short-range order and metastable state.Modeling of the nanoparticles can be obtained by various approaches. The major arrangement is the use of a model support on which metal nanoparticles can be created in a controlled way. Another approach is the use of amorphous alloy as precursor in which the ensemble of active sites (normally small metal nuclei embedded into amorphous matrix) is created.The modeling will be illustrated through the paper using SiO2/Si(100) on which several transition metals will be deposited by pulsed laser deposition. Ultraviolet photoelectron spectroscopic technique as well as transmission electron microscopic technique will be utilized in characterization of the samples. CO chemisorption and CO oxidation as test reaction will be applied to show the connection between catalytic behavior and electronic properties or morphology of nanoparticles.

Pressure dependent formation of small Cu and Ag particles during laser ablation

Abstract In this paper the possibility of application of laser ablation in inert gas atmosphere for deposition of nanosized particles is examined. Cu and Ag were deposited by laser ablation in 0–10 mbar Ar atmosphere. The samples were characterized by TEM and XPS. It was found that below 1 mbar for Cu and 2 mbar for Ag, island like thin film formation occurs. In Cu above 1 mbar lonely nanoparticles with diameters of 3–4 nm as well as large cluster aggregates were found, instead of the islands. This morphology remained unchanged up to 10 mbar. On the contrary, in Ag at 5 mbar individual particles with sizes of 4–6 nm were observed instead of the aggregates or islands. At 10 mbar the number of the 4–6 nm particles strongly decreased, and particles in the size range of 10 nm became dominant. The observed results may be related to the different decelerating rate of the evaporated Cu and Ag atoms in the Ar atmosphere.

Adsorption of amino acids on hydrophilic surfaces

Sum frequency generation vibrational spectroscopy (SFG) is a powerful tool for in situ investigation of adsorption processes at biologically important solid–liquid interfaces. In this work adsorption of selected amino acids on fused silica, calcium fluoride and titanium dioxide substrates was studied by this technique. SFG spectra taken at the amino acid solution–fused SiO2 interface revealed the lack of formation of any ordered adsorbate layer, regardless of whether acidic or other, e.g. aromatic, amino acids were used. Ex situ spectra (measured after drying the substrate) showed the formation and gradual growth of amino acid crystallites. In the case of CaF2, growth of randomly oriented aspartic acid crystallites was observed even at the solution–substrate interface. Finally, on the TiO2 substrate, acidic amino acids formed a stable, uniform, more or less ordered coating, which remained unchanged even after drying the sample. On the other hand, non-acidic amino acids like phenylalanine showed very little affinity towards TiO2, emphasizing the role of the acidic side chain in the bonding to the substrate. The fact that formation of an amino acid overlayer was observed only on titanium dioxide is probably related to its biocompatibility property.

Interaction of Carbon Monoxide with Au(111) Modified by Ion Bombardment: A Surface Spectroscopy Study under Elevated Pressure †

Gold based model systems exhibiting the structural versatility of nanoparticle ensembles and being accessible for surface spectroscopic investigations are expected to provide new information about the adsorption of carbon monoxide, a key process influencing the CO oxidation activity of this noble metal in nanoparticulate form. Accordingly, in the present work the interaction of CO is studied with an ion bombardment modified Au(111) surface by means of a combination of photoelectron spectroscopy (XPS and UPS), sum frequency generation vibrational spectroscopy (SFG), and scanning tunneling microscopy (STM). While no adsorption was found on intact Au(111), data collected on the ion bombarded surface at cryogenic temperatures indicated the presence of stable CO adsorbates below 190 K. A quantitative evaluation of the C 1s XPS spectra and the surface morphology explored by STM revealed that the step edge sites created by ion bombardment are responsible for CO adsorption. The identification of the CO binding sites was confirmed by density functional theory (DFT) calculations. Annealing experiments up to room temperature showed that at temperatures above 190 K unstable adsorbates are formed on the surface under dynamic exposure conditions that disappeared immediately when gaseous CO was removed from the system. Spectroscopic data as well as STM records revealed that prolonged CO exposure at higher pressures of up to 1 mbar around room temperature facilitates massive atomic movements on the roughened surface, leading to its strong reordering toward the structure of the intact Au(111) surface, accompanied by the loss of the CO binding capacity.

Laser ablation induced formation of nanoparticles and nanocrystal networks

Using experimental data on morphology of nanophase materials prepared by pulsed laser ablation in an inert gas atmosphere, we present a phenomenological description of their condensation process. According to our idea, in high enough background pressure a shock wave is initiated by collisions between gas and target atoms, which slows down and spatially confines the plume, while it is effectively cooled by further collisions. Thus, the plume becomes highly supersaturated and the condensed phase of the target material starts to develop via homogeneous nucleation. Later on, still in a very limited volume, nanoparticles grow via complete or incomplete coalescence forming compact objects or large networks. The pressure threshold for gas phase condensation is intimately related to the collisional energy loss characteristics of the particular gas-target combination as well as to the thermophysical properties of the target.

Pd-decorated m-BiVO4/BiOBr ternary composite with dual heterojunction for enhanced photocatalytic activity

We introduce a unique material ensemble to boost the photocatalytic activity of m-BiVO4 by creating dual heterojunction of bismuth oxybromide nanosheets and Pd nanodomains. The m-BiVO4/BiOBr/Pd ternary composite demonstrates substantially higher photocatalytic activity compared to pure m-BiVO4. We demonstrate for the first time the use of such visible light photocatalyst in highly efficient degradation of polychlorinated biphenyls.

The investigation of thin protecting layers on roughened galvanized steel surfaces produced by different coating methods

Protective, chromate substitute thin layers on roughened galvanized surfaces produced at OCAS (Arcelor, Belgium) were characterized and compared using Scanning Electron Microscopy (SEM+EDS), Atomic Force Microscopy (AFM), Nanoindentation and X-ray Photoemission Spectroscopy (XPS). EDX maps, line scans and point analyses obtained at various places of the surfaces have shown differences between the CVD and silane nanolayers in the matter of thickness distribution and composition. At cross-section specimens the thickness of the layers could be shown. The hardness differences caused by layer thickness variations are hard to follow by nanoindentation as the penetration depth of the indenter is much larger than the thickness of the coatings. XPS measurements can distinguish between the chemical states of silicon in CVD and silane coatings.

Properties of high-density amorphous carbon films deposited by laser ablation

The properties of carbon films deposited by high intensity pulsed Nd:G lass laser are reported. Different measurements like TEM, SIMS, EELS, XPS and A FM showed the formation of a high-density amorphous carbon layer with good protection against che mically aggressive alkaline solutions. We demonstrated that instead of excimer lase rs, Nd-based solid state lasers can be used successfully for preparation of high quality amorphous carbon films. Introduction Carbon layers have growing importance because of the wide range of applications. The wide variety of carbon materials based on the unique bonding property of t he carbon atoms. Carbon has 4 valence electrons (2s 2 p). Chemical bonds may include both π or σ bonds. In case of ideal diamond crystal all 4 bonds are σ, which is known as sp 3 hybridization. In this form the carbon crystal has extreme physical and chemical par meters. In case of graphite crystal 3 σ and 1 π bonds exist (it is called sp 2 hybridization). The difference in only one bond changes the properties of the material drastically. In amorphous carbon structures both of the two hybrid states (sp 2 and sp) can exist. Therefore a wide scale of properties of the a morphous carbon materials can be achieved. Preparation of sp 3 rich diamond-like carbon (DLC) or sp 2 rich graphite-like carbon (GLC) films in high quality is a great challenge now adays. Multi-technique investigation of these layers can result in enhancing their qualit y. The commonly used methods for producing DLC layers are PVD [1], plasma CVD [2-4] ion beam de position [5,6], sputtering [7] and pulsed laser deposition (PLD) [8-14]. Many of the PLD studies concentrate on excimer laser deposition techniques. Our aim was to produce good quality carbon f ilms for protective purpose with a Nd:Glass laser, which doesn’t use any toxic gase s in contrast to the excimer lasers. It may be a very important fact in environmentally friendly industri al applications. Experiment The carbon thin films were deposited on both silicon and silicon dioxide s ubstrates at room temperature in 10 -6 mbar pressure. The laser wavelength was 1054nm with 1Hz repetiti on rate. The 40ns long laser pulses were focused onto a spot of 5mm , which resulted in 10 W/cm power density. Using an excimer laser at so high energies w ll lead to formation of a predominantly graphitic layer [15] due to the high level of multiphoton i onization and inverse bremsstrahlung heating in the plasma. The dimensions of the samples were 1cm x 5cm, but larger areas can also be covered using this method. Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 127-132 doi:10.4028/www.scientific.net/MSF.414-415.127

Graphite Oxide-TiO2 Nanocomposite Type Photocatalyst for Methanol Photocatalytic Reforming Reaction

Graphite-oxide/TiO2 (GO/TiO2) composite materials were prepared by heterocoagulation method from Brodie’s graphite-oxide (GO) in order to test them as catalysts in the methanol photocatalytic reforming reaction in liquid phase. The preparation of the composite itself resulted in only little changes in the structure of GO as it was indicated by attenuated total reflection infrared (ATR-IR) and 13C magic-angle spinning nuclear magnetic resonance (13C MAS NMR) spectroscopic measurements. However, during the photocatalytic reaction, all of the GO/TiO2 samples darkened strongly indicating structural changes of GO. X-ray photoelectron spectroscopy along with NMR confirmed the loss of oxygen functionalities and emergence of graphitic species in the samples recovered from the photocatalytic reaction. Model experiments were designed to identify the key factors determining the activity of the GO/TiO2 derived photocatalysts. It was found that the emergence of a pronounced coupling between TiO2 and the graphite-like carbonaceous material is the most important contribution to get active and stable photocatalysts.