Angeliki Siokou

Effect of the tungsten oxidation states in the thermal coloration and bleaching of amorphous WO3 films

Abstract The structural, optical and electronic changes caused by thermal annealing in vacuum and in oxygen atmosphere on evaporated WO 3 films, were investigated by visible and infrared transmission spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The luminous transmittance of the WO 3 films was related to their state of oxidation at each annealing temperature, both during thermal coloration and bleaching. It was found that annealing in vacuum progressively reduces the oxide to the lower oxidation states W 5+ , W 4+ and changes the film color. Both states contribute to the coloration of the film. However, the presence of the W 4+ state was found to play a dominant role in deep blue coloration. Thus, a two step procedure model with polaron transitions from W 5+ to W 4+ and also from W 6+ to W 5+ would better describe the phenomenon.

Structural and electrochemical properties of opaque sol-gel deposited WO3 layers

Abstract The preparation of thick and opaque WO3 layers by a simple sol–gel method is presented. The structure and morphology of these films has been assessed by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The sol–gel WO3 layers were found to have oxygen replaced by a significant amount of hydroxyls and water incorporated in their structure. They consist of particles with dimensions of several microns, have high porosity and increased surface area, which renders them suitable for gas sensing applications. Their color changes reversibly from yellowish green to dark blue upon insertion of lithium ions. They are electrochemically stable and can withstand more than 1000 voltammetric coloration–bleaching cycles.

Effect of the tungsten and molybdenum oxidation states on the thermal coloration of amorphous WO3 and MoO3 films

Transmission and X-ray photoelectron spectroscopy (XPS) was used to investigate the optical and electronic changes caused by thermal annealing in UHV of amorphous WO 3 and MoO 3 thin films suitable for electrochromic applications. The core level XPS measurements showed that annealing in UHV progressively reduces the oxides to the lower oxidation states and causes changes in their color. Their luminous transmittance was related to their state of oxidation at each annealing temperature during thermal coloration. Only the 02p peak characterizes the valence band spectra of the oxides before coloration. After annealing, the 5d and 6s states of tungsten and 4d, 5s of molybdenum are partially filled by electrons released due to the reduction of the oxides. The MoO 3 film exhibits deep blue coloration below 400°C, associated to a continuous distribution between Mo 6+ and Mo 5+ states. For the WO 3 film the W 4 state, which appears above 400°C, plays the dominant role in deep coloration. At lower temperatures distinct W 5+ states are formed with a limited contribution to coloration.

CO2-Laser-Induced Growth of Epitaxial Graphene on 6H-SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large-area, low-strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single-step epitaxial growth of large-area homogeneous graphene film on the surface of SiC(0001) using an infrared CO2 laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost-effective in that it does not necessitate SiC pre-treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X-ray photoelectron spectroscopy, secondary ion-mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO2-laser-induced graphene growth and the lack of strict sample–environment conditions.

Towards the preparation of realistic model Ziegler-Natta catalysts: XPS study of the MgCl2/TiCl4 interaction with flat SiO2/Si(100)

Despite of the wide use of supported Ti based Ziegler-Natta catalysts in the olefin polymerization industry, questions concerning the role of each one of the catalyst components in the polymerization process, have not found a satisfactory answer yet. This is mainly because of the high sensitivity of these systems to oxygen and atmospheric moisture that makes their study in an atomic level rather complicated. Realistic surface science models of the pre-activated SiO2 supported MgCl2/TiCl4 and TiCl4 Ziegler-Natta catalysts were prepared by spin coating on flat conductive SiO2/Si(1 0 0) supports under inert atmosphere. This preparation technique resembles the wet chemical impregnation which is the industrial method of the catalyst preparation. XPS analysis showed that the catalyst precursor anchors on the silica surface through bonding of the Ti atoms with surface silanes or siloxanes, while Mg is attached to the Ti through chlorine bridges. Thermal treatment of the catalysts at 723 K leads to total Cl desorption when MgCl2 is not present while a significant amount of the Ti atoms is reduced to the Ti3+ state.

Controlled release of 5-fluorouracil from microporous zeolites.

UNLABELLED Zeolite particles with different pore diameter and particle size were loaded with the model anticancer drug 5-fluorouracil. The loaded zeolites were characterized by means of SEM, XRD, DSC, XPS, N2 physisorption and FT-IR. Higher loading of 5-FU was observed for NaX-FAU than BEA. Release studies were carried out in HCl 0.1N. Release of 5-FU from NaX-FAU showed exponential-type behaviour with the drug fully released within 10 min. In the case of BEA, the kinetics of 5-FU shows a multi-step profile with prolonged release over time. Molecular dynamics simulations showed that diffusion of the drug molecule through the BEA framework is lower than for NaX-FAU due to increased van der Waals interaction between the drug and the framework. The effect of zeolitic particles on the viability of Caco-2 monolayers showed that the NaX-FAU particles cause a reduction of cell viability in a more pronounced way compared with the BEA particles. FROM THE CLINICAL EDITOR This article describes zeolite-based nanoparticles in generating time-controlled release of 5-FU from zeolite preparations for anti-cancer therapy.

Growth and interfacial studies of conjugated oligomer films on Si and SiO2 substrates

The growth of [2,5-bis(4-styryl)styryl] 1,4-dioctyloxybenzene, (Ooct-OPV5) oligomer films on Si (100)-(2×1) and Si (111)-(7×7) reconstructed surfaces as well as on a SiO2 film over a Si (100) wafer was studied by x-ray photoelectron spectroscopy (XPS). Ooct-OPV5 resembles poly (p-phenylenevinylene) (PPV), a polymer that is widely used in organic light emitting diodes. High purity oligomer films of up to 18 nm thickness were prepared on the clean substrates by stepwise evaporation in ultrahigh vacuum conditions and a layerwise growth of films was observed on all substrates. The electronic structure of the oligomer interface with n-doped Si (111) was investigated by combined x ray and ultraviolet photoelectron spectroscopies (XPS) and (UPS). The C 1s XPS peak of the bulk oligomer consisted of three components, all associated with oligomer functional groups at binding energies 285.05, 285.75, and 287.15 eV, respectively. During growth, both C 1s and O 1s peaks in the film exhibited an upward BE shift of 0.45...

SEMICONDUCTING RHENIUM SILICIDE THIN FILMS ON SI(111)

The crystallographic, electronic, and optical properties of thin ReSi2 films (∼20–300 A) have been investigated in situ by low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS), ex situ by glancing incidence x‐ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650 °C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed by in situ surface techniques. For very thin films (≲35 A) LEED shows a faint (1×1) pattern after annealing at 750 °C, which improves slightly up to ∼900 °C. For thick films (∼50–300 A) only a bright background is observed. XPS indicates that the ReSi2 composition is attained upon annealing at 600 °C. In the Re‐Si bonding the charge transfer is negligible: the energy positions of the corelevels (Si 2p and Re 4f) are the same in the compound and in the elements. As the ener...

Spillover Phenomena and Its Striking Impacts in Electrocatalysis for Hydrogen and Oxygen Electrode Reactions

The core subject of the present paper represents the interrelated spillover (effusion) phenomena both of the primary oxide and the H-adatoms, their theory and practice, causes, appearances and consequences, and evidences of existence, their specific properties, and their alterpolar equilibria and kinetic behavior, structural, and resulting catalytic, and double layer charging features. The aim is to introduce electron conductive and d-d interactive individual and composite (mixed valence) hypo-d-oxide compounds, of increased altervalent capacity, or their suboxides (Magneli phases), as the interactive catalytic supports and therefrom provide (i) the strong metal-support interaction (SMSI) catalytic effect and (ii) dynamic spillover interactive transfer of primary oxides (M-OH) and free effusional H-adatoms for further electrode reactions and thereby advance the overall electrocatalytic activity. Since hypo-d-oxides feature the exchange membrane properties, the higher the altervalent capacity, the higher the spillover effect. In fact, altervalent hypo-d-oxides impose spontaneous dissociative adsorption of water molecules and then spontaneously pronounced membrane spillover transferring properties instantaneously resulting with corresponding bronze type (Pt/H xWO 3) under cathodic and/or its hydrated state (Pt/W(OH) 6), responsible for Pt-OH effusion, under anodic polarization, this way establishing instantaneous reversibly revertible alterpolar bronze features (Pt/H 0.35WO 3 ⇔ Pt/W(OH) 6) and substantially advanced electrocatalytic properties of these composite interactive electrocatalysts. Such nanostructured-type electrocatalysts, even of mixed-valence hypo-d-oxide structures (Pt/H 0.35WO 3/TiO 2/C, Pt/H xNbO 3/TiO 2/C), have for the first time been synthesized by the sol-gel methods and shown rather high stability, electron conductivity, and nonexchanged initial pure monobronze spillover and catalytic properties. Such a unique electrocatalytic system, as the striking target issue of the present paper, has been shown to be the superior for substantiation of the revertible cell assembly for spontaneous reversible alterpolar interchanges between PEMFC and WE. The main target of the present thorough review study has been to throw some specific insight light on the overall spillover phenomena and their effects in electrocatalysis of oxygen and hydrogen electrode reactions from diverse angles of view and broad contemporary experimental methods and approaches (XPS, FTIR, DRIFT, XRD, potentiodynamic spectra, UHRTEM).

An XPS and WF study of the ErSi(100) interface formation

XPS and work function measurements were used to investigate the Er /Si(100) interface formation after Er deposition up to 9 ML between 250 and 500 K and subsequent annealing up to 970 K. Erbium initially adsorbs as isolated adatoms with a dipole moment of 1.1 ± 0.5 D. The Er-Si interaction above 1 ML leads to an interface silicide plus an intermixed dilute Si phase extending up to ∼ 5 ML at 370 K. Upon annealing, the intermixed phase becomes more dense in Si, extends up to the surface and eventually turns into a silicide with a WF of 4.8 eV. The XPS results are in agreement with the literature on Er/Si(111) and other rare-earth /silicon systems, whereas the WF results demonstrate the usefulness of WF measurements in the study of RE/Si interfaces