Yvonne Joseph

Adsorption of water on FeO(111) and Fe3O4(111): identification of active sites for dissociation

The adsorption of water on epitaxial FeO(111) and Fe3O4(111) films structurally well characterized by STM and LEED was investigated with photoelectron and thermal desorption spectroscopy. On the FeO(111) surface terminated by a close-packed oxygen layer monomeric water spe- cies get physisorbed. On the Fe3O4(111) surface terminated by ¼ monolayer of Fe atoms located over a close-packed oxygen layer underneath water dissociates resulting in adsorbed OH groups. The OH saturation coverage corresponds to the number of surface Fe atoms, which is much larger than the surface defect concentrations. Therefore, the dissociation takes place at Fe sites exposed on the regular Fe3O4(111) surface, and the FeO(111) surface is chemically inert because no Fe sites exist thereon.

Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity

Gold-nanoparticle/organic films were prepared via layer-by-layer self-assembly using dodecylamine-stabilised Au-nanoparticles and poly(propyleneimine) (PPI) dendrimers of generation one to five (G1-G5) or hexadecanedithiol (HDT) as linker compounds. TEM and FE-SEM images revealed that the bulk of the films consisted of nanoparticles with diameters of about 4 nm. XPS was used to study the chemical composition of the films. The C 1s and N 1s signals of an AuPPI-G4 film were interpreted qualitatively according to the dendrimer structure. The absence of the nitrogen signal in case of an AuHDT film indicated that the dodecylamine ligands were quantitatively exchanged during film assembly. About 76% of the sulfur atoms were bound to the nanoparticles. the remainder being present as free thiol (S H) groups. All films displayed linear current voltage characteristics and Arrhenius-type activation of charge transport. The conductivities of the AuPPI films decreased exponentially over approximately two orders of magnitude (6.8 x 10(-2) to 1.0 x 10(-3) ohms(-1) cm(-1)) with a five-fold increase of the dendrimer generation number. Dosing the films with solvent vapours caused their resistances to increase. Using different solvent vapours demonstrated that the sensitivity of this response was determined by the solubility properties of the linker compounds. Microgravimetric measurements showed that absorption of analyte was consistent with a Langmuir adsorption model. These measurements also revealed a linear correlation between the electrical response (deltaR/Rini) and the concentration of absorbed analyte. The absorption of d4-methanol from a saturated vapour atmosphere was studied by neutron reflectometry with an AuPPI-G4 film. This measurement indicated condensation of methanol on top of the film and a uniform distribution of the analyte across the film thickness.

Structure and reactivity of iron oxide surfaces

Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111) substrates and used for studying structure–reactivity correlations. The film morphologies and their atomic surface structures were characterized by scanning tunneling microscopy and low energy electron diffraction including multiple scattering calculations. The adsorption of water, ethylbenzene, and styrene was investigated by temperature programmed desorption and photoelectron spectroscopy. A dissociative chemisorption of water and a molecular chemisorption of ethylbenzene and styrene is observed on all oxides that expose metal cations in their topmost layers, whereas purely oxygen-terminated FeO(111) monolayer films are chemically inert and only physisorption occurs. Regarding the technical styrene synthesis reaction, which is performed over iron oxide based catalysts, we find a decreasing chemisorption strength of the reaction product molecule styrene, if compared to ethylbenzene, when going from Fe3O4(111) over α-Fe2O3(0001) to KFexOy(111). Extrapolation of the adsorbate coverages to the technical styrene synthesis reaction conditions using the Langmuir isotherm for coadsorption suggests an increasing catalytic activity along the same direction. This result agrees with previous kinetic experiments performed at elevated gas pressures over the model systems studied here and over polycrystalline iron oxide catalyst samples. It indicates that the iron oxide surface chemistry does not change across the pressure gap and that the model systems simulate technical styrene synthesis catalysts in a realistic way.

Interaction of ethylbenzene and styrene with iron oxide model catalyst films at low coverages: A NEXAFS study

The adsorption of ethylbenzene and styrene on well ordered epitaxial iron oxide model catalyst films with different stoichiometries was investigated using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). On the iron-terminated Fe3O4(111) and α-Fe2O3(0001) surfaces chemisorption of ethylbenzene and styrene is observed occurring initially on the iron sites ia the π-electron system of the phenyl ring. This forces the molecules into an almost flat configuration (η6-like ring adsorption geometry). In the case of ethylbenzene this adsorption complex is supposed to lead to an activation of the C–H bonds, thus facilitating the dehydrogenation to styrene. The tilt angle of the aromatic ring systems increases to about 40° when approaching monolayer saturation. In contrast, the interaction with the oxygen-terminated FeO(111) surface is weak and of the physisorption type. The adsorbate–adsorbate interaction dominates and causes a tilted adsorption of the molecules from the beginning.

On the preparation and composition of potassium promoted iron oxide model catalyst films

Potassium promoted iron oxide model catalyst films were prepared by deposition of potassium onto epitaxial Fe3O4(111) films at 200 K, followed by annealing in the range 200 to 970 K. Their formation and composition were investigated by X-ray photoelectron spectroscopy (XPS) in combination with thermal desorption spectroscopy (TDS) and thermodynamic considerations. Already at 300 K a solid-state reaction occurred and the iron oxide was partly reduced. Around 700 K a KFeO2 phase was identified which transformed at higher temperatures into KxFe22O34(0.67<x<4). This transformation started from the bulk of the film so that initially a potassium-rich KFeO2 layer was formed on top of KxFe22O34. The formation of a single-crystalline KxFe22O34 (x = 0.67) layer, which is terminated by a submonolayer of potassium, is assumed to occur at 970 K. For a certain potassium content, this surface develops a well ordered phase with a (2 × 2) superstructure. The potassium containing phases are not stable in water atmosphere: In 10−8 mbar H2O, potassium hydroxide forms, then decomposes and desorbs beyond 400–500 K resulting in a potassium-depleted near-surface layer.

Adsorption of water on Fe3O4(111) studied by photoelectron and thermal desorption spectroscopy

The adsorption of water on Fe3O4(111) films grown epitaxially onto Pt(111) was investigated by ultraviolet photoelectron spectroscopy (UPS) in adsorption-desorption equilibrium and by thermal desorption spectroscopy (TDS). With increasing coverage both methods reveal the existence of three species on the surface: Dissociatively chemisorbed water (γ), physisorbed monomeric water (β) and hydrogen bonded condensed ice (α). The corresponding isosteric heats of adsorption qst and desorption energies Edes were determined by UPS and TDS, respectively. For the α- and βspecies they compare well, for the γ-species Edes is higher indicating an activation barrier for the dissociative adsorption, which is also supported by the observed slow adsorption kinetics. The dissociation is assumed to occur at iron cations with neighboring oxygen anions acting as proton acceptors.

Determination of adsorption energies and kinetic parameters by isosteric methods

If adsorption is reversible and sufficiently fast, isotherms or isobars can be measured in adsorption–desorption equilibrium at low pressures on single crystal surfaces. The isosteric heat of adsorption can easily be derived from them using the Clausius–Clapeyron equation. Reaction orders and frequency factors can, in principle, be deduced from a fit of the isobars (or isotherms) using the kinetic equations for adsorption and desorption. Immobile and mobile precursor kinetics can be included in the analysis but the fit fails when structural phase transitions in the substrate or the adlayer cause the kinetics to become complex. We review the methods, strong points and limitations of isobar (isotherm) measurements and of their kinetic fits by discussing the adsorption of water, ethylbenzene and styrene on FeO(111), Fe3O4(111) and Pt(111) and the adsorption of ammonia on germanium surfaces. Where the kinetic fit was successful, mobile precursor kinetics is quite common and frequency factors for desorption deviate considerably from the often assumed value of 1013 s−1.

Immobilization of Titanium(IV) Oxide onto 3D Spongin Scaffolds of Marine Sponge Origin According to Extreme Biomimetics Principles for Removal of C.I. Basic Blue 9

The aim of extreme biomimetics is to design a bridge between extreme biomineralization and bioinspired materials chemistry, where the basic principle is to exploit chemically and thermally stable, renewable biopolymers for the development of the next generation of biologically inspired advanced and functional composite materials. This study reports for the first time the use of proteinaceous spongin-based scaffolds isolated from marine demosponge Hippospongia communis as a three-dimensional (3D) template for the hydrothermal deposition of crystalline titanium dioxide. Scanning electron microscopy (SEM) assisted with energy dispersive X-ray spectroscopy (EDS) mapping, low temperature nitrogen sorption, thermogravimetric (TG) analysis, X-ray diffraction spectroscopy (XRD), and attenuated total reflectance–Fourier transform infrared (ATR–FTIR) spectroscopy are used as characterization techniques. It was found that, after hydrothermal treatment crystalline titania in anatase form is obtained, which forms a coating around spongin microfibers through interaction with negatively charged functional groups of the structural protein as well as via hydrogen bonding. The material was tested as a potential heterogeneous photocatalyst for removal of C.I. Basic Blue 9 dye under UV irradiation. The obtained 3D composite material shows a high efficiency of dye removal through both adsorption and photocatalysis.

Gold-nanoparticle/dithiol films as chemical sensors and first steps toward their integration on chip

Networked films comprised of Au-nanoparticles and organic dithiols were deposited onto silicon or glass substrates via repetitive self-assembly from solution. The substrates were equipped with interdigitated electrode structures for electrically addressing the films. Dodecylamine stabilized Au-nanoparticles with a core diameter of 4 (+/-0.8) nm were used for film assembly. The metal cores were networked through 1,n-alkylenedithiols with chain lengths varying from 6 to 20 methylene units. With increasing number of methylene units, the conductivity of the films decreased by several orders of magnitude without following a monoexponential decay. When dosing the films with organic vapors (toluene, 1-propanol, 4-methyl-2-pentanone) their resistance increased reversibly. The amplitudes of this response increased strongly with increasing length of the linker molecules. In contrast, the sensitivity to water vapor was marginal for all alkylenedithiol-linked films. However, insertion of polar amide groups into the backbone of the linker decreased the sensitivity to toluene and significantly enhanced the sensitivity to water. The deposition of sensor films on chip from organic solvents could be directed by using a patterned CaO mask. After film deposition, the lift-off of nanoparticles from protected parts of the substrate was achieved by dissolving the mask in aqueous solution at ambient temperature.

Extreme biomimetics: A carbonized 3D spongin scaffold as a novel support for nanostructured manganese oxide(IV) and its electrochemical applications

Composites containing biological materials with nanostructured architecture have become of great interest in modern materials science, yielding both interesting chemical properties and inspiration for biomimetic research. Herein, we describe the preparation of a novel 3D nanostructured MnO2-based composite developed using a carbonized proteinaceous spongin template by an extreme biomimetics approach. The thermal stability of the spongin-based scaffold facilitated the formation of both carbonized material (at 650 °C with exclusion of oxygen) and manganese oxide with a defined nanoscale structure under 150 °C. Remarkably, the unique network of spongin fibers was maintained after pyrolysis and hydrothermal processing, yielding a novel porous support. The MnO2-spongin composite shows a bimodal pore distribution, with macropores originating from the spongin network and mesopores from the nanostructured oxidic coating. Interestingly, the composites also showed improved electrochemical properties compared to those of MnO2. Voltammetry cycling demonstrated the good stability of the material over more than 3,000 charging/discharging cycles. Additionally, electrochemical impedance spectroscopy revealed lower charge transfer resistance in the prepared materials. We demonstrate the potential of extreme biomimetics for developing a new generation of nanostructured materials with 3D centimeter-scale architecture for the storage and conversion of energy generated from renewable natural sources.