Spyridon Zafeiratos

Nontrivial Redox Behavior of Nanosized Cobalt: New Insights from Ambient Pressure X-ray Photoelectron and Absorption Spectroscopies

The reduction and oxidation of carbon-supported cobalt nanoparticles (3.50±0.22 nm) and a Co (0001) single crystal was investigated by ambient pressure X-ray photoelectron (APPES) and X-ray absorption (XAS) spectroscopies, applied in situ under 0.2 mbar hydrogen or oxygen atmospheres and at temperatures up to 620 K. It was found that cobalt nanoparticles are readily oxidized to a distinct CoO phase, which is significantly more stable to further oxidation or reduction compared to the thick oxide films formed on the Co(0001) crystal. The nontrivial size-dependence of redox behavior is followed by a difference in the electronic structure as suggested by theoretical simulations of the Co L-edge absorption spectra. In particular, contrary to the stable rocksalt and spinel phases that exist in the bulk oxides, cobalt nanoparticles contain a significant portion of metastable wurtzite-type CoO.

Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity

Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we report a facile bottom-up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co-condensation of urea and oxamide followed by post-calcination in molten salt is shown to result in highly crystalline species with a maximum π-π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H2 evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.

Few layer graphene decorated with homogeneous magnetic Fe3O4 nanoparticles with tunable covering densities

Magnetic iron oxide nanoparticles (NPs) with narrow size distribution (8 ± 2 nm), well defined chemical composition and crystalline structure are synthesized and homogeneously dispersed onto the surface of few-layer graphene (FLG) via a solvothermal decomposition method. The iron oxide NPs are strongly anchored to the graphene surface and confer a magnetic character to the final composite. The metal oxide/support interaction is high enough to avoid the NPs coalescence and/or agglomeration and thus to preserve the NPs size and dispersion after thermal treatment up to 400 °C. The introduced iron oxide NPs on FLG also play a role of nano-spacers to prevent the re-stacking of the graphene sheets upon the drying process. It is expected that such a composite could find use in several application fields such as catalyst support for liquid-phase reactions with easy magnetic separation, in electrochemical energy storage and in waste water treatment. The ability of the synthesized iron oxide NP/FLG composite to adsorb foreign elements (organic pollutants) is demonstrated in the methylene blue (MB) adsorption and its catalytic properties are evaluated in the selective oxidation of H2S.

Synthesis of porous carbon nanotubes foam composites with a high accessible surface area and tunable porosity

The macroscopic shaping of carbon nanostructure materials with tunable porosity, morphologies, and functions, such as carbon nanotubes (CNT) or carbon nanofibers (CNF), into integrated structures is of great interest, as it allows the development of novel nanosystems with high performances in filter applications and catalysis. In the present work, we report on a low temperature chemical fusion (LTCF) method to synthesize a self-macronized carbon nanotubes foam (CNT-foam) with controlled size and shape by using CNT as a skeleton, dextrose as a carbon source, and citric acid as a carboxyl group donor reacting with the hydroxyl group present in dextrose. The obtained composite has a 3D pore structure with a high accessible surface area (>350 m2 g−1) and tunable meso- and macro-porosity formed by the addition of a variable amount of ammonium carbonate into the starting mixture followed by a direct thermal decomposition. The as-synthesized CNT-foam also exhibits a relatively high mechanical strength which facilitates its handling and transport, while the nanoscopic morphology of the CNT significantly reduces the problem of diffusion and contributes to an improvement of the effective surface area for subsequent applications. These CNT-foams are successfully employed as selective and recyclable organic absorbers with high efficiency in the field of waste water treatment.

Hysteresis and change of transition temperature in thin films of Fe{[Me2Pyrz]3BH}2, a new sublimable spin-crossover molecule.

Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)2(NCS)2, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (∼5 eV), decreasing at thickness below 50 nm, possibly due to film morphology.

Uncovering the Stabilization Mechanism in Bimetallic Ruthenium-Iridium Anodes for Proton Exchange Membrane Electrolyzers.

Proton exchange membrane (PEM) electrolyzers are attracting an increasing attention as a promising technology for the renewable electricity storage. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied for in situ monitoring of the surface state of membrane electrode assemblies with RuO2 and bimetallic Ir0.7Ru0.3O2 anodes during water splitting. We demonstrate that Ir protects Ru from the formation of an unstable hydrous Ru(IV) oxide thereby rendering bimetallic Ru-Ir oxide electrodes with higher corrosion resistance. We further show that the water splitting occurs through a surface Ru(VIII) intermediate, and, contrary to common opinion, the presence of Ir does not hinder its formation.

Single-Layer Graphene as an Effective Mediator of the Metal-Support Interaction.

Single-layer chemical vapor deposition (CVD)-grown graphene was transferred onto a ZnO (0001) substrate forming a large-area, low-defect density, protective layer. The quality of the graphene layer and its effect on the interaction between the ZnO support and vapor-deposited cobalt particles was investigated by spectroscopic and microscopic techniques. We demonstrate that the in-between graphene layer influences both the oxidation state and the morphology of cobalt upon annealing in vacuum. In particular, cobalt strongly interacts with the bare ZnO substrate forming flat particles, which are readily oxidized and redispersed upon annealing in ultrahigh vacuum conditions. In contrast, in the presence of the graphene interlayer, cobalt forms highly dispersed nanoparticles, which are resistant to oxidation, but prone to surface diffusion and agglomeration. The graphene layer exhibits remarkable stability upon cobalt deposition and vacuum annealing, while interaction with reactive gases can facilitate the formation of defects.

Carbon nanotube channels selectively filled with monodispersed Fe3−xO4 nanoparticles

Magnetic iron oxide (Fe3−xO4) nanoparticles (NPs) with high density and narrow size distribution were selectively cast inside or at the outer surface of the channels of carbon nanotubes (CNTs) by performing thermal decomposition of an organic iron precursor in liquid-phase medium in the presence of adequately pre-treated CNTs. The size of the Fe3−xO4 NPs generated by the synthesis is around 13 ± 3 nm and no agglomeration was observed thanks to the presence of oleic acid playing the role of surfactant in the synthesis medium. The selective filling of the Fe3−xO4 NPs inside the CNT channel was confirmed by X-ray diffraction, transmission electron microscopy (also in tomography mode) and X-ray photoelectron spectroscopy. Such NPs were found to be less sensitive towards oxidation compared to the same NPs synthesized without CNTs according to the powder X-ray diffraction and they display thus a higher saturation magnetization (65 emu g−1). Such a composite was found to be superparamagnetic at room temperature. Due to its new “magnetic state”, it could efficiently be employed in applications, which need both magnetic and electric properties, especially in catalysis in liquid-phase medium where the catalyst–product separation is facilitated by the magnetic properties of the catalyst.

Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media

The influence of the oxidation of Ni electrodes on the kinetics of the hydrogen oxidation (HOR) and evolution reactions (HER) has been explored by combining an experimental cyclic voltammetry study, microkinetic modeling and X-ray photoelectron spectroscopic analysis. Almost 10 times enhancement of the activity of Ni in the HOR/HER has been observed after its oxidation under the contact with air at ambient conditions and assigned to the presence of NiO species on the surface of metallic Ni. The experimental cyclic voltammetry curves have been analyzed with the help of kinetic model in order to shed light on the mechanism of the HOR/HER for two types of Ni electrodes and its dependence on the presence of NiO on the surface of the electrode. The main features of the experimental current-potential curves can be reproduced with a kinetic model assuming that the free energy of the adsorbed hydrogen intermediate is increased and that the kinetics of the Volmer step is enhanced in the presence of nickel oxide species. The kinetic model provides evidence for the switching from the Heyrovsky–Volmer mechanism on metallic Ni to Tafel–Volmer mechanism on the activated electrode, where surface oxide species co-exist with metal Ni sites.Graphical Abstract

Surface State and Catalytic Performance of Ceria‐Supported Cobalt Catalysts in the Steam Reforming of Ethanol

The effect of the particle size and the addition of a K promoter on the oxidation state of Co/CeO2 was investigated and correlated with selectivity for ethanol steam reforming performed with various H2O/EtOH molar ratios. Spectroscopic studies showed that the oxidation state of the catalyst components depends on the H2O/EtOH molar ratio. Surface oxygen‐containing species (OH and Kδ+−Oδ−) are necessary to achieve a good catalyst selectivity, to decrease the amount of coke deposited and to change the nature of the product from fully dehydrogenated C=C to CHx. Besides the surface concentration of oxygen‐containing species, the catalyst morphology and the location at which oxygen‐containing species are chemisorbed may be equally important to their abundance; the selective ceria‐supported Co catalyst should have well‐dispersed Co particles deposited on a well‐dispersed support.