Showing papers by "Gaetano Granozzi published in 2015"

Single and Multiple Doping in Graphene Quantum Dots: Unraveling the Origin of Selectivity in the Oxygen Reduction Reaction

Abstract: Singly and multiply doped graphene oxide quantum dots have been synthesized by a simple electrochemical method using water as solvent. The obtained materials have been characterized by photoemission spectroscopy and scanning tunneling microscopy, in order to get a detailed picture of their chemical and structural properties. The electrochemical activity toward the oxygen reduction reaction of the doped graphene oxide quantum dots has been investigated by cyclic voltammetry and rotating disk electrode measurements, showing a clear decrease of the overpotential as a function of the dopant according to the sequence: N ∼ B > B,N. Moreover, assisted by density functional calculations of the Gibbs free energy associated with every electron transfer, we demonstrate that the selectivity of the reaction is controlled by the oxidation states of the dopants: as-prepared graphene oxide quantum dots follow a two-electron reduction path that leads to the formation of hydrogen peroxide, whereas after the reduction with ...

Metal–Support Interaction in Platinum and Palladium Nanoparticles Loaded on Nitrogen-Doped Mesoporous Carbon for Oxygen Reduction Reaction

Abstract: Mesoporous carbons are highly porous materials, which show large surface area, chemical inertness and electrochemical performances superior to traditional carbon material. In this study, we report the preparation of nitrogen-doped and undoped mesoporous carbons by an optimized hard template procedure employing silica as template, sucrose and ammonia as carbon and nitrogen source, respectively. Surface area measurements assert a value of 900 and 600 m2 g–1 for the best doped and undoped samples, respectively. Such supports were then thoroughly characterized by surface science and electron microscopy tools. Afterward, they were decorated with Pt and Pd nanoparticles, and it was found that the presence of nitrogen defects plays a significant role in improving the metal particles dimension and dispersion. In fact, when doped supports are used, the resulting metal nanoparticles are smaller (2–4 nm) and less prone to aggregation. Photoemission measurements give evidence of a binding energy shift, which is consi...

Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction

Abstract: In this study, low-mass loadings (ca. 5 wt%) Pt/C catalysts were synthesized using the carbonyl chemical route allowing for the heterogeneous deposition of Pt nanoparticles on different carbon-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chemical nature of the carbon-based substrate on the Oxygen Reduction Reaction (ORR) kinetics at Pt nanoparticles surfaces was investigated. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles leads to formation of Pt–N chemical bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the carbon support. It is demonstrated that ca. 25% of the total amount of N atoms were bound to Pt ones. This chemical bond also revealed by the DFT analysis, induces changes in the oxygen adsorption energy at the platinum surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide is used as substrate. In conjunction with the experimental results, DFT calculations describe the interaction between supported platinum clusters and oxygen where the support was modelled accordingly with the carbon-based materials used as substrate. It is demonstrated that the presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O–O distances suggesting facilitated dissociation. Additionally, it is revealed that the strong interaction between Pt clusters and N-containing substrates leads to very slight changes of the cluster–substrate distance even when oxygen is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochemical reactions.

Fast One-Pot Synthesis of MoS2/Crumpled Graphene p–n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production

Abstract: Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS2 nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV−visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS2 nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS2 nanohybrids are sev...

Laser generation of iron-doped silver nanotruffles with magnetic and plasmonic properties

Abstract: A frontier topic in nanotechnology is the realization of multifunctional nanoparticles (NPs) via the appropriate combination of different elements of the periodic table. The coexistence of Fe and Ag in the same nanostructure, for instance, is interesting for nanophotonics, nanomedicine, and catalysis. However, alloying of Fe and Ag is inhibited for thermodynamic reasons. Here, we describe the synthesis of Fe-doped Ag NPs via laser ablation in liquid solution, bypassing thermodynamics constraints. These NPs have an innovative structure consisting of a scaffold of face-centered cubic metal Ag alternating with disordered Ag–Fe alloy domains, all arranged in a truffle-like morphology. The Fe–Ag NPs exhibit the plasmonic properties of Ag and the magnetic response of Fe-containing phases, and the surface of the Fe–Ag NPs can be functionalized in one step with thiolated molecules. Taking advantage of the multiple properties of Fe–Ag NPs, the magnetophoretic amplification of plasmonic properties is demonstrated with proof-of-concept surface-enhanced Raman scattering and photothermal heating experiments. The synthetic approach is of general applicability and virtually permits the preparation of a large variety of multi-element NPs in one step.

Multiple doping of graphene oxide foams and quantum dots: new switchable systems for oxygen reduction and water remediation

Abstract: Single- and multi-boron, nitrogen, sulphur doped graphene oxide quantum dots and three-dimensional foams are synthesized by a simple and environmentally friendly electrochemical method. The electrochemical activity of these materials in the oxygen reduction reaction is investigated by cyclic voltammetry and rotating disk electrode measurements. The experimental data demonstrate that the reaction selectivity is controlled by the oxidation degree of the materials: as-prepared graphene oxide quantum dots, which present highly oxidized functional groups, follow a two-electron reduction pathway and produce hydrogen peroxide, whereas after a reduction treatment by NaBH4, the same materials favour a four-electron reduction of oxygen to water. The high selectivity and high efficiency of the graphene oxide quantum dots for the production of hydrogen peroxide can be efficiently used for water remediation applications (phenol decomposition).

The nature of the Fe–graphene interface at the nanometer level

Abstract: The emerging fields of graphene-based magnetic and spintronic devices require a deep understanding of the interface between graphene and ferromagnetic metals. This paper reports a detailed investigation at the nanometer level of the Fe–graphene interface carried out by angle-resolved photoemission, high-resolution photoemission from core levels, near edge X-ray absorption fine structure, scanning tunnelling microscopy and spin polarized density functional theory calculations. Quasi-free-standing graphene was grown on Pt(111), and the iron film was either deposited atop or intercalated beneath graphene. Calculations and experimental results show that iron strongly modifies the graphene band structure and lifts its π band spin degeneracy.

Polyvinyl alcohol electrospun nanofibers containing Ag nanoparticles used as sensors for the detection of biogenic amines

Abstract: Polyvinyl alcohol (PVA) electrospun nanofibers containing Ag nanoparticles (NPs) have been deposited on glass substrates. The aim of the work was to test the feasibility of this approach for the detection of biogenic amines by using either the Ag localized surface plasmon resonance quenching caused by the adsorption of amines on Ag NPs or by detecting the amines by surface enhanced Raman spectroscopy (SERS) after adsorption, from the gas phase, on the metal NPs. Two different approaches have been adopted. In the first one an ethanol/water solution containing AgNO3 was used directly in the electrospinning apparatus. In this way, a simple heat treatment of the nanofibers mat was sufficient to obtain the formation of Ag NPs inside the nanofibers and a partial cross-link of PVA. In the second procedure, the Ag NPs were deposited on PVA nanofibers by using the supersonic cluster beam deposition method, so that a beam of pure Ag NPs of controlled size was obtained. Exposure of the PVA mat to the beam produced a uniform distribution of the NPs on the nanofibers surface. Ethylendiamine vapors and volatile amines released from fresh shrimp meat were chemisorbed on the nanofibers mats. A SERS spectrum characterized by a diagnostic Ag–N stretching vibration at 230 cm−1 was obtained. The results allow to compare the two different approaches in the detection of ammines.

New Strategy for the Growth of Complex Heterostructures Based on Different 2D Materials

Abstract: Tungsten disulfide (WS2) monolayers have been synthesized under ultra high vacuum (UHV) conditions on quasi-free-standing hexagonal boron nitride (h-BN) and gold deposited on Ni(111). We find that the synthesis temperature control can be used to tune the WS2 structure. As documented by in situ core level and valence band photoemission and by ex situ Raman spectroscopy, the partially disordered WS2 layer obtained at room temperature transforms to the 2H-WS2 phase at about 400 °C. Low energy electron diffraction confirms the existence of van der Waals epitaxy between WS2 and h-BN and gold substrates. The measured band structure indicates that the WS2 electronic structure is not affected by the interaction with the h-BN and gold substrates.

Thermally Induced Strains on the Catalytic Activity and Stability of Pt–M2O3/C (M=Y or Gd) Catalysts towards Oxygen Reduction Reaction

Abstract: Yttrium oxide and gadolinium oxide modified platinum nanoparticles supported on carbon (Vulcan XC-72), noted as Pt–M2O3/C (M=Y and Gd), were synthesized by water-in-oil nanoemulsion chemical route, followed by heat treatment at 100 and 300 °C under hydrogen/nitrogen (H2/N2) atmosphere and tested as electrocatalysts for the oxygen reduction reaction (ORR). As revealed by powder X-ray diffraction analysis, all obtained catalysts showed solely Pt face-centered-cubic structure, and absence of a secondary phase before and after heat treatments, indicating that Y2O3 and Gd2O3 are highly disordered (amorphous) and dispersed clusters. The surface Pt:M ratio (M=Y and Gd), as revealed by X-ray photoelectron spectroscopy, increased after heat treatment with respect to the value of as-prepared samples, for which the ratio was 1:1. Microstrain data extracted from Williamson–Hall plots for Pt–M2O3/C (M=Y and Gd) catalysts surprisingly increased after heat treatment at 100 °C, remaining nearly constant after heat treatment at 300 °C, whereas the value of pure Pt nanoparticles, noted as Pt/C, decreased after heat treatments. The mean particle size derived from TEM images for Pt–M2O3/C (M=Y and Gd) was almost unchanged after heat treatments, at variance with the Pt/C case where a clear increase is observed. Surface specific activity and mass activity towards ORR obtained with as-prepared Pt–M2O3/C (M=Y and Gd) catalysts were higher than those of as-prepared Pt/C catalyst. After heat treatment, the ORR activity of Pt–M2O3/C (M=Y and Gd) augmented, whereas that of Pt/C diminished. Moreover, after 6000 cycles between 0.5 and 0.95 V versus reversible hydrogen electrode (vs. RHE), Pt–M2O3/C (M=Y and Gd) catalysts retained a large active surface area and a high kinetic current density at 0.9 V vs. RHE in comparison with Pt/C samples. These facts assess a positive effect of the interaction between M2O3 (M=Y and Gd) and Pt catalytic centers both on the catalytic activity of the material towards ORR and on its durability.

In-Situ Carbon Doping of TiO2 Nanotubes Via Anodization in Graphene Oxide Quantum Dot Containing Electrolyte and Carburization to TiOxCy Nanotubes

Abstract: Anodic production of self-organized titania nanotubes (TNTs) in an electrolyte enriched with graphene oxide quantum dots (GOQDs) is reported. The TNT-GOQD composites grown under these conditions show in-situ carbon doping, leading to the formation of anatase TiO2 domains and to the reduction to substoichiometric oxide (TiOx) and TiC. Surface science and electrochemical techniques are used in synergy to reveal that graphitic carbon is incorporated into TiO2 upon anodic nanotube growth promoting the formation of oxygen vacancies and thus TiO2 reduction. Upon annealing in ultrahigh vacuum, titanium oxycarbide (TiOxCy) is formed at temperatures ≥400 °C, where the material changes from a semiconductor to a semimetal. At the solid/liquid interface, the apparent electron donor density increases from as-grown TNTs to as-grown TNT-GOQD composites due to the carbon doping, and the conductivity increases further with annealing temperature due to the increasing concentration of coordinatively unsaturated C atoms, crystallinity, and TiO2 reduction. The materials synthesized and characterized in this study find application in different areas ranging from visible light photocatalysis and photo-electrochemistry to use as Li-ion battery anodes and electrocatalyst supports, because it is possible to gradually tune the density of states below the Fermi level, which can be referred to as band-gap engineering.

Comparison between the Oxygen Reduction Reaction Activity of Pd5Ce and Pt5Ce: The Importance of Crystal Structure

Abstract: A set of electrochemical and X-ray spectroscopy measurements have been used conjointly with density functional theory (DFT) simulations to study the activity and stability of Pd5Ce for the oxygen reduction reaction. A polycrystalline Pd5Ce rod has been selected as a model catalyst to test if results on a several-fold activity increase of a series of Pt/rare-earth alloys hold also for Pd rare-earth alloys. Pd5Ce crystallizes in two phases, a so-called low-temperature phase, L-Pd5Ce, which has a cubic symmetry, and a high-temperature phase, H-Pd5Ce, with a hexagonal symmetry. In both cases, a several-layers-thick Pd skin forms on the surface. Preliminary DFT results show that Pd overlayers under ≥2% compressive strain should be more active than Pt. In L-Pd5Ce, the overlayer is under tensile strain, whereas in H-Pd5Ce (a structure similar to Pt5Ce), it is under compressive strain. We have confirmed that in our sample, L-Pd5Ce is the dominant phase, both in the bulk and the outermost layers, while a H-Pd5Ce-l...

Characterization of TiO2 thin films in the EUV and soft x-ray region

Abstract: In this work, three TiO2 thin films with thicknesses of 22.7, 48.5 and 102.9 nm were grown on Si (100) substrates by the technique of electron beam evaporation. The films were deposited at a substrate temperature of 150°C with a deposition rate of 0.3 - 0.5 A/sec. The films thicknesses were characterized by spectroscopic ellipsometry and profilometry. The surface roughness was measured by AFM obtaining RMS of less than 0.7nm. Investigations performed by XPS method have shown that stoichiometric TiO2 was obtained on all the samples with no suboxide presences. Reflectance measurements of the samples were performed in EUV and SX spectral regions from 25.5 to 454.2eV using synchrotron radiation. Analyzing the refractive index N=n+ik of TiO2 thin films, optical constants (n,k) in this energy range were both determined by fitting the Fresnel equations with least-square fitting methods.