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2017

35) Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

34) On-surface synthesis of different boron–nitrogen–carbon heterostructures from dimethylamine borane

33) Influence of growing conditions on the reactivity of Ni supported graphene towards CO

32) π Magnetism of Carbon Monovacancy in Graphene by Hybrid Density Functional Calculations

31) Graphene nanobubbles on TiO2 for in operando electron spectroscopy of liquid-phase chemistry 

 

2016

30) Contamination-free suspended graphene structures by a Ti-based transfer method

29) The magnetization orientation of Fe ultrathin layers in contact with graphene

28) Synthesis of graphene nanoribbons with a defined mixed edge-site sequence by surface assisted polymerization of (1,6)-dibromopyrene on Ag(110)

27) Photoexcited carriers recombination and trapping in spherical vs faceted TiO2 nanoparticles

26) REVIEW: Computational electrochemistry of doped graphene as electrocatalytic material in fuel cells

25) CO chemisorption at vacancies of supported graphene films: a candidate for a sensor ?

24) Catalysis under Cover: Enhanced Reactivity at the Interface between (Doped) Graphene and Anatase TiO2

23) REVIEW: Theoretical Studies of Oxygen Reactivity of Free-Standing and Supported Boron-Doped Graphene

22) Unveiling the Mechanisms Leading to H2 Production Promoted by Water Decomposition on Epitaxial Graphene at Room Temperature

21) REVIEW: Doping graphene with boron: a review of synthesis methods, physicochemical characterization, and emerging applications

20) Surface-Confined Polymerization of Halogenated Polyacenes: The Case of Dibromotetracene on Ag(110)

19) Charge Carriers Separation at the Graphene/(101) Anatase TiO2 Interface

18) Formation of a Quasi-Free-Standing Single Layer of Graphene and Hexagonal Boron Nitride on Pt(111) by a Single Molecular Precursor

2015

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

16) Spherical versus Faceted Anatase TiO2 Nanoparticles: A Model Study of Structural and Electronic Properties

15) On-surface photo-dissociation of C–Br bonds: towards room temperature Ullmann coupling

14) Enhanced Chemical Reactivity of Pristine Graphene Interacting Strongly with a Substrate: Chemisorbed Carbon Monoxide on Graphene/Nickel(1 1 1)

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

12) Boron, Nitrogen Doped and Co-Doped Graphene on Cu (111): a DFT+vdW Study

11) Control of the Intermolecular Coupling of Dibromotetracene on Cu(110) by the Sequential Activation of CBr and CH bonds

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

9) The dynamics of Fe intercalation on pure and nitrogen doped graphene grown on Pt(111) probed by CO adsorption

8) Oxygen reactivity on pure and B-doped graphene over crystalline Cu(111). Effects of the dopant and of the metal support

7) The nature of the Fe-graphene interface at the nanometer level

2014

6) Opto-electrochemical biorecognition by optically-transparent highly conductive graphene-modified Fluorine-doped Tin Oxide substrates

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

4) Boron-doped graphene as active electrocatalyst for oxygen reduction reaction at a fuel-cell cathode

3) Shaping graphene oxide by Electrochemistry: from Foams to Self-Assembled Molecular Materials

2) TiO2/graphene nanocomposites from the direct reduction of graphene oxide by metal evaporation

1) Boosting Graphene Reactivity with Oxygen by Boron Doping: Density Functional Theory Modeling of the Reaction Path

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Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy


M. Favaro, J. Yang, S. Nappini, E. Magnano, F. M. Toma, E. J. Crumlin, J. Yano, and I. D. Sharp

J. Am. Chem. Soc., 2017, 139 (26), 8960-8970


Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water-splitting catalysts with semiconductors. Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co3O4/Co(OH)2 biphasic electrocatalyst on Si by means of operando ambient-pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co(OH)2 and partial conversion of the spinel Co3O4 phases to CoO(OH) under precatalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core-level regions to emerge under oxygen evolution reaction conditions on CoO(OH). The operando photoelectron spectra support assignment of these newly observed features to highly active Co4+ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co3O4 catalyst supports this interpretation and reveals that the presence of Co(OH)2 enhances catalytic activity by promoting transformations to CoO(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design.

 

 

On-surface synthesis of different boron–nitrogen–carbon heterostructures from dimethylamine borane


S. Nappini, I. Píš, G. Carraro, E. Celasco, M. Smerieri, L. Savio, E. Magnano, F. Bondino

Carbon, 2017, 120, pp. 185-193

We present here a novel approach to achieve the selective formation of different in-plane boron–nitrogen–carbon heterostructures on Pt(111) using a single molecular precursor, namely dimethylamine borane (DMAB). Through thermally activated decomposition and sequential bond activation it is possible to control intermolecular coupling and obtain alternatively a quasi–free–standing continuous monolayer formed by complementary hexagonal boron nitride and graphene (G) large-size domains, or a pure or doped G monolayer, or hybridized layers with non-planar bonds. The selective bond–bond coupling is achieved by controlling the substrate temperature either during or after the deposition of DMAB. This allows on-surface synthesis of B–N–C materials with tunable composition.


Influence of growing conditions on the reactivity of Ni supported graphene towards CO 


E. Celasco, G. Carraro, M. Smerieri, L. Savio, M. Rocca and L. Vattuone

J. Chem. Phys., 2017, 146, 104704

Free standing graphene is chemically inert but, as recently demonstrated, CO chemisorption occurs at low crystal temperature on the single layer grown by ethene dehydrogenation on Ni(111). Such layer is inhomogeneous since different phases coexist, the relative abundance of which depends on the growth conditions. Here we show by X ray photoemission and high resolution electron energy loss spectroscopies that the attained CO coverage depends strongly on the relative weight of the different phases as well as on the concentration of carbon in the Ni subsurface region. Our data show that the chemical reactivity is hampered by the carbon content in the substrate. The correlation between the amount of adsorbed CO and the weight of the different graphene phases indicates that the top-fcc configuration is the most reactive.


π Magnetism of Carbon Monovacancy in Graphene by Hybrid Density Functional Calculations


C. Ronchi, M. Datteo, D. Perilli, L. Ferrighi, G. Fazio, D. Selli, and C. Di Valentin

J. Phys. Chem. C, 2017, 121 (15), 8653–8661

Understanding magnetism in defective graphene is paramount to improve and broaden its technological applications. A single vacancy in graphene is expected to lead to a magnetic moment with both a σ (1 μB) and a π (1 μB) component. Theoretical calculations based on standard LDA or GGA functional on periodic systems report a partial quenching of the π magnetization (0.5 μB) due to the crossing of two spin split bands at the Fermi level. In contrast, STS experiments ( Phys. Rev. Lett. 2016, 117, 166801) have recently proved the existence of two defect spin states that are separated in energy by 20–60 meV. In this work, we show that self-interaction corrected hybrid functional methods (B3LYP-D*) are capable of correctly reproducing this finite energy gap and, consequently, provide a π magnetization of 1 μB. The crucial role played by the exact exchange is highlighted by comparison with PBE-D2 results and by the magnetic moment dependence with the exact exchange portion in the functional used. The ground state ferromagnetic planar solution is compared to the antiferromagnetic and to the diamagnetic ones, which present an out-of-plane distortion. Periodic models are then compared to graphene nanoflakes of increasing size (up to C383H48). For large models, the triplet spin configuration (total magnetization 2 μB) is the most stable, independently of the functional used, which further corroborates the conclusions of this work and puts an end to the long-debated issue of the magnetic properties of an isolated C monovacancy in graphene.

 

Graphene nanobubbles on TiO2 for in operando electron spectroscopy of liquid-phase chemistry 


S. Nappini, A. Matruglio, D. Naumenko, S. Dal Zilio, F. Bondino, M. Lazzarino, E. Magnano

Nanoscale, 2017, 9, 4456-4466

X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chemical properties of materials. Unfortunately this information is scarce when investigating solid/liquid interfaces and chemical or photochemical reactions under ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples present in the atmosphere or in the presence of a solvent. One promising approach is the use of graphene (Gr) windows transparent to both photons and electrons. This paper proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the solution of interest during the fabrication stage. The GNBs were successfully employed to follow in-operando the thermal-induced reduction of FeCl3 to FeCl2 in aqueous solution. The electronic states of chlorine, iron and oxygen were obtained through a combination of electron spectroscopy methods (XPS and XAS) in different phases of the process. The interaction of various components in solution with solid surfaces constituting the cell was obtained, also highlighting the formation of a covalent C–Cl bond in the Gr structure. For the easiness of GNB fabrication and straightforward extension to a large variety of solutions, we envisage a broad application of the proposed approach to investigate in detail electronic mechanisms that regulate liquid/solid electron transfer in catalytic and energy conversion related applications.


Contamination-free suspended graphene structures by a Ti-based transfer method


A. Matruglio, S. Nappini, D. Naumenko, E. Magnano, F. Bondino M. Lazzarino and S. Dal Zilio

Carbon, 2016, 103, 305-310 

Minimization of contamination associated with the graphene transfer process from the growth substrate to the device surface is a major requirement for large scale CVD graphene device applications. The most widespread transfer methods are based on the use of a thin sacrificial polymeric layer such as poly(methyl methacrylate), but its complete removal after transfer is an unsolved problem; this issue is critical for suspended graphene, since the back surface often results contaminated by the dissolved polymer. Here we present a polymer-free method of commercial CVD-grown graphene transfer from the initial copper substrate to the silicon device, in which a 15 nm-thick titanium layer replaces completely the polymer film as supporting layer during the transfer process. Our approach reduces significantly the level of contaminations for supported and suspended graphene layers. Raman spectroscopy was used to prove the quality of the transferred graphene, not affected by this approach. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy were used to assess the amount of the contaminants left by the transfer process. Overall carbon contamination was reduced by a factor 2, while contaminations originating from the metal etching in hydrofluoric acid, namely titanium and fluorine, were absent within the sensitivity of the used techniques.

 

The magnetization orientation of Fe ultrathin layers in contact with graphene

M. Cattelan, I. Píš, S. Nappini, E. Magnano, F. Bondino and S. Agnoli

Phys. Chem. Chem. Phys., 2016,18, 33233-33239
In this paper, we study the magnetic and chemical properties of Fe/graphene vertically stacked ultrathin films by means of X-ray magnetic circular dichroism and X-ray photoelectron spectroscopy. We compare two systems: an iron layer deposited directly on top of the Pt(111) surface, and an intercalated Fe film sandwiched between graphene and Pt(111). The system composed of a submonolayer Fe deposited directly on Pt(111) maintains an out-of-plane easy magnetization axis, even if it has been covered by graphene that quenches effectively the magnetic orbital moment of Fe. However, when the Fe coverage is increased above 1 ML the easy magnetization axis flips in the in-plane direction.

 

 

Synthesis of graphene nanoribbons with a defined mixed edge-site sequence by surface assisted polymerization of (1,6)-dibromopyrene on Ag(110)


M. Smerieri, I Píš, L. Ferrighi, S. Nappini, A. Lusuan, C. Di Valentin, L. Vaghi, A. Papagni, M. Cattelan, S. Agnoli, E. Magnano, F. Bondino and   Letizia Savio

Nanoscale, 2016, 8, 17843 - 17853

By a combination of scanning tunneling microscopy, X-ray spectroscopic techniques and density functional theory calculations, we prove the formation of extended patterns of parallel, graphene nanoribbons with alternate zig-zag and armchair edges and selected width by surface-assisted Ullmann coupling polymerization and dehydrogenation of 1,6-dibromopyrene (C16H8Br2). Besides the relevance of these nanostructures for their possible application in nanodevices, we demonstrate the peculiarity of halogenated pyrene derivatives for the formation of nanoribbons, in particular on Ag(110). These results open the possibility of tuning the shape and dimension of nanoribbons (and hence the correlated electronic properties) by choosing suitably tailored or on-purpose designed molecular precursors.

 

 

Photoexcited carriers recombination and trapping in spherical vs faceted TiO2 nanoparticles 

G. Fazio, L. Ferrighi and C. Di Valentin

Nano Energy 2016, 27, 673–689
Nanoparticles of very small size (below 10 nm) of TiO2 material are nowadays the functional building blocks of many developing technological applications. Nano is clearly different from bulk or extended systems as regards surface area, molecular binding properties, charge separation efficiency, electron/hole transport, photochemical conversion properties, etc. In this work, we investigate the life path of energy (excitons) and charge (electrons and holes) carriers in anatase TiO2 nanoparticles of different size (2–3 nm) and shape (faceted vs spherical), by means of a wide set of hybrid density functional theory calculations. The attention is focused on the exciton/charge carriers formation, separation, recombination, self-trapping processes, which are analyzed in terms of structural deformations, energy gain or cost, charge localization/delocalization and electronic transitions involved. The computational models are corroborated by an extensive comparison with available experimental data based on photoluminescence measurements, electron paramagnetic resonance and transient absorption spectroscopies. Peculiar differences are observed for spherical nanoparticles with respect to faceted ones because of the higher disorder and larger diversity of coordination sites present on the surface. For example, charge delocalization on several lattice sites is more competitive with self-trapping processes in faceted than in spherical nanoparticles. This relates to the fact that selective compression or elongation of Ti-O bonds play a key role in determining the effectiveness of trapping sites, with spherical nanoparticles being more flexible. Moreover, hydroxyl groups on surface five-fold coordinated Ti sites are also found to be good hole trapping sites.

 

 

Computational electrochemistry of doped graphene as electrocatalytic material in fuel cells

Review Article

G. Fazio, L. Ferrighi, D. Perilli and C. Di Valentin

Int. J. Quantum Chem., 2016, 116, 1623-1640
In this work, we present an overview on how density functional theory calculations can be used to design novel electrocatalytic materials for fuel cells. In particular, we focus the attention on non-metal doped graphene systems, which were reported to present excellent performances as electrocatalysts for the oxygen reduction reaction (ORR) at the cathodic electrode of fuel cells and are, thus, considered promising substitutes of platinum or platinum alloys electrodes. The methodology, originally proposed by Nørskov et al. (J. Phys. Chem. B 2004, 108, 17886) for electrochemical processes at metal electrodes, is revisited and applied specifically to doped graphene. Finite molecular models of graphene are found to perform as well as periodic models for localized properties or reactions. Therefore, the sophisticated molecular quantum mechanics methodologies can be safely used to compute reliable Gibbs free energies of reaction in an aqueous environment for the various steps of reduction (at the cathode) or of oxidation (at the anode). Details of the reaction mechanisms and accurate cell onset- or over-potentials can be derived from the Gibbs free energy diagrams. The latter are computational quantities which can be directly compared to experimentally obtained cell overpotentials. Modeling electrocatalysis at fuel cells is, thus, an extremely powerful and convenient tool to improve our understanding of how fuel cells work and to design novel potentially active electrocatalytic materials. In this work, we present two specific applications of B-doped graphene, as electrocatalysts for the ORR at a half-cell cathode and for the methanol oxidation reaction (MOR) at a half-cell anode.

 

 

 

CO chemisorption at vacancies of supported graphene films: a candidate for a sensor ?

E. Celasco, G. Carraro, A. Lusuan, M. Smerieri, J. Pal, M. Rocca, L. Savio, L. Vattuone

Phys. Chem. Chem. Phys., 2016, 18, 18692-18696
 
We investigate CO adsorption at single vacancies of graphene supported on Ni(111) and polycrystalline Cu. The borders of the vacancies are chemically inert but, on the reactive Ni(111) substrate, CO intercalation occurs. Adsorbed CO dissociates at 380 K, leading to carbide formation and mending of the vacancies, thus preventing their effectiveness in sensor applications.

 

 

 

Catalysis under Cover: Enhanced Reactivity at the Interface between (Doped) Graphene and Anatase TiO2

 

by L. Ferrighi, M. Datteo, G. Fazio and C. Di Valentin

J. Am. Chem. Soc., 2016, 138 (23), pp 7365–737
The “catalysis under cover” involves chemical processes which take place in the confined zone between a 2D material, such as graphene, h-BN, or MoS2, and the surface of an underlying support, such as a metal or a semiconducting oxide. The hybrid interface between graphene and anatase TiO2 is extremely important for photocatalytic and catalytic applications because of the excellent and complementary properties of the two materials. We investigate and discuss the reactivity of O2 and H2O on top and at the interface of this hybrid system by means of a wide set of dispersion-corrected hybrid density functional calculations. Both pure and boron- or nitrogen-doped graphene are interfaced with the most stable (101) anatase surface of TiO2 in order to improve the chemical activity of the C-layer. Especially in the case of boron, an enhanced reactivity toward O2 dissociation is observed as a result of both the contribution of the dopant and of the confinement effect in the bidimensional area between the two surfaces. Extremely stable dissociation products are observed where the boron atom bridges the two systems by forming very stable B—O covalent bonds. Interestingly, the B defect in graphene could also act as the transfer channel of oxygen atoms from the top side across the C atomic layer into the G/TiO2 interface. On the contrary, the same conditions are not found to favor water dissociation, proving that the “catalysis under cover” is not a general effect, but rather highly depends on the interfacing material properties, on the presence of defects and impurities and on the specific reaction involved.

 

 

Theoretical Studies of Oxygen Reactivity of Free-Standing and Supported Boron-Doped Graphene

Review Article

C. Di Valentin, L. Ferrighi and G. Fazio

ChemSusChem, 2016, 9, 1061

Graphene inertness towards chemical reactivity can be considered as an accepted postulate by the research community. This limit has been recently overcome by chemically and physically modifying graphene through non-metal doping or interfacing with acceptor/donor materials (metals or semiconductors). As a result, outstanding performances as catalytic, electrocatalytic, and photocatalytic material have been observed. In this critical Review we report computational work performed, by our group, on the reactivity of free-standing, metal- and semiconductor-supported B-doped graphene towards oxygen, which is at the basis of extremely important energy-related chemical processes, such as the oxygen reduction reaction. It appears that a combination of doping and interfacing approaches for the activation of graphene can open unconventional and unprecedented reaction paths, thus boosting the potential of modified graphene in many chemical applications.

 

Unveiling the Mechanisms Leading to H2 Production Promoted by Water Decomposition on Epitaxial Graphene at Room Temperature


A. Politano, M. Cattelan, D. W. Boukhvalov, D. Campi, A. Cupolillo, S. Agnoli, N. G. Apostol, P. Lacovig, S. Lizzit, D. Farías, G. Chiarello, G. Granozzi, and R. Larciprete
ACS Nano, 2016, 10 (4), 4543-4549
By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.

 

Doping graphene with boron: a review of synthesis methods, physicochemical characterization, and emerging applications

Review Article

S. Agnoli and M. Favaro

J. Mater. Chem. A, 2016,4, 5002-5025

Graphene based materials can be effectively modified by doping in order to specifically tailor their properties toward specific applications. So far the most used and widely investigated dopant heteroatom is probably nitrogen. However, boron is also an equally important element that can induce novel and complementary properties leading to specific implementation in alternative devices and technologies. In this paper, we survey the most recent preparation methods of boron doped graphene, including materials with specific morphology such as nanoribbons, quantum dots and 3D interconnected systems. We illustrate the results of theoretical and experimental studies dealing with the description and understanding of the main structural, electronic and chemical properties of this material. The emerging applications of boron doped graphene in several technological fields such as electrochemistry, sensors, photovoltaics, catalysis and biology are extensively reviewed.


Surface-Confined Polymerization of Halogenated Polyacenes: The Case of Dibromotetracene on Ag(110)


I. Píš, L. FerrighiT. Hai NguyenS. NappiniL. VaghiA. BasagniE. MagnanoA. PapagniF. SedonaC. Di ValentinS. Agnoli, and F. Bondino

J. Phys. Chem. C, 2016, 120 (9), pp 4909–4918

On-surface synthesis of thin organic and organometallic films in a bottom-up fashion has become a promising approach for the development of new nanotechnologies. In this work we studied 5,11-dibromotetracene (C18H10Br2) as a prototypical case of rodlike polyaromatic molecules functionalized with two bromine atoms on the sides. The adsorption and temperature-stimulated transformations of dibromotetracene assemblies on Ag(110) have been investigated by a combination of synchrotron radiation X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption spectroscopy (NEXAFS), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Upon the contact with the Ag substrate, the Br–C bonds are promptly cleaved at room temperature, and Ag-coordinated protopolymers are formed along the [001] substrate direction. The organometallic dimers and trimers remain on the surface up to 523 K. The stabilization of the protopolymers is driven by the substrate anisotropy and weak interactions with nearby Br atoms. The short oligomers formed at elevated temperatures are weakly bounded to the substrate and desorb before covalent structures can be formed.

 

Charge Carriers Separation at the Graphene/(101) Anatase TiO2 Interface 


L. Ferrighi, G. Fazio and C. Di Valentin

Advanced Materials Interfaces, 2016, 3, 1500624

Graphene/TiO2 nanocomposites are successfully applied in both photocatalysis and photovoltaics. The enhanced performances are attributed to their improved interfacial charged transfer and charge separation, which reduce the recombination rate of the photoexcited charge carriers. Here, it is shown that only density functional methods which provide corrections for the spurious self-interaction and for the van der Waals forces can correctly describe the electronic structure, the adhesion energy, and the atomic equilibrium distances. It is also proven that residual O atoms at the interface largely enhances the binding energy and causes further electronic states hybridization between G and TiO2, which is expected to favor interfacial electron transfers. Finally, evidence that electrons are preferentially trapped at subsurface layers of TiO2, while holes are preferentially delocalized on the G sheet, is provided. This opposite tendency is proposed to be at the basis of the reduced recombination leading to the observed improved outcomes in photocatalytic and photovoltaic applications.



Formation of a Quasi-Free-Standing Single Layer of Graphene and Hexagonal Boron Nitride on Pt(111) by a Single Molecular Precursor

 

S. Nappini, I Píš, T.O. Menteş, A. Sala, M. Cattelan, S. Agnoli, F. Bondino and E. Magnano



It is shown that on Pt(111) it is possible to prepare hexagonal boron nitride (h-BN) and graphene (G) in-plane heterojunctions from a single molecular precursor, by thermal decomposition of dimethylamine borane (DMAB). Photoemission, near-edge X-ray absorption spectroscopy, low energy electron microscopy, and temperature programmed desorption measurements indicate that the layer fully covers the Pt(111) surface. Evidence of in-plane layer continuity and weak interaction with Pt substrate has been established. The findings demonstrate that dehydrogenation and pyrolitic decomposition of DMAB is an efficient and easy method for obtaining a continuous almost freestanding layer mostly made of G, h-BN with only a low percentage (<3%) of impurities (B and N-doped G domains or C-doped h-BN or boron carbonitride, BCN at the boundaries) in the same 2D sheet on a metal substrate, such as Pt(111), paving the way for the advancement of next-generation G-like-based electronics and novel spintronic devices.

 

 

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

 

F. Carraro, L. Calvillo, M. Cattelan, M. Favaro, M. Righetto, S. Nappini, I. Píš, V. Celorrio, D. J. Fermín, A. Martucci, S. Agnoli, and G. Granozzi

ACS Appl. Mater. Interfaces, 20157 (46), pp 25685–25692
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 seven times more efficient with respect to single MoS2because of the formation of local p–n MoS2/N-doped graphene nanojunctions, which allow an efficient charge carrier separation.

 

 

Spherical versus Faceted Anatase TiO2 Nanoparticles: A Model Study of Structural and Electronic Properties

G. Fazio, L. Ferrighi and C. Di Valentin

J. Phys. Chem. C2015, 119 (35), 20735–20746
TiO2 nanoparticles are fundamental building blocks of many TiO2-based technologies. However, most of the computational studies simulate either bulk or surface titania. Structural and electronic properties of nanoparticles are expected to differ much from extended systems. Moreover, nanoparticles of different size and shape may also present peculiar features. In this study we compare nanocrystals and nanospheres of various sizes (up to a diameter of 3 nm) in order to highlight analogies and differences. In particular, we focus the attention on the surface-to-bulk sites ratio, the surface sites coordination distribution, the atomic distortions or curvature, and the surface energies from the structural point of view. Regarding the electronic properties, we investigate the difference between Kohn–Sham and fundamental gaps of these finite-sized systems, the frontiers orbitals space distribution, ionization potentials, and electron affinities, and finally, the densities of states projected on the various coordination sites present in the nanoparticles. This detailed analysis proves that faceted and spherical nanoparticles present different structural and electronic properties, which make each of them better suited for different uses and applications.




On-surface photo-dissociation of C–Br bonds: towards room temperature Ullmann coupling

 


A. Basagni, L. Ferrighi, M. Cattelan, L. Nicolas, K. Handrup, L. Vaghi, A. Papagni, F. Sedona, C. Di Valentin, S. Agnoli and M. Sambi

Chem. Commun., 2015, 51, 12593-12596

The surface-assisted synthesis of gold-organometallic hybrids on the Au(111) surface both by thermo- and light-initiated dehalogenation of bromo-substituted tetracene is reported. Combined X-ray photoemission (XPS) and scanning tunneling microscopy (STM) data reveal a significant increase of the surface order when mild reaction conditions are combined with 405 nm light irradiation.



 

Enhanced Chemical Reactivity of Pristine Graphene Interacting Strongly with a Substrate: Chemisorbed Carbon Monoxide on Graphene/Nickel(1 1 1)


M. SmerieriE. CelascoG. CarraroA. LusuanJ. Pal, G. BraccoM. RoccaL. Savio and L. Vattuone

ChemCatChem, 2015, 7, 2328–2331

Graphene is usually considered a chemically inert material. Theoretical studies of CO adsorption on free-standing graphene predict quite low adsorption energies (<0.1 eV). However, we show here by vibrational spectroscopy and scanning tunneling microscopy that the nondissociative chemisorption of CO occurs at cold, pristine graphene grown on Ni(1 1 1). The CO adlayer remains stable up to 125 K, although some coverage survives flashes to 225 K. This unexpected result is explained qualitatively by the modification of the density of states close to the Fermi energy induced by the relatively strong graphene–substrate interaction. The value of the adsorption energy allows us to estimate an equilibrium coverage of the order of 0.1 monolayers at 10 mbar pressure, which thus paves the way for the use of graphene as a catalytically active support under realistic conditions.

 

 

 


M. Favaro, F. Carraro, M. Cattelan, L. Colazzo, C. Durante, M. Sambi, A. Gennaro, S. Agnoli and G. Granozzi 

J. Mater. Chem. A, 2015,3, 14334-14347

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).



Boron, Nitrogen Doped and Co-Doped Graphene on Cu (111): a DFT+vdW Study 


L. Ferrighi, M. Trioni and C. Di Valentin

J. Phys. Chem. C, 2015, 119 (11), 6056–6064

The electronic properties of free standing and Cu supported pristine, boron doped, nitrogen doped, and co-doped graphene have been studied by means of Density Functional Theory (DFT) with the vdW-DF2-C09x functional.

The effects of substitutional chemical doping, metal support, lattice parameter strain and  their eventual interplay have been investigated. We find that only boron doped graphene strongly interacts with the copper substrate, due to chemical bonds between the boron atom and the underlying metal. The binding energy and charge transfer from Cu are also highly enhanced compared to both pristine and nitrogen doped supported graphene. The BN co-doped system behaves similarly to pristine graphene with a weakly physisorbed state and a small charge transfer from Cu. 

However, the presence of the non-metal dopants makes the co-doped sheet extremely tunable for redox purposes, with the boron site acting as an electron acceptor and the nitrogen site as an electron donor.


 

Control of the Intermolecular Coupling of Dibromotetracene on Cu(110) by the Sequential Activation of CBr and CH bonds

L. Ferrighi, I. Pìsˇ, T. H. Nguyen, M. Cattelan, S. Nappini, A. Basagni, M. Parravicini, A. Papagni, F. Sedona, E. Magnano, F. Bondino, C. Di Valentin and S. Agnoli

Chemistry: A European Journal. 2015, 21 (15), 5826–5835

Dibromotetracene molecules are deposited on the Cu(110) surface at room temperature. The complex evolution of this system has been monitored at different temperatures (i.e., 298, 523, 673, and 723 K) by means of a variety of complementary techniques that range from STM and temperature-programmed desorption (TPD) to high-resolution X-ray spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). State-of-the-art density-functional calculations were used to determine the chemical processes that take place on the surface. After deposition at room temperature, the organic molecules are transformed into organometallic monomers through debromination and carbon-radical binding to copper adatoms. Organometallic dimers, trimers, or small oligomers, which present copper-bridged molecules, are formed by increasing the temperature. Surprisingly, further heating to 673 K causes the formation of elongated chains along the Cu(110) close-packed rows as a consequence of radical-site migration to the thermodynamically more stable molecule heads. Finally, massive dehydrogenation occurs at the highest temperature followed by ring condensation to nanographenic patches. This study is a paradigmatic example of how intermolecular coupling can be modulated by the stepwise control of a simple parameter, such as temperature, through a sequence of domino reactions.



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

 

M. Favaro, S. Leonardi, C. Valero-Vidal, S. Nappini, M. Hanzlik, S. Agnoli, J. Kunze-Liebhäuser and G. Granozzi


Advanced Materials Interfaces. Article first published online: 13 FEB 2015

 

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 TiO2domains 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.



The dynamics of Fe intercalation on pure and nitrogen doped graphene grown on Pt(111) probed by CO adsorption

M. Cattelan, E. Cavaliere, L. Artiglia, L. Gavioli, S. Agnoli and G. Granozzi 

Surface Science, 2015, 634, 49–56
In this paper we compare by temperature programmed desorption the intercalation rate of Fe nanoparticles supported on pure and nitrogen doped graphene grown on Pt (111). Carbon monoxide desorption from Fe sites is used to probe the overall quantity of Fe present onto the graphene surface. We do not observe any appreciable difference of CO desorption temperature induced by N functionalities of graphene; however we notice a faster intercalation for Fe nanoparticles deposited on N-doped graphene with respect the those supported on pure graphene. We relate this phenomenon tonanoholes created by pyridinic and pyrrolic functionalities and/or to the lower bond enthalpy of Csingle bondN with respect to Csingle bondC bonds. Scanning tunneling microscopy and X-ray photoelectron spectroscopy are used as complementary techniques to identify the N functionalities and to characterize the morphological defectivity of the graphene films.



Oxygen reactivity on pure and B-doped graphene over crystalline Cu(111). Effects of the dopant and of the metal support


L. Ferrighi and C. Di Valentin

Surface Science 2015,  Volume 634,  Pages 68–75
Molecular oxygen reactivity on pristine and B-doped graphene over crystalline Cu(111) surface has been investigated by means of density functional theory (DFT) calculations and the periodic supercell approach. Results obtained for the supported undoped and doped systems have been compared to establish the effect of B-doping on the reactivity and on the interface adhesion, which are found to be both highly boosted. Additionally, results obtained for free standing pristine and B-doped graphene have been compared to those obtained for the metal supported counterparts in order to determine how the reactivity is affected by the presence of the metal substrate. Cu is found to be an n-type donor which enhances the reactivity of pristine graphene. However, in the case of B-doped graphene, the n-type doping by the metal surpasses the p-type doping by boron causing a reduction in the reactivity. Finally, the possibility that the oxygen could dissociate at the graphene/metal interface is investigated. Some of the reaction products are found to be more stable than those obtained with oxygen dissociating on the top side of the graphene sheet. This provides some significant insight into the confinement effect on the surface chemistry of molecules underneath which is currently a hot topic in the field.



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


M. Cattelan,   G. W. Peng,   E. Cavaliere,  L. Artiglia,   A. Barinov,   L. Roling,   M. Favaro,  I. Pìs,   S. Nappini,   E. Magnano,  F. Bondino,   L. Gavioli,   S. Agnoli,  M. Mavrikakis and  G. Granozzi  

Nanoscale, 2015, 7, 2450-2460
 
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 on top of or intercalated beneath graphene. Calculations and experimental results show that iron strongly modifies the graphene band structure and lifts its π band spin degeneracy.


 

Opto-electrochemical biorecognition by optically-transparent highly conductive graphene-modified Fluorine-doped Tin Oxide substrates


F. Lamberti, L. Brigo, M. Favaro, C. Luni, A. Zoso, M. Cattelan, S. Agnoli, G. Brusatin, G. Granozzi, M. Giomo, and N. Elvassore

ACS Appl. Mater. Interfaces2014, 6 (24), 22769–22777
 
Both optical and electrochemical graphene-based sensors have gone through rapid development, reaching high sensitivity, at low cost, and with fast response time. However, the complex validating biochemical operations, needed for their consistent use, currently limits their effective application. We propose an integration strategy for opto-electrochemical detection that overcomes previous limitations of these sensors used separately. We develop an opto-electrochemical sensor for aptamer-mediated protein detection, based on few-layer graphene immobilization on selectively modified Fluorine-doped Tin Oxide (FTO) substrates. Our results show that the electrochemical properties of graphene-modified FTO samples are suitable for a complex biological detection due to stability and inertness of the engineered electrodic interface. In addition, the few-layer immobilization of graphene sheets through electrostatic linkage with electrochemically grafted FTO surface allows obtaining an optically accessible and highly conductive platform. As a proof of concept, we used insulin as the target molecule to reveal in solution. Because of its transparency and low sampling volume (few microliters), our sensing unit can be easily integrated in lab-on-a-chip cell culture systems for effective monitoring sub-nanomolar concentrations of proteins relevant for biomedical applications.


 

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


M. Favaro, L. Ferrighi, G. Fazio, L. Colazzo, C. Di Valentin, C. Durante, F. Sedona, A. Gennaro, S. Agnoli, and G. Granozzi

ACS Catalysis, 2014, 5, 129–144
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 NaBH4, the same materials favor a four-electron reduction of oxygen to water.

 

 

Boron-doped graphene as active electrocatalyst for oxygen reduction reaction at a fuel-cell cathode


G. Fazio, L. Ferrighi, C. Di Valentin

Journal of Catalysis, 2014, 318, 203-210

Boron-doped graphene was reported to be the best non-metal  doped graphene electrocatalyst for the oxygen reduction reaction  (ORR) working at an onset potential of 0.035 V (Jiao et al., 2014). In   the present DFT study, intermediates and transition structures along  the possible reaction pathways are determined. Both Langmuir-  Hinschelwood and Eley–Rideal mechanisms are discussed. Molecular oxygen binds the positively charged B atom and forms an  open shell end-on dioxygen intermediate. The associative path is favored with respect to the dissociative one. The free energy diagrams along the four-reduction steps are investigated with the methodology by Nørskov and co. (2004) in both acidic and alkaline conditions. The pH effect on the stability of the intermediates of reduction is analyzed in terms of the Pourbaix diagram. At pH = 14, we compute an onset potential value for the electrochemical ORR of U = 0.05 V, which compares very well with the experimental value in alkaline conditions.


 

Shaping graphene oxide by Electrochemistry: from Foams to Self-Assembled Molecular Materials


M. Favaro, S. Agnoli, M. Cattelan, A. Moretto, C. Durante, S. Leonardi, J. Kunze-Liebhäuser, O. Schneider, A. Gennaro and G. Granozzi

CARBON, 2014, 77, Pages 405-415

The ability to control the three-dimensional architecture of graphene-based materials following a rational design is essential for technological applications. Here we demonstrate that the electrochemical etching can be used as a surgical tool to tailor the morphology of graphene electrodes and to impart special features, like micrometric channels and controlled mesoporosity (foams). The final materials, thanks to the high surface area, can represent a promising class of carbon-based supercapacitors. Otherwise, new materials can be prepared using a bottom-up strategy that exploits the self-assembly of the graphene oxide quantum dots produced during the electrochemical erosion. The advantages of this second approach reside not only in the possibility to downscale the control over the spatial organization as compared to the use of conventional micrometric graphene sheets, but also in the introduction of the intrinsic luminescent properties of the quantum dots in the final material. As a proof of concept we report the preparation of luminescent nanospheres by exploiting the self-organization of the graphene oxide quantum dots around frozen water nuclei.


Marco Favaro, Stefano Agnoli, Cristiana Di Valentin, Cecilia Mattevi, Mattia Cattelan, Luca Artiglia, Elena Magnano, Federica Bondino, Silvia Nappini and Gaetano Granozzi. 

CARBON, 2014, 68, 319–329

We demonstrate that graphene oxide can be efficiently reduced by evaporating shapingmetal Titanium in high vacuum. A detailed description of this reaction is provided by combining in situ photoemission spectroscopy measurements and DFT calculations: the titanium atoms readily react with the oxygenated groups of graphene oxide, disrupting the C–O bonds, with the consequent formation of titania and the recovery of the sp2 hybridized carbon atoms. When all surface oxygen is consumed, titanium can react with the carbon substrate and form carbidic species. Resonant photoemission spectroscopy measurements allow identifying the presence and exact energy position in the valence band of the Ti–C and Ti–O–C states, which are supposed to control the electron and energy transfer across the TiO2/graphene interface. Therefore with this study we provide a versatile method and the rationale for controlling, at the atomic level, the nature of the interface of graphene/metal oxide nanocomposites.


 


J. Phys. Chem. C2014, 118 (1), 223–230

bg_paper
Graphene (G) reactivity toward oxygen is very poor, which limits its use as electrode for the oxygen reduction reaction (ORR). Contrarily, boron-doped graphene was found to be an excellent catalyst for the ORR. Through a density functional study, comparing molecular and periodic approaches and different functionals (B3LYP vs PBE), we show how substitutional boron in the carbon sheet can boost the reactivity with oxygen leading to the formation of bulk borates covalently bound to graphene (BO3–G) in oxygen-rich conditions. These species are highly interesting intermediates for the O═O breaking step in the reduction process of O2 to form H2O as they are energetically stable.

 

MEET US AT THESE EVENTS:

The Graphene Conference 2017

Barcelona, Spain.

March 28-31, 2017

Martina Datteo (UNIMIB) will present at the PhD students track A:

"Catalysis under cover: enhanced reactivity at the interface between

(doped) graphene and anatase TiO2"

 

See past events

NEWS

 

Master Exam 

Daniele Perilli (UNIMIB) received his Master's degree defending the thesis:

"Reactivity at the interface between graphene and the Cu(111) metal substrate"

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Fondazione Grazioli Prize

PhD student Martina Datteo (UNIMIB) has been awarded for her Master Thesis

 

 

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