17 research outputs found
Can Single Metal Atoms Trapped in Defective h-BN/Cu (111) Improve Electrocatalysis of the H2 Evolution Reaction?
Metal-supported hexagonal boron nitride monolayers (h-BN/M) are emerging as new potential electrocatalysts for various energy-related oxidation or reduction process. So far, several preparation methods have been developed to introduce, in a controlled way, defects such as vacancies or substitutional heteroatoms. Herein, we investigate by dispersion-corrected density functional theory (DFT) calculations, defective and metal-doped h-BN/Cu(111) systems as electrocatalysts for the hydrogen evolution reaction (HER). By calculating the hydrogen binding energy (ÎG*H) at different coverage conditions, we observe how the interaction between the defective/metal-doped h-BN layer and the Cu(111) substrate plays a key role in tuning the reactivity, leading to a thermoneutral hydrogen adsorption step (i.e., ÎG*H â 0). These results could be generalized to other h-BN/M interfaces and may help their rational design for an improved H2-evolving electrocatalysis
Tuning graphene doping by carbon monoxide intercalation at the Ni(111) interface
Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epitaxial graphene monolayer grown on Ni(111), getting trapped into the confined region at the interface. On the basis of ab-initio density functional theory calculations, we provide here a full characterization of the intercalated CO pattern, highlighting the modifications induced on the graphene electronic structure. For CO coverages as low as 0.14 monolayer (ML), the graphene layer is spatially decoupled from the metallic substrate, with a significant C 1s core level shift towards lower binding energies. The most relevant signature of the CO intercalation is a clear switching of the graphene doping state, which changes from n-type, when strongly interacting with the metal surface, to p-type. The shift of the Dirac cone linearly depends on the CO coverage, reaching about 0.9 eV for the saturation value of 0.57 ML. Theoretical predictions are compared with the results of scanning tunnelling microscopy, low-energy electron diffraction and photoemission spectroscopy experiments, which confirm the proposed scenario for the nearly saturated intercalated CO system.
This result opens the way to the application of the Graphene/Ni(111) interface as gas sensor to easily detect and quantify the presence of carbon monoxide
Water at the Interface Between Defective Graphene and Cu or Pt (111) Surfaces
The
presence of defects in the graphenic layers deposited on metal surfaces
modifies the nature of the interaction. Unsaturated carbon atoms,
due to vacancies in the lattice, form strong organometallic bonds
with surface metal atoms that highly enhance the binding energy between
the two materials. We investigate by means of a wide set of dispersion-corrected
density functional theory calculations how such strong chemical bonds
affect both the electronic properties of these hybrid interfaces and
the chemical reactivity with water, which is commonly present in the
working conditions. We compare different metal substrates (Cu vs Pt)
that present a different type of interaction with graphene and with
defective graphene. This comparative analysis allows us to unravel
the controlling factors of water reactivity, the role played by the
carbon vacancies and by the confinement or âgraphene cover
effectâ. Water is capable of breaking the CâCu bond
by dissociating at the undercoordinated carbon atom of the vacancy,
restoring the weak van der Waals type of interaction between the two
materials that allows for an easy detachment of graphene from the
metal, but the same is not true in the case of Pt, where CâPt
bonds are much stronger. These conclusions can be used to rationalize
water reactivity at other defective graphene/metal interfaces
Mechanism of CO Intercalation through the Graphene/Ni(111) Interface and Effect of Doping
Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry
Functionalizing TiO<sub>2</sub> Nanoparticles with Fluorescent Cyanine Dye for Photodynamic Therapy and Bioimaging: A DFT and TDDFT Study
In
the field of nanomedicine, significant attention is
directed
toward near-infrared (NIR) light-responsive inorganic nanosystems,
primarily for their applications in photodynamic therapy and fluorescence
bioimaging. The crucial role of the NIR range lies in enabling optimal
tissue penetration, which is essential for both irradiating and detecting
nanoparticles deep within the human body. In this study, we employed
density functional theory (DFT) and time-dependent DFT (TDDFT) calculations
to explore the structural and electronic properties of cyanine-functionalized
TiO2 spherical nanoparticles (NPs) with a realistic diameter
of 2.2 nm. We revealed that different adsorption configurations of
cyanine (VG20-C1) on the TiO2 NP surface exhibit
distinct features in the optical spectra. These cyanine dyes, serving
as bifunctional linkers with two carboxylic end groups, can adsorb
in either a side-on mode (binding with both end groups)
or an end-on mode (binding only one end group). In end-on adsorption structures, low-energy excitations are
exclusive to dye-to-dye electronic transitions, while side-on structures exhibit electron charge transfer excitations from the
dye to the TiO2 NP at low energy. This thorough analysis
provides a rational foundation for designing cyanine-functionalized
TiO2 nanosystems with optimal optical characteristics tailored
for specific nanomedical applications such as photodynamic therapy
or fluorescence bioimaging
Ï Magnetism of Carbon Monovacancy in Graphene by Hybrid Density Functional Calculations
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 Ï (1 ÎŒ<sub>B</sub>) and a Ï (1 ÎŒ<sub>B</sub>) component. Theoretical
calculations based on standard LDA or GGA functional on periodic systems
report a partial quenching of the Ï magnetization (0.5 ÎŒ<sub>B</sub>) 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 20â60
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 Ï magnetization
of 1 Ό<sub>B</sub>. 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 C<sub>383</sub>H<sub>48</sub>). For large models, the triplet spin configuration (total magnetization
2 Ό<sub>B</sub>) 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
VisulaitĂ e (anti) razzismo
Questo libro riflette sulle immagini che riproducono gerarchie razziali; sulle ambivalenze dei tentativi di (dis)fare la ârazzaâ, mostrandola o viceversa occultandola; infine, sulla dimensione âcontro-visualeâ delle battaglie culturali e politiche esplicitamente anti-razzist
Formation of diphenyl-bipyridine units by surface assisted cross coupling in Pd-cyclometalled complexes
The Pd cyclometallated complex [(5-bromo-2-phenylpyridine)Pd(Ό-Cl)]2 is deposited on Ag(1 1 0) at room temperature by sublimation in ultra-high vacuum. The thermal evolution of the system is followed by scanning tunnelling microscopy and X-ray photoemission spectroscopy, while the initial and final configurations are validated by ab-initio calculations. We observe the surface induced dissociation of the molecule and the occurrence of a cross coupling reaction between the two organic fragments, leading to the surface assisted synthesis of diphenyl-bipyridine molecules. Such a process, occurring with low probability at RT, is thermally activated and competes with desorption. At variance with most cross-coupling reactions at surfaces reported in literature, in this case the reactants come from the dissociation of the same compound so that only one precursor is employed, leading to a simplified preparation protocol. The Br and Cl atoms dissociated from the molecule bind to the surface and promote an extended surface reconstruction upon annealing, which was not observed previously upon deposition of halogenated aromatic compounds
Well-ordered surface metal atoms complexation by deposition of Pd cyclometallated compounds on Ag (1 1 0)
In this paper we performed the deposition and self-assembly of a Pd-cyclometallated compound on Ag(1 1 0) surface for the first time. The system is investigated from the morphological and chemical point of view by scanning tunneling microscopy and x-ray photoemission spectroscopy, respectively, and the results are validated by ab-initio calculations. Our combined experimental and theoretical study aims at elucidating the atomistic details of the chemical steps following Pd cyclometallate deposition on the metallic substrate. To do that, we analyze the electronic and chemical properties of the species present on the surface at the end of the preparation process at room temperature and at 150 degrees C. We observe an unexpected complex chemistry: on one side, the organometallic molecules are found to dissociate into fragments, forming a well-ordered metal-carbon network; on the other side, Pd atoms become buried in the bulk of the metal substrate following metal exchange with surface Ag atoms. The details of this mechanistic study reveal the active role played by the metal substrate in promoting the chemistry of the deposited Pd cyclometallates and could open new perspectives for the application of this class of materials in heterogeneous catalysis