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1.
Nanoscale ; 12(5): 3103-3111, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-31965131

RESUMO

Detailed dynamical characterization of the mechanisms responsible for the formation and growth of iron oxide nanoparticles remains a significant challenge not only for experimental techniques but also for theoretical methodologies due to the nanoparticle size, long simulation times, and complexity of the environments. In this work, we have designed a fast computational protocol based on atomistic reactive molecular dynamics, which is capable of simulating the whole synthetic and proliferation process of the nanoparticles (greater than 10 nm) in a homogeneous medium from organometallic precursors. We have defined appropriate growth accelerating strategies based on the observed reactions, which consisted of the formation of Fe-O-Fe bridges, linking separate precursors, and Fe˙ and FeO˙ radicals. This reduced drastically the computational time allowing the simulation of NPs made of thousands of atoms (full nanometric range). We have identified the most probable reaction environments and summarized them under two distinct conditions: reductive and oxidative. The first one leads to the formation of nanoparticles with FeO stoichiometry typical of wustite, whereas the second one stabilizes stoichiometries between Fe3O4 (magnetite), and Fe2O3 (maghemite). In the latter case, the obtained NPs adopted, from the very early stages of the growth process, a cubic crystalline structure, typical of the oxidized FeOx bulk phases. The excellent agreement of our results with the experimental data demonstrates that the proposed protocol can provide a powerful predictive tool to describe structural features developed by the metal oxide nanoparticles and establish clear structure-property relationships.

2.
J Phys Chem Lett ; : 5122-5127, 2019 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-31411891

RESUMO

This work aims at characterizing for the first time the 31P spin interactions determining the nuclear magnetic resonance (NMR) properties of solid black phosphorus (bP) and of its few-layer exfoliated form (fl-bP). Indeed, the knowledge of these properties is still very poor, despite the great interest received by this layered phosphorus allotrope and its exfoliated 2D form, phosphorene. By combining density functional theory (DFT) calculations and solid-state NMR experiments on suspensions of fl-bP nanoflakes and on solid bP, it has been possible to characterize the 31P homonuclear dipolar and chemical shift interactions, identifying the network of 31P nuclei more strongly dipolarly coupled and highlighting two kinds of magnetically nonequivalent 31P nuclei. These results add an important missing piece of information to the fundamental chemico-physical knowledge of bP and support future extensive applications of NMR spectroscopy to the characterization of phosphorene-based materials.

3.
Phys Chem Chem Phys ; 21(22): 11510-11536, 2019 Jun 05.
Artigo em Inglês | MEDLINE | ID: mdl-31114821

RESUMO

Oxide materials at the two-dimensional limit, in particular in the form of ultrathin films of oxides (UTOx) grown on metal surfaces, represent promising materials in view of both fundamental science and technological applications. While the former aspect is widely recognized, these systems have not yet realized their full potential in terms of the latter (technological) aspect. In the present perspective, we review the field and its basic underlying concepts, and at the same time we provide an overview of the most promising future directions with a focus on their potential toward and relationships with real-world exploitation.

4.
Front Chem ; 7: 151, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31001511

RESUMO

Tunability and selectivity of synchrotron radiation have been used to study the excitation and ionization of 2-nitroimidazole at the C, N, and O K-edges. The combination of a set of different measurements (X-ray photoelectron spectroscopy, near-edge photoabsorption spectroscopy, Resonant Auger electron spectroscopy, and mass spectrometry) and computational modeling have successfully disclosed local effects due to the chemical environment on both excitation/ionization and fragmentation of the molecule.

5.
J Chem Theory Comput ; 15(3): 2010-2021, 2019 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-30730740

RESUMO

The very first stages of nucleation and growth of ZnO nanoparticles in a plasma reactor are studied by means of a multiscale computational paradigm where the DFT-GGA approach is used to evaluate structure and electronic energy of small (ZnO) N clusters ( N ≤ 24) that are employed as a training set (TS) for the optimization of a Reactive Force Field (ReaxFF). Reactive Molecular Dynamics (RMD) simulations based on this tuned ReaxFF are carried out to reproduce nucleation and growth in a realistic environment. Inside the reaction chamber the temperature is around 1200 K, and the zinc atoms are oxidized in an oxygen-rich atmosphere at high pressure (about 20 atm), whereas in the quenching chamber where the temperature is lower (about 800 K) the ZnO embryo-nanoclusters are grown. The main processes ruling gas-phase nucleation and growth of ZnO nanoclusters are identified and discussed together with the dependence of the inception time and average stoichiometry of nanoclusters of different size on the composition of precursor material and physical parameters.

6.
Phys Chem Chem Phys ; 21(10): 5435-5447, 2019 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-30793143

RESUMO

The tendency of glycine to form polymer chains on a rutile(110) surface under wet/dry conditions (dry-wet cycles at high temperature) is studied through a conjunction of surface sensitive experimental techniques and sequential periodic multilevel calculations that mimics the experimental procedures with models of decreasing complexity and increasing accuracy. X-ray photoemission spectroscopy (XPS) and thermal desorption spectroscopy (TDS) experimentally confirmed that the dry-wet cycles lead to Gly polymerization on the oxide support. This was supported by all the theoretical characterizations. First, classical reactive molecular dynamics (MD) simulations based on the ReaxFF approach were used to reproduce the adsorption of the experimental glycine solution droplets sprayed onto an oxide support and to identify the most probable arrangement of the molecules that triggered the polymerization mechanisms. Then, quantum chemistry density functional tight binding (DF-TB) MDs and static density functional theory (DFT) calculations were carried out to further explore favorable configurations and to evaluate the energy barriers of the most promising reaction pathways for the peptide bond-formation reactions. The results confirmed the fundamental role played by the substrate to thermodynamically and kinetically favor the process and disclosed its main function as an immobilizing agent: the molecules accommodated in the surface channels close to each other were the ones starting the key events of the dimerization process and the most favorable mechanism was the one where a water molecule acted as a proton exchange mediator in the condensation process.


Assuntos
Glicina , Prebióticos , Titânio , Catálise , Glicina/química , Simulação de Dinâmica Molecular , Oxirredução , Polimerização , Titânio/química , Água/química
7.
Phys Chem Chem Phys ; 20(3): 1707-1715, 2018 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-29265136

RESUMO

Melting and sintering of silicon nanoparticles are investigated by means of classical molecular dynamics simulations to disclose the dependence of modelling on the system type, the simulation procedure and interaction potential. The capability of our parametrization of a reactive force field ReaxFF to describe such processes is assessed through a comparison with formally simpler Stillinger-Weber and Tersoff potentials, which are frequently used for simulating silicon-based materials. A substantial dependence of both the predicted melting point and its variation as a function of the nanoparticle size on the simulation model is also highlighted. The outcomes of the molecular dynamics simulations suggest that the trend of the nanoparticulate sintering/coalescence time vs. temperature could provide a valid tool to determine the melting points of nanoparticles theoretically/experimentally.

8.
Angew Chem Int Ed Engl ; 57(5): 1209-1213, 2018 01 26.
Artigo em Inglês | MEDLINE | ID: mdl-29239093

RESUMO

A combined experimental and theoretical investigation of Ag-Pt sub-nanometer clusters as heterogeneous catalysts in the CO→CO2 reaction (COox) is presented. Ag9 Pt2 and Ag9 Pt3 clusters are size-selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first-principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano-aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O2 , and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.

9.
J Chem Theory Comput ; 13(8): 3854-3861, 2017 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-28640604

RESUMO

A novel computational approach, based on classical reactive molecular dynamics simulations (RMD) and quantum chemistry (QC) global energy optimizations, is proposed for modeling large Si nanoparticles. The force field parameters, which can describe bond breaking and formation, are derived by reproducing energetic and structural properties of a set of Si clusters increasing in size. These reference models are obtained through a new protocol based on a joint high temperature RMD/low temperature Basin Hopping QC search. The different procedures of estimating optimal force field parameters and their performance are discussed in detail.

10.
Phys Chem Chem Phys ; 19(18): 11318-11325, 2017 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-28418063

RESUMO

We report ab initio results for sub-stoichiometric HfOx with different oxygen vacancy densities, useful in exploring microscopic mechanisms that govern the operation of RRAM devices. We demonstrate that oxygen vacancy filaments are energetically more stable than randomly distributed defects. Furthermore, the stability of the filaments increases with the number of confined oxygen vacancies. Energetic and structural analyses show that bonds between neighboring coordinative unsaturated Hf atoms promote filament stability, and electron trapping, due to electron injection, increases the cohesive energy until the injection is moderate. The highly oxygen deficient configuration of the filaments leads to a substantial lowering of the HfOx band gap, which locally increases the conductivity of the system. Charge injection and electric fields modify the mobility of oxygen ions in the proximity of the filament. The simulations suggest that oxygen ion diffusion can lead to an asymmetric reduction of filament thickness and thus to its progressive disruption where the vacancy cohesion energy is lower.

11.
J Phys Chem Lett ; 7(7): 1303-9, 2016 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-26988695

RESUMO

An ultrathin two-dimensional CeO2 (ceria) phase on a Cu(110) surface has been fabricated and fully characterized by high-resolution scanning tunneling microscopy, photoelectron spectroscopy, and density functional theory. The atomic lattice structure of the ceria/Cu(110) system is revealed as a hexagonal CeO2(111)-type monolayer separated from the Cu(110) surface by a partly disordered Cu-O intercalated buffer layer. The epitaxial coupling of the two-dimensional ceria overlayer to the Cu(110)-O surface leads to a nanoscopic stripe pattern, which creates defect regions of quasi-periodic lattice distortions. The symmetry and lattice mismatch at the interface is clarified to be responsible for the topographic stripe geometry and the related anisotropic strain defect regions at the ceria surface. This ceria monolayer is in a fully oxidized and thermodynamically stable state.


Assuntos
Cério/química , Cobre/química , Adsorção , Modelos Moleculares , Oxirredução , Espectroscopia Fotoeletrônica , Propriedades de Superfície , Termodinâmica
12.
J Phys Chem Lett ; 6(11): 2134-9, 2015 Jun 04.
Artigo em Inglês | MEDLINE | ID: mdl-26266515

RESUMO

Ultrastable gold nanomolecule Au144(SCH2CH2Ph)60 upon etching with excess tert-butylbenzenethiol undergoes a core-size conversion and compositional change to form an entirely new core of Au133(SPh-tBu)52. This conversion was studied using high-resolution electrospray mass spectrometry which shows that the core size conversion is initiated after 22 ligand exchanges, suggesting a relatively high stability of the Au144(SCH2CH2Ph)38(SPh-tBu)22 intermediate. The Au144 → Au133 core size conversion is surprisingly different from the Au144 → Au99 core conversion reported in the case of thiophenol, -SPh. Theoretical analysis and ab initio molecular dynamics simulations show that rigid p-tBu groups play a crucial role by reducing the cluster structural freedom, and protecting the cluster from adsorption of exogenous and reactive species, thus rationalizing the kinetic factors that stabilize the Au133 core size. This 144-atom to 133-atom nanomolecule's compositional change is reflected in optical spectroscopy and electrochemistry.

13.
Phys Chem Chem Phys ; 17(42): 27952-67, 2015 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-25875393

RESUMO

The optical properties of multi-component metal nanostructures (or nanoalloys) are the subject of an intense and rapidly growing experimental and theoretical activity. In this perspective article, we first provide a survey of the most recent developments in the field, concerning both theoretical methods, especially at the first-principles level, and novel results, distinguishing for the convenience of presentation the sub-field of monolayer-protected multi-component metal clusters from the other alloy nanosystems. We then discuss a few general concepts which can be drawn from this survey, and offer a few suggestions on the most promising directions for future research. We hope that making the point in this fast developing field will provide a framework and a perspective useful to trigger future studies and advancements.

14.
J Am Chem Soc ; 137(14): 4610-3, 2015 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-25834925

RESUMO

Crystal structure determination has revolutionized modern science in biology, chemistry, and physics. However, the difficulty in obtaining periodic crystal lattices which are needed for X-ray crystal analysis has hindered the determination of atomic structure in nanomaterials, known as the "nanostructure problem". Here, by using rigid and bulky ligands, we have overcome this limitation and successfully solved the X-ray crystallographic structure of the largest reported thiolated gold nanomolecule, Au133S52. The total composition, Au133(SPh-tBu)52, was verified using high resolution electrospray ionization mass spectrometry (ESI-MS). The experimental and simulated optical spectra show an emergent surface plasmon resonance that is more pronounced than in the slightly larger Au144(SCH2CH2Ph)60. Theoretical analysis indicates that the presence of rigid and bulky ligands is the key to the successful crystal formation.

15.
Nanoscale ; 7(10): 4514-21, 2015 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-25682836

RESUMO

Electrochemical water-based energy cycles provide a most promising alternative to fossil-fuel sources of energy. However, current electrocatalysts are not adequate (high overpotential, lack of selectivity toward O2 production, catalyst degradation). We propose here mechanistic guidelines for experimental examination of modified catalysts based on the dependence of kinetic rates on the solvent dielectric constant. To illustrate the procedure we consider the fcc(111) platinum surface and show that the individual steps for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) change systematically with the polarizability of the medium. Thus changing this environmental variable can be used to tune the rate determining steps and the barriers, providing a means for screening and validating new systems to optimize the rate determining steps for the ORR and OER reaction pathways.

16.
Nanoscale ; 7(17): 8166-7, 2015 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-24788786

RESUMO

A recent paper in this journal reported the synthesis and characterization via electrospray ionization mass spectroscopy and UV-vis spectroscopy of (Au-Ag)144(SR)60 alloy nanomolecules with different compositions, ranging from 1 : 0 to 1 : 0.75 Au : Ag ratios. The UV-vis spectra of such systems were found to exhibit absorption peaks at 310 nm, 425 nm and 560 nm, interpreted as reminiscent of the silver surface plasmon resonance band due to simple atomic replacement of Au by Ag atoms in a fixed structural framework. On the basis of a comparison of experimentally observed and theoretically simulated optical absorption spectra, we conclude that the experimental situation must be more complicated, and that further work is needed to achieve atomistic insight into these fascinating systems.

17.
Chem Sci ; 6(7): 3915-3925, 2015 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-29218162

RESUMO

Recently Debe et al. reported that Pt3Ni7 leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface ("Pt-skin effect"). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt3Ni7 while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt-Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ∼300 000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt3Ni7 case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the Oad-hydration reaction (Oad + H2Oad → OHad + OHad) but including the barriers of O2 dissociation (O2ad → Oad + Oad) and water formation (OHad + Had → H2Oad). We find that the reaction barrier for the Oad-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt-Ni 30 : 70 initial composition. These simulations explain the peak in surface reactivity at Pt3Ni7, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials.

18.
Phys Chem Chem Phys ; 16(44): 24256-65, 2014 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-25297965

RESUMO

We propose an approach to accelerate the computational exploration and the prediction of the preferred chemical ordering in alloy nanoparticles. This approach, named Grouping Global Optimization (GGO), is based on grouping atoms into equivalence sets constrained to be occupied by the same elemental species, with a consequent significant reduction in the compositional degrees of freedom of the system. The equivalence sets are defined on the basis of point group symmetry or in general of any given order parameter, thus leaving the user a great freedom in the implementation to each specific system. The GGO approach can be used within both systematic and stochastic sampling algorithms as demonstrated by tests conducted on prototypical nanoalloys, namely on Pd-Pt and Ag-Cu binary pairs, as representative of high- or low-miscibility alloys, respectively, and on particles of two different sizes, i.e., truncated octahedra composed of 586 and 4033 atoms. It is found that GGO enables an extremely quick scan of the chemical ordering in nanoalloys containing thousands of atoms and to predict low-energy chemical ordering patterns as a function of size and composition with a modest computational effort even for the larger and symmetry-broken particles. The strategy here proposed should be applicable equally well in other fields than that of nanoalloys.

19.
J Am Chem Soc ; 136(42): 14933-40, 2014 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-25308728

RESUMO

Here we present the crystal structure, experimental and theoretical characterization of a Au24(SAdm)16 nanomolecule. The composition was verified by X-ray crystallography and mass spectrometry, and its optical and electronic properties were investigated via experiments and first-principles calculations. Most importantly, the focus of this work is to demonstrate how the use of bulky thiolate ligands, such as adamantanethiol, versus the commonly studied phenylethanethiolate ligands leads to a great structural flexibility, where the metal core changes its shape from five-fold to crystalline-like motifs and can adapt to the formation of Au(24±1)(SAdm)16, namely, Au23(SAdm)16, Au24(SAdm)16, and Au25(SAdm)16. The basis for the construction of a thermodynamic phase diagram of Au nanomolecules in terms of ligands and solvent features is also outlined.

20.
J Chem Phys ; 141(4): 041108, 2014 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-25084874

RESUMO

The kinetics of elemental inter-diffusion in Ag-Cu nanoalloys of 32-34 atoms around 80:20 composition is theoretically investigated by combining analytic-potential and first-principles calculations. An extremely varied behavior is found, with transformation times ranging from tens of nanoseconds to weeks at room temperature in a narrow interval of size and composition, also depending on quantum effects in magic clusters. Predictions are consistent with time-of-flight experiments and suggest their interpretation in a new light.

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