On the mechanical response of graphene-capped copper nanoparticles.

In this study, we investigated the mechanical behavior of pristine copper (Cu) nanoparticles (NPs) and Cu@graphene (Cu@G) hybrid NPs using molecular dynamics simulations. The longitudinal engineering strain was calculated as a measure of compression until reaching 25% of the initial size of the NPs. The stress-strain curves revealed the elastic-to-plastic transition in the Cu NPs at a longitudinal strain of 3.57% with a yield strength of 6.15 GPa. On the other hand, the Cu@G NPs exhibited a maximum average load point at a longitudinal strain of 6.81% with a yield strength of 8.26 GPa. The hybrid Cu@G NPs showed increased strength and resistance to plastic deformation compared to the pure Cu NPs, while the calculation of the elastic modulus indicated a higher load resistance provided by the graphene coverage for the Cu@G NPs. Furthermore, the analysis of atomic configurations, dislocations, and stress distribution demonstrated that the graphene flakes play a crucial role in preventing dislocation events and faceting in the Cu@G NPs by acting as a shock absorber, distributing the applied force on themselves, and producing a more homogeneous stress distribution on the Cu NPs; additionally, they prevent the movement of Cu atoms, reducing the occurrence of dislocations and surface faceting, thanks to their supportive effect. Overall, our findings highlight the potential of hybrid nanomaterials, such as Cu@G, for enhancing the mechanical properties of metallic NPs, which could have significant implications for the development of advanced nanomaterials with improved performance in a variety of applications.

Electronic-Level Insight into the Adsorption and Surface Diffusion Kinetics of a Simplified Glyphosate Model on a Goethite Surface.

The diffusive processes that occur in minerals involve chemical and physical surface phenomena of great interest that allow for understanding the mobility of different anions of environmental importance. One of them is glyphosate, which is widely used as a pesticide. In this work, we performed Hubbard-corrected density functional theory (DFT + U) calculations to study the adsorption and surface diffusion of methylphosphonic acid (MPA), as a model of glyphosate, on the (010) plane of goethite (GOT), one of the most important Fe(III) minerals in soils and sediments. In particular, the MPA adsorption was studied at the GOT-water interface, finding a strong covalent character in the bond. We also corroborated the occurrence of double proton transfer (MPA to GOT and GOT to GOT). Finally, activation energy barriers were calculated to estimate the half-lives for molecular diffusion, showing that MPA moves almost 3000 times slower than water at the GOT surface.

Can graphene improve the thermal conductivity of copper nanofluids?

Copper (Cu) nanofluids (NFs) have attracted attention due to their high thermal conductivity, which has conferred a wide variety of applications. However, their high reactivity favors oxidation, corrosion and aggregation, leading them to lose their properties of interest. Copper capped by graphene (Cu@G) core@shell nanoparticles (NPs) have also attracted interest from the medical and industrial sectors because graphene can shield the Cu NPs from undesired phenomena. Additionally, they share some properties that expand the range of applications of Cu NFs. In this work, new Morse potentials are reported to reproduce the behavior of Cu@G NPs through molecular dynamics. Coordination-dependent Morse parameters were fitted for C, H, and Cu based on density functional theory calculations. Then, these parameters were implemented to evaluate the thermal conductivity of Cu@G NFs employing the Green-Kubo formalism, with NPs from 1.5 to 6.1 nm at 100 to 800 K, varying the size, the number of layers and the orientation of the graphene flakes. It was found that Cu@G NFs are stable and have an improved thermal conductivity compared to the Cu NFs, being 3.7 to 18.2 times higher at 300 K with only one graphene layer and above 26.2 times higher for the graphene-trilayered NPs. These values can be higher for temperatures below 300 K. Oppositely, the size, homogeneity and orientations of the graphene flakes did not affect the thermal conductivity of the Cu@G NFs.

On the mechanical response in nanoalloys: the case of NiCo.

In this work, nanoindentation of spherical NiCo nanoalloys with core-shell and random mixing patterns was studied, and we compared them against monometallic nanoparticles in order to investigate how the mechanical response may be influenced by the elemental distribution and the proportion of each element. Independently of the mixing patterns, plasticity begins with the nucleation of Shockley partial dislocations (SPDs) at the nanoparticle surface, on several slip planes, which leads to the appearance of sessile dislocations and either a stacking fault pyramid (SFP) or an open pyramid at the poles of the spherical nanoalloys. SPDs leave behind stacking faults but, for core-shell structures, the formation of nanotwins was also observed. It was also found that the presence of Co in the external shell of the nanoparticle has the effect of raising the yield strength, which could be interpreted in terms of unstable stacking fault energy. These results have relevance in the design of nanoalloys, since elemental distribution and stoichiometry can be used to tune the desired mechanical properties of the nanoparticle.

Effect of Nafion content and hydration level on the electrochemical area of a Pt nanocatalyst in the triple-phase boundary.

Despite the great scientific effort, there are still some aspects of a polymeric membrane-based fuel cell (PEMFC) operation that are difficult to access experimentally. This is the case of the so-called triple-phase boundary (TPB), where the ionomer (commonly Nafion) interacts with the supported nanocatalyst (commonly Pt) and is key to the catalytic activity of the system. In this work, we use molecular dynamics simulations and electrochemical experiments on a Nafion/Pt/C system. We perform a systematic analysis, at an atomistic level, to evaluate the effect of several fundamental factors and their intercorrelation on the electrochemically active area (ECSA) of the catalysts. Our results reveal that at high Nafion contents, the catalyst utilization is affected due to the strong interaction between the sulfonic groups of the ionomer and the surface of the Pt nanoparticles (NPs). On the other hand, when the hydration level of the membrane decreases, the sulfonic groups have a greater occupation on the NP surface, covering the active area with hydrophobic Nafion chains and therefore increasing the inactive area. Voltammograms can corroborate our calculations. Overall, this investigation allows us to rationalize how the catalyst utilization is affected, which is an important step in establishing the relationship between the environment and the effectiveness and durability of the PEMFC system.

The unexpected effect of vacancies and wrinkling on the electronic properties of MoS2 layers.

We report a combined experimental/theoretical approach to study the connection of S-vacancies and wrinkling on MoS2 layers, and how this feature produces significant changes in the electronic structure and reactivity of this 2D material. The MoS2 material, when used as a catalyst in operative conditions, was found to be mainly composed of thin and short 1-5 layer sheets instead of a poorly crystalline structure, as it was previously assumed. Notably wrinkled structures with S-vacancies were also found through transmission electron microscopy. Atomistic simulations revealed a natural connection between sulfur-vacancies, wrinkling and folding. Density functional calculations further revealed that such curved structures present a lower electronic band-gap and a higher reactivity towards thiophene compared to the planar MoS2 counterpart.

Avoiding oxidation with coating: graphene protected magnesium surfaces.

Magnesium is a promising material for automotive technology. Avoiding its spontaneous oxidation is, however, mandatory for a feasible industrial application of this metal. We perform computer simulations to demonstrate that a protective graphene layer can successfully avoid the oxidation of a magnesium material. This feature remains true even when the graphene layer has several simple defects, such as vacancies and Stone-Wales transformations. In fact, the defects actually increase the strength of the graphene/metal interaction, further enhancing the protective properties. These results are rationalized in terms of the low Mg cohesive energy, which allows the system to quickly reconstruct and adapt.

Atomic-level characterization and cilostazol affinity of poly(lactic acid) nanoparticles conjugated with differentially charged hydrophilic molecules.

Nanotherapeutics is a promising field for numerous diseases and represents the forefront of modern medicine. In the present work, full atomistic computer simulations were applied to study poly(lactic acid) (PLA) nanoparticles conjugated with polyethylene glycol (PEG). The formation of this complex system was simulated using the reactive polarizable force field (ReaxFF). A full picture of the morphology, charge and functional group distribution is given. We found that all terminal groups (carboxylic acid, methoxy and amino) are randomly distributed at the surface of the nanoparticles. The surface design of NPs requires that the charged groups must surround the surface region for an optimal functionalization/charge distribution, which is a key factor in determining physicochemical interactions with different biological molecules inside the organism. Another important point that was investigated was the encapsulation of drugs in these nanocarriers and the prediction of the polymer-drug interactions, which provided a better insight into structural features that could affect the effectiveness of drug loading. We employed blind docking to predict NP-drug affinity testing on an antiaggregant compound, cilostazol. The results suggest that the combination of molecular dynamics ReaxFF simulations and blind docking techniques can be used as an explorative tool prior to experiments, which is useful for rational design of new drug delivery systems.

Real-time imaging of adatom-promoted graphene growth on nickel.

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

A density functional study on the reactivity enhancement induced by gold in IrAu nanoalloys.

IrAu nanoalloys have been proven to have remarkable reactivity for several reactions. In this work, mixed IrAu nanoalloys of 8, 27, 48 and 64 total atoms were studied in different atomic compositions (Ir m Au n ) using Density Functional Theory (DFT). A notable segregation tendency is observed, where Ir atoms are located in the inner part and Au atoms in the outermost region of the nanostructure. We found that IrAu nanoalloys present a distinctive synergistic effect with respect to reactivity. In addition, the projected density of electronic states (PDOS) energies were analyzed by examining the d-band shift to estimate the reactivity of various IrAu nanoalloys. Furthermore, the adsorption energies for the CO molecule in the domains of the Ir-Au interface were evaluated. In this sense, the addition of Au atoms to Ir clusters increases the reactivity of Ir by generating unoccupied orbitals near the Fermi level as indicated by the PDOS study.

Effects of oxidation on the plasmonic properties of aluminum nanoclusters.

The scouting of alternative plasmonic materials able to enhance and extend the optical properties of noble metal nanostructures is on the rise. Aluminum is endowed with a set of interesting properties which turn it into an attractive plasmonic material. Here we present the optical and electronic features of different aluminum nanostructures stemming from a multilevel computational study. Molecular Dynamics (MD) simulations using a reactive force field (ReaxFF), carefully validated with Density Functional Theory (DFT), were employed to mimic the oxidation of icosahedral aluminum nanoclusters. Resulting structures with different oxidation degrees were then studied through the Time-Dependent Density Functional Tight Binding (TD-DFTB) method. A similar approach was used in aluminum nanoclusters with a disordered structure to study how the loss of crystallinity affects the optical properties. To the best of our knowledge, this is the first report that addresses this issue from the fully atomistic time-dependent approach by means of two different and powerful simulation tools able to describe quantum and physicochemical properties associated with nanostructured particles.

The van der Waals Interactions of n-Alkanethiol-Covered Surfaces: From Planar to Curved Surfaces.

The van der Waals (vdW) interactions of n-alkanethiols (ATs) adsorbed on planar Au(111) and Au(100) surfaces and curved Au nanoparticles of different diameters are reported. By means of electrochemical measurements and molecular dynamic calculations, the increase in the average geometrical curvature of the surface influences the global interactions, that is, decreasing vdW interactions between neighboring molecules. Small NPs do not present the same electrochemical behavior as planar surfaces. The transition between nanoparticle to flat surface electrochemical response is estimated to occur at a circa 13-20 nm diameter range.

Mechanochemical stability of sub-nm ZnO chains.

Formation of monoatomic chains by axial stretching of zinc oxide nanowires is investigated using molecular dynamics and supported by density functional calculations. Special focus is made on the mechanical properties of these structures. Using a state-of-the-art force field it was found that O2 species are commonly formed within the chain. This species drastically weakens the chain strength. Previous simulations, based on a pair potential, failed to predict O2 formation. Moreover, the superductility of zinc oxide nanowires observed in earlier studies, was found to be an artifact of the pair potential. Simulations revealed that the chain length before rupture (usually of 6 atoms) is independent of the nanowire diameter. The electronic structure and the charge distribution of the chains were also studied.

Ultra-small rhenium clusters supported on graphene.

The adsorption of very small rhenium clusters (2-13 atoms) supported on graphene was studied by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The atomic structure of the clusters was fully resolved with the aid of density functional theory calculations and STEM simulations. It was found that octahedral and tetrahedral structures work as seeds to obtain more complex morphologies. Finally, a detailed analysis of the electronic structure suggested that a higher catalytic effect can be expected in Re clusters when adsorbed on graphene than in isolated ones.

Analysis of electron beam damage of exfoliated MoS2 sheets and quantitative HAADF-STEM imaging.

In this work we examined MoS2 sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200 kV. Structural damage of the MoS2 sheets has been controlled at 80 kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS2 material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80 kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS22 layers.

One-step/one-pot decoration of oxide microparticles with silver nanoparticles.

HYPOTHESIS: Heterogeneous nucleation of silver oxide (Ag2O) onto oxide microparticles (OMPs) followed by spontaneous thermal decomposition produce nanostructures made of OMPs decorated with silver nanoparticles (OMP|AgNPs). EXPERIMENTS: Colloidal chemistry methods have been used to produce the decoration of OMPs with silver nanoparticles (AgNPs), by carrying out the Ag2O precipitation/thermal decomposition. The process is driven in water enriched acetone medium containing NaOH, NH3, AgNO3 and SiO2MPs as substrate. Optical and morphological properties of OMP|AgNPs were characterized by using STEM, EDS, HRTEM and Raman spectroscopy. FINDINGS: A new synthetic method to decorate OMPs (TiO2, SiO2) with metallic AgNPs in a single step/single pot reaction is proven effective to produce OMP|AgNPs either in aqueous or water enriched media.