Disentangling the Effects of Laser and Electron Irradiation on AgX (X = Cl, Br, and I): Insights from Quantum Chemical Calculations.

The effects on the lattice structure and electronic properties of different polymorphs of silver halide, AgX (X = Cl, Br, and I), induced by laser irradiation (LI) and electron irradiation (EI) are investigated using a first-principles approach, based on the electronic temperature (Te) within a two-temperature model (TTM) and by increasing the total number of electrons (Ne), respectively. Ab initio molecular dynamics (AIMD) simulations provide a clear visualization of how Te and Ne induce a structural and electronic transformation process during LI/EI. Our results reveal the diffusion processes of Ag and X ions, the amorphization of the AgX lattices, and a straightforward interpretation of the time evolution for the formation of Ag and X nanoclusters under high values of Te and Ne. Overall, the present work provides fine details of the underlying mechanism of LI/EI and promises to be a powerful toolbox for further cross-scale modeling of other semiconductors.

Visualization of the Final Stage of Sintering in Nanoceramics with Atomic Resolution.

The deep understanding of the sintering mechanism is pivotal to optimizing denser ceramics production. Although several models explain the sintering satisfactorily on the micrometric scale, the extrapolation for nanostructured systems is not trivial. Aiming to provide additional information about the particularities of the sintering at the nanoscale, we performed in situ experiments using high-resolution transmission electron microscopy (HRTEM). We studied the pore elimination process in a ZrO2 thin film and identified a high anisotropic pore elimination. Interestingly, there is a redistribution of the atoms from the rough surface in the solid-gas surface, followed by the atom attachment in a faceted surface. Finally, we found evidence of the pore acting as a pin, reducing the GB mobility. These findings certainly can contribute to enhance the kinetic models to describe the densification process of systems at the nanoscale.

Stability and Rupture of an Ultrathin Ionic Wire.

Using a combination of in situ high-resolution transmission electron microscopy and density functional theory, we report the formation and rupture of ZrO_{2} atomic ionic wires. Near rupture, under tensile stress, the system favors the spontaneous formation of oxygen vacancies, a critical step in the formation of the monatomic bridge. In this length scale, vacancies provide ductilelike behavior, an unexpected mechanical behavior for ionic systems. Our results add an ionic compound to the very selective list of materials that can form monatomic wires and they contribute to the fundamental understanding of the mechanical properties of ceramic materials at the nanoscale.

Active-electrode biosensor of SnO2 nanowire for cyclodextrin detection from microbial enzyme.

Cyclodextrin (CD) is a conical compound used in food and pharmaceutical industry to complexation of hydrophobic substances. It is a product of microbial enzymes which converts starch into CD during their activity. We aim to detect CD using active-electrode biosensor of SnO2. They were grown on active electrode by the VLS method. The CD consists of several glucose units which have hydroxyl groups which tend to bind to interface states present in nanowires changing their conductivity. Experimental results of electrical conductivity at different CD concentrations are presented. A model that describes the influence of adsorbed glucose on nanowires and its electrical properties is also presented. Some general observations are performed on the applicability of the CD adsorption method by the nanowire-based biosensor.

Laser-induced formation of bismuth nanoparticles.

In the current communication, the synthesis of metallic Bi nanoparticles with coexisting crystallographic structures (rhombohedral, monoclinic, and cubic) obtained via direct femtosecond laser irradiation of NaBiO3 is demonstrated for the first time. By exploring the use of high laser power values, it is revealed that the promoted laser-mediated reactions lead to the synthesis of coexisting phases in metal nanoparticles, which may be a widely occurring phenomenon in other materials under femtosecond laser irradiation, and a fundamental concern for laser-based nanofabrication.

Controlling the Formation and Structure of Nanoparticle Superlattices through Surface Ligand Behavior.

The tailoring of nanoparticle superlattices is fundamental to the design of novel nanostructured materials and devices. To obtain specific collective properties of these nanoparticle superlattices, reliable protocols for their self-assembly are required. This study provides insight into the self-assembly process by using oleate-covered CeO2 nanoparticles (cubic and polyhedral shapes) through the correlation of experimental and theoretical investigations. The self-assembly of CeO2 nanoparticles is controlled by tuning the colloid deposition parameters (temperature and evaporation rate), and the ordered structures so obtained were correlated to the Gibbs free energy variation of the system. The analysis of the interparticle force contributions for each structure showed the importance of both the effective ligand mean size and its Flory-Huggins parameter in determining the total potential energies. Additionally, the roles of ligand solubility and effective mean size were used to understand the formation of specific superlattice phases as a function of temperature and ligand accommodation in the arrangement. Furthermore, the face-to-face interactions between nanoparticles were correlated to the type of exposed crystallographic facet in each particle.

Understanding the fundamental electrical and photoelectrochemical behavior of a hematite photoanode.

Hematite is considered to be the most promising material used as a photoanode for water splitting and here we utilized a sintered hematite photoanode to address the fundamental electrical, electrochemical and photoelectrochemical behavior of this semiconductor oxide. The results presented here allowed us to conclude that the addition of Sn(4+) decreases the grain boundary resistance of the hematite polycrystalline electrode. Heat treatment in a nitrogen (N2) atmosphere also contributes to a decrease of the grain boundary resistance, supporting the evidence that the presence of oxygen is fundamental for the formation of a voltage barrier at the hematite grain boundary. The N2 atmosphere affected both doped and undoped sintered electrodes. We also observed that the heat treatment atmosphere modifies the surface states of the solid-liquid interface, changing the charge-transfer resistance. A two-step treatment, with the second being performed at a low temperature in an oxygen (O2) atmosphere, resulted in a better solid-liquid interface.

Single-Step Exfoliation and Covalent Functionalization of MoS2 Nanosheets by an Organosulfur Reaction.

A simple approach to exfoliate and functionalize MoS2 in a single-step is described, which combines the dispersion of MoS2 in polybutadiene solution and ultrasonication processes. The great advantage of this process is that a colloidal stability of MoS2 in nonpolar solvent is achieved by chemically bonding polybutadiene on the perimeter edge sites of MoS2 sheets. In addition, elastomeric nanocomposite has been prepared with singular mechanical properties using functionalized MoS2 as nanofiller in a polybutadiene matrix with a subsequent vulcanization reaction.

Thermodynamic insights into the self-assembly of capped nanoparticles using molecular dynamic simulations.

Although the molecular modeling of self-assembling processes stands as a challenging research issue, there have been a number of breakthroughs in recent years. This report describes the use of large-scale molecular dynamics simulations with coarse grained models to study the spontaneous self-assembling of capped nanoparticles in chloroform suspension. A model system comprising 125 nanoparticles in chloroform evolved spontaneously from a regular array of independent nanoparticles to a single thread-like, ramified superstructure spanning the whole simulation box. The aggregation process proceeded by means of two complementary mechanisms, the first characterized by reactive collisions between monomers and oligomers, which were permanently trapped into the growing superstructure, and the second a slow structural reorganization of the nanoparticle packing. Altogether, these aggregation processes were over after ca. 0.6 µs and the system remained structurally and energetically stable until 1 µs. The thread-like structure closely resembles the TEM images of capped ZrO2, but a better comparison with experimental results was obtained by the deposition of the suspension over a graphene solid substrate, followed by the complete solvent evaporation. The agreement between the main structural features from this simulation and those from the TEM experiment was excellent and validated the model system. In order to shed further light on the origins of the stable aggregation of the nanoparticles, the Gibbs energy of aggregation was computed, along with its enthalpy and entropy contributions, both in chloroform and in a vacuum. The thermodynamic parameters arising from the modeling are consistent with larger nanoparticles in chloroform due to the solvent-swelled organic layer and the overall effect of the solvent was the partial destabilization of the aggregated state as compared to the vacuum system. The modeling strategy has been proved effective and reliable to describe the self-assembling of capped nanoparticles, but we must acknowledge the fact that larger model systems and longer timescales will be necessary in future investigations in order to assess structural and dynamical information approaching the behavior of macroscopic systems.

Analysis of dopant atom distribution and quantification of oxygen vacancies on individual Gd-doped CeO2 nanocrystals.

This work reports the analysis of the distribution of Gd atoms and the quantification of O vacancies applied to individual CeO2 and Gd-doped CeO2 nanocrystals by electron energy-loss spectroscopy. The concentration of O vacancies measured on the undoped system (6.3±2.6 %) matches the expected value given the typical Ce(3+) content previously reported for CeO2 nanoparticles. The doped nanoparticles have an uneven distribution of dopant atoms and an atypical amount of O vacant sites (37.7±4.1 %). The measured decrease of the O content induced by Gd doping cannot be explained solely by the charge balance including Ce(3+) and Gd(3+) ions.

Prediction of dopant atom distribution on nanocrystals using thermodynamic arguments.

A theoretical approach aiming at the prediction of segregation of dopant atoms on nanocrystalline systems is discussed here. It considers the free energy minimization argument in order to provide the most likely dopant distribution as a function of the total doping level. For this, it requires as input (i) a fixed polyhedral geometry with defined facets, and (ii) a set of functions that describe the surface energy as a function of dopant content for different crystallographic planes. Two Sb-doped SnO2 nanocrystalline systems with different morphology and dopant content were selected as a case study, and the calculation of the dopant distributions expected for them is presented in detail. The obtained results were compared to previously reported characterization of this system by a combination of HRTEM and surface energy calculations, and both methods are shown to be equivalent. Considering its application pre-requisites, the present theoretical approach can provide a first estimation of doping atom distribution for a wide range of nanocrystalline systems. We expect that its use will support the reduction of experimental effort for the characterization of doped nanocrystals, and also provide a solution to the characterization of systems where even state-of-art analytical techniques are limited.

Anisotropic nanocrystal dissolution observation by in situ transmission electron microscopy.

In this study, some keys in the knowledge of nanocrystals dissolving by the direct phenomenon observations are provided through in situ transmission electron microscopy experiments. The new characteristic of anisotropic nanoparticles dissolving is discussed and correlated with the evolution of the crystal to reach a minimum surface free energy (Gibbs-Wulff theorem), which has an impact on the nanocrystal ripening models. The process whereby the ripening occurs was identified and correlated to the adparticle motion.

Back to the Basics: Probing the Role of Surfaces in the Experimentally Observed Morphological Evolution of ZnO.

Although the physics and chemistry of materials are driven by exposed surfaces in the morphology, they are fleeting, making them inherently challenging to study experimentally. The rational design of their morphology and delivery in a synthesis process remains complex because of the numerous kinetic parameters that involve the effective shocks of atoms or clusters, which end up leading to the formation of different morphologies. Herein, we combined functional density theory calculations of the surface energies of ZnO and the Wulff construction to develop a simple computational model capable of predicting its available morphologies in an attempt to guide the search for images obtained by field-emission scanning electron microscopy (FE-SEM). The figures in this morphology map agree with the experimental FE-SEM images. The mechanism of this computational model is as follows: when the model is used, a reaction pathway is designed to find a given morphology and the ideal step height in the whole morphology map in the practical experiment. This concept article provides a practical tool to understand, at the atomic level, the routes for the morphological evolution observed in experiments as well as their correlation with changes in the properties of materials based solely on theoretical calculations. The findings presented herein not only explain the occurrence of changes during the synthesis (with targeted reaction characteristics that underpin an essential structure-function relationship) but also offer deep insights into how to enhance the efficiency of other metal-oxide-based materials via matching.

Oxidized cellulose nanofibers from sugarcane bagasse obtained by microfluidization: Morphology and rheological behavior.

It is advantageous to understand the relationship between cellulose fiber morphology and the rheological behavior of its dispersions so that their application can be optimized. The goal of this study was to produce sugarcane bagasse-sourced cellulose dispersions with different numbers of high-pressure homogenization cycles. Microfluidization produced cellulose nanofibers (between 5 and 80 nm in diameter) with similar surface charge densities and crystallinities (measured on the resulting films). Oscillatory rheology showed that TEMPO-oxidized cellulose dispersions exhibited gel-like behavior. However, not only did the samples with more microfluidization cycles present a lower storage modulus, but the sample with 100 cycles completely lost the gel-like characteristic, presenting a viscous fluid rheological behavior. Thixotropy loop tests revealed the influence of nanofiber length on the dispersion's structure, as evidenced by the decrease in the hysteresis value along with fiber breakage. Therefore, our findings demonstrate that the rheological properties of the dispersion can be tuned according to the length of the nanofibers, allowing for targeted applications.

Background optimization of powder electron diffraction for implementation of the e-PDF technique and study of the local structure of iron oxide nanocrystals.

The local structural characterization of iron oxide nanoparticles is explored using a total scattering analysis method known as pair distribution function (PDF) (also known as reduced density function) analysis. The PDF profiles are derived from background-corrected powder electron diffraction patterns (the e-PDF technique). Due to the strong Coulombic interaction between the electron beam and the sample, electron diffraction generally leads to multiple scattering, causing redistribution of intensities towards higher scattering angles and an increased background in the diffraction profile. In addition to this, the electron-specimen interaction gives rise to an undesirable inelastic scattering signal that contributes primarily to the background. The present work demonstrates the efficacy of a pre-treatment of the underlying complex background function, which is a combination of both incoherent multiple and inelastic scatterings that cannot be identical for different electron beam energies. Therefore, two different background subtraction approaches are proposed for the electron diffraction patterns acquired at 80 kV and 300 kV beam energies. From the least-square refinement (small-box modelling), both approaches are found to be very promising, leading to a successful implementation of the e-PDF technique to study the local structure of the considered nanomaterial.

High-resolution scanning transmission electron microscopy (HRSTEM) techniques: high-resolution imaging and spectroscopy side by side.

This work presents an overview of high-resolution scanning transmission electron microscopy (HRSTEM) techniques and exemplifies the novel quantitative characterization possibilities that have emerged from recent advances in these methods. The synergistic combination of atomic resolution imaging and spectroscopy provided by HRSTEM is highlighted as a unique feature that can provide a comprehensive analytical description of material properties at the nanoscale. State-of-the-art high-angle annular dark field and annular bright field examples are depicted as well as the use of X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy for probing samples properties at the atomic scale. In addition, promising techniques such as cathodoluminescence, confocal HRSTEM, and diffraction mapping are introduced. The presented examples and results indicate that HRSTEM-related techniques are fundamental tools for comprehensive assessment of properties at the atomic scale.

Improved photocatalytic activity of anisotropic rutile/anatase TiO2 nanoparticles synthesized by the Ti-peroxo complex method.

Anisotropic rutile/anatase TiO2 nanoparticles (AB-TiO2) were synthesized by the Ti-peroxo complex method. Their photocatalytic activity in the degradation of Rhodamine B (RhB) was evaluated and compared to that of commercial TiO2 P25 and TiO2 obtained through the benzyl alcohol route (OB-TiO2). The samples were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR in DRIFT mode), Field-Emission Scanning Electronic Microscopy (FEG-SEM), N2 physisorption and UV-visible spectroscopy. Photodegradation of RhB was carried out under visible light and the results revealed a remarkable photocatalytic activity of the AB-TiO2 in terms of surface area. The excellent performance of the AB-TiO2 was explained in light of the synergistic effect of the coexistence of anatase/rutile phases, anisotropy and irreversible adsorption of organic species during sol-gel synthesis. UV-visible measurements also indicated that N-deethylation and photobleaching mechanisms occur to different extents, depending on the surface composition of the photocatalyst.

The Influence of Magnetic Field and Nanoparticle Concentration on the Thin Film Colloidal Deposition Process of Magnetic Nanoparticles: The Search for High-Efficiency Hematite Photoanodes.

Hematite is considered a promising photoanode material for photoelectrochemical water splitting, and the literature has shown that the photoanode production process has an impact on the final efficiency of hydrogen generation. Among the methods used to process hematite photoanode, we can highlight the thin films from the colloidal deposition process of magnetic nanoparticles. This technique leads to the production of high-performance hematite photoanode. However, little is known about the influence of the magnetic field and heat treatment parameters on the final properties of hematite photoanodes. Here, we will evaluate those processing parameters in the morphology and photoelectrochemical properties of nanostructured hematite anodes. The analysis of thickness demonstrated a relationship between the magnetic field and nanoparticles concentration utilized to prepare the thin films, showing that the higher magnetic fields decrease the thickness. The Jabs results corroborate to influence the magnetic field since the use of a higher magnetic field decreases the deposited material amount, consequently decreasing the absorption of the thin films. The PEC measurements showed that at higher concentrations, the use of higher magnetic fields increases the JPH values, and lower magnetic fields cause a decrease in JPH when using the higher nanoparticle concentrations.

Fast and efficient electrochemical thinning of ultra-large supported and free-standing MoS2 layers on gold surfaces.

Molybdenum disulfide (MoS2) is a very promising layered material for electrical, optical, and electrochemical applications because of its unique and outstanding properties. To unlock its full potential, among different preparation routes, electrochemistry has gain interest due to its simple, fast, scalable and simple instrumentation. However, obtaining large-area monolayer MoS2 that will enable the fabrication of novel electronic and electrochemical devices is still challenging. In this work, we reported a simple and fast electrochemical thinning process that results in ultra-large MoS2 down to monolayer on Au surfaces. The high affinity of MoS2 by Au surfaces enables the removal of bulk layers while preserving the first layer attached to the electrode. With a proper choice of the applied potential, more than 90% of the bulk regions can be removed from large-area MoS2 crystals, as confirmed by atomic force microscopy, photoluminescence, and Raman spectroscopy. We further address a set of contributions that are helpful to elucidate the features of MoS2, namely, the hyphenation of electrochemistry and optical microscopy for real-time observation of the thinning process that was revealed to occur from the edges to the center of the flake, an image treatment to estimate the thinning area and thinning rate, and the preparation of free-standing MoS2 layers by electrochemically thinning bulk flakes on microhole-structured Ni/Au meshes.

How lignin sticks to cellulose-insights from atomic force microscopy enhanced by machine-learning analysis and molecular dynamics simulations.

Elucidating cellulose-lignin interactions at the molecular and nanometric scales is an important research topic with impacts on several pathways of biomass valorization. Here, the interaction forces between a cellulosic substrate and lignin are investigated. Atomic force microscopy with lignin-coated tips is employed to probe the site-specific adhesion to a cellulose film in liquid water. Over seven thousand force-curves are analyzed by a machine-learning approach to cluster the experimental data into types of cellulose-tip interactions. The molecular mechanisms for distinct types of cellulose-lignin interactions are revealed by molecular dynamics simulations of lignin globules interacting with different cellulose Iß crystal facets. This unique combination of experimental force-curves, data-driven analysis, and molecular simulations opens a new approach of investigation and updates the understanding of cellulose-lignin interactions at the nanoscale.