Reversible MnO to Mn3 O4 Oxidation in Manganese Oxide Nanoparticles.

Manganese is an attractive element for sustainable solutions. It is largely available in the earth's crust, making it ideal for cost-effective and large-scale applications. Especially MnO nanoparticles have recently received attention for applications in battery technology. However, manganese has many oxidation states that are energetically very similar, indicating that they may easily transform from one to the other. Herein, the reversible oxidation of MnO nanoparticles to Mn3 O4 studied with in situ transmission electron microscopy is shown. The oxygen sublattices of MnO and Mn3 O4 are found to be perfectly aligned, and an atomic mechanism where the transformation is facilitated by the migration of Mn cations on the shared O sublattice is proposed. Even when protected with an amorphous carbon layer, MnO particles are highly unstable and oxidize to Mn3 O4 in ethanol. The poor stability of MnO lacks discussion in many battery-related works, and strategies aimed at avoiding this should be developed.

Tandem catalysis with double-shelled hollow spheres.

Metal-zeolite composites with metal (oxide) and acid sites are promising catalysts for integrating multiple reactions in tandem to produce a wide variety of wanted products without separating or purifying the intermediates. However, the conventional design of such materials often leads to uncontrolled and non-ideal spatial distributions of the metal inside/on the zeolites, limiting their catalytic performance. Here we demonstrate a simple strategy for synthesizing double-shelled, contiguous metal oxide@zeolite hollow spheres (denoted as MO@ZEO DSHSs) with controllable structural parameters and chemical compositions. This involves the self-assembly of zeolite nanocrystals onto the surface of metal ion-containing carbon spheres followed by calcination and zeolite growth steps. The step-by-step formation mechanism of the material is revealed using mainly in situ Raman spectroscopy and X-ray diffraction and ex situ electron microscopy. We demonstrate that it is due to this structure that an Fe2O3@H-ZSM-5 DSHSs-showcase catalyst exhibits superior performance compared with various conventionally structured Fe2O3-H-ZSM-5 catalysts in gasoline production by the Fischer-Tropsch synthesis. This work is expected to advance the rational synthesis and research of hierarchically hollow, core-shell, multifunctional catalyst materials.

Single Particle Deformation and Analysis of Silica-Coated Gold Nanorods before and after Femtosecond Laser Pulse Excitation.

We performed single particle deformation experiments on silica-coated gold nanorods under femtosecond (fs) illumination. Changes in the particle shape were analyzed by electron microscopy and associated changes in the plasmon resonance by electron energy loss spectroscopy. Silica-coated rods were found to be more stable compared to uncoated rods but could still be deformed via an intermediate bullet-like shape for silica shell thicknesses of 14 nm. Changes in the size ratio of the rods after fs-illumination resulted in blue-shifting of the longitudinal plasmon resonances. Two-dimensional spatial mapping of the plasmon resonances revealed that the flat side of the bullet-like particles showed a less pronounced longitudinal plasmonic electric field enhancement. These findings were confirmed by finite-difference time-domain (FDTD) simulations. Furthermore, at higher laser fluences size reduction of the particles was found as well as for particles that were not completely deformed yet.

Acetate ligands determine the crystal structure of CdSe nanoplatelets - a density functional theory study.

Cadmium selenide (CdSe) nanoplatelets of a few atomic layers thick exhibit extremely sharp photoluminescence peaks and are synthesized in the zinc blende crystal structure, whereas the most stable bulk polymorph of CdSe is the wurtzite structure. These platelets can be synthesized very monodispersely in thickness, and are covered with acetate ligands. Here, we show by means of density functional theory (DFT) calculations that these ligands play a pivoting role in the stabilization of 2D nanosheets as a whole, including the deviating crystal structure. The relative stability as a function of slab thickness, strong effects on electronic properties, and implications for synthesis are discussed.

The accurate calculation of the band gap of liquid water by means of GW corrections applied to plane-wave density functional theory molecular dynamics simulations.

Knowledge about the intrinsic electronic properties of water is imperative for understanding the behaviour of aqueous solutions that are used throughout biology, chemistry, physics, and industry. The calculation of the electronic band gap of liquids is challenging, because the most accurate ab initio approaches can be applied only to small numbers of atoms, while large numbers of atoms are required for having configurations that are representative of a liquid. Here we show that a high-accuracy value for the electronic band gap of water can be obtained by combining beyond-DFT methods and statistical time-averaging. Liquid water is simulated at 300 K using a plane-wave density functional theory molecular dynamics (PW-DFT-MD) simulation and a van der Waals density functional (optB88-vdW). After applying a self-consistent GW correction the band gap of liquid water at 300 K is calculated as 7.3 eV, in good agreement with recent experimental observations in the literature (6.9 eV). For simulations of phase transformations and chemical reactions in water or aqueous solutions whereby an accurate description of the electronic structure is required, we suggest to use these advanced GW corrections in combination with the statistical analysis of quantum mechanical MD simulations.

Solution-phase epitaxial growth of quasi-monocrystalline cuprous oxide on metal nanowires.

The epitaxial growth of monocrystalline semiconductors on metal nanostructures is interesting from both fundamental and applied perspectives. The realization of nanostructures with excellent interfaces and material properties that also have controlled optical resonances can be very challenging. Here we report the synthesis and characterization of metal-semiconductor core-shell nanowires. We demonstrate a solution-phase route to obtain stable core-shell metal-Cu2O nanowires with outstanding control over the resulting structure, in which the noble metal nanowire is used as the nucleation site for epitaxial growth of quasi-monocrystalline Cu2O shells at room temperature in aqueous solution. We use X-ray and electron diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, and absorption spectroscopy, as well as density functional theory calculations, to characterize the core-shell nanowires and verify their structure. Metal-semiconductor core-shell nanowires offer several potential advantages over thin film and traditional nanowire architectures as building blocks for photovoltaics, including efficient carrier collection in radial nanowire junctions and strong optical resonances that can be tuned to maximize absorption.

Real-time atomic scale imaging of nanostructural evolution in aluminum alloys.

We present a new approach to study the three-dimensional compositional and structural evolution of metal alloys during heat treatments such as commonly used for improving overall material properties. It relies on in situ heating in a high-resolution scanning transmission electron microscope (STEM). The approach is demonstrated using a commercial Al alloy AA2024 at 100-240 °C, showing in unparalleled detail where and how precipitates nucleate, grow, or dissolve. The observed size evolution of individual precipitates enables a separation between nucleation and growth phenomena, necessary for the development of refined growth models. We conclude that the in situ heating STEM approach opens a route to a much faster determination of the interplay between local compositions, heat treatments, microstructure, and mechanical properties of new alloys.

Atomic resolution monitoring of cation exchange in CdSe-PbSe heteronanocrystals during epitaxial solid-solid-vapor growth.

Here, we show a novel solid-solid-vapor (SSV) growth mechanism whereby epitaxial growth of heterogeneous semiconductor nanowires takes place by evaporation-induced cation exchange. During heating of PbSe-CdSe nanodumbbells inside a transmission electron microscope (TEM), we observed that PbSe nanocrystals grew epitaxially at the expense of CdSe nanodomains driven by evaporation of Cd. Analysis of atomic-resolution TEM observations and detailed atomistic simulations reveals that the growth process is mediated by vacancies.

Core-shell reconfiguration through thermal annealing in Fe(x)O/CoFe2O4 ordered 2D nanocrystal arrays.

A great variety of single- and multi-component nanocrystals (NCs) can now be synthesized and integrated into nanocrystal superlattices. However, the thermal and temporal stability of these superstructures and their components can be a limiting factor for their application as functional devices. On the other hand, temperature induced reconstructions can also reveal opportunities to manipulate properties and access new types of nanostructures. In situ atomically resolved monitoring of nanomaterials provides insight into the temperature induced evolution of the individual NC constituents within these superstructures at the atomic level. Here, we investigate the effect of temperature annealing on 2D square and hexagonal arrays of FexO/CoFe2O4 core/shell NCs by in situ heating in a transmission electron microscope (TEM). Both cubic and spherical NCs undergo a core-shell reconfiguration at a temperature of approximately 300 ° C, whereby the FexO core material segregates at the exterior of the CoFe2O4 shell, forming asymmetric dumbbells ('snowman-type' particles) with a small FexO domain attached to a larger CoFe2O4 domain. Upon continued annealing, the segregated FexO domains form bridges between the CoFe2O4 domains, followed by coalescence of all domains, resulting in loss of ordering in the 2D arrays.

A transferable force field for CdS-CdSe-PbS-PbSe solid systems.

A transferable force field for the PbSe-CdSe solid system using the partially charged rigid ion model has been successfully developed and was used to study the cation exchange in PbSe-CdSe heteronanocrystals [A. O. Yalcin et al., "Atomic resolution monitoring of cation exchange in CdSe-PbSe heteronanocrystals during epitaxial solid-solid-vapor growth," Nano Lett. 14, 3661-3667 (2014)]. In this work, we extend this force field by including another two important binary semiconductors, PbS and CdS, and provide detailed information on the validation of this force field. The parameterization combines Bader charge analysis, empirical fitting, and ab initio energy surface fitting. When compared with experimental data and density functional theory calculations, it is shown that a wide range of physical properties of bulk PbS, PbSe, CdS, CdSe, and their mixed phases can be accurately reproduced using this force field. The choice of functional forms and parameterization strategy is demonstrated to be rational and effective. This transferable force field can be used in various studies on II-VI and IV-VI semiconductor materials consisting of CdS, CdSe, PbS, and PbSe. Here, we demonstrate the applicability of the force field model by molecular dynamics simulations whereby transformations are initiated by cation exchange.

Unexpectedly high thermal stability of Au nanotriangle@mSiO2 yolk-shell nanoparticles.

The shape of Au nanoparticles (NPs) plays a crucial role for applications in, amongst others, catalysis, electronic devices, biomedicine, and sensing. Typically, the deformation of the morphology of Au NPs is the most significant cause of loss of functionality. Here, we systematically investigate the thermal stability of Au nanotriangles (NTs) coated with (mesoporous) silica shells with different morphologies (core-shell (CS): Au NT@mSiO2/yolk-shell (YS): Au NT@mSiO2) and compare these to 'bare' nanoparticles (Au NTs), by a combination of in situ and/or ex situ TEM techniques and spectroscopy methods. Au NTs with a mesoporous silica (mSiO2) coating were found to show much higher thermal stability than those without a mSiO2 coating, as the mSiO2 shell restricts the (self-)diffusion of surface atoms. For the Au NT@mSiO2 CS and YS NPs, a thicker mSiO2 shell provides better protection than uncoated Au NTs. Surprisingly, the Au NT@mSiO2 YS NPs were found to be as stable as Au NT@mSiO2 CS NPs with a core-shell morphology. We hypothesize that the only explanation for this unexpected finding was the thicker and higher density SiO2 shell of YS NPs that prevents diffusion of Au surface atoms to more thermodynamically favorable positions.

From sphere to multipod: thermally induced transitions of CdSe nanocrystals studied by molecular dynamics simulations.

Molecular dynamics (MD) simulations are used to show that a spherical zinc blende (ZB) nanocrystal (NC) can transform into a tetrapod or an octapod as a result of heating, by a local zincblende-to-wurtzite phase transformation taking place in the NC. The partial sphere-to-tetrapod or sphere-to-octapod transition occurs within simulation times of 30 ns and depends on both temperature and NC size. Surprisingly, the wurtzite (WZ) subdomains are not formed through a slip mechanism but are mediated by the formation of highly mobile Cd vacancies on the ZB{111} Cd atomic planes. The total potential energy of a tetrapod is found to be lower than that of a ZB sphere at the same numbers of atoms. The simulation results are in good agreement with experimental high-resolution transmission electron microscopy (HR-TEM) data obtained on heated colloidal CdSe NCs.

Formation Pathways of Lath-Shaped WO3 Nanosheets and Elemental W Nanoparticles from Heating of WO3 Nanocrystals Studied via In Situ TEM.

WO3 is a versatile material occurring in many polymorphs, and is used in nanostructured form in many applications, including photocatalysis, gas sensing, and energy storage. We investigated the thermal evolution of cubic-phase nanocrystals with a size range of 5-25 nm by means of in situ heating in the transmission electron microscope (TEM), and found distinct pathways for the formation of either 2D WO3 nanosheets or elemental W nanoparticles, depending on the initial concentration of deposited WO3 nanoparticles. These pristine particles were stable up to 600 °C, after which coalescence and fusion of the nanocrystals were observed. Typically, the nanocrystals transformed into faceted nanocrystals of elemental body-centered-cubic W after annealing to 900 °C. However, in areas where the concentration of dropcast WO3 nanoparticles was high, at a temperature of 900 °C, considerably larger lath-shaped nanosheets (extending for hundreds of nanometers in length and up to 100 nm in width) were formed that are concluded to be in monoclinic WO3 or WO2.7 phases. These lath-shaped 2D particles, which often curled up from their sides into folded 2D nanosheets, are most likely formed from the smaller nanoparticles through a solid-vapor-solid growth mechanism. The findings of the in situ experiments were confirmed by ex situ experiments performed in a high-vacuum chamber.

Morphology-Controlled Growth of Crystalline Ag-Pt-Alloyed Shells onto Au Nanotriangles and Their Plasmonic Properties.

The surface plasmon resonance of noble-metal nanoparticles depends on nanoscale size, morphology, and composition, and provides great opportunities for applications in biomedicine, optoelectronics, (photo)catalysis, photovoltaics, and sensing. Here, we present the results of synthesizing ternary metallic or trimetallic nanoparticles, Au nanotriangles (Au NTs) with crystalline Ag-Pt alloyed shells, the morphology of which can be adjusted from a yolk-shell to a core-shell structure by changing the concentration of AgNO3 or the concentration of Au NT seeds, while the shell thickness can be precisely controlled by adjusting the concentration of K2PtCl4. By monitoring the growth process with UV-vis spectra and scanning transmission electron microscopy (STEM), the shells on the Au NT-Ag-Pt yolk-shell nanoparticles were found to grow via a galvanic replacement synergistic route. The plasmonic properties of the as-synthesized nanoparticles were investigated by optical absorbance measurements.

Thermal Stability and Sublimation of Two-Dimensional Co9Se8 Nanosheets for Ultrathin and Flexible Nanoelectronic Devices.

An understanding of the structural and compositional stability of nanomaterials is significant from both fundamental and technological points of view. Here, we investigate the thermal stability of half-unit-cell thick two-dimensional (2D) Co9Se8 nanosheets that are exceptionally interesting because of their half-metallic ferromagnetic properties. By employing in situ heating in the transmission electron microscope (TEM), we find that the nanosheets show good structural and chemical stability without changes to the cubic crystal structure until sublimation of the nanosheets starts at temperatures between 460 and 520 °C. The real-time observations of the sublimation process show preferential removal at {110} type crystal facets. From an analysis of sublimation rates at various temperatures, we find that the sublimation occurs through noncontinuous and punctuated mass loss at lower temperatures while the sublimation is continuous and uniform at higher temperatures. Our findings provide an understanding of the nanoscale structural and compositional stability of 2D Co9Se8 nanosheets, which is of importance for their reliable application and sustained performance as ultrathin and flexible nanoelectronic devices.

Thermolysis-Driven Growth of Vanadium Oxide Nanostructures Revealed by In Situ Transmission Electron Microscopy: Implications for Battery Applications.

Understanding the growth modes of 2D transition-metal oxides through direct observation is of vital importance to tailor these materials to desired structures. Here, we demonstrate thermolysis-driven growth of 2D V2O5 nanostructures via in situ transmission electron microscopy (TEM). Various growth stages in the formation of 2D V2O5 nanostructures through thermal decomposition of a single solid-state NH4VO3 precursor are unveiled during the in situ TEM heating. Growth of orthorhombic V2O5 2D nanosheets and 1D nanobelts is observed in real time. The associated temperature ranges in thermolysis-driven growth of V2O5 nanostructures are optimized through in situ and ex situ heating. Also, the phase transformation of V2O5 to VO2 was revealed in real time by in situ TEM heating. The in situ thermolysis results were reproduced using ex situ heating, which offers opportunities for upscaling the growth of vanadium oxide-based materials. Our findings offer effective, general, and simple pathways to produce versatile 2D V2O5 nanostructures for a range of battery applications.

Thermal Reduction of MoO3 Particles and Formation of MoO2 Nanosheets Monitored by In Situ Transmission Electron Microscopy.

Nanoscale forms of molybdenum trioxide have found widespread use in optoelectronic, sensing, and battery applications. Here, we investigate the thermal evolution of micrometer-sized molybdenum trioxide particles during in situ heating in vacuum using transmission electron microscopy and observed drastic structural and chemical changes that are strongly dependent on the heating rate. Rapid heating (flash heating) of MoO3 particles to a temperature of 600 °C resulted in large-scale formation of MoO2(001) nanosheets that were formed in a wide area around the reducing MoO3 particles, within a few minutes of time frame. In contrast, when heated more gently, the initially single-crystal MoO3 particles were reduced into hollow nanostructures with polycrystalline MoO2 shells. Using density functional theory calculations employing the DFT-D3 functional, the surface energy of MoO3(010) was calculated to be 0.187 J m-2, and the activation energy for exfoliation of the van der Waals bonded MoO3 (010) layers was calculated to be 0.478 J m-2. Ab initio molecular dynamics simulations show strong fluctuations in the distance between the (010) layers, where thermal vibrations lead to additional separations of up to 1.8 Å at 600 °C. This study shows efficient pathways for the generation of either MoO2 nanosheets or hollow MoO2 nanostructures with very high effective surface areas beneficial for applications.

In situ single particle characterization of the themoresponsive and co-nonsolvent behavior of PNIPAM microgels and silica@PNIPAM core-shell colloids.

Poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM colloidal shells attract continuous strong interest due to their thermoresponsive behavior, as their size and properties can be tuned by temperature. The direct single particle observation and characterization of pure, unlabeled PNIPAM microgels in their native aqueous environment relies on imaging techniques that operate either at interfaces or in cryogenic conditions, thus limiting the observation of their dynamic nature. Liquid Cell (Scanning) Transmission Electron Microscopy (LC-(S) TEM) imaging allows the characterization of materials and dynamic processes such as nanoparticle growth, etching, and diffusion, at nanometric resolution in liquids. Here we show that via a facile post-synthetic in situ polymer labelling step with high-contrast marker core-shell Au@SiO2 nanoparticles (NPs) it is possible to determine the full volume of PNIPAM microgels in water. The labelling allowed for the successful characterization of the thermoresponsive behavior of PNIPAM microgels and core shell silica@PNIPAM hybrid microgels, as well as the co-nonsolvency of PNIPAM in aqueous alcoholic solutions. The interplay between electron beam irradiation and PNIPAM systems in water resulted in irreversible shrinkage due to beam induced water radiolysis products, which in turn also affected the thermoresponsive behavior of PNIPAM. The addition of 2-propanol as radical scavenger improved PNIPAM stability in water under electron beam irradiation.

Chemical transformation of Au-tipped CdS nanorods into AuS/Cd core/shell particles by electron beam irradiation.

We demonstrate that electron irradiation of colloidal CdS nanorods carrying Au domains causes their evolution into AuS/Cd core/shell nanoparticles as a result of a concurrent chemical and morphological transformation. The shrinkage of the CdS nanorods and the growth of the Cd shell around the Au tips are imaged in real time, while the displacement of S atoms from the CdS nanorod to the Au domains is evidenced by high-sensitivity energy-dispersive X-ray (EDX) spectroscopy. The various nanodomains display different susceptibility to the irradiation, which results in nanoconfigurations that are very different from those obtained after thermal annealing. Such physical manipulations of colloidal nanocrystals can be exploited as a tool to access novel nanocrystal heterostructures.

Three-dimensional atomic imaging of colloidal core-shell nanocrystals.

Colloidal core-shell semiconductor nanocrystals form an important class of optoelectronic materials, in which the exciton wave functions can be tailored by the atomic configuration of the core, the interfacial layers, and the shell. Here, we provide a trustful 3D characterization at the atomic scale of a free-standing PbSe(core)-CdSe(shell) nanocrystal by combining electron microscopy and discrete tomography. Our results yield unique insights for understanding the process of cation exchange, which is widely employed in the synthesis of core-shell nanocrystals. The study that we present is generally applicable to the broad range of colloidal heteronanocrystals that currently emerge as a new class of materials with technological importance.