In Situ Visualization of Self-Assembly of Charged Gold Nanoparticles

Self-assembly of Au nanoparticles (NPs) coated with positively charged cetyltrimethylammonium ions (CTA⁺) and negatively charged citrate ions in aqueous liquid cell was investigated by in situ transmission electron microscopy (TEM). Under electron illumination in TEM, the hydrated electrons will reduce the overall positive charges of the CTA⁺ covered Au NPs and decrease the repulsive electrostatic forces among NPs, leading to assembly of individual NPs into one-dimensional structures. On the contrary, the negatively charged Au NPs coated with citrate ions are steady in liquid cell regardless of electron beam intensity

PVP-Assisted Synthesis of Uniform Carbon Coated Li₂S/CB for High-Performance Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is a great alternative to the state-of-the-art lithium ion batteries due to its high energy density. However, low utilization of active materials, the insulating nature of sulfur or lithium sulfide (Li₂S), and polysulfide dissolution in organic liquid electrolyte lead to low initial capacity and fast performance degradation. Herein, we propose a facile and viable approach to address these issues. This new approach entails synthesis of Li2S/carbon black (Li2S/CB) cores encapsulated by a nitrogen-doped carbon shell with polyvinylpyrrolidone (PVP) assistance. Combining energy-filtered transmission electron microscopy (EFTEM) elemental mappings, XPS and FTIR measurements, it is confirmed that the as-synthesized material has a structure of a Li₂S/CB core with a nitrogen-doped carbon shell (denoted as Li₂S/CB@NC). The Li₂S/CB@NC cathode yields an exceptionally high initial capacity of 1020 mAh/g based on Li₂S mass at 0.1 C with stable Coulombic efficiency of 99.7% over 200 cycles. Also, cycling performance shows the capacity decay per cycle as small as 0.17%. Most importantly, to further understand the materials for battery applications, field emission transmission electron microscopy (FETEM) and elemental mapping tests without exposure to air for Li₂S samples in cycled cells are reported. Along with the first ever FETEM and field emission scanning electron microscopy (FESEM) investigations of cycled batteries, Li₂S/CB@NC cathode demonstrates the capability of robust core–shell nanostructures for different rates and improved capacity retention, revealing Li₂S/CB@NC designed here as an outstanding system for high-performance lithium–sulfur batteries

Quantifying the Nucleation and Growth Kinetics of Microwave Nanochemistry Enabled by in Situ High-Energy X‑ray Scattering

The fast reaction kinetics presented in the microwave synthesis of colloidal silver nanoparticles was quantitatively studied, for the first time, by integrating a microwave reactor with in situ X-ray diffraction at a high-energy synchrotron beamline. Comprehensive data analysis reveals two different types of reaction kinetics corresponding to the nucleation and growth of the Ag nanoparticles. The formation of seeds (nucleation) follows typical first-order reaction kinetics with activation energy of 20.34 kJ/mol, while the growth of seeds (growth) follows typical self-catalytic reaction kinetics. Varying the synthesis conditions indicates that the microwave colloidal chemistry is independent of concentration of surfactant. These discoveries reveal that the microwave synthesis of Ag nanoparticles proceeds with reaction kinetics significantly different from the synthesis present in conventional oil bath heating. The in situ X-ray diffraction technique reported in this work is promising to enable further understanding of crystalline nanomaterials formed through microwave synthesis

Silicon Nanoparticles: Stability in Aqueous Slurries and the Optimization of the Oxide Layer Thickness for Optimal Electrochemical Performance

In this study, silicon nanoparticles are oxidized in a controlled manner to obtain different thicknesses of SiO₂ layers. Their stability in aqueous slurries as well as the effect of oxide layer thickness on the electrochemical performance of the silicon anodes is evaluated. Our results show that slightly increasing the oxide layer of silicon nanoparticles significantly improves the stability of the nanoparticles in aqueous slurries and does not compromise the initial electrochemical performance of the electrodes. A careful comparison of the rate and cycle performance between 400 °C treated Si nanoparticles and pristine Si nanoparticles shows that by treating the silicon nanoparticles in air for slightly increasing the oxide layer, improvement in both rate and cycle performance can be achieved

Insight into the Structural Evolution of a High-Voltage Spinel for Lithium-Ion Batteries

With a high operating voltage and three-dimensional lithium (Li)-ion diffusion pathways, Li_<i>x</i>Ni_0.5Mn_1.5O₄ (high-voltage [HV] spinel) is considered to be a promising high-energy and high-power density cathode material for Li-ion batteries. Here, we extensively investigate the structural dependence of the spinel cathode on the stoichiometry of the cation/anion ratio through an unprecedented overstoichiometric Li intercalation. This material undergoes the well-known cubic phase transition with one Li insertion from a fully delithiated state. The further overstoichiometric Li intercalation results in a cubic-to-tetragonal phase transition when <i>x</i> reaches 3. When <i>x</i> is electrochemically pushed to ∼4, the coexistence of a rock-salt structure with a layered component is observed. The parent spinel structure is reformed upon complete deintercalation. This reversibility underscores the fact that the HV spinel has a distinct memory of its original form. The resultant phases and morphologies are identified by X-ray diffraction (XRD) and microscopy methods

Nanostructured TiO₂/Polypyrrole for Visible Light Photocatalysis

Stable TiO₂/polypyrrole nanocomposites have been synthesized by a simple one-step hydrothermal method. The nanocomposites are capable of efficient visible-light photocatalysis driven by their morphology that utilizes a high concentration of 4.5 nm TiO₂ nanoparticles electronically coupled to 200–300 nm polypyrrole granules. The polypyrrole acts as visible-light photosensitizer, and the photoactivity of nanocomposite arises from the electron transfer from excited polypyrrole to TiO₂ nanoparticles and further across nanocomposite interface. The visible-light photocatalysis is demonstrated by methylene blue degradation and by the production of H₂ from water with efficiency of 1 mmol H₂ g_catalyst^–1 h^–1 wt %(Pt)⁻¹

Visualization of the Magnetic Structure of Sculpted Three-Dimensional Cobalt Nanospirals

In this work, we report on the direct visualization of magnetic structure in sculpted three-dimensional cobalt (Co) nanospirals with a wire diameter of 20 nm and outer spiral diameter of 115 nm and on the magnetic interactions between the nanospirals, using aberration-corrected Lorentz transmission electron microscopy. By analyzing the magnetic domains in three dimensions at the nanoscale, we show that magnetic domain formation in the Co nanospirals is a result of the shape anisotropy dominating over the magnetocrystalline anisotropy of the system. We also show that the strong dipolar magnetic interactions between adjacent closely packed nanospirals leads to their magnetization directions adopting alternating directions to minimize the total magnetostatic energy of the system. Deviations from such magnetization structure can only be explained by analyzing the complex three-dimensional structure of the nanospirals. These nanostructures possess an inherent chirality due to their growth conditions and are of significant importance as nanoscale building blocks in magneto-optical devices

In Situ Focused Ion Beam-Scanning Electron Microscope Study of Crack and Nanopore Formation in Germanium Particle During (De)lithiation

Germanium has emerged as a promising high-capacity anode material for lithium ion batteries. To understand the microstructure evolution of germanium under different cycling rates, we monitored single germanium particle batteries using an in situ focused ion beam-scanning electron microscope. Our results show that both the lithium concentration and delithiation rate have an impact on nanopore formation. This study reveals that germanium electrodes with low and high cycling rates have better microstructure integrity, which leads to better cycling performance. The nanopores tend to aggregate into large porous structures during cycling which leads to particle pulverization and capacity fading of the electrode

Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets

Elevated temperature optoelectronic performance of semiconductor nanomaterials remains an important issue for applications. Here we examine 2D CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell sandwich NPs at temperatures ranging from 300 to 700 K using static and transient spectroscopies as well as in situ transmission electron microscopy. NPs exhibit reversible changes in PL intensity, spectral position, and emission line width with temperature elevation up to ∼500 K, losing a factor of ∼8 to 10 in PL intensity at 400 K relative to ambient. Temperature elevation above ∼500 K yields thickness-dependent, irreversible degradation in optical properties. Electron microscopy relates stability of the core-only NP morphology up to 555 and 600 K for the four and five monolayer NPs, respectively, followed by sintering and evaporation at still higher temperatures. Reversible PL loss, based on differences in decay dynamics between time-resolved photoluminescence and transient absorption, results primarily from hole trapping in both NPs and sandwich NPs

Kinetic Pathway of Palladium Nanoparticle Sulfidation Process at High Temperatures

A significant issue related to Palladium (Pd) based catalysts is that sulfur-containing species, such as alkanethiols, can form a PdS_<i>x</i> underlayer on nanoparticle surface and subsequently poison the catalysts. Understanding the exact reaction pathway, the degree of sulfidation, the chemical stoichiometry, and the temperature dependence of this process is critically important. Combining energy-filtered transmission electron microscopy (EFTEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy experiments at the S <i>K-</i>, Pd <i>K</i>-, and <i>L</i>_2,3-edges, we show the kinetic pathway of Pd nanoparticle sulfidation process with the addition of excess amount of octadecanethiol at different temperatures, up to 250 °C. We demonstrate that the initial polycrystalline Pd-oleylamine nanoparticles gradually become amorphous PdS_<i>x</i> nanoparticles, with the sulfur atomic concentration eventually saturating at Pd/S = 66:34 at 200 °C. This final chemical stoichiometry of the sulfurized nanoparticles closely matches that of the crystalline P₁₆S₇ phase (30.4% S), albeit being structurally amorphous. Sulfur diffusion into the nanoparticle depends strongly on the temperature. At 90 °C, sulfidation remains limited at the surface of nanoparticles even with extended heating time; whereas at higher temperatures beyond 125 °C, sulfidation occurs rapidly in the interior of the particles, far beyond what can be described as a core–shell model. This indicates sulfur diffusion from the surface to the interior of the particle is subject to a diffusion barrier and likely first go through the grain boundaries of the nanoparticle