Atomic Mechanisms of Nanocrystallization via Cluster-Clouds in Solution Studied by Liquid-Phase Scanning Transmission Electron Microscopy

Direct Imaging of the Atomic Mechanisms Governing the Growth and Shape of Bimetallic Pt-Pd Nanocrystals by In Situ Liquid Cell STEM

Understanding the atomic mechanisms governing the growth of bimetallic nanoalloys is of great interest for scientists. As a promising material for photocatalysis applications, Pt-Pd bimetallic nanoparticles (NPs) have been in the spotlight for many years due to their catalytic performance, which is typically superior to that of pure Pt NPs. In this work, we use in situ liquid cell scanning transmission electron microscopy to track the exact atomic mechanisms governing the formation of bimetallic Pt-Pd NPs. We find that the formation process of the bimetallic Pt-Pd is divided into three stages. First, the nucleation and growth of ultrasmall primary nanoclusters are formed by the agglomeration of Pt and Pd atoms. Second, the primary nanoclusters are involved in a coalescence process to form two types of bigger agglomerates, namely, amorphous (a-NC) and crystalline (c-NC) nanoclusters. In the third stage, these clusters undergo a coalescence process leading to the formation of Pt-Pd NPs, while, in parallel, monomer attachment continues. We found that the third stage contains three types of coalescence processes, a-NC-a-NC, a-NC-c-NC, and c-NC-c-NC coalescence, which eventually give rise to crystalline bimetallic alloys. However, each type of coalescence gave distinct NPs in terms of shape and defects. Our results thus reveal the exact growth mechanisms of bimetallic alloys on the atomic scale, unravel the origin of their structure, and overall are of key interest to tailor the structure of bimetallic NPs.

Aggregation-based phase transition tailored heterophase junctions of AgInS2 for boosting photocatalytic H2 evolution

Due to high defect tolerance and multiphase allowance, AgInS₂ (AIS) quantum dots (QDs) provide chances for designing new type junctions via tailoring defects, size, or phase structure. These new type junctions potentially enhance photoelectric performance, such as photocatalytic H2 evolution (PHE). Here, ultra-small AIS QDs (∼1 nm) with well-defined exciton absorption were prepared aqueously via a reverse hot-injection procedure for the first time. A coalescence or fast aggregation-based growth was observed for coarsening at 95 or 135 ℃, respectively. XRD and TEM investigations revealed that the tetragonal-orthorhombic (t-o) phase transition occurred via aggregation-based growth. The studies on phase transition kinetics resulted in fine-tailoring on AIS polymorphs, favoring t-o AIS junctions. UV-vis absorption spectra confirm the double absorption edge of the t-o heterophase junction with enhanced visible absorption. Steady and transient PL spectra suggest improvements in carriers' separation/transfer in this t-o junction. As a result, the optimized t-o AIS shows superior photocatalytic H₂ evolution rates of 1022 μmol. g^-1. h^-1, 51.1 times that of t-AIS or 3.8 times that of o-AIS. This work is expected to provide new insight for designing ternary alloyed QDs with strongly coupled interfaces for effective H₂ generations.

In situ liquid transmission electron microscopy reveals self-assembly-driven nucleation in radiolytic synthesis of iron oxide nanoparticles in organic media

We have investigated the early stages of the formation of iron oxide nanoparticles from iron stearate precursors in the presence of sodium stearate in an organic solvent by in situ liquid phase transmission electron microscopy (IL-TEM). Before nucleation, we have evidenced the spontaneous formation of vesicular assemblies made of iron polycation-based precursors sandwiched between stearate layers. Nucleation of iron oxide nanoparticles occurs within the walls of the vesicles, which subsequently collapse upon the consumption of the iron precursors and the growth of the nanoparticles. We then evidenced that fine control of the electron dose, and therefore of the local concentration of reactive iron species in the vicinity of the nuclei, enables controlling crystal growth and selecting the morphology of the resulting iron oxide nanoparticles. Such a direct observation of the nucleation process templated by vesicular assemblies in a hydrophobic organic solvent sheds new light on the formation process of metal oxide nanoparticles and therefore opens ways for the synthesis of inorganic colloidal systems with tunable shape and size.

In Situ Direct Monitoring of the Morphological Transformation of Single Au Nanostars Induced by Iodide through Dual-Laser Dark-Field Microscopy: Unexpected Mechanism and Sensing Applications

Single nanoparticle imaging is a significant technique to help reveal the reaction mechanism and provides insight into the nanoparticle transformation. Here, we monitor the in situ morphological transformation of Au nanostars (GNSs) induced by iodide (I⁻) in real time using dark-field microscopy (DFM) with 638 nm red (R) and 534 nm green (G) laser coillumination. The two lasers are selected because the longitudinal localized surface plasmon resonance of GNSs is located at 638 nm and that for GNSs after transformation is at 534 nm. Interestingly, I⁻ can interact with GNSs directly without the engagement of other reagents, and upon increasing I⁻ concentrations, GNSs undergo color changes from red to orange, yellow, and green under DFM. Accordingly, green/red channel intensities (G/R ratios) are extracted by obtaining red and green channel intensities of single nanoparticles to weigh the morphological changes and quantify I⁻. A single nanoparticle sensor is constructed for I⁻ detection with a detection limit of 6.9 nM. Finally, a novel mechanism is proposed to elucidate this shape transformation. I⁻ absorbed onto the surface of GNSs binds with Au atoms to form AuI⁻, lowering the energy of its bond with other Au atoms, which facilitates the diffusion of this atom across the nanoparticle surface to low-energy sites at the concaves, thus deforming to spherical Au nanoparticles.

Copper Phosphonate Lamella Intermediates Control the Shape of Colloidal Copper Nanocrystals

Understanding the structure and behavior of intermediates in chemical reactions is the key to developing greater control over the reaction outcome. This principle is particularly important in the synthesis of metal nanocrystals (NCs), where the reduction, nucleation, and growth of the reaction intermediates will determine the final size and shape of the product. The shape of metal NCs plays a major role in determining their catalytic, photochemical, and electronic properties and, thus, the potential applications of the material. In this work, we demonstrate that layered coordination polymers, called lamellae, are reaction intermediates in Cu NC synthesis. Importantly, we discover that the lamella structure can be fine-tuned using organic ligands of different lengths and that these structural changes control the shape of the final NC. Specifically, we show that short-chain phosphonate ligands generate lamellae that are stable enough at the reaction temperature to facilitate the growth of Cu nuclei into anisotropic Cu NCs, being primarily triangular plates. In contrast, lamellae formed from long-chain ligands lose their structure and form spherical Cu NCs. The synthetic approach presented here provides a versatile tool for the future development of metal NCs, including other anisotropic structures.

Liquid-Phase Transmission Electron Microscopy for Reliable In Situ Imaging of Nanomaterials

Liquid-phase transmission electron microscopy (LPTEM) is a powerful in situ visualization technique for directly characterizing nanomaterials in the liquid state. Despite its successful application in many fields, several challenges remain in achieving more accurate and reliable observations. We present LPTEM in chemical and biological applications, including studies for the morphological transformation and dynamics of nanoparticles, battery systems, catalysis, biomolecules, and organic systems. We describe the possible interactions and effects of the electron beam on specimens during observation and present sample-specific approaches to mitigate and control these electron-beam effects. We provide recent advances in achieving atomic-level resolution for liquid-phase investigation of structures anddynamics. Moreover, we discuss the development of liquid cell platforms and the introduction of machine-learning data processing for quantitative and objective LPTEM analysis.

Surface plasmon resonance allied applications of silver nanoflowers synthesized from Breynia vitis-idaea leaf extract

An environmentally friendly, green synthesis process has been adopted to synthesize silver nanoparticles (AgNPs) in an aqueous solution from a new remedial plant. Breynia vitis-idaea leaves act like natural capping and reducing agents. The resulting AgNPs were characterized and analyzed using different characterization techniques, such as UV-Vis spectroscopy, X-ray diffraction, zeta potential, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The UV-Vis absorption spectrum showed high stability and a surface plasmon resonance (SPR) peak around 430 nm. The effects of several processing variables, such as reaction time, temperature, concentration and pH, were analyzed. High temperature and alkaline pH intensify the ability to form flower-shaped AgNPs with enhanced properties. AgNPs were investigated for antibacterial activity against Gram-negative E. coli bacterial strains with a 10 mm zone of inhibition. These AgNPs showed dye degradation up to 88% when an aqueous crystal violet dye solution was mixed with AgNPs as the catalyst. Further, AgNPs alone were effectively used in the detection of hydrogen peroxide (H2O2) in an aqueous medium with a LOD (limit of detection) of 21 μM, limit of quantification (LOQ) of 64 μM and a decrease in absorption intensity up to 89%. Based on these results, these AgNPs were effectively used in numerous fields, such as biomedical, water purification, antibacterial and sensing of H2O2.

Multi-step atomic mechanism of platinum nanocrystals nucleation and growth revealed by in-situ liquid cell STEM

The understanding of crystal growth mechanisms has broadened substantially. One significant advancement is based in the conception that the interaction between particles plays an important role in the growth of nanomaterials. This is in contrast to the classical model, which neglects this process. Direct imaging of such processes at atomic-level in liquid-phase is essential for establishing new theoretical models that encompass the full complexity of realistic scenarios and eventually allow for tailoring nanoparticle growth. Here, we investigate at atomic-scale the exact growth mechanisms of platinum nanocrystals from single atom to final crystals by in-situ liquid phase scanning transmission electron microscopy. We show that, after nucleation, the nanocrystals grow via two main stages: atomic attachment in the first stage, where the particles initially grow by attachment of the atoms until depletion of the surrounding zone. Thereafter, follows the second stage of growth, which is based on particle attachment by different atomic pathways to finally form mature nanoparticles. The atomic mechanisms underlying these growth pathways are distinctly different and have different driving forces and kinetics as evidenced by our experimental observations.

Versatile Printing of Substantial Liquid Cells for Efficiently Imaging In Situ Liquid-Phase Dynamics

Through its ability to image liquid-phase dynamics at nano/atomic-scale resolution, liquid-cell electron microscopy is essential for a wide range of applications, including wet-chemical synthesis, catalysis, and nanoparticle tracking, for which involved structural features are critical. However, statistical investigations by usual techniques remain challenging because of the difficulty in fabricating substantial liquid cells with appreciable efficiency. Here, we report a general approach for efficiently printing huge numbers of ready-to-use liquid cells (∼9000) within 30 s by electrospinning, with the unique feature of statistical liquid-phase studies requiring only one experimental time slot. Our solution efficiently resolves a complete transition picture of bubble evolution and also the induced nanoparticle motion. We statistically quantify the effect of the electron dose rate on the bubble variation and conclude that the bubble-driven nanoparticle motion is a ballistic-like behavior insignificant to morphological asymmetries. The versatile approach here is critical for statistical research, offering great opportunities in liquid-phase-associated dynamic studies.