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

Measurement of the Dewetting, Nucleation, and Deactivation Kinetics of Carbon Nanotube Population Growth by Environmental Transmission Electron Microscopy

Understanding the collective growth of carbon nanotube (CNT) populations is key to tailoring their properties for many applications. During the initial stages of CNT growth by chemical vapor deposition, catalyst nanoparticle formation by thin-film dewetting and the subsequent CNT nucleation processes dictate the CNT diameter distribution, areal density, and alignment. Herein, we use <i>in situ</i> environmental transmission electron microscopy (E-TEM) to observe the catalyst annealing, growth, and deactivation stages for a population of CNTs grown from a thin-film catalyst. Complementary <i>in situ</i> electron diffraction and TEM imaging show that, during the annealing step in hydrogen, reduction of the iron oxide catalyst is concomitant with changes in the thin-film morphology; complete dewetting and the formation of a population of nanoparticles is only achieved upon the introduction of the carbon source, acetylene. The dewetting kinetics, i.e., the appearance of distinct nanoparticles, exhibits a sigmoidal (autocatalytic) curve with 95% of all nanoparticles appearing within 6 s. After nanoparticles form, they either nucleate CNTs or remain inactive, with incubation times measured to be as small as 3.5 s. Via E-TEM we also directly observe the crowding and self-alignment of CNTs after dewetting and nucleation. In addition, <i>in situ</i> electron energy loss spectroscopy reveals that the collective rate of carbon accumulation decays exponentially. We conclude that the kinetics of catalyst formation and CNT nucleation must both be addressed in order to achieve uniform and high CNT density, and their transient behavior may be a primary cause of the well-known nonuniform density of CNT forests

Measurement of the Dewetting, Nucleation, and Deactivation Kinetics of Carbon Nanotube Population Growth by Environmental Transmission Electron Microscopy

Understanding the collective growth of carbon nanotube (CNT) populations is key to tailoring their properties for many applications. During the initial stages of CNT growth by chemical vapor deposition, catalyst nanoparticle formation by thin-film dewetting and the subsequent CNT nucleation processes dictate the CNT diameter distribution, areal density, and alignment. Herein, we use <i>in situ</i> environmental transmission electron microscopy (E-TEM) to observe the catalyst annealing, growth, and deactivation stages for a population of CNTs grown from a thin-film catalyst. Complementary <i>in situ</i> electron diffraction and TEM imaging show that, during the annealing step in hydrogen, reduction of the iron oxide catalyst is concomitant with changes in the thin-film morphology; complete dewetting and the formation of a population of nanoparticles is only achieved upon the introduction of the carbon source, acetylene. The dewetting kinetics, i.e., the appearance of distinct nanoparticles, exhibits a sigmoidal (autocatalytic) curve with 95% of all nanoparticles appearing within 6 s. After nanoparticles form, they either nucleate CNTs or remain inactive, with incubation times measured to be as small as 3.5 s. Via E-TEM we also directly observe the crowding and self-alignment of CNTs after dewetting and nucleation. In addition, <i>in situ</i> electron energy loss spectroscopy reveals that the collective rate of carbon accumulation decays exponentially. We conclude that the kinetics of catalyst formation and CNT nucleation must both be addressed in order to achieve uniform and high CNT density, and their transient behavior may be a primary cause of the well-known nonuniform density of CNT forests

Atomically Visualizing Elemental Segregation-Induced Surface Alloying and Restructuring

Using in situ transmission electron microscopy that spatially and temporally resolves the evolution of the atomic structure in the surface and subsurface regions, we find that the surface segregation of Au atoms in a Cu­(Au) solid solution results in the nucleation and growth of a (2 × 1) missing-row reconstructed, half-unit-cell thick L1₂ Cu₃Au­(110) surface alloy. Our in situ electron microscopy observations and atomistic simulations demonstrate that the (2 × 1) reconstruction of the Cu₃Au­(110) surface alloy remains as a stable surface structure as a result of the favored Cu–Au diatom configuration

Compositional Inhomogeneity of Multinary Semiconductor Nanoparticles: A Case Study of Cu₂ZnSnS₄

Probe-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the inter- and intraparticle compositional inhomogeneity of multinary Cu₂ZnSnS₄ (CZTS) nanoparticles. CZTS nanoparticles were prepared following three distinct synthesis protocols described in the literature. Strong fluctuations in composition were observed for Cu and Zn in individual nanoparticles, independent of the synthesis method. Certain particles have regions that have compositions close to that of Cu₂SnS₃, as well as, in the extreme case, the presence of nearly pure ZnS species. This is an observation that has not been reported in prior studies of these systems and underscores the need to both more carefully study the polydispersity of multinary semiconductor nanoparticles (MSNs) and to improve synthetic protocols and characterization of MSNs. Notablydespite the observation of compositional fluctuations in individual nanoparticlesreactive sintering in Se vapor was shown to reduce the nanoscale compositional fluctuations in the resulting sintered grains, facilitating the use of these heterogeneous particles in optoelectronic devices

Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by <i>In Situ</i> Environmental TEM

Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy <i>in situ</i> for cobalt catalysts under reaction conditions. Studies reveal quantitative correlation of the cobalt valence states with the particles’ nanoporous structures. The <i>in situ</i> experiments were performed on nanoporous cobalt particles coated with silica, while a 15 mTorr hydrogen environment was maintained at various temperatures (300–600 °C). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. <i>In situ</i> mapping of the valence state and the corresponding nanoporous structures allows quantitative analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, for example, for Fischer–Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms

Atomic Insight into the Layered/Spinel Phase Transformation in Charged LiNi_0.80Co_0.15Al_0.05O₂ Cathode Particles

Layered LiNi_0.80Co_0.15Al_0.05O₂ (NCA) holds great promise as a potential cathode material for high energy density lithium ion batteries. However, its high capacity is heavily dependent on the stability of its layered structure, which suffers from a severe structure degradation resulting from a not fully understood layered → spinel phase transformation. Using high-resolution transmission electron microscopy and electron diffraction, we probe the atomic structure evolution induced by the layered → spinel phase transformation in the NCA cathode. We show that the phase transformation results in the development of a particle structure with the formation of complete spinel, spinel domains, and intermediate spinel from the surface to the subsurface region. The lattice planes of the complete and intermediate spinel phases are highly interwoven in the subsurface region. The layered → spinel transformation occurs via the migration of transition metal (TM) atoms from the TM layer into the lithium layer. Incomplete migration leads to the formation of the intermediate spinel phase, which is featured by tetrahedral occupancy of TM cations in the lithium layer. The crystallographic structure of the intermediate spinel is discussed and verified by the simulation of electron diffraction patterns

Mechanism and Enhanced Yield of Carbon Nanotube Growth on Stainless Steel by Oxygen-Induced Surface Reconstruction

It is well-known that carbon nanotubes (CNTs) can be grown directly on the surface of stainless steel (SS) alloys, because the native composition of SS contains elements that seed CNT growth upon hydrocarbon exposure at elevated temperature. Often such methods use acid immersion or oxidation in air to treat the surface prior to hydrocarbon exposure for CNT growth. However, there lacks a general understanding of how the surface chemistry and morphology influences the nucleation and growth of CNTs. Using environmental transmission electron microscopy, we observe that CNT growth is enabled by surface reconstruction of SS upon oxygen exposure at elevated temperature, followed by further breakup of the surface upon reduction, and subsequent CNT nucleation and growth upon hydrocarbon exposure. Using electron energy loss spectroscopy, we find that catalyst particles consist of both pure iron as well as iron alloys such as Fe–Cr and Fe–Ni. We use these insights to study the synthesis of CNTs on bulk net-shaped porous SS materials and show that annealing of the SS at 1000 °C in air prior to CVD using an ethylene feedstock mixture produces a 70-fold increase in CNT yield. Our findings demonstrate how process conditions can be designed for efficient manufacturing of CNT-enhanced stainless steel materials, and guide improved understanding of CNT growth on other industrially relevant metal substrates

Structural Modification of Graphene Sheets to Create a Dense Network of Defect Sites

Pt/graphene composites were synthesized by loading platinum nanoparticles onto graphene and etched at 1000 °C in a hydrogen atmosphere. This results in the formation of a dense array of nanostructured defect sites in the graphene, including trenches, nanoribbons, islands, and holes. These defect sites result in an increase in the number of unsaturated carbon atoms and, consequently, enhance the interaction of the CO₂ molecules with the etched graphene. This leads to a high capacity for storing CO₂; 1 g of the etched samples can store up to 76.3 cm³ of CO₂ at 273 K under ambient pressure