Real-Time Observation of Crystal Evaporation in a Metal Phosphate at High Temperature

A number of experimental studies on crystal growth have been performed in connection with a variety of crystalline systems ranging from simple oxides to complex organic compounds. In contrast, little is known regarding how crystals evaporate. By using a combination of real-time high-resolution electron microscopy at high temperature, image simulations, and density functional theory calculations, we demonstrate the evaporation of metal-phosphate nanocrystals with flat surfaces at atomic resolution. In situ imaging and direct comparison with image simulation results reveal that, while a layer-by-layer lateral process is macroscopically maintained, the cations preferentially evaporate over the (PO₄)^3– tetrahedral anions from shrinking ledges. The present observations provide the first atomic-scale experimental details of the evaporation of complex oxides, emphasizing the value of direct visualization in real time

Capturing Heterogeneous Nucleation of Nanoscale Pits and Subsequent Crystal Shrinkage during Ostwald Ripening of a Metal Phosphate

It has been generally accepted that crystal shrinkage during Ostwald ripening can be understood simply as a reverse process of crystal growth, and as a result, little attention has been paid to shrinkage behavior. The entire microstructure of polycrystalline materials, however, forms as a consequence of both growing and shrinking crystals. Thus, scrutiny of shrinking characteristics in addition to growth aspects is essential for a complete understanding of the evolution of microstructure during Ostwald ripening. By capturing real-time <i>in situ</i> high-resolution electron micrographs at high temperature, we herein demonstrate the shrinkage behavior of nanocrystals embedded in a solid crystalline matrix during the ripening process of a metal phosphate. Unlike typical crystal growth behavior based on two-dimensional homogeneous nucleation, heterogeneous types of nucleation with nanoscale pits at solid–solid interfaces (or crystal edges) are observed to dominantly occur during shrinkage of the crystals. The findings of this study suggest that crystal shrinkage proceeds with a lower activation energy barrier than that of crystal growth, although both crystal growth and shrinkage take place at the same time during Ostwald ripening

Capturing Heterogeneous Nucleation of Nanoscale Pits and Subsequent Crystal Shrinkage during Ostwald Ripening of a Metal Phosphate

It has been generally accepted that crystal shrinkage during Ostwald ripening can be understood simply as a reverse process of crystal growth, and as a result, little attention has been paid to shrinkage behavior. The entire microstructure of polycrystalline materials, however, forms as a consequence of both growing and shrinking crystals. Thus, scrutiny of shrinking characteristics in addition to growth aspects is essential for a complete understanding of the evolution of microstructure during Ostwald ripening. By capturing real-time <i>in situ</i> high-resolution electron micrographs at high temperature, we herein demonstrate the shrinkage behavior of nanocrystals embedded in a solid crystalline matrix during the ripening process of a metal phosphate. Unlike typical crystal growth behavior based on two-dimensional homogeneous nucleation, heterogeneous types of nucleation with nanoscale pits at solid–solid interfaces (or crystal edges) are observed to dominantly occur during shrinkage of the crystals. The findings of this study suggest that crystal shrinkage proceeds with a lower activation energy barrier than that of crystal growth, although both crystal growth and shrinkage take place at the same time during Ostwald ripening

Transformative Two-Dimensional Layered Nanocrystals

Regioselective chemical reactions and structural transformations of two-dimensional (2D) layered transition-metal chalcogenide (TMC) nanocrystals are described. Upon exposure of 2D TiS2 nanodiscs to a chemical stimulus, such as Cu ion, selective chemical reaction begins to occur at the peripheral edges. This edge reaction is followed by ion diffusion, which is facilitated by interlayer nanochannels and leads to the formation of a heteroepitaxial TiS2–Cu2S intermediate. These processes eventually result in the generation of a single-crystalline, double-convex toroidal Cu2S nanostructure. Such 2D regioselective chemical reactions also take place when other ionic reactants are used. The observations made and chemical principles uncovered in this effort indicate that a general approach exists for building various toroidal nanocrystals of substances such as Ag2S, MnS, and CdS

Heterogeneous Binuclear Complexation of 1,3-Alternate Calix[4]-Bis<i>-</i>Crown Bearing Two Different Crown Rings

Heterogeneous Binuclear Complexation of 1,3-Alternate Calix[4]-Bis-Crown Bearing Two Different Crown Ring

In Situ TEM Observation of Heterogeneous Phase Transition of a Constrained Single-Crystalline Ag₂Te Nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices

Phase-Tuned Tetrapod-Shaped CdTe Nanocrystals by Ligand Effect

We achieved extensive control of the morphology and phase of colloidal cadmium telluride (CdTe) nanocrystals (NCs) using two different types of capping ligands, namely, amines and phosphonic acids having various alkyl chains ranging from butyl (C4) to octadecyl (C18). Among the alkylamines, the largest tetrapods are preferentially produced using octylamine (C8), suggesting that the steric hindrance of the alkyl chains optimizes the growth condition of the tetrapods. The phase of the tetrapod branches can be tuned from wurtzite to zinc blende by enhancing the steric hindrance of the tertiary alkyl phosphine used as the Te activating ligand. The in situ synthesis of tetrapod CdTe NCs on carbon nanotubes (CNTs) was performed using the two types of ligands. All of the CdTe NCs grown on the CNTs consisted of wurtzite phase, and the largest tetrapods are produced using phosphonic acid having the alkyl chain, C10

In Situ TEM Observation of Heterogeneous Phase Transition of a Constrained Single-Crystalline Ag₂Te Nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices

Unveiling Chemical Reactivity and Structural Transformation of Two‑Dimensional Layered Nanocrystals

Two-dimensional (2D) layered nanostructures are emerging fast due to their exceptional materials properties. While the importance of physical approaches (e.g., guest intercalation and exfoliation) of 2D layered nanomaterials has been recognized, an understanding of basic chemical reactions of these materials, especially in nanoscale regime, is obscure. Here, we show how chemical stimuli can influence the fate of reaction pathways of 2D layered nanocrystals. Depending on the chemical characteristics (Lewis acid (¹O₂) or base (H₂O)) of external stimuli, TiS₂ nanocrystal is respectively transformed to either a TiO₂ nanodisc through a “compositional metathesis” or a TiO₂ toroid through multistage “edge-selective structural transformation” processes. These chemical reactions can serve as the new design concept for functional 2D layered nanostructures. For example, TiS_2(disc)-TiO_2(shell) nanocrystal constitutes a high performance type II heterojunction which not only a wide range solar energy coverage (∼80%) with near-infrared absorption edge, but also possesses enhanced electron transfer property

Phase-Transition Mo_1–<i>x</i>V_<i>x</i>Se₂ Alloy Nanosheets with Rich V–Se Vacancies and Their Enhanced Catalytic Performance of Hydrogen Evolution Reaction

Alloys of transition-metal dichalcogenide can display distinctive phase evolution because of their two-dimensional structures. Herein, we report the colloidal synthesis of Mo1–xVxSe2 alloy nanosheets with full composition tuning. Alloying led to a phase transition at x = 0.7 from the semiconducting 2H phase MoSe2 to the metallic 1T phase VSe2. It also produced significant V and Se vacancies, which became the richest in the 2H phase at x = 0.3–0.5. Extensive spin-polarized density functional theory calculations consistently predicted the 2H–1T phase transition at x = 0.7, in agreement with the experimental results. The vacancy formation energy also supports the formation of V and Se vacancies. Alloying in the 2H phase enhanced the electrocatalytic performance toward hydrogen evolution reaction (HER) at x = 0.3 (in 0.5 M H2SO4) or 0.4 (in 1 M KOH). The Gibbs free energy along the HER pathway indicates that this maximum performance is due to the highest concentration of active V and Se vacancy sites