Aqueous-Phase Reactions on Hollow Silica-Encapsulated Semiconductor Nanoheterostructures

We introduce a facile and robust methodology for the aggregation-free aqueous-phase synthesis of hierarchically complex metal–semiconductor heterostructures. By encapsulating semiconductor nanostructures within a porous SiO₂ shell with a hollow interior, we can isolate each individual particle while allowing it access to metal precursors for subsequent metal growth. We illustrate this by Pt deposition on CdSe-seeded CdS tetrapods, which we found to be facilitated via the surprising formation of a thin interfacial layer of PtS coated onto the original CdS surface. We took advantage of this unique architecture to perform cation exchange reactions with Ag⁺ and Pd²⁺, thus demonstrating the feasibility of achieving such transformations in complex metal–semiconductor nanoparticle systems

Understanding the Growth Mechanism of α‑Fe₂O₃ Nanoparticles through a Controlled Shape Transformation

The growth mechanism of α-Fe₂O₃ nanoparticles in solution has been elucidated from a comprehensive analysis on the shape and morphology of obtained particles. It is found that the hydrothermal synthesis of α-Fe₂O₃ nanoparticles from ferric chloride precursor follows two stages: the initial nucleation of α-Fe₂O₃ nuclei and the subsequent ripening of nuclei into various shapes. The initial nucleation involves the formation of polynuclears from hydrolysis of Fe³⁺ salt precursors, followed by the growth of β-FeOOH nanowires with an akaganeite structure, and then into two-line ferrihydrite nanoparticles through a dissolution–recrystallization process. In the subsequent ripening process, we suggest that the formation of large α-Fe₂O₃ particles follows the dissolution of two-line ferrihydrite and then precipitation and oriented aggregation of α-Fe₂O₃ nuclei rather than the oriented aggregation of ferrihydrite nanoparticles followed by phase transformation. The oriented attachment of {104} facets between α-Fe₂O₃ nuclei results in the formation of oblate spheroid nanocrystals (nanoflower-like particles) either in ethanol or in the beginning stage where the particles first undergo oriented aggregation. With the addition of water, Ostwald ripening process (dissolution–reprecipitation) will play an important role to convert the assembly of nanoflowers into a 3D rhombohedral shape with well-defined edges and surfaces. The proposed mechanism in this article not only allows us to better control the synthesis of iron oxide particles with designed shapes and structures but also provides guidance for theoretical simulations on the oriented attachment process for hematite formation

Understanding the Growth Mechanism of α‑Fe₂O₃ Nanoparticles through a Controlled Shape Transformation

The growth mechanism of α-Fe₂O₃ nanoparticles in solution has been elucidated from a comprehensive analysis on the shape and morphology of obtained particles. It is found that the hydrothermal synthesis of α-Fe₂O₃ nanoparticles from ferric chloride precursor follows two stages: the initial nucleation of α-Fe₂O₃ nuclei and the subsequent ripening of nuclei into various shapes. The initial nucleation involves the formation of polynuclears from hydrolysis of Fe³⁺ salt precursors, followed by the growth of β-FeOOH nanowires with an akaganeite structure, and then into two-line ferrihydrite nanoparticles through a dissolution–recrystallization process. In the subsequent ripening process, we suggest that the formation of large α-Fe₂O₃ particles follows the dissolution of two-line ferrihydrite and then precipitation and oriented aggregation of α-Fe₂O₃ nuclei rather than the oriented aggregation of ferrihydrite nanoparticles followed by phase transformation. The oriented attachment of {104} facets between α-Fe₂O₃ nuclei results in the formation of oblate spheroid nanocrystals (nanoflower-like particles) either in ethanol or in the beginning stage where the particles first undergo oriented aggregation. With the addition of water, Ostwald ripening process (dissolution–reprecipitation) will play an important role to convert the assembly of nanoflowers into a 3D rhombohedral shape with well-defined edges and surfaces. The proposed mechanism in this article not only allows us to better control the synthesis of iron oxide particles with designed shapes and structures but also provides guidance for theoretical simulations on the oriented attachment process for hematite formation

Unusual Selectivity of Metal Deposition on Tapered Semiconductor Nanostructures

We describe a surfactant-driven method to synthesize highly monodisperse CdSe-seeded CdS nanoheterostructures with conelike, tapered geometries in order to examine the effects of shape on the location-specific deposition of Au under ambient conditions. Although preferential metal deposition at surface defect sites are generally expected, we found suprisingly that Au growth at the side facets of tapered linear and branched structures was significantly suppressed. Further investigation revealed this to be due to a highly efficient electrochemical Ostwald ripening process which was previously thought not to occur in branched nanostructures such as tetrapods. We exploited this phenomenon to fabricate uniform asymmetrically tipped CdSe-seeded CdS tetrapods with conelike arms, where a solitary large Au tip is found on one of the arms while the other three arms bear Ag₂S tips. Importantly, this work presents a synthetic route toward the selective deposition of metals onto branched semiconductor nanostructures whose arms have nearly symmetric reactivity

Real-Time Imaging of the Formation of Au–Ag Core–Shell Nanoparticles

We study the overgrowth process of silver-on-gold nanocubes in dilute, aqueous silver nitrate solution in the presence of a reducing agent, ascorbic acid, using <i>in situ</i> liquid-cell electron microscopy. Au–Ag core–shell nanostructures were formed via two mechanistic pathways: (1) nuclei coalescence, where the Ag nanoparticles absorbed onto the Au nanocubes, and (2) monomer attachment, where the Ag atoms epitaxially deposited onto the Au nanocubes. Both pathways lead to the same Au–Ag core–shell nanostructures. Analysis of the Ag deposition rate reveals the growth modes of this process and shows that this reaction is chemically mediated by the reducing agent

Real-Time Imaging of the Formation of Au–Ag Core–Shell Nanoparticles

We study the overgrowth process of silver-on-gold nanocubes in dilute, aqueous silver nitrate solution in the presence of a reducing agent, ascorbic acid, using <i>in situ</i> liquid-cell electron microscopy. Au–Ag core–shell nanostructures were formed via two mechanistic pathways: (1) nuclei coalescence, where the Ag nanoparticles absorbed onto the Au nanocubes, and (2) monomer attachment, where the Ag atoms epitaxially deposited onto the Au nanocubes. Both pathways lead to the same Au–Ag core–shell nanostructures. Analysis of the Ag deposition rate reveals the growth modes of this process and shows that this reaction is chemically mediated by the reducing agent

Effects of pH (A) and temperature (B) on the activity of TreS.

<p>(A) The enzyme activity at various pH values were examined at the maltose concentration of 200 mM and 45 °C for 30 min. (B) The enzyme activity at various temperature were examined at the maltose concentration of 200 mM and pH 9.0 for 30 min. The average of triplicate experiments is presented. </p

Real-Time Imaging of the Formation of Au–Ag Core–Shell Nanoparticles

We study the overgrowth process of silver-on-gold nanocubes in dilute, aqueous silver nitrate solution in the presence of a reducing agent, ascorbic acid, using <i>in situ</i> liquid-cell electron microscopy. Au–Ag core–shell nanostructures were formed via two mechanistic pathways: (1) nuclei coalescence, where the Ag nanoparticles absorbed onto the Au nanocubes, and (2) monomer attachment, where the Ag atoms epitaxially deposited onto the Au nanocubes. Both pathways lead to the same Au–Ag core–shell nanostructures. Analysis of the Ag deposition rate reveals the growth modes of this process and shows that this reaction is chemically mediated by the reducing agent

SDS-PAGE analysis of the purified recombinant TreS.

<p>Lane M is protein molecular weight markers. Lane 1 is the crude extract of the recombinant strain <i>E. coli</i> BL21 with pET22b. Lane 2 is the crude extract of the recombinant strain <i>E. coli</i> BL21 with pET22b-<i>treS</i>. Lane 3 is the recombinant enzyme TreS purified using Ni-NTA affinity chromatography. The arrow indicates the recombinant TreS in this study. </p

Comparative modeling of TreS based on known trehalulose synthase template.

<p>(A) Model of TreS (blue) was superimposed with 2PWG (red). Five key amino acids (His104, Asp200, Glu254, His326, and Asp327) in the active center of 2PWG are labeled with marked sticks inside the (α/β)8 barrel catalytic domain. (B) Side view of the surface model for TreS and its five conserved amino acids (His172, Asp201, Glu251, His318, and Asp319) in the active center. The key amino acids are indicated by sticks and name of residue.</p