In Situ Atomic-Level Tracking of Heterogeneous Nucleation in Nanocrystal Growth with an Isocyanide Molecular Probe

We report the use of 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a spectroscopic probe to study the heterogeneous nucleation and deposition of Pd on Ag nanocubes under different conditions by surface-enhanced Raman scattering. As a major advantage, the spectroscopic analysis can be performed in situ and in real time with the nanoparticles still suspended in the reaction solution. The success of this method relies on the distinctive stretching frequencies (ν_NC) of the isocyanide group in 2,6-DMPI when it binds to Ag and Pd atoms through σ donation and π-back-donation, respectively. Significantly, we discovered that ν_NC was sensitive to the arrangement of Pd adatoms on the Ag surface. For example, when the isocyanide group bound to one, two, and three Pd atoms, we would observe the atop, bridge, and hollow configurations, respectively, at different ν_NC frequencies. As such, the ν_NC band could serve as a characteristic reporter for the Pd adatoms being deposited onto different types of facets on Ag nanocubes with atomic-level sensitivity. When 2,6-DMPI molecules were introduced into the reaction solution, we further demonstrated in situ tracking of heterogeneous nucleation and early stage deposition of Pd on Ag nanocubes by monitoring the evolution of ν_NC bands for both Ag and Pd surface atoms as a function of reaction time. This in situ technique opens up the opportunity to investigate the roles played by reaction temperature and the type of Pd­(II) precursor in influencing the heterogeneous nucleation and growth of bimetallic nanocrystals. The sensitivity of isocyanide group to Pd atoms helps elucidate some of the details on the reduction, deposition, and diffusion processes involved in heterogeneous nucleation

Mechanistic Roles of Hydroxide in Controlling the Deposition of Gold on Colloidal Silver Nanocrystals

This article describes a systematic study of the roles played by hydroxide in controlling the deposition of Au on Ag nanocubes for the fabrication of diversified Ag–Au bimetallic nanocrystals. The synthesis simply involves the titration of aqueous HAuCl₄ into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (H₂Asc), NaOH, and poly­(vinylpyrrolidone) at room temperature. The OH^– ions from NaOH can affect the reduction kinetics of the Au­(III) precursor in a number of ways and thereby the deposition pathways of the Au atoms. First of all, the OH^– can accelerate the reduction kinetics by neutralizing H₂Asc into ascorbate monoanion (HAsc^–), the true player behind the reduction power of ascorbic acid. Second, the OH^– can neutralize the added HAuCl₄ and progressively transform AuCl₄^– into AuCl₃(OH)⁻, AuCl₂(OH)₂^–, AuCl­(OH)₃^–, or Au­(OH)₄^– through ligand exchange, generating Au­(III) precursors with increasingly lower reduction potentials and thus lower probability for galvanic replacement reaction with Ag nanocubes than AuCl₄^–. Third, the OH^– can react with the Ag⁺ ions released from the galvanic reaction to generate Ag₂O patches at the corners of Ag nanocubes. Our results indicate that the deposition of Au on Ag nanocubes can follow two distinct pathways depending on the initial pH of the reaction solution. When the initial pH is controlled in the range of 10.3–11.9, the reduction of Au­(III) is initiated by Ag nanocubes but dominated by HAsc^– afterward, leading to the formation of Ag@Au core-frame and then core–shell nanocubes. In contrast, if the initial pH is controlled in the range of 3.2–4.8, both the galvanic replacement with Ag nanocubes and the chemical reduction by HAsc^– contribute to the conversion of Au­(III) to Au atoms. The Ag⁺ ions released from the galvanic replacement can also be reduced by HAsc^– to transform Ag nanocubes into Ag@Ag–Au concave nanocubes with hollow interiors and alloyed walls

Syntheses, Plasmonic Properties, and Catalytic Applications of Ag–Rh Core-Frame Nanocubes and Rh Nanoboxes with Highly Porous Walls

We report a simple and general method for the production of Ag–Rh bimetallic nanostructures with a unique integration of the plasmonic and catalytic properties exemplified by these two metals, respectively. When a Rh­(III) precursor is titrated into a polyol suspension of Ag nanocubes held at 110 °C in the presence of ascorbic acid and poly­(vinylpyrrolidone), Rh atoms are generated and deposited on the nanocubes. When the amount of Rh­(III) precursor is relatively low, the Rh atoms tend to nucleate from the edges of the Ag nanocubes and then follow an island growth mode because of the relatively low temperature involved and the high cohesive energy of Rh. The Rh islands can be maintained with an ultrafine size of only several nanometers, presenting an extremely large specific surface area for catalytic applications. As the amount of Rh­(III) precursor is increased, the galvanic replacement reaction between the Rh­(III) and Ag nanocubes will kick in, leading to the formation of increasingly concaved side faces and an increase in surface coverage for the Rh islands. Meanwhile, the resultant Ag⁺ ions are reduced and deposited back onto the nanocubes, but among the Rh islands. By simply controlling the amount of Rh­(III) precursor, we observe the transformation of Ag nanocubes into Ag–Rh core-frame and then Ag–Rh hollow nanocubes with a highly porous surface. Upon selective removal of Ag by wet etching, the hollow nanocubes evolve into Ag–Rh and then Rh nanoboxes with highly porous walls. Although the Ag–Rh core-frame nanocubes show a unique integration of the plasmonic and catalytic properties characteristic of Ag and Rh, respectively, the Rh nanoboxes show remarkable activity toward the catalytic degradation of environmental pollutants such as organic dyes

Galvanic Replacement-Free Deposition of Au on Ag for Core–Shell Nanocubes with Enhanced Chemical Stability and SERS Activity

We report a robust synthesis of Ag@Au core–shell nanocubes by directly depositing Au atoms on the surfaces of Ag nanocubes as conformal, ultrathin shells. Our success relies on the introduction of a strong reducing agent to compete with and thereby block the galvanic replacement between Ag and HAuCl₄. An ultrathin Au shell of 0.6 nm thick was able to protect the Ag in the core in an oxidative environment. Significantly, the core–shell nanocubes exhibited surface plasmonic properties essentially identical to those of the original Ag nanocubes, while the SERS activity showed a 5.4-fold further enhancement owing to an improvement in chemical enhancement. The combination of excellent SERS activity and chemical stability may enable a variety of new applications

HAuCl₄: A Dual Agent for Studying the Chloride-Assisted Vertical Growth of Citrate-Free Ag Nanoplates with Au Serving as a Marker

We have investigated the vertical growth of citrate-free Ag nanoplates into truncated right bipyramids and twinned cubes with truncated corners in the presence of Cl^– ions at low and high concentrations, respectively, with Au serving as a marker for electron microscopy analysis. Both the Cl^– ions and Au atoms could be introduced through the use of HAuCl₄ as a dual agent. When HAuCl₄ was added into an aqueous mixture of citrate-free Ag nanoplates, ascorbic acid (AA), and poly­(vinylpyrrolidone), Au would be immediately formed and deposited on the surfaces of the nanoplates due to the reduction by both Ag and AA. The deposited Au could be easily resolved under STEM to reveal the growth patterns of the nanoplates. We found that the presence of Au did not change the growth pattern of the original Ag nanoplates. In contrast, the Cl^– ions could deterministically direct the formation of Ag nanoplates with a triangular or hexagonal shape, followed by their further growth into truncated right bipyramids or twinned cubes with truncated corners upon the introduction of AgNO₃. This work demonstrates, for the first time, that citrate-free Ag nanoplates could be transformed into right bipyramids or twinned cubes by controlling a single experimental parameter: the concentration of Cl^– ions in the growth solution. The mechanistic understanding represents a step forward toward the rational design and shape-controlled synthesis of nanocrystals with desired properties

Shore hardness value of 60Co γ-irradiated PMMA impregnated with two media.

Shore hardness value of 60Co γ-irradiated PMMA impregnated with two media.</p

Relative molecular mass differential distribution curves of PMMA.

Relative molecular mass differential distribution curves of PMMA.</p

Transformation of Ag Nanocubes into Ag–Au Hollow Nanostructures with Enriched Ag Contents to Improve SERS Activity and Chemical Stability

We report a strategy to complement the galvanic replacement reaction between Ag nanocubes and HAuCl₄ with co-reduction by ascorbic acid (AA) for the formation of Ag–Au hollow nanostructures with greatly enhanced SERS activity. Specifically, in the early stage of synthesis, the Ag nanocubes are sharpened at corners and edges because of the selective deposition of Au and Ag atoms at these sites. In the following steps, the pure Ag in the nanocubes is constantly converted into Ag⁺ ions to generate voids owing to the galvanic reaction with HAuCl₄, but these released Ag⁺ ions are immediately reduced back to Ag atoms and are co-deposited with Au atoms onto the nanocube templates. We observe distinctive SERS properties for the Ag–Au hollow nanostructures at visible and near-infrared excitation wavelengths. When plasmon damping is eliminated by using an excitation wavelength of 785 nm, the SERS activity of the Ag–Au hollow nanostructures is 15- and 33-fold stronger than those of the original Ag nanocubes and the Ag–Au nanocages prepared by galvanic replacement without co-reduction, respectively. Additionally, Ag–Au hollow nanostructures embrace considerably improved stability in an oxidizing environment such as aqueous H₂O₂ solution. Collectively, our work suggests that the Ag–Au hollow nanostructures will find applications in SERS detection and imaging

Evolution of transmittance with various 60Co γ irradiation doses for PMMA.

Evolution of transmittance with various 60Co γ irradiation doses for PMMA.</p

Generation of Enzymatic Hydrogen Peroxide to Accelerate the Etching of Silver Nanocrystals with Selectivity

We report a simple and versatile system for generating highly concentrated H₂O₂ on the surface of nanoparticles through enzymatic oxidation of glucose. It involves immobilization of glucose oxidase, a negatively charged enzyme, on the surface of a positively charged metal nanoparticle via electrostatic attraction. Upon the introduction of glucose at a concentration of 1.7 mM, this system is able to produce enzymatic H₂O₂ on the surface of the nanoparticle, with oxidation power equivalent to that of aqueous H₂O₂ at a concentration of 5 M when it is directly added into the reaction solution. We have evaluated the system for the etching of both twinned and single-crystal Ag nanocubes. We identified that the highly localized and concentrated H₂O₂ generated on the surfaces of Ag twinned cubes would lead to selective etching from the {111} facets parallel to the twin plane, in a fashion identical to the growth process but in the reversed order. For Ag single-crystals nanocubes, the etching would initiate from the corners to gradually transform the cubes into spheres. This study offers the opportunity to control the etching of metal nanocrystals with selectivity for elucidating the mechanism and diversifying the nanocrystals