Synthesis of Ag Nanocubes 18–32 nm in Edge Length: The Effects of Polyol on Reduction Kinetics, Size Control, and Reproducibility

This article describes a robust method for the facile synthesis of small Ag nanocubes with edge lengths controlled in the range of 18–32 nm. The success of this new method relies on the substitution of ethylene glycol (EG)the solvent most commonly used in a polyol synthesiswith diethylene glycol (DEG). Owing to the increase in hydrocarbon chain length, DEG possesses a higher viscosity and a lower reducing power relative to EG. As a result, we were able to achieve a nucleation burst in the early stage to generate a large number of seeds and a relatively slow growth rate thereafter; both factors were critical to the formation of Ag nanocubes with small sizes and in high purity (>95%). The edge length of the Ag nanocubes could be easily tailored in the range of 18–32 nm by quenching the reaction at different time points. For the first time, we were able to produce uniform sub-20 nm Ag nanocubes in a hydrophilic medium and on a scale of ∼20 mg per batch. It is also worth pointing out that the present protocol was remarkably robust, showing good reproducibility between different batches and even for DEGs obtained from different vendors. Our results suggest that the high sensitivity of synthesis outcomes to the trace amounts of impurities in a polyol, a major issue for reproducibility and scale up synthesis, did not exist in the present system

Boosting the High Working Voltage of an Aqueous Symmetric Supercapacitor by a Phosphotungstic Acid-Based Coordination Polymer Coating with Polypyrrole

The low working voltage limits the energy density and feasibility of practical applications of aqueous supercapacitors (SCs) to some extent. Herein, new CuPW12@PPy (n, n = 1, 2, 3) nanocomposites were designed and fabricated by involving the hydrothermal synthesis of crystalline H4[Cu2(bix)4][PW12O40]2·8H2O (CuPW12) and subsequently pyrrole in situ oxidation polymerization on the CuPW12 surface. These were then used as electrode materials to widen the working voltage of SC. As expected, CuPW12@PPy (n, n = 1, 2, 3) can operate stably within −0.6 to 1.0 V while inhibiting the hydrogen evolution reaction, which exhibits higher specific capacitance in 2 M H3PO4. Specifically, CuPW12@PPy (2) shows a 711.2 F g–1 specific capacitance at 1.5 A g–1, attributed to the high ion/electron transportation and their synergy from the conductive PPy covering the CuPW12 surfaces. Finally, the assembled symmetric SC cell can operate at 1.6 V and deliver a 43.67 Wh kg–1 energy density and 1280 W kg–1 power density at 1.0 A g–1 and a 91.3% capacitance retention at 5 A g–1 after 10,000 cycles

Transformation of Pd Nanocubes into Octahedra with Controlled Sizes by Maneuvering the Rates of Etching and Regrowth

Palladium octahedra with controlled edge lengths were obtained from Pd cubes of a single size. The success of this synthesis relies on a transformation involving oxidative etching and regrowth. Because the {100} side faces of the Pd nanocubes were capped by Br^– ions, Pd atoms were removed from the corners during oxidative etching, and the resultant Pd²⁺ ions could be reduced and deposited back onto the nanocubes, but preferentially on the {100} facets. We could control the ratio of the etching and regrowth rates (<i>R</i>_etching and <i>R</i>_regrowth) simply by varying the amount of HCl added to the reaction solution. With a large amount of HCl, etching dominated the process (<i>R</i>_etching ≫ <i>R</i>_regrowth), resulting in the formation of Pd octahedra with an edge length equal to 70% of that of the cubes. In contrast, with a small amount of HCl, all of the newly formed Pd²⁺ ions could be quickly reduced and deposited back onto the Pd cubes. In this case, <i>R</i>_etching ≈ <i>R</i>_regrowth, and the resultant Pd octahedra had roughly the same volume as the starting cubes, together with an edge length equal to 130% of that of the cubes. When the amount of HCl was between these two extremes, we obtained Pd octahedra with intermediate edge lengths. This work not only advances our understanding of oxidative etching in nanocrystal synthesis but also offers a powerful means for controlling the shape and size of metal nanocrystals simply by adjusting the rates of etching and regrowth

Quantifying the Coverage Density of Poly(ethylene glycol) Chains on the Surface of Gold Nanostructures

The coverage density of poly(ethylene glycol) (PEG) is a key parameter in determining the efficiency of PEGylation, a process pivotal to <i>in vivo</i> delivery and targeting of nanomaterials. Here we report four complementary methods for quantifying the coverage density of PEG chains on various types of Au nanostructures by using a model system based on HS–PEG–NH₂ with different molecular weights. Specifically, the methods involve reactions with fluorescamine and ninhydrin, as well as labeling with fluorescein isothiocyanate (FITC) and Cu²⁺ ions. The first two methods use conventional amine assays to measure the number of unreacted HS–PEG–NH₂ molecules left behind in the solution after incubation with the Au nanostructures. The other two methods involve coupling between the terminal −NH₂ groups of adsorbed −S–PEG–NH₂ chains and FITC or a ligand for Cu²⁺ ion, and thus pertain to the “active” −NH₂ groups on the surface of a Au nanostructure. We found that the coverage density decreased as the length of PEG chains increased. A stronger binding affinity of the initial capping ligand to the Au surface tended to reduce the PEGylation efficiency by slowing down the ligand exchange process. For the Au nanostructures and capping ligands we have tested, the PEGylation efficiency decreased in the order of citrate-capped nanoparticles > PVP-capped nanocages ≈ CTAC-capped nanoparticles ≫ CTAB-capped nanorods, where PVP, CTAC, and CTAB stand for poly(vinyl pyrrolidone), cetyltrimethylammonium chloride, and cetyltrimethylammonium bromide, respectively

Gold–Silver Hybrid Nanostructures for Efficient Near-Infrared Photothermal Conversion: Core–Shell Configuration of Multipod and Hollow Cage

Gold–silver hybrid nanostructures have emerged as promising candidates for efficient near-infrared (NIR) photothermal conversion due to their unique optical and electronic properties. In this study, we report on the synthesis and characterization of gold–silver core–shell nanostructures with Au multipods as the core and Ag hollow cage as the shell, exhibiting strong absorption in the NIR region, which is attributed to the coupled localized surface plasmon resonance (LSPR) effect. Benefiting from its large surface area and porous structure, an optimized photothermal conversion efficiency of 68.5% is achieved, evaluated using a water suspension under an 808 nm laser at a power density of 1.0 W cm–2. The photothermal stability was also investigated, revealing good durability after multiple cycles of heating and cooling. Our study demonstrates the potential of gold–silver core–shell hybrid nanostructures involving both multipods and hollow cages for efficient NIR photothermal conversion applications. These findings pave the way for further optimization of these nanostructures for various biomedical and industrial applications

Seed-Mediated Growth of Gold Nanocrystals: Changes to the Crystallinity or Morphology as Induced by the Treatment of Seeds with a Sulfur Species

We report our observation of changes to the crystallinity or morphology during seed-mediated growth of Au nanocrystals. When single-crystal Au seeds with a spherical or rod-like shape were treated with a chemical species such as S₂O₃^2– ions, twin defects were developed during the growth process to generate multiply twinned nanostructures. X-ray photoelectron spectroscopy analysis indicated that the S₂O₃^2– ions were chemisorbed on the surfaces of the seeds during the treatment. The chemisorbed S₂O₃^2– ions somehow influenced the crystallization of Au atoms added onto the surface during a growth process, leading to the formation of twin defects. In contrast to the spherical and rod-like Au seeds, the single-crystal structure was retained to generate a concave morphology when single-crystal Au seeds with a cubic or octahedral shape were used for a similar treatment and then seed-mediated growth. The different outcomes are likely related to the difference in spatial distribution of S₂O₃^2– ions chemisorbed on the surface of a seed. This approach based on surface modification is potentially extendable to other noble metals for engineering the crystallinity and morphology of nanocrystals formed via seed-mediated growth

Hollow Graphitized Carbon Nanocage Supported Pd Catalyst with Excellent Electrocatalytic Activity for Ethanol Oxidation

Low cost, high activity and reliable stability are significant to the commercialization of fuel cell electrocatalysts. However, the synthesis of non-Pt anode catalysts with low cost, excellent performance and reliable stability is still a great challenge. Herein, we developed hollow graphitized carbon nanocages for improving the electrocatalytic performance of Pd nanoparticles (NPs) toward ethanol oxidation. A mild method was utilized for the preparation of hollow graphitized carbon nanocages (CN) using magnesium oxide as a sacrificial template without high-temperature processing. The CN can act as high-efficiency support for the distribution of Pd NPs. Pd NPs decorated on CN exhibited high catalytic performance with the current density of 2411.5 mA mg^–1 for ethanol oxidation reaction, which is 1.84 and 4.42 times higher than reduced graphene oxide (1308.5 mA mg^–1) and C (545.2 mA mg^–1) as supports, respectively. The Pd/CN with excellent catalytic performance can be attributed to the CN, including the large surface area with a mesoporous hollow structure, uniform dispersion of Pd NPs, and excellent electrical conductivity. This study may offer new insights for the development of highly effective carbon-based support for applications in ethanol oxidation