Yiqun Zheng

Controlling the Shapes of Silver Nanocrystals with Different Capping Agents

This paper provides direct evidence to support the role of a capping agent in controlling the evolution of Ag seeds into nanocrystals with different shapes. Starting with single-crystal seeds (spherical or cubic in shape), we could selectively obtain Ag octahedrons enclosed by {111} facets and nanocubes/nanobars enclosed by {100} facets by adding sodium citrate (Na(3)CA) and poly(vinyl pyrrolidone) (PVP), respectively, as a capping agent while all other parameters were kept the same. This research not only offers new insights into the role played by a capping agent in shape-controlled synthesis but also provides, for the first time, Ag octahedrons as small as 40 nm in edge length for optical and spectroscopic studies.

Silver Nanocrystals with Concave Surfaces and Their Optical and Surface-Enhanced Raman Scattering Properties

Shaped and dimpled: Silver nanocrystals enclosed by concave surfaces and thus high-index facets have been prepared by simply controlling the growth habit of Ag cubic seeds. Four types of concave nanocrystals, including octahedron, cube, octapod, and trisoctahedron, were obtained (see picture).

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 Pd(2+) 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 (R(etching) and R(regrowth)) simply by varying the amount of HCl added to the reaction solution. With a large amount of HCl, etching dominated the process (R(etching) ≫ R(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 Pd(2+) ions could be quickly reduced and deposited back onto the Pd cubes. In this case, R(etching) ≈ R(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.

Controlling the size and morphology of Au@Pd core-shell nanocrystals by manipulating the kinetics of seeded growth.

This article reports a systematic study of the seed-mediated growth of Au@Pd core-shell nanocrystals with a variety of controlled sizes and morphologies. The key to the success of this synthesis is to manipulate the reaction kinetics by tuning a set of reaction parameters, including the type and concentration of capping agent, the amount of ascorbic acid used as the reducing agent, and the injection rate used for the precursor solution. Starting from Au nanospheres of 11 nm in diameter as the seeds, Au@Pd core-shell nanocrystals with a number of morphologies, including octahedra, concave octahedra, rectangular bars, cubes, concave cubes, and dendrites, could all be obtained by simply altering the reaction rate. For the first time, it was possible to generate Au@Pd nanocrystals with concave structures on the surfaces while their sizes were kept below 20 nm. In addition, the as-prepared Au@Pd nanocubes can be used as seeds to generate Au@Pd@Au and Au@Pd@Au@Pd nanocrystals with multishelled structures.

Facile Synthesis of Gold Nanorice Enclosed by High- Index Facets and Its Application for CO Oxidation

A facile method for generating Au nanorice enclosed by high-index facets in high purity. The nanorice shows much higher catalytic activity for CO oxidation than multiply twinned particles of Au enclosed by {111} facets at temperatures below 300 °C.

Facile synthesis of Pd–Ir bimetallic octapods and nanocages through galvanic replacement and co-reduction, and their use for hydrazine decomposition

This article describes a facile synthesis of Pd-Ir bimetallic nanostructures in the forms of core-shell octapods and alloyed nanocages. The success of this synthesis relies on the use of Pd nanocubes as the sacrificial templates and interplay of two different processes: the galvanic replacement between an Ir precursor and the Pd nanocubes and the co-reduction of Pd(2+) and Ir(3+) by ethylene glycol. The galvanic replacement played a dominant role in the initial stage, through which Pd atoms were dissolved from the side faces whereas Ir atoms were deposited at the corner sites to generate Pd-Ir core-shell octapods. As the concentration of Pd(2+) in the reaction mixture was increased, co-reduction of Pd(2+) and Ir(3+) occurred in the late stage of synthesis. The resultant Pd and Ir atoms were deposited onto the octapods while the Pd atoms in the interiors continued to be etched away due to the galvanic replacement, finally leading to the formation of Pd-Ir alloyed nanocages. The octapods and nanocages were then evaluated as catalysts for the selective generation of hydrogen from the decomposition of hydrous hydrazine. The nanocages exhibited better selectivity for hydrogen generation than octapods (66% versus 29%), which can be attributed to the presence of an alloyed, porous structure on the surface.

Transformation of zincblende nanoparticles into wurtzite microrods by a dissolution–regrowth process: an intergrowth homojunction with enhanced photocatalytic activity

We report the facile one-pot synthesis of a unique cadmium sulfide (CdS) intergrowth structure in the form of zincblende nanoparticle-decorated wurtzite microrods. The success of this preparation relies on a process involving both dissolution and regrowth. The growth of CdS crystals started from rapid generation of small zincblende nanoparticles, followed by dissolution of some nascent nanoparticles due to the strong alkalinity and relatively high reaction temperature. The resultant CdS monomers, capped by ethanediamine, nucleated again and went through further one-dimensional regrowth, leading to the formation of wurtzite microrods. This transformation gives rise to the generation of a novel intergrowth homojunction that consists of zincblende nanoparticle-decorated wurtzite microrods. Such a close contacted homojunction, having a type-II band alignment, shows an enhanced photocatalytic activity without loading any co-catalyst for solar hydrogen production in comparison to the use of either nanoparticles or microrods alone. This work not only enriches our knowledge on the fundamentals of homojunction formation, but also reveals an important fact that the intergrowth of a rationally designed junction structure shall bring about enhanced photocatalytic activity and thus deserves attention.

Controlled formation of intense hot spots in Pd@Ag core-shell nanooctapods for efficient photothermal conversion

Plasmonic Ag nanostructures have been of great interest for such applications in cancer therapy and catalysis, etc. However, the relatively week Ag-Ag interaction and spontaneous atom diffusion make it very difficult to generate concaved or branched structures in Ag nanocrystals with sizes less than 100 nm, which has been considered very favorable for plasmonic effects. Herein, by employing a cubic Pd seed and a specific reducing agent to restrict the surface diffusion of Ag atoms, Pd@Ag core-shell nanooctapod structures where Ag atoms can be selectively deposited onto the corner sites of the Pd cubes were obtained. Such selective decoration enables us to precisely control the locations for the hot spot formation during light irradiation. We find that the branched nanooctapod structure shows strong absorption in the visible-light region and generates intense hot spots around the octapod arms of Ag. As such, the photothermal conversion efficiency could be significantly improved by more than 50% with a coll...