Hongjun You

Icosahedral platinum alloy nanocrystals with enhanced electrocatalytic activities.

This communication describes the synthesis of Pt-M (M = Au, Ni, Pd) icosahedral nanocrystals based on the gas reducing agent in liquid solution method. Both CO gas and organic surface capping agents play critical roles in stabilizing the icosahedral shape with {111} surfaces. Among the Pt-M alloy icosahedral nanocrystals generated, Pt(3)Ni had an impressive ORR specific activity of 1.83 mA/cm(2)(Pt) and 0.62 A/mg(Pt). Our results further show that the area-specific activity of icosahedral Pt(3)Ni catalysts was about 50% higher than that of the octahedral Pt(3)Ni catalysts (1.26 mA/cm(2)(Pt)), even though both shapes are bound by {111} facets. Density functional theory calculations and molecular dynamics simulations indicate that this improvement may arise from strain-induced electronic effects.

Synthesis of colloidal metal and metal alloy nanoparticles for electrochemical energy applications

This Review is focused on the recent progresses in the synthetic approaches to the precise control of structure, size, shape, composition and multi-functionality of metal and metal alloy nanoparticles. Many of these strategies have been developed based on colloidal methods, and to limited extent, the galvanic and other methods. The shape, size and composition often govern the chemical and catalytic properties that are important for electrochemical energy applications. The structure-property relationship and the design in controllable structures and morphologies for specific reactions such as oxygen reduction reaction (ORR) are emphasized.

Electrochemical Synthesis and Catalytic Property of Sub-10 nm Platinum Cubic Nanoboxes

We report an electrochemical synthesis of ultrafine Pt cubic nanoboxes from Pt-on-Ag heteronanostructures. These cubic nanoboxes have an average edge length of about 6 nm and a wall thickness of 1.5 nm. Several reaction parameters including the profile of applied potentials were examined to develop an optimal procedure for controlling the size, shape, and surface morphology of the nanoboxes. A strong shape-dependent catalytic property is observed for Pt cubic nanoboxes, which is 1.5 times more active than hollow spheres in terms of turn over frequency for catalytic oxidation of methanol.

Highly sensitive, uniform, and reproducible surface-enhanced Raman spectroscopy from hollow Au-Ag alloy nanourchins.

A hierarchical nanoparticle strategy to simultaneously gain super Raman signal amplification, high uniformity, and reproducibility is presented. Using hollow Au-Ag alloy nanourchins, an ultrahigh sensitivity, e.g., down to 1 fM concentrations for DEHP molecule is obtained. A small standard deviation of <10% is achieved by simply dropping and evaporating sub-100 nm nanourchins onto a substrate.

Composition-Dependent Formation of Platinum Silver Nanowires

The understanding of shape control of colloidal nanoparticles is still rather limited even after well over a decade of intensive research efforts. While surface capping agents can greatly influence the growth habit of nanocrystals in solution, the formation of certain morphology can hardly be understood based on both experimental data and simulations. Without a good understanding of the origins for shape formation, deterministic approaches to the synthesis of nanostructures can be hard to realize. In this paper, we describe the synthesis and formation of PtAg alloy nanowires in the presence of oleylamine and oleic acid through the oriented attachment. Transmission electron microscopy study shows the formation of wormlike nanowires occurs largely at the composition around Pt(50)Ag(50). Both Pt and Ag rich alloy nanostructures form sphere-like or faceted nanoparticles under the same reaction conditions. Density functional theory calculation is used to understand the interactions between the functional groups of capping agents and low index planes of PtAg alloys. The structural order of interfaces after collision between primary particles is obtained by molecular dynamic simulation. The results indicate that the formation of alloy nanowires is mostly driven by the interplay between the binding energy of capping agents on alloy surfaces and the diffusion of atoms at the interface upon the collision of primary nanoparticles.

Interface synthesis of gold mesocrystals with highly roughened surfaces for surface-enhanced Raman spectroscopy

Local electromagnetic enhancement excited from collective oscillations of free electrons on a highly roughened mental surface can induce greatly enhanced Raman scattering. Herein gold mesoparticles with various morphologies and highly roughened surfaces, including sea urchin-like, flower-like, star-like, meatball-like, and dendritic nanostructures are prepared using pentanol/water interface as a growth “bed”. The morphologies of the prepared gold mesoparticles are well controlled by varying the concentrations of additives such as gold ions, ascorbic acid (AA) and cetyltrimethylammonium bromide (CTAB). Due to the unique structures such as rough surface, high internal porosity as well as complex morphology, these as-prepared mesocrystals exhibit a remarkable performance in surface-enhanced Raman scattering (SERS) compared with polyhedral mesoparticles.

Facet-selective growth of Cu–Cu2O heterogeneous architectures

We have demonstrated a facile protocol for the synthesis of facet-selective growth of low-cost metal Cu nanoparticles on {111} facets of polyhedral 26-facet Cu2O architectures. The novel Cu–Cu2O heterogeneous architectures show better adsorption and photodegradation of methyl orange than those of the original Cu2O architectures.

Hierarchical silver mesoparticles with tunable surface topographies for highly sensitive surface-enhanced Raman spectroscopy

Using a simple aqueous synthesis method, Ag hierarchical mesoparticles with tunable surface topography were obtained. The mechanism of tuning the topographies of the hierarchical mesoparticles in this particle-mediated anisotropic growth system was studied. A series of Ag mesoparticles with well-tuned surface topographies were synthesized as an ideal research target for a systematic investigation of the effect of surface nanostructures on the performance of surface enhanced Raman spectroscopy (SERS). Highly-branched Ag mesoparticles were shown to have the highest SERS sensitivity both as single-particle SERS substrates and particle-array SERS substrates, which have an enhancement factor greater than 109. Using the finite difference time domain method, the distribution of the localized electromagnetic field near the particle surface was simulated. Based on the simulation results, the mechanism of the effect of topography on the SERS properties was systematically studied.

A green chemical approach for preparation of PtxCuy nanoparticles with a concave surface in molten salt for methanol and formic acid oxidation reactions

Molten salts are a kind of inorganic ionic liquid, which have advantages of a wide liquid range, negligible vapor pressure, good solubility characteristics, non-flammability, excellent thermal and chemical stability, and more importantly, they are low-cost and recyclable. These advantages have aroused an appealing green chemical approach to synthesize nanoparticles (NPs) with controlled morphologies. In the current research, we synthesized PtxCuy (x/y = 23/77, 51/49, 74/26, 83/17) nanoparticles with a concave and clean surface in the KNO3–LiNO3 molten salt system, in air, without using any organic solvent or capping agent. The as-prepared PtxCuy NPs were characterized by X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The results indicated that the as-obtained PtxCuy alloy had a face-centered-cubic (fcc) polycrystal structure in which the lattice parameter and composition relationship followed Vegards law in the Cu-poor regions. The results of CV on methanol and formic acid electrooxidation indicated that Pt74Cu26/C exhibited a remarkable enhancement in catalytic activities compared with the commercially used Pt/C catalyst.

Etching-limited branching growth of cuprous oxide during ethanol-assisted solution synthesis

An etching-limited branching growth mechanism has been elucidated during ethanol-assisted solution synthesis of octahedral Cu2O crystals, which is different from the conventional diffusion-limited aggregate and recent overpotential-limited branching mechanism. It provides an innovative approach for revealing the shape evolution from habit formation (octahedron) to branching growth (hexapod-like architecture) via adjusting the competition between preferential growth and selective oxidative etching, and displays a constructive model system for fundamental research of crystal growth and design.