Yaoting Wu

Monodisperse Core/Shell Ni/FePt Nanoparticles and Their Conversion to Ni/Pt to Catalyze Oxygen Reduction

We report a size-controllable synthesis of monodisperse core/shell Ni/FePt nanoparticles (NPs) via a seed-mediated growth and their subsequent conversion to Ni/Pt NPs. Preventing surface oxidation of the Ni seeds is essential for the growth of uniform FePt shells. These Ni/FePt NPs have a thin (≈1 nm) FePt shell and can be converted to Ni/Pt by acetic acid wash to yield active catalysts for oxygen reduction reaction (ORR). Tuning the core size allows the optimization of their electrocatalytic activity. The specific activity and mass activity of 4.2/0.8 nm core/shell Ni/FePt after acetic acid wash reach 1.95 mA/cm(2) and 490 mA/mgPt at 0.9 V (vs reversible hydrogen electrode), which are much higher than those of benchmark commercial Pt catalyst (0.34 mA/cm(2) and 92 mA/mgPt at 0.9 V). Our studies provide a robust approach to monodisperse core/shell NPs with nonprecious metal core, making it possible to develop advanced NP catalysts with ultralow Pt content for ORR and many other heterogeneous reactions.

Large-Area Nanoimprinted Colloidal Au Nanocrystal-Based Nanoantennas for Ultrathin Polarizing Plasmonic Metasurfaces

We report a low-cost, large-area fabrication process using solution-based nanoimprinting and compact ligand exchange of colloidal Au nanocrystals to define anisotropic, subwavelength, plasmonic nanoinclusions for optical metasurfaces. Rod-shaped, Au nanocrystal-based nanoantennas possess strong, localized, plasmonic resonances able to control polarization. We fabricate metasurfaces from rod-shaped nanoantennas tailored in size and spacing to demonstrate Au nanocrystal-based quarter-wave plates that operate with extreme bandwidths and provide high polarization conversion efficiencies in the near-to-mid infrared.

Hierarchical Materials Design by Pattern Transfer Printing of Self-Assembled Binary Nanocrystal Superlattices

We demonstrate the fabrication of hierarchical materials by controlling the structure of highly ordered binary nanocrystal superlattices (BNSLs) on multiple length scales. Combinations of magnetic, plasmonic, semiconducting, and insulating colloidal nanocrystal (NC) building blocks are self-assembled into BNSL membranes via the liquid-interfacial assembly technique. Free-standing BNSL membranes are transferred onto topographically structured poly(dimethylsiloxane) molds via the Langmuir-Schaefer technique and then deposited in patterns onto substrates via transfer printing. BNSLs with different structural motifs are successfully patterned into various meso- and microstructures such as lines, circles, and even three-dimensional grids across large-area substrates. A combination of electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) measurements confirm the ordering of NC building blocks in meso- and micropatterned BNSLs. This technique demonstrates structural diversity in the design of hierarchical materials by assembling BNSLs from NC building blocks of different composition and size by patterning BNSLs into various size and shape superstructures of interest for a broad range of applications.

Preparation and Self-Assembly of Dendronized Janus Fe3O4–Pt and Fe3O4–Au Heterodimers

Janus nanoparticles (NPs) often referred to as nanosized analogs of molecular surfactants are amphiphilic structures with potential applications in materials science, biomedicine, and catalysis, and their synthesis and self-assembly into complex architectures remain challenging. Here, we demonstrate the preparation of Janus heterodimers via asymmetric functionalization of Fe3O4-Pt and Fe3O4-Au heterodimeric NPs. The hydrophobic and hydrophilic dendritic ligands that carry phosphonic acid and disulfide surface binding groups selectively coat the iron oxide and platinum (or gold) parts of the heterodimer, respectively. Such an approach allows simple and efficient preparation of amphiphilic structures. Moreover, liquid-air interface self-assembly studies of each ligand exchange step revealed a drastic improvement in film crystallinity, suggesting the dendronization induced improvement of the whole particle polydispersity of the heterodimers.

Anisotropic Cracking of Nanocrystal Superlattices

The synthesis colloidal nanocrystals in nonpolar organic solvents has led to exceptional size- and shape-control, enabling the formation of nanocrystal superlattices isostructural to atomic lattices built with nanocrystals rather than atoms. The long aliphatic ligands (e.g., oleic acid) used to achieve this control separate nanocrystals too far in the solid state for most charge-transporting devices. Solid-state ligand exchange, which brings particles closer together and enhances conductivity, necessitates large changes in the total volume of the solid (compressive stress), which leads to film cracking. In this work, truncate octahedral lead selenide nanocrystals are shown to self-assemble into body-centered cubic superlattices in which the atomic axes of the individual nanocrystals are coaligned with the crystal axes of the superlattice. Due to this coalignment, upon ligand exchange of the superlattices, cracking is preferentially observed on ⟨011⟩ superlattice directions. This observation is related to differences in the ligand binding to exposed {100} and {111} planes of the PbSe nanocrystal surfaces. This result has implications for binary and more complex structures in which differential reactivity of the constituent elements can lead to disruption of the desired structure. In addition, cracks in PbSe superlattices occur in a semiregular spacings inversely related to the superlattice domain size and strongly influenced by the presence of twin boundaries, which serve as both emission centers and propagation barriers for fractures. This work shows that defects, similar to behavior in nanotwinned metals, could be used to engineer enhanced mechanical strength and electrical conductivity in nanocrystal superlattices.

Interplay between spherical confinement and particle shape on the self-assembly of rounded cubes

Self-assembly of nanoparticles (NPs) inside drying emulsion droplets provides a general strategy for hierarchical structuring of matter at different length scales. The local orientation of neighboring crystalline NPs can be crucial to optimize for instance the optical and electronic properties of the self-assembled superstructures. By integrating experiments and computer simulations, we demonstrate that the orientational correlations of cubic NPs inside drying emulsion droplets are significantly determined by their flat faces. We analyze the rich interplay of positional and orientational order as the particle shape changes from a sharp cube to a rounded cube. Sharp cubes strongly align to form simple-cubic superstructures whereas rounded cubes assemble into icosahedral clusters with additionally strong local orientational correlations. This demonstrates that the interplay between packing, confinement and shape can be utilized to develop new materials with novel properties.Colloidal nanoparticles self-assembled under spherical confinement can form a rich variety of structures. Here, the authors study the self-assembly of sharp and rounded nanocubes under such confinement, revealing the influence of particle and face geometry on positional and orientational behavior.

Improved Chemical and Colloidal Stability of Gold Nanoparticles Through Dendron Capping

Nanoparticle (NP) stability is imperative for commercialization of nanotechnology. In this study, we compare the stability of Au NPs with surfaces functionalized with oleylamine, dodecanethiol, and two dendritic ligands of different generations. Dendrimer ligands provide a significant increase in the chemical stability of Au NPs when analyzed by cyanide-induced NP decomposition as well as an investigation into their colloidal stability at ambient conditions. These results were supported by absorption measurements, transmission electron microscopy, thermogravimetric analysis, nuclear magnetic resonance, and small-angle transmission X-ray scattering and show that dendrimers play a key role in improving the chemical and colloidal stability of NPs.