Shouheng Sun

Exchange-coupled nanocomposite magnets by nanoparticle self-assembly

Exchange-spring magnets are nanocomposites that are composed of magnetically hard and soft phases that interact by magnetic exchange coupling. Such systems are promising for advanced permanent magnetic applications, as they have a large energy product—the combination of permanent magnet field and magnetization—compared to traditional, single-phase materials. Conventional techniques, including melt-spinning, mechanical milling and sputtering, have been explored to prepare exchange-spring magnets. However, the requirement that both the hard and soft phases are controlled at the nanometre scale, to ensure efficient exchange coupling, has posed significant preparation challenges. Here we report the fabrication of exchange-coupled nanocomposites using nanoparticle self-assembly. In this approach, both FePt and Fe3O4 particles are incorporated as nanometre-scale building blocks into binary assemblies. Subsequent annealing converts the assembly into FePt–Fe3Pt nanocomposites, where FePt is a magnetically hard phase and Fe3Pt a soft phase. An optimum exchange coupling, and therefore an optimum energy product, can be obtained by independently tuning the size and composition of the individual building blocks. We have produced exchange-coupled isotropic FePt–Fe3Pt nanocomposites with an energy product of 20.1 MG Oe, which exceeds the theoretical limit of 13 MG Oe for non-exchange-coupled isotropic FePt by over 50 per cent.

Synthesis, Functionalization, and Biomedical Applications of Multifunctional Magnetic Nanoparticles

Synthesis of multifunctional magnetic nanoparticles (MFMNPs) is one of the most active research areas in advanced materials. MFMNPs that have magnetic properties and other functionalities have been demonstrated to show great promise as multimodality imaging probes. Their multifunctional surfaces also allow rational conjugations of biological and drug molecules,making it possible to achieve target-specific diagnostics and therapeutics.This review fi rst outlines the synthesis of MNPs of metal oxides and alloy sand then focuses on recent developments in the fabrication of MFMNPs of core/shell, dumbbell, and composite hybrid type. It also summarizes the general strategies applied for NP surface functionalization. The review further highlights some exciting examples of these MFMNPs for multimodality imaging and for target-specific drug/gene delivery applications.

Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices (invited)

High temperature, solution phase reduction of cobalt chloride in the presence of stabilizing agents was employed to produce magnetic colloids (ferrofluids) of cobalt nanocrystals. We systematically synthesized and isolated nearly monodisperse nanocrystal samples ranging in size from 2 to 11 nm while maintaining better than a 7% std. dev. in diameter. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (e-Co). Annealing the nanocrystals at 300 °C converts them quantitatively to the more common hexagonal-close-packed crystal form. Deposition of these uniform cobalt particles on solid substrates by evaporation of the carrier solvent results in the spontaneous assembly of two-dimensional and three-dimensional magnetic superlattices (colloidal crystals). A combination of x-ray powder diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry were used to characterize both the d...

Colloidal synthesis of nanocrystals and nanocrystal superlattices

This paper provides an overview of the synthetic techniques used to prepare colloidal nanocrystals (NCs) of controlled composition, size, shape, and internal structure and the methods for manipulat...

Magnetic nanoparticles: synthesis, functionalization, and applications in bioimaging and magnetic energy storage

This tutorial review summarizes the recent advances in the chemical synthesis and potential applications of monodisperse magnetic nanoparticles. After a brief introduction to nanomagnetism, the review focuses on recent developments in solution phase syntheses of monodisperse MFe(2)O(4), Co, Fe, CoFe, FePt and SmCo(5) nanoparticles. The review further outlines the surface, structural, and magnetic properties of these nanoparticles for biomedicine and magnetic energy storage applications.

Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction

Advances in chemical syntheses have led to the formation of various kinds of nanoparticles (NPs) with more rational control of size, shape, composition, structure and catalysis. This review highlights recent efforts in the development of Pt and non-Pt based NPs into advanced nanocatalysts for efficient oxygen reduction reaction (ORR) under fuel-cell reaction conditions. It first outlines the shape controlled synthesis of Pt NPs and their shape-dependent ORR. Then it summarizes the studies of alloy and core-shell NPs with controlled electronic (alloying) and strain (geometric) effects for tuning ORR catalysis. It further provides a brief overview of ORR catalytic enhancement with Pt-based NPs supported on graphene and coated with an ionic liquid. The review finally introduces some non-Pt NPs as a new generation of catalysts for ORR. The reported new syntheses with NP parameter-tuning capability should pave the way for future development of highly efficient catalysts for applications in fuel cells, metal-air batteries, and even in other important chemical reactions.

FePt nanoparticles assembled on graphene as enhanced catalyst for oxygen reduction reaction.

Seven-nanometer FePt nanoparticles (NPs) were synthesized and assembled on graphene (G) by a solution-phase self-assembly method. These G/FePt NPs were a more active and durable catalyst for oxygen reduction reaction (ORR) in 0.1 M HClO(4) than the same NPs or commercial Pt NPs deposited on conventional carbon support. The G/FePt NPs annealed at 100 °C for 1 h under Ar + 5% H(2) exhibited specific ORR activities of 1.6 mA/cm(2) at 0.512 V and 0.616 mA/cm(2) at 0.557 V (vs Ag/AgCl). As a comparison, the commercial Pt NPs (2-3 nm) had specific activities of 0.271 and 0.07 mA/cm(2) at the same potentials. The G/FePt NPs were also much more stable in the ORR condition and showed nearly no activity change after 10 000 potential sweeps. The work demonstrates that G is indeed a promising support to improve NP activity and durability for practical catalytic applications.

PET/MRI Dual-Modality Tumor Imaging Using Arginine-Glycine-Aspartic (RGD)–Conjugated Radiolabeled Iron Oxide Nanoparticles

The purpose of this study was to develop a bifunctional iron oxide (IO) nanoparticle probe for PET and MRI scans of tumor integrin αvβ3 expression. Methods: Polyaspartic acid (PASP)–coated IO (PASP-IO) nanoparticles were synthesized using a coprecipitation method, and particle size and magnetic properties were measured. A phantom study was used to assess the efficacy of PASP-IO as a T2-weighted MRI contrast agent. PASP-IO nanoparticles with surface amino groups were coupled to cyclic arginine-glycine-aspartic (RGD) peptides for integrin αvβ3 targeting and macrocyclic 1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″,-tetraacetic acid (DOTA) chelators for PET after labeling with 64Cu. IO nanoparticle conjugates were further tested in vitro and in vivo to determine receptor targeting efficacy and feasibility for dual PET/MRI. Results: PASP-IO nanoparticles made by single-step reaction have a core size of 5 nm with a hydrodynamic diameter of 45 ± 10 nm. The saturation magnetization of PASP-IO nanoparticles is about 117 emu/g of iron, and the measured r2 and r2* are 105.5 and 165.5 (s·mM)−1, respectively. A displacement competitive binding assay indicates that DOTA-IO-RGD conjugates bound specifically to integrin αvβ3 in vitro. Both small-animal PET and T2-weighted MRI show integrin-specific delivery of conjugated RGD-PASP-IO nanoparticles and prominent reticuloendothelial system uptake. Conclusion: We have successfully developed an IO-based nanoprobe for simultaneous dual PET and MRI of tumor integrin expression. The success of this bifunctional imaging approach may allow for earlier tumor detection with a high degree of accuracy and provide further insight into the molecular mechanisms of cancer.

Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO2 to CO

We report selective electrocatalytic reduction of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 °C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the maximum Faradaic efficiency (FE) (up to 90% at -0.67 V vs reversible hydrogen electrode, RHE). Density functional theory calculations suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the reduction intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methylimidazolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at -0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and selective electrocatalytic reduction of CO2 to CO.

Dumbbell-like Pt−Fe3O4 Nanoparticles and Their Enhanced Catalysis for Oxygen Reduction Reaction

Monodisperse dumbbell-like Pt-Fe(3)O(4) nanoparticles are synthesized by epitaxial growth of Fe onto Pt nanoparticles followed by Fe oxidation. The nanoparticle size in the structure is tunable from 2 to 8 nm for Pt and 6 to 20 nm for Fe(3)O(4). Pt nanoparticles in the Pt-Fe(3)O(4) structure show a 20-fold increase in mass activity toward oxygen reduction reaction compared with the single component Pt nanoparticles and the commercial 3 nm Pt particles. The work proves that it is possible to maximize catalytic activity of the Pt nanoparticle catalyst through the control not only of Pt size and shape but also of its interaction with Fe(3)O(4) nanoparticles.