Other affiliations: University of Nebraska–Lincoln, Georgia Institute of Technology, Nanjing University ...read more
Bio: Shouheng Sun is an academic researcher from Brown University. The author has contributed to research in topics: Nanoparticle & Catalysis. The author has an hindex of 125, co-authored 412 publications receiving 61450 citations. Previous affiliations of Shouheng Sun include University of Nebraska–Lincoln & Georgia Institute of Technology.
Papers published on a yearly basis
TL;DR: Thermal annealing converts the internal particle structure from a chemically disordered face- centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies that can support high-density magnetization reversal transitions.
Abstract: Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.
TL;DR: As-synthesized iron oxide nanoparticles have a cubic spinel structure as characterized by HRTEM, SAED, and XRD and can be transformed into hydrophilic ones by adding bipolar surfactants, and aqueous nanoparticle dispersion is readily made.
Abstract: High-temperature solution phase reaction of iron(III) acetylacetonate, Fe(acac)3, with 1,2-hexadecanediol in the presence of oleic acid and oleylamine leads to monodisperse magnetite (Fe3O4) nanoparticles. Similarly, reaction of Fe(acac)3 and Co(acac)2 or Mn(acac)2 with the same diol results in monodisperse CoFe2O4 or MnFe2O4 nanoparticles. Particle diameter can be tuned from 3 to 20 nm by varying reaction conditions or by seed-mediated growth. The as-synthesized iron oxide nanoparticles have a cubic spinel structure as characterized by HRTEM, SAED, and XRD. Further, Fe3O4 can be oxidized to Fe2O3, as evidenced by XRD, NEXAFS spectroscopy, and SQUID magnetometry. The hydrophobic nanoparticles can be transformed into hydrophilic ones by adding bipolar surfactants, and aqueous nanoparticle dispersion is readily made. These iron oxide nanoparticles and their dispersions in various media have great potential in magnetic nanodevice and biomagnetic applications.
TL;DR: The reported procedure can be used as a general approach to various ferrite nanoparticles and nanoparticle superlattices.
Abstract: Monodisperse magnetite nanoparticles have been synthesized by high-temperature solution-phase reaction of Fe(acac)3 in phenyl ether with alcohol, oleic acid, and oleylamine. Seed-mediated growth is used to control Fe3O4 nanoparticle size, and variously sized nanoparticles from 3 to 20 nm have been produced. The as-synthesized Fe3O4 nanoparticles have inverse spinel structure, and their assemblies can be transformed into γ-Fe2O3 or α-Fe nanoparticle assemblies, depending on the annealing conditions. The reported procedure can be used as a general approach to various ferrite nanoparticles and nanoparticle superlattices.
TL;DR: The fabrication of exchange-coupled nanocomposites using nanoparticle self-assembly with an energy product that exceeds the theoretical limit of 13 MG Oe for non-exchange- coupled isotropic FePt by over 50 per cent is reported.
Abstract: 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.
TL;DR: This review 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 and summarizes the general strategies applied for NP surface functionalization.
Abstract: 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 first outlines the synthesis of MNPs of metal oxides and alloys and 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.
01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.
28 Jul 2005
01 Jan 2015
TL;DR: A unified theoretical framework highlights the need for catalyst design strategies that selectively stabilize distinct reaction intermediates relative to each other, and opens up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions.
Abstract: BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.