Showing papers by "Younan Xia published in 2017"
TL;DR: The unique capabilities of electrospun nanofibers as porous supports for heterogeneous catalysis and as functional scaffolds for tissue regeneration are demonstrated by concentrating on some of the recent results.
Abstract: ConspectusElectrospinning is a simple and versatile technique that relies on the electrostatic repulsion between surface charges to continuously draw nanofibers from a viscoelastic fluid. It has been applied to successfully produce nanofibers, with diameters down to tens of nanometers, from a rich variety of materials, including polymers, ceramics, small molecules, and their combinations. In addition to solid nanofibers with a smooth surface, electrospinning has also been adapted to generate nanofibers with a number of secondary structures, including those characterized by a porous, hollow, or core–sheath structure. The surface and/or interior of such nanofibers can be further functionalized with molecular species or nanoparticles during or after an electrospinning process. In addition, electrospun nanofibers can be assembled into ordered arrays or hierarchical structures by manipulation of their alignment, stacking, and/or folding. All of these attributes make electrospun nanofibers well-suited for a bro...
738 citations
TL;DR: This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics.
Abstract: Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.
518 citations
TL;DR: Recent progress in the synthesis of intermetallic nanocrystals with controllable sizes and well-defined shapes is highlighted and perspectives on future developments in the context of synthetic control, structure-property relationships, and applications are discussed.
Abstract: At the forefront of nanochemistry, there exists a research endeavor centered around intermetallic nanocrystals, which are unique in terms of long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure. In contrast to alloy nanocrystals with no elemental ordering, it is challenging to synthesize intermetallic nanocrystals with a tight control over their size and shape. Here, recent progress in the synthesis of intermetallic nanocrystals with controllable sizes and well-defined shapes is highlighted. A simple analysis and some insights key to the selection of experimental conditions for generating intermetallic nanocrystals are presented, followed by examples to highlight the viable use of intermetallic nanocrystals as electrocatalysts or catalysts for various reactions, with a focus on the enhanced performance relative to their alloy counterparts that lack elemental ordering. Within the conclusion, perspectives on future developments in the context of synthetic control, structure-property relationships, and applications are discussed.
324 citations
TL;DR: A novel strategy that employs oxygen-independent free radicals generated from a polymerization initiator for eradicating cancer cells by inducing apoptosis in hypoxic cancer cells is introduced.
Abstract: Anticancer modalities based on oxygen free radicals, including photodynamic therapy and radiotherapy, have emerged as promising treatments in the clinic. However, the hypoxic environment in tumor tissue prevents the formation of oxygen free radicals. Here we introduce a novel strategy that employs oxygen-independent free radicals generated from a polymerization initiator for eradicating cancer cells. The initiator is mixed with a phase-change material and loaded into the cavities of gold nanocages. Upon irradiation by a near-infrared laser, the phase-change material is melted due to the photothermal effect of gold nanocages, leading to the release and decomposition of the loaded initiator to generate free radicals. The free radicals produced in this way are highly effective in inducing apoptosis in hypoxic cancer cells.
163 citations
TL;DR: A smart release system responsive to near‐infrared (NIR) light is developed for intracellular drug delivery, demonstrating that the DOX trapped in the nanoparticles can be efficiently released into the cytosol under NIR irradiation, resulting in enhanced anticancer activity.
Abstract: A smart release system responsive to near-infrared (NIR) light is developed for intracellular drug delivery. The concept is demonstrated by coencapsulating doxorubicin (DOX) (an anticancer drug) and IR780 iodide (IR780) (an NIR-absorbing dye) into nanoparticles made of a eutectic mixture of naturally occurring fatty acids. The eutectic mixture has a well-defined melting point at 39 °C, and can be used as a biocompatible phase-change material for NIR-triggered drug release. The resultant nanoparticles exhibit prominent photothermal effect and quick drug release in response to NIR irradiation. Fluorescence microscopy analysis indicates that the DOX trapped in the nanoparticles can be efficiently released into the cytosol under NIR irradiation, resulting in enhanced anticancer activity. A new platform is thus offered for designing effective intracellular drug-release systems, holding great promise for future cancer therapy.
153 citations
TL;DR: This review highlights the unique advantages of inverse opal scaffolds over their non‐uniform counterparts and showcases their broad applications in tissue engineering and regenerative medicine, followed by a summary and perspective on future directions.
Abstract: Three-dimensional porous scaffolds play a pivotal role in tissue engineering and regenerative medicine by functioning as biomimetic substrates to manipulate cellular behaviors. While many techniques have been developed to fabricate porous scaffolds, most of them rely on stochastic processes that typically result in scaffolds with pores uncontrolled in terms of size, structure, and interconnectivity, greatly limiting their use in tissue regeneration. Inverse opal scaffolds, in contrast, possess uniform pores inheriting from the template comprised of a closely packed lattice of monodispersed microspheres. The key parameters of such scaffolds, including architecture, pore structure, porosity, and interconnectivity, can all be made uniform across the same sample and among different samples. In conjunction with a tight control over pore sizes, inverse opal scaffolds have found widespread use in biomedical applications. In this review, we provide a detailed discussion on this new class of advanced materials. After a brief introduction to their history and fabrication, we highlight the unique advantages of inverse opal scaffolds over their non-uniform counterparts. We then showcase their broad applications in tissue engineering and regenerative medicine, followed by a summary and perspective on future directions.
129 citations
TL;DR: It is demonstrated that the pathway has a correlation with the reduction kinetics involved and that the reduction pathway of PdBr42- by ascorbic acid could be switched from surface to solution by raising the reaction temperature.
Abstract: Despite the pivotal role played by the reduction of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the solution phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by reduction through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the reduction kinetics involved. Our quantitative analyses of the reduction kinetics of PdCl42– and PdBr42– by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl42– was reduced in the solution phase while PdBr42– was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr42– by ascorbic acid could be switched from surface to solution by raising the reaction temperature.
77 citations
TL;DR: Icosahedral nanocrystals of noble metals have received ever increasing interest in recent years owing to their unique features such as the size-dependent distribution of strain fields and the presence of a large number of twin boundaries as discussed by the authors.
76 citations
TL;DR: Recent progress in nanochemistry suggests that it is feasible to put the majority of metal atoms in a catalytic particle to work at optimal activity and selectivity.
Abstract: Waste not, want not. There is a pressing need to maximize the use of precious metals in catalysts to attain affordable and sustainable products. Recent progress in nanochemistry suggests that it is feasible to put the majority of metal atoms in a catalytic particle to work at optimal activity and selectivity.
74 citations
TL;DR: Nanocrystal growth is used to illustrate the concept of symmetry breaking to convey its importance from a mechanistic perspective, by which one can rationally alter the experimental conditions to manipulate the growth pattern and thus generate colloidal nanocrystals with controlled shapes, structures, and properties for various applications.
73 citations
TL;DR: The reduction rate of a salt precursor can be used as a quantitative knob for achieving deterministic synthesis of colloidal metal nanocrystals.
Abstract: Despite the incredible developments made to the synthesis of colloidal metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.
TL;DR: This study provides a generic method for improving the mechanical properties of nonwoven nanofiber mats, holding great potential in various applications.
Abstract: This communication describes a simple and effective method for welding electrospun nanofibers at the cross points to enhance the mechanical properties of their nonwoven mats. The welding is achieved by placing a nonwoven mat of the nanofibers in a capped vial with the vapor of a proper solvent. For polycaprolactone (PCL) nanofibers, the solvent is dichloromethane (DCM). The welding can be managed in a controllable fashion by simply varying the partial pressure of DCM and/or the exposure time. Relative to the pristine nanofiber mat, the mechanical strength of the welded PCL nanofiber mat can be increased by as much as 200%. Meanwhile, such a treatment does not cause any major structural changes, including morphology, fiber diameter, and pore size. This study provides a generic method for improving the mechanical properties of nonwoven nanofiber mats, holding great potential in various applications.
TL;DR: This article reports the design and successful implementation of a one-pot, polyol method for the synthesis of penta-twinned Pd nanowires with diameters below 8 nm and aspect ratios up to 100 and demonstrates that the Pdnanowires can serve as sacrificial templates for the conformal deposition of Pt atoms to generate Pd@Pt core-sheath nanowire and then Pd-Pt nanotubes with a well-defined surface structure.
Abstract: This article reports the design and successful implementation of a one-pot, polyol method for the synthesis of penta-twinned Pd nanowires with diameters below 8 nm and aspect ratios up to 100. The key to the success of this protocol is the controlled reduction of Na2PdCl4 by diethylene glycol and ascorbic acid through the introduction of NaI and HCl. The I– and H+ ions can slow the reduction kinetics by forming PdI42– and inhibiting the dissociation of ascorbic acid, respectively. When the initial reduction rate is tuned into the proper regime, Pd decahedral seeds with a penta-twinned structure appear during nucleation. In the presence of I– ions as a selective capping agent toward the Pd(100) surface, the decahedral seeds can be directed to grow axially into penta-twinned nanorods and then nanowires. The Pd nanowires are found to evolve into multiply twinned particles if the reaction time is extended beyond 1.5 h, owing to the involvement of oxidative etching. When supported on carbon, the Pd nanowires s...
TL;DR: This work systematically investigated the differentiation of BMSCs into Schwann cells on scaffolds comprising electrospun fibers to optimize the differentiation efficiency and offers a promising strategy for integrating fiber guidance with stem cell therapy to augment peripheral nerve injury repair.
Abstract: Seeding nerve guidance conduits with Schwann cells can improve the outcome of peripheral nerve injury repair. Bone marrow stem cells (BMSCs) represent a good choice of cell source as they can differentiate into Schwann cells under appropriate conditions. In this work, we systematically investigated the differentiation of BMSCs into Schwann cells on scaffolds comprising electrospun fibers. We changed the alignment, diameter, and surface properties of the fibers to optimize the differentiation efficiency. The uniaxial alignment of fibers not only promoted the differentiation of BMSCs into Schwann cells but also dictated the morphology and alignment of the derived cells. Coating the surface of aligned fibers with laminin further enhanced the differentiation and thus increased the secretion of neurotrophins. When co-cultured with PC12 cells or chick dorsal root ganglion, the as-derived Schwann cells were able to promote the outgrowth of neurites from cell bodies and direct their extension along the fibers, de...
TL;DR: It is demonstrated that autocatalytic surface reduction can be employed to enable the formation of metal nanocrystals with well-controlled and predictable shapes through seed-mediated growth, and the kinetics are quantitatively analyzed in an effort to control the evolution of nanocry crystals into predictable shapes.
Abstract: The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.
TL;DR: In this article, a reiz of synthese kolloidal Metallnanokristalle is presented, in which the reiz results from dem erstaunlichen Ausmas an Kontrolle, das uber die Grose, Form, Zusammensetzung and Struktur von Nanokristallen erzielt werden kann.
Abstract: Beim keimvermittelten Wachstum handelt es sich um eine leistungsfahige und vielseitige Vorgehensweise zur Synthese kolloidaler Metallnanokristalle. Der grose Reiz dieser Methode resultiert aus dem erstaunlichen Ausmas an Kontrolle, das uber die Grose, Form, Zusammensetzung und Struktur von Nanokristallen erzielt werden kann. Diese Parameter kontrollieren nicht nur die Eigenschaften von Nanokristallen, sondern entscheiden auch uber deren Relevanz und Leistungsfahigkeit in diversen Anwendungen. Das keimvermittelte Wachstum bietet hervorragende Aussichten fur die Erweiterung zahlreicher Anwendungen und offnet zugleich Turen fur neue Entwicklungen. Dieser Aufsatz veranschaulicht die schier endlos erweiterbare Diversitat von Metallnanokristallen auf Basis von Keimen mit genau definierten und kontrollierbaren internen Strukturen in Verbindung mit einer geeigneten Kombination aus Bedeckungsmittel und Reduktionskinetik. Daruber hinaus werden neue Einsatzmoglichkeiten und zukunftige Richtungen aufgezeigt.
TL;DR: In this Perspective, a brief overview on the evolution of oxidative etching as an alternative route to the facile synthesis of well-controlled metal nanocrystals is offered, as well as an outlook into the future directions of the field.
Abstract: Oxidative etching is a powerful tool for carving out new designs in metal nanocrystals. In this issue of ACS Nano, Jin et al. demonstrate how this tool can be applied to the fabrication of Pd nanoframes by carefully balancing the rates of etching and growth during the excavation of solid nanocrystals. In this Perspective, we offer a brief overview on the evolution of oxidative etching as an alternative route to the facile synthesis of well-controlled metal nanocrystals, as well as an outlook into the future directions of the field.
TL;DR: A facile synthesis of Ag@Au core-sheath nanowires through the conformal deposition of Au atoms onto the surface of pre-synthesized Ag Nanowires is reported, showing morphology and optical properties almost identical to the pristine Ag nanowire, but with greatly improved stability under different corrosive environments.
TL;DR: In this article, Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hexagonal close-packed (hcp) of bulk Ru, were reported.
Abstract: Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hexagonal close-packed (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temperature were all instrumental to the formation of Pd@Ru core–shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five atomic layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent dens...
TL;DR: Ex situ heating experiments are used to demonstrate that Pd@Pt4L core-shell nanoscale cubes and octahedra are promising for catalytic applications at temperatures up to 400 °C and a facet dependence for the thermal stability in terms of shape and composition is demonstrated.
Abstract: Core–shell nanocrystals offer many advantages for heterogeneous catalysis, including precise control over both the surface structure and composition, as well as reduction in loading for rare and costly metals. Although many catalytic processes are operated at elevated temperatures, the adverse impacts of heating on the shape and structure of core–shell nanocrystals are yet to be understood. In this work, we used ex situ heating experiments to demonstrate that Pd@Pt4L core–shell nanoscale cubes and octahedra are promising for catalytic applications at temperatures up to 400 °C. We also used in situ transmission electron microscopy to monitor the thermal stability of the core–shell nanocrystals in real time. Our results demonstrate a facet dependence for the thermal stability in terms of shape and composition. Specifically, the cubes enclosed by {100} facets readily deform shape at a temperature 300 °C lower than that of the octahedral counterparts enclosed by {111} facets. A reversed trend is observed for ...
TL;DR: This work systematically investigates the surface and bulk reconstructions experienced by a Pd concave icosahedron when subjected to heating up to 600 °C in vacuum, and identifies the equilibration pathways of this far-from-equilibrium structure.
Abstract: Despite the remarkable success in controlling the synthesis of metal nanocrystals, it still remains a grand challenge to stabilize and preserve the shapes or internal structures of metastable kinetic products. In this work, we address this issue by systematically investigating the surface and bulk reconstructions experienced by a Pd concave icosahedron when subjected to heating up to 600 °C in vacuum. We used in situ high-resolution transmission electron microscopy to identify the equilibration pathways of this far-from-equilibrium structure. We were able to capture key structural transformations occurring during the thermal annealing process, which were mechanistically rationalized by implementing self-consistent plane-wave density functional theory (DFT) calculations. Specifically, the concave icosahedron was found to evolve into a regular icosahedron via surface reconstruction in the range of 200–400 °C, and then transform into a pseudospherical crystalline structure through bulk reconstruction when fu...
TL;DR: A tutorial review on the development of reconstituted LDL (rLDL) particles for the targeted delivery of theranostic applications and highlights the applications of rLDL particles in the staging of disease progression, treatment of lesioned tissues, and delivery of photosensitizers for photodynamic cancer therapy.
Abstract: Low-density lipoprotein (LDL), one of the four major groups of lipoproteins for lipid transport in vivo, is emerging as an attractive carrier for the targeted delivery of theranostic agents. In contrast to the synthetic systems, LDL particles are intrinsically biocompatible and biodegradable, together with reduced immunogenicity and natural capabilities to target cancerous cells and to escape from the recognition and elimination by the reticuloendothelial system. Enticed by these attributes, a number of strategies have been developed for reconstituting LDL particles, including conjugation to the apolipoprotein, insertion into the phospholipid layer, and loading into the core. Here we present a tutorial review on the development of reconstituted LDL (rLDL) particles for theranostic applications. We start with a brief introduction to LDL and LDL receptor, as well as the advantages of using rLDL particles as a natural and versatile platform for the targeted delivery of theranostic agents. After a discussion of commonly used strategies for the reconstitution of LDL, we highlight the applications of rLDL particles in the staging of disease progression, treatment of lesioned tissues, and delivery of photosensitizers for photodynamic cancer therapy. We finish this review with a perspective on the remaining challenges and future directions.
TL;DR: This study demonstrates that LDLs reconstructed with fatty acids hold great promise to serve as effective and versatile nanocarriers for targeted cancer therapy.
Abstract: Low-density lipoproteins (LDLs) are a class of nanocarriers for the targeted delivery of therapeutics into aberrant cells that overexpress the LDL receptor. A facile procedure is used for reconstituting the hydrophobic core of LDLs with a binary fatty acid mixture. Facilitated by the tumor targeting capability of the apolipoprotein, the reconstituted, drug-loaded LDLs can effectively target cancer cells that overexpress the LDL receptor while showing minor adverse impact on normal fibroblasts. According to a hypothesized mechanism, the reconstituted LDLs can also enable metabolism-triggered drug release while preventing the payloads from lysosomal degradation. This study demonstrates that LDLs reconstructed with fatty acids hold great promise to serve as effective and versatile nanocarriers for targeted cancer therapy.
TL;DR: It is established that the adsorption of bovine serum albumin (BSA) onto nanofibers is a time- and concentration-dependent process and a simple and versatile method for generating gradients of bioactive proteins on nanofiber mats is reported.
Abstract: Electrospun nanofibers are widely used in tissue engineering owing to their capability to mimic the structures and architectures of various types of extracellular matrices. However, it has been difficult to incorporate a biochemical cue into the physical cue provided by the nanofibers. Here we report a simple and versatile method for generating gradients of bioactive proteins on nanofiber mats. We establish that the adsorption of bovine serum albumin (BSA) onto nanofibers is a time- and concentration-dependent process. By linearly increasing the volume of BSA solution introduced into a container, a gradient in BSA is readily generated across the length of a vertically oriented strip of nanofibers. Next, the bare regions uncovered by BSA can be filled with the bioactive protein of interest. In demonstrating the potential application, we examine the outgrowth of neurites from dorsal root ganglion (DRG) isolated from chick embryos and then seeded on aligned polycaprolactone nanofibers covered by nerve growth factor (NGF) with a uniform coverage or in a gradient. In the case of uniform coverage, the neurites extending from DRG show essentially the same length on either side of the DRG cell mass. For the sample with a gradient in NGF, the neurites extending along the gradient (i.e., increase of NGF concentration) were significantly longer than the neurites extending against the gradient.
01 Mar 2017
TL;DR: In this article, a one-pot method for the facile synthesis of Cu nanowires in high purity, together with ultrathin diameters well below 20nm, was reported.
Abstract: We report a one-pot method for the facile synthesis of Cu nanowires in high purity, together with ultrathin diameters well below 20 nm. Selected area electron diffraction and high-resolution transmission electron microscopy studies confirm that the Cu nanowires are grown along the direction to give pentatwinned, one-dimensional nanostructures, enclosed by five {100} facets on the side surface. A systematic study further indicates that it is critical to conduct the synthesis under an argon atmosphere in order to improve the purity and uniformity of the nanowires while keeping their diameters thinner than 20 nm. We also demonstrate the use of these nanowires as sacrificial templates for the synthesis of Au-based nanotubes through a galvanic replacement process.
TL;DR: In this paper, a systematic study of the oxidative etching of penta-twinned Pd decahedral nanocrystals under different conditions and its impact on their subsequent growth behavior is presented.
Abstract: We report a systematic study of the oxidative etching of penta-twinned Pd decahedral nanocrystals by O2/I– under different conditions and its impact on their subsequent growth behavior. Analysis by transmission electron microscopy shows significant rounding of the decahedral structure. More specifically, the etching is found to begin at the equatorial vertices, due to their high surface free energy, and proceed along the adjacent, equatorial edges through the dissolution of low-coordination atoms. Comparison of the etching behaviors under different conditions reveals the critical role of a reductive environment for the initiation of oxidative etching, possibly due to the presence of a protective oxide layer on the surface of Pd decahedra. Overgrowth on the seeds with a rounded profile generates penta-twinned Pd nanorods with an asymmetric, tapered structure as a result of simultaneous axial and radial growth. In comparison, the original decahedral seeds only show axial growth, leading to the formation of ...
01 Dec 2017
TL;DR: In this article, a facile, aqueous-phase synthesis of sub-10 nm Pt octahedral nanocrystals is reported, which relies on the use of poly(vinyl pyrrolidone) (PVP) as a mediator.
Abstract: We report a facile, aqueous-phase synthesis of sub-10 nm Pt octahedral nanocrystals. The success of this synthesis relies on the use of poly(vinyl pyrrolidone) (PVP) as a mediator. By tracking the evolution of the Pt nanocrystals throughout the growth process, we found that single-crystal seeds with a pseudo-spherical shape are formed in the early stage of the synthesis, followed by growth through the preferential deposition of Pt atoms onto the {100} facets, directing the seeds to evolve into octahedra enclosed by {111} facets. We have manipulated the atom deposition rates by tuning various experimental parameters in an effort to support the mechanism for the formation of Pt octahedra. In addition, we have investigated the catalytic activity of these Pt nanocrystals towards oxygen reduction. For the 9-nm Pt octahedra enclosed by {111} facets, they show a specific activity of 1.5 mA cm-2, much greater than that of Pt nanocubes enclosed by {100} facets, which only has a specific activity of 0.28 mA cm-2.
TL;DR: It is established that twin defects can be rationally generated by equilibrating nanoparticles of different sizes through heating and then cooling, and it is validated that Ag nanocrystals with icosahedral, decahedral, and single-crystal structures are favored at sizes below 7”nm, between 7 and 11 nm, and greater than 11’nm, respectively.
Abstract: This work demonstrates a new strategy for controlling the evolution of twin defects in metal nanocrystals by simply following thermodynamic principles. With Ag nanocrystals supported on amorphous SiO2 as a typical example, we establish that twin defects can be rationally generated by equilibrating nanoparticles of different sizes through heating and then cooling. We validate that Ag nanocrystals with icosahedral, decahedral, and single-crystal structures are favored at sizes below 7 nm, between 7 and 11 nm, and greater than 11 nm, respectively. This trend is then rationalized by computational studies based on density functional theory and molecular dynamics, which show that the excess free energy for the three equilibrium structures correlate strongly with particle size. This work not only highlights the importance of thermodynamic control but also adds another synthetic method to the ever-expanding toolbox used for generating metal nanocrystals with desired properties.
21 Jun 2017
TL;DR: In this article, waste tires were converted to hard carbon, then functionalized with catalytically active -SO3H groups on the surface through an environmentally benign process that involved the sequential treatment with L-cysteine, dithiothreitol, and H2O2.
Abstract: Many inexpensive biofuel feedstocks, including those containing free fatty acids (FFAs) in high concentrations, are typically disposed of as waste due to our inability to efficiently convert them into usable biofuels. Here we demonstrate that carbon derived from waste tires could be functionalized with sulfonic acid (-SO3H) to effectively catalyze the esterification of oleic acid or a mixture of fatty acids to usable biofuels. Waste tires were converted to hard carbon, then functionalized with catalytically active -SO3H groups on the surface through an environmentally benign process that involved the sequential treatment with L-cysteine, dithiothreitol, and H2O2. When benchmarked against the same waste-tire derived carbon material treated with concentrated sulfuric acid at 150 °C, similar catalytic activity was observed. Both catalysts could also effectively convert oleic acid or a mixture of fatty acids and soybean oil to usable biofuels at 65 °C and 1 atm without leaching of the catalytic sites.
TL;DR: The hypothesis that toughening of a fibrous scaffold arises from brittle mineral particles that crosslink the fibers, but only if the particles are free to slide relative to the fibers is supported.