Composition Of Nanocrystals Research Articles

Temperature dependence of acoustic vibrations of CdSe and CdSe-CdS core-shell nanocrystals measured by low-frequency Raman spectroscopy.

We measure the temperature dependence of breathing-mode acoustic vibrations of semiconductor nanocrystals using low-frequency Raman spectroscopy. In CdSe core-only nanocrystals, the lowest-energy l = 0 mode red-shifts with increasing temperature by ∼5% between 77-300 K. Changes to the interatomic bond distances in the inorganic crystal lattice, with corresponding changes to the bulk modulus and density of the material, contribute to the observed energy shift but do not fully explain its magnitude across all nanocrystal sizes. Invariance of the Raman linewidth over the same temperature range suggests that the acoustic breathing mode is inhomogeneously broadened. The acoustic phonons of CdSe/CdS core-shell composite nanocrystals display similar qualitative behavior. However, for large core-shell nanocrystals, we observe a higher-order Raman peak at approximately twice the energy of the l = 0 mode, which we identify as a higher spherical harmonic-the n = 2, l = 0 eigenmode-rather than a two-phonon scattering event.

(9R)-9-Hydroxystearate-Functionalized Hydroxyapatite as Antiproliferative and Cytotoxic Agent toward Osteosarcoma Cells.

The possibility to functionalize calcium phosphates with bioactive agents is a promising strategy to design innovative biomaterials for bone repair able to couple the bioactive properties of the inorganic compounds with the therapeutic effect of the functionalizing agent. The R enantiomer of the 9-hydroxystearic acid, (9R)-9-HSA, produced from Dimorphotheca sinuata L. seeds, has proven to act as a natural negative regulator of tumor cell proliferation. On this basis, hydroxyapatite was synthesized with increasing contents of (9R)-9-hydroxystearate, up to ∼8.6 wt %. The incorporation of HSA in the composite nanocrystals induces a reduction of the crystal mean dimensions and of the length of the coherently scattering crystalline domains, which suggest a preferential adsorption onto the hydroxyapatite crystal faces parallel to the c-axis direction. The composite nanocrystals were designed so that their cytostatic and cytotoxic effects toward osteosarcoma cells were modulated by hydroxystearate content. In fact, results of in vitro tests show that the presence of HSA in the composite nanocrystals provokes a significant decrease in SaOS2 osteosarcoma cells proliferation and viability as well as an increase in lactate dehydrogenase, tumor necrosis factor α, and caspase 3 levels, with a cytotoxic effect increasing with HSA content in the nanocrystals.

A Highly Efficient UV-Vis-NIR Active Ln(3+)-Doped BiPO4/BiVO4 Nanocomposite for Photocatalysis Application.

In this Article, we report the synthesis of Ln(3+) (Yb(3+), Tm(3+))-doped BiPO4/BiVO4 nanocomposite photocatalyst that shows efficient photocatalytic activity under UV-visible-near-infrared (UV-vis-NIR) illumination. Incorporation of upconverting Ln(3+) ion pairs in BiPO4 nanocrystals resulted in strong emission in the visible region upon excitation with a NIR laser (980 nm). A composite of BiPO4 nanocrystals and vanadate was prepared by the addition of vanadate source to BiPO4 nanocrystals. In the nanocomposite, the strong blue emission from Tm(3+) ions via upconversion is nonradiatively transferred to BiVO4, resulting in the production of excitons. This in turn generates reactive oxygen species and efficiently degrades methylene blue dye in aqueous medium. The nanocomposite also shows high photocatalytic activity both under the visible region (0.010 min(-1)) and under the full solar spectrum (0.047 min(-1)). The results suggest that the photocatalytic activity of the nanocomposite under both NIR as well as full solar irradiation is better compared to other reported nanocomposite photocatalysts. The choice of BiPO4 as the matrix for Ln(3+) ions has been discussed in detail, as it plays an important role in the superior NIR photocatalytic activity of the nanocomposite photocatalyst.

Antioxidant and bone repair properties of quercetin-functionalized hydroxyapatite: An in vitro osteoblast–osteoclast–endothelial cell co-culture study

Quercetin (3,3′,4′,5,7–pentahydroxy-flavone) is a flavonoid known for its pharmacological activities, which include antioxidant and anti-inflammatory properties, as well as possible beneficial action on diseases involving bone loss. In this work, we explored the possibility to functionalize hydroxyapatite (HA) with quercetin in order to obtain new materials for bone repair through local administration of the flavonoid. HA was synthesized in presence of different concentrations of quercetin according to two different procedures: direct synthesis and phase transition from monetite. Direct synthesis lead to composite nanocrystals containing up to 3.1wt% quercetin, which provokes a reduction of the crystals mean dimensions and of the length of the coherently scattering domains. Synthesis conditions provoke a partial oxidation of quercetin and, as a consequence, a significant reduction of its radical scavenging activity (RSA). On the other hand, synthesis through phase transition yields samples containing up to 1.3wt% of quercetin incorporated into hydroxyapatite, with minor structural modifications, which exhibit relevant anti-oxidant activities, as testified by their high RSA levels, (slightly lower than that of pure quercetin). The biological response to these materials was tested using an innovative triculture model involving osteoblast, osteoclast and endothelial cells, in order to mimic bone microenvironment. The results show that the presence of quercetin in the composite materials enhances human osteoblast-like MG63 proliferation and differentiation, whereas it downregulates osteoclastogenesis of osteoclast precursors 2T-110, and supports proliferation and differentiation of human umbilical vein endothelial cells (HUVEC). Statement of SignificanceThe pharmacological activities of the flavonoid quercetin include anti-oxidant and antiinflammatory properties, as well as capability to prevent bone loss. In this paper, we demonstrate that it is possible to synthesize hydroxyapatite functionalized with different amounts of quercetin and obtain new composite materials which display both the good bioactivity of the inorganic phase and the therapeutic properties of the flavonoid. The innovative in vitro model developed in this study, which involves co-culture of osteoblast, osteoclast and endothelial cells, allows to state that the new materials exert a beneficial action onto bone repair microenvironment, stimulating osteoblast proliferation and activity, downregulating osteoclastogenesis, and supporting microangiogenetic processes necessary for new bone formation.

Effect of Surface Stoichiometry on Blinking and Hole Trapping Dynamics in CdSe Nanocrystals

We measure the photoinduced carrier dynamics as the surface composition of CdSe nanocrystals is systematically varied from metal rich (∼80% surface Cd) to nearly stoichiometric (∼50% surface Cd). Using time-resolved optical spectroscopy, we determine that the luminescence lifetime is controlled by the rate of hole trapping at the newly exposed surface selenium atoms. However, the increased rate of the photoluminescence decay is not sufficient to explain the decreased photoluminescence quantum yield, and requires a growing proportion of nanocrystals in a dark or “OFF” state to explain the data. A global kinetic model is proposed that relates the fraction of selenium sites to the rate of hole trapping. A linear relationship between the rate of hole trapping and the fraction of exposed Se sites (xSe) is observed within the range of accessible stoichiometries (xSe = 0.5–0.2). Extrapolation to higher surface cadmium fractions suggests that not all Se sites serve as effective hole traps. These results explain t...

The effects of strontium on bone mineral: A review on current knowledge and microanalytical approaches

The interest in effects of strontium (Sr) on bone has greatly increased in the last decade due to the development of the promising drug strontium ranelate. This drug is used for treating osteoporosis, a major bone disease affecting hundreds of millions of people worldwide, especially postmenopausal women. The novelty of strontium ranelate compared to other treatments for osteoporosis is its unique effect on bone: it simultaneously promotes bone formation by osteoblasts and inhibits bone resorption by osteoclasts. Besides affecting bone cells, treatment with strontium ranelate also has a direct effect on the mineralized bone matrix. Due to the chemical similarities between Sr and Ca, a topic that has long been of particular interest is the incorporation of Sr into bones replacing Ca from the mineral phase, which is composed by carbonated hydroxyapatite nanocrystals. Several groups have analyzed the mineral produced during treatment; however, most analysis were done with relatively large samples containing numerous nanocrystals, resulting thus on data that represents an average of many crystalline domains. The nanoscale analysis of the bone apatite crystals containing Sr has only been described in a few studies. In this study, we review the current knowledge on the effects of Sr on bone mineral and discuss the methodological approaches that have been used in the field. In particular, we focus on the great potential that advanced microscopy and microanalytical techniques may have on the detailed analysis of the nanostructure and composition of bone apatite nanocrystals produced during treatment with strontium ranelate.

Fabrication of Cell–Hydroxyapatite Nanocrystal Composites Assisted with Layer-by-layer Nanometer-sized Extracellular Matrix Films on Individual Stem Cells

Abstract The ability to control cell function is one of the challenges for cell-based therapy in regenerative medicine. In this study, we describe a novel method to fabricate cell–hydroxyapatite (HAp) nanocrystal composites to modulate cellular microenvironment. By preparation of layer-by-layer nanometer-sized extracellular matrix films onto cell surfaces, HAp nanocrystals were fabricated onto cell surfaces without cytotoxicity. This technology can modulate cellular functions such as adhesion, growth, and differentiation.

Giant Optical Activity of Quantum Dots, Rods, and Disks with Screw Dislocations.

For centuries mankind has been modifying the optical properties of materials: first, by elaborating the geometry and composition of structures made of materials found in nature, later by structuring the existing materials at a scale smaller than the operating wavelength. Here we suggest an original approach to introduce optical activity in nanostructured materials, by theoretically demonstrating that conventional achiral semiconducting nanocrystals become optically active in the presence of screw dislocations, which can naturally develop during the nanocrystal growth. We show the new properties to emerge due to the dislocation-induced distortion of the crystal lattice and the associated alteration of the nanocrystal’s electronic subsystem, which essentially modifies its interaction with external optical fields. The g-factors of intraband transitions in our nanocrystals are found comparable with dissymmetry factors of chiral plasmonic complexes, and exceeding the typical g-factors of chiral molecules by a factor of 1000. Optically active semiconducting nanocrystals—with chiral properties controllable by the nanocrystal dimensions, morphology, composition and blending ratio—will greatly benefit chemistry, biology and medicine by advancing enantiomeric recognition, sensing and resolution of chiral molecules.

Independent Composition and Size Control for Highly Luminescent Indium-Rich Silver Indium Selenide Nanocrystals.

Ternary I-III-VI nanocrystals, such as silver indium selenide (AISe), are candidates to replace cadmium- and lead-based chalcogenide nanocrystals as efficient emitters in the visible and near IR, but, due to challenges in controlling the reactivities of the group I and III cations during synthesis, full compositional and size-dependent behavior of I-III-VI nanocrystals is not yet explored. We report an amide-promoted synthesis of AISe nanocrystals that enables independent control over nanocrystal size and composition. By systematically varying reaction time, amide concentration, and Ag- and In-precursor concentrations, we develop a predictive model for the synthesis and show that AISe sizes can be tuned from 2.4 to 6.8 nm across a broad range of indium-rich compositions from AgIn11Se17 to AgInSe2. We perform structural and optical characterization for representative AISe compositions (Ag0.85In1.05Se2, Ag3In5Se9, AgIn3Se5, and AgIn11Se17) and relate the peaks in quantum yield to stoichiometries exhibiting defect ordering in the bulk. We optimize luminescence properties to achieve a record quantum yield of 73%. Finally, time-resolved photoluminescence measurements enable us to better understand the physics of donor-acceptor emission and the role of structure and composition in luminescence.

Monodisperse Copper Chalcogenide Nanocrystals: Controllable Synthesis and the Pinning of Plasmonic Resonance Absorption.

Controllable synthesis of copper chalcogenide nanocrystals (NCs), including desired geometry, composition and surrounding environment, is of high significance for the modulation of their optoelectronic response and the corresponding applications. Herein, copper nitride nanoparticles have been used as "uncontaminated" copper precursors to synthesize copper chalcogenide NCs with high monodispersity through a one-pot strategy. In this protocol, the sizes and compositions of NCs can be readily controlled by varying the ratio of the precursors. For Cu(2-x)S NCs with different diameters, the size variations are all smaller than 5.6%. Furthermore, the plasmonic properties of the copper chalcogenide NCs are investigated under a steady state by tuning the plasmonic resonance absorption band to a limiting condition (denoted "pinning" phenomena). It is observed that the pinning frequency increases (from 1.09 to 1.23 eV) with the increment of the NC size (from 5.4 ± 0.3 to 11.1 ± 0.4 nm), explained by introducing surface scattering. Meanwhile, the frequencies of ternary alloyed copper sulfide selenide NCs blue-shift from 0.90 to 1.00 eV with the increase of selenium content from 11% to 66%, which is related to the effective mass of free carriers. Additionally, the plasmonic absorption bands of Cu(2-x)S NCs encapsulated by two single-layer graphene pin at 1525-1550 nm during the oxidation process, which is influenced by both the dielectric constant and redox potential of the surrounding environment. This study demonstrates the controllable synthesis and precise fundamental plasmonic properties of the copper chalcogenide NCs, ensuring the potential plasmonic-related techniques with high efficiency, accuracy and excellent spatial resolution.

Studies on the structural, thermal, and dielectric properties of fabricated Nylon 6,9/CaCu3Ti4O12 nanocomposites

AbstractNylon 6,9/CaCu3Ti4O12 (CCTO) nanocrystal composites with relatively high dielectric permittivity (220 at 100 Hz) were fabricated by melt mixing followed by hot pressing. The CCTO nanoceramics were synthesized using the oxalate precursor route, and the transmission electron microscopy studies exhibited that the crystallites are in the range of 20–200 nm. The nanocomposites were characterized using X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and impedance analyzer to study their structural, thermal, and dielectric properties. The introduction of CCTO nanoparticles into the matrix had influenced the thermal properties. The effective dielectric permittivity of the nanocomposite increased by the augmentation of CCTO content in the Nylon 6,9. Our experimental outcomes showed that the fixed dielectric permittivity of such two-phase composite was established above 200 when the CCTO concentration was closer to its percolation threshold. The room temperature dielectric permittivity as high as 220 at 100 Hz has been achieved when the CCTO content increased to 58 vol% in the polymer and this was increased to 3845 at 150°C. The increase in AC conductivity with the increase in the CCTO content in the polymer matrix supported the hopping of the charge carrier conduction mechanism.

Nonthermal plasma synthesis of metal sulfide nanocrystals from metalorganic vapor and elemental sulfur

Nanocrystal synthesis in nonthermal plasmas has been focused on elemental group IV semiconductors such as Si and Ge. In contrast, very little is known about plasma synthesis of compound nanocrystals and the time is ripe to extend this synthesis approach to nanocrystals comprised of two or more elements such as metal sulfides, oxides and nitrides. Towards this end, we studied, in an argon–sulfur plasma, the synthesis of ZnS, Cu2S and SnS nanocrystals from metalorganic precursors diethyl Zn(II), hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane, and tetrakis(dimethylamido) Sn(IV), respectively. In situ optical emission spectroscopy was used to observe changes in relative concentrations of various plasma species during synthesis, while ex situ material characterization was used to examine the crystal structure, elemental composition and optical absorption of these nanocrystals. For a constant metalorganic vapor feed rate, the elemental composition of the nanocrystals was found to be independent of the sulfur flow rate into the plasma, above a small threshold value. At constant sulfur flow rate, the nanocrystal composition depended on the metalorganic vapor feed rate. Specifically, the ensemble metal atomic fraction in the nanocrystals was found to increase with increasing metalorganic vapor flow rates, resulting in more metal-rich crystal phases. The metalorganic feed rate can be used to control the composition and crystal phase of the metal-sulfide nanocrystals synthesized using this plasma process.

(Invited) Colloidal Nanocrystal Composites with High and Low Thermal Conductivity

We present our work on thermal transport in colloidal nanocrystal composites within the context of thermoelectrics and thermal energy management. Colloidal nanocrystals consist of an inorganic crystalline core with short ligand molecules bound to the surface. These nanocrystals can be made with excellent control over size, shape, and composition, thereby enabling microstructural control over thermal energy transport. Our first two topics focus on low thermal conductivity materials for thermoelectricity: semiconductor nanocrystal thin films and semiconductor matrix – semiconductor nanoparticle composites. Our last topic focuses on metal matrix – metal nanoparticle composites with high thermal conductivity, which can be used to thermally manage transient power spikes in high power electronics. For the first topic, we systematically study the effect of nanocrystal diameter and surface chemistry on thermal transport in lead chalcogenide nanocrystal thin films. These samples exhibit very low thermal conductivities ranging from 0.1 to 0.5 W/m-K and we find that larger nanocrystals and shorter ligands generally lead to higher thermal conductivities. Our measurements also show that the effect of surface chemistry on thermal transport becomes more pronounced as nanocrystal diameter decreases. For the second topic, we focus on polycrystalline In2Se3 matrices with embedded CdSe nanocrystals ranging from 0 to ~100 vol%. We find that the thermal conductivities of these composites are very low over the entire nanocrystal volume fraction range and vary from 0.3 to 0.5 W/m-K. We also find that the presence of CdSe nanocrystals strongly effects the formation of the In2Se3 matrix (i.e. grain orientation and size as well as ternary phase formation). Lastly, we present our work on phase change Bi nanocrystals embedded in a solid Ag-matrix. The latent heat of the phase change Bi nanocrystals can absorb large quantities of transient heat generated during power spikes in power electronics operation. In addition, fast thermal energy transport is facilitated by the Ag-matrix, which leads to high composite thermal conductivities ranging from 101 to 102 W/m-K. We also use size-dependent melting of the phase change nanocrystals as a design variable for the thermal energy absorption temperature.

Insights into the Structure of Dot@Rod and Dot@Octapod CdSe@CdS Heterostructures

CdSe@CdS dot@rods with diameter around 6 nm and length of either 20, 27, or 30 nm and dot@octapods with pod diameters of ∼15 nm and lengths of ∼50 nm were investigated by X-ray absorption spectroscopy. These heterostructures are prepared by seed-mediated routes, where the structure, composition, and morphology of the CdSe nanocrystals used as a seed play key roles in directing the growth of the second semiconducting domain. The local structural environment of all the elements in the CdSe@CdS heterostructures was investigated at the Cd, S, and Se K-edges by taking advantage of the selectivity of X-ray absorption spectroscopy, and was compared to pure reference compounds. We found that the structural features of dot@rods are independent of the size of the rods. These structures can be described as made of a CdSe dot and a CdS rod, both in the wurtzite phase with a high crystallinity of both the core and the rod. This result supports the effectiveness of high temperature colloidal synthesis in promoting the formation of core@shell nanocrystals with very low defectivity. On the other hand, data on the CdSe@CdS with octapod morphology suggest the occurrence of a core composed of a CdSe cubic sphalerite phase with eight pods made of CdS wurtzite phase. Our findings are compared to current models proposed for the design of functional heterostructures with controlled nanoarchitecture.

Mild solvothermal synthesis of Cu2ZnSn(SxSe1−x)4 nanocrystals with tunable phase structure and composition

Cu2ZnSn(SxSe1−x)4 (CZTSSe) nanocrystals (NCs) are synthesized via a one-pot solvothermal method under the mild reaction condition. By varying the S/Se ratio in the solution, the chalcogen composition of CZTSSe NCs can be well controlled over the whole range, leading to a tunable bandgap from 1.11 to 1.49 eV. Furthermore, it reveals that the S/Se ratio of anionic precursors has a dramatic effect on the phase structure of CZTSSe NCs. When the S/(S + Se) ratio (x) is higher than 0.5, wurtzite (WZ)-derived structure is the dominant phase; on the contrary, the zinc blende (ZB)-derived structure overwhelms the former as x gradually reduces from 0.5 to 0. Our findings thus suggest a novel and facile method to obtain CZTSSe NCs with metastable WZ phase, which is distinguished from previously reported routes based on carefully choosing organic solvents and/or capping ligands. In addition, the obtained CZTSSe NCs are readily dispersed in low boiling point and low toxicity solvent to form homogeneous and stable NCs ink. The photocurrent response measurement on the CZTSSe NCs films demonstrates its potential as a low-cost, high-reproducibility and high-yield NC preparing approach towards efficient CZTSSe solar cells.

Electronic doping and redox-potential tuning in colloidal semiconductor nanocrystals.

Electronic doping is one of the most important experimental capabilities in all of semiconductor research and technology. Through electronic doping, insulating materials can be made conductive, opening doors to the formation of p-n junctions and other workhorses of modern semiconductor electronics. Recent interest in exploiting the unique physical and photophysical properties of colloidal semiconductor nanocrystals for revolutionary new device technologies has stimulated efforts to prepare electronically doped colloidal semiconductor nanocrystals with the same control as available in the corresponding bulk materials. Despite the impact that success in this endeavor would have, the development of general and reliable methods for electronic doping of colloidal semiconductor nanocrystals remains a long-standing challenge. In this Account, we review recent progress in the development and characterization of electronically doped colloidal semiconductor nanocrystals. Several successful methods for introducing excess band-like charge carriers are illustrated and discussed, including photodoping, outer-sphere electron transfer, defect doping, and electrochemical oxidation or reduction. A distinction is made between methods that yield excess band-like carriers at thermal equilibrium and those that inject excess charge carriers under thermal nonequilibrium conditions (steady state). Spectroscopic signatures of such excess carriers, accessible by both equilibrium and nonequilibrium methods, are reviewed and illustrated. A distinction is also proposed between the phenomena of electronic doping and redox-potential shifting. Electronically doped semiconductor nanocrystals possess excess band-like charge carriers at thermal equilibrium, whereas redox-potential shifting affects the potentials at which charge carriers are injected under nonequilibrium conditions, without necessarily introducing band-like charge carriers at equilibrium. Detection of the key spectroscopic signatures of band-like carriers allows distinction between these two regimes. Both electronic doping and redox-potential shifting can be powerful tools for tuning the performance of nanocrystals in electronic devices. Finally, key chemical challenges associated with nanocrystal electronic doping are briefly discussed. These challenges are centered largely on the availability of charge-carrier reservoirs with suitable redox potentials and on the relatively poor control over nanocrystal surface traps. In most cases, the Fermi levels of colloidal nanocrystals are defined by the redox properties of their surface traps. Control over nanocrystal surface chemistries is therefore essential to the development of general and reliable strategies for electronically doping colloidal semiconductor nanocrystals. Overall, recent progress in this area portends exciting future advances in controlling nanocrystal compositions, surface chemistries, redox potentials, and charge states to yield new classes of electronic nanomaterials with attractive physical properties and the potential to stimulate unprecedented new semiconductor technologies.

Modulating Exciton Dynamics in Composite Nanocrystals for Excitonic Solar Cells.

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.

Abnormal Temperature Dependence of Mobility in a Disordered System With Traps: Experiment and Theory

Mobility μ is essential in electronic and optoelectronic devices. Abnormal temperature T dependence of the carrier mobility μ of poly [2-methoxy-5-2-ethyl-hexyloxy]-p-phenylenevinylene (MEH-PPV)/InP nanocrystal composites was observed using the time-of-flight technique. The slope of ln(μ) as a function of 1/T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> dramatically changed before and after the critical temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 283 K. Using the variable range hopping theoretical mode, a new trap-filling factor (FF) was proposed as a key parameter to illustrate the effect of the trap on the mobility. According to FF, there are three regions. When FF ≫ 1 and FF≪1, the carrier behavior follows the Fermi and Boltzmann distributions, respectively. When FF~1, the derived transition temperature of 282.4 K represents a transition point from Fermi to Boltzmann distribution, which is consistent with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> in the experiment. Finally, a universal dependence of ln(μ) on T was concluded based on FF in a disordered system with traps.

Pd–Pb Alloy Nanocrystals with Tailored Composition for Semihydrogenation: Taking Advantage of Catalyst Poisoning

Metallic nanocrystals (NCs) with well-defined sizes and shapes represent a new family of model systems for establishing structure-function relationships in heterogeneous catalysis. Here in this study, we show that catalyst poisoning can be utilized as an efficient strategy for nanocrystals shape and composition control, as well as a way to tune the catalytic activity of catalysts. Lead species, a well-known poison for noble-metal catalysts, was investigated in the growth of Pd NCs. We discovered that Pb atoms can be incorporated into the lattice of Pd NCs and form Pd-Pb alloy NCs with tunable composition and crystal facets. As model catalysts, the alloy NCs with different compositions showed different selectivity in the semihydrogenation of phenylacetylene. Pd-Pb alloy NCs with better selectivity than that of the commercial Lindlar catalyst were discovered. This study exemplified that the poisoning effect in catalysis can be explored as efficient shape-directing reagents in NC growth, and more importantly, as a strategy to tailor the performance of catalysts with high selectivity.

Composites of MnO2 nanocrystals and partially graphitized hierarchically porous carbon spheres with improved rate capability for high-performance supercapacitors

Manganese dioxide/carbon nanocomposites with partially graphitized hierarchical porous structure have been designed and synthesized. A high specific capacitance of 412Fg−1 and excellent rate capability of these composites can be achieved owing to the interconnected meso- and micro-porous structure and the graphitic pore walls facilitating the ion diffusion and electron transportation, respectively, which is highly demanded for high-performance supercapacitor electrodes materials. Even at a high scan rate of 100mVs−1, a specific capacitance of 251Fg−1 can be obtained, corresponding to 61% capacitance retention. Moreover, a long cycling stability with initial capacitance retention of ∼88% is obtained after over 4000cycles at a current density of 1.0Ag−1. This work presents an efficient electrode materials design and a novel composite which holds great promise in high-performance supercapacitor applications.