David P. Young

One-Pot Synthesis of Magnetic Graphene Nanocomposites Decorated with Core@Double-shell Nanoparticles for Fast Chromium Removal

A facile thermodecomposition process to synthesize magnetic graphene nanocomposites (MGNCs) is reported. High-resolution transmission electron microscopy and energy filtered elemental mapping revealed a core@double-shell structure of the nanoparticles with crystalline iron as the core, iron oxide as the inner shell and amorphous Si-S-O compound as the outer shell. The MGNCs demonstrate an extremely fast Cr(VI) removal from the wastewater with a high removal efficiency and with an almost complete removal of Cr(VI) within 5 min. The adsorption kinetics follows the pseudo-second-order model and the novel MGNC adsorbent exhibits better Cr(VI) removal efficiency in solutions with low pH. The large saturation magnetization (96.3 emu/g) of the synthesized nanoparticles allows fast separation of the MGNCs from liquid suspension. By using a permanent magnet, the recycling process of both the MGNC adsorbents and the adsorbed Cr(VI) is more energetically and economically sustainable. The significantly reduced treatment time required to remove the Cr(VI) and the applicability in treating the solutions with low pH make MGNCs promising for the efficient removal of heavy metals from the wastewater.

High-temperature weak ferromagnetism in a low-density free-electron gas

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB6) which, we argue, show that—at a density of 7× 1019 electrons cm−3—the ground state is ferromagnetically polarized with a saturation moment of 0.07 µB per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.

Magnetic polyaniline nanocomposites toward toxic hexavalent chromium removal

The removal of toxic hexavalent chromium (Cr(VI)) from polluted water by magnetic polyaniline (PANI) polymer nanocomposites (PNCs) was investigated. The PNCs were synthesized using a facile surface initiated polymerization (SIP) method and demonstrated unique capability to remove Cr(VI) from polluted solutions with a wide pH range. Complete Cr(VI) removal from a 20.0 mL neutral solution with an initial Cr(VI) concentration of 1.0–3.0 mg L−1 was observed after a 5 min treatment period with a PNC load of 10 mg. The PNC dose of 0.6 g L−1 was found to be sufficient for complete Cr(VI) removal from 20.0 mL of 9.0 mg L−1 Cr(VI) solution. The saturation magnetization was observed to have no obvious decrease after treatment with Cr(VI) solution, and these PNCs could be easily recovered using a permanent magnet and recycled. The Cr(VI) removal kinetics were determined to follow pseudo-first-order behavior with calculated room temperature pseudo-first-order rate constants of 0.185, 0.095 and 0.156 min−1 for the solutions with pH values of 1.0, 7.0 and 11.0, respectively. The Cr(VI) removal mechanism was investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and energy-filter transmission electron microscopy (EFTEM). The results showed that PANI was partially oxidized after treatment with Cr(VI) solution, with Cr(VI) being reduced to Cr(III). The EFTEM observation indicated that the adsorbed Cr(III) had penetrated into the interior of the PNCs instead of simply adsorbing on the PNC surface. This synthesized material was found to be easily regenerated by 1.0 mol L−1p-toluene sulfonic acid (PTSA) or 1.0 mol L−1 hydrochloric acid (HCl) and efficiently reused for further Cr(VI) removal.

Carbon nanostructure-derived polyaniline metacomposites: electrical, dielectric, and giant magnetoresistive properties.

Polyaniline (PANI) nanocomposites incorporating different loadings of graphene and various other carbon nanostructures including carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been synthesized using a surface-initiated polymerization (SIP) method. Transmission electron microscopy (TEM) results indicate that the graphene has been exfoliated into a few layers (typically one, two, and three layers) during polymerization and has been uniformly dispersed in the PANI matrix. The graphene layer dispersion degree is quantified by a free-path spacing measurement (FPSM) method based on the TEM microstructures. The SIP method also demonstrates its feasibility for coating PANI on one-dimensional (1D) CNFs and CNTs without introducing additional surface functional groups. The effects of graphene size, loading level, and surface functionality on the electrical conductivity and dielectric permittivity of their corresponding nanocomposites have been systematically studied. The temperature-dependent conductivity behavior revealed a quasi-3D variable range hopping (VRH) electron transport mechanism for all the nanocomposites. Giant magnetoresistance (GMR) at room temperature is observed in pure PANI, which can be enhanced by the incorporation of a high loading of graphene (5%) due to the π-π stacking-induced efficient electron transport at the PANI/graphene interface. More interestingly, negative permittivity is found in each composite which can be easily tuned by adjusting the filler loading, morphology, and surface functionality.

Polyaniline-tungsten oxide metacomposites with tunable electronic properties†

Polyaniline (PANI) nanocomposites reinforced with tungsten oxide (WO3) nanoparticles (NPs) and nanorods (NRs) are fabricated via a facile surface-initiated-polymerization (SIP) method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations reveal the uniform coating of polymer on the filler surface and a good dispersion of the nanofillers within the polymer matrix. Unique negative permittivity is observed in pure PANI and its nanocomposites. The switching frequency (frequency where real permittivity switches from negative to positive) can be easily tuned by changing the particle loading and filler morphology. Conductivity measurements are performed from 50∼290 K, and results show that the electron transportation in the nanocomposites follows a quasi 3-d variable range hopping (VRH) conduction mechanism. The extent of charge carrier delocalization calculated from VRH well explains the dielectric response of the metacomposites. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) reveal an enhanced thermal stability of the nanocomposites with the addition of nanofillers as compared to that of pure PANI.

Polyaniline Stabilized Magnetite Nanoparticle Reinforced Epoxy Nanocomposites

Magnetic epoxy polymer nanocomposites (PNCs) reinforced with magnetite (Fe(3)O(4)) nanoparticles (NPs) have been prepared at different particle loading levels. The particle surface functionality tuned by conductive polyaniline (PANI) is achieved via a surface initiated polymerization (SIP) approach. The effects of nanoparticle loading, surface functionality, and temperature on both the viscosity and storage/loss modulus of liquid epoxy resin suspensions and the physicochemical properties of the cured solid PNCs are systematically investigated. The glass transition temperature (T(g)) of the cured epoxy filled with the functionalized NPs has shifted to the higher temperature in the dynamic mechanical analysis (DMA) compared with that of the cured pure epoxy. Enhanced mechanical properties of the cured epoxy PNCs filled with the functionalized NPs are observed in the tensile test compared with that of the cured pure epoxy and cured epoxy PNCs filled with as-received NPs. The uniform NP distribution in the cured epoxy PNCs filled with functionalized NPs is observed by scanning electron microscope (SEM). These magnetic epoxy PNCs show the good magnetic properties and can be attached by a permanent magnet. Enhanced interfacial interaction between NPs and epoxy is revealed in the fracture surface analysis. The PNCs formation mechanism is also interpreted from the comprehensive analysis based on the TGA, DSC, and FTIR in this work.

Magnetoresistance from quantum interference effects in ferromagnets

The desire to maximize the sensitivity of read/write heads (and thus the information density) of magnetic storage devices has stimulated interest in the discovery and design of new magnetic materials exhibiting magnetoresistance. Recent discoveries include the ‘colossal’ magnetoresistance in the manganites and the enhanced magnetoresistance in low-carrier-density ferromagnets. An important feature of these systems is that the electrons involved in electrical conduction are different from those responsible for the magnetism. The latter are localized and act as scattering sites for the mobile electrons, and it is the field tuning of the scattering strength that ultimately gives rise to the observed magnetoresistance. Here we argue that magnetoresistance can arise by a different mechanism in certain ferromagnets—quantum interference effects rather than simple scattering. The ferromagnets in question are disordered, low-carrier-density magnets where the same electrons are responsible for both the magnetic properties and electrical conduction. The resulting magnetoresistance is positive (that is, the resistance increases in response to an applied magnetic field) and only weakly temperature-dependent below the Curie point.

Antimonides with the half-Heusler structure: New thermoelectric materials

The thermoelectric properties near ambient temperature of half-Heusler alloys based on LnPdSb, where Ln=Ho, Er, and Dy are reported. The Seebeck coefficients are large, between 60 and 250 μV/K, and the materials are p type. The resistivities are between 0.6 and 20 mΩ cm. Thermal conductivities are between approximately 5.0 and 3.5 W/mK at 300 K, and are smallest in intentionally disordered materials. The highest ambient temperature ZT obtained is 0.06. Band-structure calculations are presented for LuPdSb. It is suggested that half-Heusler alloys with 18 electrons per formula unit may represent a large class of thermoelectric materials.

Electrical and dielectric properties of polyaniline–Al2O3 nanocomposites derived from various Al2O3 nanostructures

Four Al2O3 nanostructures (i.e. nanofiber, nanoplatelet, nanorod and nanoflake) have been successfully synthesized via hydrothermal procedures followed by a dehydration process. Subsequently, polyaniline (PANI) nanocomposites incorporating these four Al2O3 nanostructures have been fabricated using a surface initialized polymerization (SIP) method. Both TEM and SEM are used to characterize the morphologies of the Al2O3 nanostructures and PANI/Al2O3 nanocomposites. X-Ray diffraction results reveal that the morphology of the nanofiller has a significant effect on the crystallization behavior of the PANI during polymerization. The electrical conductivity and dielectric permittivity of these nanocomposites are strongly related to both the morphology of the filler and the dispersion quality. Temperature-dependent-conductivity measurements from 50–290 K show that the electron transportation of the nanocomposites follows a quasi 3-d variable range hopping (VRH) conduction mechanism. The extent of charge carrier delocalization calculated from VRH is well correlated to the dielectric response of these nanocomposites. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results reveal an enhanced thermal stability of the PANI/Al2O3 nanocomposites as compared to that of pure PANI due to the strong interaction between the nanofillers and polymer matrix. The mechanism of the SIP method is also elaborated in this work.

Large anomalous Hall effect in a silicon-based magnetic semiconductor

Magnetic semiconductors are attracting great interest because of their potential use for spintronics, a new technology that merges electronics with the manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently emerged as the most popular materials for this new technology, and although their Curie temperatures are rising towards room temperature, these materials can only be fabricated in thin-film form, are heavily defective, and are not obviously compatible with Si. We show here that it is productive to consider transition metal monosilicides as potential alternatives. In particular, we report the discovery that the bulk metallic magnets derived from doping the narrow-gap insulator FeSi with Co share the very high anomalous Hall conductance of (GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens up a new arena for spintronics, involving a bulk material based only on transition metals and Si, which displays large magnetic-field effects on its electrical properties.