Jisheng Pan

Hybridization of graphene sheets and carbon-coated Fe3O4 nanoparticles as a synergistic adsorbent of organic dyes

Magnetic graphene–Fe3O4@carbon (GFC) hybrids with hierarchical nanostructures have been synthesized and their application as an adsorbent for the removal of organic dyes has been investigated. Graphene–Fe3O4 hybrids were first prepared via a facile one-pot solvothermal process, then carbonaceous coatings on Fe3O4 nanoparticles of nanometer thickness were synthesized by a hydrothermal carbonization process using eco-friendly glucose as a carbon source. Graphene sheets acting as two-dimensional (2D) substrates can effectively prevent the Fe3O4 nanoparticles from aggregating and enable a good dispersion of these magnetic nanoparticles. The carbonaceous layer protects the Fe3O4 nanoparticles in acidic environments and greatly enhances the specific surface area of the hybrids which is beneficial for the removal of organic dyes, such as methylene blue (MB). The resultant GFC hybrids exhibit great adsorption properties not only in water but also in acidic environments, and about 86% and 77% of the dye removal efficiency can be retained after five adsorption–desorption cycles in water and 1 M HCl, respectively. The rapid and efficient adsorption of organic dyes from water as well as acid suggests that the GFC hybrids have potential environmental applications as alternatives to commercial materials in wastewater treatment for the removal of organic dyes.

Synthesis of Fe nanoparticles@graphene composites for environmental applications

Fe nanoparticles@graphene composites (FGC) are successfully synthesized by using graphene oxide (GO) as a supporting matrix. GO is first treated with Fe(3+) to form Fe(3+)@GO complexes. Then, by adding NaBH(4) solution, Fe(3+) and GO are simultaneously reduced in situ to Fe and graphene respectively, forming FGC hybrid composites. The structures, properties and applications of the hybrids thus obtained are investigated by X-ray diffraction, Raman spectroscopy, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis and magnetization measurements. The hybrids are also evaluated for decolorization of methyl blue solution, a model dye in wastewater of dyeing industry. Compared with bare Fe particles, the high removal capacities of FGC are due to the increased adsorption sites in the hybrids, which are achieved by inhibiting the particle aggregation and reducing the size of Fe nanoparticles.

Nitrogen-Doped Graphene Nanoribbons as Efficient Metal-Free Electrocatalysts for Oxygen Reduction

Nitrogen-doped graphene nanoribbon (N-GNR) nanomaterials with different nitrogen contents have been facilely prepared via high temperature pyrolysis of graphene nanoribbons (GNR)/polyaniline (PANI) composites. Here, the GNRs with excellent surface integration were prepared by longitudinally unzipping the multiwalled carbon nanotubes. With a high length-to-width ratio, the GNR sheets are prone to form a conductive network by connecting end-to-end to facilitate the transfer of electrons. Different amounts of PANI acting as a N source were deposited on the surface of GNRs via a layer-by-layer approach, resulting in the formation of N-GNR nanomaterials with different N contents after being pyrolyzed. Electrochemical characterizations reveal that the obtained N8.3-GNR nanomaterial has excellent catalytic activity toward an oxygen reduction reaction (ORR) in an alkaline electrolyte, including large kinetic-limiting current density and long-term stability as well as a desirable four-electron pathway for the formation of water. These superior properties make the N-GNR nanomaterials a promising kind of cathode catalyst for alkaline fuel cell applications.

Reaction of SiO2 with hafnium oxide in low oxygen pressure

A dynamic process consisting of a series of reactions during deposition of HfO2 films on SiO2-covered silicon under oxygen-deficient conditions is identified. The oxygen-deficient HfOx<2 layer absorbs the oxygen in the SiO2 layer to form fully oxidized metal oxide film. As a result, there is no silicate and silicon oxide formed at the interface with silicon substrate. Thermodynamic analysis indicates that even if there is a layer of silicate forming at the initial stage of deposition, the silicate layer will be decomposed with the progress of HfOx<2 deposition.

Density functional study on ferromagnetism in nitrogen-doped anatase TiO2

We report first principles calculations on the magnetism and electronic structures for nitrogen-doped anatase TiO2 (N:TiO2). Our calculations indicate that magnetic state is the ground state for N:TiO2 systems. An isolated N atom produces a total magnetic moment of 1.00μB and introduces spin-polarized 2p states in the band gap. The origin of the magnetic moments is the holes in N 2p π band of the N dopant. Several doping configurations studied suggest the existence of ferromagnetic coupling between N dopants. The ferromagnetism in N:TiO2 can be attributed to the hole-mediated double exchange through the strong p-p interaction between N and O.

Symmetrical negative differential resistance behavior of a resistive switching device

With a thin insulator sandwiched between two electrodes, the negative differential resistance (NDR) behavior has been frequently reported for its potential device applications. Here we report the experimental observation of a symmetric NDR characteristic in a resistive switching device based on TiO(2). We propose a charge storage mechanism for the NDR effect, with oxygen molecular ions working as the active source, in a thin insulating layer. Current-voltage measurements demonstrated a highly reproducible state at about 0.65 eV, and the photoelectron spectroscopy measurements showed that it complies well with the Ti3d band gap state. Our first-principle calculations confirm that charge storage and release arise from trapping and detrapping of oxygen molecular ions at the defect sites. The results and mechanism demonstrated here in a thin layer could be extended to other systems approaching molecular dimensions for device applications.

Effect of nitrogen incorporation on the electronic structure and thermal stability of HfO2 gate dielectric

The effect of nitrogen incorporation on the electronic structure and thermal stability of HfO2 gate dielectric was investigated by using photoemission study and first-principles calculation. Hafnium oxynitride (HfON) dielectric shows higher thermal stability in comparison to pure HfO2 on Si. Atomic N can passivate O vacancies in the dielectrics during nitridation process, but the N atoms incorporated into interstitial sites cause band gap reduction. Postnitridation annealing is required to activate interstitial N atoms to form stable N–Hf bonds, which will increase the band gap and band offset of as-nitrided dielectric film.

Magnetic nanomaterial derived from graphene oxide/layered double hydroxide hybrid for efficient removal of methyl orange from aqueous solution

Magnetic hybrid nanomaterials composed of reduced graphene oxide, zero-valent nickel, and NiAl-mixed metal oxides (rGO/Ni/MMO) have been synthesized by calcining graphene oxide (GO)/layered double hydroxide (LDH) hybrid in nitrogen atmosphere. Structural characterizations demonstrate that with the presence of GO substrate, NiAl-LDHs can be reduced into zero-valent Ni and NiAl-MMOs during calcination. Transmission electron microscopy (TEM) is used to investigate the morphology of the as-prepared hybrid nanomaterials, demonstrating that the introduction of GO substrate prevents the aggregation of LDHs. Magnetism characterization proves the ferromagnetic property of rGO/Ni/MMO hybrid. This magnetic hybrid nanomaterial exhibits excellent adsorption ability toward methyl orange (MO) in aqueous solutions. The kinetics of the adsorption process and the adsorption isotherm are investigated. The MO removal process is found to obey the Redlich-Peterson isotherm model, and its kinetics follows pseudo-second-order rate equation. In addition, the magnetic hybrid also exhibits good recycle ability for MO removal. This novel magnetic hybrid nanomaterial derived from GO/LDH hybrid demonstrates great potential in the applications of water treatment.

Germanium–Tin (GeSn) p-Channel MOSFETs Fabricated on (100) and (111) Surface Orientations With Sub-400

In this letter, we report the first study of the dependence of carrier mobility and drive current I_Dsat of Ge_0.958Sn_0.042 p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs) on surface orientations. Compressively strained Ge_0.958Sn_0.042 channels were grown on (100) and (111) Ge substrates. Sub-400°C Si₂H₆ treatment was introduced for the passivation of the GeSn surface prior to gate stack formation. Source/ drain series resistance and subthreshold swing S were found to be independent of surface orientation. The smallest reported S of 130 mV/decade for GeSn pMOSFETs is achieved. The (111)-oriented device demonstrates 13% higher IDsat over the (100)oriented one at a V_GS-V_TH of -0.6 V and V_DS of -0.9 V. We also found that GeSn pMOSFETs with (111) surface orientation show 18% higher hole mobility than GeSn pMOSFETs with (100) orientation.

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The nitrogen-doping induced changes in optical properties and electronic structures of SrTiO3 films have been investigated by using spectroscopic ellipsometry and x-ray photoemission spectroscopy. Combined with the first-principles calculations, it is found that the localized N 2p states above O 2p states are attributed to the new absorption edge at 500nm and the photoactivity in the visible light region. Our results are consistent with both recent experimental and theoretical studies on nitrogen-doped TiO2, where the visible light responses arise from the localized N 2p states slightly above the valence-band edge rather than the band gap narrowing.