Facile electrochemical synthesis of ultrathin iron oxyhydroxide nanosheets for the oxygen evolution reaction

We propose a facile approach to synthesise ultrathin iron oxyhydroxide nanosheets for use in catalysing the electrochemical oxygen evolution reaction. This two dimensional material lowers the overpotential and provides a platform for further performance enhancement via integration of species such as nickel into an ultrathin nanosheet structure

Co valence and possible spin transformation in diluted magnetic semiconductors Zn/sub 1-z/Mg/sub z/Co/sub 0.15/O and Zn/sub 1-x/Co/sub x/O

In this paper, possible spin transformation and Co valence in dilute magnetic semiconductors is studied. Polycrystalline samples of Zn/sub 1-x/Co/sub x/O (0.05/spl les/x/spl les/0.17) and Zn/sub 1-z/Mg/sub z/Co/sub 0.15/O are prepared by rapid oxalate decomposition technique. X-ray diffraction is used to determine phase purity of the samples. Co valence state 2+ is determined by X-ray absorption near edge spectroscopy (XANES) using synchrotron irradiation. Magnetic properties measured show that all samples are paramagnetic and magnetization hysteresis measurement indicated that there is no trace of ferromagnetism. From Curie-Weiss fittings at high temperature region, the effective magnetic moment (/spl mu//sub eff/) is 3.87/spl mu//sub B//Co which corresponds to that of tetrahedral Co/sup 2+/ high spin state. When fitting at T approaches 0 K, /spl mu//sub eff/ = 2.82/spl mu//sub B//Co is observed indicating a possible spin state transition to Co/sup 2+/ low spin state

Very strong intrinsic supercurrent carrying ability and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that single crystals of (Ba,K)Fe2As2 with Tc = 32 K have a pinning potential, U0, as high as 10^4 K, with U0 showing very little field depend-ence. In addition, the (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The rigid vortices in the two dimensional (Ba,K)Fe2As2 distinguish this compound from 2D high Tc cuprate superconductors with 2D vortices, and make it being capable of cearrying very high critical current.Flux jumping due to high Jc was also observed in large samples at low temperatures.Comment: 4 pages, 7 figures. submitte

Highly conductive carbon nanotube-graphene hybrid yarn

An efficient procedure for the fabrication of highly conductive carbon nanotube/graphene hybrid yarns has been developed. To start, arrays of vertically aligned multi-walled carbon nanotubes (MWNT) are converted into indefinitely long MWNT sheets by drawing. Graphene flakes are then deposited onto the MWNT sheets by electrospinning to form a composite structure that is transformed into yarn filaments by twisting. The process is scalable for yarn fabrication on an industrial scale. Prepared materials are characterized by electron microscopy, electrical, mechanical, and electrochemical measurements. It is found that the electrical conductivity of the composite MWNT-graphene yarns is over 900 S/cm. This value is 400% and 1250% higher than electrical conductivity of pristine MWNT yarns or graphene paper, respectively. The increase in conductivity is asssociated with the increase of the density of states near the Fermi level by a factor of 100 and a decrease in the hopping distance by an order of magnitude induced by grapene flakes. It is found also that the MWNT-graphene yarn has a strong electrochemical response with specific capacitance in excess of 111 Fg-1. This value is 425% higher than the capacitance of pristine MWNT yarn. Such substantial improvements of key properties of the hybrid material can be associated with the synergy of MWNT and graphene layers in the yarn structure. Prepared hybrid yarns can benefit such applications as high-performance supercapacitors, batteries, high current capable cables, and artificial muscles

Studies on diluted oxide magnetic semiconductors for spin electronic applications

Conventional semiconductor electronics is based on the charge of the electron. For a long time the spin of the electron has been ignored in the field of conventional electronics. Spintronics, also called spin electronics, agnetoelectronics or magnetotronics, is a newly emerging field in solid state physics and information technology. One of the major challenges for emiconductor spintronic devices is to develop suitable novel spin-polarized magnetic semiconducting materials that will effectively allow spin-polarized carriers to be injected, transported, and manipulated. Therefore, searching for new materials has become crucial from the viewpoints of both fundamental research and practical applications. Diluted magnetic semiconductors (DMS) are one of the most promising candidates for spintronic application. The research on the DMS materials which has been carried out worldwide in the past decade has been reviewed in this thesis. A DMS material can be realized when a conventional host semiconductor, such as GaAs, ZnO, etc., is doped with magnetic impurities, usually transition metal (TM) ions. For practical application, DMS material should favorably be ferromagnetic (FM) at room temperature. Early studies on DMS materials showed that FM can be induced in Mn doped III-V semiconductors. However, these materials are not suitable for practical applications as their Curie temperatures are quite low. On the other hand, some theoretical works predicted room temperature ferromagnetism in TM doped oxide semiconductors. This fact has boosted research in the field of DMS materials. The number of reports on observations of room temperature FM in Co, Mn, Ni, and Cr doped ZnO and TiO2 semiconducting oxides is constantly growing. The aim of this thesis was to study the doping effects of transition metal ions on the structure, transport, and diluted magnetic properties of various host oxide semiconductors. The oxide semiconductors investigated in this work are: ZnO, CuO, Ga2O3, and In2O3. A search for room temperature ferromagnetic semiconductors was the key point of this research. In addition, we have tried to understand and explain the possible origins of the magnetic properties of the samples produced, because at the present time there is no firm theoretical model that could explain magnetism in DMS materials. The majority of the samples studied in this research were prepared by a conventional solid state synthesis technique. We have carried out X-ray diffraction and electricalmagnetic transport measurements to determine the crystal structure, electrical and magnetic properties of our samples. In order to investigate the valence state of transition metal ions in the prepared materials, X-ray absorption near edge spectroscopy analysis was used. The major results from this PhD study are: (1) Polycrystalline Co-doped ZnO oxide samples were prepared with Co doping levels varying between 1 and 10%. All samples were found to be paramagnetic without any trace of ferromagnetism at room temperature and were insulators. Introduction of In ions into the system decreased the electrical resistivity of the samples. The spin state assessment revealed that strong spin-orbital coupling is present in In containing samples. Valence state assessment showed that in ZnO Co is present in the 2+ valence state. (2) Mn doped CuO bulk samples showed a ferromagnetic transition at 80 K. All the samples prepared were insulating. In and Zn were used as charge donors. It was found that the In solubility limit in CuO lattice is very limited, less than 1%. The magnetic properties that were measured showed a large decrease in the magnetic susceptibility of (Mn,Zn) and (Mn,In) co-doped CuO samples. This could be attributed to the formation of large amounts of antiferromagnetic impurities and phase segregation in the samples. Valence state assessment showed that Mn is present in the 2+ valence state, eliminating the possibility of a double exchange interaction mechanism in this system. (3) Various transition metal ions, such as Mn, Fe, Cr, and Ni, were doped into In2O3 and indium-tin oxide (ITO). In contrast to the reported data, our Fe doped In2O3 samples were paramagnetic. Paramagnetism was also observed in Cr doped In2O3. Mn doped In2O3 samples were insulators with a Curie temperature of 46 K, while Mn doped ITO samples were typical semiconductors with the same Curie temperature. Furthermore, these samples showed a large positive MR effect below the ferromagnetic transition temperature, reaching 20% at a temperature of 5 K. Ni doped In2O3 and ITO samples were also found to be ferromagnetic at room temperature. Electrical transport properties, though, were different in nature. Ni doped In2O3 was found to be a typical semiconductor, while the electrical conductivity of Ni doped ITO was found to be characteristic of metallic materials. (4) (Fe,Mn) co-doped In2O3 and ITO samples were ferromagnetic at room temperature, being both conducting and insulating depending on the host semiconductor. The change in lattice parameter a was very dependent on the ratio of Mn to Fe in the system, with decrease in lattice parameter a as Fe content increased. The maximum saturation magnetization was found for an In1.80Mn0.12Fe0.08O3 sample, which reached 0.35 μB/(Mn,Fe) ions at a temperature of 300 K. (Mn,Fe) co-doped In2O3 samples were insulating at room temperature, while (Fe,Cr) co-doped In2O3 samples were both conducting and ferromagnetic at room temperature. In addition, (Fe,Cr) co-doped samples showed a large positive MR effect, i.e. 5% at 5 K. On the contrary, despite being good conductors, (Mn,Fe) co-doped ITO samples did not exhibit similar MR features. (5) (RE,Fe) co-doped In2O3 polycrystalline samples were semiconducting and showed giant positive magnetoresistance at 5 K. The obtained magnetoresistance in (Eu,Fe) co-doped In2O3 reached 80 % at 5 K. This value is the largest reported MR value for any diluted magnetic semiconductor. In addition (RE,Fe) co-doped samples showed clear ferromagnetic hysteresis behavior at 300 K. TEM studies of these samples revealed that particles are well formed and are about 100 nm in size. Based on the results, among the transition metal doped oxide semiconductors studied, In2O3 and ITO are the most promising candidates for diluted semiconductor materials with possible practical applications in spintronic devices

Absence of ferromagnetism and strong orbital coupling in carrier rich Zn/sub 1-x/In/sub x/Co/sub 0.075/O

Polycrystalline samples of Zn/sub 1-x/In/sub x/Co/sub 0.075/O (0.010/spl les/x/spl les/0.020) diluted magnetic semiconductors were prepared by the rapid oxalate decomposition technique. Phase purity is analyzed by means of X-ray diffraction technique and structures are refined by Rietveld refinement technique. Bulk conductivity of the polished pellets was measured by two-point probe technique at 295 K. Magnetic properties were analyzed with magnetic property measurement system. Samples show paramagnetic behavior and Curie-Weiss fitting at high temperature range gave effective magnetic moment of Co ions. Magnetization behavior caused by applied magnetic field was also investigated. High itinerant carrier concentration was achieved but no ferromagnetism was observed in the samples

Magnetic and transport properties of transition metal doped polycrystalline In2O3

Synthesis and characterization of transition metal (TM)-doped In 2O3 oxide is reported. Most of the samples were found to be paramagnetic, and only Ye and Cr co-doped sample showed as light trace of ferromagnetism at 300 K with saturation magnetization M s = 0.35 emug/g. Measured transport properties revealed significant differences in transport among the samples. The absolute value of electrical resistivity for the In1.8Fe0.1Mn0.1O3 sample at 300 K was rho= 9.4 x 103 Omega middot cm, while In1.8Fe0.1Cr0.1O3 had rho = 62 Omega middot cm at the same temperature. Furthermore, magnetoresistance (MR) studies of In1.8Fe0.1Cr0.1O3 showed that the sign of MR changes from negative (200 K), with MR(200 K) = -0.2%, to positive T \u3c 50 K reaching maximum absolute value at 10 K, i.e., MR(10K) = 5.2 %

Absence of ferromagnetism and strong spin-orbital coupling in polycrystalline in and Co codoped Zn/sub 1-x/Co/sub 0.075/In /sub x/O oxide

Polycrystalline samples of In and Co codoped ZnO (Zn1-xInxCo0.075O; 0.010≤x≤0.020) oxide were prepared by solid-state synthesis technique. Phase purity and structure refinement done by means of the Rietveld analysis technique shows that both Co and In substitute properly into Zn positions. In doping, increased bulk conductivity of the samples at room temperature indicates an increase of charge carrier concentration. All samples showed paramagnetic behavior following Curie-Weiss law at close to room temperatures, with short range antiferromagnetic interaction with Θ≈-200 K. Effective magnetic moment (μeff) calculations showed a strong orbital contribution to the value of μeff, increasing with an increase of In content (x)