Synthesis, characterisation and properties of tantalum based inorganic nanofibres

This thesis describes the synthesis and characterisation of 1-dimensional nanometric phases using simple preparative reactions and a variety of characterisation methods. Comparison of properties between the bulk and nano-morphology has played a large part and is a common theme throughout. High aspect ratio tantalum disulfide, TaS2 nanofibres were prepared from a 1:2 stoichiometric mixture of elemental powders in a one-step synthesis utilising silica ampoules. A surface assisted growth phenomena was investigated and found to significantly increase the yield, both in quality and quantity. The resulting nanofibres were seen to retain and indeed enhance some of the bulk properties, e.g. a 50 fold increase in observed superconducting transition temperature. Changing the stoichiometry of the reactants to 1:3, produced tantalum trisulfide nanofibres. Tantalum trisulfide is of interest as it has pseudo 1-dimensional crystal structure and properties in the bulk. TEM and SAED have shown that the TaS3 unit cell is oriented with the b direction parallel to the long axis of the nanofibres, indicating the potential for the transfer of the low dimensional properties of the bulk material into the nanophase morphology. (Low dimensional properties of bulk TaS3 result from chains of tantalum atoms propagating along the b direction of the unit cell). Although the structure of the TaS3 remains illusive the preliminary investigations show these nanofibres to be metallic along their lengths, potentially leading to many applications in nanoscale electrical devices. The concept of pseudomorphic change from the disulfide nanomaterials into more functional materials such as Ta3N5 and Ta2O5 was investigated. Nanofibres were initially formed and can reversibly be inter-converted between the three different nanometric phases (TaS2, Ta3N5 and Ta2O5) using simple solid-gas reaction, without significant loss of nanofibrous morphology. Further this series of reactions shows potential for the formation of other related and potentially applicable nanometric phases such as TaN, TaO2 and TaON as well as opening the door to countless other analogous systems

Nickel-doped ceria nanoparticles : the effect of annealing on room temperature ferromagnetism

Nickel-doped cerium dioxide nanoparticles exhibit room temperature ferromagnetism due to high oxygen mobility within the doped CeO2 lattice. CeO2 is an excellent doping matrix as it can lose oxygen whilst retaining its structure. This leads to increased oxygen mobility within the fluorite CeO2 lattice, leading to the formation of Ce3+ and Ce4+ species and hence doped ceria shows a high propensity for numerous catalytic processes. Magnetic ceria are important in several applications from magnetic data storage devices to magnetically recoverable catalysts. We investigate the effect doping nickel into a CeO2 lattice has on the room temperature ferromagnetism in monodisperse cerium dioxide nanoparticles synthesised by the thermal decomposition of cerium(III) and nickel(II) oleate metal organic precursors before and after annealing. The composition of nanoparticles pre- and post-anneal were analysed using: TEM (transmission electron microscopy), XPS (X-ray photoelectron spectroscopy), EDS (energy-dispersive X-ray spectroscopy) and XRD (X-ray diffraction). Optical and magnetic properties were also studied using UV/Visible spectroscopy and SQUID (superconducting interference device) magnetometry respectively

Superconducting tantalum disulfide nanotapes; growth, structure and stoichiometry

Superconducting tantalum disulfide nanowires have been synthesised by surface-assisted chemical vapour transport (SACVT) methods and their crystal structure, morphology and stoichiometry studied by powder X-ray diffraction (PXD), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and nanodiffraction. The evolution of morphology, stoichiometry and structure of materials grown by SACVT methods in the Ta-S system with reaction temperature was investigated systematically. High-aspect-ratio, superconducting disulfide nanowires are produced at intermediate reaction temperatures (650 degrees C). The superconducting wires are single crystalline, adopt the 2H polytypic structure (hexagonal space group P6(3)/mmc: a = 3.32(2) angstrom, c = 12.159(2) angstrom; c/a = 3.66) and grow in the <2<(1)over bar>(1) over bar0> direction. The nanowires are of rectangular cross-section forming nanotapes composed of bundles of much smaller fibres that grow cooperatively. At lower reaction temperatures nanowires close to a composition of TaS3 are produced whereas elevated temperatures yield platelets of 1T TaS2

Composition analysis of Ta3N5/W18O49 nanocomposite through XPS

A characterization of a nanocomposite material consisting of Ta3N5 nanoparticles and W18O49 nanowires is presented. The material is of interest for photocatalytic applications, with a focus on pollution reduction through the photodegradation of dye waste; under white light illumination, the combination of Ta3N5 and W18O49 yielded an enhanced rate of dye degradation relative to Ta3N5 particles alone. The facile method of synthesis is thought to be a promising route for both upscale and commercial utilization of the material. X-ray photoelectron spectroscopy revealed a core–shell composite structure with W18O49 present as an overlayer on Ta3N5; the analyzed spectra for the C 1s, O 1s, Ta 4f, N 1s, W 4f, and Na 1s regions are reported. It should be noted that due to differential charging of the underlying Ta3N5 component relative to the W18O49 shell, an additional uncompensated voltage shift may exist in the Ta 4f and N 1s spectra

Structural and electronic properties of Cu4O3 (paramelaconite): the role of native impurities

Hybrid density functional theory has been used to study the phase stability and formation of native point defects in Cu4O3. This intermediate copper oxide compound, also known as paramelaconite, was observed to be difficult to synthesize due to stabilization issues between mixed-valence Cu1+ and Cu2+ ions. The stability range of Cu4O3 was investigated and shown to be realized in an extremely narrow region of phase space, with Cu2O and CuO forming readily as competing impurity phases. The origin of p-type conductivity is confirmed to arise from specific intrinsic copper vacancies occurring on the 1+ site. Away from the outlined stability region, the dominant charge carriers become oxygen interstitials, impairing the conductivity by creating deep acceptor states in the electronic band gap region and driving the formation of alternative phases. This study further demonstrates the inadequacy of native defects as a source of n-type conductivity and complements existing experimental findings

Photocatalytic degradation of rhodamine B dye and hydrogen evolution by hydrothermally synthesized NaBH4-spiked ZnS nanostructures

Metal sulphides, including zinc sulphide (ZnS), are semiconductor photocatalysts that have been investigated for the photocatalytic degradation of organic pollutants as well as their activity during the hydrogen evolution reaction and water splitting. However, devising ZnS photocatalysts with a high overall quantum efficiency has been a challenge due to the rapid recombination rates of charge carriers. Various strategies, including the control of size and morphology of ZnS nanoparticles, have been proposed to overcome these drawbacks. In this work, ZnS samples with different morphologies were prepared from zinc and sulphur powders via a facile hydrothermal method by varying the amount of sodium borohydride used as a reducing agent. The structural properties of the ZnS nanoparticles were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. All-electron hybrid density functional theory calculations were employed to elucidate the effect of sulphur and zinc vacancies occurring in the bulk as well as (220) surface on the overall electronic properties and absorption of ZnS. Considerable differences in the defect level positions were observed between the bulk and surface of ZnS while the adsorption of NaBH4 was found to be highly favourable but without any significant effect on the band gap of ZnS. The photocatalytic activity of ZnS was evaluated for the degradation of rhodamine B dye under UV irradiation and hydrogen generation from water. The ZnS nanoparticles photo-catalytically degraded Rhodamine B dye effectively, with the sample containing 0.01 mol NaBH4 being the most efficient. The samples also showed activity for hydrogen evolution, but with less H2 produced compared to when untreated samples of ZnS were used. These findings suggest that ZnS nanoparticles are effective photocatalysts for the degradation of rhodamine B dyes as well as the hydrogen evolution, but rapid recombination of charge carriers remains a factor that needs future optimization

Apparent disagreement between cyclic voltammetry and electrochemical impedance spectroscopy explained by time-domain simulation of constant phase elements

A selection of electrodes was analysed using cyclic-voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and a large apparent resistance was observed with CV that was absent with EIS. The explanation for this resistance anomaly was traced to the constant phase element (CPE) behaviour which is exhibited by the electrode double-layer capacitance. Computer simulations of the transient-response of an RQ network (where Q represents a CPE) to a voltage ramp revealed bi-exponential behaviour, with two separate time-constants. One is equal to the product of R and Q, but the other is fixed at about 0.3 s. This finding is supported by observation, by mathematical derivation, and by a novel mixed-domain five-component equivalent circuit model. In addition, example code is provided as a basis for transient simulations of constant phase elements with arbitrary voltage waveforms. This explanation assists in the correct interpretation of potentially misleading cyclic voltammetry results

From Renewable Energy to Renewable Fuel: A Sustainable Hydrogen Production

Hydrogen, a zero-emission fuel and the universal energy vector, can be easily produced from many different energy sources. It is a storable, transportable product that can be used on demand to overcome supply and demand imbalances. As of today, most of the hydrogen produced comes from natural gas; the production process itself is in fact not so pollution free. As the world is looking for a low carbon future, researchers have therefore been looking for more sustainable, environmentally friendly pathways of hydrogen production by using renewable energy sources such as solar and wind. Among the different methods, water electrolysis is a conventional and promising method of hydrogen production if renewable energy sources are to be employed in the process. Lots of progress has been made over the past few years in extending the use of hydrogen in different sectors. This perspective article briefly covers the recent developments in the hydrogen fuel-based projects and technologies and provides a description of the advantages of employing renewable energy sources for sustainable hydrogen production