Academic literature on the topic 'Nanoparticulates'

Thesis (Ph.D.)--City University of Hong Kong, 2005.<br>"Submitted to Department of Physics and Materials Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy" Includes bibliographical references.

Titanium dioxide, TiO2, has a wide range of applications as a photocatalyst in the field of solar energy conversion and environmental remediation including water purification and wastewater treatment. In general, a TiO2 photocatalytic process consists of three major steps, namely, the mass transfer process in solution, the interfacial step, and the photoelectron transport inside the catalyst. This work explores the characterisation of TiO2 photocatalysis using a photoelectrochemical method to focus on each fundamental reaction steps. Each of these steps will be used to gain an accurate understanding of processes and identify possible improvements of the overall performance of TiO2 semiconductor photocatalysts. In this work, nano-sized TiO2 semiconductor photocatalysts were prepared by the solgel method, and immobilised onto a conducting ITO glass substrate to form a photoanode. Photocatalytic studies utilising immobilised TiO2 thin films have many advantages over the suspension/slurry system, including the elimination of the separation process. Most importantly however, photocatalysis by immobilised TiO2 photocatalyst can be manipulated by applying an external potential bias to focus on understanding certain aspects of the photocatalytic process (e.g. the rate determining steps).<br>Thesis (PhD Doctorate)<br>Doctor of Philosophy (PhD)<br>Griffith School of Environment<br>Science, Environment, Engineering and Technology<br>Full Text

Nickel sulfide possesses a variety of typical structures and stoichiometries that distinguish itself from iron sulfide and exhibits unique roles in the prebiotic reactions which are proposed to be involved in the origin of life. Nickel sulfide precipitate is hydrated and nanocrystalline, modelled as a 4 nm sphere with a 1 nm crystalline and anhydrous NiS (millerite) core, surrounded by a hydrated and defective mantle phase. It is a metastable but fairly robust structural configuration. It may be formulated as NiSxFbOx approximates to 1.5 and decreases on heating. The fresh nanoparticulate nickel sulfide precipitates undergo structural transformation from the initial millerite-like NiS to the more crystalline polydymite-like Ni3S4. This reaction is accompanied by the formation of a less crystalline Ni3S2 (heazlewoodite) phase. The reaction, happening in ambient conditions, occurs more readily for the solids precipitated from acidic environments (i.e., pH 3) and may be facilitated by the hydrogen and water bonding contained in this material. The performance of nickel sulfide and iron sulfide precipitates is investigated in the formaldehyde world under ambient and sulfidic environments which mimic the ambient ancient Earth environments to some extent. The catalytic capacity of the metal sulfides is not obvious in these experiments. An interesting finding is that, trithiane, the cyclic (SCH2)3, also suppresses the pyrite formation and thus promotes the greigite formation in the reaction between FeS and H2S. This provides another cause for the greigite formation in the Earth sedimentary systems and adds information to the origin-of-life theory in the iron sulfur world. Voltammetry experiments reveal that the nickel-cysteine complex lowers the overpotential for molecular H2 evolution in sea water to -1.53 V under ambient conditions. This catalytic property of the abiotic nickel-cysteine complex apparently mimics the Ni-S core in some hydrogenase enzymes functioning in physiological conditions. This bridges the abiotic and biotic worlds and supports the idea that life originated in the prebiotic ancient ocean.

Printed silicon is an award-winning technology in the development of a large area of flexible electronics. In an investigation of the fundamental properties of printed nanoparticulate silicon composites, layers were screen printed and successfully characterised to establish their electrical performance using a Hall Measurement System (HMS). To explore properties of the nanoparticulate silicon composite a magnetoconductivity tensor model was developed and applied to extract parameters governing the electrical properties of the material. All the layers showed at least two carrier types. The effect of particle loading and temperature on the electrical properties was also investigated. Although carrier concentrations are generally low, their mobility was found to be comparable to, or even better than, similar classes of semiconductor materials.

An intriguing frontier for magnetic resonance imaging (MRI) is the development of responsive Gd³⁺-based contrast agents (CAs) that can report physiological variations in the tissue microenvironment.[1] Heterogeneous biological milieu and quantification of the CA concentration at the site of interest are the major limitations of responsive concentration-dependent protic MRI probes. A ratiometric approach based on dualmode CAs containing a ¹⁹F MRI reporter, in addition to a paramagnetic moiety, is one of the feasible strategies to overcome this drawback. Since high fluorine content is required for in vivo imaging, nanoparticles offer advantageous characteristics over molecular probes due to their large surface area and size, allowing the incorporation of high concentrations of uorinated probes and paramagnetic CAs. For emerging applications of Gd³⁺ complexes at ultra-high magnetic fields, it was found that not only octadentate (q = 1), but also highly kinetically stable nonadentate (q = 0) Gd³⁺-DOTA systems can be incorporated into the mesopores of silica nanoparticles, resulting in an 8-fold increase in proton longitudinal relaxivity from 0.7 mM^-1 s^-1 to 6.2 mM^-1 s^-1 at 4.7 T compared to their molecular counterparts. A further 7-fold increase in longitudinal relaxivity was achieved when the silica surface of such particles was functionalised with highly acidic propyl sulphonates, enabling faster proton transfer through a hydrogen-bonded hydronium network. pH responsiveness of Gd³⁺ complexes tethered to particles has, on the other hand, been achieved through either local mobility or water access regulated by pH-responsive polymers acting as gating valves on the mesopores. Moreover, symmetric ¹⁹F MRI CAs were accommodated within micron- and nanosized matrices comprised of polyelectrolytes, quaternary ammonium first generation dendrimers and centrifugable oil-containing silica nanocapsules. For these, the biggest challenge has been posed by restricted local mobility, which notably shortens transverse relaxation time and attenuates the ¹⁹F NMR signal. Finally, polymers based on uorinated quaternary ammonium, pH-sensitive and Gd³⁺-DOTA methacrylates were prepared, exhibiting a dual ¹⁹F/¹H MRI contrast with reverse pH response.

Silicon nanoparticles for the application of printed electronics were successfully synthesised and characterised. High energy milling has been proven to yield uncontaminated powder of median particle size 150 nm satisfying a lognormal distribution. Single crystalline P- and N-type silicon wafers, and metallurgical grade silicon were used as starting materials. The structural characterisation of all milled powders using X-ray diffraction (XRD), Small Angle X-ray Scattering (SAXS) and electron diffraction proved that the silicon nanoparticles are polycrystalline with a crystallite size of about 40 nm. For the first time, we have formulated printable semiconducting inks from nanoparticulate silicon. Silicon nanoparticles were mixed with organic binders, such as linseed oil and acrylic, to produce printable inks. Similarly nanoparticulate silicon ink, doped with inorganic salts, which is a different procedure to conventional impurity doping of the silicon structure, was produced with linseed oil. A home-built Hall measurement system was used to characterise layers of doped ink, for which a complete carrier type reversal was observed. Based on the result of elemental mapping, two possible models were suggested to explain the doping effect. A state-of-the-art Hall measurement system was used to perform field dependent analysis of screen printed silicon inks in van der Pauw geometry. A magnetoconductivity tensor model was developed to extract the carrier properties. All the layers were demonstrated to have at least two carrier types. Inks produced from P-type silicon maintained their carrier type, but reversal was observed for the N-type layers. The mobility of the carriers is better or comparable to similar classes of semiconducting materials. 2 More information on the interparticle connections were obtained from IV and impedance spectroscopy measurements which demonstrated the capacitive effects present in the printed layers. The capacitors originate at the interfaces between the metal and the layers and between the particles.