Jiawei Ng

Transformation of Bromine Species in TiO2 Photocatalytic System

Bromine species have six oxidation states from Br(-) to BrO(3)(-), and their transformation between each oxidation state is complex and has been a common debate topic in photocatalytic systems where oxidants (h(+) and OH.) and reducers (e(-)) coexist. In this study, the lowest oxidation state (Br(-)) and highest oxidation state (BrO(3)(-)) were used as the starting compounds in a photocatalytic reaction to investigate the transformation of bromine species. The experimental results showed that oxidation of Br(-) to BrO(3)(-) by OH. and reduction of BrO(3)(-) to Br(-) by e(-) were concurrent. However, due to a higher reaction rate for reduction of BrO(3)(-) under a pH range of 3-11, oxidation of Br(-) was totally offset and hence, only the reduction of BrO(3)(-) was observed with hydrobromous acid and hydrobromite formed as intermediates. Apart from e(-), H(2)O(2), to a certain extent, was involved in the photocatalytic reduction of BrO(3)(-). A low concentration of organic matter (3 mg/L) reacted with hydroxyl radicals to inhibit combination of holes and electrons, hence promoting photocatalytic reduction of BrO(3)(-). It is important to note that pH had an influence on the transformation of bromine species, because it affects adsorption of reactants on the photocatalyst and controls the amount of aqueous H(+) and OH(-) ions present. Photocatalytic oxidation of Br(-) became dominant under a strong acidic condition (pH 1.5) while both photocatalytic oxidation and reduction were inhibited under a strong basic condition (pH 13.5).

Hierarchical assembly of anatase nanowhiskers and evaluation of their photocatalytic efficiency in comparison to various one-dimensional TiO2 nanostructures

Well-crystallized anatase nanowhiskers were successfully grown from a Ti substrate via an anodization process followed by hydrothermal treatment. Progressive morphological and crystallographic evolution of anodic nanotubes to nanowhisker arrays was clearly elucidated. The morphology, crystallinity and optical properties of the final nanowhiskers were extensively investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV-Vis spectroscopy and photoluminescence (PL) techniques. The synthesized TiO2 photocatalyst exhibited excellent photodegradation of Acid Orange 7 (AO7), a common dye used in textile industries, under UV-A irradiation. Photocatalytic performance was evaluated against other TiO2 films of various 1D (one-dimensional) nanostructures, and benchmarked against a Degussa P25 TiO2 coated film. Owing to its unique morphology and monocrystalline defect-free lattice structure, the nanowhisker sample exhibited the highest photocatalytic efficiency. In view of presented experimental results, the 1D hierarchical nanowhisker architecture may find a variety of potential applications in solar cells, endoscopy and photonic devices, etc.

A hierarchically assembled mesoporous ZnO hemisphere array and hollow microspheres for photocatalytic membrane water filtration

A mesoporous ZnO hemisphere array has been prepared via a topotactic transition of Zn(4)(OH)(6)CO(3)·H(2)O (ZCHH) by chemical bath deposition. Each hemisphere is comprised of a radially oriented nanoflake shell grown on the hemispherical interior. Reaction time-dependent SEM analysis shows that the morphological formation of ZCHH involves a deposition-growth-secondary growth-redeposition procedure. Upon calcination, ZCHH readily decomposes to nanocrystalline wurtzite-phase ZnO without significant change in morphology, and the release of CO(2) and H(2)O from ZCHH creates an additional mesoporous structure in both hemispherical interior and nanoflake shell. A similar process but without using a substrate has been developed for synthesis of mesoporous ZnO hollow microspheres in powder form. Both the elaborated superstructured photocatalysts consisting of mesoporous nanoflakes have been demonstrated to exhibit excellent performances in the photocatalytic membrane filtration.

Simultaneous copper ion removal and hydrogen production from water over a TiO2 nanotube photocatalyst.

A one-dimensional (1-D) mesoporous TiO₂ nanotube (TiNT) was successfully synthesized via a hydrothermal-calcination process, and employed in simultaneous photocatalytic Cu(2+) removal and H₂ production. Under irradiation, Cu(2+) in the wide concentration range of 8-800 ppm, could be reduced rapidly, and the reduction was not severely impacted by co-existing inorganic ions in solution. Simultaneous with Cu(2+) reduction, noticeable H₂ was produced over the in-situ fabricated Cu incorporated TiNT (Cu-TiNT) photocatalyst, while H₂ evolution rate was controlled by the Cu(2+) reduction process, due to competition of electron capture between protons and Cu(2+). In addition, H₂ generation activity of Cu-TiNT depended on the initial Cu(2+)/Ti ratio, and could be depressed by co-existing ions in solution. Fast Cu(2+) reduction and remarkable H₂ evolution confirmed the feasibility of simultaneous Cu(2+) removal and H₂ production over a TiNT photocatalyst.