Titania-based nanocomposites for solar photocatalysis

Abstract

This thesis explores several titania-based composites for photocatalytic water treatment and hydrogen evolution. For water treatment, TiO$_2$-C core-shell composites were prepared through controlled hydrolysis of titanium alkoxides followed by calcination in a non-oxygenating atmosphere. A total of nine TiO$_2$-C samples were synthesized by combining different alkoxides and solvents. The sub-micron sized composites displayed eight times faster photocatalytic activity for degradation of aqueous methylene blue under simulated sunlight compared to pristine TiO$_2$. Particle size of the best performing TiO$_2$-C catalyst was further reduced through hydrolysis with aqueous KCl. The resulting nanoparticles (~10 nm) displayed a comparable rate of photodegradation for methylene blue with only 1/5th catalyst loading by weight. This nano-sized powder composite (nanocomposite) photocatalyst was also tested for photodegradation of other dyes, pharmaceuticals and Escherichia coli bacteria. The photo-activity was found to be at par or better than several state-of-art TiO$_2$-C materials reported in the literature and approximately twice as better as standard commercially available TiO$_2$ (P25). Second part of the thesis explores three different nickel-based cocatalysts for TiO$_2$ for visible-light driven hydrogen evolution. Composites containing Ni(OH)$_2$, NiO, and Ni$_2$P were prepared and the effects of loading concentration and synthesis method on the rate of hydrogen evolution were studied. It was observed that hydrogen evolution could be achieved in the presence of a sacrificial electron donor (ethylenediaminetetraacetic acid disodium). Catalysts with Ni(OH)$_2$ and Ni$_2$P displayed significant photoactivity, in which highest rate of hydrogen production was achieved with TiO$_2$-Ni$_2$P nanoparticles. The prepared catalyst also displayed excellent long-term stability under continuous illumination tested for seven days. The final section of this thesis reports on the synthesis and visible-light activity of a hybrid Bi$_0.5$Na$_0.5$TiO$_3$-BiOCl (BNT-BiOCl) ferroelectric photocatalyst. The parent ferroelectric BNT microparticles were prepared by solid-oxide reaction route and BiOCl growth on surface was achieved by treatment with dilute HCl. Despite a large bandgap and large particle size (>1 µm), the composite photocatalyst displayed a high rate of rhodamine B degradation under visible light. The high photoactivity was attributed to the heterojunction formed between the two phases, which improved charge separation.