Mbulanga, C. M.
CURRENT RESEARCH
Main objective:
"Fabricate titanium dioxide alloy nanotubes on Ti foil substrates with an engineered band gap that responds to visible and UV light."
Progress to date
Formation mechanism of the rutile-phase of Titanium dioxide nanorods on Ti foil substrate by gel-calcination method
Abstract
A formation mechanism that leads to the synthesis of rutile-phase titanium dioxide nanorods at high temperatures on Ti foil by NaOH-based gel-oxidation method is discussed based on a series of experimental investigations. Titanium dioxide nanostructures are prepared on Ti foil following two steps, namely gelation and oxidation. It is shown that the use of an alkali-based solution during gelation, such as NaOH, leads to the formation of tetragonal nanorods of titanium dioxide upon oxidation at high temperature (~ 800 oC). When an acidic solution that does not contain an alkali element, such as H2O2, is used during gelation, the shape of the nanostructures upon oxidation at high temperature does not display the tetragonal nanorod shape. The following formation mechanism is suggested: the high temperature oxidation of the Na based hydrogel (formed on the Ti surface during a 24-hour soak in NaOH solution) converts it into Na-titanate in the shape of tetragonal nanorods, which in turn convert into tetragonal nanorods of rutile-phase titanium dioxide when Na evaporates in the form of an oxide.
Zinc oxide (ZnO) nanorods grown by a two-step chemical bath deposition method on Si substrate is characterized. Research was conducted on ZnO nanorods for the understanding of their optical properties at room temperature (RT), with the emphasis on the visible luminescence.
In this report, the effects of thermal annealing on the room temperature (RT) photoluminescence characteristics of solution-grown ZnO nanorods (ZNs) are presented. It is shown that the near surface regions of asgrown ZNs are rich in Zn. Within the detection limit of X-ray photoelectron spectroscopy (XPS), it is confirmed that the environment of annealing affects indeed the activation of intrinsic defects. Furthermore, thermal treatment at high temperatures removes H-related defects as expected; and this removal process is found to affect significantly the RT luminescence properties of ZNs, especially when ZNs are annealed sequentially from 300 °C to ~700 °C. Specifically, the passivation of vacancy-related defects by H is demonstrated following thermal treatment in this temperature range. Finally, the green luminescence (~500 nm) that evolves following annealing above ~800 °C is assigned to Zn vacancy defects.