Titania Nanowires: synthesis, characterization and properties.

Abstract

This thesis explored the synthesis of titanium dioxide nanowires by seed assisted thermal growth in the temperature range between 600-1000 °C. In order to improve the photocatalytic performances of TiO2 NWs, we modified the samples applying two approach: doping by ion implantation of Fe+ and annealing in forming gas. It was found that the formation of pure Ti NWs is not energetically or kinetically favoured in the full range of explored temperatures (600-1000 °C). According to this observation the Au-Ti phase diagram cannot be taken unto account to depict the VLS mecanism with the help of eutectic alloys. Moreover, without catalyst the NWs growth is very difficult to be induced and controlled, only few NWs are observed in small isolated areas of the sample with no reproducibility. In conclusion, the seed assisted growth of TiO2 NWs is a very complex synthesis mecanism, as it is assisted by Ti-Au interdiffusion, Ti migration, gas carrier transport and oxygen gas reaction. We demonstrared that the same method of TiO2 NWs growth can be applied to different type of Ti substrates, opening new possibilities for different kind of applications. Fe+ implantation-induced defects formed energy levels inside the band-gap of the material, but it is crucial to keep their amount below a certain limit, as they act as recombination centres for electrons and holes. The same approach was extended to TiO2 nanowires by tuning the ion implantation energy, NWs showed to change their morphology as a consequence of ion implantation damage by bending. Four peaks were clearly resolved in DLTS spectra of samples with TiO2 NWs, with energy level below the conduction band at EC ET respectively of: 0.11 eV, 0.30 eV, 0.45eV and >0.6 eV. The latter E4 peak disappears in the spectrum of the samples of NWs treated with forming gas, which also exhibits a considerably higher amplitude of the shallow E1 peak than the other samples, while the E2, E3 peaks are substantially weaker. The identity of the E1, E2, E3 and E4 levels is not known and previous DLTS data in the literature on deep levels in TiO2 NWs are scarce (if any). However, the loss of the E4 level after the FG treatment can be correlated with the samples photocatalytic activity. We assessed the photocatalytic activity of different TiO2 NWs on various substrates by measuring the rate of photocatalytic bleaching of the MB dye in aqueous solution. The photocatalytic rate constant of the samples with NWs is from 36% up to 67% higher than the one obtained from the corresponding substrates without NWs. The treatment in forming gas triplicated the photocatalytic rate constant of the NWs sample compared to the one of the same substrate without NWs. Moreover, this increase in the photocatalytic activity after the forming gas annealing correlates with the results obtained by deep level transient spectroscopy that showed a disappearance of a deep level (E4) that could act as a competing charge carrier recombination center suppressing the catalytic effect of the photon-induced electron-hole pairs.