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Abstract

Three one-dimensional Ti–O-based nanomaterials, including the transformations among them, were studied in this work: (i) protonated titanate, (ii) titanium dioxide (TiO₂), and (iii) titanium oxynitride (Ti(O,N)).
Protonated titanate nanotubes were successfully applied as a solid acid catalyst for the aldol condensation between benzaldehyde and cyclohexanone. They also efficiently catalysed other reaction systems involving substituted benzaldehyde derivatives, as well as a large-scale (one-gram) experiment. Recycling studies showed no significant decrease in the catalytic activity of protonated titanate nanotubes over five consecutive cycles. The results demonstrate that protonated titanate nanotubes could therefore be used as a greener alternative to current industrial approaches to the aldol reaction.
In the next chapter, the transformation of protonated titanate nanoribbons (HTiNRs) into titanium dioxide (anatase) nanoribbons was studied. The transformation involves a condensation reaction that can be achieved by calcination or catalysis in an aqueous medium. Calcination of HTiNRs in air initially led to the formation of the TiO₂(B) phase, followed by the formation of anatase as the calcination temperature increased from 400 °C to 650 °C. In contrast, calcination under an ammonia flow resulted in N-doped TiO₂ nanoribbons. Despite the stability of protonated titanate in a basic aqueous medium, it transformed into TiO₂ through interactions with basic cations. TiO₂ nanoribbons prepared by hydrothermal treatment had surfaces covered with small nanocrystals, resulting in an increased specific surface area. In addition, the behaviour of protonated titanate in aqueous media across the entire pH range was explained in terms of the corresponding zeta potential of the material.
Finally, the photocatalytic activity of selected samples was evaluated by monitoring the photooxidation of isopropanol to acetone. The photocatalytic activity of TiO₂ nanoribbons prepared by calcining HTiNRs in air increased with the calcination temperature. In contrast, N-doped TiO₂ nanoribbons exhibited suppressed photocatalytic activity despite their absorption in the visible-light range. The highest photocatalytic activity was observed for TiO₂ nanoribbons initially prepared by hydrothermal treatment in deionized water and subsequently calcined in air.
In the final chapter, Ti(O,N) nanoribbons, a technologically promising material, were synthesized by calcining HTiNRs under an ammonia flow at 800 °C. A wide range of samples was prepared by varying the reaction parameters, including the ammonia flow rate and reaction time. As Ti(O,N) formed, the initially smooth surface of the HTiNRs became porous and developed numerous voids. The initial porosity arose from lattice mismatch between the phases and was subsequently enhanced by the Kirkendall effect. During the transformation, two types of disorder developed in the material: ion vacancies and random O/N occupancy of anionic sites. Both types of disorder affect the electronic properties of the material. Magnetic measurements revealed that all Ti(O,N) samples exhibited metallic behaviour, whereas only those with the lowest degree of disorder also exhibited superconductivity at low temperatures.