Download files
Abstract
Sodium titanate nanoribbons and nanotubes were synthesised by hydrothermally ageing Aeroxide P25 in 10M NaOH at temperatures between 150 and 200°C. The Na1.48H0.52Ti3O7 nanoribbons comprised a layered TiO6 nanosheet framework with sodium and hydrogen cations interspersed between the sheets. Nanoribbon/nanosheet formation occurred via an amorphous (Ti-O-Na) intermediate phase. At milder temperatures (i.e. 150°C), the kinetics of nanosheet formation are slow enough to allow for their rolling up leading to nanotube formation. Acid washing converted the structure into hydrogen titanate (H2Ti3O7) by ion-exchange and provided uniform nanoribbons and nanotubes.
Heat treatment of hydrogen titanate transformed nanotubes to anatase, while nanoribbon transformation to anatase occurred via a TiO2(B) intermediate phase. The enhanced stability provided by the nanoribbon architecture preserved a dominant {010} crystal facet at temperatures higher than 700°C. Nanotube thermal stability was improved by nitric acid treatment. Methanol and oxalic acid photodegradation performance was the highest at calcination temperatures of 500°C for nanotubes and 800°C for nanoribbons. The nanotube optimum represented a transition between the dominance of surface area and crystal phase, while nanoribbon performance was governed primarily by crystal phase. Despite both nanostructures producing particles with similar physical characteristics when calcined at 800°C the particles originating from the nanoribbons exhibited superior photoactivity. This difference was attributed to a greater percentage of {010} crystal facet in these particles which is beneficial for hydroxyl radical generation.
The photocatalytic activity of Na1.48H0.52Ti3O7, Na2Ti6O13, H2Ti3O7 nanoribbons and anatase TiO2 nanorods were compared for water splitting, oxalic acid photodegradation and H2/O2 generation using sacrificial agents. The intrinsic properties of the materials affected their performance depending on the particular reaction. The Na2Ti6O13, in the presence of RuO2 co-catalyst, outperformed the TiO2, for the water splitting reaction, generating over 10 times more H2/O2. This derived from their tunnel-like structure which provided better electron/hole separation when compared with TiO2. However, the efficient holes and electrons scavenging in the presence of sacrificial agents overwhelmed the tunnel-like structure effect. In this case photoactivity was governed by the crystal structure: TiO2>Na2Ti6O13>H2Ti3O7~Na1.48H0.52Ti3O7 and by the band gap of the semiconductor which determined its capacity to absorb photons in producing electron/hole pairs.