The surface chemistry of nanostructured titanates is more versatile compared to the relatively inert chemistry of carbon nanotubes. The latter requires special treatment under aggressive conditions in order to activate their surface, using carboxylic groups to provide a flexible route for further functionalization. However, even in this case the surface density of carboxylic groups is relatively low. By contrast, nanotubular titanates are abundant with surface -OH groups, which are characterised by weak Bronsted acidity. The coverage density of surface -OH groups is estimated from the lattice parameters of trititanates, and taking into account that the nanotube surface corresponds to the (100) plane and that there are two -OH groups in the area of 0.375 x 0.919 nm2 (see Table 3.1 in Chapter 3). These considerations suggest that a nanotube surface area of 1 nm2 contains approximately 5.8 -OH groups.
The abundance of -OH groups on the surface of titanate nanotubes largely determines their chemical behaviour and the routes for their functionalization. One dramatic difference between titanate nanotubes and TiO2 nanoparticles (such as P-25), is their reactivity with a H2O2 solution (30 wt%) at room temperature which results in the formation of titanium(iv) peroxo-complexes on the surface of the nanotubes, but not on that of the nanoparticles (see Figure 4.9). These peroxo-complexes are easily detected by a characteristic absorption band at 420 nm. Such surfaces, containing titanate nanotubes functionalized with surface peroxo-complexes, can be utilized in catalytic surface redox reactions.
The surface of titanate nanotubes is characterised by its hydrophilic properties due to a strong surface dipole moment and a high concentration of -OH groups. In order to modify the surface properties of the nanotubes to become hydrophobic, it is necessary to functionalise the surface, for example, by covering it with molecules containing long-alkane chains. The most popular hydrophobization method, adopted from silicon dioxide chemistry, is the controlled hydrolysis of trialkoxysilanes (e.g. allyltriethoxysilane30) on the nanostructure surface in anhydrous solvents (see Figure 4.9). Strong covalent Ti-O-Si-C bonds are formed, and the density of these hydrophobic groups is proportional to the density of the initial -OH groups. Another advantage of this functionalization is that the alkane chain can contain other functional groups (e.g. amino groups from g-aminopropyl trimethoxysilane,31 or alcohol groups from 3-aminopropyl-triethoxysilane32), which can be used for further polymer grafting. The disadvantage of this method is the cost and the strict high water-content requirement.
Figure 4.9 Examples of surface functionalization of titanate nanotubes using chemical o o o
Figure 4.9 Examples of surface functionalization of titanate nanotubes using chemical reactions.
Another method of titanate nanotube surface hydrophobization is based on Van-der-Waals interactions between titanate nanotubes and cationic surfactant in aqueous solutions. The treatment of nanotubes with cetyl-trimethylammonium bromide (CTAB)33 or poly(diallyldimethylammonium) chloride34 greatly increases the hydrophobic properties of nanotubes, due to a strong adsorption of surfactant on their surface. Moreover, the addition of surfactant in excess can switch over the zeta potential of nanotubes from negative to positive, due to the formation of admicelles.33
Although the surface -OH groups of titanate nanotubes are characterised by a weak acidity, it is possible to perform an esterification between them and various carboxylic acids35 in anhydrous alcohol solvent. The surface of titanate nanotubes has been shown to bond relatively well with carboxylic acid, demonstrating monodentate, as well as bidentate, modes.
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