Tailored titanium dioxide nanomaterials: anatase nanoparticles and brookite nanorods as highly active photocatalysts

High quality brookite TiO₂ nanorods have been obtained by the thermal hydrolysis of commercially available aqueous solutions of titanium bis(ammonium lactate) dihydroxide in the presence of high concentrations of urea (≥6.0 M) as an in situ OH^- source. Biphasial anatase/brookite mixtures are obtained at lower urea concentrations. The ratios between anatase and brookite can readily be tailored by the control of the urea concentration. The obtained powders have been characterized by X-ray diffraction, Raman spectroscopy, field emission-scanning electron microscopy, high-resolution transmission electron microscopy, UV-vis diffuse reflectance spectra, and nitrogen adsorption. The photocatalytic activity of pure anatase nanoparticles, of anatase/brookite mixtures, and of pure brookite nanorods has been assessed by hydrogen evolution from aqueous methanol solution as well as by the degradation of dichloroacetic acid (DCA) in aqueous solution. The results indicate that the photocatalytic hydrogen evolution activity of anatase/brookite mixtures and of pure brookite is higher than that of pure anatase nanoparticles despite of the lower surface area of the former. This behavior is explained by the fact that the conduction band edge of brookite phase TiO₂ is shifted more cathodically than that of anatase as experimentally evidenced under dark and UV-vis illumination conditions. On the contrary, in case of the photocatalytic degradation of DCA, anatase/brookite mixtures and pure brookite exhibit lower photocatalytic activity than pure anatase nanoparticles. This behavior correlates well with the surface area of the investigated powders.