Towards the hydrothermal growth of hierarchical cauliflower-like TiO₂ anatase structures

TiO₂ anatase nanostructure-based hierarchical spheres were synthesized by using a facile microwave-assisted hydrothermal method. H₂SO₄ was used as both phase inducer for the formation of anatase and capping agent to promote the self-assembly of cauliflower-like spheres. The hydrothermal synthesis was carried out at 180 °C for 2 h from an acid colloidal solution of titanium tetraisopropoxide, at pH < 2. The relationship between morphology and structure was studied by the means of X-ray diffraction, high-resolution transmision electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, nitrogen adsorption and ultraviolet-visible spectroscopy analyses. The results revealed that the acid concentration is a key factor to the controlled hydrothermal aggregation of TiO₂ anatase faceted-like nanoparticles, leading to hierarchical cauliflower-like structures. The microspheres consist of hierarchical structures of sheet-shaped TiO₂ anatase particles with exposed {001} and {101} facets. The nanostructures grow in preferential directions to become in sheet-shaped, by an oriented attachment mechanism. The sulphate groups anchored on the surface and detected by X-ray photoelectron spectroscopy, help the hydrothermal formation of hierarchical spheres and then the cauliflower-like morphology.