Maximizing the Catalytically Active {001} Facets on Anatase Nanoparticles

In applications such as photocatalysis and NO$_x$ removal, it is advantageous to have a large exposure of the high-energy {001} facets of anatase TiO$_2$ nanocrystals. However, a large-scale synthesis of such nanocrystals has proven to be difficult as it involves the use of HF or fluorine agents. Hydrothermal synthesis represents a green and versatile method for producing nanocrystalline materials with good morphology control, but all previous studies have been unable to produce high amounts of exposed {001} facets without the use of HF. Here, the hydrothermal formation mechanism of anatase TiO$_2$ nanoparticles from a cheap industrial-grade TiOSO$_4$ precursor is investigated by in situ powder X-ray diffraction. Surprisingly, it is found that the crystallite domains of the initially formed anatase nanocrystals are square platelets that can potentially have very high exposures of {001} facets. With increasing reaction time, the square platelet crystallite domains grow into bipyramids, which decreases the potential amount of {001} facets that can be exposed, and thus, the reaction time is the key parameter to controlling the morphology of the crystallite domains in this synthesis process. Ex situ autoclave synthesis demonstrates that, indeed, nanocrystals with a potentially high {001} exposure can be obtained in a laboratory scale, although the present method results in crystal agglomeration, reducing the specific surface area and absolute amount of {001} facet exposure. Analysis of the reaction kinetics indicates that crystal formation occurs through bulk nucleation with a low activation energy of 54(7) kJ/mol