《Nature communications》:Titania single crystals with a curved surface


Crystals in nature generally have regular shapes, such as sodium chloride crystals have cube shapes, which reflect the three-dimensional ordered structure inside the crystal. In 1901, Georg Wulff stated that the length of a vector drawn normal to a crystal face hi will be proportional to its surface energy γi. This is the well-known Gibbs-Wulff theorem (G. Wulff, Z. Kristallogr1901, 34, 449-530). Therefore, well-developed single crystals are generally enclosed by regular flat facets spontaneously to form a polyhedral morphology due to the well-known self-confinement principle for crystal growth. However, in nature, crystals with curved external surface such as calcitic skeleton of biological organisms, usually exhibits with unique mechanical property, light management capacity and biological compatibility. Unfortunately, the growth of continuous curved surface is really a great challenge in artificial single crystal synthesis.


Recently, Prof. Hua Gui Yang from School of Materials Science and Engineering, have demonstrated a facile synthetic strategy to prepare unconventional TiO2 single crystals with a curved surface, in which organic citric acid and inorganic hydrofluoric acid were used as synergistic capping agents.Such curved surfaces of TiO2 in anatase and rutile phase are composed of continuous high-index microfacets. Moreover, they investigated the formation mechanism of covered crystal surface by density functional theory calculations, which suggest the key role of synergistic effects of chemisorbed hydrofluoric and citric acid, and particularly, the concentration of citric acid and its competitive adsorption on the high-index surfaces (like (112)) provide unique stabilization effect on the formation of bicone-like curved anatase TiO2. This part was assisted by Prof. Xue Qing Gong from School of Chemistry and Molecular Engineering. More importantly, all curved TiO2 crystals show higher photocatalytic activities than faceted TiO2 crystals. This synthetic strategy provides the principles for designing a new family of functional crystals with non-flat surface, which have potential applications in catalysis, photonics, bio-inspired materials and chemical-mechanical planarization of advanced integrated circuits, and may also shed light on fundamental mechanisms of the organic-inorganic interactions in biomineralization process.