Anatase TiO2 single crystals with a large percentage of reactive facets

Single crystals of titanium dioxide (TiO2), with highly reactive surfaces, show promise for energy and environmental applications. Unfortunately, the highly reactive surfaces tend to disappear during crystal growth as a result of the minimization of surface energy. Most available samples of anatase, a naturally occurring crystalline form of TiO2, are therefore dominated (to over 90%) by thermodynamically stable {101} facets, rather than the more reactive {001} type. Hua Gui Yang et al. use hydrofluoric acid treatment of anatase TiO2 to remedy this situation. Based on theoretical predictions, they synthesized uniform anatase TiO2 single crystals containing 47% of the reactive {001} facets. This work may pave the way for the more general use of non-metallic atoms as surface controlling agents. Extensive first principles calculations carried out show that the relative stability of facets of anatase can be switched by terminating the surfaces with fluorine. It is then demonstrated that uniform anatase single crystals with a high percentage of {001} facets can be generated using hydrofluoric acid as a structure directing agent. Subsequently, surfaces can be freed of fluorine using a simple heat treatment. Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied1,2,3,4,5,6,7,8,9,10,11,12,13. Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. A typical example is titanium dioxide (TiO2), which has promising energy and environmental applications14,15,16,17. Most available anatase TiO2 crystals are dominated by the thermodynamically stable {101} facets (more than 94 per cent, according to the Wulff construction10), rather than the much more reactive {001} facets8,9,10,11,12,13,18,19,20. Here we demonstrate that for fluorine-terminated surfaces this relative stability is reversed: {001} is energetically preferable to {101}. We explored this effect systematically for a range of non-metallic adsorbate atoms by first-principle quantum chemical calculations. On the basis of theoretical predictions, we have synthesized uniform anatase TiO2 single crystals with a high percentage (47 per cent) of {001} facets using hydrofluoric acid as a morphology controlling agent. Moreover, the fluorated surface of anatase single crystals can easily be cleaned using heat treatment to render a fluorine-free surface without altering the crystal structure and morphology.