Anatase TiO2 single crystals with a large percentage of reactive facets

Abstract

Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied^{1,2,3,4,5,6,7,8,9,10,11,12,13}. 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 applications^14,15,16,17. 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} facets^{8,9,10,11,12,13,18,19,20}. 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.