Hydrothermal Stability of {001} Faceted Anatase TiO₂

Due to its great importance in fundamental research and practical applications, tailored synthesis of anatase TiO 2 dominated with highly energetic {001} facets has been extensively studied during the past few years. However, clean (001) surface of anatase TiO 2 has been evidenced to be unstable and usually tends to reconstruct under ultrahigh-vacuum conditions. Thus, the stability of surface structure under other ambient conditions might be one of the most critical issues for anatase TiO 2 with exposed high-reactive {001} facets. In this study, the hydrothermal stability of {001} faceted anatase TiO 2 was systematically investigated by using single-crystalline anatase TiO 2 nanosheets with 80% {001} facets as a model starting material. Under hydrothermal conditions (200 °C in deionized water), anatase TiO 2 nanosheets can grow into larger single crystals with a truncated bipyramidal shape through an oriented attachment process along the [001] crystallographic direction, driven by the minimization of surface energy. Furthermore, the coarsening behavior and growth mechanism were discussed with the assistance of standard and high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry, as well as theoretical calculations. A modified kinetic model was also developed to elucidate the asymptotic growth of anatase TiO 2 nanosheets via oriented attachment mechanism. In addition, pH value and the solvents adopted during the treatments were revealed to have significant influence on the oriented attachment-based crystal growth due to suppression of hydrolysis of Ti–F groups on the surface. For example, the anatase TiO 2 nanosheets remained their original morphology unchanged when ethanol, propanol, butanol, or 1.5 M HCl aqueous solution was used as reaction medium.