Hydrothermal Stability of {001} Faceted Anatase TiO2

Due to its great importance in fundamental research and practical applications, tailored synthesis of anatase TiO2 dominated with highly energetic {001} facets has been extensively studied during the past few years. However, clean (001) surface of anatase TiO2 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 TiO2 with exposed high-reactive {001} facets. In this study, the hydrothermal stability of {001} faceted anatase TiO2 was systematically investigated by using single-crystalline anatase TiO2 nanosheets with 80% {001} facets as a model starting material. Under hydrothermal conditions (200 degrees C in deionized water), anatase TiO2 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 TiO2 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 TiO2 nanosheets remained their original morphology unchanged when ethanol, propanol, butanol, or 1.5 M HCl aqueous solution was used as reaction medium.