Efficient photocatalytic reductive dechlorination of 4-chlorophenol to phenol on {0 0 1}/{1 0 1} facets co-exposed TiO2 nanocrystals

Abstract

4-chlorophenol could be efficiently photoreductively dechlorinated over anatase TiO2 nanocrystals with co-exposed {001} and {101} facets, which were synthesized and further characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Although fluorine could adsorb on {001} facets to decrease their surface energy, enabling TiO2 to expose high energy {001} facets, the surface bonded fluorine might depress the photoreductive dechlorination efficiency of 4-chlorophenol, attributed to the electron trapping role of surface ≡TiF groups. Due to the formation of a surface heterojunction between {101} and {001} facets in a single TiO2 nanocrystal, electrons and holes were spontaneously self-separated and selectively migrate to {101} and {001} facets, respectively. Electron trapping experiments demonstrated that photogenerated electrons are the responsible for the reductive dechlorinaton of 4-chlorophenol to phenol. To avoid the oxidative degradation of 4-chlorophenol by holes and ensure sufficient electrons to reductively dechlorinate the substrate, moderate scavengers were required in the reaction system and dissolved oxygen, which might deplete electron on TiO2, also should be removed. With the optimal scavengers, the conversion efficiency of 4-chlorophenol (4-CP) achieved 97.5% and the selectivity for phenol was 92.5%, which were much higher than that of commercial TiO2 P25.