Shape-dependent performance of TiO2 nanocrystals as adsorbents for methyl orange removal

Abstract

Two types of TiO₂ nanocrystals, namely {001} plane dominated nanoplates and {101} faceted nanobipyramids prepared by an oleylamine-containing alcohothermal method in the presence or absence of HF, have been used as adsorbents for removal of methyl orange (MO) from water. Transmission electron microscopy investigation, along with X-ray diffraction analysis, reveals that the average width of the nanoplates is about 13 nm with a thickness of ∼4 nm, and the nanobipyramids posses a length in the c-axis of about 18 nm and a central width of about 11 nm. Although the surface area of TiO₂ nanoplates and nanobipyramids are, respectively, about 125.5 and 27.6 m² g⁻¹, the latter exhibits a much better adsorption performance than the former. In the case of 200 ml of MO aqueous solution with an initial concentration of 50 μmol L⁻¹ (pH = 3), the removal efficiency of MO on TiO₂ nanobipyramids (40 mg) reaches about 95% within a contact time of 10 min whereas only ∼65% of MO is removed by TiO₂ nanoplates after 60 min treatment. Further investigations indicate that this shape-dependent adsorption performance is related to the fact that the existence of fluorine on the surface of the nanoplates is unfavorable for the adsorption of MO and the interaction between MO (through its sulfonate group) and nanoplates is weaker than that between MO and nanobipyramids. The adsorption of oleylamine on the surfaces of nanobipyramids and nanoplates that may affect the interaction between the MO and TiO₂ surface has also been discussed based on the experimental results.