Well-aligned one-dimensional-like titania-liquid crystals composite photocatalyst synthesized under magnetic field

Abstract

The synthesis of one-dimensional-like titania (TiO2) and the elucidation of its shape-photocatalytic activity relationship remain a big challenge today. It is hypothesized that this kind of material can be synthesized under magnetic field with the presence of magnetically responsive liquid crystals. This research is considered as a novel work since comprehensive studies have been carried out for the one-dimensional-like TiO2 and its photocatalytic activity. The importance of the one-dimensional-like TiO2 should be related to the electronic structures that affect the electron-hole recombination, and hence, photocatalytic activity. In this research, the synthesis of well-aligned one-dimensional-like TiO2 using liquid crystals as the structure aligning-agent was demonstrated via sol-gel method under a magnetic field. The 4-cyano-4’-pentylbiphenyl (5CB) and 4-cyano-4’-octylbiphenyl (8CB) liquid crystals have been used as the structure aligning-agents. Each of the liquid crystals have been mixed with tetra-n-butyl orthotitanate (TBOT), 2-propanol and water, and the mixtures underwent slow hydrolysis in a magnetic field (0.3 T) under ambient conditions in the open atmosphere. The obtained TiO2 composite samples were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometer, photoluminescence spectrometer, direct current electrical conductometer and Hall effect analyzer. Interesting results were observed when an external magnetic field was applied during the hydrolysis of TBOT in the presence of the liquid crystals. The TiO2-5CB composite was more well-aligned in the shape of whiskers compared to the TiO2-8CB composite when the reaction mixtures were placed under the magnetic field. This is due to the steric size of 8CB which is larger than 5CB. Meanwhile, the TiO2-5CB and TiO2-8CB composites have irregular, spherical shape when no magnetic field was applied. Other than that, it is quite surprising that the well-aligned one-dimensional-like TiO2-5CB showed the anatase peak, considering that no further treatment such as calcination was required. The intensity of the emission peaks in the photoluminescence spectrum of well-aligned one-dimensional-like TiO2-5CB composite was lower compared to the other composites. This might be caused by electrons transferring from 5CB to TiO2 in the well-aligned one-dimensional-like TiO2-5CB during ultraviolet irradiation. The plausible mechanism of electron charge transfer was elucidated by density functional theory (DFT) calculation. Based on these results, the interfacial interaction between the liquid crystal and TiO2 is the key factor to control the shape of TiO2 during the hydrolysis process. Furthermore, direct current electrical conductivity and Hall effect studies showed that the well-aligned one-dimensional-like TiO2 enhanced the electron mobility. Therefore, due to the increasing of electron mobility, the recombination of electrons and holes could be delayed, and hence, the photocatalytic activity of the well-aligned one-dimensional-like TiO2 in the oxidation of styrene was enhanced. Based on the above results, the structure-photocatalytic activity relationship of well-aligned one-dimensional-like TiO2 composite, synthesized under magnetic field was clarified in this research.