Enhanced photocatalytic activity of porous single crystal TiO₂/CNT composites by annealing process

To improve the photocatalytic performance of anatase TiO₂ (a-TiO₂), it is necessary to simultaneously increase its crystallinity and surface area. Our approach to achieve the desired morphology is to develop a porous single crystal that can be transformed from its mesocrystal form via annealing. We synthesized a-TiO₂ mesocrystals onto multiwalled CNTs using a facile one-pot chemical approach, and investigated the effect of the annealing temperature (200–600 °C) on the crystallinity, morphology, chemical bonding state, and photocatalytic performance of the TiO₂/CNT composites. The as-grown sample and sample annealed at 200 °C consisted of spindle-like a-TiO₂ mesocrystals. As the annealing temperature increased to 400 °C, the morphology of a-TiO₂ changed from mesocrystals into porous single crystals and the surface area enlarged due to the thermo-decomposition of organic residues between the subunits. The chemical bonding (Ti–O–C) between TiO₂ and CNT was also strengthened with increasing annealing temperature. On the other hand, the TiO₂ was separated from the CNT at 600 °C because of the large difference in the thermal expansion coefficients. The photocatalytic performance of the TiO₂/CNT composites was the highest at 400 °C due to the increased crystallinity, removal of the by-products, and strengthened Ti–O–C bonds, resulting in an increase in the photocatalytic active sites and efficient charge separation.