Role of pH in the Hydrothermal Synthesis of TiO2 Nanorod Photocatalysts

We report the influence of pH on the hydrothermal growth of TiO2 nanostructures, observing variations in photocatalytic activity. Using comprehensive characterization techniques, we find that nanorod growth significantly depends on the synthesis conditions and that TiO2 mainly crystallizes in the rutile phase. A red shift in the absorption edge towards the visible region was observed by changing the initial HCl : H2O molar ratio, resulting in a tangible shift in the energy band gap from 3.54 eV to 2.95 eV. The highest photocatalytic degradation efficiency of methylene blue in aqueous solution under UV irradiation was achieved with an initial HCl : H2O molar ratio of 1 : 1 (86.5 % after 210 min). In parallel to the experimental results, the crystal structure, optical and electronic structure of the rutile TiO2 system and subsequently their influence on the photocatalytic activity were modeled using density functional theory calculations. The photocatalytic enhancement is attributed to higher density of states and expanded energy bands around the Fermi level, leading to a prolonged lifetime of photoinduced charge carriers and the higher charge transfer which is also measured by electrochemical impedance spectroscopy. Our results are promising towards the judicious design and synthesis of TiO2-based photocatalysts.