Developing Active TiO₂ Nanorods by Examining the Influence of Morphological Changes from Nanorods to Nanoparticles on Photocatalytic Activity

We synthesized rutile TiO2 nanorods with high crystallinity, i.e., degree of crystallinity greater than ca. 90%, by using a hydrothermal method and studied the effect of the morphological change from rod shape to subsphere by the subsequent thermal treatment. The photocatalytic activity was estimated for O2 evolution from water oxidation because this reaction was hardly affected by the specific surface area of photocatalysts. Thermal desorption and gas chromatography–mass spectrometry measurements suggested the decomposition of residual glycolic acid in TiO2 nanorods at 272 and 590 °C. Observation with scanning electron microscopy and physical properties such as specific surface area and crystallite size indicated deformation of the rod shape to subsphere by the calcination above 600 °C. Time-resolved microwave conductivity measurements have revealed that the TiO2 nanorods possess the longest lifetime of photogenerated electrons among all samples. However, the O2 evolution rate of the TiO2 nanorods was much lower than subsphere TiO2 obtained by calcination at 800 °C (TiO2-800). Photodeposition of Pt and PbO2 showed that the oxidation and the reduction sites were separated on TiO2-800 whereas their deposition distributed uniformly on the TiO2 nanorods. The Pt loading on the surface of the TiO2 nanorods increased the O2 evolution rate whereas that on TiO2-800 was hardly affected, indicating that the Pt loading enhances the charge separation on the surface of the TiO2 nanorods. Therefore, the lower photocatalytic activity of the TiO2 nanorods is ascribed to the recombination process on the surface. The suppression of the recombination on the surface and thereby the utilization of the long lifetime of the photogenerated electrons is the key factor to develop superior TiO2 nanorods for photocatalysis