Disclosing the hidden presence of Ti3+ ions in different TiO2 crystal structures synthesized at low temperature and photocatalytic evaluation by methylene blue photobleaching

We have performed a comprehensive study on the relationship among different crystal structures of TiO2, appearance of self-doping Ti3+ ions at low synthesis temperature, and photodegradation efficiency of the organic dye methylene blue (MB). Samples with anatase and rutile phase of TiO2 were synthesized by microwave assisted hydrothermal method. The anatase structured samples promote faster MB photobleaching in comparison with the rutile one. Electron paramagnetic resonance and X-ray photoelectron spectroscopies disclosed the presence of self-doping Ti3+ ions in the anatase that are completely absent in the rutile phase. The creation of these Ti3+ ions driven by anatase structure due to its lower vacancy formation energy induces mid-gap states within the band-gap of TiO2, which in turn can enhance the visible light absorption. Understanding the precursors at low synthesis temperatures of defects and self-doping formations can shed light on both the efficiency evolution and photocatalytic nature of this important semiconducting material.