Effect of annealing on UV-visible absorption and photoluminescence behavior of liquid phase deposited TiO2 nanorods

In this work, anatase and rutile TiO2 nanorods were fabricated using one-step liquid phase deposition process, followed by heat treatment in the range 300-800 degrees C. The direct and indirect band gap of the TiO2 nanorods was estimated form optical absorption data which illustrated a red shift at higher temperatures owing to the different nature of excitons in anatase and rutile phases. The photoluminescence (PL) spectra revealed the presence of two main emission bands consisting of four peaks. It was found that two high-energy peaks located at 2.95-3.30 eV could be generated from exciton transitions from the conduction band to the valence band of TiO2 nanorods, while two low-energy peaks located at 2.43-2.64 eV may arise from surface state transitions. The PL intensity firstly increased with temperature and at 500 degrees C reached a maximum value, then decreased through increasing temperature up to 800 degrees C. These variations in the intensity of PL emission could be explained in terms of changes in phase structure, crystallinity, and amount of the oxygen vacancies, which are all dependent to the annealing temperature based on X-ray diffractometer and X-ray photoelectron spectrometer studies. These results indicated that annealing temperature allows to manipulate the properties of TiO2 nanorods for opto-electronic applications.