2.1. Synthesis of TiO2 Nanoparticles

For the synthesis of TiO₂ nanoparticles, titanium (IV) oxysulfate hydrate (TiOSO₄, Sigma Aldrich, St. Louis, MO, USA) precursor was used. Furthermore, the synthesis of TiO₂ aggregates has been performed using a hydrothermal synthesis technique. The TiOSO₄ precursor solution was prepared by dissolving 6.4 g of TiOSO₄ (2.5 M) in 16 mL of distilled water under constant stirring of 750 r/min and temperature of 45 °C for 2 h to get a clear solution. Then the solution of TiOSO₄ was transferred into a Teflon-lined stainless-steel autoclave of 25 mL capacity. The heating rate was of 2.5 °C/min, and during the synthesis, the temperature was maintained at different temperatures of 100, 200 and 220 °C for 6 h depending on the aggregate size required. After this synthesis in autoclave, a white TiO₂ powder was obtained and was washed six times in distilled water and two times in ethanol. Then the powder was dried overnight in the oven and annealed in air at temperature of 500 °C for 30 min with the heat rate of 5 °C/min. For nanourchin, nanotube, and nanobelt synthesis, 0.5 g powder of TiO₂ aggregate was introduced in a Teflon-lined autoclave of 25 mL capacity. Then, the autoclave was filled with 10 M NaOH solution up to 80% of the autoclave capacity. During the synthesis, the temperature was maintained at different temperatures of 100, 150, and 220 °C with the heating rate of 2.5 °C/min and the synthesis time of 360, 180, and 15 min, depending on the required morphology. Afterwards, synthesis nanobelt particles are subjected to the washing and annealing protocols to obtain at the end of these processes: sodium titanate. The latter product was washed many times with diluted HCl solution to attain a pH value of 1. After that, the suspension was washed with distilled water several times to reach a pH value of 7. Finally, the obtained powder was dried overnight in the oven, and annealed in air at temperature of 500 °C for 30 min, with the heat rate of 5 °C/min.

All the chemicals are of analytical grade and used without further purification. The water used in all the experiments was purified by Milli Q System (Millipore, electric resistivity 18.2 MΩ.cm).