| Abstract |
This PhD thesis investigates the enhancement of the photocatalytic performance of titanium dioxide (TiO2), through the synthesis of titania-based hybrid materials with improved properties compared to bare TiO2. In specific, though the synthetic approaches presented herein, we address important shortcomings of TiO2, including its low absorption in the visible light range, its tendency towards particle aggregation and the high charge carrier recombination rate.
Different strategies were evaluated, such as the surface modification of TiO2 with noble metal nanoparticles or/and graphitic sheets, hydrophilic polymers and the synthesis of inorganic heterostructures. The morphology and structure of the synthesized materials were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman and UV-Vis diffuse reflectance spectroscopy, zeta potential, photoluminescence (PL), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) measurements. Moreover, the photocatalytic performance of the bare and modified TiO2 photocatalysts was assessed by the decoloration rate of aqueous solutions of methylene blue dye under UV-Vis or visible light irradiation.
First, we investigated the surface modification of commercially available TiO2 particles (TiO2 P25) with Ag nanoparticles or reduced graphene oxide (rGO) sheets. These two approaches were then combined into the synthesis of Ag modified TiO2 that was hydrothermally deposited on rGO. Critical parameters that influence the photocatalytic reactions were evaluated, such as the optimum loading values of Ag nanoparticles and graphene. The hybrid materials exhibited a superior photocatalytic activity when exposed to visible light irradiation compared to conventional TiO2 P25, due to the suppression of electron-hole recombination and the extension of the absorption of TiO2 towards the visible light range.
Second, hydrophilic random copolymers synthesized by RAFT polymerization were used for the in-situ surface modification of hydrothermally synthesized TiO2 and the ex-situ modification of commercially available TiO2. The ex-situ approach resulted in low polymer grafting and no significant effect on the photocatalytic performance of TiO2, while the in-situ strategy allowed the higher binding efficiency of the polymeric chains. The in-situ polymer modified nanocatalysts demonstrated a higher photocatalytic response compared to the bare TiO2 nanoparticles, while the effects of the polymer loading and the uniformity of the polymeric chains on the photoactivity of the hybrid photocatalysts were assessed. The enhanced photoactivity of the polymer modified TiO2 was attributed to reduced aggregation and the enhanced adsorption of the dye on the organic shell.
In the third part of this thesis, inorganic core-shell heterostructures with a complex, flower-like morphology, comprising a zinc oxide (ZnO) core and a TiO2 shell were prepared. In addition, hollow TiO2 flowers with intact morphology were obtained, after etching the core in an acidic solution. The photocatalytic performance of the core-shell heterostructures was higher compared to that of bare TiO2 particles, ZnO flowers and hollow TiO2 flowers, while the titania shell thickness was found to strongly influence the photocatalytic reaction rates. The higher photoactivity observed for the core-shell photocatalysts was assigned to an efficient charge separation at the heterojunction between the two semiconductors. Additionally, the most photoactive core-shell sample exhibited outstanding superhydrophilicity without UV irradiation.
In the last part of this thesis, the extension of the absorption of the ZnO-TiO2 core-shell flower-like structures towards the visible light range was attempted. Towards this direction, the surface of the core-shell photocatalysts was hydrothermally wrapped with rGO sheets of tunable mass loadings. The rGO modified ZnO-TiO2 photocatalysts exhibited a photoactivity that was dependent on the graphene content, but was in all cases superior compared to that of the bare ZnO-TiO2 core-shell structures. This enhanced photocatalytic response can be attributed to an enhanced absorption in the visible light range, as well as a more pronounced electron and hole separation.
The simple surface modification approaches proposed in this PhD thesis can find application in the preparation of TiO2-based hybrid photocatalysts with synergistic properties for the removal of undesirable organic pollutants from aqueous matrices.
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