The Synthesis Of Modified Anatase {001}TiO₂ Nanosheets Hierarchically Structured Materials And Their Performance For Photocatalytic Hydrogen Production

With increasingly serious global energy and environmental problems,photocatalytic conversion of solar energy to hydrogen via water splitting has attracted great attention. {001} facets exposed TiO2 nanosheets hierarchical structured materials show higher photocatalytic activity than traditional TiO2 nanomaterials. In the present thesis, nitrogen and g-C3N4 modified anatase{001}TiO2 nanosheets hierarchically structured materials has been synthesized.The crystal structure, microscopic morphology and electronic state of the obtained modified photocatalysts were investigated. The photocatalytic hydrogen production activity of the photocatalysts was studied. The nature of the active sites of the modified anatase {001}TiO2 nanosheets hierarchical photocatalysts was revealed. The main contents of the thesis are as follows:(1) N-doped anatase TiO2 nanosheets hierarchical mesoporous spheres Nx{001}TiO2-t (x=0.11, 0.27, N/Ti molar ratio; t=12 h, 24 h, 36 h, reaction time) with {001} facets exposed and significantly enriched concentration of Ti3+-oxygen vacancies were synthesized using a green and moderate solvothermal method. The as-obtained hierarchical spheres photocatalysts are self-assembled by the ultrathin mesoporous nanosheets (10-13 nm) consisting of ultrafine anatase crystals (10.1-13.7 nm), resulting in uniform mesoporous distribution (2-8 nm) and high specific surface area (106-144 m2/g). In the Nx{001}TiO2-t photocatalysts, N elements are successfully incorporated into both the interstitial and substantial sites of TiO2 lattice, maybe beneficial for the concentration of oxygen vacancies (Ov) increasing. All the N-doped samples show higher Ti3+/Ti4+ ratios than {001}TiO2-24, while N0.11{001}TiO2-24 and N0.27{001}TiO2-24 presents the highest ratio,demonstrating the existence of a large number of small neutral Ti3+-oxygen vacancies. But The N0.11{001}TiO2-24 possesses the highest specific surface area (144 m2 g-1).(2) The photocatalytic H2 production activities for the series of Nx{001}TiO2-t using methanol as sacrificing agent are obviously higher than that of the pure {001}TiO2-24. The N0.11{001}TiO2-24 shows the highest photocatalytic H2 evolution rate of 23.8 mmol·h-1·g-1 and energy conversion efficiency of 4.0%. The high photocatalytic H2 production activity can be attributed to the high exposure of active sites, the high specific surface area and the increased concentration of Ti3+-oxygen vacancies. Also, the H2 evolution rate of N0.11{001}TiO2-24 in KOH solution without sacrificing agent is up to 11.8 mmol h-1 g-1, which can be attributed to the unique structure and the reduction effect of OH- anions.(3) Hierarchically structured heterojunction complex photocatalysts{001}TiO2/g-C3N4-x (x=50, 100, 150, 175, 200 mg, the addition of g-C3N4)were synthesized using solvothermal method. The appropriate addition of g-C3N4, especially {001}TiO2/g-C3N4-175 shows that ultrathin nanosheets(60-100×7-11 nm) in situ vertically grew on the surface of g-C3N4 sheets, and exhibits the highest photocatalytic H2 evolution rate of 26.1 mmol·h-1·g-1 using methanol as sacrificing agent, which is much higher than those of pure g-C3N4 and {001}TiO2-24. The excellent photocatalytic H2 production activity can be ascribed to the uniform mesoporous distribution (?2.8 nm) and high specific surface area (146 m2/g) endowed by the ultrathin nanosheets (10.3 nm)consisting of ultrafine crystallites and the surface-interface synergistic effect of between anatase TiO2 and g-C3N4.