Construction Of High Efficient Photocatalysts With Carbon Dots And Surface Oxygen Vacancies And Their Photocatalytic Mechanism

Generally,metal oxides such as titanium dioxide and iron oxide suffer from limitations of weak absorption ability of visible light and high recombination rate of photogenerated electron(e^-)and hole(h⁺).In order to overcome these limitations,modification method including carbon dots loading and surface oxygen vacancies modulation were employed.This dissertation mainly consists of carbon dots modified TiO₂,carbon dots and surface oxygen vacancies co-modified TiO₂,carbon dots and surface oxygen vacancies co-modified Fe₂O₃.Besides,the possible roles of these modifications in photocatalytic process were investigated systematically.1.Carbon dots-TiO₂ nanosheets(CDs-TNs)composites were synthesized by hydrothermal method and applied for the first time in photoreduction of Cr(VI)under sunlight illumination.The materials were characterized by a series of analytical techniques and the results confirmed that 1.78 nm of well-crystallized CDs are successfully incorporated with TNs through Ti-O-C bonds.With the introduction of CDs,CDs-TNs composites exhibit excellent photoreduction performance of Cr(VI),which is higher than that of TNs,P25 and CDs-P25.The unique upconversion property and electron transfer property of CDs play essential roles on light harvesting and charge carriers transferring in the photoreduction process.Most importantly,CDs could facilitate the formation of H₂O₂,leading to the consumption of holes.As a result,the recombination of photoinduced electron-hole pairs is hindered and photoreduction activity of CDs-TNs is promoted.2.One-step hydrothermal strategy was developed to synthesize carbon dots-surface oxygen vacancies co-modified titanium dioxide(CDs@TiO₂)composites,in which ascorbic acid served as carbon precursor and surface oxygen vacancies modulator.In CDs@TiO₂ composites,amorphous carbon dots are uniformly coated on TiO₂ with Ti-O-C bond.With the amount of CDs increasing,the thickness of amorphous layer increase whereas the amount of surface oxygen vacancies increase firstly and then decrease.With CDs decoration and surface oxygen vacancies modification,CDs@TiO₂ composites exhibit enhanced photoreduction performance.Under simulated sunlight and visible light,the reaction rate of 1.5CDs@TiO₂ is 9.6times and 216 times than that of pure TiO₂,respectively.CDs mainly contribute to high utilization of solar light by narrowing band gap and reducing conduction band position.While surface oxygen vacancies are more devoted to promote photogenerated electron-hole separation by reducing valence band position,enhancing the charge transfer ability and increasing the carrier density.Compared with CDs,surface oxygen vacancies play a more decisive role in improving the photocatalytic properties of CDs@TiO₂ composites.3.Introducing one-step hydrothermal strategy to Fe₂O₃ modification,we successfully synthesized carbon dots-surface oxygen vacancies co-modified iron oxide(CDs@Fe₂O₃)composites.Fe₂O₃nanorodes are encapsulated by 10.2wt%amorphous carbon dots with Fe-O-C bond.Compared with Fe₂O₃,CDs@Fe₂O₃ show appearance of Fe²⁺and obvious EPR signal at g=2.003,indicating the formation of surface oxygen vacancies in CDs@Fe₂O₃.Thanks to co-modification of carbon dots and surface oxygen vacancies,the photo-Fenton degradation rates of Rh B and phenol by CDs@Fe₂O₃ are 7 times and 70 times higher than that of original Fe₂O₃,respectively.The improvement of photo-Fenton properties of CDs@Fe₂O₃ composites is mainly attributed to the synergistic effect of CDs and oxygen vacancies.On one hand,they can facilitate the decomposition of H₂O₂ to produce·OH to enhance the oxidation ability;on the other hand,they can promote the charge transfer and increase carrier density of the composites,so that photogenerated electron-holes could be separated efficiently and the reaction could be accelerated.