Synthesis Of TiO₂（B）-based Heterojunction Material And Evalution Of Degradation Mechanism Of Tetracycline

The widespread use and continued release of antibiotics have led to the contamination of the water environment,posing a serious threat and challenge to human health and the ecological environment.Currently,photocatalytic technology based on semiconductors and their derivatives as a medium capable of converting renewable solar energy into chemical energy for the removal of antibiotics from water is considered to be a relatively advanced and green technology.Among many semiconductor photocatalysts,TiO₂has the most promising market application due to its non-toxic,cheap,high reactivity and good chemical stability,but its weak visible light response and electron-hole complexation tendency severely limit its large-scale application.Based on this,this thesis focuses on the use of ultrathin two-dimensional sub-stable TiO₂（B）nanosheets as an entry point to effectively increase the photoresponse range,built-in electric field strength and photogenerated electron-hole pair complexation rate of the catalyst through defect modulation and heterostructure building strategies.The intrinsic link between photocatalytic activity and the interaction between surface defects,local electronic structure,energy band structure and heterojunction interface was also explored for the removal of tetracycline.The relationship between the enhanced photocatalytic activity and surface defects,local electronic structure and heterojunction interface for the removal of tetracycline are also discussed.The main results of this study are as follows:1.A g-C₃N₄/TiO₂（B）binary heterojunction photocatalyst was prepared by in situ growth of two-dimensional（2D）TiO₂（B）nanosheets（NSs）onto the surface of 2D g-C₃N₄NSs using a mild one-step solvent method.The edge-rich TiO₂（B）flakes were combined with g-C₃N₄NSs to form a unique and ultra-thin 2D/2D heterostructure,which exhibited a large interfacial contact area and thus a shorter migration distance of photogenerated carriers to the surface,which effectively promoted the generation of reactive sites.Under visible light,g-C₃N₄/TiO₂（B）nanocomposites showed higher photocatalytic degradation performance than pure g-C₃N₄and TiO₂（B）nanosheets,up to 82.17%(the degradation rate constant k of 0.0135 min^-1).Based on the energy band structure and XPS results,it was clear that an electron transfer from TiO₂（B）to g-C₃N₄at the g-C₃N₄/TiO₂（B）interface formed a built-in electric field.At the same time,the photogenerated electrons from the conduction band（CB）of g-C₃N₄and holes from the valence band（VB）of TiO₂（B）were retained,which contributed to activate oxygen molecules and hydroxyl groups to generate reactive radicals.The improved tetracycline degradation activity can be attributed to the ultrathin2D/2D nanosheet structure and the constructed S-type heterojunction.2.Ultrathin TiO₂（B）nanosheets decorated with layered In₂S₃nanospheres were prepared by a hydrothermal-oil bath strategy by introducing the co-catalyst excess metal sulphide In₂S₃,and the constructed heterojunctions were used for the photocatalytic degradation of tetracycline.The optimized IT-1.5:1 heterojunctions obtained enhanced visible light capture,superior electron-hole pair transfer efficiency and more reactive radicals generated from the abundantly coupled heterogeneous interfaces.Meanwhile,the heterojunction achieved a removal efficiency of 97.3%for tetracycline,which was 3.2 and2.1 times higher than that of TiO₂（B）and In₂S₃,respectively.The XPS results indicated that electrons migrated from TiO₂（B）to In₂S₃,then combined with the superoxide and hydroxyl radicals generated from ESR and degradation reaction and the energy band structure obtained from DFT calculations,the TiO₂（B）/In₂S₃was thus determined to be an S-scheme heterojunction rather than a Type-II heterojunction.Furthermore,based on the intermediates identified by high performance liquid chromatography-mass spectrometry（HPLC-MS）,it was postulated that the possible degradation pathway of tetracycline was the cleavage of the central carbon and oxygen atoms susceptible to attack by reactive radicals.3.By a simple hydrothermal method using non-metallic ion F doping,S-scheme heterojunction of Cogwheel-shaped NH₂-MIL-53（Al）anchored by F-TiO₂（B）nanosheets was synthesized and used for photocatalytic degradation of tetracycline.Under the synergistic effect of F dopants,matched energy band structures and built-in electric fields,the optimized nanocomplexes obtained superior degradation rates（94.6%）and TOC mineralization efficiency（71.4%）.Furthermore,the addition of external interfering ions such as Cu²⁺or NO₃^-had almost no effect on the photoactivity.A possible degradation pathway for tetracycline is proposed on the basis of the Fuki index calculated by DFT and the main intermediates identified by high performance liquid chromatography-mass spectrometry（HPLC-MS）.At the NH₂-MIL-53（Al）/F-TiO₂（B）interface,electrons leap from F-TiO₂（B）to NH₂-MIL-53（Al）through strong electron interactions,resulting in an S-scheme structure.This heterogeneous structure facilitated rapid space charge separation,improved visible light utilisation and provides exposed active sites with high potentials.