The Study On The Preparation And Photocatlytic Properties Of N-CDs/TiO₂ Complex Catalysts

TiO₂ has attracted the attention of scholars due to its potential application value for energy and environmental fieids.However,TiO₂ is far from being a perfect photocatalyst.Because it has wide bandgap energy which can only absorb UV light and can't take advantage of solar energy.In addition,TiO₂ photocatalysts have limited applications due to its low electron transfer rate and high electron-hole recombination rate.Carbon dots?CDs?not only have the fluorescence properties of traditional semiconductor quantum dots,but also possess superior properties such as environment-friendly,light stability,biocompatibility,low toxicity etc.CDs are expected to sensitize TiO₂ which can extend their light response range and to improve the photocatalytic activity.Therefore,in this thesis,we focus on the synthesis of N-CDs,OBP-TiO₂ and TiO₂nanosheets?TNS?with?001?crystal planes which were prepared by simple solvothermal method.Then,the photocatalysts were synthesized by different preparation methods which were one-step in-situ synthesis,two-step synthesis and three-step synthesis to study their photocatalytic activity under visible light.The specific work included following sections:?1?A facile hydrothermal route was used to synthesize N doped CDs?N-CDs?by using citric acid as the carbon source and urea as N source.N-CDs/P25 photocatalysts were obtained by simple hydrothermal process.TEM,XPS,DRS confirmed N-CDs were successfully combined with P25.The photocatalytic activity of N-CDs/P25 composites were evaluated by degradation capability of RhB.Experiment showed that the photocatalytic activity of N-CDs/P25increased with N-CDs content increasing,however,further increasing N-CDs content,decreased catalytic activity.The photodegradation efficiency has reached its maximum value merely at 6mL N-CDs doping,which shows a 13.06 fold photocatalytic activity improvement over P25.The RhB photodegradation efficiency changed relatively little after 5 cycles of experiment under identical condition,indicating that the photocatalyst has superior photocatalytic stability.Finally,through trapping experiments of reactive species,we investigated the N-CDs/P25 catalytic mechanism under visible light irradiation.?2?In the one-step in-situ synthesis method,1sp-N-CDs/TiO₂ and 1sp-N-CDs/F-TiO₂ were synthesized through a solvothermal method using tetrabutyl titanate,glacial acetic acid,ethylene glycol,citric acid,urea and whether adding hydrofluoric acid as raw material.The catalysts prepared by different ratios were characterized by XRD,TEM,XPS,DRS and other properties.Experiment showed that the morphology of the 1sp-N-CDs/TiO₂ calcined before and after varied with the increase of the amount of citric acid and urea which changed from nano-sized particles with uniform size to hollow sphere.The highest photocatalytic activity has been noticed for1sp-N-CDs/TiO₂^A,which also showed good stability and repeatability.However,1sp-N-CDs/F-Ti O₂ were composed of TiO₂ nanocrystals and N-CDs which with few partially reacted precursors being deposited on the surface and between them.The vast majority of N-CDs and TiO₂ nanocrystal separated which did not well combined.As to calcined samples,the surface of TiO₂ became rougher,crystallization properties and photocatalytic performance became better than before.?3?In the two-step method,2sp-1^#N-CDs/OBP-TiO₂ and 2sp-1^#N-CDs/TNS were prepared by using OBP-TiO₂ or TNS as substrate and adding the raw materials of N-CDs.The catalysts prepared by different ratios were characterized by TEM and other properties.Experiment showed that a little of N-CDs loaded on OBP-TiO₂ or TNS and most of them separated from OBP-TiO₂ or TNS and the photocatalytic performance was slightly higher than OBP-TiO₂ or TNS.2sp-2^#N-CDs/TiO₂ and 2sp-2^#N-CDs/TNS were prepared by using N-CDs as substrate and adding the raw materials of OBP-TiO₂ or TNS.The catalysts prepared by different ratios were characterized by TEM and other properties.Experiment showed that 2sp-2^#N-CDs/TiO₂ catalysts were consist of some small particles with uniform size and the average diameter were 20 nm,2sp-2^#N-CDs/TNS catalysts showed a uniform size and a sheet structure with a size of 20-45 nm.The N-CDs and TiO₂ of two kinds of catalysts could combine closely.The degradation rate of RhB by 2sp-2^#N-CDs/Ti O₂ was 98%within 180 min,while that of 2sp-2^#N-CDs/TNS reached90%within 240 min.?4?In the three-step method,3sp-N-CDs/OBP-TiO₂ and 3sp-N-CDs/TNS photocatalysts were prepared by adding N-CDs and using OBP-TiO₂ or TNS as substrates.It was found that N-CDs can be loaded on the OBP-TiO₂ or TNS uniformly.Experiment showed that the photocatalytic activity of 3sp-N-CDs/OBP-TiO₂ and 3sp-N-CDs/TNS were significantly improved due to the addition of N-CDs and the degradation efficiency of RhB achieved the best at N-CDs 6 mL doping.The degradation efficiency of 3sp-N-CDs/OBP-TiO₂^C on RhB reached95%in 240 min.The degradation efficiency of 3sp-N-CDs/TNS^C on RhB reached 92%within180min,and which has good stability and repeatability.Finally,through trapping experiments of reactive species,we investigated the 3sp-N-CDs/TNS catalytic mechanism under visible light irradiation.?5?Comparing all the catalysts,the photocatalytic activity of the prepared composites was improved compared with pure TiO₂.Among them,?6 mL?N-CDs/P25 had the highest photocatalytic activity and could degrade the RhB solution completely within 90 min.In addition,1sp-N-CDs/TiO₂^A photocatalyst was prepared by in-situ synthesis which using tetrabutyl titanate as the titanium source,acetic acid as the solvent with suitable content of ethylene glycol and0.128 g citric acid and 0.12 g urea.1sp-N-CDs/TiO₂^A photocatalyst could completely degrade RhB in 120 min,and it has good stability and repeatability.