| With the rapid development of global economy,environmental pollution and energy shortages have become increasingly severe issues,thus people are more and more committed to seek excellent photocatalysts used for pollutants degradation and hydrogen production from water splitting for solving such problems.Titanium dioxide(TiO2)has potential applications in photocatalysis,solar cells,lithium batteries and gas sensors and so on due to its unique photochemical properties.In recent years,accumulated investigations demonstrate that the surface modification of TiO2 can significantly improve its photoelectric conversion efficiency and photocatalytic activity.TiO2 nanotube arrays(TiO2 NTAs)were prepared in this thesis as the primary structure for carrying out the controlled fabrication of TiO2-based composite nanomaterials and its application in photocatalytic degradation of organic pollutants and water splitting to obtain hydrogen.TiO2 has advantages of low price,friendly environment and outstanding physical and chemical stability,and TiO2 nanotube arrays show uniform surface morphology,regular arrangement,highly ordered orientation,adjustable length and pore size,and unique photoelectrochemical characteristics,thus TiO2 nanotube arrays and its composites are always the hotspot of photocatalysis research.Since their synthesis with anodization in 2001,TiO2 nanotube arrays have aroused great interests among researchers in related fields.However,it is worth noting that TiO2 semiconductor materials show wide band gap,3.2 e V for anatase TiO2 and 3.0 e V for rutile TiO2,thus it only displays photoresponse to ultraviolet(UV)light,and nearly no photoresponse to visible and near infrared(NIR)light,which greatly limit its efficient utilization of solar light.In addition,the effective separation and transformation transfer of charge carriers are limited because of the low conductivity of TiO2 semiconductor materials,resulting in that the photogenerated electron-hole pairs recombination reduces photoelectric conversion efficiency.Numerous of defects exist in TiO2 semiconductor materials,the photogenerated electron-hole pairs show obvious loss in alkaline solution,and the addition of electron donor can effectively consume photogenerated holes,thus restraining the electron-hole pairs recombination.Therefore,TiO2 nanotube arrays modification can significantly extend the wavelength range of light response,promote the solar light utilization,resulting in enhanced photoelectric conversion efficiency and photocatalytic activity.In order to verify the modification effect,we performed the comparative study of pristine TiO2 nanotube arrays and its based nanocomposites in optical and photoelectrochemical characteristics,and photocatalytic activity.The specific research work we performed as follows:(1)A two-step anodization was employed to prepare TiO2 nanotube arrays with uniform surface morphology,regular arrangement and highly ordered orientation,and TiO2 anatase/rutile heterojunction was obtained with the assitance of calcination.The as-synthesized samples were characterized by scanning electron microscopy,X-ray diffraction,Raman,X-ray photoelectron spectroscopy,UV-vis absorption spectroscopy,photoluminescence spectra,photoelectrochemical and photocatalytic activity measurements.The photoelectrochemical tests demonstrate that this heterojunction shows strong photoresponse to UV light,etremely weak photoresponse to visible light,and almost no photoresponse to NIR light.It is known to us that the photoelectric conversion efficiency of semiconductor materials depends on photogenerated electron-hole pairs' separation capability,thus the surface modification of TiO2 nanotube arrays to reduce its band gap and enhance its photogenerated electron-hole pairs' separation capability is the effective stratege for promoting its photoelectrochemical characteristics and photocatalytic activity.(2)A facile electrochemical approach for the large-scale synthesis of high quality carbon dots(CDs)with high purity,uniform size and excellent monodispersion,and CDs were uniformly anchored to the inner and outer wall surface of TiO2 nanotube arrays by electrodeposition to synthesize CDs/TiO2 NTAs compound structures,which efficiently enhanced TiO2 nanotube arrays' photoresponse,photoelectric conversion efficiency and photocatalytic activity.Simultaneously,it is realized that the controllable growth of CDs on TiO2 nanotube arrays by tuning the electrodeposition time.CDs show the electron donation/acceptance and up-conversion photoluminescence characteristics,which efficiently promote the photogenerated electrons' production and separation.Also,CDs can act as an electron reservoir to trap electrons emitted from TiO2 nanotube arrays due to the light irradiation,and thus hinder electron-hole pairs' recombination probability.In addition,CDs can serve as an electron scavenger to avoid the process of electron-hole recombination.Therefore,the introduction of CDs can effectively expand the photoresponse wavelength range of TiO2 nanotube arrays,enhance the solar light utilization,resulting in CDs/TiO2 NTAs compound structures' significantly enhanced photoelectric conversion efficiency and photocatalytic activity.(3)Silk fibroin was used as the precursor to prepare nitrogen-doped carbon quantum dots(N-CQDs)with the size less than 10 nm and excellent monodispersion by hydrothermal treatment,and then N-CQDs were loaded to the inner and outer wall surface of TiO2 nanotube arrays by hydrothermal process to construct N-CQDs@TiO2 NTAs compound structures.Graphitic carbon nitride with the band gap of 2.7 e V is a newly development metal free semiconductor,which has been widely used in photocatalytic pollutants degradation,water splitting to produce hydrogen,sterilization,electrocatalytic oxygen reduction reaction,bio-imaging and solar cells and so on,since it shows excellent biocompatibility,negative conduction band position for reaction and visible light response.N-CQDs extracted from silk fibroin show highly attractive water-solubility,low toxicity,biocompatibility,size-tunable photoluminescence,and green and economical synthetic approach.N-CQDs used to decorate TiO2 nanotube arrays can efficiently promote the electrons transfer on the interfaces between N-CQDs and TiO2 nanotube arrays,resulting in that N-CQDs@TiO2 NTAs compound structures show extended the photoresponse wavelength range,enhanced photoelectric conversion efficiency and photocatalytic activity.(4)Monolayer molybdenum disulfide quantum dots(MoS2 QDs)with uniform size and excellent monodispersion were synthesized by hydrothermal method,and then MoS2 QDs were successfully loaded to the inner and outer wall surface of TiO2 nanotube arrays by electrodeposition to synthesize MoS2 QDs/TiO2 NTAs heterostructures.As a two-dimensional layered transition meatal dichalcogenide,MoS2 with the hexagonal structure(2H-MoS2)shows unique light absorption,photoluminescence,thermal effect,stability and semiconductor characteristics at ambient temperature,making it to be served as a co-catalyst to decorate other wide band gap semiconductor materials for effectively reducing the band gap,resulting in promoting photogenerated electron-hole pairs separation.The optical properties of many two-dimensional materials are related to its layers number,of course including MoS2,the band gap structure of MoS2 can change from indirect band gap to direct bad gap when its thickness reduces to monolayer.Monolayer MoS2 QDs with more excellent fluorescence characteristics used as co-catalyst to decorate TiO2 nanotube arrays can effectively reduce the band gap and promote electron-hole pairs separation,resulting in MoS2 QDs/TiO2 NTAs heterostructures' significantly enlarged light absorption range,enhanced photoelectric conversion efficiency and photocatalytic activity.The photocatalytic capability of TiO2 nanotube arrays can be significantly enhanced by its modification with various quantum dots.Therefore,the design of TiO2 nanotube arrays based composite nanomaterials develop the photoelectric response study of semiconductor materials in visible-NIR light wave band,which are beneficial for further exploring nanocomposites applied in photocatalytic degradation of organic pollutants and water splitting to produce hydrogen,and promoting the industrialization of photocatalytic technology. |
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