Construction Of TiO₂ Nanofiber-based Heterojunction And The Mechanism Of Photocatalytic Enhancement

Environmental pollution and energy shortage threaten the sustainable development of human society.Photocatalytic hydrogen production and CO₂ reduction into solar fuels have been regarded as one of the most attractive strategies to solve above crises.TiO₂ as a semiconductor photocatalyst shows the merits of physiochemical stability,nontoxicity and ecofriendliness,cost effectiveness,and easy availability.But it still suffers from the rapid recombination of photogenerated electrons and holes and low utilization of solar energy due to its wide band gap.Therefore,exploring high-efficiency TiO₂-based photocatalysts and optimizing their preparation or surface modification techniques are of importance for the development of TiO₂-related photocatalytic technology.In this dissertation,1D/2D TiO₂-based hybrid nanofibers were prepared by loading appropriate cocatalysts on TiO₂ electrospun nanofibers and constructing direct Z-scheme heterojunction so as to realize the efficient separation of electron/hole pairs and enhanced photocatalytic efficiency.The photocatalytic mechanism and intrinsic structure-dependent photocatalytic performance TiO₂-based photocatalysts were also elucidated.The research contents are shown as follows:1.Direct Z-scheme TiO₂/CuInS₂ heterojunctions for CO₂ photoreduction.We have hydrothermally fabricated novel hybrid nanofibers by in-situ growing CuInS₂nanoplates onto TiO₂ nanofibers.The resulting TiO₂/CuInS₂ hybrid nanofibers show abundant pores,large specific area(102 m² g^-1)and enhanced absorbance of visible light.X-ray photoelectron spectroscopy（XPS）results and density functional theory（DFT）calculations reveal the presence of electron transfer from CuInS₂ to TiO₂ upon hybridization.Such electron transfer creates a built-in electric field at the interfaces,which is the intrinsic driving force to construct the direct Z-scheme heterojunction of TiO2/CuInS2.Guided by the electric field,the photoinduced electrons and holes transfer along a“Z”pathway across the interfaces,achieving an efficient separation of electron/hole pairs and reserving the strong redox ability of photoinduced electrons.The TiO₂/CuInS₂ exhibits superior photocatalytic activity for CO₂ reduction under irradiation,with CH₄ and CH₃OH production rates of 2.50 and 0.86μmol h^–1 g^–1,due to the enhanced light absorption,large specific area and efficient charge separation.This work may provide a new idea for the design and fabrication of direct Z-scheme TiO2-based photocatalyst for high-efficiency CO2 photoreduction.2.Direct Z-scheme TiO₂/NiS heterojunctions for photocatalytic hydrogen production.Photocatalytic water splitting for hydrogen production is a sustainable approach for solving the current energy crisis.We have prepared novel TiO₂/NiS hybrid nanofibers,where few-layered NiS nanoplates were vertically and uniformly deposited upon the TiO₂ nanofibers via electrospinning and hydrothermal methods,guaranteeing intimate contact for charge transfer.XPS analysis and DFT calculation imply that the electrons in NiS would transfer to TiO₂ upon hybridization,which creates a built-in electric field at the interfaces and thus facilitates the separation of useful electron and hole upon photoexcitation.In-situ XPS analysis directly proved that the photoexcited electrons in TiO₂ migrated to NiS under UV-visible light irradiation,suggesting that a direct Z-scheme heterojunction was formed in the NiS/TiO₂ hybrid.This direct Z-scheme mechanism greatly promotes the separation of useful electron-hole pairs and fosters efficient H₂ production.The hybrid nanofibers unveiled a high H₂-production rate of 655μmol h^-1 g^-1,which was 14.6-fold of pristine TiO₂ nanofibers.Isotope（⁴D₂O）tracer test confirmed that H₂ was produced from water,rather than from any H-containing contaminants.This work provides an alternative approach to rationally design and synthesize TiO₂-based photocatalysts with direct Z-scheme pathways toward high-efficiency photo-generation of H2.3.TiO₂/MoS₂ heterojunctions for efficient CO₂ photoreduction.MoS2,a type of2D layered material,has attracted significant attention in photoelectronics,sensors and photo/electrocatalytic water splitting owing to its remarkable properties.Nevertheless,to date,MoS₂ is barely used as（co）catalyst for CO₂ photoreduction.Herein,a novel1D/2D TiO₂/MoS₂ nanostructured hybrid with TiO₂ fibers covered by MoS₂ nanosheets（2 nm in thickness）by hydrothermal transformation method is fabricated.XPS results and DFT calculations imply the intimate chemical interaction between MoS2 and TiO2upon hybridization,which can facilitate electron–hole separation upon photoexcitation.In addition,the hierarchical TiO2/MoS2 nanostructure shows enhanced optical absorption and CO₂ adsorption,therefore,a superior photocatalytic activity for reducing CO₂ into CH₄(2.86μmol h^-1 g^-1)and CH₃OH(2.55μmol h^-1 g^-1)is achieved over the hybrid as compared to pristine TiO₂.Isotope(¹³C)tracer test confirms that the products are produced from the photocatalytic reduction of the CO₂ source instead of any organic contaminants.This work offers an alternative approach to rationally design and synthesize TiO2-based photocatalysts toward high-efficiency CO2 photoreduction.4.TiO₂/graphdiyne heterojunctions for efficient CO₂ photoreduction.As an emerging carbon allotrope,graphdiyne（GDY）features a 2D character,unique carbon-carbon bonds and photothermal effect.Exploring efficient GDY cocatalyst to boost photocatalytic CO₂ reduction is of critical importance for solar-to-fuel conversion.Herein,we report on a novel GDY cocatalyst coupled TiO₂ nanofibers for boosted photocatalytic CO₂ reduction,synthesized by an electrostatic self-assembly approach.DFT calculation and in-situ XPS measurement reveal that the delocalized electrons in GDY can hybrid with the empty orbitals in TiO2 within the TiO2/GDY network,leading to the formation of an internal electric field at the interfaces,pointing from GDY to TiO₂.Theoretical simulation further implies strong chemisorption and deformation of CO₂ molecules upon GDY,which can be verified by in-situ diffuse reflectance infrared Fourier transform spectroscopy.These effects,in combination with the photothermal effect of GDY,result in enhanced charge separation and directed electron transfer,enhanced CO₂ adsorption and activation as well as accelerated catalytic reactions over the TiO2/GDY heterostructure,thereby resulting in significantly improved CO2photoreduction efficiency with CO and CH₄ production rates of 50.53 and 2.80μmol h^-1 g^-1,respectively.This work demonstrates that GYD can function as a highly effective cocatalyst for solar energy harvesting and may be used in other catalysis processes.5.Concluding the whole dissertation and prospecting the follow-up work.