Synthesis And Photocatalytic Performance Of Ultra-thin Two-dimensional TiO₂

The research and application of ultra-thin two-dimensional nanostructure materials represented by graphene have demonstrated their excellent surface activity and electron transport properties.As a classic photocatalyst,the design and synthesis of two-dimensional nanostructure of TiO₂ has attracted more and more interest.Therefore,in this work,the large size ultra-thin graphene-like TiO₂ nanosheets were successfully synthesized by changing the thermodynamic conditions of crystallization and maturation by placing the precursor in liquid phase at low temperature.The work focuses on the growth process of TiO₂ nanosheets and the effects of nitric acid concentration,aging temperature and reactant concentration on the formation process of the two-dimensional structure.Transmission electron microscopy（TEM）is used to characterize the microscopic morphologys of the samples.The microstructures of samples are analyzed by UV-vis absorption spectroscopy,X-ray diffraction spectrum（XRD）,X-ray photoelectron spectroscopy（XPS）and fourier transform infrared spectroscopy（FT-IR）.In addition,the photocatalytic property of TiO₂ nanosheets is studied by photocatalytic reduction of Cr（VI）and photocatalytic oxidation of rhodamine B.Experimental results show that the hydrolysates of tetrabutyl orthotitanate and anhydrous ethanol in dilute nitric acid aqueous solution under ice-water bath can be peptized to form ultra-small TiO₂ nanoclusters under low temperature.Under the effect of hydrogen bonding,those TiO₂ nanoclusters are connected to form a two-dimensional atom network and further matured into ultra-thin TiO₂ nanosheet at a low temperature.The TiO₂ nanosheets have an apparent quantum size effect.Photocatalytic experiments show that the TiO₂ nanosheets have a better photocatalytic effect compared to nanocrystal particles,and the photocatalytic activity of TiO₂ nanosheets under ultraviolet light is about 6 times that of nanocrystal particles.The TiO₂ nanosheets also have good reusability,and Cr（VI）and Rhodamine B can still be degraded completely after 10 cycles.In the synthesis process,the nitric acid concentration,aging temperature and reactant concentration affect the microstructure and photocatalytic activity of TiO₂nanosheets.Without nitric acid,the hydrolyzed product can’t peptize;when the concentration of nitric acid is during 0.0217⁰.0721 mol/L,the ultra-thin anatase TiO₂nanosheets are obtained by low temperature aging;when the concentration of nitric acid is too high,the hydrolysis rate becomes slow.Due to the generation of rutile nanocrystal particles,the photocatalytic activity of the samples with the concentration of nitric acid greater than 0.0721 mol/L decreases rapidly.Because of the unstability of the hydrogen bond in high temperature,the two-dimensional structure of TiO₂ will be destroyed when the aging temperature exceeds 4 ^oC.Therefore,the photocatalytic activity of samples will decrease with the rising of aging temperature.Increasing the amount of ethanol in the reactant helps to disperse the hydrolysates and promote the formation of the two-dimensional structure.The samples with the larger amount of ethanol have higher photocatalytic activity in the same aging period.The proper volume ratio of ethanol to tetrabutyl titanate is between 10²0.In addition,during the photocatalytic reduction of Cr（VI）by TiO₂ nanosheets,the reduced product Cr³⁺is doped with nanosheets at room temperature and the band gap of the nanosheets is reduced to 2.8 eV.Therefore,the TiO₂ nanosheets response to visible light,and have a dood performance of photocatalytic reduction of Cr（VI）under natural light.The tetrabutyl titanate dispersed in ethanol hydrolyzes in nitric acid solution with a concentration during 0.0217⁰.0721 mol/L.After aged at 4 ^oC,TiO₂ nanosheets with a thickness less than 1 nm will obtained,and the products have good catalytic activity and reusability.