Preparation And Performance On CO₂ Photocatalytic Reduction Of Carbon/Titanium Dioxide Composites

With the increasing of atmospheric CO2 owing to various human activities such as energy production and transport,the temperature of earth is warmer.There are several methods to reduce CO2:the capture and geological storage of the gas to form stable carbonates,absorption into various functionalized materials,and large-scale forestation.However,those solutions are in fact energy intensive and costly.One alternative method,artificial conversion of CO2 to other useful chemical compounds using the abundant solar energy,has attracted much attention.Photocatalytic reduction of CO2 with water by photocatalysts such as TiO2 to produce solar fuels is an attractive approach to alleviate the environmental influences of greenhouse gasses and in the meantime produce valuable solar fuels.However,TiO2 has several disadvantages for CO2 photoreduction,such as the large band gap,the fast charge recombination rate and relatively insufficient CO2 adsorption capability results in the low production rate of solar fuels.Therefore,carbon materials including carbon quantum dots,graphene and carbon nanotubes have been combined with semiconductor materials for surface synthesis and reduction of manufacturing costs,while obtaining light absorption in visible light and enhanced charge separation and transmission.Based on the above reasons,we composite titanium dioxide(P25)with carbon quantum dots(zero dimension),carbon nanotubes(one dimension)and graphene(two dimension),and discuss the effect of titanium dioxide composite with different dimensions of carbon materials on the product and yield in the process of photocatalytic reduction of CO2.The details are as follows:1)CQD/TiO2 composites were successfully prepared by impregnation-calcination.The morphology and photocatalytic performance of the composites were analyzed by a series of characterization methods.Transmission electron microscopy,X-ray diffraction,Fourier transform infrared spectroscopy,and Raman spectroscopy showed that the carbon quantum dots were successfully recombined with P25.Characterization of UVvisible diffuse reflection and transient photocurrent response proves that with the increase of the content of carbon quantum dots,the composite material absorbs visible light,and the band gap decreases,effectively suppressing electron-hole recombination.Photocatalytic evaluation of CQD,CQD/TiO2 composites,and comparison with pure TiO2(P25).The yield of CQD(0.5 wt%)/P25 is as high as 23.06 ?mol·g-1,which is 9 times that of pure P25.In addition,the selectivity of pure CQD for CO is greater than 99%.2)CNT/TiO2 composites with different contents were prepared by hydrothermal method.Transmission electron microscopy,X-ray diffraction,Fourier transform infrared spectroscopy,and Raman spectroscopy proved that carbon nanotubes and titanium dioxide were successfully composited.The use of physico-chemical adsorption,ultraviolet-visible diffuse reflection,photoelectrochemical test-transient photocurrent response indicates that the composite promotes the adsorption of CO2,the absorption of visible light is enhanced,the band gap is reduced,and the electron-hole recombination is suppressed.The photocatalytic evaluation of the CNT/TiO2 composite was compared with pure TiO2(P25).The yield of CNT(1wt%)/P25 was as high as 5.89?mol·g-1,which was about twice that of pure P25.3)RGO/TiO2 composites were successfully prepared by sol-gel method in which graphite oxide with different contents was compounded with TiO2(P25).Physicochemical adsorption,transmission electron microscopy,X-ray diffraction,Fourier transform infrared spectroscopy,Raman spectroscopy,ultraviolet-visible diffuse reflection,photoelectrochemical test-transient photocurrent response,and X-ray photoelectron spectroscopy were used to analyze and characterize the composites.The results showed that graphite oxide was successfully reduced to RGO,and P25 was attached to the surface of RGO;the specific surface area increased,which increased the adsorption of CO2;the composite material's response to visible light was enhanced,which reduced the band gap of P25 and effectively suppressed electronsRecombination of holes.The photocatalytic evaluation of RGO/TiO2 composites was compared with that of pure TiO2(P25).Among them,the yield of RGO(4 wt%)/P25 was as high as 7.56 ?mol·g-1,which was about three times that of pure P25.4)By exploring the photocatalytic reduction of CO2 by CQD/TiO2,CNT/TiO2 and RGO/TiO2 composites,P25 was composited with carbon quantum dots(0-dimensional),carbon nanotubes(1-dimensional),and graphene(2-dimensional The effect of carbon materials with different dimensions after)on photocatalytic results.From the results of UV-visible diffuse reflection,it is known that the forbidden band width of CQD/TiO2,CNT/TiO2 and RGO/TiO2 gradually decreases;the results of CO2 adsorption indicate that RGO/TiO2 has higher adsorptivity;generated during photocatalysis CH4 requires more electrons,which is more difficult.From the photoelectrochemical test-transient photocurrent response,it shows that the RGO/TiO2 has a stronger photoelectric signal and shows higher selectivity to methane.The comparison of the selectivity of composite materials for different products indicates that the dimensions of the material will affect the transient photocurrent response and thus its selectivity to the products methane and carbon monoxide.