The Study Of Photoelectrocatalytic Reduction Of CO₂ Based On The Bi₂O₃ Semiconductor Nanomaterials Doped With S,Se Or Sn

November 6,2016,the World Meteorological Organization?WMO?had published the latest survey report: global temperatures would rise by 2 ? in the 21 st century.The main reason is the world's carbon dioxide emissions increase.According to the survey,the world's carbon dioxide emissions had reached record in 2012,in which the CO₂ emissions had reached 39.1 billion tons by cars and all kinds of smoke exhaust facilities.And the emissions would be far more than 35 billion tons,which was predicted as the safety of CO₂ emissions by Scientists and environmentalists.As the main culprit of global warming,the growth of CO₂ emissions in 2012 has exceeded the average growth rate of CO₂ emissions in the past decade.It has reported that carbon emissions from burning fossil fuels has been flat for three consecutive years,it seems to be a good news for the global environment.However,it is not enough to solve the problem of global warming.The high rate of CO₂ emissions is leading to the serious green island effect of the Earth,causing sea level rise,glacier melting,and global climate change.Thus reducing CO₂ emissions,building non-fossil fuels,environmentally friendly renewable energy system is increasingly popular.Among the many energy-saving and emission-reducing technologies,photoelectrocatalytic reduction technology is favored,and CO₂ photoelectrocatalytic technology is water as the H source,electrical energy and sunlight as the driving force,is clean,environment-friendly technology.The core of photoelectrocatalysis is how to build photoelectrocatalysts.In theory,there are shortcomings in the photo-induced electron-hole recombination rate and poor product selectivity,and the electrocatalysis can be compensated.The over-potential in the electrocatalysis is high,and the shortcomings of inactivation passivation,in which the photocatalysis just can be strengthened.Photoelectric excitation to produce electrons for the reduction of CO₂,reducing the input of external energy,reduce energy consumption,but also help to remove the catalyst surface of the passivation,which is conducive to the electrocatalytic activity of the applied external voltage,in addition to its own electrocatalytic activity,but also contribute to the separation of photogenerated electron holes and improve the photocatalytic activity.The increase in photoelectrocatalytic activity also enhances the selectivity and controllability of the product.Bismuth-based semiconductor photocatalyst can be excited by visible light,has good photocatalytic activity,can be degraded in visible light organic pollutants,photodegradation of water and reduction CO₂,with good prospects for development.In the bottom of the bismuth compounds conduction band Bi 6s and valence band top O 2p orbit are mutually hybrid,the formation of strong Bi-O covalent bond,short-range repulsive force decreases,the system can always reduce the compounds have good chemical and thermodynamics.Moreover,due to the effect of hybridization,the bandgap of the system is relatively small,and when the external energy is introduced,the electrons are easily excited and the migration rate is high,resulting in high catalytic activity.Among them,bismuth oxide?Bi₂O₃?is a typical representative of this kind of material.As an indirect bandgap semiconductor,the energy band gap is 2.0-3.96 eV,and the Bi₂O₃ band gap of different crystal phases is different,in which the band gap of ?-Bi₂O₃ is 2.85 eV,and the band gap of ?-Bi₂O₃ is 2.58 eV.Bi 6s orbital solvable electron-induced internal polarization field contributes to the separation of hole-electron pairs and the transfer of carriers,so Bi₂O₃ exhibits good catalytic activity.The doping of nonmetallic elements has been reported on the photoelectrocatalytic activity of semiconductors.Photocatalytic reduction of CO₂ by Bi₂O₃ catalysts doped with oxygen elements has been studied.Bi₂S₃ NRs/Bi₂O₃ NPs and Bi₂Se₃/Bi₂O₃ NPs catalysts were successfully prepared on the basis of Bi₂O₃.Firstly,on the SEM characterization analysis,the Bi₂O₃ NPs catalyst has been transformed into Bi₂S₃ NRs/Bi₂O₃ NPs nanorods,and the Bi₂Se₃/Bi₂O₃ NPs catalysts tend to be blurred and grown in a one-dimensional layered direction.The mainly reason of the growth of Bi₂S₃ and Bi₂Se₃ is along different crystal structures.UV-vis characterization of chalcogen non-metallic doping to Bi₂O₃ NPs catalyst on the absorption range of visible light from 500 nm widened to 700 nm,greatly increased the absorption of visible light and utilization.EIS characterization revealed that the band gap of the Bi₂S₃ NRs/Bi₂O₃ NPs?1.40 eV?and Bi₂Se₃/Bi₂O₃ NPs?1.53 eV?decreased by 0.88 eV and 0.75 eV,respectively,relative to Bi₂O₃ NPs?2.28 eV?,greatly increased the electricity utilization.The product was detected by photoelectrocatalytic reduction CO₂ for 6 h at the potential of-0.7V,and the product of Bi₂O₃ NPs was methanol that was only 0.684 mmol L-1 cm-2.And Bi₂S₃ NRs/Bi₂O₃ NPs were methanol and ethanol,and the yield was 4.345 mmol L-1 cm-2 and 0.883 mmol L-1 cm-2,respectively.While Bi₂Se₃/Bi₂O₃ NPs contained not only methanol and ethanol but also methanal,and the yield was 1.47 mmol L-1 cm-2,0.572 mmol L-1 cm-2 and 0.523 mmol L-1 cm-2,respectively.The results show that the selectivity of the photoelectrocatalytic reduction CO₂ products decreases with the increase of the metality of the non-metal ions,and the species diversity of the products is increased.Compared with the Bi₂O₃ NPs catalyst,Bi₂S₃ NRs/Bi₂O₃ NPs and Bi₂Se₃/Bi₂O₃ NPs show more excellent photoelectrocatalytic performance.SnO2 modified Bi-based semiconductor catalytic reduction CO₂ preparation of formic acid has been reported.About SnO2,we also do a preliminary study and found that SnO2 on the catalytic reduction CO₂ has excellent selectivity.So we designed SnX1-y?X = S,Se?doped Bi₂O₃ NPs catalyst for the photoelectrocatalytic reduction CO₂.First,we can see that with the SEM,with the metal dopant doping,the morphology of the catalyst changed from nano-particles?Bi₂O₃ NPs?to one-dimensional nanometer rod-like growth?SnS₂ NRs/Bi₂O₃ NPs?,and finally the rods tend to be blurred and turned to one-dimensional planar growth?Sn₂Se₃/Bi₂O₃ NPs?.From the UV-vis we can see that the composite catalyst to expand the absorption range of visible light 500 to 800 nm,and the SnS₂ NRs/Bi₂O₃ NPs?1.35 eV?and Sn₂Se₃/Bi₂O₃ NPs?1.43 eV?are reduced by 0.93 eV and 0.85 eV,respectively,and the AC impedance is reduced by a factor of 3,greatly increasing the use of visible light and of the consumption,and further improve the photoelectrocatalytic efficiency.The product was detected by photoelectrocatalytic reduction CO₂ for 6 h,and found that with the change of the reduction voltage,the product type and yield changed obviously.About the SnS₂ NRs/Bi₂O₃ NPs catalysts,methanol,ethanol and methanal were detected in the product.With the voltage changes from-0.5 V to-0.9 V,the maximum amount of the different species also undergoes a shadow change.The yield of methanal and ethanol reached 0.320 mmol L-1 cm-2 and 0.447 mmol L-1 cm-2 at-0.6 V,respectively.The yield of methanol reached a maximum of 2.162 mmol L-1 cm-2 at-0.7 V.Under the different voltage,1+1>2 photoelectrocatalytic synergistic effect appeared.About the Sn₂Se₃/Bi₂O₃ NPs catalyst,only ethanol and methanal were detected in the product.With the change of voltage,ethanol appears at-0.5 V,reaching a maximum of 1.698 mmol L-1 cm-2 at-0.7 V and gradually disappearing at-0.9 V,and the new product methanal appears,and the yield of methanal reaches a maximum at-1.1 V 2.254 mmol L-1 cm-2,-1.3 V gradually disappear.These phenomena indicate that the kind and yield of the photocatalytic reduction products of CO₂ by the Sn-based semiconductor catalyst are greatly affected by the voltage.With the increase of the metallic properties of the metal Sn compound,the product selectivity is improved and the variety of the product is reduced.With the change of the reduction potential,the kind and yield of the product are also adjusted accordingly.Mainly due to the different kinds of metal Sn compounds on the photoelectrocatalytic reduction CO₂ specificity,as well as different types of metal Sn compounds at different voltage under the specific catalytic results.In this paper,five new catalytic materials were prepared and applied to the catalytic reduction of CO₂,which achieved great effect.It was of great significance for the reduction of CO₂ and the recycling of CO₂ as potential carbon resources.At the same time,Photoelectrocatalysis reduction CO₂ has some guidance and reference.