Photocatalysts Design And Photoreduction Carbon Dioxide Study Based On Semiconductor Energy Band Control

In recent decades,the over utilization of fossil fuels not only quickens itself depletion shortage,but also results in ever-increasing of CO₂ concentration in the atmosphere,which seriously breaks the natural carbon balance,causing global climate change.^[1]Such issue has risen as one of the major environmental challenges for human society.In view of this,converting the CO₂ into energy hydrocarbon has been considered as a prospective solution since it could simultaneously solve the energy problems and environmental challenges.Utilizing the inexhaustible solar energy of nature,it can realize the conversion and utilization of carbon dioxide without consuming fossil energy,converting solar energy into available chemical energy.However,the efficiency of photoconversion CO₂ to usable fuel is still very low,which can not achieve large-scale practical application,mainly due to the low light absorption efficiency,photogenerated carrier separation and migration efficiency,carrier redox ability,and insufficient catalytic active sites.Therefore,based on some typical semiconductors（g-C₃N₄/Bi₂WO₆/Bi OBr）,the band structure,surface catalytic active sites,charge separation efficiency of semiconductors were optimized by structural reconstruction,surface heteroatom modification and ultrathin two-dimensional solid solution methods.The suitable catalyst model for photocatalytic CO₂ reduction was constructed.The internal mechanism of photocatalytic CO₂ reduction process was explored by combining experiments and theoretical calculation.The main contents of this paper are as follows:（1）Structural reconstruction of carbon nitride enhancing photocatalytic CO₂ reduction:Intrinsicπ-πconjugated electronic system of graphitic carbon nitride（CN）greatly determines its band structure,driving force of redox reaction,carrier dynamics,and reaction active sites.Herein,the molecule structure of carbon nitride was reconstructed by simply calcining the sodium borohydride and CN nanosheets（CNS）in a nitrogen atmosphere.Structural reconstruction of CNS leads to adjusting electronic structure and lowering the valence band（VB）position to increase the water oxidation driving force.Furthermore,the structural reconstruction of CNS can promote the in-plane separation and transport of charge carriers.In addition,the structural reconstruction of CNS optimizes the surface structure,changes charge distribution,and increases surface active sites,promoting the adsorption and activation CO₂molecule.Moreover,the structural reconstruction of CNS can decrease the barrier energy of rate-limiting step for adsorbed*COOH intermediate.Therefore,structural reconstruction of CNS manifests outstanding CO₂ reduction performance(55.3μmol g^-1 h^-1)and high CO selectivity（98.9%）without any cocatalysts and sacrificial agents.More importantly,the R-CNS-400 can selectively oxidize the Ph CH₂OH to Ph CHO（selectivity/99.3%）and simultaneously reduce CO₂ to CO,which shows a great potential to make full use of the photogenerated carriers to produce fuels and value-added chemicals.This work provides new insights for designing CN-based materials with increased water oxidation driving force and enhanced charge separation for high efficiency CO₂ photoreduction by structural reconstruction.（2）Surface halogen modified Bi₂WO₆ boosting photocatalytic CO₂ reduction:Facilitating the charge separation of semiconductor photocatalysts to increase the photocatalytic CO₂reduction activity has become a great challenge for sustainable energy conversion.Herein,the surface halogenation-modified defective Bi₂WO₆ nanosheets have been successfully prepared to address aforementioned challenge.The surface halogenation-modified can optimize surface active sites of Bi₂WO₆ nanosheets,which is beneficial to the adsorption and activation CO₂molecule.Furthermore,the surface halogenation-modified can optimize charge distribution,the enhanced charge accumulation occurs around the halogenation atom,suggesting more opportunities for the accumulation of photogenerated charge and the occurrence of more efficient charge transfer,which can achieve efficient photocatalytic CO₂ reduction.DFT calculations results disclose that the formation of*COOH intermediate radical is rate-limiting step of the whole process of CO₂ reduction,the modification of surface halogenation atoms is beneficial to decrease the activation energy of CO₂ molecules through stabilizing the*COOH intermediates.Moreover,the modification of surface halogenation atoms can optimize the band structure of Bi₂WO₆,increasing the reduction potential of conduction band enhances the reduction ability of photogenerated electrons.Therefore,all the halogenation-modified defective Bi₂WO₆ nanosheets manifested an enhanced photocatalytic CO₂ reduction activity.This finding provides a new approach to optimize the carbon dioxide reduction pathway of semiconductor photocatalysts,which is beneficial to develop highly efficient CO₂ reduction photocatalysts.（3）Two-dimensional ultrathin Bi OBr_1-xCl_x solid solution photocatalysts achieving efficient photocatalytic CO₂ reduction:Although Bi OBr can absorb visible light and capture solar energy more efficiently,its relatively low conduction band and insufficient conduction band potential can not realize photocatalytic carbon dioxide reduction.The formation of solid solution can optimize the energy band structure,further improve the driving force of carbon dioxide reduction,enhance the carrier separation efficiency,and achieve efficient photocatalytic carbon dioxide reduction.Herein,ultrathin Bi OBr_1-xCl_x nanosheets solid solution was prepared through a facile hydrothermal route,and its band structure has been changed in comparison with Bi OBr,the position of conduction band increases and the position of valence band decreases,which increase the redox driving force of photogenerated carriers.Furthermore,the charge carrier separation is improved,which is beneficial to the photocatalytic carbon dioxide reduction.The best sample Bi OBr_0.6Cl_0.4 manifests outstanding CO formation rate of 57.3μmol g^-1 h^-1 without any cocatalysts and sacrificial agents,roughly 5 folds of Bi OBr(11.3μmol g^-1h^-1)and 3 folds of Bi OCl(19.8μmol g^-1 h^-1),respectively.Moreover,Bi OBr_0.6Cl_0.4 has excellent stability.This work provides a new direction for designing highly efficient Bi OX photocatalysts for CO₂ reduction.