Development Of BiOBr-based P-n Heterojunction Photocatalysts And Its Application In Air Purification

With the further development of industrialization and the increasing global population,human society is facing an increasingly serious crisis of resource depletion and environmental degradation.The excessive consumption of fossil fuels such as oil,coal and natural gas,energy shortage and environmental pollution have become the focus of attention worldwide.Therefore,the exploration and development of renewable,sustainable and clean energy alternatives to fossil fuels has become an important direction in the field of catalysis and energy research.Among atmospheric pollutant gases,nitrogen oxides are considered to be one of the major pollutants due to the environmental problems they cause,such as photochemical smog,acid rain,haze and ozone layer depletion.In addition,humans exposed to high levels of nitrogen oxide gas can develop many health problems.Semiconductor photocatalysis is a "green" technology that uses sunlight to generate free radicals on the surface of photocatalysts to inactivate viruses or completely eliminate various pollutants,making it one of the most promising solutions to environmental problems.Bromine bismuth oxide（BiOBr）semiconductors are layered compounds with a crystal structure of [Bi₂O₂]²⁺layers interspersed with layers of Br atoms.Due to strong interlayer bonds and weak interlayer van der Waals interactions,this often leads to excellent performance and promising applications in photocatalysis.BiOBr is a p-type semiconductor with a narrow band gap（2.9 e V）,which has a strong optical response in visible light.Although the narrow bandgap BiOBr is capable of capturing visible light in the solar spectrum,its photogenerated electron-hole pairs undergo rapid recombination.Optimizing the catalytic properties of the semiconductor by designing the structure is a feasible way to improve the overall charge transfer efficiency.BiOBr can be used as a sensitizer for wide bandgap semiconductors,so the formation of heterojunctions between BiOBr and wide bandgap semiconductors can reduce photogenerated carrier recombination,extend carrier lifetime and improve photocatalytic activity.The construction of heterojunctions is not only a feasible way to develop highly active photocatalysts,but also a reasonable way to study the relationship between photogenerated carrier transfer and photocatalytic performance.Therefore,in this paper we have selected two different n-type semiconductors,La₂Ti₂O₇ and SnO₂,to construct p-n heterojunctions with BiOBr,respectively.In this study,the morphological structure and optical properties of BiOBr-based p-n heterojunctions were comprehensively investigated.By constructing BiOBr-based p-n heterojunctions,a wide bandgap n-type semiconductor was covered on the surface of BiOBr to extend the light absorption to the visible range.In addition,the charge transfer channels and directions at the BiOBr-based p-n heterojunction interface were determined by a combination of theoretical calculations and experimental studies.Within the BiOBr-based p-n heterojunction,the charge in the n-type semiconductor migrates into BiOBr through the pre-formed electron transfer channel,thus generating an internal electric field（IEF）between the n-type semiconductor and BiOBr.Under the influence of the IEF,the photogenerated electrons of BiOBr migrate from the conduction band（CB）to the CB of the n-type semiconductor,thus promoting the separation of electron-hole pairs.The intermediates and final products of NO oxidation process were monitored by in situ DRIFTS,and the oxidation pathway of NO was reasonably proposed.Meanwhile,the construction of heterojunctions not only achieved more efficient NO photocatalytic oxidation,but also inhibited the production NO₂.This work provides new insights into the reaction mechanism of interfacial charge transfer and efficient air purification of BiOBr-based p-n heterojunction photocatalysts.