Regulation Of The Behavior Of Photogenerated Carriers In Micro-nano Structures Photocatalysts And Their Photocatalytic Performances

Photocatalytic CO₂ reduction simulates the natural photosynthesis of green plants and converts H₂O and CO₂ into hydrocarbon fuel by using sunlight as power,which is one of the effective methods to solve the energy shortage and environmental pollution.As a challenging and promising method,photocatalytic reduction of CO₂ possesses the following advantages:(1)It can be carried out under relatively mild conditions(room temperature and atmospheric pressure);(2)The use of inexhaustible clean solar energy to promote the reduction of a large number of waste CO₂;(3)It can directly generate CO and short chain hydrocarbon fuels,such as CH₄,CH₃OH,C₂H₄,C₂H₆,etc.,to alleviate the increasingly tense energy crisis;(4)The technology uses CO₂ as raw material,which can alleviate environmental problems such as greenhouse effect.Therefore,photocatalytic CO₂ reduction technology can save energy and protect the environment,is no different from killing two birds with one stone.However,low photocatalytic efficiency,serious recombination of photogenerated carriers and difficult control of product selectivity are still the main scientific problems of photocatalytic technology.In this paper,from the perspective of the regulation of photocatalytic carrier behavior,the photocatalytic performance was improved by heterostructures construction,ultrathin two-dimensional materials design,defect introduction and construction of hollow structure.The specific innovation results are as follows:(1)By immobilizing black phosphorus quantum dots(BPQDs)on WO₃ nanowires to construct Z-scheme heterostructures,efficient CO conversion and massive C₂H₄generation were achieved.In the process of photocatalysis,WO₃ usually shows a high oxidation capacity,and its conduction band level is larger than that of CO₂ redox surface potential,which can't reduce CO₂ to CO.Although the band structure of BPQD was suitable for CO₂ reduction,only trace amounts of CO and C₂H₄ were generated in the experiment.This might be caused by the rapid recombination of photogenerated electrons and holes on the surface,where BPQD contained a large number of defects.In contrast,BPQD-WO₃ heterostructures exhibited obviously enhanced photocatalytic activity under continuous light irradiation.The yield of C₂H₄ increased first and then decreased with the increased amount of BPQD,indicating that BPQD played a crucial role in C₂H₄ production.The charge transfer and separation between BPQD and WO₃in a single nanowire were investigated using Kelvin probe atomic force microscopy,which confirmed the Z-scheme model.(2)Monolayer AgInP₂S₆ sheets were prepared through mechanical exfoliation,and V_S-AgInP₂S₆ sheets with rich sulfur vacancy(V_S)were obtained by H₂O₂ etching.The systematic study found that the monolayer AgInP₂S₆ nanosheets significantly enhanced the efficiency of CO generation,along with the production of CH₄ and C₂H₄,compared with AgInP₂S₆ bulk material which only produced CO in low efficiency.This improvement in photocatalytic performance could be attributed to the fact that the ultrathin structure reduced the distance of photogenerated carriers from the inside of the catalyst to the surface,thus reduced bulk recombination.In addition,the introduction of V_S enabled C₂H₄ becoming the main product of V_S-AgInP₂S₆ sheets,which realized the product selectivity regulation.This phenomenon indicated that V_Spromoted the separation of photogenerated electrons and holes further.Combining the theoretical calculation,it was found that the exposed Agatom caused by V_S introduction could enhance the adsorption of CO,promot the C-C coupling reaction and reduce the activation energy of C₂H₄ generation.(3)A semiconductor heterostructure supported by porous carbon micron tubes(PCMT)was designed to achieve efficient photocatalytic conversion of CO₂ to CO.Firstly,the PCMT with In₂O₃ nanoparticles precipitated on the surface were synthesized by hydrothermal synthesis of In-MOF,which was then annealed in an inert atmosphere.Next,Zn In₂S₄(ZIS)nanosheets were hydrothermal synthesis and deposited on the surface of In₂O₃ nanoparticle layer to obtain the In₂O₃/ZIS heterostructure supported by PCMT(PCMT@In₂O₃/Zn In₂S₄).In order to systematically study the reasons for the improvement of catalytic performance,the hollow microtube(In₂O₃ MT)composed of small In₂O₃ nanoparticles and the In₂O₃ microtube combining with ZIS(In₂O₃/ZIS)composite sample were also constructured as contrast samples.It was found that PCMT@In₂O₃/ZIS possessed the highest CO yield,and the improvement of catalytic efficiency was mainly due to the synergistic effects of the following factors:(1)The multi-stage jump of charge carriers in In₂O₃,ZIS and PCMT greatly reduced the charge recombination in In₂O₃ and ZIS.(2)The mesoporous characteristics of PCMT possessed large specific surface area and abundant active sites,which could enhance the local concentration of CO₂ in heterogeneous structures.(3)The existence of a large number of carbon defects in PCMT enhanced the activity of CO₂ adsorbtion.(4)The tubular structure of the double-opening PCMT facilitated the rapid diffusion of reactants and products,and increased the absorption of light through multiple light scattering/reflection of internal voids.(5)The unique synthesis route made the close contact between PCMT,In₂O₃,and ZIS,which was also conducive to charge transfer.