Controlled Synthesis Of Black-phosphorus-based Photocatalysts And Their Performance In Reduction Of CO₂

With green photocatalytic technology,semiconductor photocatalysts can convert CO₂ into high value-added chemicals by solar energy excitation,which can effectively realize the utilization of CO₂ and contribute to"peak CO₂ emissions,carbon neutrality".As a new type of non-metallic semiconductor catalyst,black phosphorus（BP）has the advantages of tunable band gap,high electron mobility and strong light absorption ability,which has attracted much attention.However,ultra-thin black phosphorus nanosheets are highly susceptible to oxygen and humidity of environment and cause surface oxidation or self-decomposition,meanwhile,the material itself has defects,such as high recombination rate of photogenerated carriers and few active sites in the catalytic process,which limit the photocatalytic CO₂ reduction efficiency.To address the above problems,this paper adopts various strategies to modify and stabilize BP,which effectively improves the photocatalytic CO₂ reduction performance.It is a significant exploration for the design and development of BP photocatalysts for CO₂ reduction.The main research results of this paper are as follows.1.With the help of oversized folded surface of two-dimensional graphitic carbon nitride（2D g-C₃N₄）,the crushed BP nanosheets was encapsulated finely and formed closely bonded heterostructure 2D BP/2D g-C₃N₄（FPCN）at low temperature and photoreduction performance of samples was optimized by modulating the composition of the complexes.The X-ray Photoelectron Spectroscopy（XPS）analysis revealed that the 2D g-C₃N₄ folded surface could better encapsulate the BP nanosheets,which reduced the contact possibility between the BP surface and air,and then the oxidated surface area of BP became smaller.Compared with the pure 2D g-C₃N₄,the formed FPCN composites had better light absorption ability and enhanced separation and transfer of photogenerated carriers.Under visible light illumination for 5 h,the sample of mass mixing ratio of 15%exhibited the best photocatalytic activity(187.7μmol g^-1 h^-1),which was nearly doubled compared to the pure 2D g-C₃N₄,with a high CO product selectivity of 92.55%.2.Ultrathin 2D black phosphorus nanosheets（BPs）were prepared by liquid-phase exfoliation using N,N-dimethylformamide（DMF）as the solvent.The Co₂P/BPs heterostructures were synthesized by in situ growth of transition metal phosphides on the surface of BPs,which was used as the substrate and as the only phosphorus（P）source.Meanwhile,the effect of Co₂P/BPs heterostructures on CO₂ photoreduction performance was investigated.The results showed that after the erosion of organic solvent and high temperature treatment,P defects would be formed on the surface of BPs,and then formed Co₂P.In situ growth of Co₂P effectively covered the surface of BP materials,and then reduces its affinity with oxygen,and the oxidation of BPs itself is reduced.Meanwhile,Co₂P acted as a co-catalyst to rapidly extract the photogenerated electrons generated on the surface of BPs and reduce the recombination rate of photogenerated electrons and photogenerated holes.After 5 h of visible light illumination,the CO generation rate of the Co₂P/BPs was 25.5μmol g^-1h^-1,and the activity was significantly enhanced compared with the pristine BPs,and the sample showed excellent stability.3.Bulk BP was treated with liquid nitrogen and then was exfoliated in isopropyl alcohol（IPA）solution.The thickness of BP was adjusted by intercalation of small IPA molecules,while hydroxyl groups（-OH）were in situ grafted on the BP surface,and the CO₂ photoreduction performance of hydroxyl-modified monolayer BP alkene（M-BP-OH）were investigated.The analysis of the study showed that the band gap of BP is enlarged and the conduction band position（CB）was shifted to a more negative position due to the reduction of the number of layers.More importantly,the resulting M-BP-OH exhibited excellent durability and stability in air and water,which can be attributed to the introduction of-OH during the exfoliation process.M-BP-OH could effectively and stably catalyze the conversion of CO₂ to CO under visible light with an activity as high as 112.6μmol h^-1 g^-1.Compared with monolayer BP（M-BP）,the activity of M-BP-OH was effectively enhanced and the stability was greatly improved during photocatalytic CO₂ reduction.