Construction,Photocatalytic Performance And Mechanism Research Of Two Dimensional Heterogeneous Composites

With the deepening of industrialization and the development of productivity,the demand for energy continues to increase.Fossil fuels account for most of the global energy composition,and because of the continuous extraction and use of fossil fuels,the global energy crisis is becoming increasingly serious.Among various new energy sources,hydrogen energy is considered as an ideal alternative energy source for fossil fuels,because it has the characteristics of renewable,wide sources,high combustion value,and green product.In recent years,the photocatalytic water-splitting technology that utilizes solar energy to split water into hydrogen is considered to be a cheap and feasible hydrogen production method.Therefore,the development of photocatalytic hydrogen production materials has become one of the hot research directions.P-type semiconductor cuprous oxide（Cu₂O）not only has a high conduction band potential（-1.35 V vs.Ag/Ag Cl,p H=7）,but also possesses environmental protection,weak toxicity,and rich reserves.It has been used in photocatalytic organic matter degradation,photocatalytic water decomposition,and photocatalytic CO₂ reduction.However,its narrow band gap（2-2.2 e V）will cause the rapid recombination of photogenerated electron-hole pairs,resulting in a decrease in photocatalytic efficiency.Therefore,the construction of efficient Cu₂O-based photocatalytic system is a major challenge in the field of photocatalysis.Graphene is a kind of 2D grid carbon material,which is formed by sp² carbon atoms.Due to its large specific surface area,excellent electron transport and excellent thermal stability,graphene and its oxides have been used as 2D solid electronic mediators with various semiconductors to promote the separation and migration of photogenerated charge carriers in semiconductors and enhance their photocatalytic activities.Therefore,on the basis of Cu₂O,RGO was introduced as the solid-state electronic mediator to construct Cu₂O/RGO binary composites,and the ternary composite photocatalytic systems were further constructed to make full use of the high reducibility photoelectrons of Cu₂O for hydrogen production.Besides,in recent years the graphene-like layered covalent organic framework（COF-TpPa-1）with the excellent light absorption and electron conduction has been regarded as an ideal photocatalytic material,which can accommodate the role of semiconductor and solid-state electron mediator,due toπelectron conjugates in the layers and p-p electron conjugates between the layers.However,its narrow band gap（～2.0 EV）can also lead to the rapid recombination of photochargic carriers.Therefore,COF-TpPa-1 can be seen as‘Cu₂O/RGO’,which needs to be combined with 2D semiconductor for the improvement of photocatalytic performance.The main contents and conclusions are as follows:（1）Cu₂O/RGO/BiVO₄ composite photocatalytic systemAn all-solid-state Z-scheme system of Cu₂O/RGO/BiVO₄ was prepared by a two-step solvothermal method.The tetracycline degradation and water-splitting was integrated in a system with visible-light illumination.The as-prepared Cu₂O/RGO/BiVO₄ system exhibits the excellent photocatalytic performance for the simultaneous tetracycline degradation and water-splitting.The composite showed the highest hydrogen production,which was 6 times higher than that of pure Cu₂O with the tetracycline degradation（20.3%）.In the process of total degradation,the composite also showed a tetracycline degradation efficiency of up to 96%.The improved performance of Cu₂O/RGO/BiVO₄ composite is attributed to the construction of an all-solid-state Z-scheme system.During the entire photocatalytic process,RGO was used as a solid-state electron mediator to combine the photogenerated electrons in BiVO₄ and holes in Cu₂O.It facilitates the effective separation of electron-hole pairs in Cu₂O and BiVO₄,which makes the electrons and holes in Cu₂O and BiVO₄ abundant,enhancing the redox capacity of the Cu₂O/RGO/BiVO₄ system in the photocatalytic process.（2）Cu₂O/RGO/MoS₂ composite photocatalytic systemA novel Cu₂O/RGO/MoS₂ graphene-based ternary composite was successfully prepared by a simple room-temperature method.The Cu₂O/RGO/MoS₂ composite is used for photocatalytic hydrogen production and exhibits good photocatalytic hydrogen production activity,which is 8 times that of pure Cu₂O.As an excellent electron mediator,RGO not only can effectively transfer the electrons on the conduction band of Cu₂O to the surface of RGO and can transfer the photogenerated electrons to the MoS₂ cocatalyst.The"face-to-face"communication shortens the electron transfer distance between RGO and MoS₂.Because of the two-dimensional structure of MoS₂sheet,the active sites for hydrogen production are greatly increased.（3）Cu₂O/RGO/Shewanella oneidensis MR-1 hybrid systemWe fabricated a unique whole-cell IBS by using reduced graphene oxide（RGO）to integrate Cu₂O and Shewanella oneidensis MR-1（SW）cells.The RGO provided a sufficient area for efficient photoelectron collection from Cu₂O and established a transmembrane pathway for excellent electron distribution into the cells,which was 38-and 27-fold higher than that of Cu₂O/SW and Cu₂O/MV²⁺/SW,respectively.PL analysis indicated that an excellent photoelectron transport and collection were achieved.Moreover,genetic mutagenesis analysis proved that the establishment of an RGO-MRP electron transfer pathway further improved the electron distribution from RGO to the SW cells.In addition,the density functional theory（DFT）calculations and Bader charge analysis confirm the efficient charge transfer between Cu₂O and SW.（4）2D/2D COF-TpPa-1/TiO₂ heterogeneous compositeA novel 2D/2D COF-TpPa-1/TiO₂ heterogeneous nanocomposite was successfully prepared by a simple hydrothermal method.The heterogeneous composite was used for photocatalytic hydrogen production,and it exhibited excellent hydrogen production activity.When the mass ratio of COF-TpPa-1/TiO₂ was 14:6,the composite showed the highest photocatalytic hydrogen production activity（1.30 mmol/g/h）,which was 28.7times that of pure COF-TpPa-1（0.045 mmol/g/h）.COF-TpPa-1 can not only act as a semiconductor to generate photo-generated electrons and hole pairs,but also transfer electrons as a graphene-like supporting material,because of its unique layered structure,the largeπ-electron conjugate in the layer,and the p-p conjugate between the layers.Moreover,the COF-TpPa-1 nanosheets and TiO₂ nanosheets are"face-to-face"composited,which not only shortens the electron transfer distance between different semiconductors,but also increases the active site of the photocatalytic reaction.（5）2D/2D COF-TpPa-1/ZnIn₂S₄ heterogeneous compositeA novel 2D/2D COF-TpPa-1/ZnIn₂S₄ heterogeneous nanocomposite was successfully prepared by a simple hydrothermal method.The heterocomposite is used for photocatalytic hydrogen production,and it exhibits excellent photocatalytic activity.When the mass ratio of COF-TpPa-1/ZnIn₂S₄ is 11:9,the composite shows the highest photocatalytic hydrogen production activity（1.18 mmol/g/h）,which is more than 26times higher than pure COF-TpPa-1（0.045 mmol/g/h）.The covalent organic framework COF-TpPa-1 has the functions of both semiconductor and electron carrier,because of its large specific surface area,πelectron conjugation in the layer and interlayer P-P conjugation between layers.In addition,the S atom in the ZnIn₂S₄nanosheet can form a strong chemical bond with the COF-TpPa-1 nanosheet,which can further enhance the transmission capacity between ZnIn₂S₄ and COF-TpPa-1,which can accelerate the transfer of charge carriers and improve the photocatalytic efficiency.