Structural Design,Construction And Structure-property Relationship Of High Performance Visible-light Photocatalysts Based On Two-dimensional Materials

Semiconductor photocatalysis as an emerging technology has been widely concerned by researchers in the development and utilization of clean energy and environmental governance.In this?green‘chemical reaction process,semiconductor photocatalysts absorb sunlight to generate active charge carriers,and then those with strong redox capacity can crack water molecules to produce hydrogen or decompose environmental pollutants.At the same time,semiconductor photocatalysts keep themselves stable.However,the obstacle of large-scale application of photocatalytic technology is low efficiency in visible-light utilization,rapid charge recombination rate,poor adsorption and difficult separation of the photocatalysts.Therefore,the search of visible-light photocatalysts with high efficiency,high stability and large-scale preparation is the main task of current research.In this paper,high-efficient visible-light photocatalysts with structure of zero-dimensional（0D）spherical lamination,one-dimensional（1D）nanotubes,two-dimensional（2D）porosity and three-dimensional（3D）sandwich are constructed based on two-dimensional nanosheets materials.In this paper,studies are made from the structure-property relationship of photocatalysts as following:1.0D laminated PANI/Ti_0.91O₂ spherical composite was prepared by in-situ polymerization for PANI and electrostatic self-assembly technique for Ti_0.91O₂nanosheets on the surface of spherical template.The selective self-assembling is based on an electrostatic driving force between Ti_0.91O₂ nanosheet（negatively charged）and PANI（positively charged）.The formation process of each layer of photocatalysts were monitored and characterized by SEM,XRD and FTIR.The performance tests showed that the visible-light photocatalytic activities of the laminated composites increased with the covers of shells increasing.When the laminated composite has three layers(1 g L^-1),81.7%of RhB(1×10^-5 M)can be degraded under 6 h of visible-light irradiation.Incorporating PANI in composite extends absorption optical into the range of visible-light.The mechanism shows that PANI（p-type semiconductor）and Ti_0.91O₂（n-type semiconductor）form p-n heterojunction,so that e^-from PANI is injected into the CB of Ti_0.91O₂,and h⁺remains inπorbit of PANI,which effectively prevents the recombination of e^--h⁺pairs and accelerats the reaction rate.2.In view of the complex manufacturing process of laminated structure above,the3D sandwich-structured composites from Ti_0.91O₂ nanosheets and PANI nanospheres were fabricated of the by a facile electrostatic self-assembly technique.After incorporating PANI into interlayer,the spacing between Ti_0.91O₂ layers was increased by about 11 times,making the solution diffusion easier and facilitating the internal photocatalytic reaction.Visible-light photocatalytic performance tests of sandwich composites(0.5 g L^-1)showed that RhB(1×10^-5 M)can be degraded 99.6%after 6 h of irradiation.The introduction of PANI also extends absorption optical into the range of visible-light.The results of reactive free radical trapping experiments and UV-visible spectrum analysis show that the surface charge of photocatalysts can affect the photodegradation pathway of RhB.When the surface has negative charge,it occurs deethylation.When the surface is mainly positive charge,it occurs chromophore cycloreversion.3.Due to the large band-gap of Ti_0.91O₂ nanosheets,it is unable to absorb visible-light.2D graphite nitride carbon（g-C₃N₄）nanosheets with visible-light response are obtained based on size-dependence effect.The ultrathin and small porous g-C₃N₄nanosheets were prepared by hydrothermal modification of melamine from a bottom-up method.From the results of characterization such as SEM,XRD,DTG-TGA and IR,it was found that the hydrothermal treatment of melamine caused crystal cracking and the size became smaller.Besides,phase transformation,hydrogen bond fracture and oligomerization occurred simultaneously.Hydrogen bond fracture is the main reason of these changes.There was no change in the non-polar solvothermal treatment of melamine.The key of our hypothesis is the crystal cracking into smaller species by long time hydrothermal treatment,after which accordingly forms ultrathin and small g-C₃N₄nanosheets as a result of self-size-dependent effect.Meanwhile,a large number of penetrating holes are generated.For example,when the hydrothermal time reaches 48hours,the specific surface area of the nanosheets is 177.2 m² g^-1 and the pore volume is0.921 cm³ g^-1.On the other hand,the porous mesh structure improves light-harvesting efficiency due to light scattering.In addition,the release of ammonia gas during the hydrothermal process leads to the loss of N species,which brings N defects.The visible-light performance test showed that this nanosheets(0.5 g L^-1)degraded 95.0%of RhB(2×10^-5 M)within 30 min,and its excellent photocatalytic activity was mainly due to the exposure of more active sites,increased light absorption efficiency,accelerated rate of charge transfer and effective inhibition of photogenerated charge recombination.4.In order to further improve the light absorption range and separation of photogenerated charge of g-C₃N₄ nanosheets,based on the conclusion of 1 and 2 study,a metal-free 1D PANI nanotubes/g-C₃N₄（PNCN）hollow composite was fabricated by solvent evaporation method.PANI nanotubes were chosen as the host templates and g-C₃N₄ nanosheets were promising access to uniformly loading on,for the flexible feature of exfoliated thin nanosheets.The structure of the hollow composite was confirmed by SEM,TEM,XRD and IR.The visible-light performance test showed that the hollow composite(0.5 g L^-1)had strong adsorption（52.3%）and high photocatalytic degradation capacity（95%）of RhB(1×10^-5 M)within 150 min.The optical absorption test also proved that the introduction of PANI widened the absorption range.The photocatalytic mechanism showed that h⁺transferred from VB of g-C₃N₄ to theπbonding orbital of PANI and e^-transferred fromπ*antibonding orbital of PANI to CB of g-C₃N₄,achieving effective separation of photogenerated charge.Moreover,the 1D hollow core-shell heterojunction had a short diffusion path,and the reactive free radicals could rapidly diffuse to the surface of the photocatalysts to speed up the reaction rate.