Synthesis And Performance Of G-C₃N₄ Based Z-Scheme Photocatalysts

Graphite phase carbon nitride（g-C₃N₄）,a polymer material without metal elements,has attracted wide attention due to its suitable energy band structure,non-toxicity,low cost,good stability and easy preparation.However,pure g-C₃N₄materials cannot simultaneously have a wide spectral absorption range,effective charge separation and high redox capacity.In order to further improve the photocatalytic performance,this thesis has constructed the g-C₃N₄ based Z-scheme photocatalysts,where the matched energy bands between different semiconductors and their synergy could improve the photogenerated charge separation efficiency and broaden the range of spectral response.The construction of Z-scheme photocatalysts could adjust the direction of electron transfer and improve photocatalytic performance.Then the g-C₃N₄ based Z-scheme photocatalysts wree applied to boost the selective organic transformation photocatalytic degradation of organic pollutants.The Z-scheme photocatalytic mechanism has been analyzed and explained.The main research contents of this thesis are desceied as follows:1.W₁₈O₄₉/g-C₃N₄heterostructures have been synthesized by growing W₁₈O₄₉ultrathin nanowires on g-C₃N₄ nanosheets via a convenient solvothermal process.Various characterizations were performed on the materials to understand the structure-performance relationship.The photocatalytic properties of the W₁₈O₄₉/g-C₃N₄ heterostructures were evaluated by the two oxidation reactions,phenol degradation and benzylamine oxidation,under a simulated sunlight.With tuning the W₁₈O₄₉/g-C₃N₄ mass ratio,the optimal photocatalyst of W₁₈O₄₉（30）/g-C₃N₄containing 30wt.%W₁₈O₄₉nanowires exhibited the highest activity in both the photocatalytic reactions.The generations and contributions of the active species in the photocatalytic reactions were identified by electron spin resonance（ESR）spectra and active-species-eliminating experiments.Accordingly,the photocatalytic mechanism of W₁₈O₄₉/g-C₃N₄ heterostructures has been expounded based on the direct Z-scheme electron transfer between the two semiconductors as well as the synergistic actions of active sites on W₁₈O₄₉ nanowires and g-C₃N₄ nanosheets.The significantly improved photocatalytic performance of W₁₈O₄₉/g-C₃N₄ heterostructures can be attributed to the effective separation of photogenerated carriers,the broadened spectral response range,the synergy between W₁₈O₄₉ and g-C₃N₄,and the active sites exposed on g-C₃N₄nanosheets W₁₈O₄₉ nanowire.This work demonstrates a rational paradigm to construct 1D/2D semiconductor heterostructures and provides further insights into Z-scheme photocatalytic mechanism for boosting solar-driven pollutant degradation and organic transformation.2.The CdS-EDTA/g-C₃N₄ heterostructure photocatalyst was designed according to Z-scheme photocatalytic mechanism and synthesized with assistance of EDTA chelating agent.EDTA played multiple roles in the construction of CdS-EDTA/g-C₃N₄ Z-scheme heterostructures:controlling the morphology of CdS nanostructures,linking CdS and g-C₃N₄ together,and inducing the charge transfer between two semiconductors.The optimized CdS-EDTA/g-C₃N₄（10%）photocatalyst exhibited greatly enhanced activities toward the selective reduction of nitrophenol and the selective oxidation of benzyl alcohol,compared with those of individual g-C₃N₄,individual CdS respectively synthesized without and with EDTA,and CdS/g-C₃N₄heterostructures without EDTA.The improved photocatalytic performance could be attributed to the spatial separation and suitable photoredox potentials of photoexcited charge carriers in the EDTA-bridged Z-scheme heterostructures,where EDTA is crucial to reinforce the heterointerface and to expedite the interfacial charge transfer.This work provides some enlightenment on exploring inexpensive organic electron mediators for the all-solid-state Z-scheme photocatalysts and demonstrates the development of Z-scheme photocatalytic systems for photoredox reactions of organic transformations.3.The g-C₃N₄/CoO_x/BiOBr heterostructure photocatalyst was designed according to Z-scheme photocatalytic mechanism.Frist,the g-C₃N₄/BiOBr composite was synthesized by hydrothermal method,and then CoO_x was in g-C₃N₄/BiOBr by photodeposition.The mass fraction of CoO_xwas adjusted to optimize the photocatalytic performance.It was found that CoO_x could act as an excellent co-catalyst,but also an electron mediator,which promote the electron transfer from the conduction band of BiOCl to the valence band of g-C₃N₄.The g-C₃N₄/CoO_x/BiOBr Z-scheme heterojunction was obtained.Phenol degradation was used as a model reaction to evaluate the photoactivity of the catalysts.The results showed that the g-C₃N₄/CoO_x/BiOBr with a theoretical mass fraction of CoO_x being1.5%,exhibited the optimal photocatalytic performance.The degradation rate of phenol could achieve 100%after 40 min of illumination,and the rate constant was0.0579 min^-1,which is 7.3 times that of pure g-C₃N₄(0.0079 min^-1)and 4.3 times taht of pure BiOBr(0.0135 min^-1).The improved photocatalytic performance could be attributed to the fact that CoO_xcould not only provide a large number of active sites as a cocatalyst,but also improve the photogenerated charge separation efficiency by adjusting electron transfer direction.