Strategies Of Heterojunction Construction And Molecular Engineering For The Improvement Of Photocatalytic Performance Of Graphitic Carbon Nitride

Solar（visible-light）-driven photocatalysis is a research field with broad prospects and challenges to address the current energy and environmental problems,and designing efficient photocatalysts and understanding the mechanism deeply involved in the photocatalysts-assisted redox processes are still key topics in this field.In comparison of traditional inorganic semiconductor photocatalysts,theπ-conjugated polymer semiconductor photocatalyst,graphitic carbon nitride（g-C₃N₄）,stands out as an ideal candidate for the next generation of photocatalysts with excellent visible-light performance with its visible-light response and highly flexible host for precise molecular structure modification,which renders g-C₃N₄an ideal candidate for the next generation of photocatalysts with excellent visible-light performance.However,some key barriers associated with g-C₃N₄-based photocatalytic redox reactions still exist,including low charge separation and transfer efficiency,narrow visible-light absorption range,insufficient oxidative driving force,and limited active sites,which significantly limits the practical application of bulk g-C₃N₄for sustainable environmental remediation and clean energy production.Therefore,in this doctoral dissertation,systematic and deep studies related to the improvement of the photocatalytic redox performance of g-C₃N₄are employed,via new strategies such as heterojunction construction and molecular engineering.Firstly,Z-scheme g-C₃N₄/WO₃heterojunction photocatalysts with morphology control of WO₃are facilely fabricated via a thermal-induced self-polymerization of melamine in the presence of WO₃nanowires（NWs）,nanosheets（NSs）and microflowers（MFs）,and are successfully applied in simulated sunlight photocatalytic degradation of two emerging pollutants,including p-nitrophenol（PNP）and methylparaben（MPB）.The g-C₃N₄/WO₃heterojunctions demonstrate high removal efficiency towards the target pollutants in comparison of pristine g-C₃N₄and WO₃;moreover,both the ratio and morphology of WO₃influence the photocatalytic activity of g-C₃N₄/WO₃dramatically.g-C₃N₄/WO₃NWs（0.05）exhibit the highest pollutants removal efficiency among three heterojunction photocatalysts.By combination of testing results and Density Functional Theory（DFT）calculations,the unique Z-scheme charge transfer mechanism of g-C₃N₄/WO₃heterojunctions is revealed,which not only boosts the spatial separation of charge carriers but also endows the g-C₃N₄/WO₃with supreme redox capacity.Ultimately,abundant reactive species like valence band holes(h_VB⁺),superoxide anion radicals（·O₂^-）,and hydroxyl radicals（·OH）radicals are generated and the target pollutants can be oxidized deeply.Secondly,to further improve the visible-light photocatalytic oxidation performance,porous-C=C-C（carbon chain）incorporated g-C₃N₄nanosheets（HCN-C_x）are prepared by preorganization of L-cysteine and urea under hydrothermal environment followed by thermal copolymerization.Besides,carbon doping level can be precisely adjusted by changing initial urea/L-cysteine molar ratio.The characteristic results combined with DFT calculations confirm that-C=C-C skeletons can not only narrow the bandgap,generate defect-induced midgap states,boost photoexcited charge carrier transfer dynamics but enhance the oxidative driving force of h_VB⁺,which improve the process of generation,migration,separation,and conversion of photogenerated charge.Therefore,abundant reactive oxygen species（ROS,including generated·O₂^-,¹O₂and·OH）and h_VB⁺with stronger oxidation capacity are generated,leading to notably elevated photocatalytic oxidation activity to three emerging organic pollutants including acetaminophen（APAP）,levofloxacin（LEV）,and MPB.Finally,to further clarify the impact mechanism of carbon doping and nitrogen vacancy on the intrinsic electron and band structures,mass transfer and photocatalytic activity,a systematic and in-depth study is conducted.Hereto,acetamide-or formamide-assisted freeze drying-hydrothermal treatment and thermal copolymerization strategy is designed to synthesize carbon atom self-doped g-C₃N₄（AHCN_x）and nitrogen vacancy-modified g-C₃N₄（FHCN_x）,which settles problems related to vague chemical structure definition,challenging regulation of photocatalytic performance,and unclear structure-activity relationship for defect-modified g-C₃N₄,stemming from the mismatched physical properties of acetamide（or formamide）with urea.By adjusting the initial molar ratio of urea to acetamide（or formamide）,the types,concentrations and positions of defects within the g-C₃N₄structure are precisely regulated successfully.Combination experimental results with DFT calculations,it is confirmed that both AHCN_xand FHCN_xpossess enhanced visible-light harvesting capacity and the localized charge distributions on HOMO and LUMO,leading to promoted charge separation and transfer.Therefore,both AHCN_xand FHCN_xexhibit remarkably improved visible-light photocatalytic performance in the oxidation of emerging organic pollutants（APAP and MPB）and reduction of proton to H₂,surpassing bulk g-C₃N₄.Additionally,the study reveals distinct photocatalytic redox mechanisms for AHCN_xand FHCN_x.In the AHCN_xsystem,the synergistic effect of abundant reactive species(·O₂^-,·OH and h_VB⁺)results in the complete degradation and mineralization of target organic pollutants.In contrast,the FHCN_xrelies on cooperative effect of·O₂^-radicals and the stronger oxidation capability of h _VB⁺to promote the oxidation of target organic pollutants.And both AHCN_xand FHCN_xdemonstrate rapid charge transfer and separation capabilities,providing more free electrons for photocatalytic hydrogen production.In summary,new strategies such as heterojunction construction and molecular engineering are designed and implemented to synthesize a series of novel g-C₃N₄-based photocatalytic materials to enhance the photocatalytic performance of g-C₃N₄in the oxidation of typical emerging organic pollutants in water and the reduction of water to H₂production,from both thermodynamic and kinetic perspectives.By combination of various advanced characterization techniques and theoretical calculations,the reaction mechanisms of the enhanced photocatalytic oxidation activity towards aqueous organic pollutants and reduction of water to H₂are revealed;meanwhile,the structure-activity relationship of as-prepared photocatalysts is investigated in depth.This work therefore provides some valuable insights and new ideas for the design and synthesis of novel and efficient g-C₃N₄-based photocatalytic materials,as well as offering a sustainable new technology for the efficient treatment of emerging organic pollutants in water.