Microstructure Regulation And Photocatalytic Water Splitting Properties Of Supramolecular Assembly Derived Carbon Nitride

As a green and sustainable energy conversion technology,photocatalytic water splitting into hydrogen fuels has been recognized as an effective pathway to solve the energy crisis and environmental pollution issues,but its commercial application is hampered by the high cost and low efficiency of photocatalysts.Carbon nitride（CN）has a highly delocalizedπ-conjugated system,which brings excellent optical properties and unique electronic band structure.In addition,the adjustable chemical configuration and excellent physicochemical stability give CN materials extensive prospects for photocatalytic applications.However,traditional CN obtained by one-step annealing generally suffers from low specific surface area,insufficient active sites,narrow visible light absorption range and severe recombination of photogenerated carriers,which lead to unsatisfactory photocatalytic water splitting performance.Based on this,this dissertation introduces supramolecular assembly into the precursor preparation of CN materials.The dimensionality regulation,heteroatom doping,vacancy engineering and functional group modification of the derived CN materials are realized by adjusting the morphology,structure and chemical composition of supramolecular assemblies,so as to increase the number of active centers,broaden the light absorption range,and inhibit the recombination of photogenerated carriers.Combined with a series of in-situ/ex-situ characterizations and theoretical calculations,the structure-activity relationships between CN microstructure regulation and photocatalytic water splitting performance are established.On this basis,the active H⁰ generated from water splitting is further used as the hydrogen source to realize the efficient reduction of nitrophenol by non-metallic photocatalyst,which promotes the research progress of photocatalytic transfer hydrogenation reaction.The research contents of this dissertation are listed as follows:（1）Through the functional design of triazine-based monomer（T1）,the cyano defects and Na doping modified CN（CN1）is synthesized by a bottom-up process.The hybridization of the outer electron orbitals of doping Na and N atom in CN1 leads to ion-dipole interaction,which will promote the charge transport.Moreover,the in-situ modified electron-withdrawing cyano group constructs an internal electric field in the conjugated structure of CN1,further promoting the separation and transfer of photogenerated carriers.The optimized CN1 exhibits a photocatalytic hydrogen evolution rate of 1070μmol h^-1 g^-1.In addition,three-dimensional（3D）porous CN microtubule cluster（3DCN1）is prepared via the thermal polymerization of supramolecular assembly from T1 and cyanuric acid.The unique 3D porous structure enables3DCN1 to have a larger specific surface area,more active sites,and higher separation and migration efficiency of photogenerated carriers.The corresponding photocatalytic hydrogen evolution performance of 3DCN1 reaches 1681μmol h^-1 g^-1,and further increases to 2215μmol h^-1 g^-1 after optimizing the photocatalyst concentration.（2）To further broaden the visible light absorption range and inhibit the recombination of photogenerated carriers,a thiophene-modified triazine-based monomer（Triazine-Th）is designed.The 3D porous CNs（TCN-x）modified with thiophene electron donor groups are successfully synthesized by the bottom-up thermal polymerization of supramolecular assembly using Triazine-Th,T1 and cyanuric acid as the starting monomers.The thiophene electron donor groups in TCN-x are easily adjusted by controlling the amount of Triazine-Th.The structural characterizations and photoelectrochemical tests revealed that the lone pair electrons on the S atom in thiophene group can transition to the excited state ofπ-conjugated system,realizing the n‐π*electron transition,thus extending the visible light absorption range.Furthermore,the electron donor property of thiophene group can localize the charge density distribution and improve the separation and migration of photogenerated charge carriers.The optimized TCN-3 exhibits an improved photocatalytic hydrogen evolution rate of 5758μmol h^-1 g^-1.（3）2D ultrathin CN nanosheets（2DCN-S）with an average thickness of 1.7 nm are successfully synthesized via a novel solid state supramolecular assembly strategy,which can efficiently overcome the influence of solvent molecules in traditional solvothermal methods.The structure characterizations and DFT calculations revealed the structure transformation process and the formation mechanism of 2D ultrathin nanosheets during the solid state supramolecular assembly process.The cyanuric acid-melamine（CM）supramolecular nanoplates with exposed（001）planes are firstly formed,and then polymerized into 2DCN-S ultrathin nanosheets.Synchrotron-based X-ray absorption near-edge spectroscopy（XANES）confirmed the existence of C-N interactions between the twisted carbon nitride layers of2DCN-S,which will act as a rapid photogenerated carrier transfer pathway to reduce the charge transfer distance and improve the separation efficiency of photogenerated electron-hole pairs.2DCN-S exhibits a remarkable photocatalytic hydrogen evolution rate of 6388μmol h^-1 g^-1under visible light irradiation.The solid state supramolecular assembly method is straightforward,scalable,and universally applicable,which will offer a novel pathway to the synthesis of 2D carbon-nitrogen materials.（4）In order to realize the combination of photocatalytic water splitting and nitrophenol hydrogenation reaction to synthesize high value-added chemicals,a loofah-like carbon nitride（LCN）with 2D/3D hierarchical porous structure was designed.The unique structure can improve the specific surface area and light absorption capacity.The mesopores with sizes in the tens of nanometers provide rapid mass transfer capability,while the mesopores with sizes in several nanometers offer abundant exposed active sites and shorten charge carrier diffusion distance.The conversion rate of 4-nitrophenol（4-NP）to 4-aminophenol（4-AP）achieves 96.5%using Pt/LCN as a photocatalyst in the transfer hydrogenation reaction with water as the hydrogen source.This conversion rate is significantly higher than the traditional hydrogenation reaction using Na BH₄ as the hydrogen source（8.3%）.The isotope-labeling experiments and DFT calculations are performed to reveal the reaction mechanism.The active H⁰ produced by water splitting on Pt/LCN prefers to participate in the hydrogenation of nitro groups in 4-NP rather than generate H₂,which can effectively skip the hydrogen extraction step in traditional hydrogenation reaction,thus realizing the efficient photocatalytic transfer hydrogenation reaction.