Study On The Photocatalytic Performance Of Graphitic Carbon Nitride Regulated By Hydrogen Bond

Low-cost,highly effcient,and durable photocatalysts are attracting a great deal of attention due to its potential applications of solar energy conversion and utilization.Traditional potocatalysts have some drawbacks including high recombination rate of photoexcited electron holes,narrow visible-light response,high-cost,and/or low light utilization,which adversely affect their photocatalytic efficiency.As a metal-free organic semiconductor photocatalyst,polymeric carbon nitride（CN）has been explored for ptotocatalytic water splitting because of its unique properties,such as earth-abundant,easy fabrication,visible-light response,chemical stability and favorable band positions straddling the water splitting potentials.However,the microstructure of CN prepared by direct thermal polycondensation of materials that contains carbon,nitrogen and hydrogen source is generally a tightly packed bulk structure,with small specific surface area,limited visible light absorption range below450 nm,sluggish charge mobility,resulting in low potocatalytic hydrogen evolution activity.In this paper,we have designed and fabricated CN-based ultra-thin nanosheets,self-assembled nanowire arrays and 0D/3D heterojunctions by regulating hydrogen bonds,and systematically studied the role of hydrogen bond in regulating the nanostructure and catalytic performance of CN-based photocatalysts.The main research contents of this paper are as follows:（1）A strategy to fabricate ultra-thin CN nanosheets by top-down control of hydrogen bonding is proposed.Unlike graphene,graphitic carbon nitride（CN）polymer contains a weak hydrogen bond and van der Waals（vd Ws）interactions within and between layers besides a strong covalent bond,which controls its final morphology and functionality.In the CN layer,triazine units are orderly connected by hydrogen bonds at the edge of the amino group（-NH₂）of the unsaturated polycondensation,extending the in-plane periodic structure.In view of the special structural properties of CN,we substituted hydrogen（H）on-NH₂ and carbon（C）on triazine units with boron（B）and phosphorus（P）atoms to achieve partial hydrogen bond breaking in the layer,and weakened the vd Ws interaction between neighboring layers,thus,ultrathin CN nanosheets are prepared.Systematic experimental characterization and theoretical calculations show that the fabrication of ultra-thin CN nanosheets by co-doping B/P atoms on the one hand gives a larger active surface area of CN and reduces the charge migration distance to the surface;On the other hand,B/P co-doping not only optimizes the electronic structure of CN,effectively widens the spectral response range and increases the proton reduction potential,but also increases the catalytic sites（including P⁺center as Lewis acid site）and promotes the spatial separation of electron holes,thus synergically enhancing the photocatalytic activity.The photocatalytic performance of B/P co-doped ultra-thin CN nanosheets（B/P-CNNs）under visible light irradiation shows that the hydrogen production rate is up to 10877.40μmol h^-1 g^-1,which is far better than bulk CN and most of reported CN-based catalysts.（2）A bottom-up modulating hydrogen bond supramolecular self-assembly method is developed for the synthesis of louved-like P doped CN（L-PCN）nanowire arrays.The precise bottom-up synthesis of nanostructure arrays without templates or substrates is quite challenging because of the general occurrence of homogeneous-nucleation and the difficult manipulation of noncovalent interactions.The supramolecular self-assembly approach based on modulated hydrogen bonds overcomes the template-assisted and single-directional limitations of traditional self-assembled nanostructures（vertical growth）,and expands the structural possibilities of nanostructure arrays.L-PCN nanowires prepared by this method have a good spatial separation position,which effectively integrates the inherent characteristics of single nanostructure and array stability.Under visible light irradiation,the self-assembled L-PCN nanowire arrays showed superior hydrogen production performance(1872.9μmol h^-1 g^-1),rendering a～25.6-fold enhancement compared to bulk CN,and achieved good stability during multiple cycles.In addition,the hydrogen generation quantum efficiency（AQY）of L-PCN nanowire arrays at420±15 nm is as high as 6.93%.The experimental results combined with first-principles calculations indicate that the significantly enhanced catalytic activity of L-PCN nanowire arrays can be attributed to the synergistic effects of their structural topology and dopant.（3）A high-throughput one-photon excitation strategy for high-efficiency charge transfer and spatial separation is proposed.The 0D/3D carbon dots/porous CN nanovesicles（CDs/PCN NVs）heterojunction was constructed based on this strategy and bottom-up hydrogen bonding regulated self-assembly.The experimental and computational results show that the introduction of CDs can realize the one-photon excitation pathway through energy band matching,and induce the photogenerated holes and electrons to accumulate in the valence band（VB）of PCN NVs and surface of CDs,respectively,thus realizing the spatial separation of the electron holes.In addition,due to the intrinsic optical properties of CDs,not only the absorption spectral response of CN is broadened to the near-infrared range,so that more carriers are excited in the wide spectrum,but the coupling of CDs also effectively regulates the band structure of CN,increases the proton reduction potential and accelerates the directional migration of carriers due to the construction of the built-in electric field.Finally,a high-throughput one-photon excitation pathway for efficient photocatalytic hydrogen production is realized.In particular,the photocatalytic hydrogen production performance of the optimized CDs/PCN NVs heterostructure is up to 14022μmol h^-1g^-1,which is 56.54 times higher than that of CN and is one of the best CN-based catalysts reported so far.