Induced Electron Transfer And Hierarchical Tubular Design For Enhanced Photocatalytic Hydrogen Production Of Carbon Nitride

The current environment and energy issues have become increasingly challenging.Hydrogen energy has attracted much attention as a clean renewable energy source.At the present stage,the use of photocatalytic technology to convert solar energy into hydrogen energy has become a hotspot of current research.Exploring and designing a photocatalyst with efficient visible light response and high activity is the core issue in photocatalysis technology.As the new generation of the most promising organic semiconductor photocatalyst,graphitic carbon nitride(g-C3N4)materials still suffer from the shortcomings such as low utilization of visible light,high recombination rate of photo-generated electron-hole,small specific surface area,and few active sites,which greatly hinder the development of photocatalytic materials and potential practical applications.To overcome the above-mentioned shortcomings,the directional intramolecular charge transfer,controlled electron migration and hierarchical micro-nano-structured carbon nitride material with doping and defects are designed via molecular coupling and supramolecular preassembly strategies by considering the perspectives of induced electron transfer and morphology design on molecular and nanoscale level.As a result,more efficient photocatalytic hydrogen production can be achieved in g-C3N4 and the mechanism and electronic structure can be also well revealed by the theoretical calculation.The corresponding results are summarized as the following:Iodobenzaldehyde and urea were copolymerized in one step at 550 °C to construct an asymmetric donor-?-acceptor-type porous carbon nitride material through two classic organic reactions: nucleophilic su bstitution between aromatic halogenides and urea,and Schiff base chemical reaction between aromatic aldehydes and urea.The formation of this structure was further verified by TEM,FT-IR and solid-state 13 C NMR spectra.The inserted benzene ring expands the conjugate system of the bulk structure and increases the range of electron delocalization.The loose porous structure caused by releasing gas increases the specific surface area of carbon nitride and exposes more active sites.At the same time,the donor and acceptor groups at both ends of benzene ring are polarized,resulting in intramolecular charge transfer,which provides a directional migration of the photogenerated electrons,thereby enhancing the light absorption capacity and improving the separation of photogenerated carriers.These factors synergistically improve the photocatalytic hydrogen production activity of carbon nitride.Under visible light irradiation,the activity can reach up to 2889.98 ?mol/g?h,which is about 5.1 times than that of the control carbon nitride,and the apparent quantum yield at 420 nm was 5.49%.Other conjugated structures such as pyridine,thiophene,or furan-based D-?-A structures also have good hydrogen-producing activity,further revealing the universality of this strategy in designing new photocatalysts.Benzaldehyde and fluorobenzaldehyde were connected to the carbon nitride skeleton by molecular grafting technology.XPS and theoretical calculations confirmed the precise control of local electron migration of carbon nitride materials by coupled groups.At the same time,changing the amount of coupled groups can further regulate the electron migration and optimize the performance of carbon nitride.The reset local electronic structure can enhance the photo-generated charge separation and red-shifted photoluminescence of carbon nitride materials.The photocatalytic hydrogen evolution rate of the sample with optimal electron migration is 9 times than that of the original carbon nitride,and the apparent quantum yield at 420 nm is 20.98%,and it is still 7.69% at 500 nm.In addition,the exfoliated carbon nitride suspension has stable and adjustable interference-free red-shift fluorescent emission.Considering the good biocompatibility and non-toxicity of carbon nitride,the material is eager to be used in biological imaging,phototherapy and other biomedical applications.Based on supramolecular pre-assembly strategy,by using melamine as the only precursor,a double-layered tubular carbon nitride and a hierarchical tubular carbon nitride with an internal porous coral-like architecture were successfully obtained by a self-templating method.The oxygen doping and carbon defects were introduced at the same time.For the first time,the three-dimensional structure of hierarchical tubular carbon nitride was analyzed and revealed by using electronic tomography.The rationality of doping and defects was further analyzed by UV-vis spectra,XPS and 13 C NMR spectra.Surpassing the simple hollow tubular carbon nitride,this hierarchical hollow structure can provide more contact reaction sites and cooperate to establish a new heterogeneous catalytic interface.The large specific surface area,short charge transfer distance,enhanced light scattering,fast mass transfer rate,and matched band gap with intermediate state significantly improve the photocatalytic hydrogen production performance of g-C3N4.Compared with the original carbon nitride and carbon nitride with simple structure,the hydrogen production rate is increased by 36 times and 2.6 times,respectively,and the apparent quantum yield at 420 nm can reach 32.4%.This study provides new insights for designing environmentally-friendly controllable micro-nano structures for efficient solar-to-chemical energy conversion.This paper introduces the new concept of induced electron transfer and the three-in-one hierarchical micro-nano structure to synergistically improve the photocatalytic hydrogen production performance of carbon nitride materials,and achieves some significant effects.The established theoretical method combining molecular engineering with nanoscale engineering has laid a foundation for exploring new photocatalytic mechanisms in future.