Design And Structure Control Of Graphitic Carbon Nitride Photocatalyst Toward Hydrogen Generation

Due to global energy crisis and environmental issues,developing renewable energy has already become a worldwide consensus.Hydrogen,as a green and low-carbon energy carrier,will play a key role in the clean and secure energy future.In comparison with hydrogen production from fossil fuels,solar driven photocatalysis for hydrogen evolution is a more ideal and promising technology.As a core factor of photocatalytic hydrogen evolution,designing and preparing photocatalysts with superior performance,long-term stability and low-cost has been emerging as a research hotspot in photochemical domain.In numerous photocatalysts,graphitic carbon nitride?g-C₃N₄?with a graphite-like two-dimensional layered structure has been extensively explored due to its tunable electronic band gaps,reliable physicochemical stability and low cost.This thesis focuses on designing and preparing highly active and stable g-C₃N₄ based photocatalysts and then explores their applications in solar-to-hydrogen conversion.Details are as follows:?1?g-C₃N₄ synthesized via traditional thermal polymerization strategy shows inferior photocatalytic performance owing to its narrow absorption range and serious recombination of electrons and holes.To boost visible-light absorption and inhibit recombination of electrons and holes,N vacancies modified g-C₃N₄ was fabricated by a scalable and one-step N₂H₄·H₂O-assisted thermal polymerization method.The experiment results show that N vacancies are introduced into g-C₃N₄ structure by removing N atoms in the C-containing triazine rings(N_2C)sites,which cause both bandgap and sub-bandgap narrowing and consequently extend the long wavelength visible-light harvesting.As a result,the optimal sample shows a highest H₂ evolution rate of 8171 and 3895?mol · h^-1 · g^-1 under ?>420 nm and ?>470 nm visible-light irradiation,respectively.Moreover,stability assays show that the as-prepared sample possesses satisfactory light and structure stability after ten cycling runs?4 h per cycling?.This work offers a new and effective route for fabricating high-performance N vacancies modified g-C₃N₄ photocatalyst on a large scale.?2?Except for limited visible-light absorption and recombination issue of charge carriers,photocatalytic performance of the g-C₃N₄ prepared by traditional thermal polymerization method is also restricted by its small specific surface area.To increase the surface area and further alter the electronic structure,heteroatoms C,O binary-doped g-C₃N₄ with hierarchical porous nanobelt architecture was synthesized via a template-free self-assembly method.The as-prepared sample exhibits well-defined hierarchical nanobelt structure composed of porous nanosheets,resulting in obviously enhanced specific surface area(120m²·g^-1).Experiment and theoretical calculation results show that C and O heteroatoms,introduced into the structure of g-C₃N₄ by substituting N atoms,induce further narrowed bandgap for more effective visible-light harvesting and negatively shifted conduction band position for stronger reducibility of photo-induced electrons for H₂ production.Based on this,the optimal sample shows a highest photocatalytic H₂ evolution rate of 18380 ?mol·h^-1·g^-1 under visible-light??>420 nm?irradiation,which is about 79.9 times higher than that of the bulk g-C₃N₄.In this work,g-C₃N₄ with a novel nano-structure and excellent performance is prepared,providing a new route for the development of efficient hydrogen production photocatalyst.?3?The innate drawbacks of g-C₃N₄ such as limited surface area,poor visible-light absorption and high recombination rate of photo-excited charge carriers can be sovled by structure control,morphology design and element doping,but the modified g-C₃N₄ photocatalyst system still needs noble metal Pt as co-catalysts to serve as electron sinks and provide the active sites for photocatalytic reaction.Considering the intrinsic drawbacks,i.e.rare and expensive of Pt co-catalyst,it is urgent to develop g-C₃N₄ based photocatalyst system with satisfactory photocatalytic H₂ evolution efficiency in the presence of a very low-loading amount of Pt or even without Pt.Herein,few-layer P-doped g-C₃N₄ nanosheets decorated with n-type metal oxides nanostructures,i.e.TiO₂ nanoparticles and SnO₂ nanowires are prepared via a self-assembly method.The n-type metal oxides with high electron mobility,low cost and excellent physicochemical stability uniformly distributing on P-doped g-C₃N₄ act as electron transporting channels,dramatically enhancing extraction efficiency of photogenerated electrons from P-doped g-C₃N₄.The resultant TiO₂/P-doped g-C₃N₄ and SnO₂/P-doped g-C₃N₄ in the absence of Pt under visible-light irradiation exhibit enhanced H2 evolution reaction rates of 1082 and 2090 ?mol·h^-1·g^-1 comparing with P-doped g-C₃N₄(132 ?mol·h^-1·g^-1).Besides,under a very low Pt amount?0.33 wt%?,the H₂ evolution reaction rates of TiO₂-Pt/P-doped g-C₃N₄ and SnO₂-Pt/P-doped g-C₃N₄ further climb up to 11632 and 12469 ?mol·h^-1·g^-1.These hybrid photocatalysts with elaborate architectures can be ranked as promising and low-cost candidates for high-efficiency photocatalytic water splitting materials in the presence of a very low-loading amount of Pt or even without Pt.