Study On Modification Of The Framework Of Graphitic Carbon Nitride And Its Photocatalytic Performance For Water Splitting

Increasingly severe energy crisis and environmental pollution are threatening global sustainable development.Hydrogen,as a clean and renewable energy has drawn muc h attention.Semiconductor-based photocatalytic water splitting for H2 evolution is a promising strategy for solar energy harvesting and storage.Seeking highly active photocatalysts has been a worldwide continuing endeavor.In these years,a metal-free photocatalyst graphitic carbon nitride?g-C₃N₄?has attracted worldwide attention due to its various advantages suc h as visible light response,easy preparation,and high stability.However,its known poor visible light utilization?Eg².7e V?,small specific surface area and fast recombination of charge carriers are the serious limitations.In this study,we modified melamine or urea with oxalic acid,ethylene glycol,edetic acid?EDTA?or ammonium acetate as the precursors for preparation of modified g-C₃N₄.A series of functional groups were grafted to the framework of g-C₃N₄,inducing the distortion of g-C₃N₄,which is capable of motivating the n-?* electronic transitions involving the N lone pairs to give an additional absorption region fro m 450 to 600 nm.All the modified g-C₃N₄ photocatalysts show enhanced photocatalytic performance for water splitting.The details are as follows:1.Carbon self-doped g-C₃N₄?C-g-C₃N₄?nanosheets were synthesized using supramolecule networks consisting of cyanuric acid,ethylene glycol?EG?and melamine bonded through hydrogen bonding as the thermal polycondensation precursor.EG was carbonized under 550 °C in inert atmosphere and the carbon atom was grafted to the g-C₃N₄ networks.According to XPS analysis,the doped carbon was grafted to the endmost nitrogen to connect two melem parts with distorted structure,inducing the activation of n-?* electronic transitions,and extending the utilization of visible light.The hydrogen evolution rate of C-g-C₃N₄ reaches 73.09 ?mol h-1,which is about 15 times of that over the g-C₃N₄ obtained from direct thermal polycondensation of melamine due to its extended light harvesting,negative-shifted CB position,larger specific surface area and faster separation of photogenerated charge carriers.2.Carbonyl-grafted g-C₃N₄ porous nanosheets?COCNPNS?were fabricated by a two-step thermal process using melamine and oxalic acid as starting reagents.As the C-C bond of the two conjoint carbonyls in oxalic acid molecule is unstable,one carbonyl was grafted to the g-C₃N₄ framework by connecting with two melem parts through the formati on of amido bonds.According to DFT calculations,the carbonyl-grafted structure is distorted,which not only promote the thermal exfoliation of g-C₃N₄ nanosheets but also induces the activation of n-?* electronic transitions,and extending the utilization of visible light.The obtained COCNPNS exhibited extended light harvesting ability,larger specific surface area and improved electronic conductivity for fast separation and transfer of photo-generated electrons and holes,thus enhanced photocatalytic performance?83.6 ?mol h-1?.In addition,the apparent quantum yield?AQY?at 450 nm for COCNPNS reached 6.1%.3.Modified g-C₃N₄ was synthesized by adding minor amount of ethylenediaminetetraacetic acid?EDTA?into urea as precursors for subsequent thermal polymerization.The thermal intermediate product ethylenediaminetetramethylene?EDTM?during decarboxylation of EDTA was grafted to the framework of g-C₃N₄ by connecting adjacent 3,s-triazine units.The special chelate structure of EDTA leads to the distortion of g-C₃N₄ to a nonplanar structure,which is responsible for extending the light harvesting around 500 nm,and consequently promoting the generation,separation and transfer of charge carriers.The H2 evolution rate on an optimized EDTM-grafted g-C₃N₄ photocatalyst?UCN-10?reaches 116.4 mmol h-1,which is about 3.5 times of that on the pristine g-C₃N₄?UCN?.In addition,the apparent quantum yield?AQY?at 450 nm for UCN-10 reached 8.2%.4.N deficiency was introduced into the frameworks of g-C₃N₄ by using the mixture of melamine and ammonium acetate as precursor.Acetic acid molecules were obtained during the decomposion of ammonium acetate,which would take away some amino of melamine and finally introduce some deficiencies into the frameworks of g-C₃N₄.N deficiencies resulted in the distortion of g-C₃N₄ frameworks,which was responsible for the enhanced absorption in the visible light region.The N-deficient g-C₃N₄ showed 5.8-fold enhancement compared with pristine g-C₃N₄,not only due to its larger specific surface area,but also to the extended absorption of visible light.