Turning Conjugated Structure Of Graphitic Carbon Nitride For Photocatalytic Hydrogen Evolution

Hydrogen as a clean,renewable and eco-friendly energy,can replace traditional fossil energy to meet the increasing energy demand.Converting renewable solar energy into clean hydrogen effectively and economically is one of the trends in future hydrogen development.The pioneer work of water splitting under solar irradiation over the Pt/TiO₂photoelectrochemical cell in 1973 opened the gate of photocatalytic water splitting.Since then,researchers have explored many stable visible-light photocatalysts with highly quantum efficiency.Graphitic carbon nitride?g-C₃N₄?,as a novel non-mental photocatalyst,has arosed significant interest due to its appropriate optical band position,cost-effective synthesis and eco-friendly.However,the photocatalytic performance of g-C₃N₄ is severely hampered by three obstacles:low specific surface area,unoptimized utilization of visible light and short lifetime of photoexcitons.Nowadays,various straties have been devoted to resolving these drawbacks.In this dissertation,we mainly focus on modifying the conjugated structure of g-C₃N₄,including combing with?-conjuigated carbon quantum dots or carbon spheres with g-C₃N₄,grafting with4,4?-?benzoc 1,2,5 thiadiazole-4,7-diyl?dianiline?BD?or polycyclic aromatic compounds?PA?in g-C₃N₄ framework,constructing porous g-C₃N₄ with satcking nanotubes.The modification of conjugated structure in g-C₃N₄ can extend visible light absorption range and improve separation effience of photo-generated electrons and holes.The details are as follows:1.Modification of g-C₃N₄ nanosheets by carbon quantum dotsThe fluorescent CQDs prepared by thermal condensation were deposited onto CNNS to form CNNS/CQDs composites.The morphology,composition and structure of the samples were characterized by TEM,FT-IR,XRD and XPS.CNNS/CQDs composites exhibit much higher photocatalytic activity under visible light irradiation than that of pure CNNS.The optimized CNNS/CQDs sample displays a photocatalytic H₂ production rate of 116.1?mol/h,which is three times of that over CNNS?37.8?mol/h?.The upconversion fluorescence of CQDs can improve the visible light harvesting of CNNS.Moreover,the CQDs can act as capture centers of photo-induced electrons to suppress recombination of charge carries in CNNS.2.g-C₃N₄-encapsulating carbon spheresNovel carbon spheres encapsulated in graphitic carbon nitride nanocomposites?CS/g-C₃N₄?were facilely fabricated through the polymerization of melamine from the carbon spheres.The optimized CS/g-C₃N₄ composite displays a hydrogen evolution rate of 50.2?mol/h,which is almost 5 times of that over the pure g-C₃N₄.The incorporation of carbon spheres in g-C₃N₄leads to enlarged contact area and strengthened interaction between the two?-conjugated components.The CS/g-C₃N₄ composites exhibit improved visible light utilization because of the interaction of?conjugated structures between g-C₃N₄ and CS and enhanced separation of photo-generated charge carriers due to the electron-capturing ability of CS.3.g-C₃N₄ based donor-?-acceptor-?-donor structureg-C₃N₄ based donor-?-acceptor-?-donor structure?UCN-BD?was constructured by incorporating 4,4'-?benzoc 1,2,5 thiadiazole-4,7-diyl?dianiline?BD?into the g-C₃N₄framework.The aromatic ring and benzothiadiazole were proved by FT-IR,XPS and NMR.Benefiting from the visible light band tail caused by the extended?conjugation,UCN-BD possesses expanded visible light absorption range.The BD monomer also acts as an electron acceptor,which endows UCN-BD with high degree of intramolecular charge transfer.With this unique molecular structure,the optimized UCN-BD sample exhibits much superior performance for photocatalytic hydrogen evolution upon visible light illumination?171.2?mol/h?,which is nearly 6 times of that of the pristine g-C₃N₄,while the AQE at?=450 nm was calculated to be 12.3%.4.Polycyclic aromatic compounds-modified g-C₃N₄The copolymerization between urea and polycyclic aromatic compounds?benzoic acid,naphthoic acid and anthroic acid?was applied to constructing aromatic rings-grafted g-C₃N₄photocatalysts.Incorporation of aromatic rings in the g-C₃N₄ network is an effective protocol to extend?-conjugated system for visible light absorption and improves the charge transfer efficiency for prolonging lifetime of photogenerated charge carriers in photocatalytic reactions.Remarkably,the moderate conjugated effect of aromatic rings?naphthalene?is crucial to promote charge separation.The developed naphthalene-grafted g-C₃N₄ achieves the highest water splitting performance with hydrogen evolution rate up to 102.1?mol/h,nearly 3.5 times of that of the g-C₃N₄.5.Porous g-C₃N₄ with stacking nanotubesPorous graphitic carbon nitride with stacking nanotubes was synthesized by thermal condensation of melamine-ammonium oxalate stacking sheets.The photocatalytic generation rate of hydrogen over the porous graphitic carbon nitride?99.7?mol/h?is nearly 9.5 times as that of pristine g-C₃N₄?10.5?mol/h?.Such enhancement was attributed to the large specific surface area,improved visible light absorption due to activation of n-?*electronic transitions and efficient separation of photo-generated electron and holes.