Structural Regulation Of Carbon Nitride-based Materials And Insight Into Their Enhanced Photocatalytic Activity

Photocatalysis has demonstrated great potentials for both environmental remediation and green energy production,wherein the key challenge is to find efficient and robust photocatalyst materials.Among various photocatalysts,a polymeric semiconductor,i.e.carbon nitride（C₃N₄）,emerged as a rising star because of its facile accessiblity,metal-free nature,low-cost,and environmentally benign property.However,bulk C₃N₄ only exhibits mediocre activity in photocatalytic water splitting,due to the limited optical absorption,sluggish charge carrier separation efficiency,and low specific surface area.To address the issues mentioned above,we performed systematic studies on the structural and compositional modulation of C₃N₄-based materials,and the mechanisms of enhanced photocatalytic activity.This work is expected to serve as a new paradigm in designing effective C₃N₄-based photocatalyst for environmental remediation and green energy production.In the thesis,C₃N₄ microspheres（CNMS）were firstly fabricated via a solvothermal method by using supramolecular complexes of dicyandiamide and cyanuric chloride as the precursors.The effect of solvothermal temperature on the morphology,band structure,and activity was systematically investigated.Structural characterization results indicate that the samples prepared at 180°C（CNMS-180）and 200°C（CNMS-200）possess a spherical morphology,while irregular bulk particles were obtained at 160°C（CN-160）.In addition,the band gap increases as the solvothermal temperature decreases from 200 to 160°C.In comparison with CN-160 and CNMS-200,the valence band of CNMS-180 is more positive and thus resulting in a higher photo-oxidation capability.Accordingly,CNMS-180 exhibits a higher photocatalytic degradation efficiency on Rhodamine B,stronger photocurrent response,and lower charge transfer resistance.Moreover,CNMS-180 exhibits excellent stability after four runs.This work provides a guidance for the regulation of morphology and band structure of C₃N₄-based materials prepared at low temperatures.On the basis of the optimized hollow carbon nitride microspheres（CNMS）,–OH groups（OH-CNMS）were used for further tuning the surface features.Their properties were thoroughly investigated by a number of advanced characterization methods.The as-prepared CNMS achieved an impressive p-hydroxybenzoic acid（HBA）degradation rate of 0.013 min^-1,which is 4.3 times higher than C₃N₄,even higher than some heterostructured or noble metal modified C₃N₄.The enhanced photooxidation activity of CNMS was achieved because of the optimized band structure and the deepened valence band edge,as unveiled by both experimental and density functional theory（DFT）calculation results.In addition,OH-CNMS exhibited an apparent quantum efficiency（AQE）of 3.7%at 420 nm.The improved hydrogen efficiency of OH-CNMS is ascribed to the surface functionalized–OH groups,which react with holes,and release more electrons to participate the water splitting,as well as the modified orbital configuration which facilitates the faster charge carrier transfer.Apart from CNMS,a simple solvothermal template-free approach was employed for the first time to synthesize phosphorous doped carbon nitride nanobelt（P-CN-NB）.Advanced characterizations,for instance,¹³C NMR,³¹P NMR,and XPS results indicate that P was substitutionally doped at the corner-carbon of the carbon nitride frameworks.The introduction of P dopants inhibites the polymerization between NH₂groups within P-CN-NB,enabling the decrease in nanobelt width for the exposure of more active sites.Therefore,the optimized P-CN-NB-2（derived from 0.2 m M H₃PO₄）renders enhanced HBA degradation nearly 66-fold higher than bulk C₃N₄,among the most efficient C₃N₄-based photocatalysts as reported.In addition,the P-CN-NB-1（derived from 0.02 m M H₃PO₄）exhibited about 2 times of H₂evolution rate than CN-NB.DFT calculations indicate that the P-CN-NB possessed a unique asymmetric LUMO orbital configuration favorable for electron-hole separation and hence for the enhanced degradation efficiency,accompanied by band narrowing which was predominantly driven by the contributions from the 2p bands in C,N,and 3p band in P dopants.At Last,we demonstrated that one-dimensional carbon nanotubes/two-dimensional ultrathin carbon nitride（1D SWCNT/2D C₃N₄）can serve as non-resonant plasmonic photocatalysts.The catalyst shows a stoichiometric production of H₂(49.8μmol g^-1 h^-1)and O₂(22.8μmol g^-1 h^-1)in overall water splitting,with a prominent H₂ production rate of 1346μmol g^-1 h^-1.The significantly enhanced photocatalysis is attributed to the non-resonant plasmonic effect,as confirmed by the increased spectral response within both ultraviolet and visible light regions,and the results of finite element method simulation.Moreover,the contributions from ultrathin morphology,long average carrier lifetime（2.54 ns）,and the electronic coupling effect of the nanohybrids collectively intensify the photocatalytic water splitting.