Preparation Of Carbon Nitride Heterojunction And Its Photocatalytic Performance

With the rapid consumption of fossil energy,the energy crisis and environmental pollution are becoming more and more serious,it is a great challenge to develop an efficient,clean and renewable technology to solve above problems.In recent years,as a technology to solve environmental pollution and energy shortage,semiconductor photocatalysis has attracted the attention of researchers all over the world due to its features of simple operation,low energy consumption,no secondary pollution and high efficiency.As a carbon-based material,graphite carbon nitride（CN）has been widely used in the field of photocatalysis due to its abundant earth reserves,non-toxic,appropriate band gap,good thermal and chemical stability.Generally,CN obtained by thermal polycondensation of carbon,nitrogen containing precursors（such as melamine,urea）is presented as bulk structure,the rapid recombination of photo-induced electron hole pairs,small specific surface area,poor stability and narrow visible light response range of bulk CN are seriously limit its practical application in the field of photocatalysis.In recent years,construct CN-based photocatalyst with high efficiency and stable photocatalytic performance has been a research focus.In this paper,we designed CN-based Z-type and lateral heterostructures with different nanostructures by nanostructure engineering and energy band engineering,and systematically studied the influence of these engineering on the photocatalytic performance of CN.The main research contents of this thesis are as follows:（1）Based on the theory of nanostructure and energy band engineering,by regulating the nanostructure and energy band structure of CN,we synthesized Z-scheme heterojunctions of CN quantum dots/Zn O nanosheets（CNQDs/OV-Zn O）and systematically studied the relationship between CNQDs nanostructures,energy band properties and the modified photocatalytic performance.Interaction forces（such as the van der Waals force）make CNQDs more dispersed and stable,and eventually the quantum dots are anchored on the semiconductor nanosheet（OV-Zn O）to form a zero-dimensional/two-dimensional（OD/2D）Z-scheme heterojunction.As a result,CNQDs/OV-Zn O heterojunction exhibits high photocatalytic activity for degradation of methyl blue and bisphenol A,where the kinetic constant is 11.4 and 32.5 fold of pure OV-Zn O,respectively.UV-Vis spectra,photoluminescence,electrochemical impedance spectroscopy and photocurrent verify that the 0D/2D Z-scheme heterojunction can effectively accelerate the separated of the photogenerated electron-hole pairs,broaden the visible light response range of OV-Zn O and improve carrier transport.The enhanced photocatalytic activity could be attributed to the synergistic effect of efficient Z-scheme charge separation,highly dispersed 0D CNQDs,coordinating sites of 2D OV-Zn O nanosheets and the strong coupling between them.In addition,the 3D flower-like structure constructed by 2D nanosheets greatly inhibits the leaching and loss of the photocatalyst in the recycling process,and ensures the high recycling ability of CNQDs/OV-Zn O.（2）The ultra-thin tubular lateral heterostructures（LHSs）of graphitic carbon nitride and carbon dots（CN/C-Dots）,with thickness of～8.0 nm via a facile,one-step thermal polymerization of the supramolecular complexes composed of melamine,cyanuric acid andβ-cyclodextrin（β-Cy D）as starting monomers.Interestingly,the structural topology of the CN/C-Dots LHSs can be effectively modulated by the amount ofβ-Cy D.The obtained tubular porous LHSs are nearly perfect nanotubes,which not only greatly increases the accessibility of active sites and light harvesting via high light absorption and scattering,but also shortens the route of carriers migrating towards the surface and improves the mass transport.In comparison with pure CN,the optimized ultra-thin tubular porous CN/C-Dots₁₀₀ LHSs exhibit significantly boosted visible light photocatalytic hydrogen evolution activity（up to 113 times）of 24760μmol h^-1 g^-1（λ>420 nm）,much higher than that over pure or nonmetal modified CN tubes reported to date.Moreover,the tubular porous CN/C-Dots₁₀₀ LHSs are first demonstrated to be excellent electrocatalysts for hydrogen evolution reaction（HER）and oxgen evolution reaction（OER）,achieving a geometrical catalytic current density of 10 m A cm^-2 at overpotentials 415 and 340 m V in 0.5 M H₂SO₄ and 1 M KOH versus the reversible hydrogen electrode（RHE）,respectively.These findings may provide insights into constructing highly efficient catalysts by tuning structural topology and heterojunction for different energy-related applications.