| Graphitic carbon nitride(g-C3N4)is a very important class of non-metallic polymers with a wide and important range of applications in many fields such as energy and catalysis.Because of its abundant raw materials,simple preparation,high stability,green and safe properties,g-C3N4 has been successfully applied in various energy conversion and environmental engineering and related systems such as photocatalytic hydrogen evolution reaction(HER),CO2 reduction and N2 fixation.Despite these advantages,the low visible light capture capacity and charge separation efficiency due to the rapid compounding of photogenerated electron-hole pairs in the photocatalytic reaction of unmodified g-C3N4 photocatalysts directly lead to the low photocatalytic performance which is far from the industrial standard and limit its further large-scale application.To improve the photoconductive property of the g-C3N4-based photocatalysts,some solution-phase synthetic routes have been designed for the controllable preparation of g-C3N4-based composite nanostructures via integrating some exotic component along with doping stragtegy by modulating the structure and electronic structure of the active components in the present dissertation.The novelties and the main contents of the dissertation are described below:1.Two-dimensional(2D)ReSe2/g-C3N4 heterojunction containing photocatalysts were designed and fabricated by hot colloid injection and ultrasound-assisted selfassembly,and their electronic properties were deeply refined by inducing the introduction of appropriate amount of selenium vacancies on the surface of ReSe2.The(high-resolution)transmission electron microscopy(TEM/HRTEM)images show that the composition of the 2D ReSe2/g-C3N4 composite nanomaterials is in the form of 2D/2D type heterostructures.In addition to electron microscopy studies,various studies such as X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and electron paramagnetic resonance spectroscopy(EPR)have confirmed the formation of heterojunctions.XPS,ultraviolet-visible spectroscopy(UV-Vis)and electrochemical Mott-schottky experiments have also analyzed the energy band structures of ReSe2 and g-C3N4 materials and the types of constituent heterojunctions.Density functional theory(DFT)calculations show that the exfoliated two-dimensional ReSe2 material yields more high-quality edge-active sites to which charge carriers from the g-C3N4 surface are transported for hydrogen-producing reactions through electron transfer between heterojunctions.The process substantially improves the charge transfer efficiency and suppresses the compounding of photogenerated electron-hole pairs.The tested twodimensional ReSe2/g-C3N4 material achieves a visible photocatalytic hydrogen production efficiency of 1055.50 μmol g-1 h-1.Meanwhile,the two-dimensional ReSe2/g-C3N4 material shows a better visible light response and very high stability,both of which are better than most of TMDs/g-C3N4 in the literature.2.NiCoP/g-C3N4 composite nanomaterials with embedded NiCoP nanoparticles(NPs)were designed and fabricated using a solvothermal reaction and self-assembly engineering.During the synthesis,the NiCoP nanoparticles keep NiCoP dispersed and immobilized on the surface of g-C3N4 nanosheets by forming special P(δ-)Co(δ+)/Ni(δ+)-N(δ-)bonds.Structural tests such as XPS show that the P(δ-)Co(δ+)/Ni(δ+)-N(δ-)bond formation enhances the adsorption ability of H atoms on the composite surface,generating new efficient catalytic active sites.In-depth study of the band structure by UV-Vis and ultraviolet photoelectron spectroscopy(UPS)tests shows that the NiCoP nanoparticles have metallic properties can form Schottky barriers with g-C3N4 nanosheets,modulate the electronic structure of g-C3N4 nanosheets,change the valence band of g-C3N4 nanosheets,and transfer the photogenerated electrons generated on the surface of g-C3N4 to the co-catalyst NiCoP to complete the H+reduction reaction.In the photocatalytic hydrogen production experiments,the NiCoP/g-C3N4 composite maintained a high level of stability and activity with only 1 wt%loading,and the hydrogen production efficiency reached 1067.11 μmol g-1 h-1,which was significantly better than the similar type of catalysts in terms of performance such as the utilization ability and controllability of the co-catalyst.3.The electronic properties of g-C3N4 nanosheets were tuned by using doping modification of BCN nanosheets containing a small amount of B elements prepared by a simple cohesive condensation reaction.The results of XPS and HRTEM tests verified the successful doping of boron elements and the substitution effect of B atoms on C atoms in the 3-s-triazine ring.Brunauer、Emmett&Teller specific surface area test method(BET)and TEM results showed that the doping of boron atoms led to certain changes in the material morphology,increased the specific surface area,and enhanced the charge transfer kinetics of the catalytic reaction.The optimized BCN-5 nanosheets with better fundamental properties were obtained by evaluating the doping amount of B atoms,and further analysis of their electronic properties demonstrated that the doping of B elements significantly changed the conduction band position of g-C3N4.We further assembled the BCN materials with the co-catalysts prepared in the first two parts to obtain new g-C3N4-based composites,and the hydrogen production efficiency experiments demonstrated that the photocatalytic activity of the two-dimensional ReSe2(SV)/BCN materials was steadily increased by 49%compared with the original g-C3N4-based composites,while the photocatalytic activity of the monodisperse NiCoP/BCN materials was decreased by 36%compared with the original g-C3N4-based composites by 36%.We combined the analysis of the catalytic reaction mechanism of the energy band structure of various composites in our work and successfully proposed an explanation for these two contrasting changes from the perspective of the energy band modulation direction,which provides a pathway for the design of more complex g-C3N4-based composite photocatalysts. |
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