Study On The Enhancement Of Photocatalytic Reduction Ability Of G-C₃N₄ By Hydrothermal Method Assisted By Formaldehyde,Acetaldehyde And Glyoxal

Graphitic carbon nitride?g-C₃N₄?has attracted extensive attention in the field of photocatalysis due to its non-toxicity,high stability and low cost.However,g-C₃N₄also faces the problems of low visible light utilization,easy recombination of photogenerated electrons and holes,and insufficient photocatalytic reduction ability.In this paper,the photocatalytic hydrogen production and Cr?VI?reduction ability of g-C₃N₄ were further enhanced by three ways:synthesis of nitrogen-deficient g-C₃N₄,carbon doping and nitrogen defects co-modified g-C₃N₄,oxygen doping and nitrogen defects co-modified g-C₃N₄.The main research contents are as follows:1.A liquid phase chemical reduction method of formaldehyde was used to synthesize nitrogen-deficient g-C₃N₄?FH-g-C₃N₄?by virtue of the weak reducibility of formaldehyde in weak acidic solution with high yield.The existence of nitrogen defects was confirmed by elemental analysis?EA?,electron paramagnetic resonance?EPR?,X-ray photoelectron spectroscopy?XPS?and energy spectrum?EDS?.The loss of nitrogen at N?C?₃ lattice sites is analyzed by XPS.The nitrogen-deficient sites have extra electrons,which can easily capture photogenerated holes and reduce the recombination of photogenerated electrons and holes.The best sample 30-CN?30 mL formaldehyde solution+30 mL water?photocatalytic reduction of Cr?VI?activity is58%higher than that of H-g-C₃N₄?60 mL water?and P25,and the photocatalytic hydrogen production rate?1.323 mmol/g/h?is twice as high as that of H-g-C₃N₄?0.654 mmol/g/h?.Compared with H-g-C₃N₄,30-CN has a stronger light absorption capacity,a more negative conduction band position,and a stronger reduction capacity.2.A novel carbon doping and nitrogen defects co-modified g-C₃N₄?AH-CN?was prepared by a acetaldehyde-assisted hydrothermal method with high yield.The existence of carbon doping and nitrogen defects was confirmed by EA,EPR,XPS and EDS.UV-Vis DRS analysis shows that,compared with 0A-CN,the bandgap of AH-CN decreases and the absorption of ultraviolet and visible light increases.PL and photoelectrochemical analysis show that AH-CN has a lower recombination rate of photogenerated carriers.UV-Vis DRS and XPS valence band analysis show that50A-CN?50 mL acetaldehyde+10 mL water?has a relatively negative conduction band value,so it has a stronger photocatalytic reduction ability.The photocatalytic activity of the optimum sample 50A-CN for the reduction of Cr?VI?is 39%higher than that of 0A-CN and 41%higher than that of P25.The enhancement of photocatalytic activity is due to the synergistic effect of nitrogen defects and carbon doping.3.A novel oxygen doping and nitrogen defects co-modified g-C₃N₄?GH-CN?was prepared by a glyoxal assisted hydrothermal method with high yield.The presence of oxygen doping and nitrogen defects was confirmed by EA,EPR,XPS and EDS.The absence of nitrogen at NH_x lattice points is analyzed by XPS.The analysis of UV-Vis DRS shows that,compared with 0G-CN?60 mL water?,the bandgap of GH-CN decreases and the absorption of ultraviolet and visible light increases.PL and photoelectrochemical analysis show that GH-CN has a lower recombination rate of photogenerated carriers.The photocatalytic activity of the optimum sample 15G-CN?15 mL glyoxal solution+45 mL water?for the reduction of Cr?VI?is 36%higher than that of 0G-CN and 25%higher than that of P25.The enhancement of photocatalytic activity is due to the synergistic effect of nitrogen defects and oxygen doping.