Modification Of Graphite Carbon Nitride And Its Piezoelectric Catalytic Properties

The development of green and clean new energy and environmental protection technology has become the top priority of today’s society.The piezoelectric catalytic degradation of pollutants based on piezoelectric materials has gradually become a research hotspot in recent years,and was considered to be another efficient catalytic degradation way after photocatalytic degradation of pollutants.Piezoelectric catalysis used mechanical vibration to deform materials and generate built-in electric field.Under the action of built-in electric field force,electrons and holes were separated,so as to catalyze the degradation of pollutants.There were a large number of asymmetric triangular cavities in the structural unit of graphite carbon nitride（g-C₃N₄）,which made it have excellent piezoelectric response and provide space for the regulation of its piezoelectric properties.Taking g-C₃N₄ based materials as the research object,this paper explored the piezoelectric properties,piezoelectric degradation mechanism and piezoelectric catalytic modification of g-C₃N₄ from the aspects of S doping,N defects and construction of active sites.The following research progress has been made:1.Aiming at the effects of morphology,structure and element doping on the piezoelectric properties of g-C₃N₄,four g-C₃N₄ materials were prepared with thiourea,urea,melamine and dicyandiamide as precursors.The piezoelectric properties of g-C₃N₄ synthesized by the four precursors were thiourea>melamine>dicyandiamide>urea.Thanks to the in-situ substitution of S atoms and the promotion of piezoelectric effect by loose interlayer stacking structure,g-C₃N₄ prepared with thiourea as precursor had the best catalytic performance.Under the condition of completely avoiding light,the degradation efficiency of 25 ppm Rh B solution in 120 minutes was 90.4%,and the piezoelectric degradation rate constant was 1.56 times and 3.44 times higher than that of g-C₃N₄ synthesized by melamine and dicyandiamide,respectively.2.In order to study the effect of defects on the piezoelectric properties of g-C₃N₄,g-C₃N₄ with different N defect contents was obtained by adjusting the calcination temperature with thiourea as the precursor.By destroying the structural unit of 3-s-triazine ring,N defect affected the asymmetry of triangular nano holes,resulting in the decline of piezoelectric properties of the samples.The g-C₃N₄ synthesized at 550℃had the least defect content and showed the strongest piezoelectric catalytic activity.The rate constants of Rh B degradation were 1.99,1.39 and 2.63 times higher than those synthesized at 510,530 and 570℃,respectively.Active species capture experiments showed that hydroxyl radicals played a leading role in the process of piezoelectric degradation of Rh B.3.For the sake of improving the carrier separation efficiency in the process of g-C₃N₄ piezoelectric catalysis,Pt particles were introduced into the surface of g-C₃N₄ by photo-deposition.The effect of Pt particles on the surface modification of samples was studied,and the piezoelectric catalytic reaction mechanism of Pt/g-C₃N₄ composites was proposed.The deposition of Pt particles on the surface of g-C₃N₄ formed a metal semiconductor Schottky junction with g-C₃N₄,which could promote the separation of carrier current and enhance the piezoelectric catalytic performance of the sample.The degradation rate of Rh B by optimized Pt/g-C₃N₄ reached 90.9%within 75 min,and the first-order reaction rate constant was 1.40 times that of pure g-C₃N₄.