Structural Design Of Novel Carbon Nitride-based Photocatalytic Materials And Their Multi-field Photocatalytic Performance

The continuous consumption of fossil fuels has brought about increasingly serious environmental pollution and energy shortages,making the survival and development of people face a huge challenge.Over the years,semiconductor photocatalytic technology has been considered as one of the best ways to develop renewable energy and control environmental pollution due to its outstanding performance in removing organic pollutants,photocatalytic hydrogen production,CO2 reduction and so on.Among them,graphitic carbon nitride（g-C3N4）has become a promising material in the field of photocatalysis due to its suitable energy band structure,high stability,non-toxic and non-hazardous,cheap and easy to obtain,etc.However,the photocatalytic activity of g-C3N4 prepared by direct thermal polymerization was low due to its small specific surface area（SBET）,low utilization of visible light and easy recombination of photogenerated electron（e-）and hole（h+）pairs,which seriously restricted its application in practical production.To overcome these shortcomings,a series of g-C3N4-based photocatalytic materials were prepared in this paper through morphological modulation,defect engineering,non-metallic element doping and heterostructure construction.The details and results of the study are as follows:（1）Porous g-C3N4 co-modified with cyano and nitrogen vacancies was prepared by a simple one-step thermal treatment with calcium cyanamide（CaCN2）and dilute hydrochloric acid assisted urea.During heat treatment,CaCN2 loosened the stacking structure of g-C3N4 and formed a thinner tremella structure,which could effectively shorten the migration distance between e-and h+.Its larger SBET can also expose more reactive sites and generate more photogenerated carriers.The introduction of appropriate concentration of cyano and nitrogen vacancies formed a defect energy level below the conduction band,which improved the lightharvesting efficiency and facilitated the separation of photogenerated carriers.Due to the above-mentioned triple synergy,the preferred CCN-0.02 generates more reactive radicals（superoxide radical and hydroxyl）for the photodegradation of organic pollutants,generating more photogenerated electrons for the reduction of H+to H2.Performance tests showed that the photocatalytic activity of CCN-0.02 was greatly enhanced,the hydrogen production rate was 1.7 times that of pure g-C3N4,and the degradation rate of rhodamine B（RhB）was even increased by 7.3 times,under visible light.Additionally,CCN-0.02 also demonstrated good stability and recyclability,maintaining high photocatalytic activity after five cycles in photocatalytic degradation of RhB and hydrogen production cycle experiments.（2）Bulk g-C3N4 was thermally etched in 25%ammonia atmosphere by a combination of conventional air-thermal polymerization and chemical vapor deposition（CVD）.Abundant carbon vacancies were successfully introduced into g-C3N4（MCN-25）.Under visible light irradiation,MCN-25 exhibited excellent photocatalytic activity.Not only can it effectively degrade rhodamine B（RhB）,tetracycline（TC）,and ciprofloxacin（CIP）,but its hydrogen production rate was as high as 9068 umol h-1 g-1,which was 5.7 times higher than that of bulk g-C3N4.The conversion rate of CO2 to CO also increased by 3.2 times.The remarkable improvement of photocatalytic performance was due to the synergistic effect of ultra-thin porous nanosheets and carbon vacancies,which not only increased SBET and provided more active sites for photocatalytic reaction,but also effectively enhanced the visible light absorption.Moreover,carbon vacancies can also capture photogenerated electrons and transfer them to the material surface,thereby improving the separation efficiency of e--h+ pairs.The electrons on the conduction band are involved in the generation of active radicals,hydrogen production and CO2 conversion reactions.（3）S-doped SnO2 QDs/g-C3N4 series composites were in-situ synthesized by one-step thermal polymerization strategy after mixing sulfur powder,urea and stannous chloride hydrate by simple grinding.The presence of S doping and SnO2 particles increased the SBET of g-C3N4,giving it a rich pore structure and broadening the visible response range.Furthermore,the formation of N vacancy and heterojunction effectively inhibited the recombination of e--h+ pairs.Among them,SCN-80Sn composite has the best photocatalytic activity for RhB degradation,and the degradation rate is 7.8 times higher than that of pure gC3N4（CN）.After five RhB degradation cycles,it still had high photocatalytic activity,which was proved by its excellent photostability and reusability.Mechanistic analysis shows that superoxide radical（·O2-）and hydroxyl（·OH）play a major role in the degradation of RhB.In this dissertation,a simple,convenient,and environmentally friendly method was adopted to realize the improvement of the photocatalytic performance of g-C3N4-based materials in various fields,and pointed out a promising direction for its energy conversion and environmental applications.