| In the 21 st century, energy crisis and environmental contamination have become serious challenge facing our human beings. Preparation of inexpensive and high-efficiency photocatalysts is becoming an urgent research field for scientists as photolysis has great potential in utilizing solar energy. Recent years, a metal-free photocalyst, g-C3N4 has attracted numerous attention from the public. However, the low surface area, narrow light absorption edge and low conductivity of the bulk g-C3N4 hinder its further application. Meanwhile, ultrathin 2D nanosheets has been widely used in electronic, catalysis, chemical or bio- sensor, supercapitor, and energy storage, etc., for their high surface area and unique properties. Therefore, synthesis of g-C3N4 nanosheets has become a research hotpot. The pioneer methods applied in preparing g-C3N4 nanosheets are of time-consuming process, low yield and high pollution, thus more facile and effective methods for preparing ultrathin g-C3N4 nanosheets with high quality and large quantity are highly desirable. Besides, the exfoliation mechanism of g-C3N4 during the synthesis process is far from understood. Moreover, the photoactivity of the g-C3N4 nanosheets prepared through chemical exfoliation is expected for further study.In this thesis, the literature related to the Photolysis, g-C3N4 and g-C3N4 nanosheets are first reviewed. Based on these reported work, two novel methods are presented and a series of characteristic methods are conducted for the studying of the structure of g-C3N4 nanosheets, the corresponding exfoliation mechanism and their photoactivity. The major results of this study are as follows:(1) Disintegration-Exfoliation(two steps) method was used for the preparation of large-aspect-ratio g-C3N4 nanosheets with an average lateral size of 1μm, a thickness of 2.5 nm and a specific surface area of 80 m2 g-1 for the first time. And the final yield of this method was 60%. The characterized result showed that the chemical structure of g-C3N4 was retained in the exfoliation process and the bandgap of the nanosheets increased as the result of the quantum confinement effect. What’s more, it is obvious that the photogenerated electron-hole pairs could separate and transport in the nanosheets more efficiently. The study of the preparation process showed that in the acid treatment process, g-C3N4 was protoned, disintegrated and partially delaminated. In the following sonification process, the disintegrated g-C3N4 was further exfoliated to the large-aspect-ratio g-C3N4 nanosheets. The photocatalytic production tests revealed that the hydrogen evolution rates of g-C3N4 nanosheets were 5.4 times and 3.1 times higher than that of bulk g-C3N4 under full sunlight and visible light, respectively. Moreover, the reaction-rate constants of the nanosheets were 2.6 times and 2.0 times higher than that of bulk under sunlight and visible light, respectively, in the potodegradation tests.(2) Water-added(one step) method was introduced for the synthesis of small-lateral-sized g-C3N4 nanosheets with an average lateral size of 70 nm, a thickness of 2.5 nm and a specific surface area of 86.29 m2 g-1 for the first time. And the process lasted for only about 30 min, while the final yield of this method was up to 70%. The characterized result showed that the chemical structure of g-C3N4 almost stay the same during exfoliation process and the bandgap of the nanosheets obtained in this method increased larger than the large-aspect-ratio nanosheets prepared in the previous method. Moreove, the separation and transport ability of the photogenerated electron-hole pairs in the nanosheets also showed to be higher than the bulk g-C3N4. The analysis of the synthetic process reflects concentrated sulfuric acid served as intercalation agent and exfoliation agent, and when water was added into this mixture, the rapid exothermic effect led to partial delamination and grain disintegration of g-C3N4 by the dissociation and oxidation of the weak defects. And by controlling the amount of water added, the degree of the disintegration and exfoliation of g-C3N4 could be manipulated. More importantly, the intercalation of H2SO4 into the interlayer space of g-C3N4 will accelerate the disintegration and exfoliation of g-C3N4 in return, and thus g-C3N4 nanosheets could be obtained in a very short time. The photocatalytic tests showed that the hydrogen evolution rates and the phtodegradation reaction-rate constant of nanosheets were 10 times and 1.7 times higher than that of bulk g-C3N4 under visible light, respectively. |
PDF Full Text Request