Synthesis And Property Of Carbon Nitride Based Photocatalytic Materials

Semiconductor photocatalytic technology could use solar energy for degrading many organic pollutants in water and air, splitting water into H2, reducing CO2 and organic synthesis through photocatalysts. Hence, it exhibits potential application for treating environmental pollution and solving energy dilemma. However, traditional semiconductor photocatalysts, including Ti O2 and Zn O, have some shortages, including low quantum efficiency, easy deactivation and poor solar energy utilization, which seriously restrict the large-scale applications of photocatalytic technology. Therefore, it is a urgent and challengeable topic to exploit novel and efficient photocatalysts in the photocatalytic fields.Graphitic carbon nitride（g-C₃N₄） has low band gap（2.7 e V） and specific electronic structure, and exhibits the excellent photocatalytic capacities for splitting water to H2 and degrading various organic pollutants under visible-light irradiation, hence, it is an ideal photocatalyst candidate with excellent physicochemical properties. Nevertheless, low surface area and high combination rate of photogenerated electrons and holes seriously limit its photocatalytic activity. Therefore, in this paper, we prepared g-C₃N₄ based composites possessing high seperation rates for charges by coupling with semiconductor and mesoporous carbon, and used in site template methods, such as hard template and bubble templates, to construct the porous g-C₃N₄ for increasing surface area, finally, we produced semiconductor modified porous g-C₃N₄ composites, which exhibited high surface area and low recombination efficiency of photogenerated electrons and holes. The detailed contents of this thesis were as follows:（1） A novel Ag2O/g-C₃N₄ composite catalyst was prepared. Ag2 O with sizes of 5¹0 nm were evenly attached on the surface of g-C₃N₄, which displays stronger visible absorption and effective separation of photogenerated electron–hole pairs. When the mass ratio of Ag2 O and g-C₃N₄ was 1:4, as-prepared samples exhibited optimal photocatalytic activity, its rate constant was approximately 4.5 times larger than that over g-C₃N₄ and 2.5 times larger than that over pure Ag2 O for the degradation of Rhodamine B（Rh B）. In addition, novel ordered mesoporous carbon/g-C₃N₄（OMC/g-C₃N₄） composites with larger surface area, enhanced visible light adsorption, and efficient separation of photogenerated electrons and holes were facilely synthesized and exhibited improved photocatalytic capacity. Morevoer, a possible mechanism for the photodegradation process was proposed combined with the experimental data.（2） Mesoporous graphitic carbon nitride（mg-C₃N₄） was prepared by the polyconsendation of cyanamide or urea, using the silica in situ formed from acid hydrolysis of tetraethylorthosilicate（TEOS） as mesoporous template. Compared with bulk g-C₃N₄, the resultant mg-C₃N₄ possessed larger surface area and high separation rate of photoinduced electron-hole pairs, resulting in the enhanced photocatalytic activity for degrading pollutants and splitting water to H2. Meanwhile, effect of TEOS adding amounts on surface area and photocatalytic activity were thoroughly investigated.（3） Porous g-C₃N₄ with large surface area was prepared through a simple and effective bubble template approach. Firstly, porous g-C₃N₄ with various surface areas were synthesized by polyconsendation of guanidine hydrochloride at different polymerization temperature. During the polyconsendation process, guanidine hydrochloride molecules polymerized into mesoporous g-C₃N₄ by eliminating HCl and NH3, which acted as mesoporous templates. Rh B was selected as model pollutant for determining its photacatalytic property, the experimental results indicated that the surface areas and activity can be further enhanced by increasing the condensation temperature. Secondly, porous g-C₃N₄ was prepared by thermal polymerization of the mixture using several ammonium salts as the bubble templates and melamine as the precursor. During the process, ammonium salts decomposed to gases, which acted as the templates for porous g-C₃N₄. Moreover, the versatility of the preparation method was detected. Besides, the structure and feature of as-prepared porous g-C₃N₄ were well characterized. And the photocatalytic activity for degrading pollutants and splitting water to H2, together with the mechanism of photocatalytic reaction were investigated. The relationship of porous structure and photocatalytic activity was revealed.（4） A novel Ag2 Cr O4/g-C₃N₄ composite catalyst was prepared by the coprecipitation method. And porous g-C₃N₄（pg-C₃N₄） with large surface area was prepared through polyconsendation of guanidine hydrochloride. Ag2 Cr O4 with sizes of 2⁵ nm were evenly loaded on the surface of pg-C₃N₄, causing the composite display stronger visible absorption and effective separation of photogenerated electron-hole pairs. Thus, excellent photocatalytic activities and stability for the degradation of Rhodamine B（Rh B） and phenol under visible light were obtained.