As the energy support of continuous development of human society, coal, oil and other fossil fuels which are not renewable resources in a short time, with the rapid consumption of human society development, has been difficult to support the further development of human society. Meanwhile, the consumption of fossil energy brought a series of environmental problems; therefore, looking for the clean and renewable alternative energy has become an urgent need. Solar energy is one of the most abundant renewable energy in the world, but its own low energy density hinders its direct utilization. Therefore, how to make use of the solar fuel has been becoming the hot topic for research workers. As the intermediate medium for the conversion of solar energy into the high energy density energy, photocatalysts play an important role in energy conversion process. Conventional photocatalysts, such as metal catalysts: TiO2, MoS2, ZnO, CdS, has obtained excellent catalytic activities, but still faces some challenges, such as: the stability of catalyst (photocorrosion), the limited visible light absorption (mainly UV absorption), the environmental toxicity of the catalyst and so on.
Graphitic carbon nitride (g-C3N4) is a new type of metal-free semiconductor polymer. It has huge potential in (photo)catalytic environment purification, water splitting and photoelectric conversion due to its high physical and chemical stability, low-cost and unique electronic band structure (2.7eV). However, the bulk g-C3N4 catalyst still faces some problems, such as low surface area, limited visible light absorption and rapid recombination of photogenerated carriers, which severely restricted its photocatalytic performance. How to further improve its photocatalytic activities is an urgent problem. In order to overcome these restricted factors, some functional strategies include atomic doping, micro/nanostructure design, crystalline/defect sites control and construction of heterojunction structure. The application of these strategies significantly improved the photocatalytic activities of g-C3N4. However, these methods still had some disadvantages, such as complex preparation process, high cost and environmental pollution. These problems restricted their further application. Therefore, looking for economic and environmental functional method is our urgent requirements. In this paper, we aim to realize the the wide application of g-C3N4 in the photocatalysis field by exploring the simple and effective functionalization method. Based on this, we supplemented and developed the green template, proton induced, construction of homogeneous heterojunction and iron-nitrogen doped carbon material composite to effectively to improve the photoelectric conversion efficiency and photocatalyitic activities of g-C3N4. The main research contents are listed as follows:
1. The porous g-C3N4 was prepared through CaCO3 template method and the photoelectric conversion efficiency was effectively improved by the increase of active sites. The porous g-C3N4 was often prepared by using commercially available SiO2 template. This strategy can obviously improve the photocatalytic performance of g-C3N4. However, the removal of SiO2 template often requires HF/NH4F solution, which not only brings a certain risk and environmental pollution, but also destructs the structure of catalyst. Meanwhile, it is not conductive to large-scale application due to the high cost of commercial SiO2. In this work, the high photoelectric conversion activity of porous g-C3N4 has been successfully prepared by introduction of industrial CaCO3 nanoparticles. This method is economic and environmental, which provides an effective way for large-scale preparation of high efficient photocatalyst.
2. The high efficient g-C3N4 catalyst with high crystallinity was prepared by using proton-induced hydrogen bond ordered rearrangement of intermediate structure units to regulate the crystallinity/defect sites under acidic conditions. Based on the polymerization process of g-C3N4, we first obtained ordered intermediate structure units through proton-induced hydrogen bond rearrangement, and then secondary thermal polymerization for anisotropic g-C3N4. The ordered structure improves the crystallinity of catalyst and reduces the number of the defect sites. Therefore, it effectively inhabits the rapid recombination of photogenerated carriers when taken the defect sites as recombination center. It greatly improves the photocatalytic performance, such as: the fast realization of the sewage purification treatment. Thus, this method shows that the photocatalytic activity can be enhanced by regulating the crystallinity/defect sites.
3. The heterojunction structure was constructed based on the small electronic structure difference due to the different morphology of g-C3N4. The hydrogen evolution rate was enhanced by effectively suppressing the recombination of photogenerated electrons and holes. Different from the previously reported heterojunction structure (TiO2/g-C3N4, BiVO4/ g-C3N4), we constructed homogeneous heterojunction structure based on the different morphology of g-C3N4 (nanosheet-like and nanoparticles). The successful construction of homogeneous heterojunction based on the small electronic structure difference due to the morphology difference. Compared with the reported structure, the interface of heterojunction was further optimized due to the similar work function, lattice structure and composition. Therefore, the hydrogen evolution rate was significantly improved due to the more effective separation of photogenerated carriers.
4. The Fe-Mp-N-C/g-C3N4 composite was fabricated by simple wet chemistry and its hydrogen evolution performance was improved remarkably. The co-doping of iron and nitrogen not only improved the conductivity of carbon material, but also enhanced its metal characterization. The improved conductivity of g-C3N4 composites accelerated the migration of photogenerated electrons from photocatalyst to the surface of carbon material. Meanwhile, the enhanced metal characterization further suppressed the recombination of photogenerated carriers through Schottky barriers. Both of them promoted the improvement of photocatalytic activities.