Investigation Of Photocatalytic Activities Of Ag₂O-ZnO Hybrid And Modified Graphitic Carbon Nitride

Since 21 st century, energy crisis and environment pollution have become more and more serious with population growth. Based on the analysis and statistics by scientists, the energy consumption in 2050 will increase by twice compared to current amount. However, our major energy is still limited and mostly comes from no-renewable fossil fuels. At the same time, there will be lots of organic waste from industrial production process, which is toxic or not directly bio-degradation. Without effective treatment, the waste dyes will result in water contamination and threaten our health.When semiconductors irradiated by the sunlight, the solar energy can be changed into chemical energy, which can be used to degradation of organic dyes and obtain hydrogen by water-splitting at the same time. Thus, both the energy and environmental issues can be simultaneously solved by the photocatalytic technology. Solar energy used in the photocatalytic technology is the most abundant, cheapest and cleanest energy in the world. Meanwhile, there are no new harmful substances or secondary pollutants produced under photo-degradation. And hydrogen, produced by photocatalytic reaction, is a clean secondary energy with high-energy density. Since semiconductor titanium dioxide was used in water-splitting from 1972, more and more researchers have focused on this issue, but the photocatalytic technology still does not satisfy the requirements of the practical application. There are lots of problems when the pure semiconductor materials are used in photocatalytic reactions, such as low efficiency and survival rate of photo-generated electrons, high cost, poor photo-stability and so on. In order to improve the photocatalytic activity, lots of strategies have been proposed such as modifying surface atoms to narrow the band gap, increasing the surface areas for more catalytic reaction sites, combining with other semiconductors to enhance the photo-generated electrons life. In this thesis, the results are divided into two parts as follow:Firstly, investigation of photocatalytic degradation activity for Ag₂O-ZnO nanohybrid.Pure Ag₂ O, as a narrow band gap semiconductor, can absorb visible light and shows a good performance in photocatalytic degradation. However, as a photocatalyst, Ag₂ O is too expensive for practical application. ZnO is also a good photocatalyst with very stable and high photoelectric property, which is widely used for degradation of different kinds of organic pollutants. However, ZnO can only absorb ultraviolet light which reduces the use of sunlight. We have synthesized Ag₂O-ZnO nanohybrid with a coprecipitation-assisted hydrothermal method, where this obtained nanohybrid has smaller size and possesses advantages of both semiconductors. Partial substitution of Ag₂ O by ZnO in the hybrid of Ag₂O-ZnO not only effectively lowers the cost of the catalyst but also greatly enhances its photocatalytic activity. Compared with pure Ag₂ O, ZnO, a physical mixture of two semiconductors, Ag₂O-ZnO hybrid shows the best activity under visible light or ultraviolet light irradiation.Secondly, investigation of photocatalytic hydrogen activities of different kinds of modified graphite carbon nitride（g-C₃N₄）.Polymer semiconductor g-C₃N₄ is a two-dimensional material and shows good photocatalytic activity. As a promising visible light photocatalytic hydrogen catalyst, g-C₃N₄ has a proper band position and band gap, excellent thermal and chemical stabilities. However, pure g-C₃N₄ has low photocatalytic activity in hydrogen evolution, which is not satisfied with our practical application and requires us to seek for the methods to improve its photocatalytic activity.1) We have firstly synthesized a new g-C₃N₄ with thermal polymerization method, and the precursor is melamine treated by acetic acid. The synthesis method is simple and easy to operate, and no toxic chemicals have been used during synthesis process. This new g-C₃N₄ catalyst holds porous structures and smaller size that larger surface area can provide more reaction sites for photocatalytic reaction. Meanwhile, the visible light adsorption is not decreased compared with that in the pure g-C₃N₄. Therefore, the new catalyst shows a better photocatalytic hydrogen activity, and the hydrogen evolution rate of the new catalyst can reach up to about 2.7 times that of the pure g-C₃N₄. Meanwhile, the new catalyst shows a good stability, which the hydrogen evolution rate of synthesized new g-C₃N₄ has not changed during a 12 h reaction period.2) We have altered the bandgap of g-C₃N₄ with elemental doping. First, we have synthesized a brown g-C₃N₄ by hydrothermal reaction combined with thermal polymerization process. This brown g-C₃N₄ has a narrower band gap as well as a lower recombination rate of photogenerated electron-hole pairs compared with pure g-C₃N₄, where the photocatalytic hydrogen evolution rate was improved up to about 5.1 times that of the pure g-C₃N₄. Later, we have synthesized K modified g-C₃N₄, where the band gap was also reduced for using more solar energy. Additional K of K-g-C₃N₄ reduces the bandgap of g-C₃N₄, which enhances the visible light absorption of g-C₃N₄. And the g-C₃N₄/g-C₃N₄ heterojunction in this new catalyst can transfer the charges freely and hinder photogenerated electron-hole recombination. Thus the photocatalytic hydrogen activity is greatly enhanced compared with the pure g-C₃N₄ under the same conditions, where the hydrogen evolution rate of K-g-C₃N₄ is about 13.1 times higher than that of pure g-C₃N₄ under visible light irradiation.3) In order to further enhance the photocatalytic efficiency of g-C₃N₄ in hydrogen evolution, we have synthesized Ag₂ O modified g-C₃N₄. The new catalyst was synthesized with a simple hydrothermal reaction, shown super high photocatalytic hydrogen activity. Ag₂ O can hinder the photogenerated electron-hole recombination rate from g-C₃N₄ under photocatalytic reaction. Under the same conditions, Ag₂ O modified g-C₃N₄ can generate hydrogen about 274 times higher than the pure g-C₃N₄, even higher than the same amount of Pt modified g-C₃N₄.