| Solar energy is one of the most important potential clean energy to alleviate the pressure of the present climate, energy and environmental crises. However, the practical utilization of solar energy is restricted because of its lower energy density. Semiconductor photocatalysis is an effective way to utilize the solar energy to degrade pollutants, produce hydrogen energy and reduce the carbon dioxide, which is promising to solve the energy and pollution issues. Photocatalytic technology has come a long way and great efforts have been made due to its potential application since the first discovered by Fujishima in 1972. For the moment, three key factors affect the photocatalytic activities and limit the practical application:(1) response range towards light; (2) charge separation and migration; (3) quantum yield. The development of photocatalysts with high separation efficiency of photo-generated carriers, high quantum efficiency, and wide spectrum absorption is really needed for the scientific research and practical applications. ZnS is an outstanding photocatalyst that exhibits high photocatalytic activity. However the bandgap of ZnS is too large to absorb visible light, which greatly reduces the utilization rate of solar energy; besides, the photo-generated carriers are easily recombined in ZnS, which decreases the quantum efficiency and photocatalytic activity. The main challenges to modify ZnS nano-semiconductors are enhancing the separation efficiency of photo-induced carriers and expanding the light absorption range.In this thesis, our studies are focused on energy band engineering and phase junction construction based on ZnS. Firstly, we studied photocatalytic activity of ZnS nanomaterial with compound phase prepared from zinc and sulfur powder by a hydrothermal method in concentrated NaOH solution of different concentration. Then, we synthesized two different phases Mn-Zn-S solid solutions (Mn0.05Zn0.95S and Mno.6Zno.4S). At last, we studied the effect of Mn ion doping on the energy band gap and photocatalytic activity of ZnS.In chapter one, we briefly summarized the research background, basic theory, applications, and restriction factors of semiconductor photocatalysis. Then, we reviewed the main modification methods to improve the photocatalytic activities. Next, we primarily focused on the base material ZnS, describing the crystal structure, preparation methods, research status and application in photocatalysis. We further discussed the modifying ways and the latest progress on ZnS based photocatalysts. Finally, the significance and the main research contents of this thesis were proposed.In chapter 2, multiple phase ZnS with sphalerite and wurtzite were prepared by a hydrothermal method, and the phase composition change with increasing the concentration of NaOH solution. We characterized the samples’ crystalline structures, morphologies, absorption properties by XRD, SEM, TEM, DRS. And the photocatalytic activities were measured by water splitting under visible light irradiation. We analyzed the mechanism of phase transition under concentrated NaOH solution, and disscussed the benift of phase junction and charge transfer between the interfaces, which promote the improvement of photocatalytic performance.In chapter 3, two different phases Mn-Zn-S solid solutions (Mn0.05Zn0.95S and Mn0.6Zn0.4S) were synthesized via a facile hydrothermal route. The phase structures, morphologies, chemical states and optical absorption properties were characterized. Both of the cubic and hexagonal MnxZn1-xS solid solutions exhibited increased absorption in the visible-light range. Their photocatalytic activities were evaluated by water splitting and the degradation of MB under visible light irradiation. Both of the solid solutions displayed excellent visible light photocatalysis properties than pure ZnS. Compared with ZnS, both Mn-Zn-S solid solutions can absorb visible light to produce electron-hole pairs, participating in the photoreactions. The cubic Mn0.05Zn0.95S exhibits higher photocatalytic efficiency than that of hexagonal Mno.6Zno.4S. The enhanced photocatalytic activities are attributed to the higher surface area, more appropriate content of Mn atoms and more positive VB potential. Thus, the Mn-Zn-S solid solutions are promising photocatalysts sensitive to visible light and can be used in water splitting and purification in future. Otherwise, we discussed the influence on the solid solubility and crystal structure by changing the hydrothermal temperature and the initial ratio of raw materialsIn chapter 4, we prepared Mn-ZnS microspheres with a series of Mn-doping concentration via a facile solvothermal route in the alcohol and water mixed solvent, and the doping concentration was tunable. We discussed the phase structures, morphologies, absorption properties and photocatalytic activities, and the optimal proportion of Mn is determined. The as-prepared Mn-doped ZnS exhibited the best photocatalytic performance at 7% than that of pure ZnS. The Mn-doped ZnS samples exhibited broader visible light absorption region with the increasing of Mn content. Their photocatalytic activities were evaluated by H2 production from water and reducing Cr6+ under visible light irradiation. It was supporting that the doping of Mn2+ introduced new states to narrow the band gap from the results of the state density calculation, and Mn2+ served as shallow trapping sites of photogenerated carriers to prolong the recombination time of photo-generated carries. The enhancement in photocatalytic activity can be attributed to the expansion of light absorption region and the increase in life time of photogenerated carriers. Thus, the photocatalytic activity of ZnS can be improved in the visible light region by the introduction of Mn ions and Mn-doped microspheres are promising photocatalysts for H2 production and environmental problems in future.In chapter 5, the summary and prospect to all the research in this thesis were provided. We analyzed the main conclusions and results obtained, discussed some unresolved problems and pointed out the promising plan for further research.In summary, it’s important to extend the visible light absorption of ZnS and promote the separation efficiency of photo-generated carriers. Multiple phases construction, solid solution, and element doping are effective modification means to improve the photocatalytic performance. In this thesis, we mainly explored the enhanced visible light photocatalytic performances of ZnS, and interpreted the possible mechanism. |
PDF Full Text Request