Synthesis, Characterization And Photocatalytic Performance Of Non-metal Doped Ultrathin BiOCl Nanosheets

Energy crisis and environmental pollution caused by rapid development of global economy have become the focus of the world attention. Semiconductor photocatalysis, one of the most valuable technology, as a potential solution to the worldwide energy shortage and environmental pollution problems, has attracted loads of attention, since it is an environmental-friendly method to convert carbon dioxide (CO2) or nitrogen (N2) to reusable chemical feedstock (CH4, HCOOH, CO, NH3, and so on) under mild condition. Unfortunately, up to now, the total photocatalytic reduction efficiency is relatively low and needs further promotion. The high efficiency of photocatalytic CO2 and N2 reduction are still hindered by the following three reasons:(1) the majority of photocatalysts, such as metal oxides, sulfides, and their mixed solid solutions, are limited by their short of surface adsorption sites for efficient CO2 or N2 molecules adsorption; (2) the thermodynamical energy of activation linear CO2 or N2 molecules is relative high, which is beyond of the reach of traditional photocatalysts, leading to a poor activation or reduction ability; (3) during the photocatalytic process, the majority of photocatalysts hold a low efficiency in the solar energy utilization and photo-induced carrier separation. Therefore, in this thesis, we focus on fabrication of the photocatalysts with high activation by controlling their electronic structure, redox potential and adsorption & reactive sites. The details are summarized briefly as follows:1. In the first section, we developed a new top-down strategy to controllable synthesize nitrogen doped ultrathin BiOCl nanosheets with high efficiency in photocatalytic CO2 reduction and systematically investigated the reduction mechanism. At first, DFT calculations were employed to study the electronic structures and surface charge distribution of nitrogen doped ultrathin BiOCl nanosheets. It was clearly found that the electron density surrounding doped nitrogen atoms was higher than that of other atoms on the surface, representing that the doped nitrogen atom could acted as Lewis Base site to efficiently adsorb CO2 molecular. Based on above analysis, we checked the progress of CO2 adsorption, activation and reaction over the doped nitrogen from theoretical perspectives. As a result, the carbon atom in CO2 molecular adsorbed over the doped nitrogen atom by forming a "C-N" 6 chemical bond. After the adsorption of CO2 molecular, the electrons partly transferred from the doped nitrogen to CO2 to active adsorbed CO2 molecular, which is beneficial for lowering the energy barrier for the single electron reduction process of CO2. On basis of the DFT calculation results, we designed a new top-down strategy to synthesized ultrathin BiOCl nanosheets with abundant strong Lewis basic nitrogen sites and demonstrated its efficient CO2 reduction performance under visible light in the absence of any organic scavengers. Advanced characteristic methods, such as TEM, AFM, valance XPS and DRS et al., were used to characterize the as-prepared nitrogen doped ultrathin BiOCl nanosheets. In addition, comprehensive experimental methods, including in-situ FT-IR, XPS and so on, were applied to monitor the specific CO2 adsorption and reduction progress visually over the doped nitrogen. As we expected, the designed surface Lewis basic nitrogen sites showed an excellent CO2 adsorption capacity, and the ultrathin BiOCl crystal framework exhibited a high reduction ability to overcome the forbidden thermodynamical barrier of CO2 reduction with a required reduction potential of-1.9 V. This work might open up a new way to design high efficient artificial photocatalysts for CO2 reduction.2.In the second section, we demonstrated that the carbon doped BiOCl nanosheets synthesized by top-down strategy held a high efficiency in photocatalytic N2 fixation. In consideration of the unoccupied 2p orbital of doped carbon atoms, we studied the photocatalytic mechanism of N2 fixation over carbon doped BiOCl nanosheets. First, we used DFT calculations to check the N2 adsorption & activation progress on the carbon doped BiOCl surface, we found that the N2 molecular shows a spontaneous adsorption and activation trend over the doped carbon sites. Then, we successfully synthesized carbon doped ultrathin BiOCl nanosheets by using the proposed top-down strategy and investigated the inner mechanism of N2 fixation performance enhancement in detail. All the experimental results depicted that the chemical adsorption performance of N2 had been dramatically improved as well as the electroconductivity of as-prepared samples after introducing dopant of carbon atom, which could accelerate the separation and transfer of e-/h+ pairs, giving rise to a significant boost of N2 fixation ability.