| Nowadays, people’s living standard has been greatly improved, at the same time, people also has to face the environment (i.e. water and air pollutions) and energy crisis. To solve the issues, one of promising ways is to find a clean, environmentally friendly, low cost, and renewable energy. Recently, hydrogen produced through photocatalytic water splitting using solar energy has attracted much attention. Moreover, to detec-t water pollution, possible effective methods are highly desired to clean heavy metal ions in water. Note that, with the rapid development of computor hardwares and sci-entific softwares, first principles calculations have been widely and successfully used in understanding experimental measurements as well as designing novel materials and devices. This thesis contains the following four chapters, and focuses on detecting heavy metal ions on nanomaterials and proposing novel photocatalysts.In Chapter 1, density functional theory (DFT) and related knowledge related to this thesis is briefly introduced, including several approximations and three kind of functionals for exchange and correlation. The DFT-based computational packages, i.e. VASP, are list at the end of this chapter.The Chapter 2, we focus on facet-dependent electrochemical performance of SnO2 nanocrystals toward heavy metal ions (HMIs). In experiments, SnO2 nanocrystals dis-play an obvious facet dependent electrochemical behavior for two HMIs (Cd and Pb), and the performance of SnO2 (110) surface is better than that of (221) surface. To un-derstand experimental observations, we perform extensive DFT calculations. Through calculating the adsorption energies, the charge transfer, diffusive behaviors, and tran-sition states, we find that, due to the relative short Pb-0 (Cd-O) bond length, the ad-sorption energies of HMIs on SnO2 (221) are significantly larger than that on (110) surface, indicating that more HMIs will be adsorbed on (221) surface. However, the relative low transition-state barrier results in the HMIs (Pb and Cd) fast diffusing on (110) surface. Clearly, SnO2(221) surface can capture more HMIs, while they diffuse more easily on (110) surface, consistent well with experimental observations. This kind of combination of experimental measurements and theoretical simulations is a promising strategy for enhancing electrochemical performance of nanoscale materials via exposing different well-defined facets.In Chapter 3, we propose a hybrid g-C3N4/MoS2 nanocomposite. The calculated electronic structure results show that it is a type-II band alignment between g-C3N4 monolayer and SnS2 sheet, and a polarized field within the interface region coming from the charge transfer between g-C3N4 and MoS2 is benefit for the separation of photogenerated carriers. In addition, this proposed layered nanocomposite is a good visible light harvesting semiconductor, and the band edge positions matching well with the redox potentials of water. These findings suggest that the proposed g-C3N4/MoS2 nanocomposite is a promising candidates for photocatalytic water splitting.In Chapter 4, we examine electronic structure of blue phosphorus. Firstly, the stability of blue phosphorus is examined, and we find that blue phosphorus is stable since there is not virtual vibration frequency in the calculated phonon spectra. The in-direct band gap of blue phosphorus is predicted to be 2.68 eV at the HSE06 hybrid functional, which is good for the visible light harvesting. Interestingly, straining is an effective method to modify the band structures of blue phosphorus, and the positions of the valence band maximum and the conduction band minimum of blue phosphor match well with the redox potentials of water. That is to say, blue phosphorus is a promising novel photocatalyst for visible light water splitting. |
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