Controllable Preparation Of Ultrathin Two-Dimensional Bismuth Oxyhalides Nanomaterials And Photocatalytic Properties

The two-dimensional nanomaterial is of unique electronic structure that is different from the bulk material owing to the unique properties associated with their ultra-thin thickness and two-dimensional planar morphology. Thus, not only the intrinsic characteristics are enhanced, but also new properties emerged, which provides new idea for exploring2D semiconductors in terms of improving photocatalytic performance. This dissertation focused on the studies of the relationship between the nanostructured2D semiconductor material and its photocatalytic properties. Layered BiOX (X=Br, C1) was selected as the object and ultrathin BiOX with atomic level thickness was achieved via a facile solvethermal method. Change in surface structure and bandgap caused by the atomic thickness and two-dimensional characteristic was systemically studied towards the effect of photocatalytic activity. Furthermore, incorporation with graphene for fabricating2D hetero-nanostructured composites of graphene/BiOX as a robust photocatalyst was studied. Details are listed as below:1. The photocatalytic performance could be significantly enhanced by adjusting the exposed crystal facets of semiconductor. Ultrathin BiOBr nanosheet was synthesized through a facile solvathermal method. HRTEM and SAED characterization reveals that the exposed facet are indexed to the (001) of BiOBr and the ratio is almost100%. UV-Vis spectroscopy indicates that the reduction in thickness of the nanosheet induces the change of bandgap, in which both conduction band and valance band are up-shift, and is beneficial for the charge separation of the photoelectron and holes. By contrast, BiOBr nanoplate with large thickness but similar2D size was prepared. The results of photocatalytic degradation of RhB reveal that ultrathin BiOBr exhibited higher photocatalytic activity which may be owned to the active (001) facets and the optimized band structure.2. Crystal facet engineering of semiconductors is of growing interest and an important strategy for fine-tuning the solar-driven photocatalytic activity. However, the primary factor in the exposed active facets that determines the photocatalytic property is still elusive. Inspired by the former work, we have experimentally achieved the high solar photocatalytic activity in ultrathin BiOCl nanosheets with almost fully exposed active (001) facets, and provided some new and deep-seated insights into how the defects in the exposed active facets affect the solar-driven photocatalytic property. As the thickness of the nanosheets reduces to atomic scale, the predominant defects change from isolated VBim defects to triple vacancy associates VBi(?)VO(?)VBi(?), which is unambiguously confirmed by the positron annihilation spectra. By virtue of the synergic advantages of enhanced adsorption capability, effective separation of electron-hole pairs and more reductive photoexcited electrons benefited from the VBi(?)VO(?)VBi(?) vacancy associates, the ultrathin BiOCl nanosheets show significantly promoted solar-driven photocatalytic activity even with extremely low photocatalyst loading. The finding of the existence of distinct defects (different from those in bulks) in ultrathin nanosheets undoubtedly leads to new possibilities for photocatalyst design using quasi-two-dimensional materials with high solar-driven photocatalytic activity.3. Based on the graphene of its outstanding charge transport ability and extremely high specific surface area, as well as the special surface structure of the ultrathin BiOCl nanosheet, heterostructured BiOCl-nanosheet/Graphene composites are prepared via simple solution mixing method. Such a composite exhibited improved photocatalytic activity towards dye degradation compared with bare BiOCl nanosheet and BiOCl-nanoplate/Graphene composite. The enhancement could be ascribed to the several aspects:1) the defect of VBi(?)VO(?)VBi(?) in the nanosheet is not only benefital for the dyes adsorption, but also strengths the interaction with graphene that improve the interface photoelectrons transfer.2) the more negative conduction band edge potential induced by the decreased thickness motivates the electron transfer process.3) graphene acts as electrons transporter, improving the charge separation efficiency.4). the induction of graphene extends the light adsorption range, promoting the photocatalyst excitation and indirect dye-sensitized degradation process. Overall, integration of graphene with2D semiconductor with exposed facets pave the way for fabricating graphene-based2D nanocomposites as robust photocatalyst.