CVD Growth And Properties Of Transition Metal Sulfide Nanostructures

Two-dimensional transition metal dichalcogenides（TMDs）have attracted extensive attention and have broad application prospects in photocatalysis,solar cells,and other fields due to their unique structures and excellent properties.Chemical vapor deposition（CVD）has become the preferred method due to its advantages of preparing TMDs with high crystallinity and controllable number of layers.This method usually uses the reaction of metal oxides（MO_X）with sulfur vapor to obtain the corresponding TMDs materials.During this process,the present of oxygen is usually considered to hinder the growth of the corresponding metal sulfides（MS₂）.However,the influence of oxygen in this process has not been systematically studied.In this thesis,the morphology control effect of oxygen in the CVD preparation of WS₂was explored in detail.Besides,Ti O₂/WS₂nanocomposites were also prepared by oxygen-controlled CVD.The obtained samples were analyzed by various characterization methods and they exhibited excellent performance in photocatalytic degradation.The main results are listed as follows:（1）Flower-like WS₂nanosheets were prepared by oxygen-regulated CVD growth.It was found that a certain concentration of oxygen played a key role in the growth of these nanosheets.Oxygen can tear the outer wrapping WS₂layer and promote the exposure,evaporation and transfer of WO₃in the core.Finally,many WS₂nanosheets with a thickness of several nanometers were assembled from the same center so that flower-like WS₂microspheres（F-WS₂）with a diameter of～1-4μm were obtained.Only WS₂nanoparticles（N-WS₂）are obtained without oxygen flow.The thinner thickness of the nanosheets helps to increase the specific surface area to provide more active sites,while the flat surface enhances the migration,transport and separation of photogenerated carriers.Therefore,F-WS₂exhibits excellent performance in photocatalytic tests,better than that of N-WS₂and nano-WO₃powders.This method confirms the feasibility of oxygen-regulated growth of TMDs-like materials and is of great significance for the development of efficient photocatalysts.（2）Titanium tetrachloride（Ti Cl₄）solution and ammonium tungstate solution were hydrothermally reacted to obtain Ti O₂/WO₃powder.The Ti O₂/WS₂nanocomposites were grown by an oxygen-regulated CVD method.The photocatalytic test results show that the performance of Ti O₂/WS₂has been significantly improved compared with that of Ti O₂/WO₃.This attributed to the formation of the heterojunction between Ti O₂and WS₂,which promotes the separation of photogenerated carriers and effectively inhibits the recombination of photogenerated carriers.Besides,the thinner thickness of WS₂nanosheets helps to increase the specific surface area and provides more active sites for the absorption and degradation of pollutions.At the same time,the flat surface of the nanosheets also enhances thetransport processes of photogenerated carriers,effectively reducing the recombination of photogenerated carriers.Compared with the traditional way of preparing nanosheets by using high pressure reactor,this method is simple and easy to operate,which provides a new way for the preparation of microsphere composite structures.