The Synthesis And First-principles Study Of 2D Structured Nanocomposite

Researches on two-dimensional?2D?materials such as graphene has attracted considerable attention and has been demonstrated to be powerful in next-generation electronic and photonic applications for the rich physical properties and outstanding electronic properties.However,its practical applications were hindered by the lack of a band gap in intrinsic structure,and it is hard to manipulate a sizable one without sacrificing its excellent electrical properties.Therefore,the search for the other alternative inorganic layered materials caused extensive concern.Recently,transitional metal dichalcogenides?TMDs?have attracted considerable interest for their intrinsic semiconductor characteristic and excellent mechanical properties.As a layered material,for example,the monolayer or few-layer MoS₂ structured based field-effect transistors have been reported to have excellent on/off ratio and mobility.The layered TMDs based photodetectors also show high responsivity and fast photoresponse.A single-layered TMD contains three atomic layers with a cation layer sandwiched by two anion layers,which has a significant difference with the one atom layered graphite.The neighboring layers of TMD interact with each other through weak van der Waals interactions and the corresponding electronic structure is significantly influenced by the interlayer interaction.For example,MoS₂ shows band gap ranging from about 1.2 eV with indirect band gap for the bulk structure to about 1.9 eV with direct band gap material as a monolayer.In this academic dissertation,we studied the HER performance of MoS₂ cooperated with graphene.Graphene is consisted of sp2-hybridized carbon atoms and has attracted many attentions for its excellent conductivity,stability and single-layer configuration.Based on the above features,the combination of MoS₂ with graphene should be a promising approach to enhance the durability and integral conductivity for the application in HER.We here analyze the adsorption character based on Gibbs free energy form a dynamic perspective under an excess of negative charge density in detail.The obtained results adequately explain the reason why composite structures show a better HER performance and these analytical methods also offer a certain theoretical foundation for a further study of the other HER catalysts.To verify the theoretical analyses,MoS₂/rGO composite nanostructures were synthesized by a facile hydrothermal method.We synthesized the thin nanoslice structured WS₂@reduced graphene oxide?rGO?composite via a facile hydrothermal synthesis method.The experiment data revealed that the LIBs performance has a link to the corresponding conductivity.The layered structure and morphology of the different structures were characterized by X-ray diffraction?XRD?,field emission scanning electron microscopy?FESEM?and transmission electronmicroscopy?TEM?.The WS₂@rGO composite structure demonstrated significantly enhanced rate capability performance in comparison with pristine WS₂ when used as anode material for lithium-ionbatteries?LIBs?.The specific capacity of the composite showed a capacity of 565 mAh/g after 100 cycles when cycled at 0.1 A/g and it could still deliver a stable capacity of about 337 mAh/g at 2 A/g.Electrochemical impedance spectroscopy?EIS?measurement showed that the synergy effect between WS₂ and rGO could remarkably reduce the contact resistance and improve the corresponding electrochemical performances.In order to analyze and interpret the corresponding results from a theoretical sound perspective,the first principles calculation was further performed to investigate the corresponding inner mechanism of pristine WS₂ and WS₂@graphene composite.For the first time,a systematic study using density functional theory?DFT?has been employed to survey the synergistic effect of a-MoO3@MoS₂ aiming at gaining insights into the role of this heterogeneous structure functioned in the relevant photocatalytic routine.The geometry/electronic structures as well as band edge positions of a-MoO3@MoS₂ composite were computed to explore the characteristic of the heterojunction.It revealed that the established heterogeneous structure could facilitate the separation of the photoinduced carriers into two parts around the interface.The photo-induced electron carriers injected to the conduction band minimum?CBM?of ?-MoO3 from the CBM of MoS₂ while the hole carriers transferred from valence band maximum?VBM?of a-MoO3 to the VBM of MoS₂.This separation process could markedly restrain the photogenerated electron-hole pairs'recombination and was further verified by the photocurrent and photo luminescence?PL? surveys.We fabricated flower-like MoS₂/BiVO₄ composite with heterojunction by a two-step approach.A possible formation mechanism of this heterostructure was investigated.The calculated valence band offset?VBO?and conduction band offset?CBO?of MoS₂/BiVO₄ heterojunction showed that the VBO and CBO of MoS₂/BiVO₄ are 1.4 and 0.3 eV,respectively,implying the formation of well-defined staggered type-II band alignment.The photodegradation of methyleneblue?MB?was adopted to assess the photocatalytic ability of the pristine MoS₂,BiVO₄ and MoS₂/BiVO₄ composites.It exhibited that the MoS₂/BiVO₄ composite structures performed much better than that of the pristine MoS₂ and BiVO₄,which due to the staggered band alignment formed between the two structures.For the first time,the novel dual-petals nanostructured WS₂@MOS₂ heterojunction was fabricated via a facile two-step approach and explored as photocatalyst for the photodegradation of methyleneblue?MB?.In the light of the results obtained form experiments,a reasonable formation mechanism for the nanopetals?NPs?structured WS₂ was proposed,in which the pretreatment of ball milling had played an important role for the formation of WS₂ NPs that subsequently acted as the base material to grow curly MoS₂ sub-NPs.Because the dual-petals nanostructured WS₂@MoS₂ possessed plenty of activesites that originated from its unique structural characteristic with densely stacked MoS₂ nanopetals and an effective separation of photoinduced carriers,it exhibited significantly enhanced photocatalytic activity and obviously exceeded the pristine MoS₂/WS₂.In order to analyze and interpret the corresponding properties from a theoretically sound perspective,we also conducted the first-principles calculations and the mechanisms of elections' transfer were studied detailedly.Credible conclusions achieved and the corresponding analytical methods acted as a bridge between experimental results and calculation conclusions.We believe this original and meaningful work will provide valuable theoretical guidance for the research of hybrid structures in the future.With the booming of experimental progresses,a further theoretical investigation on these TMD based materials have been in the ascendant and many interesting discoveries have been reported.For example,the strain-induced magnetism is observed in monolayer defective MoS₂ and a narrowed energy gap is achieved on the planar and smooth substrate in?-graphyne@MoSe2.These studies proved that TMD based materials as promising and feasible alternatives to graphene for transistors,photovoltaic cells,lithium batteries and photocatalysts applications.