Studies On The Graphene-like Heterostructure And Inorganic-organic Heterogeneous Materials

As a kind of two-dimensional layered nanomaterial,graphene has gained tremendous attention,owing to its novel structures and excellent properties,and it has been applied in various applications including optoelectronics,electrochemical energy conversion and storage and so on.Driven by the emergence of graphene,other typical graphene-like two-dimensional layered nanomaterials,such as hexagonal boron nitride（h-BN）and molybdenum disulfide（MoS₂）,have also aroused people’s extensive research interest.These specific characteristics make it play a crucial role in the coming post-Moore era.The atomically thin van der Waals heterostructures constituted by different layers of two-dimensional nanomaterials are becoming emerging as top candidates for material design and application in the past few years.Therefore,achieving the formation of van der Waals heterostructure between graphene-like two-dimensional layered nanomaterials and graphene will be of great significance for both theoretical research and application value.In this thesis,classic graphene and graphene-like materials such as h-BN and MoS₂ were selected as the research objects.By simple solution process,graphene-like nanosheets were designed to be compounded with other materials to become a novel type of graphene-like heterostructure material.The research focus is on exploring the relationship between structure and property of the as-prepared graphene-like heterostructure materials,improving their electrochemical energy storage and photoelectric performances as well as expanding the application of corresponding flexible devices.The main contents and results were summarized as follow:1.In the beginning,hexagonal boron nitride（h-BN）and Graphene were rapidly exfoliated using liquid phase system which urea/glycerin in a molar ratio of 1:2.Then,the graphene-like heterostructure materials（BN/G）were prepared by self-assembly of exfoliated layers via van der Waals force.The micro morphology of the BN/G was characterized by scanning electron microscopy（SEM）,atomic force microscopy（AFM）and transmission electron microscopy（TEM）.The results confirmed that about 1 um of h-BN nanosheets were uniformly distributed on the surface of few-layerd graphene and stacked into vertical heterostructure.XRD and Raman spectrum were also carried out to further convince that the h-BN and graphene sheets processed few-layered structures and controllable morphology.Besides that,our method is simple and feasible with low cost and energy consumption.The BN/G heterostructure and polyvinylidene fluoride were uniformly mixed at a mass ratio of 9:1 and used as the electrode in the supercapacitor.The energy storage behavior was studied by three-electrode test system.It was found that the specific capacitance of the recycled BN/G were calculated to be134 F/g at current density of 0.5 A/g;meanwhile possessed good stability with 96%of the initial capacitance remaining after 10000 cycles,when the current density rises to10 A/g.These results meet the characteristics of fast charge and slow discharge of electrochemical energy storage device and provides a potential avenue to improve the performance of supercapacitors.2.The preparation of metallic phase molybdenum disulfide（1T-MoS₂）/graphene heterostructure was investigated.Based on the original graphene exfoliated by melamine-assisted liquid-phase system,1T-MoS₂ were synthesized on its surface through a facile one-step hydrothermal method.In the as-prepared graphene-like heterostructure,the 1T-MoS2 nanosheets were homogeneous with few defects.The overall graphene-like heterostructure has excellent electrical conductivity,and this 1T-MoS₂/graphene heterostructure can greatly facilitate the enhancement of the performance of lithium ion batteries.First of all,such an architecture can provide a much larger atomic interfacial contact/interaction between metallic MoS₂ and graphene for lithium-ion batteries（LIBs）than the traditional MoS₂-based nanocomposite with very limited interface contact.Secondly,the intimate and continuous conductive highways between metallic MoS₂ and graphene are beneficial for effectively restraining MoS₂ nanosheets from aggregation and restacking,which helps accommodate volume expansion upon MoS₂ lithiation/delithiation processes,mitigate polysulfide shuttling.Finally,due to melamine-assisted liquid-exfoliated high-quality graphene’s excellent electrical conductivity and low basal-plane defects and oxidation,good adhesion of metallic MoS₂ nanosheets on graphene enable high-efficiency electron/ion transport pathway between MoS₂ and graphene,favoring the long-term cycling and rate performance.As a consequence of these inspiring merits,our novel1T-MoS₂/graphene hybrid graphene-like heterostructure exhibits an outstanding discharge capacity of 667 mAh/g at 2000 mA/g with excellent rate capability and long cycle life（over 500 cycles）.3.Oxygen intercalated molybdenum disulfide-graphene heterostructure materials（GMo）were prepared through hydrothermal route by using thiourea assisted liquid-exfoliated graphene dispersion and precursor（phosphomolybdic acid）.It is found that the increase of 9.6?（002）d-spacing in GMo film are highly correlated with oxygen insertion into d-spacing of MoS₂,especially the increasing layer spacing is close to the key length with S-O bond.Furthermore,the oxygen dangling bond on the sulfur sites（S-O）gives rise to the p-type doping effect in GMo because the strong electronegativity of oxygen depletes the electron majority but accumulates the holes in MoS₂.This novel designed GMo films containing oxygen-incorporated MoS₂ result in tunable work function characteristics based on hybrid 2H/1T-phase MoS₂,which could significantly improve hole extraction from the photoactive layer of organic photovoltaic devices（OPVs）.Importantly,due to high p-type vertical conductivity of the GMo hole extraction layers（HELs）,the GMo HELs can be placed in the traditional PCDTBT:PC₇₁BM organic solar cells to achieve the highest photoelectric conversion efficiency（PCE）of 4.5%,and after 1000 hours of storage,keep about 80%of the initial performance.Meanwhile,a high PCE of 9.5%can be achieved in PTB7-Th:PC₇₁BM OPVs which is superior to the performance for other PBDTTT-based OPVs employing2D materials,as well as offering better lifetime stability due to the ability of the GMo layer to prevent penetration of contaminants.Our investigations will encourage the widespread use of 2D-based nanomaterials for applications in many other emerging optoelectronics.4.We further extended the scope of the application in flexible devices incorporating graphene-like materials:polyvinylidene fluoride（PVDF）electrospun nanofibers membrane was employed to mediate the preparation of inorganic-organic heterogeneous materials by hydrothermal route in the presence of MoS₂.The as-fabricated heterogeneous materials is used as self-standing flexible composite electrodes.We confirmed the heter-membrane with good comprehensive performance.On the one hand,our inorganic-organic heterogeneous shows a structure of 3D interconnection of molybdenum disulfide,this structure has good electric and thermal conductivity as well as superior mechanical strength,and can be used in the performance of LIBs and hydrogen evolution catalysts（HER）.On the other hand,when utilizing the inorganic-organic heterogeneous material as anodes assembed in LIBs,the material not only integrates with high energy and power density,but also exhibites superior rate and long cycle stability.Based on all the above property,the inorganic-organic heterogeneous materials were further packagd with cathodes(LiNi_0.5Co_0.3Mn_0.2O₂,NCM)to make full battery.The full battery shows higher capacity（258 mAh/g）and good mechanical deformation resistance.In addition,the capability of inorganic-organic heterogeneous materials in HER activity has also been obviously enhanced.Among them,when the current density is 10 mA/cm²,the membrane show stable hydrogen evolution overpotential（119 mV）and lower tafel slope（101 mV/dec）,as well as excellent electrocatalytic stability.Therefore,this study provides a research basis for the design and construction of flexible wearable devices in the future.