The Preparation And Thermoelectric Performance Of Two-Dimension ?Mo,W?S₂ And Its Composite Materials

Energy is indispensable for industrial production and daily life.However,due to the development of the society,the world's demand for energy was much more than before.At the same time,the problem of global climate change caused by the combustion of fossil fuels is much more serious,and has caught more and more attention of researchers.Thus,developing a kind of environmentally and friendly energy is an urgent problem.Thermoelectric?TE?material,as a kind of green and functional material which can directly realize the conversion of thermal energy and electricity energy without pollution,can successfully make an access to the recycling of the waste heat and the harvest of the energy,and also can be applied in power generation and refrigeration.Compared with conventional bulk TE materials,two-dimensional?2D?nanomaterials have received a host of attention in the past decade,due to their exotic electronic structures and excellent physical properties.Thus,2D nanomaterials are thought as ideal candidate for TE applications.MoS₂ and WS₂ nanosheets as typical 2D materials,with large intrinsic band gap and high carrier mobility,show great promise in applications for TE.However,they still have low electrical conductivity compared with other inorganic and polymer TE materials.What's more,pristine TMDs are all bulk materials,which restrict their practical applications.A lot of work has been done to improve the TE performance and obtain few layered TMDs nanosheets.Prepare nanocomposites was considered an effective strategy to improve TE property by combining the advantages of each component.It is very important for TMDs to select the suitable and effective method to obtain nanosheets and find the promising composite materials.This dissertation obtained MoS₂ and WS₂ nanosheets by a chemical lithium-intercalation reaction,then integrated with PEDOT:PSS and rGO,and investigated their TE performance.1.1T phase MoS₂ nanosheets were exfoliated by the assistance of lithium intercalation,and fabricated MoS₂ micron thin-film via a direct vacuum filtration technique.The resulting show that the electrical conductivity about 1T phase MoS₂nanosheets was six magnitudes higher than the 2H phase MoS₂.In particular,the maximum Seebeck coefficient was able to reach 93.5?V K^-1.The highest figure of merit?ZT=0.01?is calculated in this experiment based on the low thermal conductivity 0.28 W m^-1 K^-1.The transition from 2H to 1T phase during exfoliation process is the main reason for the improving TE performance.2.The transferrable PW thin film with chemical exfoliated WS₂ nanosheets and PEDOT:PSS solutions was obtained through a direct vacuum filtration.The introduction of PEDOT:PSS resulted in a significantly enhanced electrical conductivity from 12.5 S cm^-1 to 1025 S cm^-1 and a declined Seebeck coefficient.It must be stressed out that such a kind of behavior is attributed to the transport of carriers along the PEDOT:PSS chains,therefore,reducing the energy barrier between the adjacent WS2 NSs.An optimized power factor of 45.2?W m^-1 K^-2 has been achieved at the ratio of PEDOT:PSS is 50 wt%,which is a four times higher than that of the pure restacked WS₂ thin film.3.rGO–TMDs composite thin-films were fabricated by a simple solution-processing method and their thermoelectric performance was investigated systematically.Addition of rGO nanosheets efficiently improved the electrical conductivity of MoS₂ and WS₂?MS₂?NSs,due to the excellent electron transport performance of rGO.Furthermore,it should be noted that an optimized content of rGO can effectively avoid direct contact between TMDs NSs by forming a rGO–TMDs heterojunction,leading to significantly increased electrical conductivity and a slight variation in its Seebeck coefficient.The optimal power factor was obtained at low rGO content.Our work obtained high thermoelectric performance heterostructures by inducing two kinds of layered materials using a simple method that may potentially be applied to other 2D layered materials to construct heterostructures for energy conversion.4.1T phase MoS₂ and WS₂ nanosheets composite thin-films were obtained by solution-processing method and their thermoelectric performance was investigated systematically.The results indicated that the heterostructure could optimize the TE performance,and post-treatment by CuCl₂ could have a further improvement.The significant improvement electrical conductivity was obtained at the low CuCl₂content,from 25 to 72 S cm^-1.The power factor has a significant improvementalthough a slight reduction in its Seebeck coefficient.The electrical conductivity was reduced at high CuCl₂ content,as well as low TE performance.The improvement TE performance maybe due to the addition of CuCl₂ could form a superionic conductor,which could accelerate the carrier transfer,and reduce the energy barrier.