Preparation And Thermoelectric Performance Of Two-dimensional TiS₂-based Composites

With the rapid development of science and technology,economy and socialization,the increasingly serious energy crisis has become a thorny problem that human beings must face.It is imperative and imminent to develop new green and sustainable energy conversion technologies.Thermoelectric conversion technology is expected to achieve the conversion between thermal energy and electrical energy directly and reversiblely.The design and development of green and efficient thermoelectric materials is very important for the application of thermoelectric conversion technology.In recent years,transition metal dichalcogenides(TMDs)have received extensive attention and in-depth research in the field of thermoelectric,because of their abundant reserves and unique physical and chemical performance which is influenced by the quantum size effect.Because of its excellent metal conductivity and high Seebeck coefficient at room temperature,titanium disulfide(TiS2),a natural n-type TMDs material,shows the broad application prospects in the current n-type thermoelectric materials which are scarce.However,the current studies on the thermoelectric performance of TiS2-based materials mainly focus on the rigid crystals with high synthesis difficulty.Although these crystalline materials generally have perfect superlattice structure and excellent thermoelectric properties,they are not suitable for the wearable thermoelectric devices with high flexibility requirements.Therefore,it is urgent to design and develop new high-performance TiS2-based thermoelectric thin-films which are easily prepared and expanded to meet the actual development needs of the current thermoelectric conversion technology.In addition,the performance optimization of existing materials is also one of the important topics in thermoelectric research.The macroscopic thermoelectric properties of materials are usually determined by their microstructures.Therefore,it is very important for the application and development of thermoelectric materials that exploring the internal relationship between the microstructure and the macroscopic thermoelectric performance,and exploring how to regulate and optimize the macroscopic thermoelectric properties by using the microstructure.In this thesis,a variety of TiS2 nanosheet-based thermoelectric films are prepared by different strategies,and the intrinsic relationship between the microstructure and thermoelectric properties of composites is analyzed and explained by characterization methods and theoretical analysis.The main research results are as follows:1.Solution-processible colloids and films composed of single-layer or few-layer TiS2 nanosheets are successfully prepared by lithium intercalation,ultrasonic dispersion and colloidal self-assembly strategies,and their oxidation and thermoelectric properties are studied.It is found that the electrical conductivity,Seebeck coefficient and power factor of TiS2 nanosheets films can highest reach 338.98S cm^-1,-84.29?V K^-1 and 240.83?W m^-1 K^-2,respectively.The excellent thermoelectric performance is mainly due to the energy band overlap,the decrease of band gap and the local charge mismatch of TiS2 nanosheets.However,due to the negative charge surface and a certain number of defect structures of TiS2 nanosheets,the colloids are easily oxidized in the water-rich environment.2.Various Alcohol-TiS2 composites are prepared by solvent replacement strategy,and the colloidal oxidation resistance and thermoelectric properties of the films are studied.It is found that the oxidation resistance of TiS2 nanosheets could be effectively improved by the solvent replacement of alcohols,which is mainly related to the change of the internal environment of the nanosheets.In particular,the improvement of antioxidant activity is positively correlated with alcohol hydroxyl content.The thermoelectric properties of thin-films are studied.It is found that solvent replacement has a significant effect on the thermoelectric properties of the films.Especially,the electrical conductivity and Seebeck coefficient of MT-TiS2 which is obtained by methanol replacement can reach 99.38 S cm^-1 and-94.38?V K^-1,respectively.The change of thermoelectric properties is mainly related to dielectric confinement effect and energy filtering effect.This work provides a feasible reference for regulating the oxidation resistance and thermoelectric properties of TMDs nanosheets.3.A pre-protection strategy is innovatively proposed to prepare(EDTA-2Na)-TiS2 composites,and the colloidal oxidation resistance and thermoelectric properties of the films are studied.It is found that the addition of protective agent EDTA-2Na can effectively improve the oxidation resistance of nanosheet colloid,which is mainly due to the inhibition of ionization of H2O molecules and the weakening of the adsorption and binding of H2O molecules at the edge and defects of nanosheets by the protective agent.Through the study on the thermoelectric properties of films,it is found that the maximum electrical conductivity of the E-TiS2 containing the protective agent can reach 1.51×10⁴ S m^-1.The change of electrical conductivity is mainly related to the decrease of the limited H2O molecule content in the nanosheets and the enhancement of the local electron density.This work provides an effective new way to regulate the physical and chemical properties of TMDs nanosheets.4.A variety of PEDOT:PSS-TiS2 composite films are prepared by the simple assembly strategies,and their thermoelectric properties are studied.It is found that the unique p-n interfaces are conducive to improve the Seebeck coefficient,which is related to the energy filtering effect caused by the charge transfer between the two materials with different conductive mechanisms.In particular,the power factors of P1-TiS₂and T₄₅-P₁₀-T₄₅ can reach 248.51?W m^-1 K^-2and 368.58?W m^-1 K^-2,respectively,which are much higher than those of most TMDs-based thermoelectric materials.This work provides a unique design idea for optimizing the thermoelectric properties of TMDs nanosheets by using heterogeneous interfaces.