Flexible Thermoelectric/Moisture Electricity Generation Property And Device Investigation On Two-dimensional Transition Metal Dichalcogenides

Flexible materials with continuous electrical energy supply are a hot issue in materials and electronics research today,and have significant applications in portable electronic devices,especially wearable electronics.Two-dimensional（2D）transition metal dichalcogenides（TMDC）materials have many optical and electrical properties differing from their bulk materials on one hand,and have superior mechanical flexibility as well,making them commonly regarded as one of the ideal materials for flexible power generation.To date,various clean electricity generation methods have been developed based on 2D TMDC materials,and two types of power generation methods,thermoelectricity and moisture electricity generation,are most suitable for wearable electronic devices in practical application situations.However,there are still problems of low power generation performance,poor stability and high requirements of experimental conditions with 2D TMDC materials as electricity generation materials.In the case of molybdenum disulfide（MoS₂）,a typical TMDC material,an applied voltage or change in elemental composition is often used to improve thermoelectric properties when doing thermoelectric research,but the complex external equipment and preparation process will limit its practical application.The application of 2D TMDC materials in moisture electricity generation is less researched and the performance is not enough for practical application,which has great potential for improvement.To address these issues,this thesis improves the flexible thermoelectric properties of 2D TMDC nanosheet films through a simple surface modification method.The effects of surface modification on the morphology,stacking patterns,surface electronic structure and thermoelectric parameters of 2D TMDC nanosheet films and the intrinsic modulation mechanisms are systematically described.Meanwhile,the moisture electricity generation capability of 2D TMDC nanosheets is improved based on the difference in electronic structure and wettability between different phases,and its working principle and power generation performance were investigated as well as its device application.The details of the study are as follows:（1）The modification treatment with Lewis acid precisely modulated the electronic structure of the surface of the restacked MoS₂ nanosheet films and enhanced their thermoelectric power factor.Using aluminum chloride（AlCl₃）solution as a typical example,the effects of Al³⁺ions on the surface morphology,stacking mode,electronic structure and work function of MoS₂ nanosheets were analyzed.By optimizing the concentration of AlCl₃ solution,at 1 mmol/L concentration the room temperature thermoelectric power factor of the treated MoS₂ nanosheet restacked films reaches 122μW m^-1K^-2,which is 1.8 times higher than that before treatment.In addition,flexible thermoelectric flake devices were prepared based on the treated MoS₂ nanosheets,which showed excellent mechanical properties.A wearable thermoelectric bracelet was prepared using the flexible thermoelectric flakes,and a voltage output of about 5 m V was achieved using the human wrist portion as the heat source.（2）An n-type flexible titanium disulfide（TiS₂）nanosheet restacked film with a high thermoelectric power factor was fabricated.The modification of TiS₂ nanosheets using AlCl₃ solution resulted in the improvement of the room temperature thermoelectric power factor of TiS₂ nanosheet restacked films to 516μW m^-1K^-2,which is about 14 times of that before the modification.The band structure variations of TiS₂nanosheets before and after modification were calculated using density functional theory（DFT）to elucidate the decoupling phenomenon of simultaneous increase of conductivity and Seebeck coefficient in the modified TiS₂ nanosheet films,and the mechanism of"energy filtering"effect of Al³⁺ions on the charge transport of TiS₂nanosheets is proposed.A flexible thermoelectric handband was fabricated with modified MoS₂ nanosheets as p-type thermoelectric material and modified TiS₂nanosheets as n-type thermoelectric material to absorb arm heat achieving about 4.2m V voltage output.（3）In response to the limitations of flexible thermoelectric technology in practical applications,2D TMDC nanosheet materials were attempted to applying in moisture electric generation.A 2H-1T hybrid phase MoS₂ nanosheet moisture electric generation unit based on phase gradient was prepared with MoS₂ nanosheets as a typical example.With the modification of NaOH solution,the maximum open-circuit voltage and short-circuit current of about 430 m V and 62μA/cm² were achieved,and the duration of current greater than 10μA/cm² was more than 1 hour,which surpassed most of the currently developed moisture electric generation materials in terms of comprehensive performance.The effects of annealing temperature,sodium hydroxide（NaOH）solution concentration,and immersion on moisture electric generation performance were analyzed,and a series of comparative experiments were designed to investigate the principle of moisture electric generation in 2H-1T phase MoS₂nanosheets after NaOH modification treatment,proposing the intrinsic mechanism of forming a concentration difference of movable ions under the effect of water molecules and generating electricity by spontaneous diffusion of ions.Based on the modified 2H-1T phase MoS₂ nanosheets,a micro moisture electric generator was prepared with an output voltage of 10 V and successfully realized to power small electronic devices.In addition,other TMDC materials such as Tungsten Disulfide（WS₂）,Molybdenum diselenide（Mo Se₂）,etc.were extended for moisture electric generation applications and the performance differences and mechanisms were analyzed.