Preparation Of Bi₂S₃-based Low-dimensional Carbon Nanocomposites By Hydrothermal Method And Their Thermoelectric Propertie

Since the beginning of the 21st century,with the intensification of fossil energy consumption,a series of problems such as energy shortage and environmental pollution have gradually emerged.In order to further alleviate the above crisis,the exploration and development of new energy materials gradually become a research hotspot in the field of scientific research.Thermoelectric material is a kind of energy functional material,which can realize the mutual conversion of heat and electric energy and is wi dely used in the fields of waste heat power generation and thermoelectric refrigeration.Thermoelectric devices based on thermoelectric materials can effectively solve the existing problems such as energy crisis and environmental pollution.Compared with t he traditional commercial thermoelectric material tellurium（Te）,sulfur（S）based thermoelectric materials have the advantages of abundant reserves,non-toxic and cheap,and their development in the field of thermoelectric has attracted much attention in recent years.Among many S-based thermoelectric materials,Bi₂S₃ is considered to be one of the most promising thermoelectric materials in middle temperature zone due to its low thermal conductivity and high Seebeck coefficient.However,with a band gap of 1.3e V at room temperature,Bi₂S₃ has a poor conductivity,which makes its thermoelectric value（ZT）lower than that of most Te-based thermoelectric materials.At present,researchers mainly use energy band engineering,ion co-doping,grain boundary engineering,energy filtration effect and composite materials to regulate the electrical and thermal parameters of Bi₂S₃ to obtain higher ZT value.Low dimensional carbon nanomaterials（two-dimensional,one-dimensional,and zero-dimensional）are known to possess many excellent physical and chemical properties.Among other systems of thermoelectric materials,low-dimensional carbon nanomaterials are often compounded as a second phase to optimize their thermoelectric properties.At present,the application of ion doping to optimize the thermoelectric properties of Bi₂S₃ based materials has been very mature,but the use of composite materials to optimize the thermoelectric properties of Bi ₂S₃is relatively few.In this paper,Bi₂S₃ based composite thermoelectric materials with excellent thermoelectric properties are prepared by combining simple hydrothermal synthesis technology and tube sintering technology to combine Bi₂S₃ with low dimensional carbon nanomaterials.Specific research contents are as follows:The pure phase Bi₂S₃ powders with different hydrothermal reaction temperatures were solidified under the same tube furnace sintering conditions by using the control variable method.Through the analysis of electrical transport performance,it is determined that the optimum hydrothermal reaction temperature is 100℃.Pure phase Bi₂S₃ powders were prepared at the optimum hydrothermal reaction temperature and solidified at different tube furnace sintering temperatures.Through the analysis of thermoelectric properties,the optimum sintering temperature is 350℃.Two-dimensional carbon nanomaterials-graphene oxide（GO）and reduced graphene oxide（r GO）:A stable Bi₂S₃/GO composite was formed by hydrothermal incorporation of GO containing hydrophilic groups into Bi₂S₃.GO reduces part of the hydrophilic groups in the hydrothermal process to form a certain two-dimensional conductive network,and the conductivity of Bi₂S₃/GO composite is partially optimized,up to 1106S/m.In addition,GO as the second phase increases phonon scattering,which makes the thermal conductivity of Bi₂S₃/GO composite as low as 0.33WK^-2m^-1.After calculation,the ZT value of Bi₂S₃/GO composite can reach 0.15,which is 15%higher than that of pure phase Bi ₂S₃.In addition,GO,which has been weakly reduced by glucose,was incorporated into Bi₂S₃ by hydrothermal synthesis technology.r GO with better conductivity formed more conductive networks,which resulted in the highest conductivity of Bi ₂S₃+10ml r GO composite up to1284S/m.In addition,r GO is incorporated into Bi₂S₃ in the form of the second phase,which forms a more uniform second phase in the matrix.As a result,the scattering of interfacial phonons in Bi₂S₃/r GO composite is intensified,and the thermal conductivity of Bi ₂S₃/RGo composite is reduced to 0.12WK^-2m^-1.Finally,the ZT value of Bi₂S₃/r GO composite is calculated to be up to 0.22,which is 69%higher than that of pure Bi ₂S₃.One-dimensional carbon nanomaterial-Multi-walled Carbon nanotubes（MWCNTs）:After acidification of MWCNTs,functional multi-walled carbon nanotubes（f-MWCNTs）have certain hydrophilicity.After that,f-MWCNTs were mixed into Bi₂S₃ by hydrothermal method.The functionalization of the strong acid does not affect the structure of th e MWCNTs,which preserves the excellent electrical conductivity of the MWCNTs.f-MWCNTs formed many conductive channels in Bi₂S₃ matrix and the existence of energy filtration effect optimized the transport of electrons in Bi₂S₃,which made the PF value of Bi₂S₃+0.5wt%f-MWCNTs composite up to 151m WK^-2m^-1.In addition,the interface between Bi₂S₃ and f-MWCNTs enhances phonon scattering and reduces the thermal conductivity of Bi ₂S₃/f-MWCNTs composites,which can be as low as 0.27WK^-2m^-1.Finally,the calculated ZT value of Bi₂S₃/f-MWCNTs composites can reach 0.33,which is 154%higher than that of pure phase Bi ₂S₃.Zero-dimensional carbon nanomaterial fullerene(C₆₀):C₆₀ is functionalized,which gives it a certain hydrophilicity without changing the zero-dimensional structure of C₆₀.Bi₂S₃/C₆₀composite material was formed by mixing functional C₆₀ into Bi₂S₃ by hydrothermal method.Thanks to C₆₀’s regulation of electron and phonon transport in Bi ₂S₃ matrix,the electrical and thermal transport of Bi₂S₃/C₆₀ composite has achieved ideal results.On the one hand,the flow of electrons from C₆₀ into Bi₂S₃ improves the conductivity of Bi₂S₃/C₆₀ composites,up to 5212S/m.On the other hand,due to the large elastic modulus of C ₆₀,the phonon scattering center in Bi₂S₃/C₆₀ composite is enriched,and the thermal conductivity of Bi ₂S₃/C₆₀composite is as low as 0.16WK^-2m^-1.Finally,the ZT value of Bi₂S₃/C₆₀ composite can reach0.38,nearly 3 times that of pure Bi₂S₃.In this study,Bi₂S₃-based low-dimensional carbon composites were prepared by combining two-dimensional carbon nanomaterials GO and r GO,one-dimensional carbon nanomaterials MWCNTs and zero-dimensional carbon nanomaterials C₆₀ into Bi₂S₃-based materials by a simple hydrothermal method combined with tubular f urnace sintering.The thermoelectric properties of Bi₂S₃-based carbon composites have been optimized to varying degrees thanks to the interfacial strong phonon scattering,the formation of conductive channels and networks,and the energy filtering effect.This research further expands the availability of non-toxic and inexpensive Bi₂S₃-based materials,which could contribute to their commercial application.