Synthesis And Thermoelectric Properties Of Ag-doped MnO₂ Nanowires

Thermoelectric?TE?materials,capable of converting a temperature gradient into electric power directly,have wide promising applications in energy savings and environmental protection.Devices made of TE materials can be utilized for power generation or refrigeration.TE devices have the following attractive characteristics:no moving parts,quiet operation,low environmental impact,high reliability,and long service life.Therefore,TE materials have attracted more and more attention as the new type of green pollution-free energy conversion materials.The efficiency of TE materials is determined by the dimensionless figure of merit?ZT?,defined as ZT=??²?/??T,where S is the Seebeck coefficient,?is the electrical conductivity,?is the thermal conductivity and T is the absolute temperature.Typically,there are two basic approaches aimed at increasing ZT:either the power factor??²??is maximized or the thermal conductivity is minimized.The low TE efficiency is the main problem that limits their practical commercial applications.How to get the new material with high ZT value or increase the ZT value of the existing materials has become an active topic.Recently,many deliberate attempts have been made to increase the ZT,including the exploration of new TE materials using low-dimensional systems and optimization of existing materials through doping.Compared with the traditional TE materials telluride/sulfide alloys/compounds,metal oxides have attracted more attention due to their environment friendly,high temperature stability,oxidation resistance and low cost.Manganese dioxide is a low band gap,high optical constant semiconductor that has wide applications in catalysis,electrochemical batteries and capacitors.The research shows that MnO₂ has larger Seebeck coefficient,which can be used as potential TE materials.But very few researches have been carried out on their TE properties due to the relatively low electrical conductivity.In this paper,the effect of nanostructure and Ag doping on the crystal structure and the TE performance for MnO₂ were investigated.The main results are as follows:1.MnO₂ nanowires were successfully synthesized by a simple chemical vapor deposition?CVD?method.The structure,morphology and thermoelectric properties of the as-synthesized Mn O₂ were investigated.The results indicate that the synthesized products consist of a large quantity of MnO₂nanowires with uniform morphology and good crystallinity.The diameter is about 100 nm and the length can reach tens of micrometers.The thermoelectric measurement demonstrates that the thermoelectric performance of the as-synthesized MnO₂ have been increased,which is due to the enhancement of the grain boundary scattering and the decrease of the thermal conductivity.Compared with the traditional TE materials,the ZT values of Mn O₂ nanowires are limited by relatively low electrical conductivity.As a result,it is not suitable for large-sale commercial applications.2.Ag was used as the dopant to fabricate the Ag-doped MnO₂ nanowires by a simple chemical vapor deposition?CVD?method and the effect of Ag doping on the morphology,crystal structure and the TE performance for MnO₂ were investigated.The results indicate that Ag doping has little effect on the morphology of as-synthesized samples,and the products are still composed of a large number of nanowires with a diameter of about 100 nm.Besides,the crystal structure also has no significant change when the doping density is low.However,the impurity phase of Ag₂O will occur in the products with increasing of Ag doped density.Compared to the undoped samples,the as-obtained Ag-doped MnO₂ nanowires exhibit high-performance TE properties and a maximum ZT value of 2.18 at 580 K is achieved for the 2%Ag-doped sample,which can be attributed to a strongly enhanced power factor and a significant reduction in the thermal conductivity.Meanwhile,it can be observed that while at low doping concentration?2 and4%?,the values of Seebeck coefficient are positive,meaning that the samples change from n-type to p-type semiconductor.