Chemical Synthesis Of Bi₂Te₃ And Bi₂Se₃ Nanopowders

Thermoelectric(TE) materials are functional materials,which can directly convert electricity into heat energy or reversely.They are promising materials in engergy area and have been applied in power generators and cooling/heating devices.Bi₂Te₃ and its alloys are the best TE materials available near room temperature currently.Bi₂Te₃ and Bi₂Se₃ both have wide applications in TE area and Bi₂Se₃ can be used in optical area additionally.In the present work,hydrothermal method is applied to synthesize Bi₂Te₃ and Bi₂Se₃ nanopowders.X-ray diffraction(XRD),high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy(FESEM) are used to investigated phases and morphologies of products respectively.Chemical and crystalline mechanisms during the synthesis were investigated.Some important results of the work are listed below.Bi₂Se₃ and Bi₂Te₃ nanopowders have been successfully synthesized using an easy chemical method.Nanoparticles of Bi₂Se₃ and Bi₂Te₃ as small as several tens nanometers were obtained at 105℃.Intact hexagonal Bi₂Se₃ crystals are observed in the product reacting at 190℃more than 16 hours with several hundreds nanometers in diameter.32 hours are necessary to react completely,for avoiding impurities.The grain sizes of Bi₂Se₃ increased with the increasing of temperature obviously.Intact hexagonal Bi₂Se₃ nanoplates were successfully synthesized,using ethylene glycol (EG) as solution.EG is solution,reductant and dispersant in this process.Bi₂Se₃ nuclei form gradually since the reduction of EG is weak,crystals can grow slowly with enough building-blocks,thus there are few defects.Influence of some important additions were studied.Experimental result shows that the reactive order,morphologies of products are directly affected by the amount of NaOH.Also, the amount of NaBH₄ should not more than two times to the amount of reactants.Too much NaBH₄ leads to impurities such as BiTe or Bi₄Se₃.Ordering Bi₂Se₃ nano-arrays were fabricated by controlling the amount of NaOH.These arrays are composed of nanoplates about 30 nm thick while the whole arrays are 20μm in size.XRD and TEM were used to analyze the crystalline mechanism of these nano-arrays. The results showed that NaOH was the key of the nano-arrays.These nano-arrays are formed on the by-product Bi₂SeO₂,which are considered as natural templates.The(0 1 5) surface of Bi₂Se₃ and(1 0 1) surface of Bi₂SeO₂ match each other and Bi₂Se₃ crystals grow along their a or b-axis.