Morphology, Structure And Thermoelectric Properties Of Bi-Te Thin Films Prepared By Electrodepositon

Thermoelectric materials which can directly convert heat and electricity are a kind of functional materials with superior performance, and their researchs have received increasing attention in recent decades. Bi-Te-based compounds possess the best performance of thermoelectric materials in the current room temperature, and low-dimensional and appropriate doping can further improve the thermoelectric properties of materials. In this article, Bi2Te3 thermoelectric thin films with ideal stoichiometric ratio was prepared by electrochemical method which has the advantages of low cost, simple process and macro prepareration. The research also explored the electrodeposion process and precipitation mechanisms of the thin films, and focused on the impact of the addition of Hexadecyl trimethyl ammonium Bromide (CTAB) or copper on the morphology, structure, composition and thermoelectric properties of the thin films.We could achieve precise control of morphology, structure, composition of the Bi2Te3 film by adjusting the deposition potentials, deposition time and electrolyte concentrations. The thermoelectric thin films with nice morphology, uniform structure and an ideal stoichiometric ratio were prepared at-0.1V(vs.SCE),30min, 100mg·L-1 CTAB,7.5mM Bi3+, lOmM HTeO2+ and 1mM HNO3 conditions. In addition, we had also made a further exploration on the electrodeposition precipitation mechanism of simple Bi3+ solutions, simple HTeO2+ solutions, and their mixed solutions. The results showed that the formation of the Bi2Te3 thin films was conducted step by step. Bi3+ first obtained a portion of the electrons to produce an intermediate product Bix+, then the Bix+ was reduced to form elemental Bi. And the HTeO2+ was first adsorbed onto the surface of the substrate, afterwards obtained a portion of the electrons to produce Tex+ which was finally reduced to form elemental Te. At last, the elemental Bi and Te combined to generate Bi-Te films. At the same time, we investigated the influences of deposition time and electrolyte concentration on film composition, morphology and crystal orientation, and successfully achieved the pure phase Bi2Te3.Cetyltrimethyl ammonium bromide (CTAB) as a special additive was applied to favor the deposition of bismuth telluride (Bi2Te3) thin film in this reasearch. XRD, EDS and SEM analyses demonstrated that a proper concentration of CTAB additive (100mg·L-1) in the electrolyte containing 7.5mM Bi3+,10mM HTeO2+ and 1mM HNO3 could greatly contribute to the controllable growth of Bi2Te3 thin film along (110) orientation at potential of-0.1V vs. SCE. The films possessed a uniform surface with a porous lamellar structure. Thermoelectric performance analysis showed the resistance reduced about five times, the Seebeck coefficient and power factor also got improved, and the thermoelectric properties of the film obviously optimized in the presence of CTAB. Electrochemical studies indicated that the polarity part of CTAB would be adsorbed on the surface of copper substrate and Bi2Te3 nanoparticles formed, which changed the surface energy of the nanoparticles, and the deposition potentials of Bi and Te had both moved to the negative. The effect made the thin films no longer a single vertical stack growth, but a porous lamellar uniform growth.Cu was chose as the third doping element to prepare a ternary thermoelectric thin films of CuxBi2Te3 in this reasearch, and the impact of doping concentration of Cu source on the morphology, structure and composion of the thin films had been studied. The results showed that the addition of Cu source could effectively thin the lamellas forming the thin films, and improved the inter-particle density. The resistivity of the materials decreased obviously, and their Seebeck coefficient and power factor increased significantly after doping Cu,leading to an effectively increasing of thermoelectric properties.