Effects of Potential, Electrolyte Composition, and pH on the Characteristics of Electrodeposited Bi-Te Films
Date Issued
2012
Date
2012
Author(s)
Kang, Wan-Shan
Abstract
Bi2Te3 is the thermoelectric material most widely used at low temperature ranges. The effects of electrodeposition parameters on deposited layer, such as potential, electrolyte concentration, electrolyte pH, and anion species, is discussed in this study. The Bi-Te film plated at -0.1 V presents a dense morphology; however, the others show a dendrite structure within the potential range of -0.475 V < E < -0.1 V (vs. SCE) in the nitric acid system. By varying the electrolyte HTeO2+ concentration from 8 to 20 mM at a constant Bi3+ concentration of 11 mM, the deposits with Te contents ranging from 45 to 67 at% can be made, specifically the deposit Te content varies linearly with the electrolyte HTeO2+ concentration. The electrolyte, in which the deposit close to stoichiometric Bi2Te3 was electroplated, was further subjected to study the effect of the potential (-0.1 V to +0.02 V) and electrolyte pH. It is found that the potential markedly influences the grain size but hardly affects the composition of the Bi-Te deposits. In contrast, the solution pH strongly influences the color, morphology, and adhesion of the deposits. The Bi2Te3 plated at -0.1 V in the solution containing 0.7 M HNO3 shows a gray color, needle-like morphology with sufficient adhesion. In contrast, the deposit plated at -0.1 V in 1.5 M HNO3 displays a black color, loose granular morphology with inferior adhesion. The preparation of electrolyte becomes easier due to the dissolution rate of TeO2 can be increased using hydrochloric acid instead of nitric acid. The deposit composition becomes Te-rich at higher applied potentials in the HCl system. The applicable potential range of the HCl system is smaller than that of the HNO3 system although a flat, dense Bi2Te3 film can be obtained at -0.02 V in 0.35 M HCl.
Cyclic voltammetry results show that with increasing HNO3 and HCl concentration the reduction potential of BiIII shifts toward negative direction, but the reduction potential of TeIV shifts along positive direction in the solution solely composed of BiIII or TeIV. With increasing HNO3 concentration and decreasing HCl concentration in the solution containing of BiIII and TeIV, the reduction peak shifts along positive direction. In the HCl system, the reduction of BiIII becomes more difficult than in the HNO3 system because stable complexant ion BiCl4- forms but the reduction of TeIV becomes easier due to the fact that Cl- ion bridge promotes electrochemical reaction. Owing to the pure underpotential deposition (PUD), the reduction potential of the solution containing BiIII and TeIV is more positive than the solution only containing BiIII or TeIV.
Subjects
Bi2Te3
electrodeposition
thermoelectric materials
cyclic voltammetry
underpotential deposition
Type
thesis
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