Compositional analysis of electrodeposited bismuth telluride thermoelectric thin films using combined electrochemical quartz crystal microgravimetry-stripping voltammetry

Bismuth telluride ([Bi.sub.2][Te.sub.3]) is a benchmark material for thermoelectric power generation and cooling applications. Electrodeposition is a versatile technique for preparing thin films of this material; however, it affords films of variable composition depending on the preparation history. A simple and rapid assay of electrodeposited films, therefore, has both fundamental and practical importance. In this study, a new protocol for the electroanalysis of [Bi.sub.2][Te.sub.3] thin films is presented by combining the two powerful and complementary techniques of electrochemical quartz crystal microgravimetry (EQCM) and stripping voltammetry. First, any free (and excess) tellurium in the electrodeposited film was reduced to soluble [Te.sup.2-] species by scanning to negative potentials in a 0.1 M [Na.sub.2][SO.sub.4] electrolyte, and the accompanying frequency increase (mass loss) was used to determine the content of free tellurium. The film was again subjected to cathodic stripping in the same medium (to generate [Bi.sup.o] and soluble [Te.sup.2-] from the [Bi.sub.2][Te.sub.3] film component of interest), and the EQCM frequency change was used to determine the content of chemically bound Te in the [Bi.sub.2][Te.sub.3] thin film and thereby the compound stoichiometry. Finally, the EQCM frequency change during Bi oxidation to [Bi.sup.3+] and the difference between total Bi and Bi in [Bi.sub.2][Te.sub.3] resulted in the assay of free (excess) Bi in the electrodeposited film. Problems associated with the chemical/electrochemical stability of the free Bi species were circumvented by a flow electroanalysis approach. Data are also presented on the sensitivity of electrodeposited [Bi.sub.2][Te.sub.3] film composition to the electrodeposition potential. This newly developed method can be used for the compositional analysis of other thermoelectric thin-film material candidates in general.