Microstructure and thermoelectric properties of Bi2Te3 electrodeposits plated in nitric and hydrochloric acid baths

The reduction behaviors of unitary and binary bismuth cation (BiIII), tellurium cation (TeIV) in nitric acid and hydrochloric acid solutions were studied at different acid concentrations. Compared to the nitrate bath, the reduction of BiIII was retarded and the reduction of TeIV was promoted in hydrochloric acid. As a result, the difference in the reduction potential of BiIII and TeIV was markedly reduced in the presence of chloride ion. The acid concentration affected the morphologies of the Bi-Te deposit; specifically a rougher deposit was electroplated at higher acid concentrations. A 25 μm Bi2Te3 film obtained from 0.7 M HNO3 at − 20 mV versus saturated calomel electrode (SCE) had columnar structure with (110) preferred orientation and its Seebeck coefficient and power factor were − 72.3 μV/K and 732 μW/m·K2, respectively. A 25 μm smooth, compact, nano-grained Bi2Te3 film was fabricated in 0.35 M HCl at − 20 mV versus SCE and its Seebeck coefficient and power factor were − 105.0 μV/K and 169 μW/m·K2, respectively.