Investigation of reaction mechanisms of bismuth tellurium selenide nanomaterials for simple reaction manipulation causing effective adjustment of thermoelectric properties

We synthesized ternary n-type bismuth tellurium selenide nanomaterials for thermoelectric applications via a water-based chemical reaction under an atmospheric environment. In this work, bismuth nitrate was employed as a bismuth precursor and was hydrolyzed to form bismuth hydroxide in an aqueous solution. Ascorbic acid was used to dissolve the bismuth hydroxide and give a reactive bismuth source (Bi³⁺ ions) that was able to react with anion sources (Te^2-/Se^2- ions). Ascorbic acid played a role in reducing bismuth hydroxide to an unreactive bismuth source (bismuth particles, Bi ⁰). We confirmed that ascorbic acid dissolved or reduced bismuth hydroxide depending on the solution pH. Because either Bi³⁺ ions or bismuth particles were generated depending on the pH, the nanomaterial stoichiometry was pH dependent. Nanomaterials prepared at various pH levels were individually sintered using a spark plasma sintering process to measure their thermoelectric transport properties (i.e., carrier concentration, electrical resistivity, Seebeck coefficient, and thermal conductivity). We observed how the transport properties were affected through adjustment of the pH of the reaction and found an appropriate pH for optimizing the transport properties, which resulted in enhancement of the thermoelectric performance.