Mechanism of Bi−Ni Phase Formation in a Microwave-Assisted Polyol Process

Abstract

Typically, intermetallic phases are obtained in solid-state reactions or crystallization from melts, which are highly energy and time consuming. The polyol process takes advantage of low temperatures and short reaction times using easily obtainable starting materials. The formation mechanism of these intermetallic particles has received little attention so far, even though a deeper understanding should allow for better synthesis planning. In this study, we therefore investigated the formation of BiNi particles in ethylene glycol in a microwave-assisted polyol process mechanistically. The coordination behavior in solution was analyzed using HPLC-MS and UV-Vis. Tracking the reaction with PXRD measurements, FT-IR spectroscopy and HR-TEM revealed a successive reduction of Bi3+ and Ni2+, leading to novel spherical core-shell structure in a first reaction step. Bismuth particles are encased in a matrix of nickel nanoparticles of 2 nm to 6 nm in diameter and oxidation products of ethylene glycol. Step-wise diffusion of nickel into the bismuth particle intermediately results in the bismuth-rich compound Bi3Ni, which consecutively transforms into the BiNi phase as the reaction progresses. The impacts of the anion type, temperature and pH value were also investigated. © 2020 The Authors. Published by Wiley-VCH GmbH