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.

In situ investigation of the formation mechanism of α-Bi2Rh nanoparticles in polyol reductions

“…The increased stability of metal chlorides against reduction has been reported by Balachandran et al and by us and can be attributed to the Lewis acidity of the chloride ion. 19,34 Co-reduction of Bi(NO 3 ) 3 and Rh 2 (OAc) 4 in EG In order to better separate time and temperature effects on the diffusion and reaction processes, experiments were performed with constant 10 minutes reaction time at selected temperatures. Heating a solution of Bi(NO 3 ) 3 and Rh 2 (OAc) 4 (Bi : Rh = 1 : 2) to 160 °C within a approximately 10 s yielded a black precipitate consisting of BiRh (Fig.…”

“…Since the diffusion occurs rapidly, the limiting factor is the rate of reduction of bismuth cations. 34 To identify the source of bismuth for the transition from BiRh to α-Bi 2 Rh, the supernatant of each reaction step was collected and again heated in the microwave to 250 °C. Indeed, the precipitates then consisted solely of elemental bismuth in decreasing amounts with increased temperature (Fig.…”

“…16 Previous works of our group elucidated the formation mechanisms of BiNi and Bi 2 Ir particles. 33,34 The mechanisms differ heavily depending on the metal combination and revealed a variety of interesting intermediates. BiNi particle formation follows a successive reduction of bismuth and nickel cations, whereby 2-10 nm sized particles encase the bismuth particle forming the first reported self-assembled Bi@Ni core-shell structure.…”

Trendbericht Festkörperchemie 2022

“…Der Prozess verläuft über Bi-Partikel mit einer Hülle aus Ni-Nanopartikeln, die in einem zweiten Schritt in das Bi eindringen und Bi 3 Ni bilden, bevor BiNi entsteht. 24) Abb. 5.…”

“…7. a) TEM-Aufnahme eines Bi-Partikels mit Hülle aus Ni-Nanopartikeln. Details in b); c) EDXS-Scan (Ni orange, Bi grün); d) ein Elektronenbeugungsbild mit den Reflexlagen von Ni 24).…”