La_0.67Ce_0.03Sr_0.3MnO₃-coupled microwave assisted ultra-fast synthesis of nanocrystalline Cobalt oxide and Bismuth oxide

Abstract

Bismuth hydroxide and Cobalt hydroxide gels were freshly prepared by adding aqueous sodium hydroxide to their corresponding nitrates under basic conditions. Then they were oven dried at 373 K and found to be amorphous in X-ray diffraction (XRD) study. They crystallized into their respective oxides by microwave irradiation (2.45 GHz) in merely 2 min using La_0.67Ce_0.03Sr_0.3MnO₃ (LCSMO) as couplant in a microwave domestic oven (~ 100 W). This is the lowest temperature treatment and synthesis time so far reported in the literature for the formation of these systems. Formation of nanocrystallites of Bi₂O₃ (10 nm) and Co₃O₄ (15 nm) has been confirmed by XRD. Normally these hydroxides produce oxides only on heating at temperature > 673 K or digesting them at 373 K for 4 – 6 hours as reported by us earlier. Microwave assisted heating proves to be a novel technology whose applications can be carefully harnessed due to its unique effects compared to the conventional heating, such as rapid volumetric heating, shortened reaction time and energy saving.

Introduction

In recent times, microwave assisted synthesis route has emerged as a new method for production of various oxide materials [1], [2]. In this synthesis approach, pre-processed precursors are kept in microwave chamber (typically, commercial 2.45 GHz, 100 W), and are subjected to microwave irradiation for a duration of few tens of minutes. This method is increasingly gaining importance due to its unique set of properties, namely rapid heating for large quantities; thereby increasing the reaction rates and consequently leading to low energy requirements. Furthermore, due to its extremely small reaction time, the particle sizes can be imagined to be smaller and monodispersed.

The technique is not new and there are many research inputs on these lines, explaining the possible mechanism therein. As reported by Cirera et al. [3], the absorption of microwave energy varies with the composition and structure of the material. Jansen et al. [4] suggested that the advantages offered by microwave can be attributed to fast homogeneous nucleation and the easy dissolution of the gel. However, the exact nature of the interaction of the microwaves with the reactants during the synthesis of materials is somewhat unclear and speculative. Nonetheless, in most of the references, the precursors are water-based. The effect of heating using microwaves is basically created by the interaction of polar water molecule with the high frequency electromagnetic interaction.

In this context, we report on synthesis of two different oxides, namely, Co₃O₄ (CoO) and Bi₂O₃ (BiO) using their respective non-aqueous precursors. A couplant system of La_0.67Ce_0.03Sr_0.3MnO₃ (LCSMO) is used in our setup, which makes up a mini-cavity, in which the precursors are placed and the assembly is subjected to microwave irradiation. LCSMO has been synthesised using citrate-gel route [5]. Nanocrystalline CoO and BiO powders are formed in as small a duration as 2 min. We also discuss the possible mechanism involved in the Ce–LSMO coupled microwave system to work as a mini-furnace.

Cobalt oxide (Co₃O₄) powders find wide range of applications in anode materials for rechargeable Li-ion batteries, catalyst, magnetic materials and varistors [6], [7]. Similarly, Bismuth oxide as a component finds use in wide applications in varistors, catalyst and gas sensors [8], [9], [10]. As is well known, functional properties of these materials depend on particle size, morphology, purity and chemical composition. Using microwave-assisted preparation route, we have synthesised nanoparticles of CoO and BiO. The phase purity and particle size is evaluated using X-Ray Diffraction (XRD, Philips PW 1710 Diffractometer) technique and Scanning Electron Microscopy (SEM, Leica Cambridge 440 microscope).