Extensive analysis of the formation mechanism of BaSnO₃ by solid-state reaction between BaCO₃ and SnO₂

Abstract

In this paper, the phase evolution, microstructure changes, and element diffusion for the formation of BaSnO₃ from BaCO₃ and SnO₂ were extensively investigated. It was found that BaCO₃ rapidly diffused on surface of SnO₂ at the initial stage of the reaction (approximately 700 °C), forming a uniform BaSnO₃ shell. Subsequently, BaSnO₃ grew via two different routes depending on the calcination temperature. When the reaction was carried out below 820 °C, BaSnO₃ phase further grew by diffusion of barium ions across BaSnO₃ layer to SnO₂ phase, since the BaSnO₃ interlayer prevented the direct reaction between surface BaCO₃ and SnO₂ core. Once a higher temperature above 820 °C was provided, Ba₂SnO₄ were generated by reaction between BaCO₃ and BaSnO₃ shell. Under such circumstances, pure BaSnO₃ was obtained by diffusion of barium ions from Ba₂SnO₄ across BaSnO₃ to SnO₂. The rate-determining step in both cases was assigned to the diffusion of barium ions.

Introduction

Barium stannate (BaSnO₃), a kind of ceramic material with ideal cubic perovskite structure, has attracted increasing attention in the past decade for its outstanding optical and electrical nature [1], [2], [3], [4], [5], [6]. Up to now, this material has been widely applied in various fields including capacitors, sensors, photocatalysts, and photoanode materials for dye-sensitized solar cells (DSSCs) et al. [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Traditionally, BaSnO₃ powders was prepared by solid-state reaction (SSR) method between BaCO₃ and SnO₂ at high temperature above 1300 °C [17]. However, the high temperature calcination normally resulted in serious agglomerations of the BaSnO₃ particles, which greatly limited the application of BaSnO₃ in the fields that chasing smaller particle size for better performance, such as capacitor, gas sensor or photo-anode material in DSSCs [8], [12], [16], [18]. Some impurities, mostly Ba₂SnO₄, were also observed in the products obtained by this method, which greatly weakened the electrical and optical performance of this material [19], [20].

Some recent studies showed that fine BaSnO₃ nanopowders with high purity could be prepared at reduced temperature by wet chemistry approaches, such as coprecipitation method, sol-gel process, and micro-emulsion method et al. [8], [21], [22], [23], [24], [25], [26], [27], [28], [29]. It was noteworthy that the precursors obtained from the sol-gel and oxalate coprecipitation methods, two commonly used methods in fundamental research, would decompose to BaCO₃ and SnO₂ before BaSnO₃ phase was generated, which means the pure BaSnO₃ nanopowders are intrinsically formed by reaction between BaCO₃ and SnO₂ in the above two routes [30], [31]. These facts suggested that quality of BaSnO₃ product could be greatly influenced by the state of the raw material (BaCO₃ and SnO₂). Thus, it was important to uncover the formation mechanism of BaSnO₃ from BaCO₃-SnO₂ precursor in order to guide the synthesis process and control the grain size and purity of BaSnO₃.

Although a number of studies concerning the synthesis of BaSnO₃ have been done over the last 20 years, few reports focused on the formation mechanism of BaSnO₃. Gallagher and Johnson [32] studied the reaction between physical mixture of BaCO₃ and SnO₂ by X-ray diffraction (XRD) techniques, and reported that the reaction started at temperature above 900 °C and Ba₂SnO₄ was generated as an intermediate. Berbenni et al. [30] carefully investigated the process from BaC₂O₄-SnC₂O₄ raw material to final BaSnO₃ product with XRD and thermogravimetric-differential scanning calorimetric (TG-DSC) analysis. They found that the oxalate precursor was converted to BaSnO₃ through BaCO₃ and SnO₂ intermediate, and mechanical activation of the raw material could not only effectively reduce the starting temperature of the reaction between BaCO₃ and SnO₂ to below 750 °C, but also notably suppressed the formation of Ba₂SnO₄ impurities. Köferstein et al. [31] followed the phase evolution from [Ba(C₂H₆O₂)₄][Sn(C₂H₄O₂)₃] complex precursor to BaSnO₃ by XRD, Fourier transformed infrared (FT-IR) and electron energy loss spectroscopy (EELS). They confirmed that the complex precursor transformed to BaCO₃ and SnO₂ via barium tin oxycarbonate intermediate, and then generated BaSnO₃ at 600–800 °C with no Ba₂SnO₄ being detected. It could be noticed that the above mentioned works mainly focused on the phase evolution in the production of pure BaSnO₃. However, as far as we know, detailed information on the processes at the interfaces, nucleation and further growth of BaSnO₃ (diffusion of Ba, Sn and O element), which were crucial for thorough comprehension of the formation mechanism of BaSnO₃ were rarely reported.

Mössbauer spectroscopy is one of the most sensitive tools for probing the chemical state (valence and coordination) of target element [33], [34]. In the present work, ¹¹⁹Sn Mössbauer spectroscopy was introduced for the first time to explore the evolution of Sn from the starting BaCO₃-SnO₂ mixture to final BaSnO₃ product. Besides, the microstructural changes and element diffusion from ball-milled BaCO₃-SnO₂ mixture to BaSnO₃ was extensively investigated at the nanoscale utilizing the Mössbauer analysis, combined with XRD, TG, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). The dynamic variation of the SnO₂, BaSnO₃, and the Ba₂SnO₄ impurities during the reaction between BaCO₃ and SnO₂ at isothermal conditions was determined using ¹¹⁹Sn Mössbauer spectroscopy to uncover the rate limiting step for the production of BaSnO₃. Therefore, a clear picture on the formation mechanism of BaSnO₃ was given. Finally, some suggestions on how to reduce the synthetic temperature of pure BaSnO₃ and inhibit the generation of Ba₂SnO₄ impurities were proposed.