Abstract
Sodium niobate (NaNbO3) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb2O5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa7Nb6O19·15H2O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na2Nb2O6·H2O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO3. The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution–precipitation mechanism.
Highlights
Hydrothermal synthesis is a low-temperature environmentally friendly route to a variety of functional oxides reducing challenges with evaporation, agglomeration, and coarsening, which often takes place at higher temperatures.[1−6] Still, the development of the method has been mostly achieved through a trial-and-error approach as the conventional autoclave design, not penetrable by X-rays, makes it inherently challenging to study the synthesis in real time
NaNbO3 nanowires formed by hydrothermal synthesis and subsequent calcination have proven useful in lead-free piezoelectric nanogenerator applications.[11]
Ex situ studies of the hydrothermal synthesis of NaNbO312−17 have shown that the reaction starts with the transformation of the T-Nb2O5 precursor into sodium hexaniobate (HNa7Nb6O19·15H2O), with the main building block consisting of the Lindqvist ion, [Nb6O19]8−
Summary
Hydrothermal synthesis is a low-temperature environmentally friendly route to a variety of functional oxides reducing challenges with evaporation, agglomeration, and coarsening, which often takes place at higher temperatures.[1−6] Still, the development of the method has been mostly achieved through a trial-and-error approach as the conventional autoclave design, not penetrable by X-rays, makes it inherently challenging to study the synthesis in real time. For temperatures below 285 °C for both NaOH concentrations, both of the polyoxoniobates forming in 9 and 12 M transform into Na2Nb2O6·H2O, previously observed under similar conditions.[13,16,20,22] This phase can be described as staircase-like chains where each step is a [Nb4O16]12− unit, and the chains are separated by water and Na+. The PDFs for the 1 h aged 12 M suspension can be described well with the T-Nb2O5 structure, with only a small contribution from HNa7Nb6O19·15H2O, similar to the expected PDF of a Lindqvist ion The revealing of two distinctly different growth regimes may offer important insight when untangling the growth mechanisms, leading to the various sizes and morphologies resulting from the hydrothermal synthesis of NaNbO3
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