Crystallinity-driven ferroelectric and piezoelectric properties of hydrothermally synthesized zinc stannate: exploring the substantial role of mineralizer NaOH

Abstract

Zinc stannate (ZS) belongs to a category of ternary semiconducting type perovskite-based nanomaterials which have accrued significant interest for their ferroelectric and piezoelectric properties. Such properties can be tuned by suitably modifying their unique crystal structures, which are influenced by the presence of different reactants used for synthesis. In this aspect, the mineralizer plays a significant role in tuning different crystal transitions of ZS by facilitating growth along specific crystal facets, along with other factors like thermodynamics and reaction kinetics playing an important role in determining the crystal structure. Herein, we have studied the effect of one of the widely used mineralizers namely NaOH at different concentrations varying from 0 M to 0.25 M and its effect on ferroelectric and piezoelectric properties of hydrothermally synthesized ZS. We have observed the existence of a polycrystalline phase without mineralizer, which gradually transitioned to a single crystalline material (face centred cubic perovskite ZnSn(OH)₆ and ZnSnO₃) at 0.25 M NaOH, as affirmed by XRD and TEM analysis. Initially, in the absence of a mineralizer, ZS in the form of a cluster of nanoparticles was observed which transitioned to well-defined nanocubes at 0.25 M NaOH as evidenced by SEM and TEM micrographs. Furthermore, we observed a band gap transition from ∼3.4 eV to ∼1.63 eV, with an increase in NaOH concentration, as studied by UV-vis spectroscopy. Most importantly, the change in crystal transitions led to lossy ferroelectric behaviour at 0 M and an ideal ferroelectric state at 0.25 M mineralizer concentration. The dielectric and piezoelectric properties corroborated the ferroelectric behaviour. Thus, optimization of the mineralizer concentration gives us an insight and in-depth understanding of crystalline polymorphism in ZS nanostructures which can be further tailored to specific ferroelectric and piezoelectric applications.