Unveiling hidden epitaxial interfaces in novel SnO2/Zn2SnO4 core–shell nanowires with a multi-domain shield via cross-sectional transmission electron microscopy

Abstract

Wrapping SnO₂ nanowires with appropriate oxide shell materials could produce a significant impact on their photoelectrical and gas sensing performance. In this work, a novel highly crystalline SnO₂/Zn₂SnO₄ core–shell nanowire has been fabricated using a sequential chemical vapor deposition (CVD) method. Cross-sectional transmission electron microscopy (TEM) reveals that the SnO₂ core is surrounded by five or six Zn₂SnO₄ domains, displaying a characteristic quasi-hexagonal cross section. Extensive crystallographic orientation study shows that the growth direction of the core is along the SnO₂ [101] direction, parallel to that of the shell Zn₂SnO₄ [101], i.e. SnO₂ [101]∥Zn₂SnO₄ [101]. While the Zn₂SnO₄ outer shells have two crystallographic relationships between the SnO₂ core, SnO₂ (10)∥Zn₂SnO₄ (13) and SnO₂ (020)∥Zn₂SnO₄ (1), one group of the domains sticks to the SnO₂ {111} plane and the other group grows on the SnO₂ {020} plane. All the Zn₂SnO₄ shell domains expose their {111} plane as the ending surface. The tetragonal SnO₂ core growing along the [101] direction makes it nearly invisible when collecting electron diffraction from the side view of the core–shell nanowire. The geometrical model deduced from cross-sectional TEM results suggests that there is only one low index zone axis for the core nanowire, i.e. SnO₂ [010], along which Moiré fringes and the very rarely seen multiple double-diffraction effect were observed and discussed in detail. We would like to call the attention of researchers to be careful in correlating the microstructure of core–shell nanowires with their physical properties, because merely observing from the radial direction might not be able to reveal the true interface structure.