Effect of size and aspect ratio on structural parameters and evidence of shape transition in zinc oxide nanostructures

TL;DR

This study investigates how size and shape transitions in ZnO nanostructures affect their internal structural parameters, revealing a shape-induced change in lattice properties linked to surface effects and electrostatic relaxation.

Contribution

It provides a comprehensive analysis of size and aspect ratio effects on ZnO nanostructures, highlighting the shape transition from spherical to rod-like forms and its impact on structural parameters.

Findings

Shape transition from spherical to rod-like occurs at AR~1.3

Structural parameters change linearly with aspect ratio

Surface effects influence internal lattice parameters

Abstract

Dependence of structural parameters on the size of nanoparticles is a topic of general interest where the effect of shape is often neglected. We report a comprehensive study on size dependent structural parameters of ZnO nanostructures (NS) having a wide range of aspect ratios (length/diameter). It reveals that with increase in size, ZnO NS undergo a shape transition from spherical to rod like morphology that induces sudden change in internal parameter (u) which represents relative position of two hexagonal close-packed sublattices. The change in u introduces concomitant changes in anion-cation (Zn-O) bond lengths as well as bond angles and thereby bears a linear dependence with the aspect ratio (AR). Further, the unit cell volume and microstrain decrease with increase in particle size and show a drastic reduction when flat crystal faces begin to appear at the spherical surface (AR~1.3). The significant change in structural parameters associated with the shape transition arises due to surface dipole induced electrostatic relaxation that may be further influenced by interaction with the ambient gases as evidenced from the Extended X-ray Absorption Fine Structure (EXAFS) measurement. The present study addresses the underlying reasons of shape induced change in structural and electronic properties of ZnO NS.