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

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 (NSs) having a wide range of aspect ratios (AR: length/diameter). With increase in size, ZnO NSs undergo a shape transition from spherical to rod-like morphology that induces a sudden change in the internal parameter (u) which represents the relative position of two hexagonal close-packed sublattices. The change in u is associated with the changes in anion–cation (Zn–O) bond lengths as well as bond angles and thereby bears a linear dependence on the 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 for shape-induced change in structural and electronic properties of ZnO NSs.