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

• Size- and shape-dependent structural parameters of ZnO nanostructures have been investigated experimentally as well as by theoretical modelling.
• Flat faces appear as the size of the nanosphere gradually increases due to relaxation of built-up strain.
• The apparent change in morphology of nanostructures is linked with specific changes in various structural parameters.
• Cell volume contracts for smaller nanoparticles but it remains almost constant for bigger nanorods.
• The size- and shape-dependent structural properties of ZnO nanostructures are explained on the basis of dipolar interaction with the ambient gases.

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.

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