Abstract
This paper reports on a molecular simulation study of ZnO nanostructures confined within carbon nanotubes. Both the effects of confinement (by varying the pore size) and degree of pore filling (by varying the number of confined ZnO monomers) on the structure of the nanomaterial are addressed. None of the nanostructures exhibits the ideal structure of one of the ZnO bulk crystal phases (rocksalt, blende and wurtzite), but some crystalline features with significant correlations for the first and second nearest neighbours are observed. Close inspection of the location of the peaks in the pair correlation functions, of the angle distributions between Zn–O nearest neighbours and of some corresponding molecular configurations suggest that the confined nanoparticles possess mainly the local ordering of wurtzite. We also found evidence for defects such as Zn atoms that are involved in both a four-atom ring (characteristic of a cubic phase) and a six-atom ring (characteristic of wurtzite). Due to the smaller coordination number of atoms located at the interface between the nanostructure and the nanotube, the number of nearest neighbours of like and unlike atoms is smaller than that of the bulk. It is also found that the morphology of the nanostructures embedded in the carbon nanotubes depends in a subtle way on the different parameters involved in the synthesis (nanotube size, filling density). ZnO arranges itself to form either nanorods (quasi-1D systems) or an isolated nanoparticle (dimensionality 0D). When the filling density is low, further calculations suggest that the isolated particle is more well ordered and is more stable. We also show that the O atoms are polarised along the radial direction Δr>0 (i.e. towards the external free interface). Our results also show that the use of a symmetrical nanopore as a template imposes that the confined particles exhibit surfaces terminated with both Zn and O atoms in the same stoichiometry.