Size-and phase-dependent structure of copper(ii) oxide nanoparticles

Abstract

Copper(II) oxide (CuO) nanoparticles (NPs) have found numerous applications in electronics, optics, catalysis, energy storage, health, and water purification. Controlled synthesis of CuO NPs requires information on their nanoscale structure, which is expected to vary depending on the size, shape, phase, and most importantly, on the surface morphology. In this work, we report a detailed analysis of the structure of solid and melted CuO nanoparticles as functions of size and temperature at global and local scales, using molecular dynamics simulations. Comparisons of simulated X-ray diffraction profiles, mean bond lengths, average coordination numbers, and melting points with available experimental data support the modeling results. Melting points of CuO NPs vary linearly with the reciprocal of the diameters of NPs. The long-range order seen in solid nanoparticles with diameters greater than 6 nm gradually vanishes as size decreases, indicating the loss of translational symmetry of the lattice structure and the emergence of amorphous-like structure even below the melting point. Melted nanoparticles show liquid-like characteristics with only a short-range order. Mean bond lengths and average Cu–O coordination numbers of both solid and melted NPs indicate weakening of the structural stabilization for smaller NPs that leads to an increased deformation in the local atomic arrangement because of the lack of long-range interactions. For the cases studied, most of the structural features are independent of temperature, with the notable exception of the number of oxygen atoms coordinated to Cu. This latter quantity is indeed indicative of melting phase transition and can be used to compute the melting point accurately. Atoms on the surface of solid NPs show amorphous-like behavior even at temperatures well below the melting point of the NPs due to the limited coordination environment. This study represents a useful step towards the establishment of a structure–property relationship for CuO nanoparticles.