Control of growth orientation and shape for epitaxially grown In2O3 nanowires on a-plane sapphire

Vertically aligned indium oxide nanowires were grown on a-plane sapphire substrate by the method of catalyst-assisted carbothermal reduction. The morphology and crystal structure of the nanowires are determined by X-ray diffraction, transmission electron microscopy and field-emission scanning electron microscopy. Two types of In2O3 nanowires were found by controlling the growth conditions. The nanowires with a hexagonal cross-section were shown to grow in [1 1 1] direction, whereas those with a square cross-section grow in [0 0 1] direction. In addition to the temperature effects, the concept of supersaturation in Au catalyst is proposed to explain the formation of these two types of nanowires. Besides, tapering, which is explained with the interplay between the vapor–liquid–solid and vapor–solid growth mechanisms, is observed in the nanowires.