  We report the growth of Al-catalysed ZnO nanowires (NWs) using a thermal
evaporation technique.Before the growth, the substrates were covered with a
distribution of Al nano-island that act as seeds. We found that the density of
NWs increases as the density of seeds is increased, confirming the catalyst
properties of Al. The critical parameters of growth are the substrate
temperature, oxygen partial pressure and the thickness of the initial Al layer
from which the seeds are formed. The results showed that the oxygen pressure
has a strong influence on the structural characteristics: the nanowires exhibit
a preferential orientation in the (00l)-planes when they are grown at low
oxygen flow, and they become polycrystalline when a high concentration of
oxygen in the flow is used. We consider that the growth occurs via a
vapor-solid-solid (VSS) process as the predominant growth mechanism.
