The successful synthesis of monoclinic Nb2O5 nanotube arrays (see figure) from pseudo-hexagonal Nb2O5 nanorod arrays is reported; the synthesis is based on phase transformation accompanied by void formation, where voids can be intriguingly generated during phase transformation. A key parameter for achieving nanotube growth is the energy difference between the pseudohexagonal and inonoclinic Nb2O5 nanostructures, which determines the phase transformation.