A gas phase nanowire growth apparatus including a reaction chamber (200), a first input and a second input (202 B, 202 A). The first input is located concentrically within the second input and the first and second input are configured such that a second fluid delivered from the second input provides a sheath between a first fluid delivered from the first input and a wall of the reaction chamber. An aerosol of catalyst particles may be used to grow the nanowires.