Chirally pure single-walled carbon nanotubes (SWCNTs) are required for various applications ranging from nanoelectronics to nanomedicine(1). Although significant efforts have been directed towards separation of SWCNT mixtures, including density-gradient ultracentrifugation(2), chromatography(3) and electrophoresis(4), the initial chirality distribution is determined during growth and must be controlled for non-destructive, scalable and economical production. Here, we show that the chirality distribution of as-grown SWCNTs can be altered by varying the composition of NixFe1-x nanocatalysts. Precise tuning of the nanocatalyst composition at constant size is achieved by a new gas-phase synthesis route based on an atmospheric-pressure microplasma. The link between the composition-dependent crystal structure of the nanocatalysts and the resulting nanotube chirality supports epitaxial models and is a step towards chiral-selective growth of SWCNTs.