We study the properties of Ly a emitters in a cosmological framework by computing the escape of Ly a photons through galactic outflows. We combine the galform semi-analytical model of galaxy formation with a Monte Carlo Ly a radiative transfer code. The properties of Ly a emitters at 0 < z < 7 are predicted using two outflow geometries: a shell of neutral gas and a wind ejecting material, both expanding at constant velocity. We characterize the differences in the Ly a line profiles predicted by the two outflow geometries in terms of their width, asymmetry and shift from the line centre for a set of outflows with different hydrogen column densities, expansion velocities and metallicities. In general, the Ly a line profile of the Shell geometry is broader and more asymmetric, and the Ly a escape fraction is lower than with the Wind geometry for the same set of parameters. In order to implement the outflow geometries in the semi-analytical model galform, a number of free parameters in the outflow model are set by matching the luminosity function of Ly a emitters over the whole observed redshift range. The resulting neutral hydrogen column densities of the outflows for observed Ly a emitters are predicted to be in the range similar to 10181023?cm-2. The models are consistent with the observationally inferred Ly a escape fractions, and with the shape of the Ly a line from composite spectra. Interestingly, our predicted UV luminosity function of Ly a emitters and the fraction of Ly a emitters in Lyman-break galaxy samples at high redshift are in partial agreement with observations. We predict that Ly a emitters constitute a subset of the galaxy population with lower metallicities, lower instantaneous star formation rates and larger sizes than the overall population at the same UV luminosity.