We study by low temperature photoluminescence measurements the electronic states of silicon nanowires obtained by copper catalyzed chemical vapor deposition and compare them with those of wires made by etching silicon on the insulator structure. Thermal oxidation of nanowires appears to be absolutely necessary to passivate surface states and to enhance radiative recombinations at the silicon band gap. The study of the behavior of this transition as a function of temperature and pump power demonstrates that it involves the phonon assisted recombination of free carriers. The recombination energy appears at the silicon band gap, renormalized by exchange and correlation interactions favored by spatial confinement.