Mono-dispersed free-standing Co3O4 single crystals were synthesized by hydrothermally treating Co(NO3)2·6H2O aqueous solutions with addition of NaNO3 and tetraethylammonium hydroxide (TENOH). For comparison, samples were prepared using sodium hydroxide (NaOH) instead of TENOH. The obtained phases were greatly dependent on the types of alkalis and their concentrations. TEM images of TENOH-derived samples showed that the formation of free standing spinel Co3O4 nanocubes with sizes of 40-60 nm dominate over all the observation ranges for the samples with lower TENOH concentrations. While brucite-like Co(OH)2, Co3O4 and rod-like CoOOH appeared in the NaOH-derived sample, TENOH favoured the formation of Co3O4 nanocubes compared with NaOH.