At low temperature phenol reacts with Br2 in CS2 to form

In presence of non-polar solvent (CS2) the ionization of phenol is suppressed. The ring is slightly activated and hence mono substitution occurs.On the other hand with Br2 and water, phenol forms 2,4,6-tribromo phenol.In aqueous solution phenol ionizes to give phenoxide ion. Due to the presence of negative charge on oxygen the benzene ring is highly activated and hence trisubstituted product is obtained.