ZnO nanocrystals of different shapes and sizes have been synthesized using an innovative, simple and efficient dry reactive milling methodology using Zn(NO₃)₂ and various polysaccharides as sacrificial templates. Optimum results were achieved using extracted agar from the red seaweed Gracilaria gracilis. Upon template removal after calcination at 600ºC, the protocol gave rise to a range of porous metal oxide nanomaterials of different shapes and nanoparticle sizes which were found to have excelling photocatalytic properties in aqueous phenol degradation as compared to commercial P25 Evonik titania.