A simple facile method, i.e., thermal decarbonation of ZnCO3 hydroxides, was used to prepare a series of pure ZnO photocatalysts with controlled crystallite sizes, particle sizes, and morphologies. The ZnCO3 precursor was synthesized by direct wet carbonation in the presence of growth-control additives, i.e., organic solvents, surfactants, and low molecular weight polymers. The thermal decarbonation allows for producing ZnO photocatalysts with sizes and shapes varying from 80 +/- 20 nm nonporous rhombohedral nanoparticles to 5 +/- 0.5 A mu m porous particles, for a constant crystallite size of 64 +/- 3 nm. The porous ZnO particles (5 +/- 0.5 A mu m) exhibit two times larger photocatalytic activity for methanol oxidation than the nonporous ZnO nanoparticles (similar to 180 +/- 30 nm). The reasons for the higher photocatalytic activity are further investigated in this work. A possible mechanism for the formation of ZnCO3 hydroxides and their transformation into porous microsized ZnO particles and nonporous nanoparticles are carefully discussed.