A comparison of the photocatalytic activity between commercial and synthesized mesoporous and nanocrystalline titanium dioxide for 4-nitrophenol degradation: Effect of phase composition, particle size, and addition of carbon nanotubes

•Nanocrystalline TiO2 are the most active photocatalysts for the degradation of 4-NP.•A balanced amount of anatase and rutile forms the most active photocatalysts.•More active photocatalysts could be prepared by addition of C material.•Carbon nanotubes or graphene increases the activity of polyphasic titania.•C material should be in good electrical contact with the anatase/rutile particles.