"UV in natural sunlight represents only 5%–8% of the solar spectrum at sea level and this provides a limitation and the requirement of artificial illumination of the catalysts... The need for more stable, efficient catalysts which can be activated by natural sunlight is now the goal of researchers. ...[P]recise control of the of the catalytic metal oxides and mixed metal oxides, addition of dopants, particle shape and pore topology are all critical factors for catalyst developers."