Elements Affecting Bandgap and Photocatalytic Activity

However, when the particle size is decreased, the surface area of the material is increased, as is the number of active sites where molecules can adsorb and react to form radicals. The surface properties of the semiconductors are important in 

Some other studies showed that the combination of the three polymorphs with reduced crystallite size and high surface area can lead to the best photocatalysts for 4-chlorophenol degradation [37], or that particles in the dimension range 25-40 nm give the best performances [38]. Therefore, many elements contribute to the final photocatalytic activity and sometimes the increased contribution of one parameter can compensate for the decrease of another one. For example, better photocatalytic activity can be obtained even if the surface area decreases, with a concomitant increase in the crystallinity of the sample, which finally results in a higher number of electron-hole pairs formed on the surface by UV illumination and in their increased lifetime (slower recombination) [39]. Better crystallinity can be obtained with the use of ionic liquids during the synthesis [39], with a consequent increase of activity. 

The influence of different factors on photocatalytic performance is well documented also for systems not based on titania. For instance, ZnO hollow spheres show 

---