Heterogeneous photocatalysis for environmental cleanup has been one of the most active areas, which is considered to be a green approach without creating harmful by-products. Simultaneously with the generation of electron—hole pairs, a spontaneous adsorption occurs according to the redox potential (or energy level) of each adsorbate in the presence of a fluid phase (gas or liquid): an electron transfer proceeds toward acceptor molecules, whereas positive photoholes are transferred to donor molecules. The photocatalytic reaction occurs, and the organic and inorganic compounds or even microorganisms can be degraded or transformed into less harmful substances. As similar to that for photocatalytic water-splitting, TiO2 with high crystallinity and large surface area is required to achieve a high photocatalysis activity.