"The field of heterogeneous photocatalysis... can be defined as the acceleration of photoreaction in the presence of a catalyst. The two most significant applications of photocatalysis have been in solar water splitting and the purification of air and water containing low concentrations of pollutants. ...[I]nterest in heterogeneous photocatalysis can be traced back to ...when Fujishima and Honda discovered in 1972 the photochemical splitting of water into and oxygen in the presence of TiO2."