One of the most exciting and most studied uses of photocatalysis is the photo-oxidation of organics as it applies to environmental decontamination. 
In contrast to gas phase interactions with the solid surface, the vast number of variables associated with the liquid solid interface (i.e. solution pH, photocatalyst concentration, solvent effects, diffusion rate, etc.) calls for greater care to be taken to control these variables to produce consistent experimental results. A greater variety of reactions also become possible due to the ability of solutions to stabilize charged species, making it possible to add an electron from the metal to a neutral species producing an anion that can go on to further react, or a hole to remove an electron, producing a cation that goes on to further react in solution.