In the case of back-side illumination of porous coatings as the case here, i.e., light coming from the substrate side of the coating, there are opposing gradients in absorbed photon flux versus reactant availability. The majority of photons is absorbed in the region nearest to the substate and furthest from the solution. Consequently, the thickness of the coating as well as the charge carrier transmission within it are very critical parameters as a good coating will allow those charge carriers generated near the coating substrate to reach to surface in contact with the solution, where the concentration of reactants is highest. However, the determined coating thickness does not vary significantly between the P25-derived coatings versus the ones originating from TTIP. A possible explanation for the much higher activity of the latter may be a better charge carrier transmission within it due to less grain boundaries versus the coating obtained from pre-sintered particles.