Holes and electrons are generated by absorption of light in bulk TiO2, shown in Figure as a set of 10 30 nm thick gray layers. The incident light intensity range modeled in this work is 1×10 11 ~1×10 18 photons•cm -2 s -1 , corresponding to 5.79×10 -5 ~5.79×10 2 mW•cm -2 . This overlaps the light intensity range used in the experiments and extends it to lower and higher intensities to examine trends. The total thickness of these compartments is 300 nm, which is the 1/e 2 absorption depth of 365nm light. The Beer-Lambert law is used to calculate the rate coefficient for photocarrier formation in each compartment using an estimated reflectivity of 20% and absorption coefficient α of 10 4 cm -1 (SI Section 1). Each photoabsorption event generates one electron and one hole. The charge carriers diffuse between compartments with a rate given by Fick's law, and irreversibly escape into bulk TiO2 from the lowest compartment. Charge carrier diffusion coefficients of 1.35×10 -3 cm 2 s -1 for electrons and 10 -5 cm 2 s -1 for holes are taken based on the estimation from theoretical studies by Deskins and Dupuis. The calculations indicate that a hole diffusion coefficient between 10 -5 and 10 -7 cm 2 s -1 is a reasonable value: both yield similar trends in the simulation results except for the very lowest light intensities (SI Section 5 and Table ).