Three types of holes are tracked in our model: surface free holes, surface trapped holes, and bulk holes. Total hole populations are shown in Figure , and the concentrations of specific types of surface holes as a function of light intensity are shown in Figure . In Model 1 (Figure (a)), the total concentration of trapped holes on the surface, which are the Ti-OH and Ti-OO --Ti intermediates in the photo-OER reaction, gradually decreases by 1.510 17 h + cm -3 as the light intensity increases. This reflects the shift in populations of reactive intermediates on the surface from hole traps to species that can only react thermally, as discussed in Section D below. At the same time, the concentrations of bulk holes and surface free holes increase. The concentration of the bulk holes is about 1 order of magnitude higher than that of surface free holes, where the concentration of bulk holes is the sum of the steady-state hole concentration of the 10 bulk layers (entire sample 300 nm) as shown in Our simulations cover the light intensities from very low to high, forming a detailed picture of the relationship between photocurrent (O2 production rate) and hole concentrations. A higher order dependence as observed experimentally appears in a narrow range of light intensities (Figure ). The mechanistic origin of a higher order dependence can be evaluated using the simulations. One possibility is that the photocurrent depends on specific hole populations involved in well-defined reaction steps, including the trapping species Ti-OH and Ti-OO --Ti, with an overall apparent high order. Plots of the relationship between the concentrations of the three types of holes and photocurrent (Figure ) show primarily a firstorder dependence, however.