The simulations generate a complete time history of all surface species present during the reaction for both Models as shown in Figure . In pathway I of Model 1, the concentrations of Ti-O -, Ti-OH and Ti-H2O increase sharply with the increase of light intensity at first, then transition to near-constant values when the light intensity is greater than 1 mW•cm -2 . In contrast, the concentrations of the three species Ti-OH -, Ti-OO --Ti, and Ti-OO 2--Ti continually decrease as light intensity increases. The concentrations of these species vary widely, from 10 -8 to 10 -3 during photo-OER on TiO2 nanoparticles in contact with aqueous base. Assignments for the peak at 838 cm -1 peak is Ti-OOH and the shoulders at 928 and 812 cm -1 are surface peroxo and Ti-OO-Ti respectively. A report for this reaction on a mixture of anatase and rutile phases at neutral pH by Roberts et al. note that the spectrum is pH-dependent, and that the 812 cm -1 feature is not present in pure water, which is the simulation condition used here. They assign the band at 838 cm -1 (834 cm -1 in their measurements) to the O-O stretch of an adsorbed Ti-OOH - species. This assignment is consistent with our prediction of a buildup of this intermediate under Model 2, pathway II (Figure (d)), and suggests that less mobile surface holes may dominate the overall reaction. Wang et al. assigned the peak to a Ti-OO -species in pathway I. In our simulations this species is also present in Model 1, however only at low light intensities. We have calculated total coverages of oxo, peroxo, superoxo, hydroperoxo and hydroxy intermediates as a function of light intensity from the simulation results as shown in Table and Figure .