Reaction-diffusion simulations are performed to generate a detailed picture of the state of the light-TiO2-water system as a function of space and time. Our simulated structure of surface and bulk crystalline rutile TiO2 (110) consists of 11 compartments in a 1-D array, as shown in Figure (a). This structure assumes that each compartment is well-mixed. The 1st compartment (red layer) is the (110) surface of TiO2, where OER occurs, as illustrated in Figure ). Its thickness is assumed to be 1 nm, too thin for there to be significant light absorption. The 10 compartments underneath it are pure crystalline TiO2, where light absorption, charge carrier diffusion, and charge carrier losses occur.