Quantum yields of the photocatalytic oxidation of formate in aqueous TiO2 suspensions under continuous and periodic illumination

Quantum yields phi (F) for the oxidation of formate in periodically illumin ated TiO2 suspensions are always smaller than, but approach, at sufficientl y high intermittence, the phi (F)'s measured under continuous exposure at e quivalent average photon absorption rates [I-a], We find that phi (F,cont) = (0.031 +/- 0.003) x I-a(-0.39+/-0.03) in the range 0.089 less than or equ al to I-a/mu einstein L-1. s(-1) less than or equal to 2.02. Under periodic illumination, phi (F) begins to rise from its minimum value: phi (F,long t au) = phi (F,cont)(I-a,I-max) = 0.021, for light periods tau (L) less than or equal to 1 s, regardless of the duty cycle gamma. Thereafter, phi (F) cl imbs to its upper limit: phi (F,short tau) = phi (F,cont) ([I-a] = gammaI(a ,max)), after a single inflection at tau (L) similar to 200 ms for gamma = 0.35, bur, only after a second inflection at tau (L) similar to 10 ms for g amma = 0.05. Thus, the photocatalytic oxidation of formate in similar to 10 nm TiO2 nanoparticle suspensions under periodic illumination behaves kinet ically as a homogeneous photochemical system; i.e., phi (F)'s are not limit ed by mass diffusion, or by adsorption/desorption, but by carrier recombina tion. The latter has a characteristic time of about 0.1 s under present con ditions. Sparse carriers, such as those present in gamma = 0.05 experiments at short tau (L)'s, are deactivated within similar to6 ms. Therefore, phot ocatalytic quantum yields on nanoparticle surfaces are actually insensitive to events in the sub-milliseconds domain.