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 illuminated TiO2 suspensions are always smaller than, but approach, at sufficiently high intermittence, the phi (F)'s measured under continuous exposure at equivalent 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 equal 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 tau) = 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) climbs 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 gamma = 0.05. Thus, the photocatalytic oxidation of formate in similar to 10 nm TiO2 nanoparticle suspensions under periodic illumination behaves kinetically as a homogeneous photochemical system; i.e., phi (F)'s are not limited by mass diffusion, or by adsorption/desorption, but by carrier recombination. The latter has a characteristic time of about 0.1 s under present conditions. Sparse carriers, such as those present in gamma = 0.05 experiments at short tau (L)'s, are deactivated within similar to6 ms. Therefore, photocatalytic quantum yields on nanoparticle surfaces are actually insensitive to events in the sub-milliseconds domain.