Quantum Yields of the Photocatalytic Oxidation of Formate in Aqueous TiO2 Suspensions under Continuous and Periodic Illumination

Quantum yields φ F for the oxidation of formate in periodically illuminated TiO 2 suspensions are always smaller than, but approach, at sufficiently high intermittence, the φ F 's measured under continuous exposure at equivalent average photon absorption rates 〈 I a 〉. We find that φ F,cont = (0.031 ± 0.003)× I a -0.39±0.03 in the range 0.089 ≤ I a /μeinstein L -1 s -1 ≤ 2.02. Under periodic illumination, φ F begins to rise from its minimum value: φ F,long τ = φ F,cont ( I a,max ) = 0.021, for light periods τ L ≤ 1 s, regardless of the duty cycle γ. Thereafter, φ F climbs to its upper limit: φ F,short τ = φ F,cont (〈 I a 〉 = γ I a,max ), after a single inflection at τ L ∼ 200 ms for γ = 0.35, but only after a second inflection at τ L ∼ 10 ms for γ = 0.05. Thus, the photocatalytic oxidation of formate in ∼10 nm TiO 2 nanoparticle suspensions under periodic illumination behaves kinetically as a homogeneous photochemical system; i.e., φ 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 γ = 0.05 experiments at short τ L 's, are deactivated within ∼6 ms. Therefore, photocatalytic quantum yields on nanoparticle surfaces are actually insensitive to events in the sub-milliseconds domain.