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

Abstract

Quantum yields φF for the oxidation of formate in periodically illuminated TiO2 suspensions are always smaller than, but approach, at sufficiently high intermittence, the φF's measured under continuous exposure at equivalent average photon absorption rates 〈Ia〉. We find that φF,cont = (0.031 ± 0.003)×Ia-0.39±0.03 in the range 0.089 ≤ Ia/μeinstein L-1 s-1 ≤ 2.02. Under periodic illumination, φF begins to rise from its minimum value: φF,long τ = φF,cont(Ia,max) = 0.021, for light periods τL ≤ 1 s, regardless of the duty cycle γ. Thereafter, φF climbs to its upper limit: φF,short τ = φF,cont (〈Ia〉 = γIa,max), after a single inflection at τL ∼ 200 ms for γ = 0.35, but only after a second inflection at τL ∼ 10 ms for γ = 0.05. Thus, the photocatalytic oxidation of formate in ∼10 nm TiO2 nanoparticle suspensions under periodic illumination behaves kinetically as a homogeneous photochemical system; i.e., φF'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 τL's, are deactivated within ∼6 ms. Therefore, photocatalytic quantum yields on nanoparticle surfaces are actually insensitive to events in the sub-milliseconds domain.