Quantum Yields of the Photocatalytic Oxidation of Formate in Aqueous TiO_2 Suspensions under Continuous and Periodic Illumination

Abstract

Quantum yields φF 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〉 = γI_(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.

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