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 〈I a〉. We find that φF,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, φF begins to rise from its minimum value:  φF,long τ = φF,cont(I a,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 (〈I a〉 = γI a,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.

The quantum yield, φ, of methyl orange photocatalytic oxidation on TiO2 nanoparticles under periodic illumination drops from φτ L → 0 to φτ L → ∞ in two well-resolved steps at increasingly longer bright intervals, τL, when they are separated by sufficiently long dark periods, τD. The {τL1 < τL2} values at which the inflections occur depend exponentially, but with opposite trends, on the solution pH. The condition τD ≫ τL ensures charge sparsity, leads to a stochastic kinetics regime in which carrier recombination is minimized, and lets carriers manifest their dissimilar redox reactivities. The more reactive intermediates in acid (basic) media are ascribed to the photogenerated holes (electrons), the crossover occurring ca. pH 8. We found that φτ L → 0 and φτ L → ∞ coincide with the φ values measured under steady illumination at (γI a) and I a photon absorption rates, respectively, γ = τL/(τL + τD) being the duty cycle. The similar φ vs τL behaviors observed for methyl orange and formate are traceable to the dynamics of interfacial species. The photochemically relevant intermediates persist longer than a few milliseconds under typical conditions, i.e., several orders of magnitude longer that the plethora of transient spectroscopy signals (half-lives down to <1 ps) previously associated with these species.

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