Time Scales and pH Dependences of the Redox Processes Determining the Photocatalytic Efficiency of TiO_2 Nanoparticles from Periodic Illumination Experiments in the Stochastic Regime

Abstract

The quantum yield, φ, of methyl orange photocatalytic oxidation on TiO_2 nanoparticles under periodic illumination drops from φ_(τ_L)→0 to φ_(τ_L)→∞ in two well-resolved steps at increasingly longer bright intervals, τ_L, when they are separated by sufficiently long dark periods, τ_D. The {τ_(L1) < τ_(L2)} values at which the inflections occur depend exponentially, but with opposite trends, on the solution pH. The condition τ_D ≫ τ_L 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 φ_(τ_L)→0 and φ_(τ_L)→∞ coincide with the φ values measured under steady illumination at (γI_a) and I_a photon absorption rates, respectively, γ = τ_L/(τ_L + τ_D) being the duty cycle. The similar φ vs τ_L 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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