Time Scales and pH Dependences of the Redox Processes Determining the Photocatalytic Efficiency of TiO2 Nanoparticles from Periodic Illumination Experiments in the Stochastic Regime

Abstract

The quantum yield, φ, of methyl orange photocatalytic oxidation on TiO2 nanoparticles under periodic illumination drops from φτL→0 to φτL→∞ 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 φτL→0 and φτL→∞ coincide with the φ values measured under steady illumination at (γIa) and Ia 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...