Effects of spatial topologies and electron Fermi-level gradient on the photocatalytic efficiency of nano-particulate semiconductors

Abstract

Nanocrystalline (nc) semiconductor materials are important in photocatalysis. The nanoparticle (NP) topologies and electron Fermi-level (E_F) gradient along the interconnected NPs affect the photocatalytic efficiency (η) of the nc-materials because of the charge carrier interparticle transport (IPT). However, the detailed physiochemical kinetic mechanism remains unclear. Based on the kinetic analysis and the numerical Monte-Carlo simulation of random walks, the statistical probability distributions p_Rec(t) and p_it(t) for the recombination time and interfacial transfer (IT) time have been proposed in this study. The recombination lifetime (τ_Red) and IT lifetime (τ_IT) were calculated by averaging p_Rec(t) and p_it(t). The characteristic time τ_e of the entire electron kinetics was defined using τ_Re and τ_IT, and η was calculated by dividing τ_e by τ_IT. The simulation results show that the p_Rec(t) clearly shows the IPT of electrons. Both the kinetic factors (NP spatial topologies and boundary barrier) and the thermodynamic factor (electron E_F gradient) can affect the IPT. It was observed that the increase in IPT cannot lead to a monotonous increase in η although it can prohibit recombination. Whether the IPT can increase the η is dependent on ratio of the back IPT for recombination and the forward IPT for IT. The existence of an electron E_F gradient from the electron generation site to the active site can increase η by promoting the forward IPT.