Lifetime and dynamics of charge carriers in carbon-incorporated ZnO nanostructures for water treatment under visible light: Femtosecond transient absorption and photoluminescence study

The fabrication of high performance photocatalysts working under visible light to fully degrade the organic pollutants is one of ultimate goals in environmental applications. We fabricated very high active carbon-incorporated ZnO nanowire-based photocatalyst by a simple low temperature (300°C) annealing process involved zinc acetate precursor and poly vinyl pyrrolidone (PVP). This novel carbon-incorporated ZnO nanowires decomposed 98% of MB only within 90min under sun-simulated irradiation. To understand the reason of this high activity, the effect of carbon content on dynamics and lifetime of charge carriers were investigated by Femtosecond transient absorption and Photoluminescence spectroscopy besides using SEM, XRD, UV-vis, BET, and FTIR to probe structural and physical properties. The femtosecond transient absorption spectroscopy revealed that there are four decay components for each of pure ZnO and carbon-incorporated ZnO photocatalyst. The different lifetimes can be assigned for electron-hole recombination at different levels. The fastest component in pure ZnO (τ1=50ps) represents the first stage of recombination of shallow-trapped electrons on the surface of the pure ZnO nanoparticles with holes in the valence band. This component is slower in the case of carbon-incorporated ZnO (τ1=90ps), indicating longer residence time of the electrons in the conduction band. The PL spectroscopy exhibited that interstitial oxygen and interstitial zinc are very abundant in carbon-incorporated ZnO samples and the FTIR spectroscopy showed that fragments of acetate and PVP are responsible for good adsorption of organic pollutants and also surface modification of ZnO surface.