Reliable carbon dioxide photoreduction by a rational electron transfer cycle formed on a nanorod-shaped CdS/Fe2O3 heterojunction catalyst

Abstract

Herein we focused on developing a narrow-bandgap oxide catalyst as a light absorber and on improving the selectivity of the desired product upon conversion to CO2. CdS/Fe2O3 catalysts were prepared by heterogeneously joining CdS nanorods, which have well-known activity as a water splitting catalyst, and cheap Fe2O3 particles, which are easily absorbed at wavelength range 550–600 nm. After 10 h of UV irradiation, the cumulative amount of CO2 intermediate and CH4 product in the 1CdS/1Fe2O3 heterojunction catalyst was 16.5 μmolg−1 after 10 h. The amount of CO increased with increasing Fe content. Conversely, the amount of total reduction product was greatly reduced, to 0.41 μmolg−1 after 10 h of visible irradiation, but the CH4 produced increased more than 5 times compared with the amount of CO. GC–MS spectrometry analysis of 13C labeling showed that the CH4 conversion of CO2 on the CdS/Fe2O3 heterojunction catalyst proceeds by the proton-assisted multi-electron reduction via the carbene pathway. From the results of PL and photocurrent analysis, we conclude that in CdS/Fe2O3 heterojunction particles, the excited electrons move between the two particles to inhibit the recombination of electrons and holes, whereby the catalytic activity is greatly improved. In particular, the movement of electrons in the heterojunction particles was observed flowing naturally without clogging based on the Z-scheme system. Furthermore, the electrons emitted from CdS do not recombine with their holes, but move to the VBs of adjacent Fe2O3 and fill them, exciting more electrons. Eventually, the rational electron transfer cycle between Cd2+/Fe3+ redox couples in the CdS/Fe2O3 heterojunction catalyst could act a driving force for promoting the photocatalytic reaction by naturally filling or moving electrons and holes in the photocatalyst system.