Selectivity of photoelectrochemical $CO_2$ reduction modulated with electron transfer by controlling the size of CdSe nanocrystals

Abstract

We investigate the product selectivity of $CO_2$ reduction using NiO photocathodes decorated with CdSe quantum dots (QDs) of varying size in a photoelectrochemical (PEC) cell. Size-tunable and quantized energy states of conduction band in CdSe QDs enable systematic control of electron transfer kinetics from CdSe QDs to NiO. It turns out that different size of CdSe QDs results in variation in product selectivity for CO2 reduction. The energy gap between conduction band edge and redox potential of each reduction product (e.g., CO and $CH_4$) correlates with their production rate. The size dependence of the electron transfer rate estimated from the energy gap is in agreement with the selectivity of $CO_2$ reduction products for all reduction products but CO. The deviation in the case of CO is attributed to sequential conversion of CO into $CH_4$ with CO adsorbed on electrode surface. Based on a premise that the CdSe QDs would exhibit similar surface configuration regardless of QD size, it is concluded that the electron transfer kinetics proves to alter the selectivity of $CO_2$ reduction.