Nanorod length-dependent photodriven H2 production in 1D CdS-Pt heterostructures.

Abstract

Colloidal quantum confined semiconductor-metal heterostructures are promising candidates for solar energy conversion because their light absorbing semiconductor and catalytic components can be independently tuned and optimized. Although the light-to-hydrogen efficiencies of such systems have shown interesting dependences on the morphologies of the semiconductor and metal domains, the mechanisms of such dependences are poorly understood. Here, we use Pt tipped 0D CdS quantum dots (with ∼4.6nm diameter) and 1D CdS nanorods (of ∼13.8, 27.8, 66.6, and 88.9nm average rod lengths) as a model system to study the distance-dependence of charge separation and charge recombination times and their impacts on photo-driven H2 production. The H2 generation quantum efficiency increases from 0.2% ± 0.0% in quantum dots to 28.9% ± 0.4% at a rod length of 28nm and shows negligible changes at longer rod lengths. The half-life time of electron transfer from CdS to Pt increases monotonically with rod length, from 0.7 ± 0.1 in quantum dots to 170.2 ± 29.5ps in the longest rods, corresponding to a slight decrease in electron transfer quantum efficiency from 92% to 81%. The amplitude-weighted average lifetime of charge recombination of the electron in Pt with the hole in CdS increases from 4.7 ± 0.4 µs in quantum dots to 149 ± 34 µs in 28nm nanorods, and the lifetime does not increase further in longer rods, resembling the trend in the observed H2 generation quantum efficiency. Our result suggests that the competition of the charge recombination process with the hole removal by the sacrificial electron donor plays a dominant role in the observed nanorod length dependent overall light driven H2 generation quantum efficiency.