Size-controlled electron transfer rates determine hydrogen generation efficiency in colloidal Pt-decorated CdS...

Semiconducting quantum dots (QDs) have been considered as promising building blocks of solar energy harvesting systems because of size-dependent electronic structures, e.g. QD-metal heterostructures for solar-driven H2 production. In order to design improved systems, it is crucial to understand size-dependent QD-metal interfacial electron transfer dynamics, picosecond processes in particular. Here, we report that the transfer rates of photogenerated electrons in Pt-decorated CdS QDs can be varied over more than two orders of magnitude by controlling the QD size. In small QDs (2.8 nm diameter), conduction band electrons transfer to Pt sites in an average timescale of ∼30 ps, giving a transfer rate of 2.9 × 1010 s-1 while in significantly larger particles (4.8 nm diameter) the transfer rates decrease to 1.4 × 108 s-1. We attribute this to the tuning of the electron transfer driving force via the quantum confinement-controlled energetic off-set between the involved electronic states of the QDs and the co-catalyst. The same size-dependent trend is observed in the presence of an electron acceptor in solution. With methyl viologen present, electrons leave the QDs within 1 ps for 2.8 nm QDs while for 4.6 nm QDs this process takes nearly 40 ps. The transfer rates are directly correlated with H2 generation efficiencies: faster electron transfer leads to higher H2 generation efficiencies. 2.8 nm QDs display a H2 generation quantum efficiency of 17.3%, much higher than the 11.4% for their 4.6 nm diameter counterpart. We explain these differences by the fact that slower electron transfer cannot compete as efficiently as faster electron transfer with recombination and other losses.