Competitive Hole Transfer from CdSe Quantum Dots to Thiol Ligands in CdSe-Cobaloxime Sensitized NiO Films Used as Photocathodes for H2 Evolution

Quantum dot (QD) sensitized NiO photocathodes rely on efficient photoinduced hole injection into the NiO valence band. A system of a mesoporous NiO film co-sensitized with CdSe QDs and a molecular protonreduction catalyst was studied. While successful electron transfer from the excited QDs to the catalyst is observed, most of the photogenerated holes are instead quenched very rapidly (ps) by hole trapping at the surface thiols of the capping agent used as linker molecules. We confirmed our conclusion by first using a thiol free capping agent and second varying the thiol concentration on the QD’s surface. The later resulted in faster hole trapping as the thiol concentration increased. We suggest that this hole trapping by the linker limits the H2 yield for this photocathode in a device. Visible-light-driven H2 evolution has been attracting considerable attention from both environmental and energy points of view and also as a scientific challenge in the artificial photosynthesis community. Many systems have been employed to produce H2 from sunlight−water splitting in a similar way to the natural photosynthetic systems. In general, these systems contain a combination of light harvesting material and an active catalyst that is able to produce H2 after charge transfer from the light harvester. Therefore, a good understanding of the factors limiting photoinduced charge separation is needed to improve the catalytic activity of these systems. Semiconductors quantum dots (QDs) like CdSe or CdS are popular sensitizers for solar energy applications including solar fuels. Here we report the photoinduced charge dynamics of both electrons and holes in an artificial system for H2 evolution that contains CdSe QDs as the light harvester and the wellknown molecular CoP catalyst ([CoCl(dmgH)2(pyridyl-4hydrophosphonate)]) (Figure 1). Both the QDs and the CoP are chemically attached to a mesoporous NiO film via thioglycolic acid (TGA) and phosphonate groups, respectively. This system was reported recently by some of us as an efficient and stable photocathode for H2 production. 4 CdSe QDs have been chosen as light harvesters due to their high extinction coefficient, broad absorption spectrum, sizedependent tunability of the band gap, and relatively simple preparation methods. Moreover, the CoP catalyst belongs to the best transition metal complexes known to produce H2 and is also easy to prepare. Herein, we investigate the photoinduced charge transfer dynamics upon excitation in this photocathode and find evidence for ultrafast hole transfer reactions that compete with productive hole injection into NiO. A one-pot adsorption and reaction (OPAR) method was used to prepare the TGA-capped CdSe QDs (see the SI for more details), while the CoP catalyst was prepared according to the published procedure. In this Letter, we find that the capping agent (TGA) of the QDs works as a trapping site for the photogenerated holes by using femtosecond transient absorption (TA) spectroscopy in the visible region. While the electrons are transferred from the excited QDs to the CoP catalyst as intended, most of the holes are trapped by the thiol groups and/or sulfide ions from the capping agent. We Received: August 11, 2017 Accepted: October 16, 2017 Published: October 16, 2017 Leter http://pubs.acs.org/journal/aelccp © 2017 American Chemical Society 2576 DOI: 10.1021/acsenergylett.7b00730 ACS Energy Lett. 2017, 2, 2576−2580 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.