Hole Removal Rate Limits Photodriven H₂ Generation Efficiency in CdS-Pt and CdSe/CdS-Pt Semiconductor Nanorod–Metal Tip Heterostructures

Abstract

Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.

Photophysical Properties of CdSe/CdS core/shell quantum dots with tunable surface composition

“…The inefficiencies of CdS as a hole confining shell, combined with the accessible hole trap states on the surface, make CdSe/CdS systems with sulfur termination viable candidates for QD-sensitized photocatalytic hydrogen generation systems 9, 10, 72 , where hole quenching in QDs by a sacrificial donor has been shown to be the rate limiting step 73 .…”

Efficiency of Hole Transfer from Photoexcited Quantum Dots to Covalently Linked Molecular Species

“…(8) Electron transfer studies (8)(9)(10)(11)(12)(13) outnumber hole studies, (14)(15)(16)(17)(18)(19) despite hole transfer being the limiting factor in the efficiencies of QD sensitized solar cells and in QD-based colloidal photocatalytic hydrogen evolving systems. (20,21) To establish a sound model for charge transfer from nanocrystals to molecular acceptors, we must address the features of this system that make the process more difficult to characterize than that of the pure molecular case. In addition to the intrinsic intensive parameters of the Marcus model, such as the driving force, the electronic coupling between donor and acceptor, and inner-sphere and outersphere reorganization energies, in QDs one must also contend with the possible presence of trap states of unknown energetic and spatial distribution on the QD surface, (22,23) and with the need to precisely quantify the number of molecular acceptors attached to the QD.…”

Rational design of CdS@ZnO core-shell structure via atomic layer deposition for drastically enhanced photocatalytic H2 evolution with excellent photostability

“…Photocatalytic H 2 evolution from water splitting, which directly converts solar energy into clean chemical energy without pollution, has attracted much attention [6][7][8][9][10][11]. Since the pioneering work discovered by Fujishima and Honda [12], there has been considerable development in the design and construction of highly-efficient semiconductor photocatalysts, such as TiO 2 [13][14][15][16], ZnO [19][20][21], CdS [22][23][24][25][26][27], etc.…”