Energy transfer-enhanced photocatalytic reduction of protons within quantum dot light-harvesting–catalyst assemblies

Abstract

Excitonic energy transfer (EnT) is the mechanism by which natural photosynthetic systems funnel energy from hundreds of antenna pigments to a single reaction center, which allows multielectron redox reactions to proceed with high efficiencies in low-flux natural light. This paper describes the use of electrostatically assembled CdSe quantum dot (QD) aggregates as artificial light harvesting-reaction center units for the photocatalytic reduction of H to H, where excitons are funneled through EnT from sensitizer QDs (sQDs) to catalyst QDs (cQDs). Upon increasing the sensitizer-to-catalyst ratio in the aggregates from 1:2 to 20:1, the number of excitons delivered to each cQD (via EnT) per excitation of the system increases by a factor of nine. At the optimized sensitizer-to-catalyst ratio of 4:1, the internal quantum efficiency (IQE) of the reaction system is 4.0 ± 0.3%, a factor of 13 greater than the IQE of a sample that is identical except that EnT is suppressed due to the relative core sizes of the sQDs and cQDs. A kinetic model supports the proposed exciton funneling mechanism for enhancement of the catalytic activity.

Quantum Dot Assembly for Light‐Driven Multielectron Redox Reactions, such as Hydrogen Evolution and CO2 Reduction

“…As aresult of the ultra-fast and consecutive electron transfer from multiple QDs to as ingle FeTMA cocatalyst, the sensitization efficiency( TONp er Joule of photons absorbed) of the assembled system was afactor of 11 larger than that of the uncharged CuInS 2 /ZnS QDs/FeTMA physical mixture in DMSO.N otably,m ost of the photosystems reported so far can only reduce CO 2 to CO.F urther study to realize highly effective and selective conversion of CO 2 into deep reduction products (e.g.,m ethanol and methane) is highly desirable.R ecently,W eiss et al revealed that the promoted excitonic energy transfer in the electrostatic interaction induced QD assembly could also effectively enhance the efficiencyofs olar-to-H 2 evolution. [38] To integrate the single-photon/electron events with multielectron reduction reactions (e.g., H 2 generation, CO 2 reduction), construction of assemblies consisting of multiple QDs and asingle cocatalyst is regarded as an effective pathway.As shown in the proposed model of Figure 5a,multiple QDs are assembled around as ingle cocatalyst. Thed istance (d) between QDs and catalytic center is readily controlled by modulating the length of the linker.Inaddition, by adjusting the size distribution and surface properties of the two components,t he ratio of QDs to cocatalysts in the assembly can be well controlled.…”

“…a) Assembled architecture of multiple QDs and asingle cocatalysti nwater.b )Photocatalytic H 2 evolution of aC dSe/CdS QDs/ Pt nanoparticle assembly and aphysicalm ixture of QDs and Pt nanoparticles under the same conditions. The inset is arepresentation of the assembly.Reproduced from Ref [38]. with the permission from ACS.…”

Colloidally Stable CdS Quantum Dots in Water with Electrostatically Stabilized Weak‐Binding, Sulfur‐Free Ligands

“…[1][2][3] Among the range of nanoparticles that photocatalyze aqueous reactions, semiconductor quantumd ots (QDs) stand out for their chemical and electronic tunability and versatility.Q Ds have demonstrated unprecedented activity in photocatalytic H 2 evolution, [4][5][6][7] unprecedented sensitizatione fficiency for CO 2 reduction, [8] the ability to serve as triplet exciton donors and scaffolds for stereoselective cycloadditions, [9,10] and the electrochemical potentials to act as both photo-oxidant and photo-reductant in CÀCc oupling schemes with no sacrificial reagents. [11,12] In water,p roperly functionalized colloidal QDs form colloidally stable aggregates to mimic the exciton funneling function of photosystemII, [13] and perform chemoselective alcohol oxidations. [14] The best-quality as-synthesizedQ Ds are capped with hydrophobic ligands, and surface modification is necessary to transfer the QDs from nonpolars olvents to aqueous solution.…”

Impact of FRET between Molecular Aggregates and Quantum Dots

“…The FRET process is well‐known in a range of systems, such as metal‐nanoparticle/dye surfaces, metal‐semiconductor–heterostructure composites, and semiconductor–semiconductor systems (solid state, which is distance‐independent) . Most importantly, the FRET process has been well‐demonstrated in photovoltaic devices for molecular‐adsorbate–QD composite systems, QD–QD systems, and recently for enhancing the reduction reaction in photocatalytic water splitting . The FRET process from a semiconductor nanocrystal (NC) to the excited state of a molecular species has been extensively studied .…”

“…[27,28] Most importantly,t he FRET process has been well-demonstrated in photovoltaic devices for molecular-adsorbate-QD composite systems, [29][30][31][32][33][34][35][36] QD-QD systems, [19,24,37] andr ecently for enhancing the reduction reactioni np hotocatalytic water splitting. [38] The FRET process from as emiconductor nanocrystal (NC) to the excited state of am olecular speciesh as been extensively studied. [29,32,[39][40][41][42][43] However,i na ll of these studies, monomeric species of the dye molecules acted as either the acceptoro rt he donor,d epending on their spectroscopic nature.H owever,c ertain dye molecules, which have been extensively used in photovoltaic applicationso wing to their chromophoric properties, have at endency to form aggregated states, even at normal concentrations.…”