People: Shichen Lian, Kevin McClelland, Zhengyi Zhang, Kaitlyn Perez, Yishu Jiang, Wooje Chang, Woody Rosenberg
In photoredox-based catalysis, photochemical potential (generated by absorption of photons) is used to drive a chemical transformation through excited-state single or multi-charge transfer from the catalyst to the substrate of interest. Here, we term the phrase “colloidal photocatalysis” to refer to photocatalytic systems that involve colloidal nanocrystals; colloidal photocatalysis represents a new paradigm for light-induced chemical transformations that stands alongside homogenous and heterogenous photocatalysis.
The use of quantum dots (QDs) in photocatalysis is driven by their many attractive properties: their high photostability, their tunable optoelectronic properties, large extinction coefficients (>106), high PL quantum yields, and tunable surface chemistry. Our research is focused upon developing new reaction schemes that take advantage of the unique properties of QDs.
Quantum Dots as Photosensitizers
QDs are frequently used as photosensitizers for molecular (or metallic) co-catalysts to drive chemical transformations. The goal is that a photo-excited QD transfers its photo-generated carriers to a redox-active catalyst that can then perform a useful reaction. Examples include hole transfer to a water oxidation catalyst or electron transfer to proton reduction catalyst to make molecular hydrogen as a solar fuel. However, many reactions that produce, or facilitate the production of, fuels require multiple reductions (or oxidations) of the substrate and have large kinetic over-potentials that limit the efficiencies of thermodynamically feasible reactions. We believe that QDs used in combination with molecular catalysts can be used to efficiently generate solar fuels as well as understand the fundamental processes entailed in the sensitization of the catalyst. Below, we highlight some new sensitization schemes involving QDs.
Quantum Dot Assemblies for Photocatalysis
Photosensitization of molecular catalysts to reduce CO2 to CO is a sustainable and desirable route to production of solar fuels. Crucial to the sensitization process is the highly efficient transfer of multiple redox equivalents from sensitizer to catalyst. Therefore, the ideal sensitization scheme involves multiple QD sensitizers per catalyst.
Here we demonstrate that the photosensitization of a model CO2 reduction catalyst, meso-tetraphenylporphyrin iron(III) chloride (FeTPP) catalyst is achieved by colloidal, heavy metal-free CuInS2/ZnS (CIS) QDs to reduce CO2 to CO using 450 nm light. The sensitization efficiency (turnover number per absorbed unit of photon energy) of the QD system is a factor of 198 greater than that of an analogous system with a fac-tris(2-phenylpyridine)iridium sensitizer. This high efficiency originates in ultrafast electron transfer between the QDs and FeTPP, enabled by formation of electrostatic QD/FeTPP complexes. The QDs and FeTPP molecule are negatively and positively charge, respectively, which allows for their spontaneous electrostatic assembly in water. Optical spectroscopy reveals that the electron-transfer processes occur on the ultrafast timescale.
However, photocatalysis in water (the ideal solvent for the production of solar fuels) introduces additional difficulties such as the self-aggregation of QDs. We have further optimized the earlier-shown QD-catalyst system with QDs that are capped with negatively charge ligands (3-mercaptopropionic acid, MPA) and the catalysts are functionalized with a positively charged amine group. Adding potassium modulates the electrostatic interaction between the QD sensitizers and catalysts and enhances the overall catalytic efficiency. This systems serves a photosynthetic mimic where we have multiple QD sensitizers per catalyst resulting in the high efficiency of CO2 reduction.
Similarly, we can also create electrostatic QD assemblies where light energy is funneled through these superstructures to catalytic QDs through excitonic energy transfer (EnT), akin to how natural photosynthesis operates. In these QD assemblies, we observe that the addition of QD sensitizers results in an increase of the IQE of the catalyst QDs by a factor of 15.
We can take these QD assemblies a step further by combining two different material QDs that assembly through hydrogen bonding interactions in a redox potential Z-scheme. Both exciting both of the QDs simultaneously with visible light, we can achieve effective upconversion of light with enhanced redox potentials that could not be obtained without the use of UV light.
Quantum Dots as Triplet Energy Transfer Donors
Due to the extremely small (~10 meV) splitting of the singlet-like and triplet-like excitons in QDs, we can use these high energy triplet-like “dark” excitons to access reactive triplet states of molecules that have singlet states that are too short-live to be precursors for chemical transformations. By doing triplet EnT from a QD to a molecular substrate, we can induce chemical reactions such as a 2+2 cycloaddition reaction that wouldn’t otherwise occur.
Quantum Dots Serving as Solo Photocatalysts
One under-utilized property of QDs in photocatalysis is their tunable surface chemistry. QDs have large potentially catalytic surfaces (as compared to bulk materials) with the ability to bind to multiple substrate molecules (whereas the typical homogeneous catalyst can only bind one or two), which allows colloidal photocatalysis to eliminate the traditional photosensitizer-catalyst paradigm found in homogenous catalysis. By using QDs as the sensitizer and catalyst, we create simpler catalytic systems that can perform a wide range of chemical transformations.
QD Surfaces can bind substrates with high-affinity
Here, we demonstrated that CdS QDs photocatalyze the reduction of protonated nitrobenzene ([NB H]+) to aniline ([AN H]+) through six sequential proton coupled electron transfer steps, with nitrosobenzene ([NSB H]+) and phenylhydroxylamine ([PHA H]+) intermediates, and using 3-mercaptopropionic acid (MPA) and MeOH as proton and terminal electron sources. Oxidation of MPA produces the corresponding disulfide (“MPA-MPA”) and serves as the primary sacrificial electron donor. Nuclear magnetic resonance (NMR) experiments showed that the reagents were adsorbed to the QD surface throughout the catalytic cycle such that electron transfer from the QD never became diffusion-controlled (on the order of microseconds). This allows us to analyze measured rate constants for each catalytic step in terms of energetic driving force and kinetic barriers for the reaction.
Ligand-exchange can increase the catalytic surface area
One advantage of QDs as photocatalysts is that simple ligand exchanges can tune the structure of the particle surface and increase the efficacy for substrate binding with minimal changes in the electronic or redox properties of the QD core. Ligand exchange can be performed to change the permeability of the ligand shell, its hydrophobicity, or its charge density to optimize interactions with the QD and the substrate.
In one of the very few examples of QD-catalyzed C-C coupling, we adjusted the surface chemistry of CdS QDs to tune their photocatalytic with regard to a C-C coupling reaction, the addition of 1-phenyl pyrrolidine and phenyl trans-styryl sulfone to form 1-phenyl-2-(2-phenylethenyl) pyrrolidine. A partial ligand exchange of the QDs’ native ligands, oleate, for octylphosphonate increase the disorder of the QD surface. We suspect the increased surface disorder lead to an increase area for substrate binding, which was supported by PL measurements, and an increased turnover rate for the system.
Ligand-exchange can also induce enantioselective C-C coupling
We can use CdS QDs to enantioselective photocatalyze the classical α-alkylation of aldehydes, where QDs can not only serve as the photosensitizers but also provide steric hindrance for inducing chiral selectivity for the reaction. Compared to previously reported photocatalytic systems employing molecular dye sensitizers and separated enantioselective catalysts, the QD photocatalyst we have developed has three advantages:
1) the highly absorbing QDs replace previously used precious metal complexes, such as Ru(bpy).
2) the on-site enantio-inducing ligands minimize the diffusion distance for the intermediates, and hence improve the reactivity and can reduce the loading of enantioselective catalysts by two orders of magnitude (as compared to other systems in the literature)
3) the facile ligand exchange procedure leads to easier controls of enantiofacial discrimination as compared to organocatalysts that require extensive synthetic efforts..
QD-ligand systems can serve as probe for the local proton concentration for PCET reactions
As we have shown (and others have reported), QDs are efficient photocatalysts for proton-coupled electron transfer (PCET) reactions that require one or multiple photo-redox equivalents, provided that the substrate is close enough to the QD to undergo charge transfer. Tracking the kinetics of charge transfer between the QD and the substrate is easily achieve using standard time-resolved spectroscopy such as transient absorption, but tracking the kinetics of proton transfer is more analytically challenging. We have shown previously that the protonation state of phosphonate ligands affects the first exciton absorbance energy of the quantum dot. By calibrating the degree of the first exciton absorbance energy shift to the exact number of protons proximate to the surface of the quantum dot, we can develop a QD-based local proton sensor that functions in polar aprotic solvents (in which proton-coupled electron transfer reactions are typically performed). The first exciton absorbance energy of the quantum dot can then serve as analytical probe of the surface-proximate protons available for participation in quantum dot-catalyzed proton-coupled electron transfer reactions and provide insights into the timescale of proton transfer in these systems.
Tuning the size of QDs to modify the selectivity
We can also modify the size of the QDs as a method of modifying the selectivity of the QDs. Since QDs are subject to “quantum confinement effects” that increase the energies of the conduction and valence band of the QDs (as compared to bulk semiconductors), we can easily tune the energy levels of the QDs by modifying the size. This allows us to change the reactivity of the QDs for different photoredox-based reactions.
Below, we demonstrate the photocatalytic oxidation of benzyl alcohol with cadmium sulfide quantum dots (CdS QDs), and control over the selectivity of this process for either a two-electron oxidation of one alcohol molecule to benzaldehyde or two one-electron oxidations of two alcohol molecules followed by carbon-carbon coupling to hydrobenzoin. Product distributions are controlled by changing the QD:alcohol ratio and the size of the QD and result in over 80% selectivity for either pathway.