Designing the Surfaces of Semiconductor Quantum Dots for Colloidal Photocatalysis Emily A. Weiss* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States ABSTRACT: This Perspective reviews strategies for tuning the surface chemistry of colloidal semiconductor nanocrystals (quantum dots, QDs) to function as photoredox catalysts or sensitizers of redox catalysts for organic transformations. These strategies include (i) tuning surface charge density to encourage high-affinity interactions between the QD and substrate (or co-catalyst) in the absence of a covalent linkage, (ii) maximizing the QD’s catalytic surface area through ligand exchange, (iii) using “hole shuttle” ligands to efficiently extract oxidative equivalents from the QD core, and (iv) controlling the concentration of protons on the QD surface to lower the kinetic barrier for proton-coupled electron-transfer reactions.
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explored for this purpose 30 years ago. We do not yet know the full potentialor the full set of idiosyncrasiesof QDs with
any reactions that produce, or facilitate the production of, fuelssuch as the reduction of CO2 to methanol or methane, the reduction of protons (from water) to H2, the breakdown of lignin to access cellulose/hemicellulose that is hydrolyzed to methanol, or the fixation of nitrogen to ammoniarequire multiple reductions or oxidations, or both, of small-molecule substrates. These “redox” reactions can be conducted more sustainably by powering them with sunlight, through the use of a photoredox catalyst. In most cases, the relocation of electrons is accompanied by relocation of protons, and the combined requirements of multielectron transfer and multiproton transfer result in high kinetic barriers for these reactions, even when they are thermodynamically achievable. CO2 reduction to methanol or methane, for example, requires multiple highbarrier steps to occur in series, preferably all at the same catalytic site. An ideal photoredox catalyst for fuel production addresses both the thermodynamic and kinetic challenges of these reactions; it uses visible or lower-energy light to produce excited-state electrons (reductive equivalents) and holes (oxidative equivalents) with adequate potentials to form the final product and has means of lowering kinetic barriers for proton-coupled electron-transfer (PCET) reactions. Soluble metal chalcogenide nanocrystals, or quantum dots (QDs), when properly functionalized, are in many ways ideal colloidal photoredox catalysts due to their intrinsically high catalytic surface area, the electronic structure of their cores, and the chemical tunability of both their cores and their surfaces. Due to the synthesis of new quantum-confined materials and recent advancements in tuning the surface chemistry of QDs for selective interactions and stability, we must consider QDs as a new material for photocatalysis and not extrapolate their catalytic function and properties from those of bulk semiconductors, or even those of semiconductor nanoparticles © 2017 American Chemical Society
We do not yet know the full potential or the full set of idiosyncrasiesof QDs with respect to photocatalysis; therefore, it is our contention that every fundamental study of these properties, even if those studies do not utilize fuelrelevant substrates, is valuable in order to determine the capabilities and limitations of QDs as sensitizers, scaffolds, and electron and hole donors for proton-coupled, multielectron reactions. respect to photocatalysis; therefore, it is our contention that every fundamental study of these properties, even if those studies do not utilize fuel-relevant substrates, is valuable in order to determine the capabilities and limitations of QDs as sensitizers, scaffolds, and electron and hole donors for protoncoupled, multielectron reactions. This type of targeted mechanistic surveyusing the most sophisticated synthetic techniques, postsynthetic functionalization chemistry, and analytical tools availableis, in our opinion, the most useful strategy for developing QDs into usable, scalable photoReceived: January 20, 2017 Accepted: April 7, 2017 Published: April 7, 2017 1005
DOI: 10.1021/acsenergylett.7b00061 ACS Energy Lett. 2017, 2, 1005−1013
Perspective
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
Perspective
catalysts. I note here that, in this Perspective, I use the word “photocatalysis” to encompass both photocatalytic reactions, more precisely defined as those for which ΔGrxn < 0 but which have high kinetic barriers that light energy and other aspects of the catalyst must overcome such that the reaction proceeds at a reasonable rate, and photosynthetic reactions, for which ΔGrxn > 0, and, regardless of kinetic barriers, light energy is needed to make the reaction thermodynamically feasible. There are several excellent papers describing this distinction in more detail.1−3 The advantages of the electronic structure of QDstunable absorption profiles, large extinction coefficients, and enhanced redox potentials relative to those of bulk semiconductorsfor photoredox catalysis are well-documented.4−6 The excitonic nature of the QD excited state is also an advantage of QDs over small (i.e., completely absorptive) metal nanoparticles as photocatalysts and sensitizers, at least for now; extraction of a redox equivalent from an exciton is currently a betterunderstood process than extraction of a carrier from a plasmon. A desirable property of colloidal QDs as photosensitizers or direct catalysts for energy-relevant reactions that is lessdiscussed, perhaps because it is still in development, is their tunable surface chemistry. Upon going from millimeter-sized particles to nanocrystals (with diameters of ∼5 nm), the ratio of potentially catalytically active surface area to volume increases by a factor of ∼106. While the typical homogeneous catalyst has one or two binding sites for catalytic substrate molecules, QDs have tens to hundreds of sites that can, in principle, be functionalized to bind substrates in reactive geometries. Here, we outline four ways in which QD surfaces can be designed to overcome the kinetic overpotentials often present for even thermodynamically feasible redox reactions, barriers associated with electronic coupling between the catalyst and substrate (or sensitizer and catalyst), the availability of protons for PCET reactions, and the extraction of the hole from the QD, which, if inefficient, leads to both side products and photodegradation of the particle. High-Af f inity QD−Molecule Complexes for Ultrafast Delivery of Redox Equivalents. A desirable property of a catalyst particle is the ability to bind substrates in high-affinity, high-coupling geometries without covalent linking chemistry, in order to facilitate ultrafast multielectron transfer that does not rely on collisions between freely diffusing species. It is also important to find conditions that maximize the coverage of the substrate on the QD surface because the rate of charge transfer from an exciton to an adsorbed molecule scales linearly with the number of available ET pathways.7 Faster redox processes also increase the selectivity of catalytic reactions because fewer side products form from idle intermediates.8 Electrostatic interactions between a molecule and QD surface are an important mechanism for high-affinity adsorption, and a QD’s surface charge is conveniently tunable.9−11 For instance, viologens serve as efficient redox shuttles between QD photosensitizers and molecular catalysts12−15 and multielectron acceptors from QDs,16 even though they do not, in general, have a group that binds covalently to the QD surface. We determined that the adsorption geometry for the QD−viologen complex that leads to ultrafast electron extraction by the viologen is “face-on”, such that the bipyridyl core is oriented in a plane parallel to that of the QD surface,17 and is driven by electrostatic interactions, Figure 1. These interactions are enhanced when we synthesize QDs with chalcogenide-enriched
Figure 1. (A) Schematic diagram depicting the proposed adsorption geometries of the functionalized viologen ligand and two possible photoinduced electron-transfer (PET) pathways (through-bond (red) and through-space (blue)). If the PET pathway is through-bond, increasing the length of the alkyl chain will decrease the rate of PET by decreasing the donor−acceptor electronic coupling (dotted line in B). If the PET pathway is through overlap of the QD and the orbitals of the viologen core, then lengthening the alkyl chain will not change the donor− acceptor distance or the rate of PET (data points in B). From ref 17.
surfaces by injecting an excess of the intended terminating ion immediately prior to arresting growth. The affinity of viologen-based molecules for QD surfaces makes them superior charge-transfer partners and one of the few classes of acceptors that efficiently dissociate multiexciton states of QDs.18−20 For instance, we observed simultaneous two-electron transfer from the biexciton state of a CdS QD (created by multiphoton absorption) to a single viologen cyclophane, cyclobis(4,4′-(1,4-phenylene) bipyridin-1-ium-1,4phenylenebis(methylene)) tetrachloride (“ExBox4+”), Figure 2.21 The ultrafast multielectron transfer to ExBox4+ was enabled by its close contact to and strong electronic coupling with the CdS surface. The ligands for the QDs in this case, 3mercaptopropionic acid, are both short and negatively charged and therefore encouraged adsorption of the positively charged ExBox4+ molecule even without explicit covalent linkage. High-affinity binding of a QD with an organic molecule is also critical in the sensitization of meso-tetraphenylporphyrin iron(III) chloride (FeTPP) by CuInS2/ZnS QDs for the catalytic conversion of CO2 to CO using visible light (unpublished results). Without linking the QD to FeTPP, we observe the successive donation of three electrons from the QD to FeTPP (the first two in
