Photocatalytically Active Superstructures of Quantum Dots and Iron Porphyrins for Reduction of CO₂ to CO in Water

Abstract

This paper describes the use of electrostatic assemblies of negatively charged colloidal CuInS/ZnS quantum dot (QD) sensitizers and positively charged, trimethylamino-functionalized iron tetraphenylporphyrin catalysts (FeTMA) to photoreduce CO to CO in water upon illumination with 450 nm light. This system achieves a turnover number (TON) of CO (per FeTMA) of 450 after 30 h of illumination, with a selectivity of 99%. Its sensitization efficiency (TON per Joule of photons absorbed) is a factor of 11 larger than the previous record for photosensitization of an iron porphyrin catalyst for this reaction, held by a system in which both QDs and metal porphyrin were uncharged. Steady-state and time-resolved optical spectroscopy provides evidence for electrostatic assembly of QDs and FeTMA. Control of the size of the assemblies with addition of a screening counterion, K, and a correlation between their measured size and their catalytic activity, indicates that the enhancement in performance of this system over the analogous uncharged system is due to the proximity of the FeTMA catalyst to multiple light-absorbing QDs and the selective formation of QD-FeTMA contacts (rather than QD-QD or FeTMA-FeTMA contacts). This system therefore shows the ability to funnel photoinduced electrons to a reaction center, which is crucial for carrying out reactions that require multistep redox processes under low photon flux, and thus is an important advance in developing artificial photocatalytic systems that function in natural light.

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

“…Forinstance,the positively charged CO 2 reduction cocatalysts of trimethylamino-functionalized iron tetraphenylporphyrin (FeTMA) were assembled with negatively charged CuInS 2 / ZnS QDs under mild conditions (Figure 4b,i nset). [37] Under the optimal conditions,the system evolved CO with aTON of 450 per catalyst after 30 hi llumination and as electivity of 99 % (Figure 4b). 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.…”

Water‐Soluble Conjugated Molecule for Solar‐Driven Hydrogen Evolution from Salt Water

“…Taken together, a water‐soluble conjugated small molecule would be a favorable choice to overcome these drawbacks . In this case, unfortunately, the loading of cocatalyst on a single‐molecule catalyst is relatively difficult due to the limited interface contact area . As a result, the diffusion‐controlled bimolecular electron transfer process between the individually separated small‐molecule catalyst and cocatalyst becomes the bottleneck to achieve high photocatalytic efficiency …”

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

“…b) Plot of TON vs. absorbed photon energy (bottom axis) and vs. illumination time (top axis) reproduced from Ref. with the permission from ACS.…”