2 research outputs found

    Computational Design Principles of Two-Center First-Row Transition Metal Oxide Oxygen Evolution Catalysts

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    Computational screens for oxygen evolution reaction (OER) catalysts based on Sabatier analysis have seen great success in recent years; however, the concept of using chemical descriptors to form a reaction coordinate has not been put under scrutiny for complex systems. In this paper, we examine critically the use of chemical descriptors as a method for conducting catalytic screens. Applying density functional theory calculations to a two-center metal oxide model system, we show that the Sabatier analysis is quite successful for predicting activities and capturing the chemical periodic trends expected for the first-row transition metal series, independent of the proposed mechanism. We then extend this analysis to heterodimer metallic systemsmetal oxide catalysts with two different catalytically active metal centersand find signs that the Sabatier analysis may not hold for these more complex systems. By performing a principal component analysis on the computed redox potentials, we show (1) that a single chemical descriptor inadequately describes heterodimer overpotentials and (2) mixed-metal overpotentials cannot be predicted using only pure-metal redox potentials. We believe that the analysis presented in this article shows a need to move beyond the simple chemical descriptor picture when studying more complex mixed metal oxide OER catalysts

    Speed Limit for Triplet-Exciton Transfer in Solid-State PbS Nanocrystal-Sensitized Photon Upconversion

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    Hybrid interfaces combining inorganic and organic materials underpin the operation of many optoelectronic and photocatalytic systems and allow for innovative approaches to photon up- and down-conversion. However, the mechanism of exchange-mediated energy transfer of spin-triplet excitons across these interfaces remains obscure, particularly when both the macroscopic donor and acceptor are composed of many separately interacting nanoscopic moieties. Here, we study the transfer of excitons from colloidal lead sulfide (PbS) nanocrystals to the spin-triplet state of rubrene molecules. By reducing the length of the carboxylic acid ligands on the nanocrystal surface from 18 to 4 carbon atoms, thinning the effective ligand shell from 13 to 6 Å, we are able to increase the characteristic transfer rate by an order of magnitude. However, we observe that the energy transfer rate asymptotes for shorter separation distances (≤10 Å) which we attribute to the reduced Dexter coupling brought on by the increased effective dielectric constant of these solid-state devices when the aliphatic ligands are short. This implies that the shortest ligands, which hinder long-term colloidal stability, offer little advantage for energy transfer. Indeed, we find that hexanoic acid ligands are already sufficient for near-unity transfer efficiency. Using nanocrystals with these optimal-length ligands in an improved solid-state device structure, we obtain an upconversion efficiency of (7 ± 1)% with excitation at λ = 808 nm