Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

Abstract

We demonstrate that metal carboxylate complexes (L–M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors like sterics and chelation, though also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L–M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd:Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se-2S3/2h absorption and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)2 at room temperature (~60 %) and fully reversed at elevated temperature. A model is proposed where electron accepting M(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.

Efficient Water Splitting Cascade Photoanodes with Ligand‐Engineered MnO Cocatalysts

“…Therefore, most of the previously reported results related to BiVO 4 ‐based photoanodes13, 14, 16, 19, 47, 51, 52, 53 were measured in sulfite oxidation condition to show photo‐electrochemical properties of BiVO 4 ‐based electrodes independently of its poor water oxidation kinetics, as shown in Table S2 (Supporting Information). The photo‐electrochemical current densities of the BiVO 4 ‐based photoanodes4, 13, 14, 16, 17, 19, 20, 21, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 68, 69, 77 were plotted as a function of potential versus RHE. Thus, we measured PEC properties of BiVO 4 ‐based anodes under sulfite oxidation to figure out the effect of ligand engineering.…”

Repairing Nanoparticle Surface Defects

“…The direct observation of the development and propagation of surface defects is enabled by the exceptionally thin NPs as the removal of just a few atoms from the (001) facets strains the lattice, inducing perforations. 8a While the aqueous solution of NH 4 OH, itself an L‐type (two‐electron donor) promoter ligand, enables the LE with thiostannate ligands to proceed rapidly, 9a it also supports degradation, likely following a process known as L‐promoted Z‐type ligand displacement. Calibrated HAADF STEM images showed that at least 50 % of the material was removed from the damaged areas (Figure S4 d, e).…”

“…The displacement potency of a ligand depends on multiple attributes, such as electronic effects, chelation, and steric properties. 9a We varied the latter by replacing NH 4 OH with another promoter ligand pyridine, 3‐methylpyridine, or 2,6‐dimethylpyridine (DMP), which are all L‐type ligands, and found that all LE treatments resulted in perforation of the NPs; furthermore, treatment with DMP also resulted in material rearrangement from the NP core onto the surface (Figure S2). This structural damage always correlated with a dramatic decrease in PLQY (ca.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…They provide ''trap states'' for charge carriers, i.e., energy levels within the bandgap where the charge carrier is spatially localized. Indeed, the quantum efficiency of NC emission improves when the NC surface is covered with a protective shell of high-bandgap material [13,15,16,61,74,[243][244][245][246], or when ligands saturate chemical bonds on the surface [16,123,[247][248][249][250][251].…”

“…The simplest picture is that quenching can be suppressed by saturating chemical bonds of the surface atoms [15,16,61,[243][244][245][246][247][248][249]251]. This can be achieved by overcoating the NC either by a shell of another semiconductor or by a ligand layer.…”