Unraveling Changes to PbS Nanocrystal Surfaces Induced by Thiols

Abstract

Ligand exchange reactions are commonly employed to modify the surfaces of colloidal semiconductor nanocrystals (NCs), yet multiple reaction pathways induced by the introduction of thiols to NCs have been proposed in the literature to occur independently of each other. Here, we report three separate ligand-based reaction mechanisms between oleate-capped PbS NCs and the exchange thiol ligand, undec-10ene-1-thiol (UDT), and unveil how these three pathways are intertwined. Experimental evidence for these mechanisms is provided by the 1 H NMR spectroscopic handles of oleate-and UDT-based species as well as Pb species quantified by inductively coupled plasma mass spectrometry. We find that Z-type Pb(oleate) 2 ligands are displaced at low quantities of added UDT (0−100 equiv per NC) alongside the coupled binding of L-type thiol ligands to the NC surface. With increasing amounts of added UDT (100−500 equiv), L-type binding of thiol ligands not associated with Z-type displacement is inferred from comparison of the numbers of each bound and free ligand throughout the 1 H NMR spectroscopic titration. Lastly, a substantial quantity of oleic acid is liberated by the reaction of NCs with 100−500 equiv of UDT, suggesting an X-type exchange mechanism, supported by Fourier transform infrared (FTIR) spectroscopic titration. This work provides fundamental insight into changes to the NC surface upon ligand exchange with thiols and proposes site-specific reactivity for each mechanism contextualized within the prior art.

Effects of Ligand Shell Composition on Surface Reduction in PbS Quantum Dots

“…As previously demonstrated by Giansante et al, exchange of oleate-capped PbS QDs with triethylammonium p -toluate proceeds through an X-type exchange mechanism akin to a salt metathesis that liberates triethylammonium oleate upon toluate binding . In contrast to carboxylic acid-terminated ligands, alkyl thiols have been established to undergo a variety of surface ligand reactions with oleate-capped QDs, including X-type exchange, ,, L-type ligand binding (as a two-electron donor), ,, and L-type-promoted Z-type ligand displacement (liberating a Pb­(oleate) 2 ligand). , Detailed investigations of the reaction between UDT and oleate-capped PbS QDs from our lab show that each of the three listed mechanisms occurs . From this complementary work, we expect that a substantial portion of bound UDT is L-type thiol with 100 equiv UDT added, formed through a dominant L-type-promoted Z-type ligand displacement mechanism at low UDT concentrations (<100 equiv added per QD) and L-type binding to undercoordinated sites at moderate UDT concentrations (100–500 equiv added per QD).…”

“…36,37 Detailed investigations of the reaction between UDT and oleate-capped PbS QDs from our lab show that each of the three listed mechanisms occurs. 38 From this complementary work, we expect that a substantial portion of bound UDT is L-type thiol with 100 equiv UDT added, formed through a dominant Ltype-promoted Z-type ligand displacement mechanism at low UDT concentrations (<100 equiv added per QD) and L-type binding to undercoordinated sites at moderate UDT concentrations (100−500 equiv added per QD). X-type exchange also occurs under these reaction conditions, resulting in mixed-shell UDT/oleate-PbS QDs passivated with a mixture of thiol and thiolate ligands.…”

“…The concurrent L-type-promoted Z-type displacement of lead oleate (one UDT bound, two oleates liberated), L-type binding to undercoordinated sites (one UDT bound, zero oleates liberated), and X-type exchange (one UDT bound, one oleate liberated) under the reaction conditions give an apparent averaged 1:1 exchange ratio, consistent with other studies in our lab. 38 Comparing Surface Reduction of Mixed-Shell QDs by CoCp 2 . The redox reactivity of PbS QD surfaces with excess electrons added via chemical reductants has been recently demonstrated; displacement of native X-type oleate ligands in response to reduction of populations of surface Pb 2+ ions with the molecular reductant cobaltocene (CoCp 2 , E°′ = −1.3 V vs Fc +/0 ) has been reported in oleate-capped PbS QDs to yield proposed Pb 0 surface sites and displaced oleate ligands charge balanced with oxidized [CoCp 2 ] + counterions (Scheme 3).…”

“…Further, UDT is predominately bound as an L-type ligand under the conditions employed to generate mixed-shell QDs; L-type UDT is not expected to participate in an electron-promoted displacement mechanism. 21,24,38 For the UDT bound as a Pb 2+ -thiolate moiety, the relative energetics of bonding and antibonding orbitals are expected to be substantially different from bonding and antibonding orbitals associated with Pb 2+oleate bonds (see the Supporting Information). Thiolates are established to bind more tightly to CdSe and PbS QDs in comparison to carboxylates.…”

“…Similarly, the introduction of UDT to the ligand shell also results in a marked increase in the ε 400 value for UDT/oleate-PbS. As UDT maintains an electronically similar ligand backbone to oleate (concluded from the UDA/oleate system), the difference arises from the exchange of carboxylates for thiolates, as well as the binding of L-type thiols concomitantly with loss of Z-type lead oleate . An increase in absorptivity with thiol-terminated ligands has been attributed to greater ligand–QD state mixing in the covalent surface bonding of thiol-reacted PbS QDs compared to carboxylate-capped QDs. ,, These observations are collectively consistent with in situ ligand exchange experiments conducted on PbS QDs ,, as well as the UV–vis–NIR absorbance spectra in Figure .…”