Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.043
Publication date: 22nd December 2022
Ligands play a crucial role in the synthesis and/or stabilization of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, of which oleic acid being the most typical example. Here we show that monoalkyl phosphinic acids are another ligand class.
We put forward monoalkyl phosphinic acids as an alternative ligand class, similar to carboxylic acids and phosphonic acids. After presenting the ligand synthesis for several selected substrates, we proceed to show the intermediate reactivity of the phosphinic acids in CdSe quantum dot syntheses. The nanocrystals synthesized with phosphinic acids are also easier to purify since there is no gel formation. Very small (2–3 nm) CdSe quantum dots with low polydispersity and high photoluminescence quantum yields can be easily accessed with phosphinic acid ligands. CdSe and CdS nanorods were also synthesized using phosphinic acids, whereby the rods showed high purity an d uniformity.
In addition, we investigate the ligand-NC interaction on both metal oxide (i.e., HfO2) as metal chalcogenide (i.e., CdSe and ZnS) NCs. Given their intermediate acidity, phosphinic acids (pKa ≈ 3.08) bind to the surface with an affinity between carboxylic and phosphonic acids. Using solution NMR, we quantify the X-for-X ligand exchange via the alkene resonance of the oleyl chain (carboxylic and phosphonic acid), and the ether resonance of the 6-(hexyloxy)hexyl phosphinic acid. We conclude that the monoalkyl phosphinic acids quantitatively displace carboxylate ligands and are in equilibrium with phosphonates (although phosphonate binding is favored). This results show that monoalkyl phosphinic acids are suitable reagents to efficiently functionalize nanocrystal surfaces.
In conclusion, by careful design new type of ligands can be created, and can tailored towards specific functionalities such as solubility matching or intermediate reactivity and binding strength.
The authors acknowledge the FWO Vlaanderen (FWO-SB grant 1S28820N and 1SA4221N), Special Research Fund/Concerted Research Actions project (BOF2015/GOA/007), Ghent University, and Basel University for financial support. The authors thank Jan Goeman for the GC/LC-MS and Funda Aliç for elemental analysis. The authors acknowledge funding from the Danish Ministry of Higher Education and Science through the SMART Lighthouse. The authors thank DANSCATT (supported by the Danish Agency for Science and Higher Education) for support. The authors acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at P02.1, and the authors thank Martin Etter for assistance in using the beamline.
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