Water-Dispersible Copper Sulfide Nanocrystals via Ligand Exchange of 1-Dodecanethiol

In colloidal Cu 2-x S nanocrystal synthesis, thiols are often used as organic ligands and the sulfur source, as they yield high-quality nanocrystals. However, thiol ligands on Cu 2-x S nanocrystals are difficult to exchange, limiting the applications of these nanocrystals in photovoltaics, biomedical sensing, and photocatalysis. Here, we present an effective and facile procedure to exchange native 1-dodecanethiol on Cu 2-x S nanocrystals by 3-mercaptopropionate, 11-mercaptoundecanoate, and S 2-in formamide under inert atmosphere. The product hydrophilic Cu 2-x S nanocrystals have excellent colloidal stability in formamide. Furthermore, the size, shape, and optical properties of the nanocrystals are not significantly affected by the ligand exchange. Water-dispersible Cu 2-x S nanocrystals are easily obtained by precipitation of the nanocrystals capped by S 2-, 3-mercaptopropionate, or 11-mercaptoundecanoate from formamide, followed by redispersion in water. Interestingly, the ligand exchange rates for Cu 2-x S nanocrystals capped with 1-dodecanethiol are observed to depend on the preparation method, being much slower for Cu 2-x S nanocrystals prepared through heating-up than through hot-injection synthesis protocols. XPS studies reveal that the differences in the ligand exchange rates are due to the surface chemistry of the Cu 2-x S nanocrystals, where the nanocrystals prepared via hot-injection synthesis have a less dense ligand layer due to the presence of trioctylphosphine oxide during synthesis. A model is proposed that explains the observed differences in the ligand exchange rates. The facile ligand exchange procedures reported here enable the use of high-quality colloidal Cu 2-x S nanocrystals prepared in the presence of 1-dodecanethiol in various applications.