Crystal-Bound <i>vs</i> Surface-Bound Thiols on Nanocrystals

The use of thiol ligands as a sulfur source for nanocrystal synthesis has recently come <i>en vogue</i>, as the products are often high quality. A comparative study was performed of dodecanethiol-capped Cu₂S prepared with elemental sulfur and thiol sulfur reagents. XPS and TGA-MS provide evidence for differing binding modes of the capping thiols. Under conditions where the thiol acts only as a ligand, the capping thiols are “surface-bound” and bond to surface cations in low coordination number sites. In contrast, when thiols are used as a sulfur source, “crystal-bound” thiols result that sit in high coordination sites and are the terminal S layer of the crystal. A ¹H NMR study shows suppressed surface reactivity and ligand exchange with crystal-bound thiols, which could limit further application of the particles. To address the challenge and opportunity of nonlabile ligands, dodecyl-3-mercaptopropanoate, a molecule possessing both a thiol and an ester, was used as the sulfur source for the synthesis of Cu₂S and CuInS₂. A postsynthetic base hydrolysis cleaves the ester, leaving a carboxylate corona around the nanocrystals and rendering the particles water-soluble