Ligand-Dependent Optical Properties of Colloidal Ternary Spinel Oxide Nanocrystals Containing Transition Metals

Ternary spinel oxides of formula AB₂O₄ are semiconductors that possess compositionally and structurally tunable magnetic and optoelectronic properties that, when coupled with their extraordinary chemical and thermal stability, offer functional materials with applications in the fields of photocatalysis, solar energy conversion, gas sensing, and photoelectrochemistry. Nanocrystals of these materials offer the additional advantages of high surface area-to-volume ratios and the ability to use surface functionalization as a plausible strategy for tailoring their optoelectronic properties to improve their function in a specific application. Here, we demonstrate that surface-bound species can dominate the absorption spectra of colloidal ternary spinel oxide nanocrystals. We show that the surface functionalization of cobalt-containing systems with thiol ligands leads to the growth of an intense peak centered at 2.4 eV (518 nm) in their absorption spectra, which arises due to the formation of cobalt-thiolate linkages on the nanocrystal surface. We demonstrate that the observed optical change can be used to track ligand exchange reactions and assess the relative binding affinity of thiol, amine, and carboxylate ligands to the nanocrystal surface. This work highlights the significant role that surface chemistry can play in determining the optical properties of ternary spinel oxide nanocrystals.