Co-Operative Cation Exchange Reaction in CdS Magic Size Cluster

Cation exchange has been developed as a post-synthetic method to explore a wide range of nanoparticle composition, phase, and morphology. Traditionally, the cation exchange reaction is explained by the reaction zone theory where the substitutional cations diffuse into the nanoparticle and form a reaction zone “shell” within the nanoparticle. Reaction zone theory implies the cation exchange is diffusion-controlled and the extent of cation exchange reaction is thus controlled by the propagation of the diffusion zone. However, several recent studies in nanoclusters (size &lt;2 nm) find that cation exchange reactions take on discrete transformation steps. For example, the CdSe/Cu cation exchange shows a two steps mechanism, starting with the slow exchange of 1-3 Cu atoms and follow by an avalanche reaction which forms completely exchanged Cu₂Se nanoclusters.<br/>Using the CdS magic size cluster (MSC) as a model system, we investigated the cation exchange reaction mechanism in nanoclusters. The MSC cation exchange with Ag ions reveals a similar two steps reaction mechanism. The cation exchange reaction initiates with 1 to 2 Ag atoms incorporated into the MSC. Once the Ag concentration reaches the critical concentration (~10% Ag), a rapid increase of Ag content is observed followed by a complete phase transformation to Ag₂S. Using MALDI-TOF spectroscopy, we successfully identified two Ag exchanged reaction intermediates (Ag2Cd32S33, Ag1Cd33S33) as well as the fully exchanged nanocluster. X-ray diffraction and X-ray photoelectron spectroscopy also demonstrated a sudden change in crystal structure and electronic structure at the critical Ag concentration and corroborate the MALDI-TOF results. More interestingly, we found the Ag exchanged reaction intermediate has a low energy barrier for further cation exchange with other elements which were difficult to cation exchange. As a result, we used knowledge of this cluster exchange mechanism to successfully exchange Mn into AgCd33S33, to introduce interesting optomagnetic properties.