Reversed Nanoscale Kirkendall Effect in Au–InAs Hybrid Nanoparticles (original) (raw)
2016, Chemistry of Materials
Metal-semiconductor hybrid nanoparticles (NPs) offer interesting synergistic properties, leading to unique behaviors that have already been exploited in photocatalysis, electrical, and optoelectronic applications. A fundamental aspect in the synthesis of metal-semiconductor hybrid NPs is the possible diffusion of the metal species through the semiconductor lattice. The importance of understanding and controlling the co-diffusion of different constituents is demonstrated in the synthesis of various hollow structured NPs via the Kirkendall effect. Here, we used a post-synthesis, room-temperature reaction between AuCl 3 and InAs nanocrystals (NCs) to form metal-semiconductor core-shell hybrid NPs, through the "reverse Kirkendall effect". In the presented system the diffusion rate of the inward diffusing specie (Au) is faster than that of the outward diffusion specie (InAs), which results in the formation of a crystalline metallic Au core surrounded by an amorphous, oxidized InAs shell containing nanoscale voids. We used time-resolved (TR) x-ray absorption fine-structure (XAFS) spectroscopy to monitor the diffusion process and found that both the size of the Au core and the extent of the disorder of the InAs shell depend strongly on the Auto -NC ratio. We have determined, based on multi-element fit analysis, that Au diffuses into the NC via the kick-out mechanism, substituting for In host atoms: this compromises the structural stability of the lattice and triggers the formation of In-O bonds. These bonds were used as markers to follow the diffusion process and indicate the extent of degradation of the NC lattice. Time-resolved x-ray diffraction (XRD) was used to measure the changes in crystal structures of InAs and the nanoscale Au phases. By combining the results of XAFS, XRD and electron microscopy, we correlated the changes in the local structure around Au, As and In atoms, and the changes in the overall InAs crystal structure. This correlative analysis revealed co-dependency of different structural consequences when introducing Au into the InAs NCs. This study of diffusion effects in nanocrystals therefore has relevance to powerful concepts in solid-state nanochemistry related to processes of cation exchange, doping reactions, and diffusion mechanisms.