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 postsynthesis room-temperature reaction between AuCl3 and InAs nanocrystals (NCs) to form metal-semiconductor core-shell hybrid NPs through the "reversed Kirkendall effect". In the presented system, the diffusion rate of the inward diffusing species (Au) is faster than that of the outward diffusing species (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 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 Au-to-NC ratio. We have determined, based on multielement 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 the 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 a co-dependence of different structural consequences when introducing Au into the InAs NCs. Therefore, this study of diffusion effects in nanocrystals has relevance to powerful concepts in solid-state nanochemistry related to processes of cation exchange, doping reactions, and diffusion mechanisms.
Bibliographical noteFunding Information:
The research leading to these results received funding through the NSF-BSF International Collaboration in Chemistry program. J.L., Y.A., A.I.F., and U.B. acknowledge support of this work by NSF Grant No. CHE-1413937 and BSF Grant No. 2013/610. U.B. thanks the Alfred and Erica Larisch Memorial Chair. This research used Hitachi 2700C of the Center for Functional Nanomaterials for STEM/EELS studies and X-rays from the XPD beamline of National Synchrotron Light Source II, both of which are U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.
© 2016 American Chemical Society.