AVS 62nd International Symposium & Exhibition
    In-Situ Spectroscopy and Microscopy Focus Topic Tuesday Sessions
       Session IS-TuP

Paper IS-TuP3
Bimetallic Nanoparticles Stability Investigated by In Situ XPS and TEM

Tuesday, October 20, 2015, 6:30 pm, Room Hall 3

Session: In-Situ Spectroscopy and Microscopy Poster Session
Presenter: Cecile Bonifacio, University of Pittsburgh
Authors: C.S. Bonifacio, University of Pittsburgh
S.C. Carenco, Lawrence Berkeley National Laboratory
M.B. Salmeron, Lawrence Berkeley National Laboratory
J.C. Yang, University of Pittsburgh
Correspondent: Click to Email
Bimetallic Ni and Co nanoparticles (NPs) possess intriguing potential for industrial applications due their remarkable catalytic activity for methane production and selective CO2 reduction. Synthesizing these NPs as a core-shell structure presents a cost-effective way to produce catalysts by using the less expensive metal in the core and the more expensive but active metal catalyst in the shell. The core-shell structure can also allow for the fine tuning of reactivity, aversion of sintering issues for the core material, and even enhancement of tolerance to high temperatures. Unfortunately, NPs tend to be unstable at temperatures far below their bulk melting temperature. In this work, the structural stability of Ni-Co core-shell was studied using in situ techniques. The morphological, structural, and chemical changes involved in the core-shell reconfiguration were examined during in situ annealing through simultaneous imaging and acquisition of elemental maps in the transmission electron microscope (TEM), and acquisition of O 1s, Ni 3p, and Co 3p x-ray photoelectron spectra (XPS). By observing the structural rearrangements of the nanoparticles in situ as a function of temperature, we were able to determine the thermal conditions under which the as-synthesized structure is stable and the minimum temperature where significant elemental diffusion and aggregation occurs. The NPs were annealed in situ in the TEM and XPS from 80°C to 650°C under vacuum conditions. Energy-dispersive x-ray spectroscopy (EDS) maps were acquired after each annealing step, yielding the quantitative distribution of Ni and Co within the NPs. Our results show that core-shell reconfiguration occurred in a stepwise process of surface oxide removal and metal segregation. Removal of the stabilizing surface oxide occurred from 320°C to 440°C, initiating the core-shell reconfiguration. Above 440°C, Ni migration from the core to the shell disrupted the core-shell structure of the NP leading to the subsequent formation of a homogeneous Ni-Co mixed alloy at 600°C. This work impacts other fields besides catalysis – including metallurgy, magnetism, etc. – where nanoparticle stability under operating conditions is essential for their technical viability and long-term durability.