MS-1-P-2172 Equilibrium shape changes of PtCu alloy nanoparticles across the order-disorder transformation - An in-situ TEM study
Platinum alloy nanoparticles find application in the field of electrocatalysis and gas phase reaction catalysis. The activity and stability of the catalyst
nanoparticles depend on the exposed crystal facets. Hence, the shape changes of the catalyst nanoparticles affect their surface properties like adsorption of gases and
hence the catalytic activity. Shapes assumed by the particles can be either kinetic or thermodynamic. If the particles are equilibrated under fixed conditions of
temperature, pressure, volume and composition, they attain their thermodynamic or equilibrium shape which is given by the Wulff construction. While the shape changes
with temperature has been studied for monometallic particles, there are very few studies on alloy nanoparticles. In particular, the shape changes associated with
changes in ordering of nanopartciles has not been investigated.
We have chosen a class of bimetallic A50B50 type alloy system which undergoes order to disorder phase transformation with the increase of temperature. The equilibrium
shape changes of the alloy systems having a varied range of heat of mixing across the transition from B2 ordered to A2 disordered structure and L1o ordered to A1
disordered structure have been studied theoretically. Shape change in terms of change in the area of the exposed facet was observed with changing degree of order in
the alloy system (Figure 1).
Experimentally equilibrium shape changes have been observed for PtCu system which undergoes transformation from ordered rhombohedral structure to disordered cubic
structure using in-situ heating techniques in transmission electron microscope (TEM). The system has been designed such that the alloy nanoparticles nucleated on the
MgO cubes appear edge-on on the face of the cube when tilted to its [001] zone axis and the shapes of the equilibrated alloy particles at different temperatures
implying different order parameters have been imaged at high resolution. The facet lengths of the equilibrated particles were observed to change monotonically with
decreasing order parameter that corresponds to the facet area change observed in the theoretical study (Figure 3).
The equilibrium shape change with the degree of order in an alloy system has been observed for the first time and its implication is immense, not only in terms of its
fundamental basis but also in terms of the application of the alloy particles undergoing order-disorder phase transformation where the property of equilibrium shape
change with degree of order could be exploited in the field of catalysis.
Financial support from DST is acknowledged. The electron microscopes are a part of the Advanced Facility for Microscopy and Microanalysis at IISc.
Fig. 1: Figure 1. Theoretically derived equilibrium shapes for B2 ordered to A2 disordered phase transition wherein the relative area of 110 and 100 facets vary with decreasing order parameter. The effect of different heat of mixing is also shown in this figure. |
Fig. 2: Figure 2. (a) Low magnification image of MgO cubes with PtCu alloy nanoparticles nucleated on the cubes. (b) and (c) faceted ordered alloy nanoparticles appearing edge-on on MgO substrate. |
Fig. 3: Figure 3. PtCu alloy nanoparticle on MgO substrate equilibrated at three different temperatures showing changes in the facet length in two-dimensional projection which translates to facet area change in the three-dimensional particle. |
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