First-Principles Study on Stability and Electronic Structures of Pt-Rh Bimetallic Nanoparticles

Abstract

Energetic stability and electronic structures of Pt atoms in Pt-Rh nanoparticle is investigated by first-principles calculation. Pt atom energetically prefer surface sites (vertex, edge, and (100)) rather than subsurface and core site, which is attributed to lower Pt surface energy compared with Rh. Vertex of nanoparticle is the most favorable site for Pt atom, which has lowest coordination numbers. Band center of d-state electronic contribution for Pt atom measured from the Fermi energy exhibit negative dependence with respect to Pt coordination number. This can be attributed to positive dependence of second-order moment of density of states for the Pt d-band on the coordination number. Pt segregation to the surface is expected due mainly to contribution from Pt on-site segregation energy compared with weak ordering tendency of Pt-Rh unlike-atom pairs.

Stability and Electronic Structures of Pt-Rh Icosahedral Nanoparticles

“…This can be attributed to the fact that Pt atoms segregate to sites with lower coordination number in order to decrease surface energy of the nanoparticle in terms of effective medium approximation up to 2nd order moment. 39) This tendency perfectly holds for cuboctahedral nanoparticles as described in Table 1, while it has an exception for icosahedron: Edge sites with higher coordination number of Z ¼ 8 have larger on-site energy in negative sign than vertex sites with lower coordination number of Z ¼ 6. In order to interpret this, we exhibit average interatomic distances for the Pt 1 Rh 54 and Rh 55 nanoparticles as described in Table 1.…”

“…Most of the interaction energies (8 of 9) exhibit positive sign with larger value compared with that in negative sign, indicating that Pt-Pt pairs are energetically unfavored and Pt-Rh unlike-atom pairs are favored in the PtRh nanoparticles, which is an ordering tendency similar to the bulk Pt-Rh alloy as well as the Pt-Rh cuboctahedral nanoparticle. 29,39) Another important point in Fig. 6 is that E I{J s are in one-order smaller than Pt on-site energies described in Table 1.…”

“…Therefore, Pt-Rh icosahedral nanoparticles are expected to show segregation behavior similar to the Pt-Rh bulk surface and the Pt-Rh cuboctahedral nanoparticle, where strong Pt segregation is mainly attributed to Pt on-site energy contribution. 29,39) …”

“…In order to understand ordering tendency including energetic preference of Pt atoms, we introduce Pt-Pt pair interaction energy, E I{J , defined by our previous work. 39) Here, E I{J < 0 indicates preference of Pt pair at I and J sites, and E I{J > 0 disfavor the Pt-Pt pair. We should note here that in order to apply the defined pair interaction energies to interpreting preference of individual atomic pairs, it would be required that structural differences between Pt 1 Rh 54 and Pt 2 Rh 53 nanoparticles can be small.…”

“…However, very recently, we investigate stability, electronic structure and quantitative segregation profile at finite temperature for Pt-Rh cuboctahedral nanoparticles at equiatomic and Pt-dilute composition. 39,40) We find that on-site segregation energy for Pt-dilute nanoparticle can be attributed to lower-order moment of the d-state electronic density of states, and that the segregation behavior can be qualitatively extended to Pt-Rh equiatomic nanoparticles, where multibody interaction should play significant role for quantitative estimation of segregation. With these considerations, further theoretical assessment for electronic structures and stability of different shape of nanoparticles should be highly desirable in order to essentially understand dependence of segregation behavior on the shape.…”