Studies of Iridium Nanoparticles Using Density Functional Theory Calculations

Abstract

The energetics and the electronic and magnetic properties of iridium nanoparticles in the range of 2-64 atoms were investigated using density functional theory calculations. A variety of different geometric configurations were studied, including planar, three-dimensional, nanowire, and single-walled nanotube. The binding energy per atom increases with size and dimensionality from 2.53 eV/atom for the iridium dimer to 6.09 eV/atom for the 64-atom cluster. The most stable geometry is planar until four atoms are reached and three-dimensional thereafter. The simple cubic structure is the most stable geometric building block until a strikingly large 48-atom cluster, when the most stable geometry transitions to face-centered cubic, as found in the bulk metal. The strong preference for cubic structure among small clusters demonstrates their rigidity. This result indicates that iridium nanoparticles intrinsically do not favor the coalescence process. Nanowires formed from linear atomic chains of up to 4-atom rings were studied, and the wires formed from 4-atom rings were extremely stable. Single-walled nanotubes were also studied. These nanotubes were formed by stacking 5- and 6-atom rings to form a tube. The ring stacking with each atom directly above the previous atom is more stable than if the alternate rings are rotated.

First principles studies of Fe m Ir n (2 ≤ m + n ≤ 4) nano clusters

“…The Ir 4 clusters show average and local magnetic moment of 2.13l B /atom, in agreement with experimental values (2.04l B /atom) reported for the tetrahedral Ir 4 cluster (Stevanovic et al 2012). Pawluk et al (2005) also reported 2l B /atom for the square isomer, however, they have obtained zero magnetic moment for the tetrahedral structure. For the tetramers also, the presence of Fe in the neighborhood enhances the Ir moments and the average magnetic moment increases with Fe concentration.…”

“…The predicted stable geometries in theoretical studies include tetrahedral (Feng et al 1997), a butterfly structure (Bussai et al 2005), rhombus (Pawluk et al 2005), and square (Stevanovic et al 2012) whereas experiments has showed tetrahedral as the most stable structure having a bond length of 2.71 Å (Deutsch et al 1997). We have seen that the tetrahedral structure is the most stable structure in agreement with this experimental result and theoretical study which obtained tetrahedron having bond length of 2.79 Å (Feng et al 1997).…”

“…The calculated bond length (2.44 Å ) and the magnetic moment/atom (2.13l B ) of Ir 4 , is comparable to that obtained by Stevanovic et al (2012) (2.31 Å and 2.00l B , respectively), however, they have obtained octahedral geometry as the stable structure. Yet another study showed the square planar structure with large binding energy (Pawluk et al 2005) while we have observed that the square planar structure upon complete geometry optimization transforms into a tetrahedral geometry. These variations in stability can be attributed to the difference in approximations involved in the choice of pseudo potential and exchange correlation functional.…”

“…Studies have shown that there exists a low translational energy pathway for the dissociation of methane using Ir as the catalyst (Reeves et al 2001). Iridium clusters of various sizes are being studied both experimentally (Watzky and Finke 1997;Wang and Ehrlich 1997) and theoretically (Feng et al 1997;Bussai et al 2005;Pawluk et al 2005;Stevanovic et al 2012). Feng et al (1997) investigated the reactivity of Ir clusters from 4 to 10 atoms.…”

“…Bussai et al's (2005) analysis on the binding energies of four atom iridium clusters showed a square planar configuration as the most stable structure using a scalar relativistic variant of density functional theory (DFT) method. Another DFT study of Ir clusters with sizes ranging n = 2-64 (Pawluk et al 2005) showed that the magnetic moments and binding energies showed oscillatory behavior with increase in cluster size. On the other hand, an increase in binding energy with cluster size is also reported in DFT studies (Stevanovic et al 2012).…”