Article pubs.acs.org/JPCC
Exotic High Activity Surface Patterns in PtAu Nanoclusters Junais Habeeb Mokkath and Udo Schwingenschlögl* King Abdullah University of Science and Technology, Physical Science and Engineering Division, Thuwal 23955-6900, Saudi Arabia ABSTRACT: The structure and chemical ordering of PtAu nanoclusters of 79, 135, and 201 atoms are studied via a combination of a basin hopping atom-exchange technique (to locate the lowest energy homotops at fixed composition), a symmetry orbit technique (to find the high symmetry isomers), and density functional theory local reoptimization (for determining the most stable homotop). The interatomic interactions between Pt and Au are derived from the empirical Gupta potential. The lowest energy structures show a marked tendency toward PtcoreAushell chemical ordering by enrichment of the more cohesive Pt in the core region and of Au in the shell region. We observe a preferential segregation of Pt atoms to (111) facets and Au atoms to (100) facets of the truncated octahedron cluster motif. Exotic surface patterns are obtained particularly for Pt-rich compositions, where Pt atoms are being surrounded by Au atoms. These surface arrangements boost the catalytic activity by creating a large number of active sites.
I. INTRODUCTION Nanosize bimetallic clusters have attracted significant interest owing to their promising applications in optics, magnetism, and catalysis.1,2 This is because their physio-chemical properties can be tuned not only by varying size, thereby increasing the surface to volume ratio, but also by varying composition and the chemical order. Bimetallic nanoclusters made of noble metals, in particular PtAu nanoclusters, show attractive catalytic3−7 and optical properties.8 Recent investigations indicate that nanoengineered PtAu materials with different compositions and surface atomic arrangements can be used to develop highly active bifunctional catalysts for applications in fuel cells9,10 as well as methanol and formic acid oxidation.3,11 Among the optical properties, particularly the surface plasmon resonance can be tuned via the chemical ordering or atomic mixing pattern (such as core−shell, random, multishell, ordered, and disordered arrangements).8 Bulk PtAu exhibits a miscibility gap at 20−90% Au at low temperatures and a preferential segregation of Au atoms to the surface.12 However, at high temperatures, a continuous PtAu solid solution is formed.4 Finding the lowest energy configuration of a given nanoalloy cluster will be extremely difficult for both experiment and theory due to the huge complexity in the structure, composition, and chemical order. A number of stable configurations have been reported experimentally for PtAu nanoclusters3,8,13−17 because in experiments different growth and/or synthesis conditions can lead to different chemical orders, which are governed not just by energetics but by kinetic processes as well. Postsynthesis manipulations can induce different degrees of intermixing, including surface diffusion and disorder, for example. In a recent investigation, by combining element specific high-energy X-ray diffraction experiments and atomic pair distribution function analysis, Petkov et al. have predicted the formation of PtAu alloys with random chemical ordering.3 In another experiment using γ-ray radiolysis, Belloni et al. have produced both AucorePtshell and PtcoreAushell bilayered nanoparticles14 and reported that the former is energetically more stable, though © 2013 American Chemical Society
surface and cohesive energies would favor the latter. Braidy et al. have reported that the AucorePtshell configuration is most stable when the PtAu cluster was thermally annealed at 300 °C for 24 h but transformed to a layered structure after annealing at 600 °C.6 Although there have been many experimental works on PtAu nanoclusters, the theoretical studies have been limited to small size clusters. The structural and electronic properties of very small clusters of PtAu dimer, Pt3Au, and PtAu6 have been investigated using ab initio density functional theory (DFT).18,19 In another DFT study, the icosahedral PtcoreAushell structure was reported to be the most stable for 13-atom clusters,20 where the single Pt atom in the PtAu12 cluster prefers being in the core and the Au atom of Pt12Au favors the cluster surface. Another computational study on larger size AumPtm (m = 1−50) clusters, by combining the empirical Gupta potential (EP) with genetic algorithm sampling and DFT optimization for m = 1−10, has been performed by Logsdail et al.21 PtcoreAushell chemical ordering was found to be energetically stable. Moreover, DFT local relaxation also has validated this result for small clusters up to 20 atoms. Another computational study of Tran et al.22 on PtAu clusters with 40 atoms using the empirical Gupta potential and DFT reoptimization has confirmed that PtcoreAushell chemical ordering is most stable. Besides a general interest from the perspective of sampling the vast configurational space, PtAu nanoclusters are particularly appealing because of their exciting catalytic activities. They enhance greatly the kinetics of the oxygen reduction and evolution reactions and offer the lowest charging voltage and highest round trip efficiency of rechargeable Li−O2 cells.23 The enhanced catalytic activity can be attributed to complex surface patterns. Thus, tuning the surface pattern of PtAu nanoparticles is a highly promising strategy to materialize robust bifunctional catalytic activities. By combining scanning Received: February 9, 2013 Revised: April 10, 2013 Published: April 16, 2013 9275
dx.doi.org/10.1021/jp401466h | J. Phys. Chem. C 2013, 117, 9275−9280
The Journal of Physical Chemistry C
Article
transmission electron microscopy and X-ray diffraction, Lu et al.23 were able to determine the geometrical structure of a specific PtAu nanoparticle and calculate the corresponding surface atomic ratio and average particle composition as 60 ± 2%/40 ± 2% and 56 ± 5%/44 ± 5%, respectively. However, detailed surface information is missing. In another investigation using Fourier transform infrared probing of CO adsorption on the surface of PtAu nanoparticles, Wanjala et al.15 have observed strong electron donation to the CO band by a Pt atom at top site surrounded by Au atoms as compared to monometallic Pt surfaces. They speculate that this sort of bimetallic alloyed surface patterns would enhance the catalytic activities of PtAu alloys by shifting up the d-band center for Pt atoms surrounded by Au atoms. However, for predicting and improving the catalytic properties, atomically resolved surface information is required, which can serve as a characteristic fingerprint. Finding the most stable atomic arrangement for a given nanoalloy is a difficult computational task, especially at the first principles level, due to the combinatorial increase in the number of possible homotops24 (isomers having the same structure and composition but different atomic arrangements). Recently, Barcaro et al.25 have introduced a novel method, the symmetry orbit shell optimization technique, and have successfully employed it to find the most stable configurations of PdPt nanoclusters with greatly reduced computational time. In the present study, we adopt the symmetry orbit shell optimization technique combined with the EP-DFT approach to investigate the chemical ordering in PtAu clusters of 79, 135, and 201 atoms.
Table 1. Parameters of the Empirical Gupta Potential
i
(1)
Ebi and Eri can be written as Eib
1/2 ⎧ ⎡ ⎛ rij ⎞⎤⎫ ⎪ ⎪ 2 = −⎨ ∑ ζ exp⎢ −2q⎜ − 1⎟⎥⎬ ⎢⎣ ⎪ j≠i,r
