Letter pubs.acs.org/NanoLett
Thermal Stability of Metal Nanocrystals: An Investigation of the Surface and Bulk Reconstructions of Pd Concave Icosahedra Kyle D. Gilroy,†,⊥ Ahmed O. Elnabawy,‡,⊥ Tung-Han Yang,†,⊥ Luke T. Roling,‡ Jane Howe,§ Manos Mavrikakis,*,‡ and Younan Xia*,†,∥ †
The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States ‡ Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States § Hitachi High-Technologies Canada, Toronto, Ontario M9W 6A4, Canada ∥ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: Despite the remarkable success in controlling the synthesis of metal nanocrystals, it still remains a grand challenge to stabilize and preserve the shapes or internal structures of metastable kinetic products. In this work, we address this issue by systematically investigating the surface and bulk reconstructions experienced by a Pd concave icosahedron when subjected to heating up to 600 °C in vacuum. We used in situ highresolution transmission electron microscopy to identify the equilibration pathways of this far-from-equilibrium structure. We were able to capture key structural transformations occurring during the thermal annealing process, which were mechanistically rationalized by implementing self-consistent plane-wave density functional theory (DFT) calculations. Specifically, the concave icosahedron was found to evolve into a regular icosahedron via surface reconstruction in the range of 200−400 °C, and then transform into a pseudospherical crystalline structure through bulk reconstruction when further heated to 600 °C. The mechanistic understanding may lead to the development of strategies for enhancing the thermal stability of metal nanocrystals. KEYWORDS: Palladium, concave icosahedron, nanocrystal, thermal stability, in situ electron microscopy, density functional theory
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faults, and/or screw dislocations.10−16 These features are susceptible to changes at elevated temperatures, as thermal fluctuations can endow the atoms with sufficient kinetic energy to escape from their initial locations and migrate to more stable sites. This is especially problematic for nanocatalysts used in many industrial processes such as CO oxidation,17 partial oxidation,18 and cracking of hydrocarbons,19 as well as combustion reactions20 where operating conditions demand temperatures exceeding 300 °C.21 To this end, it is critically important that we begin to develop methods for quantifying the thermal stability of metal nanocrystals. The physical underpinnings of thermal stability are related to the coordination number of the atoms, all the possible diffusion pathways, and the system’s energy landscape. To date, the difficulty in understanding and quantifying the thermal stability of nanocrystals can be attributed to our inability to directly observe the structural transformations that occur during
etal nanocrystals have found use in a variety of applications, including those in photonics, electronics, catalysis, and sensing.1−4 In general, the merit of each application depends critically on the size, shape, internal twin structure, and composition of the nanocrystals. It is these individual or combined features that define how nanocrystals interact with light for photonic applications, adsorb and transform reactants during catalytic reactions, and conduct charge carriers in electronic applications.5−7 In particular, the strong correlation between function and surface atomic structure has motivated a research front centered on the development of wet-chemical methods for generating uniform metal nanocrystals with well-controlled shapes (or faceting) and internal twin structures.8,9 Such controls allow one to define how the atoms situated on the surface of a nanocrystal are coordinated to their nearest neighbors and thus the landscape of the surface electronic structure. However, most of these nanocrystals are far-from-equilibrium products that have low-coordination surface atoms located at edges, corners, vertices, and high-index facets, as well as significant lattice strains caused by defects such as twin boundaries, stacking © XXXX American Chemical Society
Received: February 27, 2017 Revised: April 26, 2017 Published: April 27, 2017 A
DOI: 10.1021/acs.nanolett.7b00844 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters thermodynamic equilibration. The recent advancement in microscopy, namely in situ high-resolution transmission electron microscopy (HRTEM), has provided a viable route toward overcoming this problem by allowing for the direct characterization of nanocrystals upon exposure to variable experimental conditions.22−28 For example, by heating Au nanocrystals (with either single-crystal or icosahedral structure) with sizes between 5−12 nm to 400 °C, Kirkland and coworkers directly observed their equilibration to the decahedral structure.22 To rationalize this behavior, a quantitative phase map derived from relativistic ab initio calculations can be used to predict which shape and internal defect structure is most stable at a given size and temperature.23 Most notably, for Au nanocrystals under ambient temperatures, the icosahedral structure is most thermodynamically stable at sizes ≲2 nm, while the decahedral structure is most favored at sizes between 2 and 15 nm, followed by the single-crystal structure when the size is greater than 15 nm. These studies clearly demonstrate that, when the temperature is within a sufficient range, the atoms in a nanocrystal can gain the kinetic energy necessary to rearrange and give rise to a new structure with internal structure and shape having lower overall free energy. The equilibration pathways are not always obvious or straightforward to elucidate as metal nanocrystals are becoming increasingly complex, often engineered to express a multitude of features that may contribute to the instability, such as the void spaces in highly porous structures,29 ultrafine features like those found in nanoframes,30 or high-index facets derived from kinetically controlled syntheses.10−12 Currently, the equilibration pathways of nanocrystals that contain such features are still unknown, along with the temperatures and respective activation energies that bring about their removal. Therefore, it is of critical importance that methods are established for directly observing and quantitatively analyzing such processes. Motivated by this importance, we rationally designed and synthesized model nanostructures that have two of the most commonly observed metastable features: those on the surface in the form of high-index facets, and those in the bulk in the form of internal defect structures. This model structure is referred to as the concave icosahedron throughout the article. The synthesis of Pd concave icosahedra involves a two-step wet-chemical process, with the formation of regular icosahedra as an intermediate product. Figure 1a,b shows a schematic and typical TEM images of the regular icosahedra, which were about 13 nm in size (defined as the distance between two opposite faces, see Figure S1 for a size histogram). This size is well beyond the stable range predicted using theoretical calculations as Pd icosahedra are only stable when comprised of 100 atoms or fewer, corresponding to a size of approximately 1.5 nm.31 Between 100 and 6500 atoms, the 5-fold twinned decahedral structure is supposed to be most stable while particles containing more than 6500 atoms (>6 nm) favor a single-crystal structure. These three equilibrium structures are markedly different in terms of symmetry, faceting, the extent and distribution of stored strain energy, and most importantly the size-dependent function that defines their excess free energies. For a multiply twinned structure such as the icosahedron and decahedron, the excess energy increases with the number of atoms as N1/3 for N → ∞.31 As a result, it is the relatively high strain energy that contributes most to the intrinsic instability of the 13 nm icosahedral nanocrystals. To further push the structure away from equilibrium, we used the regular icosahedral nanocrystals as seeds in a
Figure 1. (a) Schematic illustration showing the synthesis of Pd concave icosahedron, a far-from-equilibrium structure favored by kinetics but not thermodynamics. The synthesis involves first the homogeneous nucleation and growth of Pd atoms into regular icosahedra, which then serve as seeds for the formation of concave icosahedra. (b,c) TEM images of the regular and concave icosahedra, respectively. The scale bars in the insets correspond to 20 nm.
kinetically controlled process for the synthesis of icosahedra with concave surfaces (Figure 1a,c). In this case, a high-index, low-coordination surface structure was brought about by depositing metal atoms on the surface of a seed at a rate greater than the surface diffusion of adatoms, generating a tip at the point of deposition and adjacent concavity.32 In essence, it is the difficulty for adatoms to diffuse toward near-equilibrium, high-coordination sites on the surface that helps maintain the metastable shape or morphology. The Pd concave icosahedra were then dispersed on the surface of a silicon nitride chip and loaded into a high-resolution TEM with heating capability. Because TEM imaging only provides a two-dimensional projection of a three-dimensional structure and the facets are not necessarily perpendicular to the viewing axis, we could not confidently index the facets. Instead, we used the angle between the oblique facets (when viewed along the [112]-projection) as a means to monitor the restructuring process. The initial angle (θi) between adjacent facets varied between 120° and 160° for the concave structures at room temperature (see Figure S2). As the temperature was increased, the concavity constantly decreased and the angle between adjacent facets eventually increased to a final angle (θf) of 180°. We were able to quantify the degree of surface restructuring by monitoring individual particles at elevated temperatures in real time. To this end, two neighboring (but not contiguous) nanocrystals were rapidly heated from room temperature to 200 °C at a rate of ∼3000 °C/min, and immediately after focus was obtained (
