Understanding Atom Probe Tomography of Oxide-Supported Metal

Letter pubs.acs.org/JPCL

Understanding Atom Probe Tomography of Oxide-Supported Metal Nanoparticles by Correlation with Atomic-Resolution Electron Microscopy and Field Evaporation Simulation Arun Devaraj,*,†,# Robert Colby,†,# François Vurpillot,‡ and Suntharampillai Thevuthasan† †

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ‡ Groupe de Physique des Matériaux, UMR 6634 CNRS, Université et INSA de Rouen, 76801 St Etienne du Rouvray, France S Supporting Information *

ABSTRACT: Oxide-supported metal nanoparticles are widely used in heterogeneous catalysis. The increasingly detailed design of such catalysts necessitates three-dimensional characterization with high spatial resolution and elemental selectivity. Laser-assisted atom probe tomography (APT) is uniquely suited to the task but faces challenges with the evaporation of metal/insulator systems. Correlation of APT with aberration-corrected scanning transmission electron microscopy (STEM), for Au nanoparticles embedded in MgO, reveals preferential evaporation of the MgO and an inaccurate assessment of nanoparticle composition. Finite element field evaporation modeling is used to illustrate the evolution of the evaporation front. Nanoparticle composition is most accurately predicted when the MgO is treated as having a locally variable evaporation field, indicating the importance of considering laser−oxide interactions and the evaporation of various molecular oxide ions. These results demonstrate the viability of APT for analysis of oxidesupported metal nanoparticles, highlighting the need for developing a theoretical framework for the evaporation of heterogeneous materials. SECTION: Spectroscopy, Photochemistry, and Excited States

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old nanoparticles supported on oxides including MgO,1,2 TiO2,3,4 NiO,5 Al2O3, and so forth have been of great interest in catalysis since the discovery of unusual activity of Au nanoparticles for low-temperature CO oxidation and propylene epoxidation on such materials.2,6−8 Several different mechanisms for CO oxidation on supported Au catalysts are proposed, including negative charging of Au clusters,9 increased step density and strain on Au surfaces,10 and availability of lowcoordination Au atoms.11 Thus, the catalyst performance depends critically upon the shape, size, and distribution of nanoparticles on the oxide support. Development of improved catalysts of single metals as well as alloy and core−shell bimetallic or multielement nanoparticles supported on metal oxides therefore hinges upon an ability to characterize these materials in three dimensions with high spatial resolution and a high level of compositional discrimination.12 Atom probe tomography (APT) is a uniquely capable technique that is rapidly becoming popular for three-dimensional characterization of materials providing spatially resolved mass spectra that can be reconstructed with subnanometer spatial resolution and part-per-million level mass sensitivities.13−15 This combination of spatial and mass resolution provides complementary information to approaches such as high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM)16 and tomography.17−23 APT is complementary to HAADF-STEM imaging, in particular, for © 2014 American Chemical Society

materials with similar atomic numbers and for electron-beamsensitive materials.24 Also, APT can be uniquely suited for three-dimensional characterization of very small metal nanoparticles and single-to-few atom clusters on an oxide support. While APT analysis is uniquely suited for providing 3D chemical analysis of a variety of materials at subnanometer spatial resolution, there are challenges for analyzing heterogeneous materials. The trajectories of ions escaping the surface of an APT sample under an applied external field will depend upon the evaporation field of the sample’s constituents, such as for precipitates or interfaces. When a local variation in the evaporation field results in a broadening or narrowing of the escaping ion trajectories, it is commonly referred to as a local magnification artifact.25−28 For example, for a small precipitate with a high evaporation field in a matrix with a lower evaporation field, the trajectory of ions escaping from the precipitate and matrix will overlap, resulting in an incorrect measure of interface width.26,29 Such local magnification artifacts have been postulated but not directly demonstrated experimentally. Theoretical modeling projects a maximum compositional overlap of less than 2 nm at the interface for an evaporation field difference of ±15% between the matrix and Received: February 5, 2014 Accepted: March 26, 2014 Published: March 26, 2014 1361

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precipitate regions.26 For metal−dielectric composites, the dielectric can induce severe electric field distortions at the interface and exacerbate the ion trajectory deviation.30 As yet, there are only a limited number of studies aimed at understanding these challenging aspects of APT analysis of metal−dielectric composites,30 even though such materials are widely employed for energy and environmental applications, in which accurate APT analysis would have a substantial impact. To address this need, we directly correlated aberrationcorrected STEM and APT to analyze a model nanocomposite system composed of Au nanoparticles embedded in MgO, chosen due to its wide scale application as a catalyst in the oxidation of CO.1,9,31 APT needle specimens are imaged both before and after APT analysis to demonstrate the resulting changes in the surface topography, as well as to correlate the geometry of the volume analyzed with the reconstructed APT data. The experimental result was compared with a simulated field evaporation generated using a finite element (FE)-based model, demonstrating the magnitude of local evaporation field differences and elucidating the sources of artifacts observed in experimental APT reconstructions. The nanocomposite consisting of Au nanoparticles embedded in MgO were synthesized by implantation of Au+ ions at 1 MeV into a single-crystal MgO(001) substrate, followed by a 10 h anneal at 1000 °C in air. Samples were prepared for APT and STEM using an FEI Helios dual-beam focused ion beam/ scanning electron microscope (FIB/SEM) equipped with an Omniprobe, by a typical lift-out procedure.32 STEM imaging was performed using a probe-corrected FEI Titan 80-300 operated at 300 kV. APT analysis was performed using a Cameca LEAP 4000XHR with a 355 nm UV laser, using 20 pJ laser pulse energy at a specimen temperature of 30 K with the evaporation rate maintained at 0.005 atoms/pulse. HAADF-STEM images of the Au−MgO nanocomposite viewed in cross section along the MgO[100] zone axis show a varying density of the Au nanoparticles as a function of depth from the surface, following a typical profile for samples created by ion implantation (Figure 1a). The gold particles range in shape from cubic to truncated octahedral but generally have well-defined facets. The Au/MgO interface appears very abrupt in HAADF-STEM measurements. While it is difficult to achieve an atomic-resolution determination of the interface abruptness in three dimensions with HAADF-STEM, the Au/MgO interfaces appear to be atomically abrupt when imaged edgeon across well-aligned facets, such as for the top and bottom (001) surfaces of the particle in Figure 1b,c, viewed along the MgO[110] direction. As the particle edges are often truncated, a particle’s Au/MgO interfaces cannot all be viewed perfectly edge-on from any given angle; those interfaces that appear less abrupt in HAADF-STEM are attributed to such facets viewed at an angle (e.g., for a (100) interface viewed from the [110] direction). The band of Au nanoparticles is also clearly evident in HAADF-STEM images of sample prepared for APT, prior to atom probe analysis (Figure 2a). The topmost layer capping the needle-shaped sample is a thick Pt/C film deposited to protect the Au/MgO from Ga ion implantation during FIB-based sample preparation. Initial APT analysis was conducted using 20 pJ laser energy to obtain 100 million ion counts and then interrupted for a second round of HAADF-STEM imaging. Before and after images, shown in Figure 2a, were viewed from the same MgO zone axis, such that the height of the volume removed could be determined for accurate scaling of the

Figure 1. (a) HAADF-STEM images of a 10 h annealed sample viewed along the MgO[100] zone axis showing the varying density of the Au nanoparticles as a function of depth from the surface (the surface is toward the top of the image). (b) High-resolution image of one Au nanoparticle viewed along the MgO[110] direction and (c) a one-atomic-column-wide intensity profile across the particle in the [001] direction, in the location indicated by the arrow, showing the nominally abrupt interface between the Au nanoparticle and MgO matrix.

subsequent APT reconstruction. The ionic view of the entire reconstruction is shown in Figure 2b, with cyan points corresponding to O, magenta to Mg, and yellow to Au. Molecular species evaporation of MgO, MgO2, Mg2O, and O2 were also observed in the APT mass spectra similar to past APT studies on UV laser−MgO interaction.15 Au-rich regions within the reconstructed data are visualized by tracing 10 atom % isocomposition surfaces (Figure 2c). The number and density of the Au-rich regions are qualitatively consistent with HAADFSTEM images. The shape of individual Au nanoparticles in APT reconstruction appears to be distorted in comparison to HAADF-STEM images in Figure 1a. This axial distortion is consistent with past comparisons between HAADF-STEM tomography and APT for embedded metal precipitates, though it is a more extreme case owing to the larger field difference between MgO and Au.33 A proximity histogram across the Au 10 atom % isocomposition surfaces of the 30 largest Au nanoparticles shows that the core of the Au-rich regions reaches a maximum average concentration of only 22.19 ± 0.75 atom % Au (Figure 2d). There is a coincident minimum of Mg (38.01 ± 0.88 atom %) and O (39.58 ± 0.89 atom %). The compositional 1362

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Figure 2. Results of cross-correlation of APT and aberration-corrected STEM imaging (a) HAADF-STEM images of the APT sample before and after APT analysis. The topmost layer of the tip before APT is a FIB-deposited Pt/C protective layer. (b) Ionic view of the APT reconstruction (108.7 × 105.8 × 323.2 nm), where cyan points correspond to O, magenta to Mg, and yellow to Au. (c) Au-rich regions in the reconstruction are visualized by tracing 10 atom % isocomposition surfaces. (d) Proximity histogram plotted across the 30 largest Au nanoparticles obtained using the Au 10 atom % isocomposition surface showing roughly 22 atom % Au concentration at the center of the Au-rich regions. (e) The distribution of ions of Au (yellow) ions and ions from MgO (blue) shown by a 15 × 15 × 2 nm slice across a representative (141 nm3) Au nanoparticle showing the diffuse interface.

corner atoms, consistent with an argument for local field enhancement. The protrusion of Au particles is typically on the order a few (001) layers, less than 1 nm, on both the apex and sides of the needle-shaped sample. Correlated APT-STEM imaging confirms that nonuniform roughening of the sample surface after APT analysis can occur for Au−MgO, resulting in protruding Au nanoparticles on the top and sides of the APT specimen. This new topography would then be expected to further exacerbate the nonuniform field and the resulting evaporation. To better understand the continuously iterative interplay between evaporation field differences between Au and MgO and topographical field enhancement, electrostatic FE simulations of the evaporation process have been performed for a single 4 nm cubic Au nanoparticle embedded within a homogeneous needle-shaped MgO matrix. The field evaporation process is simulated as the removal of surface atoms, considered as metallic cells, under the influence of a high surface electric field. Each atom is defined as an assembly of numerical potential points. Once an atom is field-evaporated, the atomic cell is removed, and an ion (charge ne) leaves the surface. The trajectories of ions are used to determine the tip to detector image projection. With this model, the gradual evolution of the field, the tip shape, and the resulting ion impacts on the detector are all simulated. More

distribution can be clearly visualized by extracting a 2 nm thick slice through a representative Au nanoparticle region (141 nm3 volume), as shown in Figure 2e, with MgO matrix ions represented by blue dots and Au ions in yellow. The apparent extent of Au/MgO interdiffusion scales with the size of the Aurich regions, with smaller particles generally having a lower maximum concentration of Au at the core and higher concentrations of Mg and O. The incorporation of Mg and O into the Au particles seems suspicious in general given the resistance of Au to oxidation,34,35 particularly given that HAADF-STEM results are consistent with abrupt Au/MgO interfaces. Additionally, optical studies of Au implanted in MgO and similar oxides have indicated that particles were metallic in nature.36,37 The surface of the Au/MgO needle specimen shown in Figure 2a was further examined in detail both before and after APT analysis. Prior to APT analysis, the sides of the needleshaped sample were nominally smooth, without protrusions or pits in the vicinity of Au/MgO interfaces, as shown in Figure 3a. After partial evaporation in the atom probe system, Au particles were found to protrude from both the sides and top of the needle-shaped sample (Figure 3b,c). Additionally, a sharpening effect is observed on the exposed edges of the protruding nanoparticles, suggesting preferential evaporation of 1363

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shown in yellow. The overall atomic density inside of the Au nanoparticle regions is observed to be much lower than the atomic density in MgO regions, presumably because of the higher evaporation field of Au. The density of ion detection at the Au nanoparticle−MgO interface appears to be quite high from the simulation results, indicating high distortion of ion trajectories at the particle/matrix interface, leading to significant ion trajectory overlap. On the basis of this simulation result, the trajectory overlap is expected only for ions located within ∼0.5 nm of the particle/matrix interface (Figure 4b). The data from the core of the nanoparticle showed 100% Au, and no presence of MgO ions was detected based on the composition profile across the Au nanoparticle. STEM images in Figure 1a indicate that the size of Au nanoparticles ranges up to nearly 10 nm. Even for the largest nanoparticles, APT reconstruction results in Figure 2d,e indicate a core with a substantial concentration of MgO. This points to a greater extent of ion trajectory overlap than is predicted by evaporation field differences alone. This discrepancy may be related to the impact of nonhomogeneous field evaporation inside of the MgO as the evaporation of both single atoms (Mg and O) and molecules (MgO, Mg2O, O2, etc.) is observed experimentally during laser-assisted APT due to the combined effect of photoexcitation and applied electric field.15 Therefore, a single evaporation field constant for MgO can be a crude simplification.15,45 To eliminate this assumption, the simulation was modified to consider the laser-assisted evaporation of oxide molecules by assigning 50% of the MgO as having a lower evaporation field and 50% a higher evaporation field. The variable evaporation field is limited to a minimum 1:5 ratio between MgO and Au while keeping the average constrained to the expected 1:2 ratio. A snapshot of this simulation, at an instant when a protruding Au nanoparticle forms on the top surface, is given in Figure 4c. The increased roughening of the MgO surface observable in Figure 4c is consistent with previously reported aberration-corrected TEM images of pure MgO after laser-assisted APT analysis,15 indicating that a simulation with variable evaporation fields in MgO might be a more realistic simulation of the MgO evaporation process. These simulations likewise result in a greater overlap between the MgO and the Au particle, effectively reducing the Au concentration in the Au nanoparticle region (Figure 4d). This can also be observed by comparing simulated detector maps (inset in Figure 4b,d), in which there are clearly more MgO ions (blue dots) within the Au nanoparticle region for the variable-field simulation. The improved correlation to experimental observations strongly implies that the evaporation of molecular ions along with elemental ions from the oxide matrix can influence the overall spatial resolution of laser-assisted APT analysis and thus effect the accuracy of compositional measurement for oxidesupported metallic nanoparticles. There are additional factors that have not yet been considered in the simulation, such as the dielectric nature of MgO, which could induce additional field distortions at the Au/ MgO interface.30 Field- and temperature-driven diffusion of atoms at the tip surface may also degrade the spatial resolution. Additionally, metallic nanoparticles in a dielectric matrix can act as plasmonic resonators under UV laser illumination,48,49 which may result in local heating around the Au nanoparticles. For ion-implanted Au nanoparticles, there are vacancies within the MgO50 that could further complicate the ion trajectories during APT analysis; this is a practical concern for catalysts, in which

Figure 3. HAADF-STEM images of the sides of the APT sample (a) before APT analysis, showing the absence of protruding Au nanoparticles, and (b) of the same sample after APT analysis, showing protruding Au nanoparticles. (c) The tip of the APT sample has exposed the Au/MgO after APT analysis and also has protruding Au nanoparticles.

details concerning the model can be found in the literature.30,38−44 Due to technical limitations, tips with end radii of about 20−25 nm were modeled, but for such small particles, the tip size influence is only of second order. The initial simulations treat both materials as metallic, with the embedded particle having an evaporation field higher than the matrix. The evaporation field of MgO (FMgO) was estimated by Kreuzer et al.45 between 20 and 25 V/nm, in good agreement with the estimation of Marquis et al. (FMgO ≈ 22−25 V/nm).43 The evaporation field of Au (FAu) is given by Tsong to be 53 V/nm.46 This 1:2 ratio between the evaporation field of MgO compared to Au was confirmed by experimental APT studies on a Fe/MgO/Fe/Au multilayer material.47 The simulations reproduce the experimentally observed development of a protruding Au nanoparticle over the course of the field evaporation. A movie of the simulation showing the dynamic development of protruding Au nanoparticles during evaporation is provided in the Supporting Information. Figure 4a shows a simulation snapshot of the evaporating specimen when the Au nanoparticles have protruded out of the top surface of the specimen. The corresponding ion detection map on the detector is shown as inset in Figure 4b, where ions from MgO are shown in blue and ions from the Au nanoparticle are 1364

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Figure 4. (a) Snapshot of the simulated tip profile when the Au nanoparticle protrudes out of the top surface during the evaporation of a 4 nm nanocube of Au, embedded in a MgO matrix. The evaporation field of the matrix was fixed to FMgO = (1/2)FAu. (b) The concentration profile across the particle after reconstruction shows low density of atoms in the Au nanoparticle region and high atomic density at the Au−MgO interface. The inset shows the ion distribution on the detector with MgO ions in blue and Au ions in yellow. (c) Simulated tip profile during the evaporation of a nanocube of Au, embedded in a MgO matrix with an average FMgO = (1/2)FAu, but 50% of the atoms have a lower evaporation field, and 50% a higher (see the text for details). (d) The concentration profile across the particle after reconstruction shows a lower decrease in atomic density and a degradation of the spatial resolution (about 1−2 nm). The numbers of atoms in (b) and (d) are plotted on an arbitrary scale for relative comparison.

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supports are often created porous to increase surface area. These additional factors could readily explain the remaining difference between the simulated and experimentally observed Au nanoparticle composition. In summary, a metal−dielectric composite consisting of Au nanoparticles embedded in MgO has been examined by laserassisted APT, coupled with high-resolution STEM measurements before and after atom probe analysis, and the evaporation behavior has been studied by FE simulations. The difference in evaporation field between Au and MgO can lead to unequal evaporation rates of Au and MgO regions, resulting in formation of protruding Au nanoparticles on the APT specimen surface. This observation of locally protruding Au nanoparticles beyond the tip surface implies a heterogeneous evaporation front during APT analysis. This nonuniform specimen topography can subsequently lead to further local field enhancement, resulting in an escalating deviation from an ideal field evaporation model. FE simulation results indicate that even the field difference between the different molecular species detected during laser-assisted APT analysis of oxides can deteriorate the effective spatial resolution and accuracy of composition quantification of embedded metal nanoparticles in oxides. This study presents the first detailed experimental observation of hypothesized topographical variations during APT analysis of heterogeneous materials.25,26,33,38,39,41,44,51−55 This understanding can be utilized to develop strategies for understanding and correcting the trajectory aberrations during atom probe tomography reconstructions, paving the way to more accurate and reliable APT analysis results of these technologically important classes of metal−dielectric materials.

ASSOCIATED CONTENT

S Supporting Information *

A movie of the finite element field evaporation simulation, showing the dynamic development of protruding Au nanoparticles during evaporation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions #

A.D. and R.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Laboratory Directed Research and Development (LDRD) program of Pacific Northwest National Laboratory as a part of the Chemical Imaging Initiative. The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research located at PNNL, and with the assistance of the William R. Wiley postdoctoral fellowship. The authors would also like to acknowledge Wayne P. Hess for useful scientific discussions. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830

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The Journal of Physical Chemistry Letters

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dx.doi.org/10.1021/jz500259c | J. Phys. Chem. Lett. 2014, 5, 1361−1367