Strain Field in Ultrasmall Gold Nanoparticles Supported on Cerium

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Strain Field in Ultra Small Gold Nanoparticles Supported on Ceriumbased Mixed Oxides. Key Influence of the Support Redox State. Miguel Lopez-Haro, Kenta Yoshida, Eloy del Rio, Jose A. Perez-Omil, Edward D. Boyes, Susana Trasobares, Jian-Min Zuo, PRATIBHA L GAI, and José J. Calvino Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00758 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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Strain Field in Ultra Small Gold Nanoparticles Supported on Cerium-based Mixed Oxides. Key Influence of the Support Redox State Miguel López-Haro1, 2*, Kenta Yoshida3, Eloy del Río1, José A. Pérez-Omil1, Edward D. Boyes3, Susana Trasobares1, Jian-Min Zuo4, Pratibha L. Gai3, José J. Calvino1 1

Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica. Facultad

Ciencias. Universidad de Cádiz. Campus Rio San Pedro. Puerto Real, 11510-Cádiz, Spain. 2

Univ. Grenoble Alpes, F-38000 Grenoble, France CEA-INAC/UJF-Grenoble 1 UMR-E, SP2M, LEMMA, Minatec Grenoble, F-38054, France

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Departments of Chemistry and Physics, University of York, The York JEOL Nanocentre, Heslington,

York YO10 5DD, UK. 4

Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, 1304

West Green Street, Urbana, Illinois 61801, United States

KEYWORDS: Gold catalysts, Nanoparticles, Strain, AC-TEM, Redox state

ABSTRACT.

Using a method that combines experimental and simulated Aberration-Corrected High Resolution Electron Microscopy images with digital image processing and structure modeling, strain distribution ACS Paragon Plus Environment

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maps within gold nanoparticles relevant to real powder type catalysts, i.e. smaller than 3 nanometers, and supported on a ceria-based mixed oxide have been determined. The influence of the reduction state of the support and particle size has been examined. At this respect, it has been proved that reduction even at low temperatures induces a much larger compressive strain on the first {111} planes at the interface. This increase in compression fully explains, in accordance with previous DFT calculations, the loss of CO adsorption capacity of the interface area previously reported for Au supported on ceriabased oxides.

INTRODUCTION Gold nanoparticles supported on ceria and closely related cerium-mixed oxides are recognized as an interesting family of materials with excellent catalytic activity in a number of reactions of major technological interest, as are those related to the production of hydrogen for fuel cell applications, e.g. Low Temperature Water Gas Shift (LT-WGS), Preferential Oxidation of CO in the presence of large amounts of hydrogen (PROX) or Low-Temperature CO oxidation (OXI-CO). Since all these reaction involve CO as a reactant, adsorption of CO on gold has been the focus of intense basic research.1-11 Thus, a significant progress has been made in the understanding of the chemical principles governing CO interaction with gold surfaces.12-15 In connection with this, recent experiments have shown that the CO-adsorption capacity of ceria-zirconia gold catalysts could be strongly inhibited by application of a rather mild reduction treatment at 473 K.16 This deactivation was found to occur in parallel with subtle structural changes taking place at the nanometer-sized Au-ceria oxide interfaces induced by charge transfer effects between the reduced support and the supported gold nanoparticles. Thus, these treatments play a key role in the chemical behaviour of these catalysts. It is well known that compressive and tensile strain induce modifications on the structure of nanoparticles and, therefore, they may also influence their chemical and catalytic properties.17, 18 In fact, previous works have already reported such correlation.19-21 Thus, on the basis of DFT calculations,

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Maveriakis et al., 22 predicted an enhancement of the CO and O chemisorption on strained Ru surfaces, whilst Gsell et al. provided experimental clues about the preferential adsorption of oxygen atoms on strained sites of Ru(0001) surfaces.23 Likewise, in the particular case of gold catalysts, Xu et al., on the basis of DFT calculations, also showed that a 10% tensile strain substantially improved O2 activation on Au (111) layers supported on TiO2.24 Determining the strain state of supported gold catalysts seems, therefore, a requirement to better understand their performance. The limited-size of gold nanoparticles and the intrinsically low values expected for the corresponding atomic displacements in these nanostructures, demand for highly localized and accurate characterization techniques, able to provide structural information at the atomic scale. This is the case of AberrationCorrected Electron Microscopy (AC-EM) techniques, which nowadays allow recording structural information with sub-Ångstrom spatial resolution. Some recent results25 illustrate the high potential of both ex-situ and in-situ AC-EM in the detailed characterization of metal supported catalysts.In particular, the development of spherical aberration correctors has allowed not only improving resolution but also, very important, overcoming contrast delocalization effects which limited the interpretation of conventional, non-corrected, HREM images in terms of position of atomic columns and, at the same time, improving the image contrasts. All these features open the possibility of locating with high accuracy the position of atomic columns in AC-HREM images, this in turn, allowing us to map strain fields in a variety of nanoparticles.26-31 Walsh et al. have thus developed a methodology, using aberration corrected high resolution transmission electron microcopy images (AC-HREM), to show that ultrasmall decahedral Au nanoparticles present an average surface expansion of 5.6%. Likewise, on the basis of DFT calculations they predicted an enhanced activity for CO oxidation due to this effect.31 Though this paper clearly illustrates the influence of lattice strain on catalytic performance of nanoparticles, the morphology studied in this work is far away from those characteristic of supported gold catalysts prepared by deposition-precipitation (DP) methods.32 Moreover the presence and possible influence on strain of the support is not considered. ACS Paragon Plus Environment

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Geometric phase analysis (GPA) and peak pairs algorithm (PPA) methods have become the most suitable techniques for studying stress and strain at nanometer scale.33, 34 However, these approaches have not provided satisfactory results when measuring the strain lattice in nanoparticles.31 Consequently, the determination of strain distribution in gold nanoparticles present in powdered catalytic systems, typically with diameters below 3 nm, still remains as a major challenge, relevant for an improved understanding of their catalytic activity. This paper is aimed at measuring the strain field of gold nanoparticles supported on ceria-based oxides and, more particularly, at unveiling the influence of redox treatments on strain, an aspect not considered yet, to the best of our knowledge. To achieve this major goal, we have used a methodology that combines AC-HREM images, digital image processing, computer modeling and HREM image simulation. The investigated catalyst consisted of a 1.5wt%Au/Ce0.5Tb0.12 Zr0.38O2 (Au/CTZ) sample studied in previous papers. This catalyst was part of a research plan aimed at investigating the influence of a second reducible lanthanide element incorporated into cerium-zirconium mixed oxides on both redox and catalytic performance in different reactions. Previous works35, 36 showed that Tb-doped CeO2 oxides depict a better redox response at low temperatures than pure CeO2 and this led us to estimating Tb effects in ceria-zirconia supports. The improved low temperature reducibility on ternary systems37, 38 prompted us to study interactions with gold nanoparticles in these catalysts after different pretreatment conditions. Details of the preparation and characterization of the catalysts are given in the experimental section and references therein. To consider the influence of the redox state of the support, images of the catalyst after being submitted to both an oxidation treatment and a reducing one were analyzed. The first treatment consisted in heating the catalyst at 523 K under an O2 (5%)/He mixture flowing at atmospheric pressure. For the reducing treatment, the catalyst was treated under flowing H2/(5%)/Ar at 473 K for 1h and then it was oxidized at room temperature. This last step is necessary to avoid the uncontrolled reoxidation of the reduced catalyst during re-exposure to air in its transfer from the reactor into the electron microscope chamber. Previous studies12 have indicated that after the oxidation treatment the CTZ support is fully oxidized, whereas after reduction and mild-reoxidation the support ACS Paragon Plus Environment

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retains a 18% of reduced cerium species (Ce3+). Moreover EELS analysis of the reduced sample pointed to the accumulation of the reduced species in the areas just beneath the gold nanoparticles, which could contain up to a 60% Ce3+.16 Therefore, in the oxidized catalyst the metal nanoparticles sit on top of a fully oxidized (Ce4+) support structure, whereas in the case of the reduced catalyst, the metal nanoparticles are in contact with a highly reduced CTZ oxide, containing a large fraction of, larger, Ce3+ species. These local modifications of the support lattice structure could be expected to induce subtle structural changes on the metal nanoparticles, modifying possibly their strain state, especially in the areas close to the interface. Unveiling such changes is the major goal of this contribution.

EXPERIMENTAL SECTION

The Au(1.5 wt%)/Ce0.5Tb0.12 Zr0.38O2 (Au/CTZ) catalyst, with a BET surface area of 16 m2g-1, was prepared by the deposition-precipitation method, starting from a commercial CTZ mixed oxide support kindly donated by Grace Davison. The gold precursor was 99.99%H[AuCl4], from Alfa Aesar. An aqueous solution of urea was used as precipitating agent. Further details about the preparation procedure are reported in ref.39 The metal loading was confirmed by ICP analysis. The electron microscopy sample grid was prepared by depositing a small amount of the sample powder directly onto holey-carbon coated Cu grids. Excess powder was removed from the grids by gentle blowing with a nozzle. The HREM images were recorded in a JEOL 2200FS double aberration corrected transmission electron microscope (AC-TEM/STEM), operating at 200 kV. Using the Zemlin tableau, the defocus and the third order aberration coefficients of the objective lens were measured and adequately compensated for. The acquisition time was 1 second per image in order to guarantee the smallest damage possible to the sample. The magnification used for recording the 2k x 2k digital images was in the range of 600k-1M. These magnifications allowed us to achieve a pixel size ranging from 0.015 down to 0.0098 nm. The plug-in HREM Filters Lite v1.7 for Digital Micrograph was used to


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apply the Wiener filter. The structural models were built with RHODIUS, a software developed in our lab.40 The AC-HREM image simulations were carried out using the TEMSIM software.41 The treatment and analysis of the images was done using ImageJ and home-developed Python routines.

RESULTS AND DISCUSSION

Figure 1 illustrates raw and Wiener filtered AC-HREM images recorded in profile view on the Au/CTZ representative of the structure of the catalyst after two different pretreatments: 1) oxidation at 523K (Figures 1a, b, c, g, h, i ) and 2) reduction at 473K followed by a re-oxidation at 298K (Figures 1d, e, f, j, k l ). The images were acquired using a slightly negative value of spherical aberration and a slightly positive defocus value to reduce to a minimum contrast delocalization effects as well as to improve the contrast.42 Likewise, it is worth mentioning that the use of the Wiener filter allows not only improving the signal-to-noise ratio of the images, a key issue to face strain measurements with high accuracy, but also to enhance the image contrasts, in particular at edges and surfaces without introducing additional artifacts.43, 44 In the following we will refer to the nanoparticles observed in these images as to Ox-1, Ox-2, Ox-3 Figures 1a, 1b, 1c and Rd-1, Rd-2 and Rd-3 Figures 1d, 1e and 1f, respectively. Note first how the nanoparticles observed in these images are all well faceted sitting on a flat surface, depicting a truncated-cuboctahedron like morphology and a size in the 0.7 - 1.6 nm range. As shown in Figure SI-1, the particle size distributions of the two catalysts are very similar and the particles collected in Figure 1 are those covering the size range corresponding to the most frequent ones in both catalysts. The crystallographic information of these Au NPs was extracted from the results of Fourier analysis, Figure SI-2. Thus, the spots lighting up in the Digital Diffraction Patterns (DDPs) show in all cases the distances and angles characteristic of {111} and {200} planes of a cubic lattice in orientation [011]. This result is in good agreement with previous analysis performed in refs14, 32, 45 which showed that after


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these treatments, the Au nanoparticles present a metallic f.c.c. structure and grow under a parallel epitaxy relationship on top of the {111} planes of the support. To investigate structural distortions in nanomaterials, a reference network must be first defined to which refer any atomic displacement. In general this reference is selected from a defect-free crystal region. However, in systems such as nanoparticles, formed by a very limited number of, e.g. {111}, planes, such possibility is not feasible. Consequently, an alternative approach becomes compulsory which allows dealing with this very peculiar case.28,

31

In order to propose such reference lattice,

structural models of non-relaxed gold nanoparticles with truncated cuboctahedron morphology in [011] zone axis and under epitaxial relationship with the support were built, Figures 1g-l. It is important to note that in these models, the crystallographic parameters characteristic of bulk gold were considered, i.e., an f.c.c. structure with Fm-3m symmetry and a lattice parameter value of 0.408 nm. This crystallographic configuration is that corresponding to distances between {111} planes equal to 0.235 nm and 0.204 nm between {002} planes. The observation of hundreds of particles in experimental profile view images of the Au/CTZ catalysts investigated here, along different zone axis orientations, indicates that the very small Au nanoparticles supported on the CTZ crystalline support depict atomically flat surfaces. Therefore, the starting models for the image simulations can reasonably consider a non-rough faceting for the Au nanoparticles. In other words, it is quite reasonable assuming that the facets imaged down the zone axis are as flat as those observed edge-on. From these models, several important features could be determined like: their equivalent diameter, deq (diameter of the hemisphere which contains the same number of atoms as the particle); the number of {111} planes which stack parallel to the interface plane; N1-11 and the aspect ratio (W/H). All these values are gathered in Table SI-1 (Supporting Information). Thus the Au NP observed in Figure 1a, Ox1, has an equivalent diameter equal to 2.47 nm, whereas Au NP observed in Figure 1d, Rd-1, which corresponds to an equivalent diameter equal to 1.74 nm. Such differences in terms of size could cause differences in the details of their spatial arrangement since smaller nanoparticles contain a higher


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fraction of, more loosely bounded, low coordinated atoms located both at the surfaces and at the interface with the support. Given that AC-HREM imaging is sensitive to local thickness values, a reference-simulated image is necessary for each analyzed particle, starting from the model which best takes into account its observed shape and dimensions. Sets of AC-HREM images were simulated, which considered both defocus and the influence of a slight tilt around the exact zone axis orientation condition. Pixel size values identical to those of the experimental images and the following electron-optical parameters were used; Accelerating voltage=200kV, Cs=-5 µm, ∆f= -10 to 10 nm. Figure 1m-r shows the simulated images that best fitted the experimental ones. Note the excellent agreement between the contrasts in the experimental and simulated images. Therefore, these simulated AC-HREM images clearly prove that the projection of atomic columns on the experimental images can be interpreted as those existing in the experimental Au NPs. To estimate the position of contrast maxima (minima) in the images, 2D bicubic interpolation was used. This image processing technique provides sub-pixel resolution and therefore allows increasing the accuracy of the analysis (see Figure SI-3).34 After interpolation, the geometric centroid was calculated, since the images are not affected by any anisotropic effect. Nevertheless, we can observe how the x-y positions of the atomic columns in the experimental images are clearly shifted with respect to those in the simulated ones Figure SI-4. Before attempting to compare the position of atomic columns between these pairs of images (simulated and experimental), they were first aligned by minimization of the total atomic displacement at the interface. This procedure provides the best fitting between pairs of images without any bias. The following important question arise at this point; do the position of the intensity maxima observed in the simulated images correspond to those of the corresponding atomic columns in the models? In order to answer this question we determined the x-y coordinates of all the intensity maxima (minima) observed in the simulated images and they were compared with those corresponding to the positions of atomic columns in the model, Figure SI-5a,b. ACS Paragon Plus Environment

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Note how, after applying the procedure described above, the position determined for the atomic columns in the simulated images are in fact quite close to those in the models, within distance differences ranging between 3.2±1.2-5.9±3.0 pm in average. Actually, though the values of these differences slightly depend both on Au NP size and on the exact location of the column within the particle (larger values being in general observed at the top-most surface and interface positions, see Figure SI-5c,d), they are always below 3% the value of the spacing of Au {111} planes (0.235 nm). Moreover, the average value of these differences, considering the whole particles, is only about 2%. Quite important, these results also prove that the imaging process introduces, though small in amplitude, some shifts in the position of atomic columns, which have to be properly accounted for when determining displacements in experimental images linked to actual structural effects. Therefore, we propose using the contrasts in the simulated images as the reference network. By comparing the position of contrasts in experimental and simulated images it would be possible to measure atomic displacement taking place in supported nanoparticles not due to image artifacts but, instead, to actual structural modifications. It also should be noticed that in the particular cases of particles Ox-1 and Rd-3 the projection of the atomic columns in the experimental image are showed as black contrasts, so, the column position map was built in this particular case considering the local minima of each column instead of maxima. After extracting the position of atomic columns from experimental and simulated images the local displacement vectors δij (as explained in Figure SI-6, the i index refers to the number of the (1-11) plane parallel to the interface, starting at the interface; whereas the j index refers to the horizontal position within this plane) were calculated, Figure 2. These vectors, drawn from the position of atomic columns in the reference lattice to the position in the experimental image, provide a description of the strain field in the Au nanoparticles. Results are shown for the nanoparticles supported on the fully oxidized CTZ support (Figures 2a, b, c) as well as on the reduced ones (Figures 2d, e, f). Note how the change in the orientation of the displacement vectors coincide in the whole set of analyzed particles.


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It is also important to highlight that the values of the modulus of the displacement vectors between the experimental images and the reference lattice, though changing with exact position within the particles, are much larger than those detected between the simulated images and the exact column positions in the unrelaxed model. The former differences are in the range 1-90 pm, which is one order of magnitude larger than the latter. This clearly indicates that the particles are actually relaxed in both catalysts. To get a further insight into the deformation field, the local strain parallel (εxx) and perpendicular (εyy) to the interface were calculated, Figure 3. First, this calculation was performed on the simulated image of a particle of medium size, Figure SI-7. In this particular case the reference lattice was that corresponding to the exact positions in Au f.c.c. As expected from previous comments, the imaging process itself gives rise to an “apparent” strain. The shifts in the position of the atomic columns due to the imaging process are very small, so they cannot be appreciated by eye in the vector plot of Figure SI7(d). Nevertheless, the average values, εxx and εyy, estimated for the whole particle were in this case 0.31 and -0.85 respectively, Figures SI-7(d-e). These results clearly indicate that the positions in the perfect lattice cannot be used as a reference to measure strain in nanoparticles, since the imaging process itself, apart from any other influence related to additional factors, induces certain background values of “apparent” strain. Once this point clarified, the influence of the presence of residual noise in the experimental and Wiener-filtered images was also investigated, Figures SI-7(g-i). To this purpose, it is necessary to start from a simulated image, which contains no noise, and then evaluate the effect of adding noise and applying the same Wiener filter employed with the experimental images. It is also important to note here that, in order to evaluate just the influence of noise, the reference lattice in this case has to be the simulated image. By doing so we are correcting for the apparent strain due to the imaging process, i.e. we are removing the shifts in the atomic columns due to imaging and retaining only those due to residual noise. With this in mind, Poisson type noise was added to the simulated image in Figure SI7(a), Figure SI-7(b), and then it was partially removed using the same Wiener filter employed to


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evaluate the experimental images, Figure SI-7(c). Figures SI-7(g-h) show the distribution of strain in the image with residual noise. Note that the influence of noise on strain amounts roughly to -0.2 in the two main directions. Very similar values were obtained on many other trial images obtained after adding noise repeated times to the same simulated image, which clearly indicates that these strain values are representative. Note also that the atom column shifts are still hardly visible on the vector map, Figure SI-7(g). Additionally, these results have been used to estimate error bars. It is also important to stress that, in any case, both the local and the average strain values determined on the experimental images, Figures SI-7(j-l) are well above those on the simulated ones containing residual noise. This is specially so in the case of εyy. Importantly, the strain sign changes from negative in the simulated image with residual noise to positive in the experimental images. Now, the displacement vectors are clearly observed in the map of Figure SI-7(j). Likewise, the intensity of the colour images increases when going from the simulated images with noise to the experimental one. After all these considerations, Figure 3 shows the strain maps corresponding to the whole set of nanoparticles investigated in this work. Concerning the spatial distribution of strain, note that, in general, the largest strain values are often observed at surfaces and interfaces. At this respect, there is also consistency among the whole set of studied particles. To better compare the deformation observed in the different particles, average values of strain parallel and perpendicular to the interface were calculated from the strain maps for both the whole particles (εxxtot and εyytot ) and for the atomic columns at the interface plane (εxx1 and εyy1), Table 1. As we will see later, these last values are of paramount importance to understand the influence of strain on chemical properties. For general discussion purposes, we have also estimated the average values of these parameters considering the three particles of each type: (εxx, avtot, εyy, avtot) and (εxx, av1 and εyy, av1). From Table 1, it becomes clear that the nature of the redox pretreatment of the catalyst plays a major influence on the strain state of the nanoparticles. Thus particles supported on the oxidized support (Ox) show lower absolute values of total average strain (εxx,

tot av

and εyy,

tot av )

than those deposited on the


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reduced support (Rd). In the case of average total strain along the direction parallel to the interface, the particles on the oxidized support manifest a slight (εxx,

tot av

= 0.43) tensile strain along that direction,

whereas those on the reduced support suffer from a large compressive strain (εxx,

tot av

= -1.11). Along

the direction perpendicular to the interface, both types of particles manifest a tensile strain, though the average value observed for the Ox particles (εyy,

tot av

= 1.34) is much smaller; about a third that of the

Rd ones (εyy, avtot = 3.66). In Figure SI-8 we plotted the different average strain values on a linear strain scale to illustrate their relative positions. Note that in the case of the average strain values parallel to the interface, the points corresponding to the oxidized catalyst cover a strain range located at higher values than those of the reduced catalyst, both in the case of εxxtot and εxx1. In the case of the average strain values perpendicular to the interface, the opposite is observed. The scatter in the strain values observed on both catalyst stems most likely from the fact that we are analyzing particles which are not identical in size, aspect ratio or which are located on areas of the support differing in reduction states, but it is not related to intrinsic errors related to the methodology used to determine strain. To clarify this point, the position of the average values, εxx,avtot, εyy,avtot, εxx,av1 and εyy,av1 have been added to the graph in Figure SI-8 including actual errors bars. Note that in no case there is an overlap of these averages. Therefore, this graph clearly illustrates that changing the catalyst pretreatment from oxidizing to reducing leads to meaningful modifications in the strain state of the gold nanoparticles. To the best of our knowledge, these are the first results clearly demonstrating the key influence of catalyst redox pretreatments on the strain state of supported nanoparticles with sizes relevant to real powder type catalysts. The comparative analysis of the strain field at the level of the Au NPs || Support interface provides also quite interesting ideas. At this respect, our dataset clearly shows that in general, as previously described for the behavior of the particles as a whole, the absolute values of strain at the level of the interface are much higher for the particles supported on the reduced CTZ oxide. This points out a good ACS Paragon Plus Environment

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consistency of the whole dataset. Concerning strain at this particular boundary of the catalysts, the average values observed in the direction parallel to the interface (εxx, av1) are in both cases negative, this indicating that the particles suffer from compression at the first metal plane in direct contact with the oxide support surface. Note however that the compression state along this direction is much higher in the case of the particles supported on the reduced oxide. The average value observed for the Rd particles, εxx, av1= - 1.59, is roughly twofold that observed as average for the Ox ones, εxx, av1= -0.78. A coincidence with respect to the type of distortion at the interface is also observed between the two types of pretreatments along the direction perpendicular to the interface. Thus, both εyy, av1 values are in this case positive, indicating a tensile distortion. But, again, the distortion occurring in the reduced particles is in average much larger, εxx, av1 = 11.42, than that detected in the particles supported on the oxidized support, εxx, av1 = 0.27. Summing up at this point, our strain analysis shows a very consistent set of data that evidences large differences between the behavior of particles supported on the oxidized and the reduced supports. Both, as a whole and at the interface level, the particles supported on the reduced support manifest larger strain values than those sitting on the surface of the oxidized one. In general, the distortions along the direction parallel to the interface tend to be compressive in nature whereas strain in the direction perpendicular to the interface tends to be of tensile type. There seems to be in general, from this point of view, a compensation effect between strains along these two mutually perpendicular directions. Figure 4 shows two plots that allow visualizing the influence of AuNP size on average strain values. In particular these plots show the variation of εxxtot and εyytot with size, expressed in this case as the total number of (1-11) planes comprising each particle, N1-11. In the case of εxxtot values, particle size seems to influence in the same manner both type of catalysts. The average total strain in this direction seems to decrease with the number of (1-11) planes. Thus, in the case of the Rd particles the compressive strain decreases with increasing N1-11, i.e. εxxtot values tend to be less negative. This is the same observed in


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the case of the Ox particles, for which increasingly positive εxxtot numbers indicate an increase in tensile strain. In other words, increasing size decreases compression or, alternatively, increases tension. In any case, the Rd nanoparticles are more compressed along the direction parallel to the interface than the Ox ones (εxxtot values always smaller). Regarding the variation of εyytot with AuNP size, an opposite trend is observed between Rd and Ox nanoparticles. For the former, εyytot tend to increase with N1-11, whereas a steady decrease is observed in the Ox particles. This is an additional aspect in which these two types of particles differ. In fact, this experimental description of strain is in very good agreement with the predictions made on the basis of previous DFT calculations for 2D Au surface models.

46, 47

These calculations

predicted that the Au (111) surface suffers a significant outward expansion of the topmost layer compared to the subsurface layer. Our results are also in good agreement with the study performed by Kawasaki et al., which showed large distortions on the surface and interface of Au NPs supported on TiO2.48 They attributed this deformation to the presence of the atom missing-row and the strong interaction with the TiO2 substrate. Huang et al.49 combined coherent electron diffraction and molecular dynamics calculations to analyse the surface structure of Au nanoparticles. They showed that the displacements of atoms on the surface was dependent on their specific coordination and described a contraction of the {100} and {111} surfaces in a 4 nm-size AuNP, i.e. a particle larger than Ox-1. These results are also in good agreement with those found by us for Ox-1. In any case it should be stressed that in our case the nanoparticles are supported under an epitaxial relationship. Due to its relation with macroscopically measured chemical properties, it becomes quite relevant comparing the values of εxxi and εyyi for the planes just at the interface, i.e. the εxx1 and εyy1 values, as a function of redox treatment, Figure 4(c,d) and Table 1. Note at this respect that the εxx1 values corresponding to the three nanoparticles on the reduced support, from the smallest to the largest, are much more negative than those observed for the three particles on the oxidized support. This indicates ACS Paragon Plus Environment

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that the interface plane is suffering a much larger compression in the particles supported on the reduced support. This higher compression along the direction parallel to the interface observed for the plane just at the interface in the nanoparticles on the reduced support could explain the drop in the CO adsorption capacity of Au/CTZ after reduction at temperatures as low as 473 K. Previous studies have clearly shown at this respect that, in quantitative terms, the drop in CO adsorption observed after reduction treatments in catalysts of this family corresponds very closely to the fraction of atoms at the interface between gold nanoparticles and the CTZ support.16 The much larger compression detected on the interface plane of the Au nanoparticles supported on reduced supports and the previously recognized effect of compressive strain on CO adsorption capacity of noble metals would perfectly explain the changes in CO adsorption after reduction.23, 31 Concerning the comparison of εyy1 between the two catalysts, again the values detected for the Au NPs on the reduced support are largely differing from those of the particles on the oxidized one. Much more positive values of εyy1 are observed for the former nanoparticles. This indicates that the distance between the first (1-11) metal plane at the interface and the one just on top of that is larger in the Rd nanoparticles. At this respect it is important to recall that in a previous study16 an electronic transfer between the Au metal nanoparticles and the CTZ support was evidenced in the case of the reduced catalyst. This electronic transfer was accompanied by a substantial approach between the metal and support planes at the interface, which come closer than in the oxidized catalyst. Such an approach, which correlates with a tighter binding between metal and support, adds up to that observed in this study due to compressive strain in the first atomic (1-11) plane at the interface, which also points out to a tighter binding between the Au metal atoms in that plane. If we consider both effects simultaneously, it appears reasonable at first instance that binding to the adjacent (1-11) metal plane may suffer some weakening, this very likely explaining the observed increase in the εyy1 value in the Rd nanoparticles.


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CONCLUSION

In conclusion, we have developed a methodology which combines experimental AC-HREM images, digital image processing, computer modeling and HREM image simulation that allows measuring the strain field of AuNPs with dimensions as small as 2 nm supported on a carrier oxide. Comparison with structural models confirmed that simulated images could be used as the reference lattice to which local displacements of atomic columns inside the nanoparticles could be measured. The strain fields determined for gold nanoparticles supported on a CTZ support could be characterized in terms of different parameters: local displacement vectors (δ δij), average total and interface strain along two directions (εxxtot, εyytot, εxx1, εyy1) as well as in terms of average strain over individual (1-11) planes stacked parallel to the interface (εxxi, εyyi). Comparison of strain maps obtained from simulations and experimental images clearly indicate that, regardless the redox state of the support, the nanoparticles epitaxially grown on the support become relaxed. Our results clearly evidence a strong influence of the support redox state on the strain induced on the supported nanoparticles. Clear differences are observed both in the average strain values of the whole particles and those corresponding to the interface plane. In particular, reduction of the support induces a much larger compressive strain of the (1-11) Au plane at the interface. This large structural change at the interface could explain, in accordance to previous DFT calculations, the loss of CO adsorption capacity of the interface observed in these catalysts after reduction at 473K. Our data also prove that the strain state of nanoparticles cannot be considered an intrinsic property of the particles themselves, showing dependence only on particle size, but that it is largely influenced by the support and, as such, it is a property of the whole Metal||Support system instead. This is possibly quite a different perspective. The presence of oxygen vacancies and Ln3+ species in the areas of the partially reduced CTZ support underneath the gold nanoparticles must induce a local strain. Thus, the change in the strain state in the nanoparticles after reduction must very likely be induced by the ACS Paragon Plus Environment

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appearance of strain in the reduced support as well as to the occurrence of electron transfer phenomena through the interface. On the other hand, the number of cases analysed in this contribution seems adequate to capture the influence of redox pre-treatments on the strain state of the nanoparticles, since these cover the size range corresponding to the most frequent gold particles in both catalysts. However, it seem reasonable that to reach a more in-depth understanding of the simultaneous influence of other parameters like particle shape or aspect ratio, which as our own data reveal superimpose over that of the reduction state of the support, further experimental studies considering a larger number of particles would be necessary. Finally it is worth highlighting once more the large potential of modern Aberration-Corrected Electron Microscopy techniques as probes of the local structure of complex systems, as it is the case of supported metal catalysts, as well as to provide unique information to rationalize their macroscopic performance.


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Table 1. Average values of the strain calculated for the whole particles (εxxtot and εyytot) and for the interface planes (εxxint and εyyint ) of Au NPs as a function of the treatment applied. εxxtot εyytot εxx1 εyy1 Treatment Au Np

Ox

Rd

1

0.94

-1.18

-1.13

3.00

2

0.10

4.14

-0.55

0.94

3

0.25

1.07

-0.65

-3.14

Average values*

0.43±0.10 (εxx, av tot)

1.34±0.60 (εyy, av tot)

-0.78 ±0.07 (εxx ,av 1)

0.27±2.40 (εyy, av 1)

1

-1.77

2.19

-1.92

3.03

2

-0.82

6.15

-1.90

8.22

3

-0.74

2.63

-0.96

23.00

Average values*

-1.11±0.10 (εxx, av tot)

3.66±0.60 (εyy, av tot)

-1.59±0.07 (εxx, av 1)

11.42±2.40 (εyy, av 1)

* In this average the values corresponding to the three particles of each type have been considered. The error bars are estimated on the basis of the analysis of the influence of de-noised images by Wiener filtering.


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Figure 1

Lopez-Haro et al.

Figure 1. Raw and Wiener filtered AC-HREM images of the Au nanoparticles oxidize at 523K (a,g(Ox1), b,h(Ox-2), c,i (Ox-3)) and after reduction at 473K and reoxidation at 298K (d,j (Rd-1), e,k(Rd-2), f,l(Rd-3). Non-strained gold nanoparticle models with truncated cuboctahedron morphology in zone axis [011] and epitaxial relationships with the support (m-r). The best matching simulated AC-HREM images (s-x). The scale bar corresponds to 1 nm.


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Figure 2

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Lopez-Haro et al.

Figure 2. Displacement fields of the gold nanoparticles on the oxidized catalyst (a: Ox-1, b: Ox-2, c: Ox-3) and on the reduced one (d: Rd-1, e: Rd-2, f:Rd-3).


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Figure 3

Lopez-Haro et al.

Figure 3. Strain map of the gold nanoparticles on the oxidized catalyst (left hand-side) and on the reduced one (right hand-side) parallel (εxx: Ox-1(a), Ox-2 (b), Ox-3 (c), Rd-1 (g), Rd-2(h), Rd-3(i)) and perpendicular to the interface (εyy: Ox-1(d), Ox-2 (e), Ox-3 (f), Rd-1 (j), Rd-2(k), Rd-3(l)).


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Figure 4

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Lopez-Haro et al.

Figure 4. Average values of the strain calculated for the whole particles (εxxtot and εyytot) and for the interface planes (εxxint and εyyint ) of Au NPs as a function of the treatment applied and total number of {1-11} planes stacked parallel to the interface. Au Nps submitted at oxidation treatment is displayed in blue circles and in red triangles the reduced one. The error bars are estimated on the basis of the analysis of the influence of de-noised images by Wiener filtering.


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ASSOCIATED CONTENT Supporting Information. Particle size distribution, digital diffraction pattern, characteristics of the modeled gold nanoparticles, peak intensity analysis, index of the atomic columns, strain map for simulated images and study of the influence of Poisson noise on strain determination.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias. Universidad de Cadiz. Campus Río San Pedro. E-11510. Puerto Real, Cádiz (Spain). FAX: +34-956-016288 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Authors acknowledge funding from MINECO/FEDER (MAT2013-40823R and CSD09-00013). Financial resources from the European Union Seventh Framework Programme under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative – I3) is also acknowledged. MLH acknowledges funding from MINECO Juan de la Cierva Program (Ref: IJCI-2014-19367).


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